Relationship between hydraulic properties and plant coverage of the closed-landfill soils in Piacenza (Po Valley, Italy)

Abstract

In this paper the results of a study of soil hydraulic properties and plant coverage of a landfill located in Piacenza (Po Valley, Italy) are presented, together with the attempt to relate the hydraulic properties in relation with plant coverage. The measured soil water retention curve was first compared with the output of pedotransfer functions taken from the literature and then compared with the output of the same pedotransfer functions applied to a reference soil. The landfill plant coverage was also studied. The relationship between soil hydraulic properties and plant coverage showed that the landfill soils have a low water content available for plants. The soils' low water content, together with a lack of depth and a compacted structure, justifies the presence of a nitrophilous, disturbed-soil vegetation type, dominated by ephemeral annual species (therophytes).

Solid Earth, 6, 929943, 2015 www.solid-earth.net/6/929/2015/ doi:10.5194/se-6-929-2015 Author(s) 2015. CC Attribution 3.0 License.

C. Cassinari1, P. Manfredi2, L. Giupponi3, M. Trevisan1, and C. Piccini4

1Istituto di Chimica Agraria ed Ambientale, Universit Cattolica del Sacro Cuore, Via Emilia Parmense 84, 29122 Piacenza, Italy

2m.c.m. Ecosistemi s.r.l. localit Faggiola, 29027 Gariga di Podenzano, Piacenza, Italy

3Centro Interdipartimentale di Studi Applicati per la Gestione Sostenibile e la Difesa della Montagna, Universit della Montagna, Universit degli Studi di Milano, Via Morino 8, 25048 Edolo, Brescia, Italy

4CRA Consiglio per la Ricerca e la Sperimentazione in Agricoltura, Centro di ricerca per lo studio delle relazioni tra pianta e suolo, Via della Navicella 24, 00184 Rome, Italy

Correspondence to: C. Cassinari ([email protected])

Received: 13 January 2015 Published in Solid Earth Discuss.: 20 February 2015 Revised: 2 July 2015 Accepted: 7 July 2015 Published: 28 July 2015

Abstract. In this paper the results of a study of soil hydraulic properties and plant coverage of a landll located in Piacenza (Po Valley, Italy) are presented, together with the attempt to relate the hydraulic properties in relation with plant coverage. The measured soil water retention curve was rst compared with the output of pedotransfer functions taken from the literature and then compared with the output of the same pedotransfer functions applied to a reference soil. The landll plant coverage was also studied. The relationship between soil hydraulic properties and plant coverage showed that the landll soils have a low water content available for plants. The soils low water content, together with a lack of depth and a compacted structure, justies the presence of a nitrophilous, disturbed-soil vegetation type, dominated by ephemeral annual species (therophytes).

1 Introduction

Soil water is a fundamental resource for the components of the ecosystem; it plays a vital role in determining the functioning of plants and other soil biota (Brevik et al., 2015). Soil physics is largely related to the interaction between soil and water; therefore the physical, chemical and biological processes that take place in soil depend on the amount and composition of water (Brevik et al., 2015). With this in mind, knowledge of the hydraulic properties of soils is important in

Relationship between hydraulic properties and plant coverage of the closed-landll soils in Piacenza (Po Valley, Italy)

many scientic disciplines, from agriculture to ecology, since the amount of water, and the strength by which it is held by soil, represents the characteristics of soil behaviour and for the vegetation and all other organisms development.

Land use can signicantly affect soil properties, such as bulk density, saturated hydraulic conductivity, inltration rate and available soil water content (Haghighi et al., 2010), and it has been shown to be one of the main factors controlling soil water variability (Qui et al., 2001; Pan and Wang, 2009). Because soil properties are the main factor controlling soil water variation (Vachaud et al., 1985, Famiglietti et al, 1998; Hu et al., 2010), land use could inuence soil water variations by changing soil properties (Gao et al, 2014). In recent studies, the effects of land use on soil water variation have been investigated via statistical analysis (Fu et al., 2003; Chen et al., 2007; Gross et al., 2008), simulation, or physical-based models (Li et al., 2009). Gao et al. (2014) demonstrate, through a study in a small catchment of the Chinese Loess Plateau, that land use can lead to spatial variation of soil water but has a negligible effect on soil water temporal patterns.

Soil moisture inuences soil behaviour; it takes an important role, for example, in soil erosion. Antecedent soil moisture content, together with rainfall intensity, slope steepness and land use/land cover are factors inuencing soil erosion and runoff (Ziadat and Taimeh, 2013). The effect of antecedent soil water content on soil erosion is still a matter of debate, as an opposing effect has been reported on aggre-

Published by Copernicus Publications on behalf of the European Geosciences Union.

930 C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza

gate breakdown and seal formation (Vermang et al., 2009).

Wet soils double the runoff coefcient and shorten the time to runoff, compared with the same soil when dry (Li et al., 2011). Greater soil erosion was observed during the wet season in Spain (Cerd, 2002; Ziadat and Taimeh, 2013).

Soil water/moisture is also a key factor affecting vegetation structure in a water-limited environment (Rodriguez-Iturbe et al., 1999); in turn, vegetation exerts vital control on the entire water balance (Rodriguez-Iturbe et al., 2001) via complex and mutually interacting hydrological processes (Porporato et a., 2002; Gao et al. 2014).

In this paper, the study of hydraulic properties of soil is presented, by laboratory analysis, by using predictive models and by studying soil vegetation cover.

