Effects of agricultural management on surface soil properties and soil–water losses in eastern Spain

Soil & Tillage Research 106 (2009) 117–123

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Effects of agricultural management on surface soil properties and soil–water losses in eastern Spain F. Garcı´a-Orenes a,*, A. Cerda` b, J. Mataix-Solera a, C. Guerrero a, M.B. Bodı´ a,b, V. Arcenegui a, R. Zornoza a, J.G. Sempere a a b

GEA - Grupo de Edafologı´a Ambiental, Departamento de Agroquı´mica y Medio Ambiente, Universidad Miguel Herna´ndez, Avda. de la Universidad s/n, E-03202-Elche, Alicante, Spain Departament de Geografia. Universitat de Vale`ncia Blasco Iba`n˜ez, 28, 46010 Valencia, Spain

A R T I C L E I N F O

A B S T R A C T

Article history: Received 25 May 2009 Accepted 1 June 2009

In Spain, agriculture triggers soil degradation and erosion processes. New strategies have to be developed to reduce soil losses and recover or maintain soil functionality in order to achieve a sustainable agriculture. An experiment was designed to evaluate the effect of different agricultural management on soil properties and soil erosion. Five different treatments (ploughing, herbicide, control, straw mulch and chipped pruned branches) were established in ‘‘El Teularet experimental station’’ located in the Sierra de Enguera (Valencia, Spain). Soil sampling was conducted prior to treatment establishment, and again after 16 months, to determine soil organic matter content (OM), aggregate stability (AS), and microbial biomass carbon content (Cmic). Fifty rainfall simulations tests (55 mm during one hour, 5-year return period) were applied to measure soil and water losses under each treatment. The highest values of OM, AS and Cmic were observed in the straw-covered plot, where soil and water losses were negligible. On the contrary, the plot treated with herbicides had the highest soil losses and a slight reduction in Cmic. Soil erosion control was effective after 16 months on the plots where vegetation was present while on the ploughed and herbicide-treated plots, the practices were not sustainable due to large water and soil losses. Except for the straw mulch plot, soil properties (OM, AS, Cmic) were not enhanced by the new land managements, but soil erosion control was achieved on three of the five plots used (weeds, weeds plus straw and weeds plus chipped pruned branches). Erosion control strategies such as weeds, weeds plus straw mulch and weeds plus chipped branches mulch are highly efficient in reducing soil losses on traditional herbicide-treated and ploughed agricultural land. However, it takes longer to recover other soil properties such as OM, AS, and Cmic. Published by Elsevier B.V.

Keywords: Aggregate stability Microbial biomass Soil erosion Agriculture management Herbicide Plough Straw mulch Weeds Chipped pruned branches Spain

1. Introduction Soil erosion is a major problem in the Mediterranean region due to the arid conditions and torrential rainfalls, which contribute to the degradation of agricultural land (Lo´pez-Bermu´dez and Albadalejo, 1990; Lal, 1999). This is because soil losses on agricultural land are enhanced by the high susceptibility of soil particles and aggregates to become detached and transported by erosive agents, as vegetation is usually absent (Boardman et al., 1990; Go´mez et al., 1999). In fact, the highest erosion rates are usually found on agricultural land where vegetation cover is low. Research conducted in Spain since the 1980s confirms that sediments are coming from agricultural land, as soil erosion from grass, forest and scrubland is low (Garcı´a-Ruiz et al., 1995; Lo´pez

* Corresponding author. Tel.: +34 966658948; fax: +34 966658532. E-mail address: [email protected] (F. Garcı´a-Orenes). 0167-1987/$ – see front matter . Published by Elsevier B.V. doi:10.1016/j.still.2009.06.002

Bermu´dez et al., 1998; Cerda`, 2001, 2002). This is why soil erosion control strategies are largely being applied to agricultural land. Catch crops, no-tillage or reduced tillage, chipped pruned branches, straw mulch, geotextils and weed control by herbicides (or no-tillage techniques) are some of the land managements currently being applied on experimental farms to reduce the usually high erosion rates on rainfed agricultural land in eastern Spain. These soil erosion control managements are being tested at the El Teularet—Sierra de Enguera Soil Erosion Experimental Station (TESEES) as the first step in applying the most sustainable agriculture management under Mediterranean climatic conditions. The research area selected is the Macizo del Caroig region, Western Valencia province in eastern Spain where climate is typical Mediterranean: warm winters, hot and dry summers, and intense rainfall events in autumn (Cerda`, 2005). New soil management practices will not only affect the erosion processes, but also the soil properties. Several studies show that agricultural

