Evolution of CO 2 and soil carbon dynamics in biologically managed, row-crop agroecosystems

Abstract

Field CO 2 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 included: conventional and organic-based row crops, woody and herbaceous perennial crops and historically tilled and never tilled successional fields. The CO 2 efflux in corn and soybeans was affected by crop residues from previous years and growing season temperatures but not soil moisture. Overwinter cover crops and perennials such as alfalfa and poplar, resulted in fairly uniform fluxes of approximately 20 kg CO 2–C ha −1 day −1 throughout the non-frozen period. Highest fluxes occurred in alfalfa, historically tilled successional and never tilled, grassland successional treatments, although, highest aboveground productivity occurred in the corn and poplar. Summed, field CO 2 fluxes were similar to residue-C inputs. Measurement of CO 2 mineralized 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 residue of acid hydrolysis defined the size of the resistant pool C r. Carbon dating determined its mean residence time (MRT). Curve analyses of CO 2 evolution 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 correction factors provided field MRTs. The active pool of this coarse textured 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 soil has the greatest MRT for both the active and slow pools. The system approach to land use sustainability (SALUS) model, which predicts CO 2 evolution from decomposition in the field as part of a plant growth – soil process model, was tested using the decomposition parameters determined by incubation and 14 C dating. The model satisfactorily predicted the intra and inter year differences in field CO 2 but over predicted fluxes from residues in the fall. It does not yet adequately consider a lag period during which the residues lose their hydrophobicity, are comminuted and colonized.