Organic Matter of a Soil Amended with Composted Sludges and Affected by Simulated Processes of Soil Degradation

Organic matter of a soil amended with composted sludges and affected by simulated processes of soil degradation

F. Ingelmol, R. ~lbiach*& S. ~am6n'

'~e~artarnentode Degradacibn y Consewacibn de Suelos, Centro de Investigaciones sobre Desertijicacidn-CIDE (CSIC-UVEG-GV) 2 Departamento de Recursos Naturales,

Instituto Valenciano de Investigaciones Agrarias, IVIA

Abstract

Under the dry Mediterranean climate, continuous degradation of soil structure is caused by water-erosion and wildfire because of the removal or destruction of the organic matter of the soil surface. These effects, normally, are avoided with the organic amendment of the soil surface. To model this process, indices of organic matter content of a forest soil classified as Rendzic Leptosol have been investigated in representative soil samples of Zarra (Valencia, Spain). Under laboratory conditions, soil samples amended with three rates of composted sludges (0, 5 and 10% wlw) were subjected to three energy levels of simulated water-erosion (EO, El and E2, corresponding to 0, 100 and 600 ~.crn-~, respectively) and two levels of heat (F0 and F1, corresponding to 25 "C and 500 "C of highest soil temperature, respectively) in a randomised complete block experimental design. For all treatments, the relative index of organic matter (rate of variation referenced to the content in the control) and the soil macroaggregates/microaggregatesratio were calculated. After the highest level of simulated degradative agents (the combination of E2 and F1 treatments), the relative organic matter index ranged from a 33% decrease in the unamended soil to a 15% increase in the soil which received the highest rate of sludge. The amendment of this coarse-textured soil with composted sludges at the rates of 5 and 10% (wlw) diminished the breakdown of macroaggregates into microaggregates but their organic matter decreased more slowly than in the unamended soil. As a consequence of the simulated soil water

erosion process, the organic matter in the unamended soil was concentrated around the particles of fine sand and that would provide their combustion by the heating induced treatment than for the amended soil samples.

1 Introduction

Under the dry Mediterranean climate, continuous degradation of soil structure is caused by water erosion and wildfires because of the removal or destruction of the organic matter of the soil surface (Molina and Sanroque [17]). These effects, normally, were avoided with the organic amendment of the soil surface, which ameliorates their main physical properties and the water infiltration (Albiach et al. [l]; Ingelmo and IbBfiez [ll]; Holz et al. [IO]). Changes in water-stable aggregation have generally been correlated with changes in the total organic matter (OM) content in soils (Tisdall and Oades [25])and with changes in particular OM fractions (Oades and Waters [18]; Angers and Giroux [4]). The use of sludges as organic amendment or fertilizer in the soil is under debate since a low quality of the product may lead to a wide range of problems. One of these problems is their high level of moisture which results in poor mechanical properties (Holz et al. [10]). The use of a composted mixture of anaerobic sewage sludges and grape marc, in the proportion of 1:2 in volume, is a preferable option because this amendment has better properties for his application in the soil (Ingelmo et al. [12]). In the water erosion process the rainfall shears off soil aggregated particles by splash impact, and breaks the main structural units in macroaggregates, microaggregates and primary particles. As shown by Schulten et al. [23], the particle size distribution and their C and N content is affected by the energy of the dispersion. Schmidt et al. [21] has demonstrated that between 450 and 500 ~.cm-~ofdispersion energy was enough to disperse all samples investigated and, that in the range 30-590 ~.cm-~no detachment of soil organic matter from primary organomineral complexes and no redistribution between particle size fractions could be detected. Fire is a major factor controlling forest landscapes in Southern Europe, especially in the Mediterranean basin and so it is considered a major environmental problem. Different intensity of heating means different level of combustion of organic matter, and that may negatively alter the soil structure (Gimeno-Garcia et al. [9]). Regardless of the losses of organic matter by combustion, Ingelmo et al. [l31 indicates that temperatures of heating higher than 400 "C may alter the soil texture because of carbonates and clay transformations, which provoke the re-aggregation of clay, silt and sand-sized fractions. In this paper we examine through laboratory test, indices of soil organic matter (SOM) and related structural characteristics, in representative soil samples of a coarse-textured forest soil amended with three rates of composted sludges and subjected to simulated actions of soil water erosion and soil heating.