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management has an important influence on chemical, physical and biological parameters (Garcia et al., 1997; Caravaca et al., 2002; Marinari et al., 2006), as microbial populations and activities are fundamental for maintaining soil quality by mediating the processes of organic matter turnover and nutrient cycling (Doran and Parkin, 1994). This paper focuses on both processes in order to understand how new soil conservation strategies to be applied in the near future by farmers in the Valencia Region will affect the soil properties. Soil erosion is largely controlled by vegetation and residue cover, and this is why the three conservation land management strategies applied were based on weed cover (control), on weeds plus straw mulch, and weeds plus chipped pruned branches. Rainfall simulation experiments were carried out to quantify the soil loss as natural rainfall has a large temporal variability. As an example, during the last 50 years annual rainfall decreased from 1051 mm in 1959 to 202 mm in 1979 at the nearest meteorological station. Thus, experiments based on plots and natural rainfall would delay the research for decades as the high magnitude—low frequency rainfall events, which contribute to the highest soil and water losses, are rare. There are several reports that show the influence of soil management on soil erosion in orchards in different Mediterranean zones (Pastor and Castro, 1995; Kosmas et al., 1997; Francia Martı´nez et al., 2006), and they agree with the high erosion rates and the need for the development and testing of strategies to control soil erosion. This paper contributes to the knowledge of how to control soil losses from Mediterranean agricultural land, and what impact these new strategies may have on soil properties. We evaluate the effect of five different agricultural managements on soil erosion and soil properties in rainfed orchards in eastern Spain. Rainfall simulation experiments were conducted in order to quantify the soil losses under 5-year return period thunderstorms, which are those that trigger the surface wash and particle removal. Measurements of aggregate stability (physical changes), organic matter (chemical changes) and microbial biomass carbon (biological changes) will be indicative of how soil management influences soil properties. 2. Material and methods 2.1. Study area A rainfed orchard located in the Sierra de Enguera (388500 N; 08420 W) was selected as representative of typical rainfed mountainous eastern Spain agriculture for the installation of the El Teularet Soil Erosion Experimental Station (TESEES) (Cerda`, 2006). The parent material is Cretaceous Marls, the soil is a Typic Xerorthent (Soil Survey Staff, 1998), the agricultural terrace had a slope gradient of 5% and the previous land management was almond and wheat crop farming. Intense ploughing has been applied at the site for centuries. The soil was ploughed in autumn 2003 to install the TESEES experimental station plots, collectors and deposits, and the management treatments were initiated in February 2004. Climate is typical Mediterranean with 3–5 months of summer drought, usually from late June–September. Mean annual rainfall at the study area range from 479 mm at the Enguera—Las Arenas meteorological station to 590 mm at the Enguera Confederacio´n Hidrogra´fica del Jucar (CHJ) meteorological station. The rain gauges located at the experimental station measured rainfall (0.2 mm of accuracy) amount of 715.8 mm in 2004 and 247.4 mm in 2005. The mean annual days of rainfall at the study area is 37.9 (Las Arenas meteorological station) and 40.7 (CHJ meteorological station). Rainfall intensities during the study period (February 2004–June 2005) were characterized by a low intensity as the daily

Table 1 Seasonal precipitation (mm) (1961–1990) at the Sierra de Enguera meteorological stations. Meterorological station

Enguera. CHJ Enguera. La Matea Enguera. Las Arenas El Teularet (2004)

Seasonal precipitation (mm) Spring

Summer

Autumn

Winter

158.4 150.1 132.9 340.0

57.9 65.6 55.0 75.0

210.7 182.2 153.6 196.0

163.0 139.4 137.7 104.8

Table 2 Mean, maximum and minimum temperatures (1961–1990) at the Sierra de Enguera meteorological stations. Meteorological station

Enguera. La Matea Enguera. Las Arenas El Teularet (2004)

