Enhancing Sustainability in Intensive Dill Cropping: Comparative Effects of Biobased Fertilizers vs. Inorganic Commodities on Greenhouse Gas Emissions, Crop Yield, and Soil Properties

Abstract

The treatment and valorization of organic solid waste has become a promising alternative to increase intensive crop productivity while reducing its environmental impact. Currently, reusing improved organic waste as novel biofertilizers is a vital tool to adapt semiarid agricultural regions to climate change, but this has been scarcely studied in aromatic crops. The present study aims to assess the greenhouse gas emissions, soil properties, and crop yield of a dill crop using a drip irrigation system with a normalized N application rate of 160 kg N ha⁻¹. We compare eight different fertilizing scenarios grouped into organic-based (manures and compost) and inorganic-based inputs (NPK commodities and slow-release formulations). GHG fluxes were measured during the 57-day fertigation period using static chambers. Key soil properties were measured previous to fertilizer applications and at harvest, coinciding with crop yield estimations. An increase in soil organic carbon was observed with stabilized organic treatments at 0–20 cm soil depth. The results show that stabilized organic-based materials lowered NO₃⁻ concentrations in dill biomass more than synthetic fertilizers, producing similar yields to those with synthetic fertilizers. In general, N₂O emissions were positively affected by the treatments. Local specific emission factors for N₂O were determined (0.08%), which were substantially lower than the default value (0.51%) of IPCC. The cumulative CO₂ emissions were high in all the organic scenarios compared to the control treatment (277 kg C-CO₂ ha⁻¹), probably due to differences in labile organic C contents. Organic-based treatments showed multiple positive effects on crop quality, crop yields, and GHG mitigation potential. The use of organic amendments is an optimized N fertilizing strategy to promote circular economy and sustainability.

1. Introduction

Anethum graveolens L. (dill) is an aromatic herbaceous plant that originated as far east as India, although some authors say it is native to the Mediterranean [1]. It is cultivated industrially using improved varieties in Central and Eastern European countries, the Middle East, and North America. However, it can be found spontaneously in the Mediterranean area and some parts of Asia [2]. Cultivating aromatic plants is a growing industry that can be economically relevant (due to stable prices and long-term profitability) and promote rural development, especially to meet the increasing global demands of the food, pharmaceutical, and cosmetic industries. Dill is usually found in warm to temperate climates, but it can also withstand slightly colder temperatures. Its spatial distribution reaches up to 600 m in altitude, and it prefers sunny, light, humid, fertile, well-drained soils [3]. It is a crop with a short vegetative cycle; the duration of the crop is usually between 50 and 80 days after sowing to obtain leaves and between 100 and 120 days to obtain fully developed seeds. The fertilizing requirements of dill in intensive plantations are usually performed by fertigation at a rate of 120 kg N ha⁻¹, 50 kg P ha⁻¹, and 50 kg K ha⁻¹ to obtain the green part of the plant. Nitrogen (N) is the limiting criterion for calculating fertilizer dosage [4].

Intensive production systems in European Mediterranean regions contribute to the loss of soil fertility. Increasing threats have pushed soils to the critical limits of their ecosystems [5]. Reversing the trend of the soil organic carbon (SOC) loss resulting from long periods of agronomic activity with mineral fertilization can help improve soil health. The possibility of increasing SOC content by changing agronomic practices can play an essential role in facing climate change [6]. Maintaining organic matter (OM) is important not only to capture and mitigate greenhouse gases (GHG) [7] but also to significantly influence the physical, chemical, and biological properties of soil [8]. Pardo et al. [9] estimated an increase in SOC of 0.13 Mg ha⁻¹ yr⁻¹ if the entire agricultural surface of the Mediterranean region of Spain was amended with available, underutilized, exogenous organic materials (urban waste and composted agroindustrial by-products).

The valorization of organic matter as a source of nutrients is a strategy to reduce dependence on chemical fertilizers. Using stabilized and mature organic amendments can maintain and even increase C stocks in soils and improve N availability to crops [10]. As a result, they are nowadays very important inputs to consider to tackle post-COVID-19 recovery and the consequences of the global energy shortage [11]. With the long-term use of bio-stabilized organic amendments, the residual effects on crop production and soil properties can last for several years, as only a fraction of N and other nutrients are available to plants after application [12]. Using organic amendments would contribute to substituting inputs for the agroecological transition of aromatic crops, which consumers currently demand. In addition, producing compost from organic waste would increase the move toward a circular economy [13]. In this way, we can help establish better fertilizing strategies for agronomy, the environment, and sustainability.

Also, for GHG emissions, N-balanced fertilization is proposed as a strategy to reduce N₂O emissions in Mediterranean agricultural systems without affecting crop yields [14]. An additional mitigation effect could be achieved by applying already existing N (organic fertilizer) when possible or with the use of nitrification and urease inhibitors [15]. However, the incorporation of fertilizers (either organic or inorganic) with high ammonia (NH₄⁺) concentration and irrigation promotes nitrification and O₂ consumption (CO₂ release), generating suboxic conditions, increasing nitrifier denitrification (NO₂⁻ accumulation promotes reduced N₂O) [16]. There is a scarcity of data on emissions in aromatic crop production systems, so more field studies should be conducted to verify the emission factors proposed by other authors in intensive crops [17,18].

