Variability of soil carbon and nitrogen stocks after conversion of natural forest to plantations in Eastern China

Abstract

Forest plantation, either through afforestation or reforestation, has been suggested to reverse and mitigate the process of deforestation. However, uncertainties remain in the potential of plantation forest (PF) to sequestrate carbon (C) and nitrogen (N) compared to natural forest (NF). Soil C and N stocks require a critical and updated look at what is happening especially in the context of increasing rate of land use change and climate change. The current study was conducted in China’s Eastern forest to estimate soil C and N stocks in six depth layers (0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm) and two forest types (NF and PF) at four sites along climate factors gradient. The results showed that the overall mean soil C and N amounts to a depth of 20 cm ranged from 2.6 ± 1.1 Mg ha⁻¹ to 38.6 ± 23.1 Mg ha⁻¹, and soil nitrogen stock ranged from 0.2 ± 0.1 Mg ha⁻¹ to 3.3 ± 1.5 Mg ha⁻¹. Moreover, a loss of C stock was observed at Qingyuan (QY) by −7%, Dinghushan (DH) by −26%, Jianfengling (JF) by −13% while that of N stock was observed at QY (−8%), DH (−19%) and JF (−12%) at both depth layers. These results indicate that NFs have a better capacity to accumulate soil C and N. The soil C and N decreased from the southeast to the northeast and increased from tropical to temperate mixed forests zone in the eastern part of the study area. The C and N stock mainly occurred in the topsoil and decreased significantly with depth. Moreover, soil C and N stocks increased with age of plantation. This study provides an overview of the current spatial distribution and soil stocks of C and N, as well as the effects of environmental factors on soil C and N stocks. It also indicated that, although mean annual temperature and mean annual precipitation are the key factors affecting the variations in soil C and N, their vertical and horizontal distribution differed in various aspects.

Introduction

Soil is the major carbon sink of global terrestrial ecosystems, storing about 1,550 Pg of carbon (C), which is twice the atmospheric carbon pool (Lal, 2004). The C stock is closely linked to the soil nitrogen (N) which could increase soil C stocks through promoting plant growth and improving the net primary productivity of terrestrial ecosystem (Solberg et al., 2004). However, it is limited by the accessibility of soil N in forest ecosystem due to the fact that N dynamics can regulate terrestrial carbon sequestration, for example, increasing N inputs leads to sustainable C sequestration (Reich & Oleksyn, 2004; Deng et al., 2016). Soil N does not only have a substantial impact on soil carbon sinks through the interaction between soil nitrogen and carbon, but also on maintaining the ecological function of plantation ecosystem (Reich et al., 2006; Liu et al., 2016). Furthermore, the C stability of terrestrial ecosystems is notably sensitive and can be directly affected to impact human activities in the short term, including deforestation, biomass burning, land-use changes, forest management practices and environmental pollution (Batjes & Dijkshoorn, 1999; Stockmann et al., 2013). It has been recognized that small fluctuations of soil organic carbon (SOC) pool could have large impacts on the atmospheric carbon dioxide (CO₂) concentration by implication in the control of the greenhouse effect (Lal, 2004; Powlson, Whitmore & Goulding, 2011).

Due to the importance of soil C and N as a source or as a sink to atmospheric CO₂, several studies have assessed soil C and N stocks (Batjes, 2002; Maquere et al., 2008; Gross et al., 2018; Gao et al., 2019b; Li et al., 2019; Da Silva Santana et al., 2019). Besides, an assessment of soil C stock is crucial for the evaluation of the capacity of soil to sequester atmospheric C. It has been demonstrated that soil C and N stocks are influenced by the complex interactions of tree species, management, climate, vegetation cover, land use (LU), bulk density (BD), soil type and texture (Batjes, 1996; Lal, 2005). For example, numerous studies have reported that tree species can alter soil C and N stocks by many processes such as changes in litter quantity and quality, turnover rate of roots and exudates, microbial communities and soil physicochemical properties (Paul et al., 2002; Pérez-Cruzado et al., 2012; Wang et al., 2013; Hoogmoed et al., 2014; Deng & Shangguan, 2017). Globally, LU change is the second largest source of greenhouse gas emissions (GHG) after burning fossil fuels and the first for the tropical region (Houghton, 2003). Deforestation through the conversion of natural forests (NF) to other LUs contributes to GHGs emission and could alter soil C and N cycles by variations in the amount of forest floors and may have a significant impact on the total amount of GHG emissions (Minasny et al., 2017; Gao et al., 2019a). Ensuring the stability of forest carbon stocks is an essential global challenge in the future. Instituting control measures to reduce GHG from forests is therefore essential for maintenance of stable stock of forest carbon.

Plantation forest (PF) either through afforestation or reforestation have been suggested to overcome this problem and ensure the sustainability of the forests through the process of deforestation, which consequently favors the accumulation of C (Metz et al., 2007) as PF have potential to contribute to “Kyoto Protocol” targets for reducing net national GHG emissions (Bolin, 1998) as large quantities of atmospheric CO₂ can be fixed into tree biomass for a long period of time (Hoen & Solberg, 1994). Although, the assessment of the soil C and N stock is complicated, it is crucial to assess if all PF necessarily result in carbon sequestration in the soil or whether this sequestration is significant. In other words, it is essential to know if it can play the same role as PF of storing soil carbon. Therefore, quantitative assessment of soil C and N stocks and their dynamics is crucial in understanding the carbon sink capacity of terrestrial ecosystems in the context of the climate change. We hypothesized that the conversion of NF–PF would result in a loss of C and N stocks, and that topsoil has higher potential accumulation than subsoil in both forest types. Also, we expected that C, N stocks will decline with depth, but would be influenced by mean annual temperature (MAT) and mean annual precipitation (MAP) especially in NF. Therefore, the objectives of this study were to (1) estimate the soil C and N stocks within the 0–100 cm soil layer along and compare their variations throughout the profile under different LU types; (2) investigate the effects of forest conversion in soil C and N stocks and assess the interaction between soil C and N changes and (3) analyze the effects of environmental factors and main factors influencing soil C and N stocks.

