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Published online 23 May 2006
Published in Soil Sci Soc Am J 70:1178-1188 (2006)
DOI: 10.2136/sssaj2005.0115
© 2006 Soil Science Society of America
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Forest, Range & Wildland Soils

Short-term Effects of Sewage-Sludge Compost on a Degraded Mediterranean Soil

Marie Larchevêque, Virginie Baldy*, Nicolas Montès, Catherine Fernandez, Gilles Bonin and Christine Ballini

Institut Méditerranéen d'Ecologie et de Paléoécologie, UMR CNRS 6116, Université de Provence; Case 421, FST St Jérôme; 13397 Marseille Cedex 20– France

* Corresponding author (virginie.baldy{at}up.univ.mrs.fr)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In Mediterranean areas, often characterized by degraded soil due to recurrent fires and violent precipitation events, sewage sludge compost could improve soil properties and stimulate plant succession processes. Most of the studies dealing with compost effects on soil properties only take into account the mineral horizon compartment, without studying compost effects on organic horizon properties. In this study, we monitored the patterns of change in the litter, humus, and mineral horizon compartments in a 7-yr-old garrigue over 2 yr after compost spreading. Three treatments were studied: control, 50 Mg ha–1, and 100 Mg ha–1 (Fresh Mass) of cocomposted sewage sludge and green wastes. The major changes occur in the humus and litter compartments, whereas the mineral horizon is slightly affected. The mineral horizon is enriched in exchangeable K, Mg, and sporadically in P (total and available) after amendment, slightly improving overall fertility, while the humus and litter compartments keep accumulated compost organic matter and its associated nutrients, showing the slow release of elements from the chosen mature compost. Compost amendment probably enhances the humus decomposition process, first because it decreases this compartment's C/N ratio, and second because it decreases its Ni and Cr concentrations. In contrast to humus, compost should reduce the litter decomposition process, by increasing C/N ratio, pH, and concentrations of potentially toxic trace elements such as Cu, Zn, Ni, Cr, and Pb. Compost greatly enriched humus and litter in P, Cu, and Zn, and only Cu moves from organic to mineral soil horizon.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
BOTH long dry summers and strong dry winds characterize the Mediterranean climate, and favor recurrent forest fires (Scarascia-Mugnozza et al., 2000; De Luis et al., 2001). Fire consumes organic matter (aboveground vegetation, litter, superficial soil layers) and part of the nutrients are volatilized and thus lost from the soil (Grogan et al., 2000). Other parts of the nutrients are deposited in the ash and are prone to erosion or leaching to ground water if they are not rapidly absorbed by plants. Violent precipitation events are frequent in the Mediterranean region (Scarascia-Mugnozza et al., 2000) and induce soil impoverishment, especially when occurring after a fire. Thus, the Mediterranean soils are often deficient in organic matter, N, and P (Archibold, 1995), and have low water availability (Le Houerou, 1973). The soils of the most degraded calcareous areas are often colonized by Quercus coccifera garrigues (Barbero, 1990). These evergreen stands might be responsible for maintaining the low-resource level of the sites they colonize (Aerts, 1995). The sclerophyllous nature of evergreen leaves promotes gradual litterfall and slow rates of decomposition, thus moderating pulses of nutrients back into the soil and promoting tighter cycles of nutrient flow (Rundel, 1988). The occurrence of broad-leaved tree species, which develop under higher fertility levels, is considerably delayed, and these garrigues can be considered as a blocking stage of succession (Lepart and Escarré, 1983; Quézel and Médail, 2003).

Sewage sludge is a source of organic matter and plant nutrients (Brockway, 1983; Martinez et al., 2003), and it can improve soil physical and chemical properties (McKay and Moffat, 2001; Caravaca et al., 2002). Moreover, spreading these biosolids on natural or arable soils constitutes an alternative to landfill disposal. However, their use presents potential environmental risks, such as accumulation of heavy metal and organic contaminants in soils (Brockway, 1983), as well as the discharge of nutrients, especially N and P, to surface and ground water (Martinez et al., 2003). Composting could stabilize the organic matter concentration of sewage sludge and thereby decrease the risks of heavy metal and salt leaching (Garcia et al., 1990; Planquart et al., 1999). In addition, the use of other organic wastes with high C/N ratios (such as green wastes) in mixture with sewage sludge can reduce the rate of N mineralization thereby decreasing the risk of leaching (McKay and Moffat, 2001) while providing durable release to plants.