The main objective of this work is to comparatively evaluate the effect of different fertilizing materials (bio-stabilized organic, fresh organic, and inorganic) on a dill crop and to study the different conditions generated by each of them on the crop itself, the soil, and the GHGs emitted into the atmosphere during cultivation. The starting hypotheses were that: (i) fertilization treatments with N isodose have equal crop yields, (ii) GHG emissions during the cultivation period are larger from conventional fertilization than from organic amendments, and (iii) organic amendments improve soil properties more than inorganic fertilization.

2. Materials and Methods

2.1. Experimental Site

The study was carried out in a productive commercial field in Librilla, Murcia, Spain (37°55′ N, 1°17′ W; 180 m a.s.l.). The climate in this area is classified as warm semiarid according to the climate classification (Köppen) due to scarce annual rainfall (303 mm). During the field experiment (57 days), the average temperature was 17.4 °C, two rainfall events (total of 13.5 mm) were recorded, and the average relative humidity was 63.2%. The weather data were obtained from a meteorological station belonging to the Agro-meteorological Information System of the Murcia Region (SIAM-IMIDA) (Figure 1).

The soil at the field site is Calcaric Fluvisol [19], clay loam (34.3% sand, 12.6% silt, 53.1% clay; color 10YR 7/3 light brown). The (0–30 cm depth) physicochemical characteristics of the soil are: 0.63% SOC, 0.08% total N (TN), 117 mg kg⁻¹ N-NO₃⁻, 47.2 mg kg⁻¹ extractable phosphorus (Pe), 0.34 dS m⁻¹ electrical conductivity (EC), pH 8.4, 1.44 kg m⁻³ bulk density, 13.9 meq 100 g⁻¹ cation exchanging capacity (CEC), and 259 g kg⁻¹ water holding capacity (WHC).

2.2. Crop Management

The Anethum graveolens L. seeds (var. N18 SR22) were direct-seeded mechanically on 22nd March and hand harvested on 16th May (55-day growing season). The sowing density was 200 plants per m⁻². Soil and seedbed preparation (1.40 m; 30 cm between beds) were performed mechanically. Drip irrigation was installed with 4 L ha⁻¹ droppers, with the total irrigation applied over 579 m³ ha⁻¹ and a cumulated irrigation of 57.9 L m⁻² (Table 1). The water quality shows an adequate Ca:Mg ratio and a moderate–high risk of salinization [20]. Pest and disease management was performed conventionally with authorized products, respecting the recommended use and dosage.

2.3. Experimental Design and Treatment

A completely randomized design with four replicates was established. Eight fertilizing treatments were applied in the plots (5.93 m²) according to a normalized N application rate of 160 kg N ha⁻¹: (i) compost (HP), prepared from the mixture of wastewater treatment biosolid + Phoenix dactilifera leaf prunings (40–60% d.m.w.); (ii) compost (HP-2), made from a mixture of agrifood sludge from pear processing wastewater + Phoenix dactilifera leaf prunings (40–60% d.m.w.); (iii) vermicompost (VT), produced from cow manure; (iv) biosolid (LO) from the wastewater treatment plant in Orihuela (Alicante, Spain); (v) cow manure (EST), from a dairy farm with 500 heads of producing cattle; (vi) NPK inorganic fertilizer (15-15-15, N-P₂O5-K₂O richness in %) (NOLI) with 85.15% ammoniacal N; (vii) NPK + DMPP inorganic fertilizer (LI-2), (21-8-11, N-P₂O₅-K₂O richness in %), with slow-releasing capacities linked to 0.8% DMPP (3,4- dimethylpyrazole phosphate) as a nitrification inhibitor; (viii) fertigation (FERTI), combining two different strategies of fertilization, NPK (14-7-17, N-P₂O₅-K₂O richness in %) for basal fertilization and liquid fertilizer injected via fertigation applying the remaining N during the growing season; and control (B), plots without fertilization. All the treatments were applied manually with a rake to deeply fertilize the soil (0–10 cm) before planting.

2.4. Soil Analysis and Crop Measurements

The physicochemical and chemical characteristics of the fertilizers and amendments used were analyzed according to the methods described by [21]. Inorganic fertilizers (NOLI and LI-2) were analyzed following the methods described in Regulation (EC) 2003/2003 regarding fertilizers (Table 2). All the analyses were made in triplicate. The effects of the fertilizers on the soil samples were taken in each plot at 0 and 55 days after application to determine the residual effects of the organic amendments in the soil. The samples were collected from topsoil (0–20 cm), then air-dried and sieved to 2 mm. The pH was measured in a 1:2.5 soil:water (w/v) extract and the electrical conductivity (EC) in a 1:5 (w/v) soil:water extract [22]. The oxidizing organic carbon (COT) was measured following Wakley and Black [23]. The N total Kjeldahl (TN), nitrate (NO₃⁻-N), and ammonium (NH₄⁺-N) were analyzed in soil samples following the methods used by [21]. Available P (Pe) was determined colorimetrically using the method of [24].

The plant material sampling was carried out before flowering (40–50 cm height). All the plants from each plot were harvested, cutting all the aerial biomass of the dill, which was weighed to determine the yield on a fresh weight basis (kg m⁻²). The dry matter was determined by drying plant samples in an oven at 45 °C to constant weight (kg m⁻²).

The mineral composition of the plants was established using nitric perchloric acid digestion [25]. The samples were made in triplicate. The carbon and N in the tissue samples were measured in an automatic elemental micro analyzer (EuroVector Elemental Analyser). P was measured in a spectrophotometer UV-V; K and Na were determined using a photoelectric flame photometer. The NO₃⁻-N in fresh leaves was determined following the European Union (2006) regulation and the EN12014–4: 2005 Standard Reference [26]. The nutrient extraction in plant aerial tissues was measured and expressed as kg ha⁻¹ to establish the overall effect of the treatments.