Materials and Methods Study site and soil sampling

The study sites were located in Chinese eastern forest which extends from Hainan Island to China’s northern border, ranging from 105°E to 130°E at latitudes and from 10°N to 50°N (Fig. 1). This zone provides an ideal platform to research carbon and nitrogen of forest ecosystems in East Asia’s monsoon region (Yu et al., 2008). Along the Eastern forest of China, we chose four forests which included four forest sites were selected as experimental plots including Qingyuan (QY), Huitong (HT), Dinghushan (DH) and Jianfengling (JF) across eastern China forest (Fig. 1). Vegetation sequence distribution includes temperate mixed forests, evergreen broadleaved forest, subtropical evergreen coniferous forest and tropical rainforest from north to south (Sheng et al., 2014). The specific characteristics of the sampling sites are described in Table 1. A total of 168 samples were collected in August 2017 from NF to PF stand of each site. Three pits were dug at one m depth for soil sampling and mineral soil samples were collected at six depth layers 0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm along soil profiles. MAT and MAP data collected from 1960 to 2014 in the adjacent climate monitoring stations are utilized herein. Soil C and N content were measured using an isotope ratio mass spectrometer (IsoPrime 100; Isoprime Ltd., Cheadle, UK), connected to a CN elemental analyzer (Vario MICRO cube; Elementar, Langenselbold, Germany). Soil pH was determined using a digital potentiometric pH meter in 1:3 soil suspensions in both 0.01 M CaCl₂ solution and deionized water. Soil samples were extracted using a stainless-steel cylinder of 100 cm³ in volume of the undisturbed soil samples and BD was calculated by dividing the oven-dried weight of fine earth by the volume of the core (Table 2).

Figure 1: The location of forest stands at four sites across eastern China. QY, Qingyuan; HT, Huitong; DH, Dinghushan; JF, Jianfengling. DOI: 10.7717/peerj.8377/fig-1

Table 1: Location and characteristics of forest stands at four sites across the eastern China.

Sites	Location	pH		Province	Elevation (m)	Soil type	Climate zones	MAT (°C)	MAP (mm)	Natural land use	Plantation type	Plantation age
NF	PF
Qingyuan (QY)	N 41.85 E 124.93	5.57	5.54	Liaoning	597	Brown forest soil	Mid temperate	5.91	794	DBL	Pinus koraiensis.	38
Huitong (HT)	N 26.85 E 109.60	4.58	4.69	Hunan	427	Lateritic red soil, yellow soil	Mid subtropical	17.17	1,256	EBL	Cunninghamia lanceolata	33
Dinghushan (DH)	N 23.17 E 113.52	4.02	4.37	Guangdong	275	Lateritic red soil, yellow soil	Southern subtropical	21.08	1,955	MEB	Pinus massoniana	30
Jianfengling (JF)	N 18.44 E 108.01	4.52	4.69	Hainan	800	Yellow soil	Tropical	19.80	2,499	TMF	Pinus caribaea	30

DOI: 10.7717/peerj.8377/table-1

Note:

DBL, Deciduous broad-leaved forest; EBL, Evergreen broad-leaved forest; MEB, Monsoon evergreen broad-leaved forest; TMF, Tropical monsoon forest. NF, Natural Forest; PF, Plantation Forest.

Table 2: Soil C and N content, Soil BD under different forest types.

	Soil depth (cm)	QY		HT		JF		DH
	NF	PF	NF	PF	NF	PF	NF	PF
C content (g kg−1)	0–10	48.95	41.06	17.09	15.15	25.38	19.39	25.77	14.67
	10–20	26.67	23.41	10.03	12.87	15.83	12.48	12.53	5.05
	20–40	12.84	12.99	8.54	9.43	11	6.93	6.97	3.93
	40–60	6.97	5.82	5.47	6.02	6.58	6.21	5.14	3.03
	60–80	4.38	3.86	4.54	5.07	4.61	8.52	4.7	6.21
	80–100	2.76	3.07	4.06	4.31	3.34	3.28	3.57	3.41
N content (g kg−1)	0–10	4.23	3.23	1.99	1.66	1.98	1.51	2.13	1.24
	10–20	2.6	2.42	1.3	1.48	1.32	1.13	1.13	0.6
	20–40	1.33	1.49	1.18	1.17	0.97	0.64	0.75	0.49
	40–60	0.79	0.69	1	0.97	0.66	0.64	0.67	0.45
	60–80	0.51	0.49	0.94	0.91	0.5	1.17	0.63	1.39
	80–100	0.34	0.36	0.92	0.9	0.36	0.35	0.57	0.56
BD (g cm−3)	0–10	0.80	0.95	1.10	1.26	0.98	1.08	0.98	1.54
	10–20	1.03	1.09	1.12	1.21	1.14	1.31	1.30	1.66
	20–40	1.27	1.26	1.13	1.28	1.24	1.48	1.41	1.69
	40–60	1.27	1.32	1.16	1.26	1.34	1.41	1.39	1.51
	60–80	1.28	0.92	1.18	1.37	1.54	1.36	1.52	1.48
	80–100	1.20	0.76	1.25	1.33	1.43	1.41	1.46	1.55

DOI: 10.7717/peerj.8377/table-2

Notes:

Data are means of three plots.

NF, Natural forest; PF, Plantation forest; QY, Qingyuan; HT, Huitong; DH, Dinghushan; JF, Jianfengling; BD, Bulk density.

Statistical analysis

Soil C and N stocks were determined on a weight to area basis (kg of C/N per m² of soil) per soil depth class was calculated using the following formula: [Formula omitted. See PDF.] where TX is soil C or N stock (kg C m⁻²); X%_i is C or N in percentage at depth i; BD_i is bulk density at depth i; and v_i is volume of soil at each horizon; a is the instrument’s typical precision ±0.005% for C and ±0.001% for N according to the manufacturer’s standard material (Assefa et al., 2017). The carbon and nitrogen content data were obtained from our previous manuscript (Ngaba et al., 2019) (Table 3).

Table 3: Two-way ANOVA results for all soil variables in both forest type.

		NF		PF
	Variables	F	P	F	P
Site	C stock	15.16	***	3.31	**
	N stock	28.99	***	4.85	**
Depth	C stock	5.35	**	36.33	***
	N stock	4.17	n.s	36.39	***
Site * Depth	C stock	0.31	n.s	2.24	n.s
	N stock	0.32	n.s	2.71	n.s

DOI: 10.7717/peerj.8377/table-3

Notes:

n = 15 (Depths), n = 4 (Site).

*Indicate a significant level at P < 0.05.

**Indicate a significant level at P < 0.01.

***Indicate a significant level at P < 0.001, respectively.

n.s, non significant.

Two-way ANOVA method was used to test the significance of differences in site, depth and their interactions on soil C and N stocks under NF and PF using a significance level of α = 0.05. Pearson correlation analysis was used to analyze the relationships between soil C or N stocks and pH, BD, elevation, age of plantation, MAT and MAP. All statistical analyses were performed using the SPSS version 20.0 (Systat Statistical Software Package for Windows) (Coakes & Steed, 2009).