Compost amendment has been frequently shown to increase soil fertility (Caravaca et al., 2002; Martinez et al., 2003), plant biomass (Guerrero et al., 2001; Moreno-Peñaranda et al., 2004), plant nutrition (Moreno et al., 1996), and plant cover (Larchevêque et al., 2005b). As soils of garrigue ecosystems are degraded, compost amendment could recreate favorable conditions for broad-leaved tree species occurrence by raising their fertility. These species are characteristic of mature stages of the succession, more resistant to fire and of great interest from a biodiversity perspective.

We spread sewage sludge and green wastes mature compost on a degraded calcareous Mediterranean soil colonized by a 7-yr old Quercus coccifera L. garrigue, and we monitored the patterns of change in litter, humus, and mineral horizon characteristics during the 2 yr following amendment. Our aims were (i) to determine the effect of compost amendment on the dynamics of some fertility parameters in mineral horizon, humus, and litter; and (ii) to study possible soil contamination by trace metals after amendment.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Site and Experimental Design
The experiment was performed over 6000 m2 on the plateau of Arbois (Southern Provence, France; 5°18'6''E– 43°29'10''N in WSG-84 Norm) at 240 m above sea level and under Mediterranean climatic conditions (Fig. 1 ). The silty-clayey chalky soil is a rendosol according to the referentiel pédologique (AFES, 1995), a Rendzic Leptosol according to FAO (FAO, 1998) and classed as Rendoll in Soil Taxonomy (Soil Survey Staff, 1999), with a high percentage of stones (77%) and a low average depth (around 24 cm). The last fire occurred in June 1995 and the site was colonized by Mediterranean sclerophyllous vegetation, with a 70% total cover, Quercus coccifera L. and Brachypodium retusum Pers. being the two dominant species. This natural vegetation belongs to the holm oak (Quercus ilex L.) successional series.


Figure 1
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  		Fig. 1. Mean air temperature and rainfall from January 2002 to March 2004 (Météo France).

 
Compost was surface applied in January 2002. It was placed in a tipper-wagon pulled by a tractor. The experimental design was a complete randomized block of twelve plots of 500 m2 each. Four plots did not receive any compost (D0 = control), four plots received 50 Mg ha–1 (D50), and four plots received 100 Mg ha–1 of fresh compost (D100).

Compost
The compost was produced by a local company (Biotechna, Ensuès, Southern Provence) and is certified to be in conformity with French legal standards for composts made with materials of sewage treatment origin (NF U 44–095, 2002), for pathogenic microorganisms, organic trace elements, and heavy metals. This compost was made with greenwastes (1/3 volume), pine barks (1/3 volume), and local municipal sewage sludge (1/3 volume). The mixture was composted for 30 d at 75°C to kill pathogenic microorganisms and decompose phytotoxic substances, and then sieved (<20-mm mesh) to remove large bark pieces and stored in swathes. The swathes were mixed several times during the following 6 mo to promote organic matter humification. Characteristics of soil before amendment and compost are shown in Table 1.


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  		Table 1. Soil mineral horizon (0–24 cm: maximal depth; N = 12) and compost (N = 3) physicochemical characteristics (Mean and Standard Error). DM: Dry Matter. FM: Fresh Mass. D50: 50 Mg ha–1 of compost.

 
Field Procedures
Mineral and organic horizons were sampled thirteen times from March 2002 to March 2004, approximately 2 yr after amendment. Sampling started only 1 mo and a half after amendment, allowing for the compost deposited on the vegetation during spreading to fall to the soil. Organic horizon was hand-collected on a 20 by 20 cm square. Then, all accessible mineral horizon down to bedrock was collected with a drill. Each analyzed soil and organic sample was a composite of three samples randomly collected on each 500-m2 plot.