Nitrogen use efficiency (NUE; expressed as the percentage of the applied N-fertilizer taken up in the dill) was calculated using the N difference method, with the total N of the unfertilized treatment as the control [27]. Phosphorus and potassium use efficiency (PUE and KUE, respectively) were calculated in the same way.

2.5. GHG Measurements

Static chamber methods were used to measure the gaseous emission surface fluxes of CO₂, CH₄, and N₂O during the experiment. The gas samples were taken on days 0, 1, 2, 3, 5, 8, and then weekly [28], with a total of 12 samplings. Opaque chambers (18.3 L and 0.108 m²) placed in the center of each plot were used for this purpose. The gas was collected from the headspace of each chamber at 0, 30, and 60 min with 60 mL syringes injected into gas-tight 12 mL pre-evacuated vials with overpressure [29]. The gas samples were always taken during the same hours of the morning (10–12 a.m.) to minimize the effects of diurnal variations on the emissions. The concentrations of gas in the samples were established using a gas chromatograph (HP 6890, Agilent Technologies) equipped with an electron-capture detector (ECD) to analyze N₂O and a flame-ionization detector (FID) to measure CO₂ and CH₄. Three gas standards comprising a mixture of gases (high with 1500 ± 7.50 ppm CO₂, 10 ± 0.25 ppm CH₄, and 2 ± 0.05 ppm N₂O; medium with 600 ± 5.50 ppm CO₂, 5 ± 0.15 ppm CH₄, and 1.5 ± 0.03 ppm N₂O; and low with 200 ± 1 ppm CO₂, 1 ± 0.05 ppm CH₄, and 1000 ± 25 ppb N₂O) were used to establish a standard curve (R² = 0.99) for each gas [30].

The flow rate emissions (mg m⁻² day⁻¹) on the surface of the soil were calculated from the changes in gas concentrations during the sampling period (60 min) following [31]. The cumulative gas emissions during the sampling period were calculated by averaging the flux in two successive determinations and multiplying that average flux by the length of the period between samplings [32]. N₂O EF was calculated following Aguilera et al. [33]. These parameters indicate the proportion of the N applied in fertilizer released as N₂O after discounting the emissions from the control treatment without fertilization. Yield-scaled N₂O emissions were expressed as the ratio of the amount of N₂O-N emitted to the uptake of N by the crop tissue [34].

Finally, the total GHG emissions were estimated as carbon dioxide equivalents (CO₂eq). The net cumulative CO₂ emissions were added to the CH₄ and N₂O emissions from each treatment previously converted to CO₂eq using a global warming potential value of 27.2 and 273, respectively [35].

2.6. Statistical Analyses

Analyses of variance at p < 0.05 were performed for all the variables of the experiment. Daily fluxes were analyzed using a generalized linear mixed model, with the fertilizer treatments and sampling occasions as fixed factors, while each plot was considered a random factor. The Least Significant Difference (LSD) test was used for multiple comparisons between the means. Differences between treatments were analyzed using the LSD Fisher contrast (p < 0.05). The data distribution normality of the GHG fluxes and soil variables was verified using the Kolmogorov–Smirnov test. All the analyses were performed using the Infostat v.2021 statistical software package [36].

3. Results

3.1. Soil Properties

The effects of the fertilizing treatments on soil pH at the beginning of dill cultivation showed an initial edaphic condition with a slightly basic pH. Soil pH values decreased significantly compared to the control, tending mostly to basic values. The soil pH in all the treatments decreased slightly in the later sampling (day 55) with the incorporation of the fertilizing treatments (Table 3). This decrease was less marked in the HP compost treatment because this fertilizer had the lowest pH value in the trial (5.8). Carbonates and limestone in the soil have a buffer effect, making the soil resistant to pH changes.

All the treatments increased the salinity range of the soil. This effect was lower in treatments where organic amendments were applied. The solid inorganic (NOLI and LI-2) and bio-stabilized organic (HP, HP-2, and VT) treatments increased the EC of soluble nutrients compared to the control at the beginning of the experiment (day 0). However, the soil EC value of the fresh organic treatments showed no significant statistical differences from the control.

At the beginning of the trial, all the soils showed a significantly higher Pe concentration than the control treatment, except for the FERTI treatment, as was expected. At the end of the experiment, the NOLI, LI-2, VT, and HP treatments increased the Pe levels in the soil compared to the control, with VT presenting a significantly higher value than the rest.

The TN content increased significantly for all the applied treatments compared to the control soil, except for the FERTI treatment. Since the fertilization of the fertigation crop is carried out gradually during the crop cycle, at the beginning of the experiment, the NPK content was similar to the control. The increase in TN from the FERTI treatment was very slight since the N supply is mainly in the form of NO₃⁻-N, which is assimilated by the plant and not detected in the NT Kjeldhal measurement.

Regarding the evolution of the NO₃⁻-N contents in the soil, an increase was observed in all the plots, except for the inorganic fertilizer (NOLI and FERTI). The LI-2 treatment showed the highest increase, which was expected since it is the material with the highest inorganic form of N. Compost and VT also obtained a statistically higher value than the control (B). At 55 days into the experiment, the control soil, stabilized organic (HP, HP-2, and VT), and EST treatments reduced their NO₃⁻-N content: significantly so in the case of the EST treatment. However, the soil to which the higher NH₄⁺-N treatments (NOLI and LO) were applied had higher NO₃⁻-N concentrations at the end of the trial. This indicates that an intense nitrification process had taken place in the soil, allowing the biotransformation of part of the N.