Direct measurements of soil hydraulic properties are rarely performed because they require lengthy and costly analysis; as an alternative, analysis of existing databases of measured soil hydraulic data may result in pedotransfer functions (PTFs) (Wsten et al., 2001). These functions often prove to be good predictors for missing soil hydraulic data. The PTFs show empirical relationships between soil hydraulic properties and some more easily measurable basic soil properties such as texture, bulk density and organic carbon content (Baker, 2008; Bouma and van Lanen, 1986; Pachepsky and Rawls, 2004; Vereecken et al., 2010; Wsten et al., 2001). To derive the PTFs, databases of soils from all over the world were used. Generally soil databases emphasize soil taxonomy and provide limited un-saturated soil hydraulic data. With this in mind, the international Unsaturated Soil Database (UNSODA) (Leij et al., 1996) and subsequently, the European database of soil hydraulic properties (HYPRES) (Nemes et al., 2001a; Wsten et al., 1999; Wsten and Lilly, 2004) were developed. Both these databases contain a wealth of information about soil hydraulic data, measurement methods and other relevant soil data (Nemes et al., 2001a).

The processing of the PTFs can be performed using computer programs such as CalcPTF 3.0 (Guber and Pachepsky, 2010), ROSETTA (Schaap M.G., et al., 2001) (which is available as stand-alone program and also as a part of the simulation model HYDRUS 1D; Simunek, et al., 2008), SOILPAR (Acutis and Donatelli, 2002) and SPAW (Saxton and Willey, 2006).

The relationship between volumetric water content and matric potential is shown by the soil water retention curve, which allows the derivation of available water for plants by comparing the water content at different applied suction (negative pressure) values.

In recent decades the increase in human population and human activity has resulted in an ongoing depletion of soil resources, to the point that the authorities have included the recovery of degraded areas in their priorities. The lower ability to make water available for plants and microorganisms is characteristic of a degraded soil; thus, in order to carry out

soil restoration, it is important to know its hydraulic proper

ties.

In this work a degraded cover soil of a landll located in Piacenza was studied. The soil used to closed the landll is a natural soil, which comes from different areas near Piacenza, and it can be classied as an Anthrosol (FAO World Reference Base for Soil Resources): a soil formed or profoundly modied through long-term human activity, such as from addition of organic waste or household waste, irrigation or cultivation. This soil has shown very low fertility for more than 30 years; there is no chemical contamination justifying its condition, so the soil can be described as a degraded soil.

Recently the landll soils and the vegetation were studied, and so the site environmental quality is described, including the relationship between soil chemical analysis and ecological indicators (Manfredi et al., 2012), the oristic vegetational indexes (Giupponi et al., 2013b) and the presence and development of Onopordum acanthium subsp. acanthium (Giupponi et al., 2013a). The area is actually involved in a Life + project (Life 10 ENV/IT/000400

New Life, http://www.lifeplusecosistemi.eu

Web End =http://www.lifeplusecosistemi.eu ), which includes among its objectives the treatment of degraded soils through an innovative reconstitution method to improve their quality, and the restoration of the closed landll.

Restoration of closed landlls is essential to minimize the adverse effect on the environment and to render the land-lls safe for further use (Chen et al., 2015). A lot of studies on landlls can be found in the literature about root contamination by gas (Gilman et al., 1982), methane production (Themelis and Ulloa, 2007), microbiological studies (Boeckx et al., 1996) and ecological performance after the restoration of plant and animal communities (Chen et al., 2015; Wong et al., 2015) but nothing can be found about hydrological properties of cover soil in relation to plant coverage.

The study of the vegetation cover of an area can be very useful as a tool to compare and corroborate the results of chemical and physical analysis performed on soil. The integrated study of different scientic disciplines in the description of an area is always preferred in order to have as complete an overview as possible. The study of the relationship between soil hydraulic properties and plant coverage, and the use of plant communities to assess the soil quality is a very interesting research eld. This new way of studying an area can be applicable not only in describing degraded areas, but also when applied in other research elds, such as rural areas, road embankments, mining, badlands or border areas.

Novara et al. (2011) demonstrate that an appropriate choice of cover crops assists in very effective soil management in vineyards in Sicily. A natural reforestation following a different land use on the ood plain in the Dragonja sedimentation basins has resulted in a change in sedimentation rate (Keesstra, 2007; Keesstra et al., 2009). A study performed in a restoration program in South East New Territories landll in Hong Kong, between pioneer species and

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C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza 931

native species including the investigation of different planting techniques, the use of different types of soil ameliorants and a focus on understanding what factors are most important in the growth of plants provided valuable information for restoring subtropical engineering landlls (Wong et al., 2015). The vegetation, climate and environmental change have been used to explain the reasons of the severe soil erosion in the Loess Plateau of China (Zhao et al., 2013).Zornoza et al. (2015) use vegetation cover for the development of an index to assess the state of human disturbances in alpine grassland with different levels of degradation based on plant cover, production, proportion of primary plant and height of the plant. Studies of the vegetation biodiversity, together with chemical and physical soil analyses and the testing of the presence/abundance of soil microbes and soil fauna, are also used to assess the soil quality in some farms in Iceland and Austria (van Leeuwen et al., 2015).

Considering the importance of soil moisture and vegetation cover, the aim of this work is to relate the hydraulic properties of landll soil with its vegetation, and to assess whether predictive systems (PTFs) are suitable for predicting these data.

