Cultivation-induced effects on belowground biomass and organic carbon

Land conversion and cultivation effects on belowground biomass and C were assessed at a fescue (Festuca altaica Trin. ssp. hallii) prairie site and an adjacent cultivated field seeded to spring wheat (Triticum aestivum L.). Belowground biomass and soil organic C (SOC) were measured in three landform element groups (upper level and convex shoulder elements [UL], low-catchment-area footslope elements [FS], and low-elevation level and high-catchment-area footslope elements [LL]) at both sites. Upper level, FS, and LL landscape positions in the prairie had, respectively, eight, 12, and 13 times more belowground biomass in the top 60 cm of soil than their cultivated field counterparts. In the upper 1.8 m of soil, belowground biomass in the prairie grassland increased downslope (UL = 1849 [+ or -] 306, FS = 2533 [+ or -] 899, and LL = 3663 [+ or -] 1248 g [m.sup.-1]). In the cultivated field, LL landscape positions had higher levels of belowground biomass than either UL or FS positions (UL = 228 [+ or -] 54.5, FS = 200 [+ or -] 47.0, and LL = 294 [+ or -] 76.8 g [m.sup.-2]). Losses of C from belowground biomass accounted for 17.6% of total SOC losses in UL landscape positions, and 71.7% of losses in FS positions after cultivation. Lower level positions had the greatest loss of belowground plant C (-15.6 Mg [ha.sup.-1]) but overall the SOC had increased by 9.0 Mg [ha.sup.-1]. In these position the loss of belowground C was offset by significantly higher SOC additions to the surface through soil redistribution. On a whole landscape basis, the C loss because of changes in belowground biomass is a large part of the C change that occurs during the transition from native prairie to arable agriculture.