Population energetics of bacterial-feeding nematodes: Carbon and nitrogen budgets

Bacterial-feeding nematodes participate in nitrogen mineralization in decomposition food webs to an extent determined by metabolic and behavioral attributes, by life history, and by the relative C-to-N ratios of the nematodes and their bacterial prey. The mean C-to-N ratio for eight nematode species cultured on Escherichia coli on agar was 5.89 (range 5.16–6.83). The mean C-to-N ratio was similar, although with greater variability among species, when nematodes were cultured in soil on a range of soil bacteria. The mean C-to-N ratio of five isolates of soil bacteria and E. coli was 4.12 (range 3.65–4.92). Where food was not limiting, production-to-assimilation ratios ranged from 0.58 to 0.86 and respiration-to-assimilation ratios from 0.14 to 0.42. The excess N assimilated during growth and egg production, and the excess N assimilated to meet the C needs of respiration, were similar across species at 20°C. The excess N from both sources provides an estimate of the amount of N excreted by each nematode during the life course. The weight of bacteria necessary to meet the growth and respiration costs at 20°C ranged from 0.87 μg μg-nematode−1 d−1 for Cephalobus persegnis to 1.99 μg μg-nematode−1 d−1 for Bursilla labiata, providing estimated consumption rates between 6.61 × 105 and 15.22 × 105 bacterial cells μg-nematode−1 d−1. At a field site, we estimate that the bacterial-feeding nematode community in the top 15 cm soil mineralized N at rates increasing to 1.01 μg-N g-soil−1 d−1 in rhizosphere soil. On a monthly basis, the community contributed 0.28 kg-N ha−1 in April, 0.98 kg-N ha−1 in May and 1.38 kg-N ha−1 in June in bulk soil. Contributions in the rhizosphere would be considerably greater depending on the ratio of rhizosphere to bulk soil. The contribution of individual species to N mineralization in the rhizosphere varied through the first 3 months of the summer growing season as a function of their abundance and their metabolic and development rates in relation to temperature. Rhabditis cucumeris was the predominant contributor in April; there were similar contributions by Acrobeloides bodenheimeri, B. labiata, Cruznema tripartitum, and R. cucumeris in May; A. bodenheimeri and B. labiata were the major contributors in June.