SOIL CARBON POOLS AND FLUXES FOLLOWING LAND CONVERSION IN A SEMI-ARID SHRUB-STEPPE ECOSYSTEM

Authors: Cochran, Rebecca; Collins, Harold; Kennedy, Ann; BEZDICEK, D - WSU, PULLMAN

Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2006
Publication Date: 5/24/2006
Citation: Cochran, R.L., Collins, H.P., Kennedy, A.C., Bezdicek, D.F. 2006. Soil carbon pools and fluxes following land conversion in a semi-arid shrub-steppe ecosystem. Biology and Fertility of Soils. 43:479-489.

Interpretive Summary: Worldwide soil carbon (C) losses associated with agricultural expansion and intensification during the past 150 years has contributed significantly to increased atmospheric CO2. Soil disturbances resulting from land use changes have been shown to modify the turnover of C and the formation of soil organic matter (SOM). The conversion of native ecosystems to agricultural developments usually results in a loss of SOM. The use of extended laboratory incubations of soil with measurements of CO2 has been widely used to differentiate the active and passive functional C pools in residues and soil. This method constitutes a biological fractionation of SOM, where labile fractions are mineralized rapidly by soil microorganisms with subsequent soil C more slowly mineralized. Carbon mineralized during the early stages of incubation consisted of C from the labile C pool and reflected variable accumulations of labile C from the incorporation of corn residues. This pool contained 3-5% of the total C and had an average turnover of approximately 28 d. Conversion from the native shrub steppe to a managed irrigated agricultural system resulted in, increases in pH and TOC and TN. Rates of C mineralization were greater after conversion with a significant difference between native and cultivated sites. The size and turnover rate of the slow pool of C increased with years of cultivation. Positive influences on C storage in this study included diverse crop rotations, addition of organic compost, residue incorporation, which all contributed to the increase of SOC over the native shrub steppe vegetation.

Technical Abstract: Worldwide soil carbon (C) losses associated with agricultural expansion and intensification during the past 150 years has contributed significantly to increased atmospheric CO2. Soil disturbances resulting from land use changes have been shown to modify the turnover of C and the formation of soil organic matter (SOM). A native semi-arid shrub-steppe ecosystem recently converted into an irrigated agricultural development in the Columbia Basin of Washington state was evaluated for several soil 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 carbon and nitrogen storage, C- and N-mineralization and C turnover. Total organic carbon and nitrogen increased 48 and 60 %, respectively above native sites after three years of cultivation. Soil organic C was divided into three pools: an active pool (Ca) consisting of labile C (simple sugars, organic acids, the microbial biomass and metabolic compounds of incorporated plant residues) with an Mean Residence Time (MRT) of days; a slow pool (Cs) consisting of structural plant residues and physically stabilized C, and a resistant fraction (Cr) 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 Ca and Cs 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. The mean residence time of the SOM following conversion increased by 2 years.