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 Californi a, Davis were investigated in microcosm systems. Nitrogen-free sand in the columns was amended with ground alfalfa and cellulose, with total N held co nstant, 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 determi ned by measuring ammonium and nitrate concentrations in the leachate from t he columns at 3-d intervals. Nematode population numbers and the fatty acid 18:2 omega 6c, a fungal biomass indicator, were monitored by destructive s ampling 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 ne matode(-1) d(-1) was 1.8 ng for A. avenae and 3.3 ng for A. composticola wh en feeding on R. solani. As the C-to-N ratios of organic substrates increas ed, total mineral N decreased with R. solani alone, but in general remained the same in the presence of nematodes. Initial and average nematode popula tion densities were significantly higher in columns containing R. solani th an in those with Trichoderma sp. Both nematode species reduced the fungal f atty acid 18:2 omega 6c in Trichoderma columns on d 21. The fatty acid 18:2 omega 6c was lower in columns containing both R. solani and A. composticol a on d 0 and 7 and higher on d 14 and 21 than those in the absence of nemat odes. (C) 1999 Elsevier Science Ltd. All rights reserved.