2 Materials and methods

2.1 Study area

The closed landll of municipal solid waste of Borgotrebbia is located in the territory of Piacenza (Po Valley, Italy, coordinates: 45 03[prime]58[prime][prime] N, 09 39[prime]06[prime][prime] E) at an altitude of 60 m.

It has an area of 200 000 m2 and is located along the right bank of the Trebbia River near its conuence with the Po River. Climatic data show that the average annual temperature is 13.3 C, while the average annual rainfall amounts to 778 mm, most of which is concentrated in the periods of

March and September.

The landll was opened from 1972 to 1985 and then was closed and covered with a layer, about 50 cm in depth, of different degraded soils left to be spontaneously colonized by plants. The soils used to close the landll are loamy soils with a predominantly multi-faceted structure; they have low porosity and, by their nature, they are compact. Further compaction of the soil was induced by compression, caused by operations carried out in order to close the landll so that the leakage of gas and inltration by rain could be avoided.

2.2 Soil

2.3 Physical-chemical analysis of the soil

Eleven sampling points were chosen as being representative of the closed landll area after a preliminary study. Initially they were sampled in the area at 51 points, following a grid division NESW, NWSE; and the distribution of the ob-served different vegetation types the plant communities dif-

fer in structure and oristic composition according to the different environmental factors such the type of soil. By statistical elaboration of the 51 chemical analyses, 11 soils resulted in being the most representative of the area.

The 11 soil samples were taken at 25 cm depth and chemical and physical analyses were carried out based on the Methods of Soil Chemical and Physical Analysis as described in the Ofcial Gazette of the Italian Republic: texture and grain size (Italian position Method II.5 Suppl. Ord. G.U. no. 248/21.10.1999; international position ISO 11277), primary and secondary structure, organic carbon (Italian position Method VII.3, Suppl. Ord. G.U. no. 248/21.10.1999, Walkley-Black,), salinity (Italian position Method IV.1 Suppl. Ord. G.U. no. 248/21.10.1999, international position ISO 11265, aqueous extract 5 : 1), total limestone (Italian position Method V.1, Suppl. Ord. G.U. no. 248/21.10.1999, international position ISO 10693) and water potential (Italian position Method VIII.3, Suppl. Ord. G.U. no. 173/02.09.1997, international position ISO/DIS 11274, sand box and Richards plates; measurements performed on undisturbed samples). The results of the physicalchemical analyses were used as input for the elaboration of 18 different PTFs (Tables 1 and 2). As the bulk and particle density of samples arent measured, the literature values for loamy soils were used: bulk density 1.3 g cm3 and particle density2.3 g cm3.

2.3.1 Water retention models

Most mathematical models that describe soil hydrologic behaviour are based on non-linear relationships between the volumetric water content in the soil, , the suction applied by the soil, h, and the hydraulic conductivity (Hillel, 1998); the functions (h) and K(h) describe the hydraulic properties of a soil through a parametric equation (Leij et al., 1997). Some predictive methods for estimating hydraulic conductivity are based on direct observations of water content in the soil measured at different values of suction (Romano and Palladino, 2002). To compensate for all the cases in which it is not possible to measure it, a group of functions called pedotransfer functions (PTFs) has been developed. PTFs correlate the water retention and hydraulic conductivity with some easily measurable chemical and physical properties of the soil such as texture, density, porosity, and organic carbon content (Elsenbeer, 2001; Tietje and Hennings, 1996; Tapkenhinrichs and Tietje, 1993). Most PTFs are regression equations that are derived from data collected during specic campaigns and are reliable for describing the soil hydraulic properties (Romano and Palladino, 2002).

In this work, the measured water retention curves were compared with those obtained using 17 PTFs proposed in the literature that are based on databases of soils distributed worldwide following two models: Brooks and Corey (1964) and van Genuchten (1980), (Rawls et al., 1998, 1992, 1982a;Saxton and Rawls, 2006; Saxton et al., 1986; Tanij, 1990).

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932 C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza

Table 1. Results of chemical and physical analyses performed on soils. Legend: A.B.: angular blocky; Sa.B.: subangular blocky; G.: granular;P.: platy; S.G.: single grain.

Sample Organic CaCO3 Electrical Sand Silt Clay Soil Structure carbon conductivity thickness of soil content% g kg1 ds m1 % % % cm

1 1.94 130.2 0.197 21.9 12.3 65.8 55 A.B.Sa.B. 2 4.13 147.7 0.212 17.5 12.9 69.6 30 G.Sa.B.3 4.14 190.4 0.152 27.9 12.3 59.8 60 G.Sa.B.4 1.67 38.5 0.232 11.5 14.7 73.8 30 Sa.B.G.5 1.04 134.8 0.167 12.2 12.4 75.4 62 P.6 1.35 57.4 0.196 10.3 14.7 75 32 Sa.B.G.7 1.92 229.8 0.130 33.3 12.5 54.2 45 S.G.Sa.B. 8 4.10 266.7 0.288 16.7 16.8 66.5 47 A.B.G.9 2.35 138.1 0.252 25 12.3 62.7 47 A.B.Sa.B. 10 2.68 59.9 0.136 18 9.8 72.2 50 Sa.B.A.B. 11 3.63 128.9 0.248 17.8 12.3 69.9 40 Sa.B.G.

Table 2. Volumetric water content ( %) from instrumental analysis at different suction values.