Results Soil C and N stocks

For all the sampling sites evaluated, soil C stock ranged from 2.6 ± 1.1 Mg ha⁻¹ to 38.6 ± 23.1 Mg ha⁻¹ and soil nitrogen stock ranged from 0.2 ± 0.1 Mg ha⁻¹ to 3.3 ± 1.5 Mg ha⁻¹. Following the land use change (LUC) in the two depth layers, a gain of soil C was observed at HT (+16%) while a loss of C was observed at QY (−7%), DH (−26%) and JF (−13%) (Fig. 2), similar result was found for N stock. The current study reported a loss in depth layers at QY (−8%), DH (−19%) and JF (−12%) (Fig. 3). The pattern distribution of N stock at HT site changed following the soil depth. A loss of −5% was observed in the topsoil while a gain of +24% was reported in the subsoil. Moreover, the Northeast region (QY) had the highest C and N stock followed by the Southeast (JF) (Figs. 2 and 3). Generally, the spatial distribution of C and N stock increased from South to North with the decreasing climate factors. Soil C and N stock was significantly (P < 0.001) higher in the topsoil compared to subsoil in both forest types. The concentration of C in soil among 0–20 cm layers varied in the order of QY > JF > DH > HT in NF and in the order of QY > JF > HT > DH in PF. Similar result was observed for N stock. The highest and lowest concentrations of N stock were found at QY, respectively in both forest types.

Figure 2: Soil C stocks under different forests in the 0–100 cm soil layers along the study sites and land use. NF, Natural forest; PF, Plantation forest; (A) QY, Qingyuan; (B) HT, Huitong; (C) DH, Dinghushan; (D) JF, Jianfengling. Error bars are the standard errors of the means (n = 3). DOI: 10.7717/peerj.8377/fig-2

Figure 3: Soil N stocks under different forests in the 0–100 cm soil layers along the study sites and land use. NF, Natural forest; PF, Plantation forest; (A) QY, Qingyuan; (B) HT, Huitong; (C) DH, Dinghushan; (D) JF, Jianfengling. Error bars are the standard errors of the means (n = 3). DOI: 10.7717/peerj.8377/fig-3

Relationship between soil C, N stocks, and underlying factors

Soil C and N stock varied significantly (P < 0.05, for all) with sites, soil depth under NF and PF but not their interactions (Table 3). The statistical analysis showed a strong correlation between soil C and N stock (R²= 0.93, P < 0.001 and R²= 0.71, P < 0.001 for NF and PF, respectively). However, soil pH under the NF did not significantly change in all depths compared to that under PF which was negatively correlated with soil C and N stock in the subsoil of PF. In general, the BD increased progressively with increasing depth in both depth layers and did not have a strong effect on soil C and N stock in the topsoil of PF (Table 2). Plantation age and elevation were positively corelated with soil C and N except at 0–10 cm of PF where elevation was negatively correlated with their values (Table 4).

[ Table omitted. See PDF. ]

DOI: 10.7717/peerj.8377/table-4

Notes:

**Correlation is significant at the 0.01 level.

*Correlation is significant at the 0.05 level.

n.s, not significant; n = 15; BD, Bulk density; NF, Natural forest; PF, Plantation forest; MAT, Mean annual temperature; MAP, Mean annual precipitation.

Relationship between soil C, N stocks and climate factors

Soil C stock increased with increasing N stock in NF and PF LU types (Fig. 4). Moreover, we observed a significant relationship between these variables in NF (R² = 0.93, R² = 0.71, P < 0.001 for all in PF and NF, respectively). In general, soil C was negatively correlated to environmental factors following NF in both depth layer (Table 4), whereas soil N stock was significantly correlated with MAT in NF of both depth layers (P < 0.05, for all).

Figure 4: Correlation between soil carbon stock and nitrogen following land use. NF, Natural forest; PF, Plantation forest. DOI: 10.7717/peerj.8377/fig-4

Discussion

Specifically, we focused our attention on soil C and N at the topsoil (0–20 cm) depth layer in different LU type because topsoils accumulate more C and N than deeper soil layers (Deng et al., 2016; Angst et al., 2018; Zhang et al., 2019) and the subsoil generally contains less soil C (Rumpel & Kögel-Knabner, 2011; Kunlanit, Butnan & Vityakon, 2019).

Effects of land-use change on soil C and N stock

Our results indicated that LUC from NF to PF is one of the key variables explaining the variation in soil C and N stock. The statistical analysis showed that soil C and N stock was significantly different across forest types (P < 0.01). Although a gain of C and N was observed at HT, the current study showed a clear decrease by −7%, −13% and −26% at QY, JF and DH, respectively, following the conversion from NF to PF. These findings suggest that LUC from NF to PF influences C and N inputs into the soil followed by a decrease in soil C and N stocks, which is in line with previous studies. For example, Guo & Gifford (2002) reported a decrease in soil C stocks after conversion forest to plantation by −13% whereas Girmay et al. (2008) observed a decrease by −83% in topsoil (0–10 cm). The patterns of gain and loss in soil C and N can vary according to a broad variety of soil types, shifts in abundance of woody and herbaceous vegetation, microbial activities, altered soil water, and temperature regimes which accelerate decomposition (Covington, 1981; Johnson et al., 1995; Jackson et al., 2000; Turner, Lambert & Johnson, 2005). The change of litter fall/input, the amount and type of plant residues associated with microbial activities produced by plants can explain, in part, the change of C and N stock. According to Arevalo et al. (2009), the lack of mixing mineral soil with the surface litter material for example, resulted in low C stocks and the accumulation of litter in the humus layer. On the other hand, soil N stock is attributed with the change of N input such as atmospheric N deposition, biological N fixation and output such as N uptake by plant, N emission to groundwater or the atmosphere (Li, Niu & Luo, 2012).

Changes in soil C and N are associated with changes of trees species and their diversity through variation in litter quantity and quality, exudates and turnover rate of roots (Wang et al., 2013; Hoogmoed et al., 2014; Deng & Shangguan, 2017). Our findings largely confirm this trend as it was observed that Pinus koraiensis stock five times more soil C than Metasequoia glyptostroboides and two times more than Cunninghamia lanceolata in PF. Similar results were observed for N stock. In addition, C. lanceolata stocks four times more N than M. glyptostroboides. Variation in soil C and N values with tree species as demonstrated in the present study has been also reported previously. For example, Demessie, Singh & Lal (2011) argued that coniferous species (Pinus patula and Juniperous procera) accrue more stock of C and N compared to the Eucalyptus species. Other studies suggested that soil C stock was generally larger under coniferous species than broadleaf species (Augusto et al., 2002; Kasel & Bennett, 2007; Schulp et al., 2008) or increases linearly with tree species diversity (Montagnini & Porras, 1998). According to Wang et al. (2013), SOC stock in 0–20 cm layer was significantly higher in the mixed plantation than in the monoculture plantations. Thus, the potential of C and N sequestration in soil varies from one species to another within an area.