Laboratory Procedures
Before analysis, samples of mineral and organic horizons were 2-mm mesh sieved and oven dried at 45°C to constant weight. Three fractions were then separated before analyses. The organic fraction >2 mm corresponded to the litter compartment and contained compost pine barks and wood pieces, and coarse plant litter. The organic fraction <2 mm was designated as the humus compartment and contained humified organic matter and composted sewage sludge. Analyses are performed according to French or International norms (AFNOR, 2004; Table 2).


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  		Table 2. Norms used for soil, humus, and litter analysis.

 
The pHH2O was measured in distilled water (PHM 240, Radiometer SAS, Neuilly Plaisance, France). Cation exchange capacity (CEC) was measured by soil percolation with an ammonium acetate solution (Metson, 1956) and organic C was determined using sulfuric-chromic oxidation and spectrocolorimetry (Cary 50, VARIAN, Les Ulis, France) (Anne, 1945). Exchangeable K and Mg were 1 M NH4Ac extracted and analyzed by atomic absorption spectrophotometry (FS220, VARIAN, Les Ulis, France) (Thomas, 1982). Total N was determined by dry combustion and thermic conductimetry (FP 428, LECO, Garges-Les-Gonesse, France) (Dumas, 1831). Available P was determined in a sodium bicarbonate solution using spectrophotometry (Olsen et al., 1954) (Cary 50 VARIAN, Les Ulis, France). Copper and Zn available forms were extracted using EDTA and concentrations were assessed by atomic absorption spectrophotometry (VISTA Radial, VARIAN, Les Ulis, France) (Gambrell and Patrick, 1982). To measure total concentrations of Cd, Cr, Cu, Ni, Pb, phosphorus (P2O5), and Zn, samples were digested in aqua regia and analyzed using plasma emission spectrophotometry (VISTA Axial, VARIAN, Les Ulis, France).

Statistical Analyses
Two way ANOVA combined with Tukey test (Zar, 1984) were used to study compost rate and sampling date effects on the different soil, humus, and litter parameters. If any interaction occurred between the two studied factors (compost, date), one way ANOVA were performed at each sampling date (2002: March, April, May, June, July, September, October, November, December; 2003: March, June, October; 2004: March) to study compost rate effect. Significant level was considered to be 95%. The software Statgraphics plus (Statistical Graphics Corp., Copyright 1994–1996) and Minitab (Minitab Inc., 2000) were used.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
pH, Organic Matter, Cation Exchange Capacity, Total Nitrogen, and Carbon/Nitrogen Ratio
Given the numerous parameters studied, only the statistically significant results are shown on Fig. 2 .


Figure 2
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  		Fig. 2. (A) Organic matter (OM), total N, C/N ratio, cation exchange capacity of humus, and (B) OM, total N, C/N ratio, pHH2O of litter, from March 2002 to March 2004, for D0 (control), D50 (50 Mg ha–1 of compost), and D100 (100 Mg ha–1 of compost) plots. Mean ± Standard Error, N = 4. ANOVA: *p < 0.05; **p < 0.01; ***p < 0.001.

 
The compost had no significant effect on mineral horizon pH, CEC, or organic matter content. Similarly, mineral horizon N concentration and C/N ratio were not affected by compost amendment during the 2 yr following amendment.

In contrast, compost strongly and significantly increased the total N concentrations in both humus (two way ANOVA, p < 0.0001) and litter (two way ANOVA, p < 0.0001) compartments. The total N concentrations nearly doubled on plots amended at both rates compared with control, and this effect persisted 2 yr after amendment (March 2004) (Fig. 2).

However, the compost had an opposite effect on these two compartments for the C/N ratio. This ratio increased significantly (one way ANOVA, June, July, September, and October 2002, p < 0.05) in the litter compartment of amended plots, while it decreased significantly in humus (one way ANOVA, April, May, June, July, September, and December 2002, March and June 2003, p < 0.05). The second year (2003), the C/N ratio decrease in humus of amended plots disappeared from October 2003 to March 2004. Moreover, throughout 2003, the litter C/N ratio strongly increased in control plots and reached higher values than those measured on amended plots. In March 2004, litter showed similar C/N ratio values on amended and control plots.

