FATE OF CARBON AND NITROGEN IN WATER-STABLE AGGREGATES DURING DECOMPOSITION OF (CN)-C-13-N-15-LABELED WHEAT-STRAW IN-SITU

When incorporated in soil, plant residues and their decomposition prod ucts are in close contact with mineral particles with which they can b e bound to form aggregates. We measured the incorporation of carbon (C ) and nitrogen (N) derived from crop residues in water-stable aggregat e fractions of a silty soil in a field experiment in Northern France u sing (CN)-C-13-N-15-labelled wheat straw (Triticum aestivum L.). Soil samples were taken seven times for 18 months and separated into slakin g-resistant aggregate size fractions which were analysed for total C a nd N contents, and C-13 and N-15 enrichments, During the early stages of decomposition (approximately 200 days), the enrichment of C-13 incr eased rapidly in the macroaggregates (> 250 mu m) but decreased therea fter. The macroaggregates represented only <20% of the soil mass and a t any one time, they accounted for <25% of the residual C-13 in the so il. The proportion of C-13 recovered in the <50-mu m and 50-250-mu fra ctions increased during decomposition of the residues; at day 574, the 50-250-mu m fraction accounted for close to 50% of the residual C-13. A greater proportion of N-15 than C-13 was recovered in the <50-mu m fraction. The results indicate that during decomposition in soil, C an d N from crop residues become rapidly associated with stable aggregate s. In this silty soil the 50-250-mu m stable aggregates appear to be i nvolved in the storage and stabilization of C from residues.