The results obtained for soil organic carbon, SOC, show significant statistical differences from the soil without treatment (B) and the soil amended with composts and vermicompost at day 0 of the experiment. These were the only ones that increased the percentage of SOC in the soil (Figure 2). In particular, the SOC value of the VT treatment (0.95%) was much higher than the fertigation treatment (0.60%). However, there are no statistical differences between the control, inorganic, and fresh organic treatments (LO and EST). At the end of the experiment (day 55), an increase in SOC was observed in most of the organic treatments.

3.2. GHG Emissions

The GHGs measured in the soil surface of the dill crop across the 57-day monitoring period were influenced by the different fertilizing treatments applied (p < 0.0001). The CO₂ emissions during the first 10 days were very high for the VT (62 kg CO₂-C ha⁻² d⁻¹) and LO (65 kg CO₂-C ha⁻² d⁻¹) treatments (Figure 3a). The VT emissions then decreased to values similar to all the treatments, and the LO treatment emissions remained high until day 25. In addition, the LO and VT total cumulated emissions (382 and 349 kg CO₂-C ha⁻¹, respectively) were able to stimulate the soil microbiota through the contribution of labile nutrients, causing an increase in their metabolic activity at the beginning of the trial. This was reflected in greater CO₂ production and emissions. The results for cumulative CO₂ emissions were variable, with the FERTI crop obtaining the lowest cumulative emissions (195 kg CO₂-C ha⁻¹) without significant differences from the control, NOLI, and HP plots.

The LO treatment had the highest N₂O fluxes (1.81 kg N₂O-N ha⁻² d⁻¹), followed by the NOLI treatment (0.32 kg N₂O-N ha⁻² d⁻¹). N₂O peaks were observed from the LO treatment on days 7 (13.46 mg N₂O-N m⁻² d⁻¹) and 21 (7.25 mg N₂O-N m⁻² d⁻¹), while the NOLI treatment only showed fluxes on day 7 (3.06 mg N₂O-N m⁻² d⁻¹) (Figure 3b). Daily and cumulative N₂O emissions were reduced for most treatments and were mainly associated with nitrification processes, as fertilizers with higher N-NH₄⁺/N-NO₃⁻ ratios (LO, NOLI, and LI-2) showed higher emission fluxes, especially during the first 30 days of the experiment. From day 30 onwards, there were no statistically significant differences between treatments, probably due to the reduced presence of N₂O precursors (labile C and nitrate) and the increased demand for NO₃⁻N and water by the plants, which reduced N₂O emissions. The highest yield-scaled N₂O emissions were measured for LO, followed by NPK, compared to the other fertilizing scenarios (Figure 4).

Although significant differences were found between the treatments in methane (CH₄) emission flux, there was a general sink effect (negative cumulative emission balance) in all the fertilizing scenarios except for LO (0.16 kg C-CH₄ ha⁻¹) and VT (0.06 kg C-CH₄ ha⁻¹). These showed reduced positive cumulative CH₄ emissions. In our study, and in most of the studies mentioned above, conditions favoring this effect normally appeared during the first part of the experiment (Figure 3c)

The organic fertilizers produced significantly lower CO₂eq than LO, NOLI, and LI-2 (Table 4). The non-stabilized organic amendments and non-specialized inorganic fertilizers produced significant N losses as N₂O. However, LO had significant increases compared to the other treatments (up to six times higher than the nearest treatment in terms of GWP). Vico et al. [18] reported similar results using biosolid amendments. Composted materials were useful to abate GHG losses (lower GWP) without crop yield penalties. This was reflected in their yield-scaled N₂O emissions, which were lower than for LO and NOLI (Figure 4). These did not increase enough to compensate for greater N₂O losses. Applying DMPP in LI-2 decreased N2O emissions compared to the control and led to higher N uptake by the crop.

3.3. Dill Yield and Nutrient Content

The fresh biomass production values of the aerial part of the dill plants with different fertilizer scenarios show significant variation. The FERTI treatment had the greatest biomass production, with 28.9 Mg ha⁻¹. The stabilized organic treatments (HP, HP-2, VT, and LO) were second, producing slightly less than the FERTI treatment, with 25.8 Mg ha⁻¹, 26.3 Mg ha⁻¹, 26.2 Mg ha⁻¹, and 25.6 Mg ha⁻¹, respectively The treatments that obtained the worst agronomic results were EST and the treatment without fertilization (B), both with productions of 20.6 Mg ha⁻¹ (Table 5).

In addition to yield, it is essential to know the nutritional content of the biomass. Nitrate content is considered one of the most important elements of vegetable quality. Both environmental and agrotechnical factors influence NO₃⁻-N concentrations in plants. Typically, leafy green vegetables accumulate NO₃⁻-N, with concentrations reaching up to 6000 mg kg⁻¹. Dill is a vegetable with medium NO₃⁻-N content. Significant differences were found in nitrate contents depending on the treatments applied (Table 5). No differences were observed between the fresh and stabilized organic treatments and the control treatment.

In terms of the macronutrient compounds extracted from the dill biomass, a low absorption N-rate of fresh organic fertilizers was observed (Table 6). Most of the N in these treatments is organic and must be mineralized to be assimilated by the plants. Stabilized organic and inorganic treatments (NOLI and FERTI) showed similar behavior and were less efficient than the LI-2 treatment. The NUE values reported significant differences, with the highest efficiency for LI-2 (30%) and the lowest for LO (2%).