The same pattern was roughly similar between litter organic matter content and litter C/N ratio, with a litter organic matter content significantly higher overall (two way ANOVA, p < 0.0001) on amended plots during the experiment, and sporadically exceeded by control values in March 2003.

Finally, the litter pH (two way ANOVA, p < 0.0001) and the humus CEC (two way ANOVA, p < 0.001) significantly increased for both compost rates (Fig. 2).

In addition, we did not observe any significant difference between the two rates of compost for all the parameters.

Phosphorus
Compost induced very strong increases in available and total P concentrations in humus and litter, while this increase was less marked in the mineral horizon (Fig. 3 ).


Figure 3
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  		Fig. 3. Phosphorus concentrations in (A) mineral horizon, (B) humus, and (C) litter, from March 2002 to March 2004, for D0 (control), D50 (50 Mg ha–1 of compost), and D100 (100 Mg ha–1 of compost) plots. Mean ± Standard Error, N = 4. ANOVA: *p < 0.05; **p < 0.01; ***p < 0.001.

 
In June 2003, total and available P concentrations in the mineral horizon were significantly higher (one way ANOVA, p < 0.05 for total P, p < 0.01 for available P) on amended plots than on control plots. However, in the mineral horizon, compost effect was more pronounced on available P concentrations than on total P concentrations, with significant increase on amended plots in March, May, June, July 2002, March and October 2003 (one way ANOVA p < 0.05) (Fig. 3). In the mineral horizon, we did not observe any difference in P concentrations between the two rates of compost.

Total P concentration in humus and litter were significantly 7 to 26 times higher on amended plots than on control plots over the whole experiment (humus: two way ANOVA, p < 0.0001; litter: one way ANOVA, all dates, p < 0.01). Humus total P concentration was significantly higher in D100 plots than in D50 plots (two way ANOVA, p < 0.05), although it was similar between rates in litter. For amended plots, the total P concentration was stable in humus (about 0.12 g kg–1 DM), whereas it showed seasonal variations in litter. The first year, maximum concentrations in litter were reached in spring and early summer (May, June, and July 2002). However, in 2003, total P concentrations were at a minimum in June, while they were twice as high in March and October.

Available P concentrations were about 15 times higher on amended plots than on control plots for humus (two way ANOVA, at all dates except March 2003, p < 0.0001), and between seven and eight times for litter (two way ANOVA, p < 0.0001), over the whole experiment. However, the dynamics were different in litter and humus of amended plots. In March and June 2003, available P concentrations were highest for humus, whereas they were lowest for litter. In humus and litter compartments, we did not observe any difference in available P concentration between the two rates of compost.

Cations
Compost had an overall significant improving effect on exchangeable K and Mg concentrations in both mineral horizon (two way ANOVA, p < 0.01) and humus (two way ANOVA, p < 0.0001) (Fig. 4 ). Humus exchangeable K was three times higher on amended plots than on control plots, 1.5 mo after amendment (March 2002). This compost-linked increase in humus remained significant 2 yr after amendment (March 2004). In the mineral horizon, the increase was slighter, as exchangeable K concentrations increased by 20% in amended plots compared with control, in March 2003 (one way ANOVA, p < 0.01).


Figure 4
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  		Fig. 4. Potassium and Mg concentrations in (A) mineral horizon and (B) humus, in March 2002, 2003, and 2004, for D0 (control), D50 (50 Mg ha–1 of compost), and D100 (100 Mg ha–1 of compost) plots. Mean ± Standard Error, N = 4. ANOVA: *p < 0.05; **p < 0.01; ***p < 0.001. Results of the comparison are given by a letter: values that do not differ at the 0.05 level are noted with the same letter (a < b < c).

