Consequences of land use change from conventional agriculture to perennial grassland for soil carbon and nitrogen dynamics.

摘要

Land use change, including the conversion of agricultural land to perennial grassland, alters levels of soil organic matter (SOM). SOM influences soil moisture, structure, and nutrient supply, and it helps to regulate atmospheric carbon dioxide concentrations. SOM formation is therefore a key process to understanding ecosystem development, yet little is known about the rate, pattern, and type of SOM formed on decadal time scales. In addition, few studies have examined the role of vegetation and soil texture on SOM accumulation at this time scale. I used 40-year chronosequences established on former agricultural fields depleted in SOM that differed in perennial grassland species used to establish vegetation and soil parent material to determine rates and patterns of SOM accumulation, changes in nitrogen (N) transformation, and changes in soil structure over time. I also evaluated how vegetation type and soil clay concentration affect these processes.; On glacial till, the rate of soil organic carbon (SOC) accumulation was 62.0 g−1 m−2 y−1. This linear rate of SOC accumulation did not slow as levels of SOC increased, and it will take 55–75 years for former agricultural fields to attain levels of SOC equal to those of unplowed native prairie. Both labile soil carbon (C) with a short turnover time, as measured by microbial biomass C, light fraction C, and a laboratory incubation, and recalcitrant soil C with a long turnover time, as measured by unhydrolyzable C, pools began to increase immediately after cessation of agriculture, although recalcitrant C pools increased faster. This suggests that some of the new C formed is stable. Several different methods of measuring labile SOC correspond to one another, although the absolute value determined by different techniques varied by an order of magnitude. Vegetation type and clay concentration influenced potential net N mineralization and soil aggregate size, but not soil C pools or dynamics. This work suggests that former agricultural fields are a temporally predictable C sink that will be saturated within a century, and that vegetation and soil texture have minimal capacity to alter the rate of SOM accumulation on these fields.