The dill plants that performed the best in P extraction were those fertilized with VT and compost. In general, dill plants with stabilized organic fertilizers showed higher N, P, and K content than the inorganic treatments. The worst P extraction performance was with the FERTI treatment, which may be due to the fact that less P was supplied because, in each fertigation, N could be dosed independently. Concerning the biomass levels of K extraction, we can observe significant differences between treatments following the same tendency as P with lower extraction levels. The efficiency indicators PUE and KUE showed the same trend: with EST, there is very low efficiency compared to the other treatments.

4. Discussion

4.1. Effect of Fertilization Treatments on Soil Properties

At the end of the experiment (day 55), the pH of the soil had decreased compared to the beginning of the experiment with the different treatments. This was more extreme in the NOLI, LI-2, and EST treatments, probably due to the more intense action of nitrifying bacteria that can consume alkalinity through their metabolic pathway, as has been demonstrated in other studies [10,37]. The soil with the compost treatment (HP) was the only one that increased its value at the end of the growing cycle, ending the experiment with a pH very similar to the control (7.99). An increase in undesirable ions is one of the main constraints of organic fertilizers. Scotti et al. [38] explain the increase in soil salinity due to the effects of direct ion solubilization and the release of soluble mineral nutrients with compost mineralization [39,40]. At the end of the experiment, the soil EC value of the different treatments increased, especially in the NOLI treatment, probably due to overfertilization.

In general, applying organic amendments increases soil Pe content [18,41], coinciding with the results obtained in this experiment. The bio-stabilized organic treatments showed the most variation for NT with respect to the control treatment. The inorganic treatments and the fresh organic treatments also increased soil TN, but the increase was significantly lower. These differences found at the beginning of the trial may be due to the higher organic N supply from the compost [42]. One of the limitations of using compost as a fertilizer is the uncertainty about the amount of nutrients available to the plant, especially N and P, due to the presence of both inorganic and organic matter [43]. At the end of the experiment (day 55), all the treatments had reduced soil TN content, except fertigation, although all had significantly higher values than the control. By analyzing the bio-stabilized organic treatments, the reduction in TN shows how these organic fertilizers were able to slowly release inorganic N in combination with the nutritional needs of the crop [12], despite dill being a winter and short-cycle crop.

In relation to N-NO₃⁻ dynamics, the results coincide with those Wang et al. [44] found in winter crop fertilization trials in different soils under the same soil and climatic conditions. They detected more intense nitrification in soils with moisture conditions alternating between saturation and unsaturation than in permanently saturated soils or in soils with long periods of drought. These variable conditions between saturation–unsaturation have been observed in the soil under study due to the drip irrigation method. However, in the LI-2 treatment, we can observe a very slight NH₄⁺-N variation between the beginning and the end of the experiment, despite the fact that this material also provided a large amount of NH₄⁺-N. This could be because the DMPP nitrification inhibiting actor slowed down the enzymes responsible for the first step of nitrification. The effectiveness of DMPP in irrigated soils is influenced by the soil texture [45], pH [46], or even SOC [47], which, if found in very labile forms, can promote denitrification processes given the heterotrophic nature of the bacteria responsible for this process [48]. Finally, the FERTI treatment showed the greatest increase in NO₃⁻-N concentrations, which is evidence of N over-fertilization. This excess of NO₃⁻-N in the soil after crop harvesting can lead to leaching and even groundwater contamination [49].

An increase in SOC was observed due to the different stimulation of the soil microbiota as a consequence of adding OM [50]. The VT treatment, which had the highest value at the beginning of the experiment, was the only organic treatment that did not increase its SOC value at the end of the experiment, possibly due to its labile condition and particle size, which favors intense OM mineralization in amended soil.

4.2. Effect of Fertilization Treatments on GHG Emissions

We can observe that high CO₂ emissions indicate high soil activity, correlated with labile C content [51]: fresh organic fertilizers > stabilized organic fertilizers > inorganic fertilizers. These results have been observed in other works on the application of organic amendments to soil. De la Fuente et al. [52] detected an increase in CO₂ emissions after applying organic amendments to clay loam soil. Subsequently, from day 30 of the trial, all the treatments gradually increased CO₂ emissions until the end of the trial, which could be due to the respiration and gas exchange of the dill plants’ root systems.

Comparing the results obtained in the treatments with inorganic fertilization, we can see that in the case of the FERTI treatment, the emission values were the lowest of all those in the experiment because this treatment did not provide N-NH₄⁺. With NOLI and LI-2, which contributed a proportion of N-NH₄⁺, the emissions were higher in the plots treated with NOLI. This could be a consequence of the nitrification inhibitor substances (DMPP) in the LI-2 fertilizer, since these substances have been shown to be effective in mitigating N₂O emissions in soil. According to Huérfano et al. [53], between 30 and 50% of N₂O emissions can be mitigated by using nitrification inhibiting fertilizers.

This sink effect was caused by the creation of methanotrophic process conditions. Under aerobic conditions, methane is converted into CO₂ by the enzymatic activity of aerobic methanotrophic bacteria that use methane as a source of C and energy for their growth [54]. Similar results were observed by Liu et al. [55] in a study on CH₄ emissions in agro-pastoral soils, where they also detected higher CH₄ production in soil after applying fresh organic fertilizers. The treatments with a significantly higher sink effect were LI-2 and the control plot (−0.11 and −0.12 kg C-CH₄ ha⁻¹, respectively). This CH₄ sink effect has been reported in other works on soils from Mediterranean environments [56,57]. In agricultural soils, it has been related to several factors: (1) soil type [58,59], which influences the drainage and porosity of the soil; (2) N fertilization rates [60], as other bacteria involved in the N cycle can compete with methanotrophic bacteria for oxygen; (3) the type of irrigation [61], with flood-type irrigation leading to greater soil saturation and the formation of anaerobic microsites; (4) or soil temperature [55], as there is a direct relationship between increased temperatures and CH₄ production.

