Soil carbon pools and fluxes after land conversion in a semiarid shrub-steppe ecosystem

Worldwide soil carbon (C) losses associated with agricultural expansion and intensification have contributed significantly to increased atmospheric CO₂. Soil disturbances resulting from land use changes were shown to modify the turnover of C and the formation of soil organic matter. A native semiarid shrub-steppe ecosystem recently converted into an irrigated agricultural development in the Columbia Basin of Washington State was evaluated for several abiotic indicators that might signal changes in an ecosystem during the initial stages of conversion and disturbance. Soil samples were collected in March of 2003 and 2004 from nine sites that included native shrub-steppe and agricultural fields converted in 2001 and 2002. Disturbance from conversion to irrigated crop production influenced total organic C and nitrogen (N) storage, C and N mineralization, and C turnover. Cultivated fields had greater concentrations of total organic C and N and higher cumulative C and N mineralization than native sites after 3 years of cultivation. Soil organic C was divided into three pools: an active pool (C _a) consisting of labile C (simple sugars, organic acids, the microbial biomass, and metabolic compounds of incorporated plant residues) with a mean residence time of days, an intermediate or slow pool (C _s) consisting of structural plant residues and physically stabilized C, and a resistant fraction (C _r) consisting of lignin and chemically stabilized C. Extended laboratory incubations of soil with measurements of CO₂ were used to differentiate the size and turnover of the C _a and C _s functional C pools. The active pools were determined to be 4.5 and 6.5% and slow pools averaged 44 and 47% of the total C in native and cultivated fields, respectively. Cultivation, crop residue incorporation, and dairy manure compost amendments contributed to the increase in total soil C.