Effects of soil depth on soil C and N stock

In general, soil C and N stocks decrease with increase of soil depth. For example, the topsoil of C accounts for 59% (QY) and 57% (JF) in NF, and similar results were observed in PF. In addition, soil N stock accounted for 68% in NF and 53% in PF of HT which is consistent with previous studies. Similar to our results, Li et al. (2019) and Wang et al. (2016) found a decrease of soil C and N stocks with depth. Therefore, we proposed the hypothesis that topsoil is most biologically active than subsoil and thus, has higher soil C and N accumulation. In addition, our results also provide some indication that soil C and N stock do not only reduce in the topsoil (0–10 cm), but also in the subsoil (10–20 cm). The statistical analysis showed a significant variation and negative correlation of soil C and N following soil depth indicating that soil C and N sequestration mechanisms varied with the soil profile. This result could be due to destruction of the existing litter layer and an increasing soil organic C mineralization rate through to the low rate of canopy covers which exposed the soil in PF to the solar radiation (Chen & Wang, 2007). On the other hand, soil disturbance on vegetation can also, in part, explain this result by way of differences in root distribution and its management. Obviously, different vegetation types will have different litter decomposition processes consequently leading to differences in release of C and N in the soil (Zhang et al., 2013). Jobbágy & Jackson (2000) for example, argued that root distributions affect the vertical placement of C in the soil, and above- and below-ground allocation affects the relative amount of C that eventually falls to the soil surface from shoots. Moreover, the action of deep roots which create pores and facilitate the movement of this nutrient could explain the larger soil N stock observed in NF (Poirier, Roumet & Munson, 2018). In addition, vegetation management influences the balance of forest C by desired crop tree species and by increasing the rate of C storage and biomass accumulation (Colombo et al., 2005). Consequently, in the current study soil disturbance affected the balance of C entering via plant exudates and residues and C output through mineralization in the soil and on N stock.

The variation in soil C and N with soil depth likely results from type of soil as postulated in other studies. Da Silva Santana et al. (2019) proposed that C and N stocks was lower in Planosols than Acrisol and Ferralsol in Brazil partly due to their shallowness and their lower capacity to stabilize N in the organic form. West & Six (2007) further suggested that soil type influenced the equilibrium between the C inputs and outputs. The current study showed that the level of C and N stock in lateritic yellow soil (Ultisols in the US Soil Classification) of JF was significantly higher with those found in mountain reddish yellow earth of HT site. However, no significant difference has been observed between the soil type at DH and JF. This finding indicates that the residence time and capacity of sequestration varies with soil types.

Effects of BD and soil pH on soil C and N stock

The statistical analyses showed that soil BD and pH vary significantly with soil depth. Soil pH tends to decrease with an increasing soil depth under different forest types while the opposite has been observed for BD. Our study showed that BD was strongly correlated with soil C and N stock except in the topsoil of PF, but only significantly correlated with pH in the subsoil of PF . This result suggests that the pattern distribution of soil BD and pH affects soil function such as soil microbial community and microbial activity (Thomas, 1996) which are closely related to soil C and N stock. It has been suggested that low soil pH can decrease microbial biomass and activity (Blagodatskaya & Anderson, 1998) and might also lead to the accumulation of soil C (Beets, Oliver & Clinton, 2002, Chen, Xu & Mathers, 2004). In addition, soil pH is influenced by trees species that can directly affect the pattern distribution of soil C and N stock since tree species have different capacity of sequestration in the soil. Generally, a low BD at total depth of 0–20 cm in both forest types was observed and it was significantly influenced by type of LU and not by soil depth. The lowest rate of BD was observed in DH where curiously the lowest values of soil C and N stock has been observed. This finding prompts us to hypothesize that the rate of BD may influence C and N input in the soil. Consistent with our findings, Demessie, Singh & Lal (2011) suggested that the lowest C and N stocks compared to the reference under Cupressus lusitanica compared to the other plantation sites may be partly ascribed to the lower BD along the profile.

Effects of elevation and stand age on soil C and N stock

The current study showed that elevation is one of the main factors controlling soil C and N stock variation. We observed that elevation favors C and N accumulation in soils particularly in both depth layers of NF while it had a negative correlation in the topsoil of PF. Saby et al. (2008) found that elevation was a control factor on SOC in a French region. Previous studies reported a lower carbon and nitrogen stock at lower elevation (Tesfaye et al., 2016). This trend was confirmed in the current study as the lowest values in soil C and N stock were found at DH which is located at the lowest elevation (275 m). Besides, the pattern of distribution of soil C and N stock following the elevation showed an increase of those variables with an increase in elevation. This finding is in line which the results obtained by Jones et al. (2005) who reported that the largest SOC stocks occur in high elevation areas of Europe.

The mechanism of soil carbon sequestration with vegetation restoration is more complex (Wang et al., 2019). Although it has been reported that there is a high rate of growth and carbon uptake in young trees, our finding reported a higher soil C and N stock in NF compared to PF yet it has younger vegetation. This result could be explained by the stand age. In the current study, NF has much longer stand age than PF, hence soil C and N inputs have probably more time to accumulate in the soil of NF through litter and roots in the long term compared to PF. Our findings are therefore consistent with the results found by Arevalo et al. (2009) who reported an increase of SOC stock with plantation age. In addition, our results showed that plantation age positively affected C and N stock with time. Interestingly, in PF, the highest rate of C and N stock was found at the QY which is the oldest PF site with 33 years old and the lowest values were observed at the youngest site of DH. Consequently, older stand general has a higher soil C stock because of the long-term soil C accumulation (Sariyildiz, Savaci & Kravkaz, 2015). Deng et al. (2017) reported that soil C stocks of Caragana korshinskii plantations increased remarkably with stand age from young to mature plantations in the Loess Plateau, whereas Laganière, Angers & Paré (2010) reported a gain of 6.1% and 18.6% in mature stage (10–30 years) and older stage (>30 years) plantations, respectively, with increase of time. Therefore, PF might need the same time or longer time period to accumulate the same level of NF in soil C and N stock in the soil. However, the verification of this hypothesis could be difficult because C stored in NF has a longer residence time and has a greater susceptibility to loss in PF (Mackey et al., 2008). Moreover, the difference observed could be also due to site management such as site preparation, the effects of harvesting, the type and level of silviculture activities and the antecedent soil fertility.