 
Exchangeable Mg concentrations were significantly and durably improved in the humus compartment for the two compost rates. Exchangeable Mg concentrations were multiplied by a factor between 2 and 3 for the three dates on amended plots compared to control. The compost effect on exchangeable Mg concentrations was lower in the mineral horizon compartment (+ 50% on amended plots compared with the control). In addition, this effect was very progressive and increased with time (two way ANOVA, p < 0.0001). In March 2004, exchangeable Mg concentrations were the highest on D100.

Trace Metals
The mineral horizon was overall not enriched in trace metals after compost amendment, during the 2 yr of experimentation. Only available Cu concentrations slightly and significantly increased (two way ANOVA, p < 0.05) on D50 compared with control, for the three sampling dates (March 2002, 2003, and 2004) (Table 3).


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  		Table 3. Trace metal concentrations in mineral horizon in March 2002, 2003, and 2004 for D0 (control), D50 (50 Mg ha–1 of compost), and D100 (100 Mg ha–1 of compost) plots. Mean (SE), N = 4. Results of the comparison are given by an exponent letter: values that do not differ at the 0.05 level are noted with the same letter (Tukey test).

 
In the humus compartment (Fig. 5 ), total Cd, Cu, and Zn, respectively, increased by a maximum factor of about two, five, and three on amended plots in March 2003 (two way ANOVA, p < 0.0001) compared with control plots. At the end of the experiment, these compost effects remained significant for Cu and Zn at both rates. Compost had no significant effect on humus total Pb concentrations.


Figure 5
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  		Fig. 5. Trace metal concentrations in humus, in March 2002, 2003, and 2004, for D0 (control), D50 (50 Mg ha–1 of compost), and D100 (100 Mg ha–1 of compost) plots. Mean ± Standard Error, N = 4. ANOVA: *p < 0.05; **p < 0.01; ***p < 0.001. Results of the comparison are given by a letter: values that do not differ at the 0.05 level are noted with the same letter (a < b < c).

 
Total Ni and Cr concentrations significantly decreased (two way ANOVA, p < 0.0001 for both elements) in humus of amended plots compared with control plots. Concentrations of these elements were 25%, and 30 to 50% lower on plots amended at both rates, respectively for Cr and Ni, compared with control.

In the litter compartment (Fig. 6 ), compost significantly increased total Cd, Cu, and Zn concentrations on amended plots (two way ANOVA, p < 0.0001) compared with control plots. The increase was higher in litter than in humus for Cu (5 to 10 times higher in litter against 5 times higher in humus). The maximum concentration values were reached in March 2003 on amended plots for the three elements.


Figure 6
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  		Fig. 6. Trace metal concentrations in litter, in March 2002, 2003, and 2004, for D0 (control), D50 (50 Mg ha–1 of compost), and D100 (100 Mg ha–1 of compost) plots. Mean ± Standard Error, N = 4. ANOVA: *p < 0.05; **p < 0.01; ***p < 0.001. Results of the comparison are given by a letter: values that do not differ at the 0.05 level are noted with the same letter (a < b < c).

 
However, in contrast to humus, total Cr, Ni, and Pb concentrations reached higher values in the litter of amended plots (one way ANOVA, p < 0.001 for the three elements) than in control plots in March 2003. Concentrations of these elements approximately doubled on D50 compared with control in March 2003. Moreover, the control concentrations of these three elements in litter increased in March 2004.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Compost Fertilization Effects
As sewage sludge contains large amounts of N, amendment greatly enriches humus and litter in this element. However, the compost used has been subjected to a 6-mo maturation phase that led to a very stable and humified product, and limited the mineral horizon N enrichment by percolation.

The sewage sludge used for compost composition comes from a sewage plant that applies a dephosphatation process to sewage. Consequently, P is concentrated in the residual sludge, leading to a P-rich compost that strongly and durably increases total and available P concentrations in both humus and litter compartments. In our experiment, in contrast to other authors (Martinez et al., 2003), this element does not seem to be very mobile down into the soil. P concentrations are only sporadically higher in the mineral horizon of amended plots than in control plots, whereas they remain persistently at high levels in the humus and litter compartments of amended plots. In Ca-rich humus, as in our study (about 13 mg of CaO per g of humus dry mass), P may form inorganic calcium phosphate (Khanna and Ulrich, 1984), which decreases its solubility.

