Short‐term kinetics of residual wheat straw C and N under field conditions: characterization by 13C15N tracing and soil particle size fractionation

Abstract

Summary A clear understanding of the short‐term decomposition and fate of crop residues is necessary to predict the availability of mineral N in soil. The fate of 13 C15N‐labelled wheat straw in a silty soil (Typic Hapludalf) was studied using particle size fractionation and in situ incubation in which the equivalent of 8 t dry matter per ha of straw was incorporated into the soil over 574 days. Soil samples were separated into five particle‐size fractions by wet sieving after disruption of aggregates. The weight, C and N contents, and 13C and 15N atom excess of each fraction were determined. Straw‐derived C disappeared rapidly from the > 2000‐μm fraction with an estimated half‐life of 53 ‘normalized’ days (equivalent of 10°C and −0−01 MPA water potential). Straw‐derived C appeared to be only temporarily stored in the intermediate fractions (1000–2000 and 200–1000 pm). The maximum net 13C accumulation in the 50–200‐μm fraction was 4·4% of added 13C. Straw‐derived C accumulated most rapidly and preferentially in the 50‐μm fraction, which stabilized after 265 days and accounted for 70% of the residual 13C on day 574. Although there was more residual 15N than 13C, the distributions and kinetics of the two isotopes in the fractions were similar.