The effects of nematode grazing on nitrogen mineralization during fungal decomposition of organic matter

Soil fungi and nematodes isolated from a long-term Sustainable Agriculture Farming Systems (SAFS) project research site at the University of California, Davis were investigated in microcosm systems. Nitrogen-free sand in the columns was amended with ground alfalfa and cellulose, with total N held constant, to create C-to-N ratios of 11:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1 and 45:1. Nitrogen mineralization by Aphelenchus avenae and Aphelenchoides composticola feeding on Rhizoctonia solani and Trichoderma sp. was determined by measuring ammonium and nitrate concentrations in the leachate from the columns at 3-d intervals. Nematode population numbers and the fatty acid 18:2 ω6c, a fungal biomass indicator, were monitored by destructive sampling on d 0, 7, 14 and 21. For R. solani, but not Trichoderma sp., there was significantly more N extracted from columns in the presence of either nematode species than in the absence of nematodes. Average N-mineralized nematode−1 d−1 was 1.8 ng for A. avenae and 3.3 ng for A. composticola when feeding on R. solani. As the C-to-N ratios of organic substrates increased, total mineral N decreased with R. solani alone, but in general remained the same in the presence of nematodes. Initial and average nematode population densities were significantly higher in columns containing R. solani than in those with Trichoderma sp. Both nematode species reduced the fungal fatty acid 18:2 ω6c in Trichoderma columns on d 21. The fatty acid 18:2 ω6c was lower in columns containing both R. solani and A. composticola on d 0 and 7 and higher on d 14 and 21 than those in the absence of nematodes.