Effects of forest management on soil C and N stock

Forest management can also affect the change of soil C and N stock through soil erosion or deposition. Certain management practices such as the site preparation by breaking down the physical protection of soil C can significantly increase the decomposition of soil organic C (Guo & Gifford, 2002). Thus, increasing the frequency of erosion after rainfall consequently influences soil structural stability and porosity. Our finding indicates that although it has been demonstrated that SOC does not accumulate indefinitely (Johnston, Poulton & Coleman, 2009), this alternative of replacement of NF with PF can be effective if the right measures are implemented avoiding as site preparation with burnt treatment, soil erosion, vegetation burning, wood products were harvested and (/or) an increased output as plantations (Harmon, Ferrell & Franklin, 1990; Berthrong, Jobbagy & Jackson, 2009; Liao et al., 2010) and maximizing litter inputs to soils. Otherwise, it will be not a sustainable measure because its performance can decrease over time partially if proper management actions are not taken. Luo & Zhang (2006) for example, reported that soil organic C stock decreased by 10% from the first to the second rotation for C. lanceolata plantations, and by 15% from second to the third rotation. The application of good management practices in PF could run up soil C and N sequestration after several years through maximizing litter inputs and might be valued to gain of C and N storage in the soil. The low values in soil C and N stock observed at DH could be as a result of the high level of site disturbance in this site which considerably decreases the rate of fine root biomass and affects the balance between biomass production and decomposition, root distribution or vegetation communities. According to Jobbágy & Jackson (2000) root distributions affect the vertical placement of C in the soil and Yimer, Ledin & Abdelkadir (2006) reported in the Bale Mountains that the mean SOC stocks were lower among the vegetation communities in the western and northern aspects than in the southern and the eastern aspects both in the upper 0.3 m soil layer.

Effects of climate factors on soil C and N stock

Soil C and N stocks are influenced by the complex interactions of climate (Lal, 2005) due to its impact on microbial activity and forest growth, consequently on the quantity and quality of organic residue soil inputs and on the rates of soil organic matter mineralization and litter decomposition (Quideau et al., 2001; Heviaa, Buschiazzoa & Heppera, 2003). Higher soil temperatures for example, increase microbial decomposition of organic matter (Conant et al., 2011) and high precipitation can also lead to C transport down the soil profile as dissolved and/or particulate organic matter (Borken & Matzner, 2009). According to Post et al. (1982), the influence of climate factors results from their influence on the balance of carbon inputs from plant production and outputs through decomposition in soil. The current study reported a significant impact of climate factors on the soil C and N stock in both depth layers. Soil C and N was negatively linked with climate factors; we observed a decrease in soil C and N stocks with MAT and MAP following soil depth layer. This finding is consistent with previous studies. Vieira et al. (2011) for example, reported a significant inverse correlation between carbon and nitrogen stocks and soil temperatures whereas Quideau et al. (2001) and Heviaa, Buschiazzoa & Heppera (2003) argued that this is probably due to their effects on the rates of soil organic matter mineralization and litter decomposition and on the quantity and quality of organic residue soil inputs.

Previous studies reported a decrease in SOC levels followed by an increase of MAT (Wang et al., 2004), a trend that was partially confirmed in our study. The highest values of soil C and N stock were found at the coldest area, particularly at QY. However, we also observed an increase in soil C and N stock from Southeast to Northeast, particularly from JF to QY. The highest values were found at QY probably due to lower temperature conditions favoring accumulation of organic matter in these vegetation communities (Post et al., 1982).

Interestingly, we observed a similar pattern of distribution between soil C and N stock and climate factors. Soil C and N stock decreased with decreasing precipitation and temperature from the Southeast to the Northeast. This trend is in line with previous studies (Jobbágy & Jackson, 2000; Amundson, 2001; Homann, Kapchinske & Boyce, 2007), Townsend, Vitousek & Trumbore (1995) and Trumbore, Chadwick & Amundson (1996) who indicated that lower temperatures could result in reduced SOC breakdown, thereby increasing SOC accumulation. By contrast, we observed that the highest soil C and N were found at QY under the lowest precipitations followed by JF which was under the highest precipitations. This differential response could be attributed to low decomposition of litter under those precipitation regimes (Seneviratne, Van Holm & Kulasooriya, 1997) and the complex interactions which exist between plant species, soil conditions, microorganisms and climatic conditions. According to Leifeld, Bassin & Fuhrer (2005), it could be due to the couple effect of lower temperatures and higher altitudes which probably limit C turnover, which results in increased C accumulation even under conditions of smaller productivity and C inputs. Although, it is generally accepted that MAT and MAP temperature are the main factors influencing the potential of C and N sequestration, we could not find consensus on the specific effects of climate factors in soil C and N.

Correlation between C and N stock

Similar to previous studies (Li, Niu & Luo, 2012; Deng, Shangguan & Sweeney, 2013; Deng et al., 2016), we observed a significant positive correlation between soil C and N stock in both forest types. The spatial distribution of soil C and N stock was similar with the largest values observed in the Northeast (QY) followed by Southeast (JF). We also observed an increase of N stock following an increase of C stock, indicating that the C budget is limited by the availability of soil N due to the coupling effect between C and N cycles in forest ecosystem (Reich & Oleksyn, 2004; Melillo et al., 2011). Besides, the relationship of soil C and N stock in NF was significantly stronger in the topsoil than in the subsoil probably due to the effect of rainfall on N vertical distribution. According to Deng et al. (2016), rainfall can facilitate the migration of N into deeper soils, increase N accumulation in the subsoil during revegetation process and vegetation restoration, and thereby decreased soil C–N relations. This finding provides support to the trend observed below, N dynamics regulate terrestrial carbon sequestration for example, increasing N inputs lead to sustainable C sequestration (Deng et al., 2016). Thereby, high soil N concentration stimulates tree growth, which potentially increases carbon inputs into soils through litterfall and rhizo deposition, and promotes SOC sequestration by decreasing decomposition rates of old litter and recalcitrant soil organic matter by suppression of soil microbes and by chemical stabilization (Jandl et al., 2007; Mo et al., 2008).

Conclusions

Land-use changes from NF to PF significantly affected soil C and N input although controlling factors induced differences in C and N stocks. The destruction of NF contributes to the loss of C and N from soil and a net increase of carbon in the atmospheric CO₂, which aggravates climate change. Our study demonstrated that although PFs can lead to higher C and N sequestration and have been promoted as a measure to mitigate future climate change, NFs have below-ground C and N processes which promote better accumulation in the soil. Although PF is one obvious approach of maintaining or increasing future wood supply and mitigating the impacts related to their destruction, it cannot replace the ecological roles played by NF especially if we consider animal biodiversity and the loss of their habitat. The best way to protect and preserve NFs for a healthy environment is to encourage the reforestation of degraded environments. The results of the currents study showed also the complex interaction existing between abiotic and biotic factors and soil C and N input.

Supplemental Information

Raw data.

DOI: 10.7717/peerj.8377/supp-1

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Additional Information and Declarations

Competing Interests

The authors declare that they have no competing interests.

Author Contributions

Mbezele Junior Yannick Ngaba conceived and designed the experiments, performed the experiments, analyzed the data, prepared figures and/or tables, and approved the final draft.

Xiang-Qing Ma conceived and designed the experiments, performed the experiments, authored or reviewed drafts of the paper, and approved the final draft.

Ya-Lin Hu conceived and designed the experiments, authored or reviewed drafts of the paper, and approved the final draft.

Data Availability

The following information was supplied regarding data availability:

Raw data is available as a Supplemental File.

