Incorporation of root C and fertilizer N into the food web of an arable field: Variations with functional group and energy channel

Agroecosystems occupy large areas of the land surface and arable soil food webs are of significant importance for global cycling of carbon (C) and nitrogen (N). In a field experiment we labeled maize plants (Zea mays L.) in 13CO2 atmosphere and by K15NO3 fertilization. During 25days, the incorporation of 13C and 15N was traced in plant compartments, soil and soil arthropods, as well as 13C in microbial phospholipid fatty acids (PLFAs) and nematodes. Highlighting the importance of root-derived resources in agroecosystems, 13C was incorporated into all food web compartments, including microorganisms (PLFAs), nematodes and arthropods. The amount of incorporated 13C (as compared to unlabeled samples) markedly decreased along the food chain with ∆13C decreasing from 500‰ in plant roots and 900‰ in microbial PLFAs, to less than 40‰ in nematodes and arthropods. Incorporation of 13C into fungal PLFAs considerably exceeded that into bacterial PLFAs, highlighting the importance of soil fungi as compared to bacteria in C cycling. Fertilizer-derived 15N uniformly increased with time in plant compartments and soil arthropods, indicating that N is distributed homogeneously in the soil food web. High channeling of both root-derived 13C and fertilizer-derived 15N to higher trophic levels by fungi, and intensive feeding on fungi by soil animals highlight the central role of saprotrophic fungi in C and nutrient fluxes in soil food webs of arable ecosystems.