Temperatures (8C) Mean

Maximum

Minimum

14.2 12.7 14.7

19.8 15.7 19.7

8.7 9.8 9.7

rainfall was always less than 60 mm and the I30 was below 25 mm h1. Rainfall is distributed homogenously amongst spring, autumn and winter, while the summer is extremely dry due to high temperatures and lack of rainfall. Mean annual temperature ranges from 12.7 8C to 14.2 8C within the La Matea and Las Arenas meteorological stations. The hottest month is August with an average monthly temperature of 23 8C, while the coldest is January with an average monthly temperature of 7.3 8C (Tables 1 and 2). 2.2. Experimental plots In October 2003, five plots (each of them 200 m2; 10 m  20 m) were established. The main characteristics of the original soil before the application of the various agricultural management treatments are shown in Table 3. After ploughing, the five treatments were applied in February 2004 (Table 4), and represent typical farming strategies in the region of the Macizo del Caroig, where rainfed agriculture is the most common farming activity (olive, almond and cereal crops). The treatments were systemic herbicide (SH), plough (P), control (weeds with no-tillage, C), straw mulch (weeds plus oats straw, SM) and chipped pruned branches (weeds plus chipped olive branches, CPB). More details are shown in Table 4. 2.3. Soil sampling and analysis The soil in the plots was sampled twice. The first soil samples were collected before the land management treatments were applied (in February 2004) in order to characterize the original soil

Table 3 Main characteristics of the original soil (0–5 cm depth). Parameter OM (%) CaCO3 equivalent (%) pH EC (mS/cm) WHC (%) Texturea(%)

2.0 59.8 8.3 185 49.5 39; 38; 23

OM: Organic matter; EC: Electrical Conductivity; WHC: Water Holding Capacity. a Sand: 2–0.02 mm; Silt: 0.02–0.002 mm; Clay: < 0.002 mm.

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Table 4 Agriculture land managements applied on the different plots. Code

Treatment

Description

SH

Systemic herbicide

P

Plough

C

Control

SM

Straw mulch

CPB

Chipped pruned branches

Four times during the study period. May 2004, July 2004, October 2004 and May 2005. Six ploughings during the study period. February 2004, May 2004, July 2004, October 2004, February 2005 and May 2005 (tractor). Weeds. No-tillage. Weeds chopped in May 2004, July 2004, October 2004 and May 2005. Weeds. No-tillage. Oat straw: 250 g m2 year1 Weeds chopped in May 2004, July 2004, October 2004 and May 2005. Weeds. No-tillage. Chipped branches (<2 cm diameter) 50 g m2 year1. Weeds chopped in May 2004, July 2004, October 2004 and May 2005.

in each plot. The second set of soil samples was collected in June 2005, 16 months after initiating the land management treatments. Three soil samples per treatment were collected to determine the organic matter content (OM), aggregate stability (AS) and microbial biomass carbon content (Cmic). Soil was sampled from the surface layer (0–5 cm). Soil OM was determined by the potassium dichromate oxidation method (Nelson and Sommers, 1982). AS was measured with the method of Rolda´n et al. (1994), based on the method of Benito et al. (1986). This method examines the proportion of aggregates that remain stable after a soil sample (sieved between 4 mm and 0.25 mm) is subjected to an artificial rainfall of known energy (270 J m2). Cmic was determined by the fumigationextraction method (Vance et al., 1987). 2.4. Rainfall simulation experiments Ten rainfall simulation experiments were carried out in June– July 2005 under dry conditions at each plot in order to determine the soil and water losses. Fifty experiments (10 experiments  5 treatments) were conducted during the summer drought period when soil moisture is low. Deionised water was applied from a height of two meters onto a 1 m2 sub-plot, and runoff was collected from a bordered circular 0.25 m2 area in the centre of the sub-plot. Simulated rainfall duration was 1 h at a rate of 55 mm h1, simulating the rainfall from a thunderstorm, which in these study areas would occur once every 5 years. Rainfall characteristics were 93.24% of Christiansen Coefficient for the rainfall distribution at 55 mm h1, with a mean drop-size of 2.53 mm (D50), mean drop velocity of 3.4 m s1, and a Kinetic energy of 7.1 J m2 mm1. A detailed information on the distribution of those parameters can be found in Cerda` et al. (1997) Overland flow from the circular collection area was measured at 1-min intervals. Every tenth 1-min runoff sample was collected for laboratory analysis in order to determine sediment concentration. Runoff rates and sediment concentration were used to calculate the sediment yield, total runoff, runoff coefficient, infiltration, and erosion rates (Cerda`, 1999). Vegetation, litter and rock fragment cover were measured in the field as % of the soil surface covered by plants. 2.5. Statistical analysis The fitting of the data to a normal distribution for all properties measured was checked with the Kolmogorov–Smirnov test. A Student’s t-test was applied in order to assess significant differences between first sampling (the start of the experiment) and second sampling (16 months after the application of treatments) with respect to the soil parameters analyzed. A oneway ANOVA was carried out with soil parameters of the second

