SHORT-TERM KINETICS OF RESIDUAL WHEAT-STRAW C AND N UNDER FIELD CONDITIONS - CHARACTERIZATION BY (CN)-C-13-N-15 TRACING AND SOIL PARTICLE-SIZE FRACTIONATION

A clear understanding of the short-term decomposition and fate of crop residues is necessary to predict the availability of mineral N in soi l. The fate of (CN)-C-13-N-15-labelled wheat straw in a silty soil (Ty pic Hapludalf) was studied using particle size fractionation and in si tu incubation in which the equivalent of 8 t dry matter per ha of stra w was incorporated into the soil over 574 days. Soil samples were sepa rated into five particle-size fractions by wet sieving after disruptio n of aggregates. The weight, C and N contents, and C-13 and N-15 atom excess of each fraction were determined. Straw-derived C disappeared r apidly from the >2000-mu m fraction with an estimated half-life of 53 'normalized' days (equivalent of 10 degrees C and -0.01 MPa water pote ntial). Straw-derived C appeared to be only temporarily stored in the intermediate fractions (1000-2000 and 200-1000 mu m). The maximum net C-13 accumulation in the 50-200-mu m fraction was 4.4% of added C-13. Straw-derived C accumulated most rapidly and preferentially in the <50 -mu m fraction, which stabilized after 265 days and accounted for 70% of the residual C-13 on day 574. Although there was more residual N-15 than C-13, the distributions and kinetics of the two isotopes in the fractions were similar.