Archibold (1995) notes that N and P may be the most limiting elements for plant growth in Mediterranean ecosystems, and both elements have critical importance in plant nutrition (Martin-Prével et al., 1984; Rundel, 1988). Despite low enrichment of the mineral horizon in N and P, the compost mulch may have greatly improved vegetation nutrition. Most feeding roots can directly dip into litter and humus nutrient reserves (Miller, 1984). In the present study, we observed significantly higher nutrient content in three dominant species of the study site (Brachypodium ramosum Pers., Cistus albidus L. and Quercus coccifera L., unpublished data) and a significantly higher cover and biomass of Brachypodium ramosum Pers. in D100 plots (Larcheveque et al., 2005b). These results indicate a higher nutrient uptake by the plant on D100 plots, which can attenuate the two-fold nutrient supply in D50 and D100 and thus explain the lack of difference in soil properties between the two rates of compost. Moreover, D100 plots contained twice as much pine bark, which is low-nutrient residue (Alifragis et al., 2001), inducing a dilution of nutrients in these plots. Brockway (1983) also observed similar nutrient and heavy metal concentrations in different soil horizons with the two highest rates of biosolids used. Significant differences in soil properties between rates could only appear after a longer period, according to the spatial heterogeneity of the ecosystem and the longer time required for the response in nutrient cycling.

Compost increases exchangeable K and Mg concentrations in humus, inducing the enrichment of the mineral horizon in these elements, probably by percolation. Likewise, several authors have noted an improvement of soil fertility after compost amendment (Guerrero et al., 2001; Korboulewsky et al., 2002; Martinez et al., 2003). This assumption is confirmed by the increase of humus CEC on amended plots, which is probably due to organic humigenic compounds of the compost, showing the incorporation of the compost organic matter into the soil (Gobat et al., 2003). However, the compost's improving effect on Mg concentration in the mineral horizon and humus is more durable than for K, and much faster and greater in humus, whereas the compost initial inputs in these two elements are roughly similar. As the compost greatly increases humus P concentration, the formation of relatively insoluble magnesium phosphate which is resistant to leaching (Embleton, 1966) could explain the durability of the increase effect.

Compost Effect on Organic Horizon Decomposability
Amendment greatly enriches humus in N, and consequently the C/N ratio decreases in amended plots during the first year. However, the compost effect on humus C/N ratio disappeared at the end of the study period, probably due to the depletion of the N pool supplied by compost. Several authors have shown the durability of the fertilizing effects to be limited for different types of biosolids (Guerrero et al., 2001; Debosz et al., 2002; Martinez et al., 2003). In the litter compartment, compost amendment introduces pine bark and green wastes that have a high C/N ratio, and consequently the C/N ratio increases on amended plots, despite simultaneous N enrichment. As C/N ratio and litter decay rate have been shown to follow an inverse linear relationship (Gosz, 1984), the amendment could reduce the litter decomposition process, whereas it could enhance the humus decomposition process. This hypothesis is confirmed by our results on microbial colonization associated with Quercus coccifera L. leaf litter at the same site (Larchevêque et al., 2005a). We observed a significant decrease in colonization of Quercus coccifera L. leaf litter by fungi on plots amended with 100 Mg ha–1. However, these effects of compost on litter and humus are not durable and probably should not affect the decomposition process in the long term.

Another factor that could affect decomposition process in the humus and litter compartments are total Cu and Zn concentrations in both humus and litter on amended plots. These concentrations are higher than concentrations reported to have toxic effects on microbial activity in soil, especially for Zn (Kabata-Pendias and Pendias, 1992). However, the solubility of these elements in the soil solution is decreased when pH and CEC increase, as a result of the increasing binding capacity of the soil (Römkens and Salomons, 1998). High pH and CEC may thus limit the availability of Cu and Zn and their toxic effects on microorganisms.