sampling to assess the differences between treatments after 16 months of land managements, and the separation of means was made according to Tukey’s honestly significant difference test (hsd) at a = 0.05 for all the parameters studied. Pearson’s correlation coefficients (r) were calculated to assess the relationship between parameters. 3. Results and discussion 3.1. Soil properties Table 5 shows OM content, AS and Cmic at the start of the experiments in February 2004, before the initiation of the land management treatments. Initially the OM matter distribution within the experimental field was very homogeneous (2%) ranging from 1.9 on the chipped pruned branches plot and the 2.2 on the ploughed plots. The Cmic was also similar in all the plots ranging from 230 mg to 349 mg of Cmic per kg of dry soil, and the percentage of AS found in the plots ranged between 52% and 62%. No statistical differences were found between the plots for OM, AS and Cmic before the treatments were applied. The application of straw mulch to the soil produced a significant increase in OM (from 2.0% to 3.2%), Cmic (from 282 mg C kg1 to 726 mgC kg1) and also in AS (from 62% to 75%) after 16 months of treatment with respect to the initial conditions, which confirm the interaction of those three selected properties. Other studies related to farming systems showed significant increases of microbial biomass carbon content with organic management, but no Table 5 Organic matter content, microbial biomass carbon content and aggregate stability in soil samples taken from the surface top layer (0–5 cm) in February 2004 before treatment application for the five agriculture land managements: SH, Systemic Herbicide; P, Ploughing; C, Control (weeds); SM, Straw Mulch (weeds plus 250 g m2 year1 of straw); and CPB, Chipped pruned branches (weeds plus 50 g m2 year1 of chipped branches). Treatment SH Mean Std. dev. P Mean Std. Dev. C Mean Std. dev. SM Mean Std. dev. CPB Mean Std. dev.

OM (%)

Cmic (mg C kg1)

AS (%)

2.0 0.2

318 37

52 1.3

2.2 0.2

339 23

61 0.2

2.0 0.1

269 4

54 1

2.0 0.1

283 15

61 2.4

1.9 0.1

232 27

60 0.6

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Fig. 1. Soil organic matter content (OM) in samples taken from the surface of top soil layer (0–5 cm) after 16 months of land management treatments (Mean  Standard deviation). SH, Systemic Herbicide; P, Ploughing; C, Control (weeds); SM, Straw Mulch (weeds plus 250 g m2 year1 of straw); and CPB, Chipped pruned branches (weeds plus 50 g m2 year1 of chipped branches). Different letters indicate statistical differences between treatments using Tukey’s hsd test (a = 0.05).

significant differences in soil organic carbon content after 14 months (Marinari et al., 2006). At the El Teularet research site after 16 months, the higher values for Cmic, OM and AS for the straw mulch treatment can be seen as compared with the other treatments (Figs. 1–3). In this experiment, OM was only measured for the soil surface (0–5 cm) and there was only a significant OM increase with the addition of straw mulch. Other authors (BlancoCanqui and Lal, 2008) have demonstrated that the balance of total organic soil carbon varies with depth, and in many cases soil management (tillage or no-till) will not cause significant differences in the total content of carbon through the soil profile. The plough treatment resulted in a significant reduction in aggregate stability (from 61.1% to 49.1%) due to the rupture of aggregates, which confirm the findings of Green et al. (2007). A significantly lower Cmic (223 mg C kg1) was found in the plot treated with herbicide. Neither the chipped pruned branches nor the control plot showed any significant change in OM, Cmic and AS, and any difference between these treatments was found in the second sampling for the three studied soil properties (Figs. 1–3). Pearson correlation coefficients demonstrate the clear correlation between the three soil properties selected: OM (chemical), Cmic (biological) and AS (physical). Positive correlations between the studied soil properties were found. The OM content is related with Cmic (0.953**); and soil structure (as indicated by the AS) is related with the OM (0.754**) and Cmic (0.789**); greater levels of OM and Cmic result in greater percentages of AS. Similar positive relationships between OM, Cmic and AS were found in other Mediterranean type-ecosystems (Garcı´a-Orenes et al., 2005;

