Carbon and nitrogen budgets of plant-feeding nematodes in grasslands of different productivity

The amounts of carbon and nitrogen that are transferred from the plant biomass to the soil by the feeding activity of herbivorous nematodes in four grasslands were calculated. The grasslands differed in the length of time since last fertiliser addition, which resulted in a gradient of nutrient-rich, high-production to nutrient-poor, low-production grasslands. The biomass of plant-feeding nematodes decreased with time of non-fertilisation, from 200 mg dry weight (DW) m-2 after 6 years without fertilisation to 50 mg DW m-2 after 28 years without fertilisation. Consequently, the total CO2 respiration rate of plant-feeding nematodes decreased with time of non-fertilisation from 4.2 to 1.6 l CO2 m-2 per year. This corresponded to carbon consumption of 14.88–5.84 g C m-2per year and nitrogen consumption of 1.98–0.78 g N m-2 per year. The direct contribution of plant-feeding nematodes to N mineralisation was approximately 2–5 f the total N mineralisation. It is suggested, however, that the indirect contribution of plant-feeding nematodes to N mineralisation may be much higher due to the release of relatively high amounts of organic nitrogen to the soil by defecation, biomass turnover and increased root exudation. The biomass consumption of plant-feeding nematodes was approximately 3–4 f the total standing biomass in all four grasslands and 5–8 f the root standing biomass. Within a grassland, however, large local differences in the numbers of plant-feeding nematodes resulted in consumption values of 1–40 f the root standing biomass. These large differences in consumption values indicate that the spatial distribution of plant-feeding nematodes can be an important factor affecting vegetation dynamics