Immobilization and remineralization of N following addition of wheat straw into soil:: determination of gross N transformation rates by 15N-ammonium isotope dilution technique

N dynamics in soil where wheat straw was incorporated were investigated by a soil incubation experiment using N-15-labelled nitrate or N-15-labelled wheat straw. The incubated soils were sampled after 7, 28, 54 days from the incorporation of wheat straw, respectively, and gross rates of N transformations including N remineralization and temporal changes in the amount of microbial biomass were determined. Following the addition of wheat straw into soils, rapid decrease of nitrate content in soil and increase of microbial biomass C and N occurred within the first week from onset of the experiment. Both the gross rates of mineralization and immobilization determined by N-15-ammonium isotope dilution technique were remarkably enhanced by the addition of wheat straw, and gradually decreased with time. Remineralization rate of N derived from N-15-labelled nitrate, and mineralization rate of N derived from N-15-labelled wheat straw was estimated by N-15 isotope dilution technique using non-labelled ammonium. Remineralization rates of N derived from N-15-labelled nitrate were calculated to be 0.71 mg N kg(-1) d(-1) after 7 days, 0.55 mg N kg(-1) d(-1) after 28 days, and 0.29 mg N kg(-1) d(-1) after 54 days. Nearly 10% of the N-15-labelled N originally contained in the wheat straw was held in the microbial biomass irrespective of the sampling time. The amount of inorganic N in soil which was derived from N-15-labelled wheat straw ranged between 1.93 and 2.37 mg N kg(-1). Rates of N transformations in soil with N-15-labelled wheat straw were obtained by assuming that the k value was equal to the N-15 abundance of biomass N, and the obtained values were considered to be valid. (C) 2004 Published by Elsevier Ltd.