Effects of soil texture on soil carbon and nitrogen dynamics after cessation of agriculture

Soil organic matter (SOM) is an important ecosystem carbon (C) pool that is often depleted by agriculture. SOM content tends to be positively correlated with soil clay concentration among sites, but it is unknown how clay concentration affects the rate of SOM accumulation over time after cessation of agriculture. I used a 40-year chronosequence of 62 former agricultural fields in western Minnesota to determine the influence of clay concentration on the accumulation of soil C pools following agricultural abandonment. As time since cessation of agriculture increased, total soil organic carbon (SOC), unhydrolyzable C, microbial biomass C, labile C calculated from a laboratory soil incubation (Cl), and aggregate size all increased, while potential net nitrogen (N) mineralization and the decay constant of the labile C pool, kl, decreased. However, clay concentration had no effect on total soil C pool sizes or rate of accumulation. Clay concentration correlated positively with aggregate size and the rate of aggregate accumulation, and it correlated negatively with potential net N mineralization rates regardless of field age. These results indicate that on former agricultural fields converted to perennial grassland, soil texture may not be a significant factor influencing SOM accumulation rates on decadal time scales.