Improvement of soil structure formation by degradation of coarse organic matter

Exogenous organic matter has been shown to have a positive impact on the physical properties of agricultural soils. In some situations, the wide availability of organic matter and the necessity of highly restructuring the soils lead to the addition of very large quantities of organic matter (up to 40% v/v, or 30 times more than under agricultural conditions). The long-term effects of the evolution of this large quantity of organic matter on the aggregation dynamics and structural characteristics of the soil are not well known. We monitored the granulometric evolution of the organic matter and the stabilisation of the structure of urban soils placed in 600 L containers. The top layer was a sandy loam amended with 20% or 40% of organic compost — sewage sludge and wood chip compost or a green waste compost — laying on a layer of sandy loam. The organisation and the evolution of the pore space of the mixtures were quantified by image analysis. Our results showed that the total C content of the mixtures rapidly decreases during the first year. This decrease in C is high for coarse fractions (> 1 mm) and is accompanied by an increase in C of the fine fractions (< 0.05 mm). At the same time, we measured an increase in C content in aggregates greater than 3 mm. This preferential carbon sequestration in the macroaggregates leads to a significant increase in the structural stability over time and a modification of the aggregate organisation and pore distribution. After 5 months, the arrangement of the pores is unimodal and consists mainly of pores with a 300 μm radius. At the end of the first year, the distribution becomes bimodal for the mixtures, with the appearance of a new porosity corresponding to elongated pores (radius > 550 μm). In 40% v/v mixtures, this neoporosity corresponds to 39% and 62%, respectively, of the total surface porosity observed at 12 and 24 months. It results from the fractionation and the alteration of organic matter particles. We showed that when organic particles break down, they surround the aggregates already present in the soil with a fine film and protect them from degradation. In our study, aggregates are dynamic entities whose stability increases over time in relation to the breakdown of organic matter particles introduced in large quantities.

Research highlights
► Degradation of organic matter great quantities induces soil structuring mechanisms.
► Organic matter coarse fraction degradation increases aggregate carbon content.
► Organic matter decomposition modifies aggregate organisation and pore distribution.