Concerning Cr and Ni, they are known to be toxic to microorganisms (Moreno et al., 2003; Shi et al., 2002), especially Ni that is toxic at lower levels than other trace metals (Kabata-Pendias and Pendias, 1992). However, the compost is less contaminated than the control humus in Cr and Ni, because our experimental site had relatively high Cr, Ni, and Pb concentrations before amendment, which is consistent with studies that assessed the contamination of French forest soils by trace elements (Hernandez et al., 2003). The origin of this contamination is assumed to be atmospheric, as the area receives significant acid atmospheric pollution (AIRMARAIX, 1999). Compost spreading could have diluted concentrations of these elements in humus and probably have a less negative effect on the decomposition process compared with control.

The litter compartment is temporarily enriched by compost in Pb, Ni, Cd, and Cr. This enrichment was probably due to the compost itself although it has low concentrations in these trace elements, and not due to litter produced by plants during the study. These elements are of very low availability to plants particularly in calcareous soils (Kabata-Pendias and Pendias, 1992), and thus plants provide a very low level of contaminated litter in these elements. This result is in accordance with data that we obtained on plant species growing on the experimental site: we observed that standing leaf tissues were not contaminated by trace elements (unpublished data). So, the litter concentrations in Pb, Ni, Cd, and Cr could have been diluted over time by leaf litter produced during the study period.

Interaction between Compost and Drought
The Year 2003 was particularly dry and warm in Southern France, with very low rainfall (24.4 mm) from May to September. During early spring and summer 2003, litter organic matter content and C/N ratios reached very high values, on control and amended plots. This is a period with maximum recorded litterfall in Mediterranean sclerophyllous ecosystems (Floret et al., 1989). Moreover, from March to June 2003, drought effect seemed to be more important than fertilization effect, because we observed similar organic matter and C/N ratios dynamics on control and amended plots. However, fertilization has been shown to increase plant sensitivity to drought by increasing the size of transpiring shoots (Van Den Driessche, 1984). Consequently, compost amendment could induce a higher plant defoliation rate under drought stress on amended plots, leading to an increase in fresh litter production. This additional litterfall could induce a greater dilution of nutrients in D100 plots and partly explain the fact that we observed similar results in litter nutrient content between D50 and D100 amended plots.

In June 2003, P concentrations were maximum in the mineral horizon of amended plots, for total and available forms. The severe drought during this period probably decreased P leaching and absorption by plants and consequently accumulation of this element occurred in the mineral horizon, whatever the rate of compost amended. In contrast, P concentrations decrease in litter compartment, probably because fresh litter has a lower P concentration than decomposed litter (Chauvet, 1987), and dilutes the litter P. In addition, low humidity induced very low P solubilization to available forms in litter in June 2003.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
In a short time-scale experiment, compost spreading induces modifications of organic superficial layers (humus and litter), and has only a slight effect on mineral horizon fertility and contamination. Moreover, the compost effect is very different depending on the compartment considered. Fertilizing effects are marked in humus, and to a lesser degree in the mineral horizon. In contrast, compost could have rather depreciative effects on the decomposition process of the litter compartment. However, organic matter and P accumulation due to the 2003 drought clearly indicates that the ecosystem functioning in the Mediterranean region is mostly influenced by climatic factors, especially by water availability. Finally, the persistence of contaminants such as trace elements in the system for long periods means that there is a need for longer time-scale surveys, especially in the case of mature compost that slowly decomposes.


   ACKNOWLEDGMENTS
 
This research was supported by the Conseil Général des Bouches-du-Rhône (France), the ADEME (Agence De l'Environnement et de la Maîtrise de l'Energie), the Conseil Régional Provence-Alpes-Côte-d'Azur, and the Rhône-Méditerranée-Corse French Water Agency. Sylvie Dupouyet and Stéphane Greff are gratefully acknowledged for field and laboratory assistance.We also thank Mr. Michael Paul for revision of English, Thierry Gauquelin, and three anonymous reviewers for comments on the manuscript.

Received for publication April 8, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome