Greater adoption of conservation practices has resulted in increases in earthworm population density in Inland Pacific Northwest agroecosystems. Organic matter incorporation into the soil profile during earthworm foraging and burrowing activity has been shown to significantly increase nitrogen (N) mineralization in agricultural soils. Additional decomposition and organic N transformation occurs in the earthworm gut, producing plant-available inorganic N forms which are deposited in the soil as labile forms in casts and middens. Microbial activity has also been shown to increase in the presence of earthworms, stimulating nitrification. The influence of earthworm species on the fate of litter-derived N from surface deposition to decomposition was proposed for study. A 13-week long mesocom study was conducted to characterize N mineralization and nitrogen-15 (N15) response through soil N pools. N15 labelled wheat straw was applied to mesocosms containing the endogeic earthworm Aporrectodea trapezoides (AT) and the anecic earthworm Lumbricus terrestris (LT), separately and combined (B). Mesocosms were destructively sampled on weeks 1, 2, 4, 7, 10, and 13. Total carbon and nitrogen and the N14/N15 ratios of bulk soil, casts, earthworms, and microbial biomass were measured at two depths (0-10 and 10-20 cm).
At the end of the study (Week 13), total soil N in the 0-10 cm depth was 11% greater in treatments (AT, LT, and B) with earthworms as compared to the controls. In the 10-20 cm depth, AT treatments yielded the greatest increase (~17% greater than controls) compared to LT and B treatments (~7% greater). Similar trends were observed in total soil C. Mean NO3- concentration in the 0-10 cm depth of LT (19.0 mg/kg) treatments at week 13 were greater than that measured in AT treatments (12.3 mg/kg). Conversely, AT produced the greatest week 1 ammonium concentrations across all treatments. Nitrate and ammonium trends strongly indicate the occurrence of nitrification and net mineralization. Species-specific results are currently undergoing analysis. Isotope data from earthworms, casts, and inorganic N are forthcoming and may shed light on the fate of litter-derived N. Current data suggest species-dependent effects on N mineralization rates, and future analysis should reveal N sources and fluxes in biotic and abiotic pools over time.