Sample Suction (kPa)0.10 0.25 1 3 6 10 33 1500

1 49.45 43.58 39.21 37.23 35.88 34.54 27.60 24.66 2 48.75 44.27 41.05 38.62 37.61 36.98 28.46 27.91 3 47.77 45.12 41.83 37.00 34.80 33.83 25.71 13.57 4 49.42 45.87 40.40 35.46 32.77 31.13 22.91 22.32 5 44.09 41.77 37.31 33.07 31.20 30.01 21.73 18.92 6 47.46 45.06 41.60 38.08 36.02 34.85 25.29 14.59 7 44.55 40.98 38.32 33.25 30.97 29.48 19.37 10.86 8 45.63 45.15 44.21 43.46 42.71 42.30 37.02 26.50 9 51.01 47.71 42.76 37.37 33.58 30.55 23.27 20.84 10 54.43 52.41 47.81 41.39 38.38 35.18 26.08 14.02 11 52.16 43.94 39.52 37.90 37.27 36.78 29.09 25.69

The functions used to describe water retention properties are the following: the van Genuchten (1980) water retention equation, as in

r

s r =

value, (cm1); m,n are the empirical shape-dening parameters in the van Genuchten equation, (dimensionless).

The values of the parameters ( , r, s, , , hb, , m,n) are predicted by PTFs, which are developed from the measured data set (Wsten et al., 2001).

In this study the processing of the PTFs was performed using the program CalcPTF 3.0 (Guber and Pachepsky, 2010).This contains a class of PTFs generated from the HYPRES database; see Table 3.

CalcPTF 3.0 is a computer program developed to calculate PTFs in order to estimate parameters of the Brooks and Corey model and the van Genuchten model. The inputs used in this program are: soil texture, organic carbon content, bulk density and particle density.

The database HYPRES (Hydraulic Properties of European Solis; Wsten et al., 1999) draws together some basic soil information and soil hydraulic data from which PTFs applicable to Europe can be derived (Nemes et

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1 m (1)

and the Brooks and Corey (1964) equation:

r

r = [braceleftBigg] [parenleftBig]

h

hb

, h > hb 1, h hb,

(2)

where is the volumetric soil water content (cm3 cm3); r is the residual soil water content (cm3 cm3); s is the saturated soil water content, (cm3 cm3); is the soil porosity, (cm3 cm3); is the pore size distribution index (dimensionless); h is the capillary pressure (cm); hb is the air-entry pressure (cm); is the parameter of the van Genuchten equation, corresponding approximately to the inverse of the air-entry

C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza 933

Table 3. Authors and localization of database and model used for the different PTFs. Legend: VG represents van Genuchten, BC represents BrooksCorey.

PTF Region Model

HYPRES Europe VG Saxton et al. (1986) USA, nationwide BC Campbell and Shiosawa (1992) None particular BC Rawls and Brakensiek (1985) USA, nationwide BC Williams et al. (1992) Australia BC Williams et al. (1992) Australia BC Oosterveld and Chang (1980) Canada, Alberta BC Mayr and Jarvice (1999) UK BC Wsten et al. (1999) Europe VG Varallyay et al. (1982) Hungary VG Vereecken et al. (1989) Belgium VG Wsten et al. (1999) Europe VG Tomasella and Hodnett (1998) Brazil VG Rawls et al. (1982b) USA, nationwide VG (corrected for OM accordingto Nemes et al., 2009)

Gupta and Larson (1979) Central USA VG Rajkai and Varallyay (1992) Hungary VG Rawls et al. (1983) USA, nationwide VG (corrected for OM according toNemes et al., 2009)

al., 2001b). Using the HYPRES database, two different sets of PTFs were derived: class pedotransfer functions and continuous pedotransfer functions. Class PTFs predict the hydraulic characteristics for each of the ve texture classes (coarse: clay < 18 % and sand > 65 %, 18 % < clay < 35 % and 15 % < sand; medium: clay < 18 % and 15 % < sand < 65 %; medium ne: clay < 35 % and sand < 15 %; ne: 35 % < clay < 60 %; very ne: 60 % < clay) and for two pedological classes within them (topsoils and subsoils) plus an additional class which encompassed the organic soil horizons. Continuous pedotransfer functions can predict hydraulic properties from individual measurements of soil texture, organic carbon content and bulk density.

The goodness of the PTFs and their ability to describe the hydraulic characteristics of the landll coverage soils was calculated through the root mean square error (RMSE) test based on the difference between the values of volumetric content of water at different suction values, measured and estimated, starting from the following equation:

RMSE =

physical properties are chosen to describe a non-degraded natural soil with the same texture, i.e. silt loam, with a bulk and particle density of respectively 1.3 and 2.3 g cm3 of landll soils but with an average organic carbon content of 1 %, which is typical of Piacenza natural soils, well structured and with a depth of 1 m. The volumetric water content of the reference soil at different suction values was calculated through the arithmetic mean of the water contents from the17 PTFs, so it is possible to achieve an estimate of available water content.

2.4 Flora and vegetation

The vegetation data were collected by making up 52 phytosociological relevs using the ZurichMontpellier school method (Braun-Blanquet, 1964). The sampling sites were selected to summarize the vegetation of the whole area. Each relev included an area of 16 m2 (4 m [notdef] 4 m) and was geo-

referenced. For each sampling site, the plant species present were listed and their cover was estimated using the values of the BraunBlanquet conventional scale (r = sporadic

species; + = < 1 %, 1 = 15 %, 2 = 525 %, 3 = 2550 %,

4 = 5075 %, 5 = 75100 %). The relevs were periodically

monitored from April to September 2012.

