Effects Of Earthworms On Soil Nitrogen Mineralization And Microbial Biomass

Earthworms are the most important macrofauna in soil ecosystem, which may influence soil nitrogen cycling and transformation indirectly through the alteration of soil physical, chemical and biological properties, and directly through the release of nitrogen from metabolic processes and mortality. Currently, the effects of earthworms on nitrogen mineralization are estimated directly through measuring soil mineral-N contents changed by earthworms, but this method can not differentiate the nitrogen that excreted by earthworms and derived from soil native pools, and could not precisely estimate the contribution of earthworms to soil nitrogen mineralization. Therefore, we used ¹⁵N-labeled method to determine the contribution of earthworm to soil nitrogen pool by excreted nitrogen, and to explain the effects and mechanisms of earthworms on soil native nitrogen mineralization.There are totally eight treatments: (1) fluvo-aquic soil without earthworms or straw-control treatment (CS); (2) fluvo-aquic soil only with earthworms (CE); (3) fluvo-aquic soil only with straw (CO); (4) fluvo-aquic soil with earthworms and straw (COE); (5) red soil without earthworms or straw-control treatment (RS); (6) red soil only with earthworms (RE); (7) red soil only with straw (RO); (8) red soil with earthworms and straw (ROE).Each treatment had five replicates. Earthworms (Pheretima guillelmi) were labeled with ¹⁵N by providing earthworms with ¹⁵N-labled organic substrates. The experiment was studied in laboratory with constant temperature. Soil samples were destructively collected periodically on D6, D12, D18, D24 and D30 after incubation. We measured soil NH₄⁺-N, NO₃^--N, microbial biomass carbon and nitrogen, and soil respiration. The main results were as follows:1. Earthworm could greatly enhance soil N mineralization in both types of soils, soil NH₄⁺-N contents were significantly higher in earthworm treatments than those in their corresponding control treatments (p<0.05) , but the increased amounts were affected by incubation time, soil treatments and soil types. In fluvo-acquic soil, the contents of NO₃^--N...