The IPCC [62] provides a methodological guide for GHG emission inventories. It proposes a default N₂O emission factor (EF N₂O) of 1% TN for irrigated crops in Mediterranean climate areas. Other works on horticulture in Mediterranean environments have reported that the 1% emission factor the IPCC advises by default overestimates the emissions of this gas in Mediterranean areas [17,63]. These studies seem to agree with the emission ratios measured in this work (VT 0.15%, HP 0.14%, HP-2 0.14%, NOLI 0.34%, LI-2 0.20%), which indicate that N2O emissions are not directly related to the amount of nitrogen applied [64] but to other factors, such as management methodology [65], the irrigation system [17], or the forms of nitrogen applied, which depend on the nature of the fertilizing material.

4.3. Effect of Fertilization Treatments on Dill Yield and Nutrient Content

Comparing the yield between the different fertilizing scenarios (stabilized organic fertilizers, fresh organic fertilizers, and inorganic fertilizers), the bio-stabilized organic treatments obtained significantly higher production than the inorganic treatments. This includes fertilizers with nitrification inhibitors, LI-2, and the conventional treatment (NOLI), with the production of 22.4 Mg ha⁻¹ and 22.1 Mg ha⁻¹, respectively. The low performance of the manure treatment could have been due to: (1) NPK content that was not balanced to the nutritional needs of dill, causing a deficiency in some of these elements, (2) the freshness of the material, making homogeneous application difficult, or (3) a phytotoxic effect on dill seed germination. The high content of NH₄⁺-N present in this type of livestock waste [66] has been shown to have a negative effect on germination and the early stages of herbaceous plants. El-Zaeddi et al. [67] obtained a somewhat lower average production (22.5 Mg ha⁻¹) than that obtained in this study (24.3 Mg ha⁻¹) in their field study on dill production.

The treatment with the highest dry matter content with a yield of 4.7 t ha⁻¹ was VT, similar to the report by Fjelkner-Modig et al. [68], who found a higher dry weight yield in a dill crop with organic fertilization. As with the fresh biomass yield, the lowest results were found for the crop without fertilization (B) and EST, with 3.6 t ha⁻¹.

Acceptable daily nitrate intake is 3.7 mg kg⁻¹-body weight [69]. Herbs such as dill are consumed in small quantities, so their daily intake is not comparable to leafy vegetables. On average, the nitrate contents obtained in this trial were similar to those found by other authors [70] for dill crops. The inorganic treatments (NOLI, LI-2, and FERTI) showed the highest nitrate content, which differs from De Martin and Restani [71], who found significantly higher NO₃⁻-N content in organically fertilized crops than in those conventionally produced.

The low mineralization rate of fresh fertilizers after short-term application to the soil [72] could have caused the low N extraction efficiency of the EST treatment. This low yield could be caused by the loss of nitrates through leaching [37] and denitrification [72]. The use of DMPP produced a two-fold increase in NUE [18]. In general, NUE values reported low values of efficiency compared to the average values obtained by other authors [73]. The inorganic P applied in the FERTI, LI-2, and NOLI treatments could have been partly immobilized by limestone in the soil, forming insoluble calcium phosphates [74]. The organic forms in the organic fertilizers could have avoided or slowed down this precipitation/insolubilization.

5. Conclusions

Applying the proposed bio-stabilized organic amendments to irrigated dill cropping was an efficient management strategy to preserve the sustainability of intensive horticultural cropping systems in the Mediterranean area. Comparing organic and inorganic-based fertilizing scenarios has provided useful results to optimize the commercial production of aromatic plants without yield or environmental penalties.

In addition to achieving yields comparable to those obtained using inorganic fertilizers, the organic amendments have a positive effect on maintaining and restoring soil carbon, with the expected consequences of soil sustainability. Directly applying fresh organic amendments has been shown to be a less recommendable option (less yield, more GHG emissions, lower nutrient indexes) than using bio-stabilized organic fertilizers. In this particular study, the limitations of the experiment included the sampling frequency. Likewise, a longer sampling period of soil and GHGs (pre-planting and post-harvest) would also have been useful to relate to the data obtained.

Inorganic advanced slow-release fertilizers, which are increasingly being used in intensive scenarios, are agronomically efficient and also have low GHG emissions. However, with long-term repeated use, a loss of organic fertility, C sinks, and, especially, poor soil properties can be expected, with overall losses in sustainability and problems adapting to climate change. To corroborate this, experiments in long-term trials are recommended for this type of study, relating the effects of treatments and climatic events.

Author Contributions

Conceptualization, R.M. and M.D.P.-M.; methodology, F.J.A.-R.; software, L.O.; validation, E.A., J.M.-T., J.S.-T. and E.M.-S.; formal analysis, E.M.-S. and J.S.-T.; investigation, E.M.-S., J.S.-T., F.J.A.-R. and E.A.; resources M.D.P.-M.; data curation, M.D.P.-M.; writing—original draft preparation, L.O.; writing—review and editing, M.Á.B.; visualization, J.M.-T.; supervision, M.Á.B. and J.M.-T.; project administration, R.M.; funding acquisition, R.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to thank SME Hermanos Lucas for sharing its production fields for this experiment.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Mean temperature (°C), WFPS: water-filled pore space (%), rainfall (L m−2), daily irrigation (L m−2), and accumulated irrigation (L m−2) at the experimental site.

