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

Abstract Agroecosystemsoccupy large areas of the land surface and arable soil food websare of significant importance for global cycling of carbon (C) and nitrogen (N). In a field experiment we labeled maize plants ( Zea mays L.) in 13 CO 2 atmosphere and by K 15 NO 3 fertilization. During 25 days, the incorporation of 13 C and 15 N was traced in plant compartments, soil and soil arthropods, as well as 13 C in microbial phospholipid fatty acids (PLFAs) and nematodes. Highlighting the importance of root-derived resources in agroecosystems, 13 C was incorporated into all food webcompartments, including microorganisms (PLFAs), nematodes and arthropods. The amount of incorporated 13 C (as compared to unlabeled samples) markedly decreased along the food chain with ∆ 13 C decreasing from 500‰ in plant roots and 900‰ in microbial PLFAs, to less than 40‰ in nematodes and arthropods. Incorporation of 13 C into fungal PLFAs considerably exceeded that into bacterial PLFAs, highlighting the importance of soil fungi as compared to bacteria in C cycling. Fertilizer-derived 15 N 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 13 C and fertilizer-derived 15 N to higher trophic levelsby fungi, and intensive feeding on fungi by soil animals highlight the central role of saprotrophic fungi in C and nutrient fluxes in soil food websof arable ecosystems.