Evolution of CO2 and soil carbon dynamics in biologically managed, row-crop agroecosystems

Field CO2 production was related to soil carbon pools and fluxes determined by laboratory incubation of soils from agroecosystems designed to test the possibility of substituting biological for chemical inputs. Treatments inc luded: conventional and organic-based row crops, woody and herbaceous peren nial crops and historically tilled and never tilled successional fields. Th e CO2 efflux in corn and soybeans was affected by crop residues from previo us years and growing season temperatures but not soil moisture. Overwinter cover crops and perennials such as alfalfa and poplar, resulted in fairly u niform fluxes of approximately 20 kg CO2-C ha(-1) day(-1) throughout the no n-frozen period. Highest fluxes occurred in alfalfa, historically tilled su ccessional and never tilled, grassland successional treatments, although, h ighest aboveground productivity occurred in the corn and poplar. Summed, fi eld CO2 fluxes were similar to residue-C inputs. Measurement of CO2 mineral ized in extended incubations in the laboratory made it possible to use soil enzyme activity to determine the size and dynamics of soil C pools. The re sidue of acid hydrolysis defined the size of the resistant pool C-r. Carbon dating determined its mean residence time (MRT). Curve analyses of CO2 evo lution plotted on a per unit time basis gave the active (C-a) and slow (C-s ) pool sizes and decomposition rate constants k(a) and k(s). Temperature co rrection factors provided field MRTs. The active pool of this coarse textur ed soil represents 2% of the soil C with a MRT of 30-66 days. The slow pool represents 40-45% of the SOC with field MRTs of 9-13 years. The poplar soi l has the greatest MRT for both the active and slow pools. The system appro ach to land use sustainability (SALUS) model, which predicts CO2 evolution from decomposition in the field as part of a plant growth - soil process mo del, was tested using the decomposition parameters determined by incubation and C-14 dating. The model satisfactorily predicted the intra and inter ye ar differences in field CO2 but over predicted fluxes from residues in the fall. It does not yet adequately consider a lag period during which the res idues lose their hydrophobicity, are comminuted and colonized. (C) 1999 Els evier Science B.V.