Funding

This study was sponsored by the National Natural Science Foundation of China (No. U1805243), State Key Laboratory of Soil and Sustainable Agriculture and China Scholarship Council (CSC) University Scholarship Program. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

References

Amundson R. 2001. The carbon budget in soils. Annual Review of Earth and Planetary Sciences 29(1):535-562

Angst G, Messinger J, Greiner M, Häusler W, Hertel D, Kirfel K, Kögel-Knabner I, Leuschner C, Rethemeyer J, Mueller CW. 2018. Soil organic carbon stocks in topsoil and subsoil controlled by parent material, carbon input in the rhizosphere, and microbial-derived compounds. Soil Biology and Biochemistry 122:19-30

Arevalo CBM, Bhatti JS, Chang SX, Sidders D. 2009. Ecosystem carbon stocks and distribution under different land-uses in north central Alberta, Canada. Forest Ecology and Management 257(8):1776-1785

Assefa D, Rewald B, Sandén H, Rosinger C, Abiyu A, Yitaferu B, Godbold DL. 2017. Deforestation and land use strongly effect soil organic carbon and nitrogen stock in Northwest Ethiopia. CATENA 153:89-99

Augusto L, Ranger J, Binkley D, Rothe A. 2002. Impact of several common tree species of European temperate forests on soil fertility. Annals of Forest Science 59(3):233-253

Batjes NH. 1996. Total carbon and nitrogen in the soils of the world. European Journal of Soil Science 47(2):151-163

Batjes NH. 2002. Carbon and nitrogen stocks in the soils of Central and Eastern Europe. Soil Use and Management 18(4):324-329

Batjes NH, Dijkshoorn JA. 1999. Carbon and nitrogen stocks in the soils of the Amazon Region. Geoderma 89(3–4):273-286

Beets PN, Oliver GR, Clinton PW. 2002. Soil carbon protection in podocarp/hardwood forest, and effects of conversion to pasture and exotic pine forest. Environmental Pollution 116:S63-S73

Berthrong ST, Jobbagy EG, Jackson RB. 2009. A global meta-analysis of soil exchangeable cations, pH, carbon, and nitrogen with afforestation. Ecological Applications 19(8):2228-2241

Blagodatskaya EV, Anderson TH. 1998. Interactive effects of pH and substrate quality on the fungal-to-bacterial ratio and qCO2 of microbial communities in forest soils. Soil Biology and Biochemistry 30(10–11):1269-1274

Bolin B. 1998. CLIMATE: the Kyoto negotiations on climate change—a science perspective. Science 279(5349):330-331

Borken W, Matzner E. 2009. Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils. Global Change Biology 15(4):808-824

Chen JN, Wang SL. 2007. Dynamic of soil carbon pool in Cunninghamia lanceolata plantation: recent advances and future prospects. Guangxi Forestry Science 36:147-151

Chen CR, Xu ZH, Mathers NJ. 2004. Soil carbon pools in adjacent natural and plantation forests of subtropical Australia. Soil Science Society of America Journal 68(1):282-291

Coakes SJ, Steed L. 2009. SPSS: analysis without anguish using SPSS version 14.0 for windows. Hoboken: John Wiley and Sons.

Colombo SJ, Parker WC, Luckai N, Dang Q, Cai T. 2005. The effects of forest management on carbon storage in Ontario’s forests. In: Climate Change Research Report (CCRR-03). Marie: Ontario Forest Research Institute.

Conant RT, Ryan MG, Ågren GI, Birge HE, Davidson EA, Eliasson PE, Evans SE, Frey SD, Giardina CP, Hopkins FM, Hyvönen R, Kirschbaum MUF, Lavallee JM, Leifeld J, Parton WJ, Megan Steinweg J, Wallenstein MD, Martin Wetterstedt JÅ, Bradford MA+9 more. 2011. Temperature and soil organic matter decomposition rates—synthesis of current knowledge and a way forward. Global Change Biology 17(11):3392-3404

Covington WW. 1981. Changes in forest floor organic matter and nutrient content following clear cutting in northern hardwoods. Ecology 62(1):41-48

Da Silva Santana M, De Sá Barretto Sampaio EV, Giongo V, Menezes RSC, De Jesus KN, De Albuquerque ERGM, Do Nascimento DM, Pareyn FGC, Cunha TJF, Sampaio RMB, Primo DC+1 more. 2019. Carbon and nitrogen stocks of soils under different land uses in Pernambuco state, Brazil. Geoderma Regional 16:e00205

Demessie A, Singh BR, Lal R. 2011. Soil carbon and nitrogen stocks under plantations in Gambo District, Southern Ethiopia. Journal of Sustainable Forestry 30(6):496-517

Deng L, Han Q-S, Zhang C, Tang Z-S, Shangguan Z-P. 2017. Above-ground and belowground ecosystem biomass accumulation and carbon sequestration with Caragana korshinskii Kom plantation development. Land Degradation & Development 28(3):906-917

Deng L, Shangguan Z-P. 2017. Afforestation drives soil carbon and nitrogen changes in China. Land Degradation & Development 28(1):151-165

Deng L, Shangguan ZP, Sweeney S. 2013. Changes in soil carbon and nitrogen following land abandonment of farmland on the Loess Plateau, China. PLOS ONE 8(8):e71923

Deng L, Wang G-L, Liu G-B, Shangguan Z-P. 2016. Effects of age and land-use changes on soil carbon and nitrogen sequestrations following cropland abandonment on the Loess Plateau, China. Ecological Engineering 90:105-112

Gao W, Huang S, Huang Y, Yue X, Ye G. 2019a. Effects of tree species on soil carbon and nitrogen stocks in a coastal sand dune of southern subtropical China. Vegetos 32(2):142-150

Gao P, Zheng X, Wang L, Zhong C, Zhang S. 2019b. Changes in soil organic carbon and total nitrogen stocks in response to walnut plantation in the southern region of the Loess Plateau, China. Arid Land Research and Management 34(1):1-16

Girmay G, Singh BR, Mitiku H, Borresen T, Lal R. 2008. Carbon stocks in Ethiopian soils in relation to land use and soil management. Land Degradation & Development 19(4):351-367

Gross CD, James JN, Turnblom EC, Harrison RB. 2018. Thinning treatments reduce deep soil carbon and nitrogen stocks in a coastal pacific northwest forest. Forests 9(5):238

Guo LB, Gifford RM. 2002. Soil carbon stocks and land use change: a meta analysis. Global Change Biology 8(4):345-360

Heviaa GG, Buschiazzoa DE, Heppera EN. 2003. Organic matter in size fractions of soils of the semiarid Argentina: effects of climate, soil texture and management. Geoderma 116(3–4):265-277

Hoen HF, Solberg B. 1994. Potential and economic efficiency of carbon sequestration in forest biomass through silvicultural management. Forest Science 40:429-451