Fig. 3. Aggregate stability (AS) in samples taken from the surface of top soil layer (0– 5 cm) after 16 months of land management treatments (Mean  Standard deviation). SH, Systemic Herbicide; P, Ploughing; C, Control (weeds); SM, Straw Mulch (weeds plus 250 g m2 year1 of straw); and CPB, Chipped pruned branches (weeds plus 50 g m2 year1 of chipped branches). Different letters indicate statistical differences between treatments using Tukey’s hsd test (a = 0.05).

Zornoza et al., 2007). It has been reported that the addition of a readily available substrate can cause a rapid stimulation of the soil microflora, and with this an increase in aggregate stability (Rolda´n et al., 1994; Lax et al., 1997). Other treatments such as a legume green fallow also improve degraded soils through an increase in the soil microbial populations (Biederbeck et al., 2005) which finally result in a more stable soil structure. This was found at the TESEES experimental station when oats straw mulch was applied but not with the other treatments (herbicide, ploughing, weeds or chipped pruned branches), where no significant changes were observed after 16 months of the different agricultural treatments. 3.2. Soil surface properties changes The most dynamic soil position on agricultural land is the surface, where the treatments are applied and where soil erosion processes take place. The original soil surface was ploughed in February 2004, when no vegetation was present and rock fragments were mixed with the fine material. After 16 months of different land management treatments, vegetation cover was negligible on the herbicide plot, zero on the ploughed plot, but high on the control (59.5%) and chipped pruned branches (60.6%) plots, and very high (83.8%) on the straw mulch covered soil. The 16-month time period also resulted in a large increase in surface rock fragments on the herbicide plot. However, due to tillage (mixing of fine and coarse material) or vegetation cover (minimizing the removal of fine materials) the amount of surface rock fragments on the other four plots was very low (Table 6). The high rock fragment cover on the herbicide plots was also related to the high erosion rates on these plots, as measured in the rainfall Table 6 Mean vegetation cover (including straw, chipped branches and litter), rock fragment cover and slope gradient (n = 10) for the rainfall simulation sub-plots. SH, Systemic Herbicide; P, Ploughing; C, Control (weeds); SM, Straw Mulch (weeds plus 250 g m2 year1 of straw); and CPB, Chipped pruned branches (weeds plus 50 g m2 year1 of chipped branches). Treatment SH

Fig. 2. Microbial biomass carbon content (Cmic) in samples taken from the surface of top soil layer (0–5 cm) after 16 months of land management treatments (Mean  Standard deviation). SH, Systemic Herbicide; P, Ploughing; C, Control (weeds); SM, Straw Mulch (weeds plus 250 g m2 year1 of straw); and CPB, Chipped pruned branches (weeds plus 50 g m2 year1 of chipped branches) Different letters indicate statistical differences between treatments using Tukey’s hsd test (a = 0.05).

Vegetation Mean (%) Std. dev. (%) Rock fragments Mean (%) Std. dev. (%) Slope gradient Mean (%) Std. dev. (%)

P

C

SM

CPB

1.4 1.6

0.0 0.0

59.5 10.3

83.8 12.6

60.6 17.2

26.7 5.3

3.1 2.3

8.2 2.5

1.0 1.3

3.9 1.9

4.9 1.8

5.0 1.8

4.9 1.8

5.0 1.8

5.0 1.8

F. Garcı´a-Orenes et al. / Soil & Tillage Research 106 (2009) 117–123 Table 7 Mean time to ponding (Tp), time to runoff (Tr) and Tr  Tp for the rainfall simulation sub-plots (n = 10). SH, Systemic Herbicide; P, Ploughing; C, Control (weeds); SM, Straw Mulch (weeds plus 250 g m2 year1 of straw); and CPB, Chipped pruned branches (weeds plus 50 g m2 year1 of chipped branches). Treatment SH Tp (s) Mean Std. dev. Tr(s) Mean Std. dev. Tr  Tp (s) Mean Std. dev. a b