Figure 2. Evolution of soil organic carbon during the experiment. HP: wastewater treatment biosolid + Phoenix dactilifera leaf prunings; HP-2: agrifood sludge from pear processing wastewater + Phoenix dactilifera leaf prunings; VT: vermicompost cow manure; LO: biosolid; EST: cow manure; NOLI: NPK inorganic fertilizer; LI-2: NPK + DMPP inorganic fertilizer; FERTI: fertigation. Different letters within a column indicate significant differences between treatments (p < 0.05). Bars represent the standard error of the mean.

Figure 3. Daily fluxes of (a) CO2, (b) N2O, and (c) CH4 from treatment applications during the 55-day monitoring period. HP: wastewater treatment biosolid + Phoenix dactilifera leaf prunings; HP-2: agrifood sludge from pear processing wastewater + Phoenix dactilifera leaf prunings; VT: vermicompost cow manure; LO: biosolid; EST: cow manure; NOLI: NPK inorganic fertilizer; LI-2: NPK + DMPP inorganic fertilizer; FERTI: fertigation. Bars represent the standard error of the mean.

Figure 4. Yield-scaled N2O according to treatment. Different letters indicate significant differences between treatments (p < 0.05). Bars represent standard errors. HP: wastewater treatment biosolid + Phoenix dactilifera leaf prunings; HP-2: agrifood sludge from pear processing wastewater + Phoenix dactilifera leaf prunings; VT: vermicompost cow manure; LO: biosolid; EST: cow manure; NOLI: NPK inorganic fertilizer; LI-2: NPK + DMPP inorganic fertilizer; FERTI: fertigation.

Table 1

Physicochemical characteristics of the irrigation water used in the experiment.

pH	EC	Na+	HCO3−	Cl−	SAR
	(dS m−1)	(mg L−1)	(mg L−1)	(mg L−1)
8.64	2.96	243	192	408	4.2

EC: electrical conductivity; SAR: sodium adsorption ratio. The table shows the average of a composite sample.

Table 2

Characteristics of the fertilizers used in the experiment (dry weight basis).

Parameters		Treatment
		HP	HP-2	VT	LO	EST	NOLI	LI-2
Moisture	(%)	51.4	34.9	27.6	77.2	76.7	nd	nd
pH		5.8	7.4	8.2	6.8	9.6	nd	5.5
EC	(dS m−1)	9.0	8.3	4.1	4.0	6.9	nd	nd
COT	(g kg−1)	314	371	173	332	408	nd	nd
TN	(g kg−1)	31.9	28.7	14.7	49.7	27.4	150	210
NO3−-N	(g kg−1)	4.66	1.20	0.92	0.04	0.03	20	99
NH4+-N	(g kg−1)	0.06	0.07	0.02	2.16	nd	130	111
COT/TN		9.8	12.9	11.8	6.7	23.4	nd	nd
P	(g kg−1)	11.1	4.1	8.6	13.5	3.0	65.5	34.8
K	(g kg−1)	11.4	22.5	10.2	3.5	27.7	125	91.6
Na	(g kg−1)	7.4	10.7	4.4	2.3	9.2	nd	nd

EC: electrical conductivity; COT: total organic carbon; TN: total nitrogen; nd: not determined. HP: wastewater treatment biosolid + Phoenix dactilifera leaf prunings; HP-2: agrifood sludge from pear processing wastewater + Phoenix dactilifera leaf prunings; VT: vermicompost cow manure; LO: biosolid; EST: cow manure; NOLI: NPK inorganic fertilizer; LI-2: NPK + DMPP inorganic fertilizer; FERTI: fertigation. The table shows the average of a composite sample.

Table 3

Effect of fertilizing treatments on soil properties.

Treatment	pH		EC(dS m−1)		Pe(mg kg−1)		TN(g kg−1)		NO3−-N(mg kg−1)
	0 d	55 d	0 d	55 d	0 d	55 d	0 d	55 d	0 d	55 d
B	8.39 c	7.99 bc	0.33 a	0.57 b	62 ab	77 b	0.72 a	0.67 a	18 a	11 a
HP	7.94 a	7.97 abc	0.58 b	0.67 c	76 cd	92 cd	1.14 d	0.92 d	60 de	23 b
HP-2	8.11 c	8.04 bc	0.51 b	0.59 bc	82 cde	85 cd	1.01 c	0.82 bc	55 cd	18 ab
VT	8.18 c	8.06 c	0.57 b	0.66 c	180 g	137 e	1.12 d	0.84 cd	52 c	14 a
LO	8.18 c	8.17 d	0.37 a	0.59 bc	72 bc	85 bc	0.95 c	0.85 d	28 b	31 c
EST	8.26 c	7.99 bc	0.39 a	0.57 bc	89 de	87 bc	0.82 cd	0.83 d	23 ab	13 a
NOLI	8.29 c	7.87 a	0.67 c	0.90 d	105 f	102 d	0,94 c	0.78 c	20 a	50 d
LI-2	8.19 c	7.85 a	0.73 d	0.56 b	93 fe	96 cd	1.04 c	0.83 d	65 e	60 e
FERTI	8.22 bc	8.18 d	0.32 a	0.45 a	51 a	64 a	0.67 a	0.72 b	17 a	78 f
F-anova	7.7 ***	14.4 ***	57.3 ***	29.3 ***	133 ***	41.1 ***	45.71 ***	8.35 ***	165 ***	191 ***

