Aggregation of a soil with different cropping histories following the addition of organic materials

Abstract. Soil management studies show that intensive arable agriculture can lead to a decline in both organic matter levels and the stability of the soil structure. It is a priority to understand how soil structure responds when fresh organic materials are added to poor quality degraded arable soils. This is of particular interest because of its implication for carbon sequestration. We investigate whether the addition of organic materials can form stable aggregates in a degraded soil. Grass or peat residues were added to samples of soil obtained from the continuous grassland and arable plots of the long-term experiment at highfield, IACR-Rothamsted (UK) and incubated at 2° and 24°C, for upto 8 weeks at -5 kPa. At 1 day and at 2, 4 and 8 weeks the soil was slaked in de-ionised water and the aggregate size distributions were measured. The data was used to calculate mean weight diameters (MWD). The treatments with added grass showed increased aggregated relative to the control; the treatments with added peat did not. At 24°C the value of MWD increased with the incubation period, but at 2°C there was no further aggregation beyond week two. Respiration measurements were made and the samples that released the most CO₂ were also those which re-aggregated the most. This suggests that the process of aggregation is microbiologically mediated. The results are discussed within the broader context of the implications of soil organic matter content on soil management.