P

C

SM

Table 8 Mean values of soil and water losses for the five land management treatments from the rainfall simulation sub-plots (n = 10). SH, Systemic Herbicide; P, Ploughing; C, Control (weeds); SM, Straw Mulch (weeds plus 250 g m2 year1 of straw); and CPB, Chipped Pruned Branches (weeds plus 50 g m2 year1 of chipped branches). No runoff was observed on the SM sub-plots. Treatment

CPB

SH a

190.6 75.0

1031.1 229.0

1628.1 262.6

3331.7 90.2

254.3 82.1

1319.8 293.4

1850.7 263.6

no runoff no runoff

2849.1b 519.1

63.7 38.6

288.7 135.7

222.6 1260

no runoff no runoff

903.9b 422.2

2070.8 433.1

Seven out of ten plots with no ponding. Three out of ten plots with no runoff.

simulation experiments. The surface wash removed the fine material and thus the amount of rock fragments on soil surface increased. No significant differences between slope gradients were found, as the orchard was very homogenous due to centuries of ploughing and levelling work. After 16 months, the five plots demonstrated a striking contrast: bare soil on the herbicide and tillage treatments versus considerable vegetation on the chipped pruned branches, straw mulch and control (weeds) treatments (Table 6). 3.3. Runoff initiation Runoff generation is a key point in soil erosion processes as it determines the time and volume of water running off the soil surface, and controls soil detachment rates and sediment transport capacity. Time to ponding (Tp) was reached in an average of 190.6 s at the herbicide sites, and was greatly delayed on the ploughed soils (1030 s). However, ponding was also greatly delayed on the control, straw and chipped sub-plots, and Tp was not even reached on 7 of the 10 straw sub-plots. Time to runoff (Tr) was 254.3 s for the herbicide treatment and 1319.8 s for the plough treatment (Table 7). The other three treatments had delayed or no runoff. The Tr  Tp gives information on the time necessary to transform the ponds into runoff, and this was much quicker for the herbicide treatment than for any of the other land managements. This is a key point, as after only an average of 63.7 s the ponding was transformed into overland flow ready to contribute with water and sediment. Tr  Tp was much greater for the other treatments, thus identifying the herbicide treatment as the one most likely to contribute high rates of runoff and sediment during rainfall events. 3.4. Water losses Average total runoff collected from the 0.25 m2 sub-plots was zero from the straw mulch management, negligible from the chipped pruned branches (0.13 l), very low from the control subplots (0.37 l), low from the ploughed sub-plots (1.01 l) and high from the herbicide treatment (3.95 l). Runoff coefficients show that the herbicide treatment lost 28.28% of the rain to runoff; while for ploughing it was 7.31%, control it was 2.68% and for the chipped branches treatment it was 0.96%. The straw-covered plots did not contribute any runoff, thus 100% of the 13.75 l of rainfall applied to the sub-plots infiltrated into the soil (Table 8). 3.5. Soil losses Sediment concentration in runoff provides information on the soil’s susceptibility to erosion (erodibility). The ploughed soil had

121

Total runoff (l) Mean Standard deviation Runoff coefficient (%) mean Standard deviation Sediment conc. (g l1) Mean Standard deviation Sediment yield (g) Mean Standard deviation Soil loss (g m2 h1) Mean Standard deviation Soil loss (Mg ha1 h1) Mean Standard deviation a

P

C

SM

CPB

3.9 2.1

1.1 0.5

0.4 0.3

0 0

0.1a 0.1a

28.3 15.1

7.3 3.3

2.7 1.8

0 0

1.0a 0.8a

2.6 2.1

8.4 4.9

1.5 0.5

0 0

0.4a 0.3a

12.6 18.6

9.5 8.5

0.5 0.4

0 0

0.1a 0.1a

50.2 74.2

37.9 33.9

2.2 1.5

0 0

0.4a 0.3a

0.5 0.7

0.4 0.3

0.0 0.0

0 0

0.0a 0.0a

Three out of ten plots with no runoff.