2 Materials and methods

The soil sampling was carried out along a Southeast oriented hillslope with a

10% inclination, in the A-horizon (0-20 cm) of a Rendzic Leptosol. The landscape is characterised by its distribution in mosaic (i.e. areas with shrubs of Rosrnarinus oficinalis and Erica multijlora, herbaceous vegetation of Brachypodium retusum, and bare soil areas). For this study, composed samples of five subsamples under vegetation and in bare soil were obtained in Zarra (Valencia, Spain) at 850-m a.s.1 The site has suffered a continuous degradation of soil structure caused by water-erosion and wildfire. Actually, the Government of the Valencia

Community undertakes projects of restoration in this area (Vallejo [26]) which include the amendment of the soil with anaerobic sewage sludges. In a randomised complete block experimental design with four replicates, the air-dried fine soil (< 2mm) was amended with three rates of composted sludges (0, 5 and 10% wlw) and was successively subjected under controlled conditions of the laboratory to three energy levels of simulated water-erosion (EO, El and E2, corresponding to 0, 100 and 600 ~.cm*~,respectively). The water erosion process was simulated by shaking end-over end the soil samples in the presence of water (1:2.5 proportions), following the procedure described in Schnitzer and

Ivarson [22]. Afterwards, the sediments were dried up to allow their rupture in aggregates from 1 to 2 mm in diameter and soil samples were subjected to two levels of heat (F0 and F1, corresponding to 25 "C and 500 "C of highest soil temperature, respectively (Ingelmo et al. [13])). Approximately, 1 Kg of soil aggregates (1-2 mm in diameter) of soil was subjected to the treatments F0 and F1. The main physical, chemical and physicochernical characteristics of the original soil and the organic amendment, and the organic matter content in the bulk soil samples and in macroaggregates and microaggregates were determined for each treatment and each replicate (CIDE [6]). For all treatments, relative indexes of soil organic matter (SOM) in the bulk soil sample (rate of variation referenced to the content in the combination of E0 and F0 treatments, considered as control treatment), the soil macroaggregates/microaggregates ratio and SOM in microaggregates (referenced to SOM in the bulk soil sample), were calculated.

3 Results and discussion

Data in Table 1 show the main physical, chemical and physicochemical characteristics of the original soil and of the composted sludges. This loamy sand soil has a high pH and carbonates content. The values of electrical conductivity in the extract of saturation (EC) and of organic Carbon (OC) and total Nitrogen

(N) are in the normal range for forest soils in this area (Antolin [3]. The particle distribution of the composted sludges is typical for substrates. This product is obtained by an aerobic composting procedure of a mixture of an anaerobically digested sewage sludges and grape mac, in the proportion of 1:2 in volume.

Their chemical and physicochemical characteristics are in the lowest range of the organic amendments used in horticultural soils Albiach et al. [2]; their use as a substrate for production of forest plants in nurseries was described in Ingelmo et al. [14].

Table 1: Main physical, chemical and physicochemical characteristics of the soil and the composted sludges.

Characteristic Soil I Composted sludges Particle size distribution I

Sand (%) 59.8

(%) I Silt \, 23.9 Clay (%) I 16.4 1 pH 7.83 6.90

EC (dS/m) 0.53 3.18 OC (%) 3.26 23.78 N W), 0.26 1.72 Carbonates 49.3

Table 2 shows the decrease with the soil water erosion of the SOM in the bulk soil and their increase in microaggregates. As the level of energy increased from 100 to 600 ~.crn-~,soil aggregates were broken down into microaggregates and primary particles, which concentrated the SOM. The amended soils have significantly highest values of SOM in bulk soil samples and in microaggregates for all the treatments. The highest rate of the sludges provoked a significant decrease in the total rnicroaggregates content. Data in table show that the heating of soil provoked the combustion of the 3 SOM in the bulk soil and in microaggregates. By reference to data in table 2, the heating of the soil produced a significant decrease in the total content of microaggregates and in their SOM. The lowest decay of SOM in microaggregates (6%) was obtained for the rate of sludges of 10 %, in the treatment E2, and the highest decrease (26%) for the unamended soil in the same treatment. This effect of the organic amendments on the preservation of the SOM in soils can be observed in the data of the figure 1. After the highest level of simulated degradative agents (the combination of E2 and F1 treatments), the relative organic matter index (ROMI) ranged from a 33% decrease in the unamended soil to a 15% increase in the soil which received the highest rate of sludge. An explanation for these results resides in differences in the composition and particle size distribution of the OM in the soil and in the composted sludges.

Table 2: Mean values of organic matter content in the bulk soil and in microaggregates, and particle size distribution of microaggregates for each rate of cornposted sludge and treatment of erosion, in experiments without heating. Values in the same followed by the

same letter are not significantly different at the 95 % level of probability.

