Soil dissolved organic carbon: Influences of water-filled pore space and red clover addition and relationships with microbial biomass carbon

Soil dissolved organic carbon (SDOC) plays an important role in organic C cycling and translocation of nutrients and pollutants in the soil profile. Soil microbial biomass C (MBC) has been used as an indicator of soil quality. Both SDOC and MBC may be affected by management practices and indigenous soil properties, which however are not fully understood. Using a laboratory incubation technique, we determined the effects of red clover (Trifolium pratense L.) addition and soil water saturation as expressed in water-filled pore space (WFPS, 20-95%) on soil SDOC and MBC in three soils from Ontario. The levels of SDOC were the greatest at 20% WFPS, and decreased with increase s in WFPS up to 95%. In comparison with the control, addition of red clover increased SDOC by up to 72% at 20% WFPS, but the effect was minimal or insignificant at WFPS above 50%. Reduction of SDOC with increases of WFPS both with and without red clover was attributed to the increased mineralization of labile organic C, as indicated by CO2 production. Regardless of the legume amendment, soil available N (e.g., mineral N), labile organic C (e.g. initial level of SDOC) or the variable derived from these two measurements, available C:N ratio, were the factors predominately affecting dynamics of SDOC at WPFS from 20 to 50% after 1-mo incubation and at WFPS from 20 to 65% with extended incubation to 3-mo. Soil factors affecting SDOC at WFPS above 85% were total N and pH without red clover, but changed to organic C and soil labile organic C with red clover. High levels of MBC were found to occur mostly with the high WFPS, and were enhanced by red clover addition only in the Perth silt loam. Soil dissolved organic C was significantly related to MBC with WFPS from 20 to 65% without red clover. No relationships between SDOC and MBC were found at WFPS above 65% without red clover and at WFPS from 20 to 95% with red clover. Soil factors affecting SDOC and the availabili ty of SDOC to microbial activity are WFPS dependent and related to soil legume amendment. Key words: Red clover, water-filled pore space, dissolved organic C, microbial biomass C, CO2 emission