Changes in soil organic carbon and total nitrogen stocks after conversion of meadow to cropland in Northeast China

Grassland conversion to cropland (GCC) may result in loss of a large amount of soil organic carbon (SOC). However, the assessment of such loss of SOC still involves large uncertainty due to shallow sampling depth, soil bulk density estimation and spatial heterogeneity. Our objectives were to quantify changes in SOC, soil total nitrogen (STN) and C:N ratio in 0-100 cm soil profile after GCC and to clarify factors influencing the SOC change. A nest-paired sampling design was used in six sites along a temperature gradient in Northeast China. SOC change after GCC ranged from -17 to 0 Mg ha(-1) in 0-30 cm soil layer, recommended by IPCC, across the six sites, but ranged from -30 to 7 Mg ha(-1) when considering 0-100 cm. We found a linear relationship between SOC change in 30-100 cm and that in 0-30 cm profile (Delta C30-100 = 0.35 Delta C0-30, P < 0.001), suggesting that SOC change in 0-100 cm was averagely 35 % higher than that in 0-30 cm. The change in STN showed a similar pattern to SOC, and soil C:N ratio did not change at most of sites. On the other hand, SOC loss after GCC was greater in soils with higher initial SOC content or in croplands without applying chemical fertilizers. Furthermore, SOC loss after GCC decreased with falling mean annual temperature (MAT), and even vanished in the coldest sites. The magnitude of SOC loss following GCC in Northeast China is lower than the global average value, partly due to low MAT here. However, the current low SOC loss can be intensified by remarkable climate warming in this region.