Fungal translocation as a mechanism for soil nitrogen inputs to surface residue decomposition in a no-tillage agroecosystem

Additions of ((NH4)-N-15)(2)SO4 to the soil inorganic nitrogen (N) pool wer e used to measure rates of N flux from the mineral soil to surface-applied wheat straw decomposing in intact soil cores collected from a no-tillage (N T) field. Half of the soil cores were treated with a fungicide to reduce fu ngal populations. Fungicide application significantly reduced fungal biomas s, decomposition rates, and net N immobilization in surface residues. Net N immobilization over the study period was estimated to be 1.5 and 0.9 g N m (-2) for untreated and fungicide-treated residues, respectively. The rate o f N-15 transfer averaged 13.4 mu g N-15 g(-1) residue d(-1) for untreated w heat straw. Fungal inhibition reduced N-15 flux by 59-78%, reductions of si milar magnitude to those observed for fungal biomass. Nitrogen transfer in sterilized soil cores accounted for only 7.8% of the total upward N transpo rt in control cores, indicating that abiotic processes did not contribute s ubstantially to N flux. We estimate a total annual fungal-mediated N flux o f 2.4 g m(-2), which is nearly equivalent to the N immobilization potential predicted, based on initial N and lignin content, for the wheat straw used in this study. We conclude that fungal N translocation is a significant me chanism for soil N input and can account for the observed net N immobilized by surface residues decomposing in the field. Both residue quality and N a vailability appear to be important controls on fungal, biomass associated w ith surface residues and rates of soil-to-residue N translocation. (C) 2000 Elsevier Science Ltd. All rights reserved.