Homann PS, Kapchinske JS, Boyce A. 2007. Relations of mineral-soil C and N to climate and texture: regional differences within the conterminous USA. Biogeochemistry 85(3):303-316

Hoogmoed M, Cunningham SC, Baker PJ, Beringer J, Cavagnaro TR. 2014. Is there more soil carbon under nitrogen-fixing trees than under non-nitrogen-fixing trees in mixed-species restoration plantings? Agriculture, Ecosystems & Environment 188:80-84

Houghton RA. 2003. Why are estimates of the terrestrial carbon balance so different? Global Change Biology 9(4):500-509

Jackson RB, Schenk HJ, Jobbágy EG, Canadell J, Colello GD, Dickinson RE, Field CB, Friedlingstein P, Heimann M, Hibbard K, Kicklighter DW, Kleidon A, Neilson RP, Parton WJ, Sala OE, Sykes MT+6 more. 2000. Below-ground consequences of vegetation change and their treatment in models. Ecological Applications 10(2):470-483

Jandl R, Lindner M, Vesterdal L, Bauwens B, Baritz R, Hagedorn F, Johnson DW, Minkkinen K, Byrne KA. 2007. How strongly can forest management influence soil carbon sequestration? Geoderma 137(3–4):253-268

Jobbágy EG, Jackson RB. 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications 10(2):423-436

Johnson CE, Driscoll CT, Fahey TJ, Siccama TG, Hughes JW. 1995. Carbon dynamics following clear cutting of a northern hardwood forest. In: McFee WW, Kelly JM, eds. Carbon Forms and Functions in Forest Soils. Madison: Soil Science Society American. 463-487

Johnston AE, Poulton PR, Coleman K. 2009. Soil organic matter: its importance in sustainable agriculture and carbon dioxide fluxes. Advances in Agronomy 101:1-57

Jones RJA, Hiederer R, Rusco E, Montanarella L. 2005. Estimating organic carbon in the soils of Europe for policy support. European Journal of Soil Science 56(5):655-671

Harmon ME, Ferrell WK, Franklin JF. 1990. Effects on carbon storage of conversion of old growth forests to young forests. Science 247:699-702

Kasel S, Bennett LT. 2007. Land-use history, forest conversion, and soil organic carbon in pine plantations and native forests of south eastern Australia. Geoderma 137(3–4):401-413

Kunlanit B, Butnan S, Vityakon P. 2019. Land—use changes influencing C sequestration and quality in topsoil and subsoil. Agronomy 9(9):520

Laganière J, Angers DA, Paré D. 2010. Carbon accumulation in agricultural soils after afforestation: a meta analysis. Global Change Biology 16(1):439-453

Lal R. 2004. Soil carbon sequestration impacts on global climate change and food security. Science 304(5677):1623-1627

Lal R. 2005. Forest soils and carbon sequestration. Forest Ecology and Management 220(1–3):242-258

Leifeld J, Bassin S, Fuhrer J. 2005. Carbon stocks in Swiss agricultural soils predicted by land-use, soil characteristics, and altitude. Agriculture, Ecosystems & Environment 105(1–2):255-266

Li D, Niu S, Luo Y. 2012. Global patterns of the dynamics of soil carbon and nitrogen stocks following afforestation: A meta-analysis. New Phytologist 195:172-181

Li Q, Yang D, Jia Z, Zhang L, Zhang Y, Feng L, He L, Yang K, Dai J, Chen J, Zhao X+1 more. 2019. Changes in soil organic carbon and total nitrogen stocks along a chronosequence of Caragana intermedia plantations in alpine sandy land. Ecological Engineering 133:53-59

Liao C, Luo Y, Fang C, Li B. 2010. Ecosystem carbon stock influenced by plantation practice: implications for planting forests as a measure of climate change mitigation. PLOS ONE 5(5):e10867

Liu Y, Li P, Wang G, Liu G, Li Z. 2016. Above and below ground biomass distribution and morphological characteristics respond to nitrogen addition in Pinus tabuliformis. New Zealand Journal of Forestry Science 46(1):25

Luo YJ, Zhang XQ. 2006. Carbon stock changes of successive rotations of plantations. Forest Research 19:791-798

Mackey B, Keith H, Berry SL, Lindenmayer DB. 2008. Green carbon part 1: the role of natural forests in carbon storage. Canberra: ANUE Press. 43-47

Maquere V, Laclau JP, Bernoux M, Saint-Andre L, Gonçalves JLM, Cerri CC, Piccolo MC, Ranger J. 2008. Influence of land use (savanna, pasture, Eucalyptus plantations) on soil carbon and nitrogen stocks in Brazil. European Journal of Soil Science 59(5):863-877

Melillo JM, Butler S, Johnson J, Mohan J, Steudler P, Lux H, Vario C. 2011. Soil warming, carbon–nitrogen interactions, and forest carbon budgets. Proceedings of The National Academy of Sciences of The United States of America 108(23):9508-9512

Metz B, Davidson OR, Bosch PR, Dave R, Meyer LA. 2007. Contribution of working group III to the fourth assessment report of the intergovernmental panel on climate change. Cambridge and New York: Cambridge University Press.

Minasny B, Malone BP, McBratney AB, Angers DA, Arrouays D, Chambers A, Chaplot V, Chen ZS, Cheng K, Das BS, Field DJ, Gimona A, Hedley CB, Hong SY, Mandal B, Marchant BP, Martin M, McConkey BG, Mulder VL, O’Rourke S, Richer-de-Forges AC, Odeh I, Padarian J, Paustian K, Pan G, Poggio L, Savin I, Stolbovoy V, Stockmann U, Sulaeman Y, Tsui CC, Vågen TG, Van Wesemael B, Winowiecki L+24 more. 2017. Soil carbon 4 per mille. Geoderma 292:59-86

Mo J, Zhang W, Zhu W, Gundersen P, Fang Y, Li D, Wang H. 2008. Nitrogen addition reduces soil respiration in a mature tropical forest in southern China. Global Change Biology 14(2):403-412

Montagnini F, Porras C. 1998. Evaluating the role of plantations as carbon sinks: an example of an integrative approach from the humid tropics. Environmental Management 22(3):459-470

Ngaba MJY, Hu Y-L, Bol R, Ma X-Q, Jin S-F, Mgelwa AS. 2019. Effects of land use change from natural forest to plantation on C, N and natural abundance of 13C and 15N along a climate gradient in eastern China. Scientific Reports 9(1):1-12

Paul KI, Polglase PJ, Nyakuengama JG, Khanna PK. 2002. Change in soil carbon following afforestation. Forest Ecology and Management 168(1–3):241-257

Poirier V, Roumet C, Munson AD. 2018. The root of the matter: linking root traits and soil organic matter stabilization processes. Soil Biology and Biochemistry 120:246-259

Post WM, Emanuel WR, Zinke PJ, Stangenberger AG. 1982. Soil carbon pools and world life zones. Nature 298(5870):156-159

