Impacts of agricultural land-use change on soil organic carbon storage in Danjiangkou Reservoir area, China

Background/Question/Methods :Land use change may alter soil carbon (C) stock in terrestrial ecosystems, but its effects remain poorly quantified. Thus, we investigated the effects of agricultural land use change following the conversion of cropland to woodland plantation (Platycladns orientalis) and shrubland plantation (Robinia pseudoacacia and Amorpha fruticosa) on soil organic carbon (SOC) and recalcitrant C (RC) in Danjiangkou Reservoir area, China. Soil C and N recalcitrance indexes (RIC and RIN) were calculated as the ratio of unhydrolyzable C and N to total C and N.

Results/Conclusions :We found that plantations increased SOC stock by an average of 608.1 g C m^-2 in the afforested soils and by 303.4 g C m^-2 in the shrubland soils. The C accumulation following plantation was largely induced by the increasing biomass input and decreasing soil organic matter (SOM) decomposition. The C:N ratios changed from low to high in an order of the cropped, shrubland and afforested soils due to increased inputs of recalcitrant shrub and forest residuals into soils. The RIC decreased from the afforested to shrub-land to the cropped soils, but RIN exhibited an opposite trend, demonstrating the priming effect on the SOC and RC stocks induced by the low-quality inputs derived from shrub and forest plantations. The δ¹⁵N values of SOM enriched from the afforested to shrubland to cropped soils, indicating the increased N loss from the cropped soils compared with afforested or shrubland soils. Change in δ¹³C ratio revealed that the proportion of SOC derived from forest and shrub residuals averaged 43.6% and 22.2% in the 0-30 cm soil layers, respectively. Overall, these results suggest that shifts in vegetation under land use change could alter both the quantity and quality of SOM and thus have potential effects on ecosystem function.