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

Additions of (15NH4)2SO4 to the soil inorganic nitrogen (N) pool were 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 (NT) field. Half of the soil cores were treated with a fungicide to reduce fungal populations. Fungicide application significantly reduced fungal biomass, 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 of 15N transfer averaged 13.4 μg 15N g−1 residue d−1 for untreated wheat straw. Fungal inhibition reduced 15N flux by 59–78%, reductions of similar magnitude to those observed for fungal biomass. Nitrogen transfer in sterilized soil cores accounted for only 7.8% of the total upward N transport in control cores, indicating that abiotic processes did not contribute substantially to N flux. We estimate a total annual fungal-mediated N flux of 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 mechanism 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 availability appear to be important controls on fungal biomass associated with surface residues and rates of soil-to-residue N translocation.