EC: electrical conductivity; Pe: extractable phosphorus; TN: total Kjeldahl N; NO₃⁻-N: nitrate. 0 d: 0 days; 55 d: 55 days; HP: wastewater treatment biosolid + Phoenix dactilifera leaf prunings; HP-2: agrifood sludge from pear processing wastewater + Phoenix dactilifera leaf prunings; VT: vermicompost cow manure; LO: biosolid; EST: cow manure; NOLI: NPK inorganic fertilizer; LI-2: NPK + DMPP inorganic fertilizer; FERTI: fertigation. *** Significant difference between treatments at p < 0.001. Different letters within a column indicate significant differences between treatments (p < 0.05).

Table 4

Estimated cumulative fluxes during the 55-day measuring period.

Treatment	CH4	N2O	CO2	CO2eq
	(mg m−2)	(mg m−2)	(mg m−2)	(kg ha−1)
B	−12.07 a	9 a	27,740 bc	20 a
HP	−6.53 ab	12 ab	28,671 cd	30 ab
HP-2	−6.33 ab	13 ab	31,072 de	33 ab
VT	6.23 c	13 ab	34,868 f	36 ab
LO	16.07 d	181 d	38,236 g	485 d
EST	−0.30 bc	10 ab	32,590 ef	29 ab
NOLI	−8.00 ab	31 c	26,551 bc	82 c
LI-2	−10.67 a	18 b	24,851 b	47 b
FERTI	−4.07 ab	8 a	19,518 a	20 a
SEM	3.2	3	1073	9
F-anova	7.7 ***	327 ***	27 ***	303 ***

CO₂eq: CO₂eq equivalent added C0₂, N₂O, and CH₄ emissions with the corresponding GWP; SEM: standard error of the mean. HP: wastewater treatment biosolid + Phoenix dactilifera leaf prunings; HP-2: agrifood sludge from pear processing wastewater + Phoenix dactilifera leaf prunings; VT: vermicompost cow manure; LO: biosolid; EST: cow manure; NOLI: NPK inorganic fertilizer; LI-2: NPK + DMPP inorganic fertilizer; FERTI: fertigation. *** Significant difference between treatments at p < 0.001. Treatments with different letters within the same column represent significant differences (p < 0.05) tested separately for each gas.

Table 5

Dill yield according to treatment.

Treatment	Fresh Weight Basis	Dry Weight Basis	NO3−-N
	(Mg ha−1)		(mg kg −1)
B	20.6 a	3.6 a	829b
HP	25.8 bc	4.5 bcd	703 b
HP-2	26.3 bc	4.6 cd	505 ab
VT	26.2 bc	4.7 d	469 ab
LO	25.6 bc	4.4 bcd	650 ab
EST	20.6 a	3.6 a	201 a
NOLI	22.1 ab	3.9 ab	1373 c
LI-2	22.4 ab	3.9 ab	5387 e
FERTI	28.9 c	4.6 d	2896 d
F-anova	11.5 ***	10.28 ***	109 ***

N-NO₃⁻: expressed on a dry matter basis. HP: wastewater treatment biosolid + Phoenix dactilifera leaf prunings; HP-2: agrifood sludge from pear processing wastewater + Phoenix dactilifera leaf prunings; VT: vermicompost cow manure; LO: biosolid; EST: cow manure; NOLI: NPK inorganic fertilizer; LI-2: NPK + DMPP inorganic fertilizer; FERTI: fertigation. *** Significant difference between treatments at p < 0.001. Different letters within a column indicate significant differences between treatments (p < 0.05).

Table 6

N, P, K extraction by dill, according to treatment.

Treatment	N	P	K	NUE	PUE	KUE
	(kg ha−1)			% index
B	79 a	5.89 a	25.5 a	-	-	-
HP	116 cde	8.09 d	35.3 d	23 bc	19 a	1.67 ab
HP-2	104 bcd	8.03 cd	35.9 de	15 b	40 a	0.67 a
VT	95 ab	8.24 d	35.7 de	13 ab	12 a	1.00 a
LO	104 bcd	6.69 ab	26.9 ab	15 b	53 ab	6.33 c
EST	83 a	7.19 bcd	35.1 d	2 a	12 a	0.33 a
NOLI	101 bc	6.87 abc	30.7 bc	14 b	61 ab	3.00 b
LI-2	127 e	7.86 cd	32.9 cd	30 c	219 c	6.00 c
FERTI	119 de	6.39 ab	39.2 e	25 bc	183 bc	25.67 d
F-anova	6.67 ***	4.55 **	11.18 ***	5.18 **	3.13 *	223.08 ***

NUE/PUE/KUE: nitrogen/phosphorus/potassium use efficiency. HP: wastewater treatment biosolid + Phoenix dactilifera leaf prunings; HP-2: agrifood sludge from pear processing wastewater + Phoenix dactilifera leaf prunings; VT: vermicompost cow manure; LO: biosolid; EST: cow manure; NOLI: NPK inorganic fertilizer; LI-2: NPK + DMPP inorganic fertilizer; FERTI: fertigation. *, ** and *** Significant difference between treatments at p-value 0.01, 0.001, and <0.0001, respectively. Different letters within a column indicate significant differences between treatments (p < 0.05).

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