Response of soil organic carbon and nitrogen stocks to soil erosion and land use types in the Loess hilly-gully region of China

- SOC and TSN contents are mainly concentrated in the surface layer among land types.
- Soil erosion, land-use change and landscape position have great effects on SOC and TSN stocks at a landscape scale.
- Artificial forestland is an optimal choice to increase C and N sequestration.

Erosion influences the vertical and horizontal distribution patterns of soil and soil organic carbon (SOC) at a landscape scale. To further understand the effect of erosion on SOC and total soil nitrogen (TSN) stocks in relation to land use types after the implementation of the “Grain for Green” program in the Loess hill-gully region, the SOC, TSN, and Caesium-137 (137Cs) contents were analyzed at three selected landscape positions under three land-use types: artificial grassland (AGL), native grassland (NGL) and artificial plantation of Robinia pseudoacacia (AFL). The results showed that all land uses experienced considerable net erosion since the mid-1950s, with an average total loss depth of 2.05 cm for AFL, 1.49 cm for AGL, and 0.54 cm for NGL. The SOC stocks in AFL and NGL were 72.3% and 26.2% lower, respectively, than that in AGL in the 0-100 cm soil layer, and significant positive correlation between SOC and TSN stocks on each layer in the soil profile was observed (R2 > 0.90). The result showed that compared with other land-use types, AGL had a greater SOC and TSN sequestration capacity. The contents of SOC and TSN were positively correlated with the amount of 137Cs in AFL and NGL (R2 = 0.97, 0.97 for AFL, respectively, and R2 = 0.90, 0.90 for NGL, respectively; n = 3), whereas no significant correlation was found in AGL (R2 = 0.41, 0.01, respectively; n = 3). The results indicated that AGL was an optimal choice to mitigate soil carbon and nitrogen loss and to increase C and N sequestration in the Loess hilly-gully region. A complex process should be considered for the distribution patterns of SOC and TSN after afforestation since 1999.