Soil Carbon Dynamics in Soybean Cropland and Forests in Mato Grosso, Brazil

Abstract

Climate and land use models predict that tropical deforestation and conversion to cropland will produce a large flux of soil carbon (C) to the atmosphere from accelerated decomposition of soil organic matter (SOM). However, the C flux from the deep tropical soils on which most intensive crop agriculture is now expanding remains poorly constrained. To quantify the effect of intensive agriculture on tropical soil C, we compared C stocks, radiocarbon, and stable C isotopes to 2ￂﾠm depth from forests and soybean cropland created from former pasture in Mato Grosso, Brazil. We hypothesized that soil disturbance, higher soil temperatures (+2ￂﾰC), and lower OM inputs from soybeans would increase soil C turnover and deplete C stocks relative to nearby forest soils. However, we found reduced C concentrations and stocks only in surface soils (0￢ﾀﾓ10ￂﾠcm) of soybean cropland compared with forests, and these differences could be explained by soil mixing during plowing. The amount and ￎﾔ14C of respired CO2 to 50ￂﾠcm depth were significantly lower from soybean soils, yet CO2 production at 2ￂﾠm deep was low in both forest and soybean soils. Mean surface soil ￎﾴ13C decreased by 0.5￢ﾀﾰ between 2009 and 2013 in soybean cropland, suggesting low OM inputs from soybeans. Together these findings suggest the following: (1) soil C is relatively resistant to changes in land use and (2) conversion to cropland caused a small, measurable reduction in the fast￢ﾀﾐcycling C pool through reduced OM inputs, mobilization of older C from soil mixing, and/or destabilization of SOM in surface soils.