SOIL, NITROGEN, PHOSPHORUS, AND ORGANIC-MATTER PROCESSING BY EARTHWORMS IN TALLGRASS PRAIRIE

Earthworms represent the dominant fraction of soil macroinvertebrate biomass in Kansas tallgrass prairie and thus may have a large impact on nutrient budgets. To evaluate this impact I estimated cast production and rates of nutrient processing by the native genus Diplocardia and by exotic Lumbricidae in permanent plots on two soil types at Konza Prairie Research Natural Area. Total cast production was estimated from field population estimates, soil climate data, and cast production-temperature relationships obtained in the laboratory. The organic matter and mineral N and P contents of surface casts by month were used to estimate total processing of these substances in the field. Earthworms present in the study sites were Diplocardia longiseta, D. kansensis, D. rugosa, D. singularis, D. smithii, D. verrucosa, and the European species Aporrectodea caliginosa and Octolasion cyaneum. Estimated total cast production and nutrient processing by Diplocardia spp. exceeded that of exotics in both soil types, in spite of higher populations of Lumbricidae in one soil. Total annual soil consumption by all earthworms was 4-10% of the A-horizon, depending on soil type. Organic matter equivalent to 10% of total soil organic matter in the top 15 cm or to 100-300% of plant annual belowground production appears to pass through the earthworms each year. Mineral N processed was almost-equal-to 10-12% of annual plant N uptake, comparable to half of the input from precipitation, while the P processed was equivalent to 50% of annual uptake. In contrast to other grassland systems studied, the introduction of Lumbricidae had a negative effect on soil turnover and nutrient mineralization due to the lower throughput of Lumbricidae and their relative intolerance of summer soil temperatures.