Fate of carbon and nitrogen in water-stable aggregates during decomposition of 13C15N-labelled wheat straw in situ

When incorporated in soil, plant residues and their decomposition products are in close contact with mineral particles with which they can be bound to form aggregates. We measured the incorporation of carbon (C) and nitrogen (N) derived from crop residues in water-stable aggregate fractions of a silty soil in a field experiment in Northern France using ¹³C¹⁵N-labelled wheat straw (Triticum aestivum L.). Soil samples were taken seven times for 18 months and separated into slaking-resistant aggregate size fractions which were analysed for total C and N contents, and ¹³C and ¹⁵N enrichments. During the early stages of decomposition (approximately 200 days), the enrichment of ¹³C increased rapidly in the macro aggregates (> 250 pm) but decreased thereafter. The macro aggregates represented only < 20% of the soil mass and at any one time, they accounted for <25% of the residual ¹³C in the soil. The proportion of ¹³C recovered in the <50-μm and 50–250-μm fractions increased during decomposition of the residues; at day 574, the 50–250-μm fraction accounted for close to 50% of the residual ¹³C. A greater proportion of ¹⁵N than ¹³C was recovered in the <50-μm fraction. The results indicate that during decomposition in soil, C and N from crop residues become rapidly associated with stable aggregates. In this silty soil the 50–250-μm stable aggregates appear to be involved in the storage and stabilization of C from residues.