Powlson DS, Whitmore AP, Goulding KWT. 2011. Soil carbon sequestration to mitigate climate change: a critical re-examination to identify the true and the false. European Journal of Soil Science 62(1):42-55

Pérez-Cruzado C, Mansilla-Salinero P, Rodríguez-Soalleiro R, Merino A. 2012. Influence of tree species on carbon sequestration in afforested pastures in a humid temperate region. Plant and Soil 353(1–2):333-353

Quideau SA, Chadwick OA, Benesi A, Graham RC, Anderson MA. 2001. A direct link between forest vegetation type and soil organic matter composition. Geoderma 104(1–2):41-60

Reich PB, Hobbie SE, Lee T, Ellsworth DS, West JB, Tilman D, Knops JMH, Naeem S, Trost J. 2006. Nitrogen limitation constrains sustainability of ecosystem response to CO2. Nature 440(7086):922-925

Reich PB, Oleksyn J. 2004. Global patterns of plant leaf N and P in relation to temperature and latitude. Proceedings of the National Academy of Sciences USA 101(30):11001-11006

Rumpel C, Kögel-Knabner I. 2011. Deep soil organic matter—a key but poorly understood component of terrestrial C cycle. Plant and Soil 338(1–2):143-158

Saby NPA, Arrouays D, Antoni V, Lemercier B, Follain S, Walter C, Schvartz C. 2008. Changes in soil organic carbon in a mountainous French region, 1990–2004. Soil Use and Management 24(3):254-262

Sariyildiz T, Savaci G, Kravkaz IS. 2015. Effects of tree species, stand age and land-use change on soil carbon and nitrogen stock rates in northwestern Turkey. iForest: Biogeosciences and Forestry 9(1):165-170

Schulp CJE, Nabuurs G-J, Verburg PH, De Waal RW. 2008. Effect of tree species on carbon stocks in forest floor and mineral soil and implications for soil carbon inventories. Forest Ecology and Management 256(3):482-490

Seneviratne G, Van Holm LHJ, Kulasooriya SA. 1997. Quality of different mulch materials and their decomposition and N release under low moisture regimes. Biol Fertil Soils 26:136-140

Sheng W, Yu G, Fang H, Liu Y, Wang Q, Chen Z, Zhang L. 2014. Regional patterns of 15N natural abundance in forest ecosystems along a large transect in eastern China. Scientific Reports 4(1):4249

Solberg S, Andreassen K, Clarke N, Torseth K, Tveito OE, Strand GH, Tomter S. 2004. The possible influence of nitrogen and acid deposition on forest growth in Norway. Forest Ecology and Management 192(2–3):241-249

Stockmann U, Adams MA, Crawford JW, Field DJ, Henakaarchchi N, Jenkins M, Minasnya B, McBratney AB, De Remy De Courcelles V, Singh K, Wheeler I, Abbott L, Angers DA, Baldock J, Bird M, Brookes PC, Chenu C, Jastrow JD, Lal R, Lehmann J, O’Donnell AG, Parton WJ, Whitehead D, Zimmermann M+14 more. 2013. The knowns, known unknowns and unknowns of sequestration of soil organic carbon. Agriculture, Ecosystems & Environment 164:80-99

Tesfaye MA, Bravo F, Ruiz-Peinado R, Pando V, Bravo-Oviedo A. 2016. Impact of changes in land use, species and elevation on soil organic carbon and total nitrogen in Ethiopian Central Highlands. Geoderma 261:70-79

Thomas GW. 1996. Soil pH and soil acidity. In: Sparks DL, ed. Methods of Soil Analysis Part 3: Chemical Methods. Madison: Soil Science Society of America. 475-490

Townsend AR, Vitousek PM, Trumbore SE. 1995. Soil organic-matter dynamics along gradients in temperature and land-use on the Island of Hawaii. Ecology 76(3):721-733

Trumbore SE, Chadwick OA, Amundson R. 1996. Rapid exchange between soil carbon and atmospheric carbon dioxide driven by temperature change. Science 272(5260):393-396

Turner J, Lambert MJ, Johnson DW. 2005. Experience with patterns of change in soil carbon resulting from forest plantation establishment in eastern Australia. Forest Ecology and Management 220(1–3):259-269

Vieira SA, Alves LF, Duarte-Neto PJ, Martins SC, Veiga LG, Scaranello MA, Picollo MC, Camargo PB, Do Carmo JB, Neto ES, Santos FAM, Joly CA, Martinelli LA+3 more. 2011. Stocks of carbon and nitrogen and partitioning between above-and belowground pools in the Brazilian coastal Atlantic Forest elevation range. Ecology and Evolution 1(3):421-434

Wang S, Huang M, Shao X, Mickler RA, Li K, Ji J. 2004. Vertical distribution of soil organic carbon in China. Environmental Management 33(S1):200-209

Wang T, Kang F, Cheng X, Han H, Ji W. 2016. Soil organic carbon and total nitrogen stocks under different land uses in a hilly ecological restoration area of North China. Soil and Tillage Research 163:176-184

Wang H, Liu S, Wang J, Shi Z, Lu L, Zeng J, Ming A, Tang J, Yu H. 2013. Effects of tree species mixture on soil organic carbon stocks and greenhouse gas fluxes in subtropical plantations in China. Forest Ecology and Management 300:4-13

Wang Y, Liu L, Yue F, Li D. 2019. Dynamics of carbon and nitrogen storage in two typical plantation ecosystems of different stand ages on the Loess Plateau of China. PeerJ 7(11):e7708

West TO, Six J. 2007. Considering the influence of sequestration duration and carbon saturation on estimates of soil carbon capacity. Climatic Change 80(1–2):25-41

Yimer F, Ledin S, Abdelkadir A. 2006. Soil organic carbon and total nitrogen stocks as affected by topographic aspect and vegetation in the Bale Mountains, Ethiopia. Geoderma 135:335-344

Yu G-R, Zhang L-M, Sun X-M, Fu Y-L, Wen X-F, Wang Q-F, Li S-G, Ren C-Y, Song X, Liu Y-F, Han S-J, Yan J-H+2 more. 2008. Environmental controls over carbon exchange of three forest ecosystems in eastern China. Global Change Biology 14(11):2555-2571

Zhang Z-S, Song X-L, Lu X-G, Xue Z-S. 2013. Ecological stoichiometry of carbon, nitrogen, and phosphorus in estuarine wetland soils: influences of vegetation coverage, plant communities, geomorphology, and seawalls. Journal of Soils and Sediments 13(6):1043-1051

Zhang Y, Tang S, Xie S, Liu K, Li J, Chen Q, Huang D, Wang K. 2019. Effects of species-dominated patches on soil organic carbon and total nitrogen storage in a degraded grassland in China. PeerJ 7(8):e6897