Table 3: Mean values of organic matter content in the bulk soil and in rnicroaggregates, and particle size distribution of microaggregates for each rate of cornposted sludge and treatment of erosion, in experiments with heating. For each treatment, values in the same column followed by the same letter are not significantly different at - the 95 % level of

Treatment Rate of OM (%) Microaggregates (96) sludges m) Bulk soil Microaggregate (200-50) < 50 pm S W 0 4.23 a 4.64 a 7.0 b 4.0 b

5 5.40 b 5.55 b 6.8 b 3.2 b 10 6.00 c 6.02 c 4. la 1.9 a 0 3.95 a 5.28 a 6.0 b 12.2 c 5 4.96 b 6.21 b 6.lb 10.0 b

According to Pagliai and Vittori [19], the amendment of the soil with organic wastes means an amelioration of the macrostructure and microstructure of the soil. The unamended soil has a coarse texture and concentrates their SOM as water soluble organics, litter and as light fraction around the fine sand particles, and stable humus into the aggregates with low water structural stability, whereas

the composted sludges may have particulate organic matter (POM) and other organic fractions such as microbial biomass that can increase the water stability of the amended soil (Tisdall and Oades [25];0ades and Waters [18]; Angers and

Giroux [4]).

". EOFO ElFO E2FO EOFl ElFl E2F1 l Treatments

Figure 1: Effect of the rate of composted sludges on the relative organic matter index for each treatment.

The decomposing OM of the composted sludge in the amended soil samples can affect the associations between organic substrates and mineral soil components, which in turn determine the abundance and activity of microorganisms and fauna in the soil sample (Christensen [g]). As a consequence, the amended soil samples would have the highest soil macroaggregates/microaggregates ratio. Carbon in the form of particulate organic matter (Six et al. [24]) derived from the composted sludge can contribute to a highest soil macroaggregates/microaggregates ratio in the amended soil samples. Data in figure 2 show the influence of the rate of composted sludges in the macroaggregates/microaggregates ratio for each treatment. The highest rate of composted sludges significantly increased this ratio for uneroded soil samples (treatment EO). When the water erosion energy increases from 0 to 100 ~.crn-~it can be see that an abrupt decay of this ratio was produced. This decrease was more gradual when the energy of water erosion increased from 100 to 600 ~.crn-~. As pointed by Roscoe et al. [20], the energy input to separate organic matter from soil (i.e. by water erosion) depends of the water stability of the aggregates and has to be sufficiently large to disrupt macroaggregates into microaggregates and primary particles. As shows by Balesdent et al. [5] if too much energy is used then plant debris of the soil may be broken down resulting in a transfer of C from coarse to fine fractions.

EOFO ElFO E2FO Treatments

Figure 2: Effect of the rate of composted sludges on the macroaggregates1 rnicroaggregates ratio for each treatment.

The heating from F0 to F1 produced a smaller decrease of this ratio for uneroded treatment, and a weak increase for eroded soils. Kemper et al. 1151 indicates an increase in water-stable aggregation because of carbonates precipitation, dehydration of organic compounds and hardening of clay particles thus affecting the soil cohesion. As suggested by Christensen [7],the soil organic matter (SOM) associated with macroaggregates was more easily decomposable than that present in rnicroaggregates (theoretically more recalcitrant and stable). From this concept, the organic matter content in the eroded soils with the lowest soil macroaggregates/microaggregates ratio would therefore be more protected against the heat-induced combustion. That can not be confirmed with the data in figure 3 where the effect of the rate of sludges in the relative organic mater content in microaggregates (SOM in microaggregates, referenced to SOM in the bulk soil) is presented. The unamended soil, which displayed the highest decrease in OM with heating, showed the highest values of this index. A possible explanation of this can reside in the high content of sand in this soil. In this case, SOM would be concentrated around the coarsest particles of the soil and that would provide their combustion by the heating induced treatment. Results of Martinez-Mena et al. [l61 obtained in Mediterranean environments suggest that in amended soils, clay, silt and sand-sized fractions of soils will be transported as aggregates, and soils without amendment, the silt and sand-sized fractions would be transported as primary particles or as aggregates depending of the energy of the rainwater.

EOFO ElFO E2FO EOFl ElFl E2F1 l Treatments l

Figure 3: Effect of the rate of composted sludges on the relative organic matter in microaggregates for each treatment.

4 Conclusions

The amendment of this coarse-textured soil with composted sludges at the rates of 5 and 10% (wlw) diminished the breakdown of macroaggregates into microaggregates but their organic matter decreased more slowly than in the unamended soil. As a consequence of the simulated soil water erosion process, the SOM in the unamended soil was concentrated around the particles of fine sand and that would provide more for their combustion by the heating induced treatment than for the amended soil samples. The obtained results through the simulated processes in the laboratory, which allow us to go deeper into the complex nature of soils and degradation processes in the Mediterranean areas, are useful for modelling them and for the understanding of each individual process.

Acknowledgements

This research was partially supported by the Centro de Investigaciones sobre Desertificacibn, CIDE (CSIC-UVEG-GV) and by the Ministerio de Ciencia y Tecnologia (I+D project: 1FD97-1117-C05-02).

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