Pignatti (1982) was consulted for the identication of the species, while the specic nomenclature is according to Conti et al. (2005). In order to process the biological spectrum of the plant list, the data concerning the biological form according to Raunkiaer (1934) (therophytes T: annual herbs; hemicryptophytes H: perennial herbs; geophytes G: perennial herbs with underground storage organs; chamaephytes Ch: woody plants with buds at no more than 25 cm above the soil surface; phanerophytes P: trees and shrubs with buds over 25 cm above the soil surface) were taken from Romani and Alessandrini (2001).

Landolts F index (soil moisture) (Landolt, 1977), updated by Landolt et al. (2010), provides a guide on the need of water by plant species during their growth period. The F values range from 1 to 5 (1 is very dry, 1.5 is dry, 2 is moderately dry, 2.5 is fresh, 3 is moderately moist, 3.5 is moist, 4 is very moist, 4.5 is wet and 5 is ooded or submerged) and were attributed to all the species, recorded in order, to obtain information on the degree of humidity of the landll soil cover. To each species was also assigned its respective life strategy according to Grime (2001, 1979) (c describes competitive strategists, r describes ruderal strategists and s describes stress-tolerant strategists); this information was retrieved from Landolt et al. (2010), according to the adjustments proposed by the author. Starting from the climate, soil and vegetation data, reference crop evapotranspiration (ETo), the total available moisture (TAM) and the readily available moisture (RAM) were calculated using the CropWat 8.0 software (FAO 2009) according to Allen et al. (1998) and Doorenbos and Kassam (1979).

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[radicaltp]

[radicalvertex]

[radicalvertex]

[radicalbt]

1 N

N

Xi=1

i i

2, (3)

where N is the number of measurements; i and i is the

volumetric water content ( %), measured and estimated.

The hydraulic data of the landll cover soils obtained instrumentally and through PTFs were also compared with those of a reference soil. The reference soil chemical-

934 C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza

for suction values of 1500 kPa for 12 cases the measured value is higher than the predicted one.

To identify which of the authors, and thus of the models, are more accurate in describing the hydraulic behaviour of the landll soils, samples of chemicalphysical data are used as inputs of PTFs, so all water retention curves are developed and then the RMSE test was conducted (Fig. 3, Table 4). It emerges that the curve by Wsten al. (1999), showing a continuous pedotransfer function, is the closest to the measured data. The results of this test and the comparisons indicate a need to conduct studies to develop new parameter values which are able to describe the behaviour of degraded soils.

To compare natural soils with a reference one, reference soil water retention curves were developed using the PTFs.The reference soil water retention curve is described as the arithmetic mean of volumetric water content at different suction values obtained from processing PTFs. The sample 5 water retention curve is compared with the reference one (Fig. 4). This comparison reveals that the reference soil PTFs data always overestimate the measured data for all suction values lower than 100 kPa, whereas for suction values higher than 300 kPa, measured data are greater than the reference soil.

To compare the measured hydraulic properties of the land-ll soil with the reference soil, their volumetric water contents at suctions 0.10 kPa, at eld capacity, at wilting point and the available water for plants are compared. The histogram in Fig. 5 shows the water content at a suction of0.10 kPa; soils have values similar to each other (average % is 48.61 %, SD 3.18 %), and also similar to the reference soil ( % is 46.32 %).

The eld capacity is described as the optimal relationship between water and air in the soil; this condition is veried when the micropore volume is entirely occupied by water while macropore volume is entirely occupied by air. In the literature the eld capacity is represented by the water content at suction values in the range of 10 kPa and 33 kPa (10 kPa for sandy soil and 33 kPa for other soils). At eld capacity (histogram Fig. 6), the sample soil average % is 26.05, SD 4.68 %; this value is lower than that of the reference soil ( % is 30.16 %).

The histogram in Fig. 7 shows the soils at a suction of 1500 kPa (wilting point); the average of volumetric water content of soils sampled is % is 19.98 %, SD 5.97 %; the trend in this case is very variable, with one soil that has a water content of % = 27.91 % and another % is 10.86 %.

The reference soil instead has a value of % is 13.66 %; in nine soils the water content is higher than that of the reference soil.

In general terms, the available water for plants is dened as the difference between soil water content at suction 33 kPa soil water content at eld capacity and 1500 kPa soil water content at wilting point (histogram Fig. 8). For the investigated soils the average amount of available water has a value of % = 6.06 %, very high SD 4.70 %, with a minimum

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Figure 1. Water retention curves of sampled soils.

Figure 2. Sample 5: actual (black) and PTF water retention curves; the curves by Wsten et al. 1999 are highlighted.

3 Results

3.1 Soil

By the measurement of volumetric water content, it is possible to describe the water retention curve for all of the samples. Table 2 shows the measured volumetric water contents at the different suction values investigated and Fig. 1 shows their water retention curves. The water retention curves, with the exception of sample 8, display a similar trend. For suction values less than 10 kPa, the values are not very different, while in the end part when the suction is high there are some differences. The curves slope increases from 10 to 33 kPa due to the different water extractor used a sand box for 10 kPa and a Richards plate for 33 kPa.

As one of the study aims is to compare the landll soil with a natural reference soil, in the rst part of the paper, sample 5 is analysed. Sample 5 is the only landll soil showing the same amount of organic carbon as the reference one.

Using sample 5 chemicalphysical data as inputs of PTFs, the sample 5 predictive water retention curve is compared with the measured one. This comparison is shown in Fig. 2; in this Figure, the curves by Wsten et al. (1999)(PTFs applicable to Europe soils) are highlighted. From the comparison it clearly emerges that for suction values lower than 100 kPa, all PTFs except one overestimate the measured data, whereas

C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza 935

Figure 3. Matrix representing the result of the RMSE test each pixel for a combination of the soils PTF and RMSE.

Table 4. Results of the calculation of RMSE.

PTF RMSE % (for samples)

1 2 3 4 5 6 7 8 9 10 11

HYPRES 4.6 4.2 3.6 5.5 6.2 3.2 5.3 2.3 4.5 3.7 4.4 Saxton et al. (1986) 5.9 6.3 3.2 6.7 6.8 4.0 4.5 5.6 6.0 5.6 6.2 Campbell and Shiosawa (1992) 3.7 3.6 2.8 5.7 6.0 4.6 4.8 2.4 4.7 5.2 3.9 Rawls and Brakensiek (1985) 5.4 5.9 2.3 6.0 5.9 3.4 2.9 5.5 4.9 5.0 5.9 Williams et al. (1992) 3.6 4.0 2.0 4.2 4.2 2.8 4.2 3.8 4.0 4.8 4.2 Williams et al. (1992) 4.5 5.0 2.7 5.0 5.0 2.7 3.0 3.5 4.5 4.8 5.0 Oosterveld and Chang (1980) 4.4 5.1 1.7 5.0 4.8 2.4 2.8 4.9 4.0 4.8 5.1 Mayr and Jarvice (1999) 14.5 16.0 12.7 12.6 11.2 13.2 10.0 18.9 12.6 14.5 15.6 Wsten et al. (1999) 3.7 5.7 1.9 5.7 5.6 3.2 4.5 5.4 4.0 4.8 4.4 Varallyay et al. (1982) 6.5 7.7 3.7 4.7 3.6 3.2 1.5 8.2 5.4 7.9 7.5 Vereecken et al. (1989) 4.8 4.7 3.1 7.5 6.4 5.5 5.2 2.0 5.0 4.4 4.7 Wsten et al. (1999) 4.7 4.3 3.0 5.2 5.7 2.6 4.6 3.2 4.5 4.2 4.5 Tomasella and Hodnett (1998) 13.6 15.2 12.2 17.5 19.4 16.8 12.4 13.1 12.4 12.1 14.8 Rawls et al. (1982b) 5.5 7.1 6.5 7.2 6.8 4.7 5.9 4.1 6.4 5.5 6.6

Gupta and Larson (1979) 8.3 9.2 8.4 11.5 12.5 10.3 9.6 6.3 8.9 7.8 8.9 Rajkai and Varallyay (1992) 9.8 7.3 8.8 12.2 14.5 11.6 12.4 4.2 10.5 9.4 8.0 Rawls et al. (1983) 4.7 5.4 4.6 6.1 5.6 3.6 4.8 2.7 5.5 5.3 5.3

corrected for OM according to Nemes et al. (2009).

value of % = 0.55 % and a maximum of % = 12.14 %; the

reference soil has a value of % = 16.50 %.

All the sampled soils have a much lower available water % than the reference soil, despite having an organic carbon

content about twice as high as in the reference soil. Generally, high values of organic carbon correspond to high levels of organic matter, which enhances permeability and water availability in the soil. It would be interesting to study

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936 C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza

Figure 4. Comparison between sample 5 water retention curve and the reference curve described as the arithmetic mean of PTFs values.

Figure 5. Volumetric water content ( %) at suction 0.10 kPa: comparison between reference soil and landll soils.

why a soil presenting characters of physical degradation i.e. compaction, associated with a lack of organic carbon content, has, on the contrary, a high organic carbon content. With this in mind, it would be interesting, also, to study the carbon decomposition in humic and fulvic acids in association with limestone content.

3.2 Flora and vegetation

The total number of plant species sampled amounts to 90 (see Appendix A); almost all of them are very common and abundant in the province of Piacenza (Bracchi and Romani, 2010; Romani and Alessandrini, 2001). Most of the species were found to be competitiveruderal (43 %) and ruderal (13 %) (Grime, 2001) and belonging to the phytosociological class Stellarietea mediae R. Tx. Lohm. et PRSG. in Tx. 1950 which includes nitrophilous annual vegetation (Mucina et al., 1993; Oberdorfer, 1993; Ubaldi, 2008).

Table 5 shows a list of the ora biological spectrum. The study area has a particularly high percentage of therophytes (45 %) when compared to the values of the biological range of the province of Piacenza (23 %; Romani and Alessandrini, 2001) and Emilia-Romagna (28 %; Pignatti et al., 2001). Typically, ephemeral annual species tend to be concentrated in urban environments (Sukopp and Werner, 1983) and in Italy, regardless of human disturbance, their percentage increases gradually from north to south in response

Figure 6. Volumetric water content ( %) at eld capacity: comparison between reference soil and landll soils.

Figure 7. Volumetric water content ( %) at a suction of 1500 kPa: comparison between reference soil and landll soils.

Figure 8. Available water to plants ( %): comparison between reference soil and landll soils.

to the emergence of a distinctly arid climate (Pignatti, 1994, 1976).

Fig. 9 represents the monthly rainfall and evapotranspiration and it should be noted that the ETo is greater than the rainfall in the period from May to August, indicating a summer drought.

The histogram referring to the F index (Fig. 10) shows that most of the species found require soils with a moisture content ranging from moderately dry to moderately moist. The typically xerophyte species and those found in submerged soils are absent, while there are two ( Bolboschoenus maritimus (L.) Palla and Eleocharis palustris (L.) Roem. & Schult) that need wet soil.

In Fig. 11 the graphs referring to the amount of water lost from a common agricultural soil of medium texture 1 m deep (a), and the soil cover of the landll (b) are presented, considering both the climatic conditions of Piacenza and the cover of grassland vegetation of perennial grasses (cool season grass varieties including bluegrass, fescue and ryegrass;

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C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza 937

Table 5. Biological spectrum of ora.

Biological spectrum of ora (%)

Therophytes 45 Hemicryptophytes 41 Geophytes 11 Phanerophytes 3

Figure 10. F index (soil moisture). Percentages are weighted by the frequency of the species in the monitoring sites (see column Presence in the Appendix). Legend: 1 is very dry, 1.5 is dry, 2 is moderately dry, 2.5 is fresh, 3 is moderately moist, 3.5 is moist, 4 is very moist, 4.5 is wet and 5 is ooded or submerged.

Figure 9. Monthly rainfall and evapotranspiration (ETo). Climate data source: San Lazzaro Alberoni weather station (Piacenza 1961 2005).

Allen et al., 1998). The soil of the landll has less water available to vegetation compared to agricultural soil.

4 Discussion and conclusions

In this study, the attempt to relate the hydraulic properties of degraded soil with plant coverage is presented.

The hydrological properties of a degraded soil are described through a comparison between the laboratory tests and the results of predictive systems by PTFs, showing that the PTFs are not able to describe them.

The study of the hydraulic properties of landll cover soils has outlined that these soils have less water content available in comparison with a natural reference soil; this is a characteristic of degradation.

On the basis of PTFs, some conclusions can be formulated. PTFs have the advantage of being relatively inexpensive and easy to derive and use, but for application at a specic point and for soils that are outside the range of soils used to derive them, prediction using PTFs might be inadequate. In this case, direct measurement is the only option (Wsten et al., 2001) and it can be interesting to conduct studies to develop degraded soils using new PTFs parameters and to relate them to the type of soil organic content. Generally, high values of soil organic carbon correspond to high levels of organic matter, which enhances permeability and water availability. With this in mind, it would be interesting to study why a soil presenting characters of physical degradation i.e. compaction, associated with a lack of organic carbon content, has, on the

Figure 11. Water lost from agricultural soil (a) and from the landll cover soil (b) by Crop Wat 8.0 software. Legend: RAM represents readily available moisture; TAM represents total available moisture.

contrary, a high organic carbon content. It would be interesting, also, to study the carbon decomposition in humic and fulvic acids in association with limestone content.

Analysing vegetation, it can be said that the landll vegetation is mainly related to the soil character. The low water content, together with the lack of depth and compacted structure, would justify the current presence of a vegetation cover which consists predominantly of therophytes instead of a more developed and stable perennial vegetation with shrubs and trees, as observed for other landlls several years after their coverage (El-Sheikh et al., 2012; Huber-Humer and Klug-Pmpel, 2004; Rebele and Lehmann, 2002). The high frequency of therophyte does not seem to be justied by

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938 C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza

summer drought and by the low level of human disturbance that affected the area in recent years, given that, under the same climatic conditions, the potential vegetation of the area should be represented by riparian forests of Populetalia albae Br.-Bl. 1935 (Puppi et al., 2010). These forests, although not very widespread, are present and contiguous to the land-ll.

The presence of Bolboschoenus maritimus (L.) and Eleocharis palustris (L.), which need wet soil, is explained by the fact that F refers to soil water availability during the time of year when the species carry out their vegetative cycle (Landolt et al., 2010). In this case the above-mentioned hydrophilic plants were detected only in the spring months when the monthly evapotranspiration is less than or equal to rainfall.

In comparison with agricultural soil in the same climatic conditions, the landll soil has less water available to vegetation, and this contributes to water stress for plants over a longer period (March to September) and is more pronounced as the amount of water absorbed by plants during the summer is close to their permanent wilting point (TAM line).

The low water content in association with high organic carbon, the lack of depth, the compacted structure of these soils and the current presence of a vegetation cover, which consists predominantly of therophytes, are important studied aspects of the aims of the New Life project, which seeks to establish a treatment for restoring degraded soils. This treatment the reconstitution produces a new soil, called reconstituted soil. The comparison between chemical-physical characters of degraded and reconstituted soil is very important. In this comparison it will be interesting to study their hydraulic properties in relation to their plant coverage.