the greatest sediment concentrations, with a mean value of 8.4 g l1. The herbicide-treated sub-plots had mean sediment concentration values of 2.6 g l1, the control value was 1.48 g l1, and the chipped branches mulch resulted in a low 0.4 g l1 concentration (Table 8). For the straw mulch treatment there was no runoff, and thus no sediment. Mean soil losses for each of the land management indicated that the ploughing and herbicide treatments contributed the greatest amounts (50.2 g m2 h1 and 37.9 g m2 h1), while the control (2.17 g m2 h1), chipped branches (0.29 g m2 h1) and the straw mulch (0 g m2 h1) had negligible or no soil loss (Table 8). 3.6. Soil properties versus soil and water losses The higher values for the three soil properties studied (OM, Cmic and AS) in the plots with higher vegetation cover (VC) indicated that the improvement of soil properties was related to the increase in vegetation cover (which also includes straw mulch and chipped pruned branches mulch). Runoff, sediment yield, sediment concentration and soil loss rates were greatest in the plots with lower OM, Cmic and AS contents. This confirms that where organic matter content, microbial biomass carbon content and aggregate stability were lower, the soil was more vulnerable to erosion processes (Table 9), as was observed on the ploughed and herbicide-treated soils. Soil and water losses were very low on the three vegetated land uses with no significant differences between them. It is worth noting that the straw mulch treatment did not have any observed

Table 9 Pearson correlation coefficients (r) between soil parameters related to soil loss (n = 50).

Veg. Cover (%) Runoff (%) Sed. Conc. (g l1) Soil Loss (g m2 h1)

Veg. cover (%)

Runoff (%)

Sed. conc. (g l1)

Soil loss (g m2 h1)

1.0000

0.6131** 1.0000

0.6152** 0.2720 1.0000

0.4813** 0.7590** 0.6001** 1.0000

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soil and water losses in the rainfall simulation experiment. This is a similar response as that of agricultural soils abandoned in the Northern-Mediterranean basin since the 1950s, in which the absence of crop production and tillage of the soil has resulted in an increase in organic matter content, soil structure and infiltration ˜ o et al., 1991; Garcı´arate, and a reduction in soil erosion (Ruiz-Flan Ruiz et al., 1995). The highest soil erosion losses were observed for the herbicide and plough treatments. The herbicide treatment contributed the highest runoff discharge while ploughing resulted in the greatest sediment concentration. The application of straw mulch was very effective in reducing soil and water losses, although the control plot (weeds) and the chipped pruned branches mulch treatment also reduced the soil and water losses to negligible values. It is often assumed that soil resistance to erosion is closely related to soil quality. This paper compared how soil erosion and soil quality evolved on rainfed Mediterranean orchards. As soil quality depends on a large number of physical, chemical and biological soil properties, key soil quality indicators were selected to shed light on the effect of land management (Eliott, 1994). On one hand, biological indicators such as Cmic, while on the other hand, physical and chemical indicators such as the aggregate stability and organic matter content, were used to study the soil changes under different land managements (Bending et al., 2004; Doran and Parkin, 1994; Dick and Gupta, 1994). Several studies show that agricultural management has an important influence on chemical, physical and biological parameters (Garcia et al., 1997; Caravaca et al., 2002; Marinari et al., 2006) as microbial populations are fundamental for maintaining soil quality by mediating the processes of organic matter turnover and nutrient cycling (Doran and Parkin, 1994). This paper confirms those ideas and strengthens the need to increase the amount of vegetation cover on Mediterranean agricultural land. 4. Conclusions Land management of Mediterranean rainfed orchards is a key factor in developing a sustainable agriculture. A 16-month experiment with five different treatments demonstrated that herbicides contributed to soil degradation and soil and water losses. Ploughing reduced aggregate stability and increased soil erodibility. No-tillage and weeds reduced soil losses by one order of magnitude. Weeds plus chipped pruned branches mulching reduced soil losses by two orders of magnitude; and application of straw mulch prevented all soil and water losses from the 5-year return period rainfall simulation events. Only the straw mulch was able to significantly improve soil properties during the 16 months study period. This highlights the fact that soil property changes by land management will take much longer than the control of soil erosion achieved after 16 months. Acknowledgements The authors are grateful to CGL2008-02879 project, the Masia d’Agricultura i Ramaderia Ecolo`gica from El Teularet, and the Massı´s del Caroig (Leader II) for providing financial support. Field work support of Maria Burguet, Javier Garcı´a, Carlos Jovani and Miguel Segura is gratefully acknowledged. Dennis Flannagan kindly reviewed and improved the manuscript. References Bending, G.D., Turner, M.K., Rayns, F., Marx, M.C., Wood, M., 2004. Microbial biochemical soil quality indicators and their potential for differentiating areas under contrasting agricultural management regimes. Soil Biol. Biochem. 36, 1785–1792.

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