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Parameters involved in the structural stability dynamic

The structural stability of soil is the result of numerous interactions of soil parameters. Micro-organisms, polysaccharides, lipids and humic substances in the soil are known to favour the stability of aggregates. They increase cohesion between soil particles, thus improving the resistance of aggregates to break-up and mechanical deaggregation under the impact of raindrops. These parameters were monitored in soils and compared with the results of structural stability tests.

Parameters involved in the structural stability dynamic

The structural stability of soil is the result of numerous interactions of soil parameters. Micro-organisms, polysaccharides, lipids and humic substances in the soil are known to favour the stability of aggregates. They increase cohesion between soil particles, thus improving the resistance of aggregates to break-up and mechanical deaggregation under the impact of raindrops. These parameters were monitored in soils and compared with the results of structural stability tests.

Effects of OWPs on the parameters involved in the structural stability dynamic

The parameters involved in the structural stability dynamic have been monitored since the start of the QualiAgro field experiment. They enable the action of OWPs to be identified. The values of these parameters are compared with the DMP indices measured to evaluate the stability of the stucture of aggregates.

Total Organic Carbon (TOCag) in aggregates

At the start of the test, the TOC varied between 0.75% and 0.89%. Over time, TOCag levels increase in all soils to which rganic waste products (OWPs) have been applied. In 2005, after four spreadings, the soils in these plots all show higher levels than the soils in control plots (no spreading).
In the QualiAgro field experiment, a positive relationship is observed in the plots receiving GWS and FYM treatments, showing that TOC explains some of the results on aggregate stability. Meanwhile, in the case of MSW compost, the low TOC content in the soils is not sufficient explanation for its significant effect measured on aggregate stability. This compost's effect on the structural stability of the soil is more likely to be a result of the fact that it stimulates the microbial biomass due to its easily biodegradeable organic matter.

Total microbial biomass

In 2003, the level of microbial biomass is low in all treatments (~100 mg C/kg DM), which can undoubtedly be attributed to the dry conditions that preceded the removal of soil. In 2004, the microbial biomass increased in all organic treatments, but the differences are only significant with MSW compost. In 2007, the levels of microbial biomass are significantly higher in the soils receiving OWPs (~300 mg/kg DM) compared with the control soil (~ 200 mg/kg DM).

Polysaccharides extractible with warm water

In 2003, only the level of polysaccharides in the FYM treatment is significantly higher than the level in the control plots. These low levels observed in 2003 are undoubtedly linked with the low microbial activity, due to the low microbial biomass measured during this year. In 2004, no significant difference was observed between the treatments. In 2007, the polysaccharide concentrations in the MSW and BIO treatments are significantly lower than in the other treatments, which themselves are not significantly different. The polysaccharides indicate a stimulation of microbial activity by the organic matter (OM) from the OWPs.

Hydrophobicity of aggregates

In 2003 and 2004, the organic treatments are significantly more hydrophic than the control. Although the input of OWPs increased the hydrophobicity of aggregates, the values achieved classify the soil as non-hydrophobic according to the King classification (1981).

The size of OWP particles

The size of OWP particles may affect their action in the soil. Manure, which forms lumps, is for example distributed differently from composts, which are made up of finer elements. MSW compost, which is the finest OWP, distributes the best in the soil. Its elements, which are finer, achieve a larger area of contact with the soil than other OWPs, which could favour the effect of this compost on the microbial biomass in the soil and allow the formation of stable aggregates.

Earthworms

Earthworms have a positive action on aggregate stability, either directly by producing mucilage and generating castings, or indirectly by breaking up organic residue into finer particles, which then stimulate the development of micro-organisms. In the field test, the OWPs had a positive effect on the earthworm biomass, in the following order: MSW > FYMA > BIO > GWS > CTR. Its is very likely that these effects explain some of the aggregate stability results observed.

The action of plants

The action of plants on structural stability is also a parameter that may be involved, directly thanks to the stabilising action of roots or indirectly by increasing the TOC stock in the soil. In the test, the effect of OWPs on yields is observed and is at its maximum level for MSW compost.