Edited by: A. Cerd

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C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza 939

Appendix A

Table A1. Species, life form, F index, plant strategies and presence of the sampled plant.

n Species Life form F index Plant strategy Presence

1 Abutilon theophrasti Medik. T 2.5 cr 3/522 Agrimonia eupatoria L. T 2 cr 2/523 Allium spp. 1/524 Alopecurus myosuroides Huds. T 3 r 10/525 Alopecurus pratensis L. T 3.5 cs 5/526 Alopecurus rendlei Eig T 3 crs 7/527 Amaranthus retroexus L. T 2.5 cr 18/528 Ambrosia artemisiifolia L. T 2 cr 15/529 Amorpha fruticosa L. H 3.5 crs 1/5210 Aristolochia clematitis L. G 3.5 cr 2/5211 Arrhenatherum elatius (L.) P. Beauv. ex J. and C. Presl H 3 cr 21/5212 Artemisia vulgaris L. G 2.5 crs 13/5213 Atriplex patula L. T 2.5 cr 10/5214 Avena fatua L. T 2.5 cr 14/5215 Ballota nigra L. T 2.5 cr 4/5216 Bolboschoenus maritimus (L.) Palla T 4.5 cs 1/5217 Bromus hordeaceus L. T 3 cr 14/5218 Bromus sterilis L. T 2 r 30/5219 Capsella bursa-pastoris (L.) Medik. T 2 r 6/5220 Cardamine hirsuta L. T 3 rs 3/5221 Cerastium spp. 9/5222 Chenopodium album L. T 2 r 27/5223 Cichorium intybus L. T 2.5 crs 2/5224 Cirsium arvense (L.) Scop. T 3 cr 6/5225 Cirsium vulgare (Savi) Ten. T 3 cr 1/5226 Convolvulus arvensis L. T 2.5 cr 50/5227 Crepis setosa Haller f. H 1.5 r 5/5228 Crepis vesicaria L. T 2 cr 2/5229 Cynodon dactylon (L.) Pers. T 2 cs 44/5230 Dactylis glomerata L. H 3 crs 6/5231 Dipsacus fullonum L. T 3.5 cr 1/5232 Echinochloa crusgalli (L.) P. Beauv. G 3.5 cr 3/5233 Eleocharis palustris (L.) Roem. and Schult. H 4.5 crs 2/5234 Elymus repens (L.) Gould T 3 cs 52/5235 Erigeron annuus (L.) Desf. H 2.5 cr 2/5236 Euphorbia cyparissias L. H 2 crs 1/5237 Galium aparine L. G 3 cr 8/5238 Galium verum L. H 2.5 crs 2/5239 Geranium dissectum L. T 3 cr 17/5240 Geranium molle L. H 2.5 cr 9/5241 Hordeum murinum L. T 2 r 23/5242 Humulus japonicus Siebold and Zucc. T 3.5 cr 1/5243 Hypericum perforatum L. G 3 crs 2/5244 Lactuca serriola L. H 2 cr 9/5245 Lamium purpureum L. T 3 r 7/5246 Lapsana communis L. T 3.5 cr 2/5247 Lepidium draba L. G 2 cr 3/5248 Lolium perenne L. H 3 cr 4/5249 Lythrum salicaria L. T 4 cs 1/5250 Malva alcea L. T 2.5 cs 2/52

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940 C. Cassinari et al.: Hydraulic properties and plant coverage of a closed-landll soils in Piacenza

Table A1. Continued.

n Species Life form F index Plant strategy Presence

51 Malva sylvestris L. T 2.5 crs 2/5252 Matricaria chamomilla L. H 3 r 2/5253 Medicago lupulina L. T 2 rs 3/5254 Medicago sativa L. H 2 cs 8/5255 Melilotus albus Medik. H 2.5 cr 3/5256 Mentha arvensis L. H 3.5 crs 2/5257 Myosotis arvensis (L.) Hill T 2 cr 2/5258 Onopordum acanthium L. T 2 cr 2/5259 Ornithogalum umbellatum L. H 3 crs 1/5260 Papaver rhoeas L. H 2 r 1/5261 Persicaria lapathifolia (L.) Delarbre H 2.5 cr 2/5262 Plantago lanceolata L. H 3.5 crs 8/5263 Poa pratensis L. T 3.3 crs 1/5264 Poa trivialis L. H 3.5 crs 14/5265 Polygonum aviculare L. T 3.5 r 23/5266 Portulaca oleracea L. H 2.5 r 1/5267 Potentilla reptans L. H 3 crs 3/5268 Ranunculus bulbosus L. H 2 crs 10/5269 Robinia pseudoacacia L. H 2.5 c 1/5270 Rumex crispus L. H 3.5 cr 44/5271 Rumex pulcher L. H 3 crs 5/5272 Salix alba L. T 4.5 c 1/5273 Salvia pratensis L. H 2 crs 2/5274 Solanum nigrum L. G 3 r 2/5275 Sonchus asper (L.) Hill H 3.5 cr 3/5276 Sonchus oleraceus L. H 3 cr 2/5277 Sorghum halepense (L.) Pers. H 2 c 2/5278 Stellaria media (L.) Vill. H 3 cr 14/5279 Tanacetum vulgare L. H 3.5 c 2/5280 Taraxacum ofcinale Weber G 3 crs 3/5281 Torilis arvensis (Huds.) Link H 2 cr 2/5282 Trifolium fragiferum L. H 3 crs 2/5283 Trifolium pratense L. G 3 crs 3/5284 Trifolium repens L. H 3 crs 4/5285 Valerianella spp. 2/5286 Verbascum thapsus L. P 2.5 crs 4/5287 Verbena ofcinalis L. P 3 cr 8/5288 Veronica persica Poir. P 3 cr 15/5289 Vicia sativa L. T 3 cr 19/5290 Xanthium orientale L. subsp. italicum (Moretti) Greuter G 3 cr 4/52

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