Carbon and Nitrogen Dynamics During the Decomposition of Litter and Roots of a Chihuahuan Desert Annual, Lepidium Lasiocarpum

Abstract

Carbon and nitrogen dynamics were analyzed during the decomposition of litter and roots of the desert ephemeral pepperweed (Lepidium lasiocarpum). We treated litter bags with the insecticide chlordane and the fungicides benomyl and captan to eliminate or restrict groups of soil biota. The mass losses of buried litter (51, 39, and 25% for untreated, insecticide—treated, and fungicide—insecticide—treated material, respectively) were higher than those of the respective root treatments (35, 18, and 15%) at 96 d. The mass loss of untreated material was correlated with numbers of detritivorous—fungivorous microarthropods, and only a small percentage of this loss was as CO2: 27 and 42% for litter and roots, respectively. In the absence of microarthropods a higher percentage of mass—loss carbon could be accounted for as CO2: 33 and 76% for litter and roots, respectively, indicating that mass loss was due primarily to litter removal by microarthropod activity and not to mineralization. Litter removal by microarthropods was less dependent on abiotic constraints such as soil moisture (r = 0.65, P < .001) than was mass loss when microarthropods were absent (r = 0.79, P < .001). In the absence of microarthropods, mass loss was more closely coupled with biomass of grazers, such as nematodes, which require free water for activity (r = 0.99, P < .0001). Unlike mass loss, carbon mineralization was highest in untreated roots, suggesting a stimulation of microbial activity by microarthropods, while in untreated litter no stimulation was observed when compared to insecticide treatments. This difference was primarily a function of fungivorous microarthropod density, with overgrazing occurring in the untreated litter. Nitrogen budgets indicated the importance of microarthropods in the turnover of root nitrogen. In the presence of microarthropods 132% of the initial root nitrogen could be accounted for after 96 d, while in the absence of microarthropods 270% could be accounted for. This net immobilization of nitrogen was primarily in the soil organic fraction around the roots and was associated with fungal development. Data from this study re—emphasize the importance of microarthropods as regulators of decomposition in deserts and suggest that predation by nematodes or protozoa on bacteria and fungi contributes to rate regulation. Nitrogen flux data suggest that when spring ephemeral plant production is high, decomposition of ephemeral roots with attendant nitrogen immobilization can reduce the nitrogen available to creosotebush, Larrea tridentata, thus reducing shrub production. Higher taxa of soil biota, i.e., nematodes and microarthropods, may thus be important regulators of nitrogen fluxes and of mass loss in decomposition.