Soil Carbon Dynamics During Natural Grass Restoration On Abandoned Sloping Cropland In The Hilly Area Of The Loess Plateau

The “Grain for Green” programme on the Loess Plateau has improved ecological restoration. However, for a long run, the growth of planted tree species depletes soil water, resulting in surface soil dessication and consequently in degradation of tree plantation. Natural vegetation succession behaves better in eco-sustainability, and accelerating the process of natural succession can enhance ecological restoration. The present study combined field investigation and laboratory analysis, focused on naturally restored grassland on the Loess Plateau of China, selected typical community grasslands, and investigated the carbon dynamics of vegetation and soil. Based on the research, we recommend strategies to accelerate grassland restoration and improve soil carbon storage. The study therefore provides important scientific background information for soil carbon sequestration and ecological restoration. The main results are as follows:(1) Grass restoration enhanced the accumulation of SOC and SIC storage, but with inverse proportional contributions to STC storage. Compared to cropland, restored grassland significantly decreased SOC storage(0-100 cm) at the early restoration(5yr), whereas SIC storage(0-100 cm) increased at the same time. And thus, SIC storage has increased faster than SOC storage since cropland was abandoned. Compared to cropland, the increments of SOC storage(0-100 cm) were-0.039(5yr), 0.037(13yr), 0.840(24yr) and 1.082 kg m-2(32yr);whereas the increments of SIC storage(0-100 cm) were 1.209(5yr), 6.000(13yr), 6.608(24yr) and 4.212 kg m-2(32yr)。During the process of grassland restoration following sloping land abandonment, the increment SIC storage was more than that of SOC storage.(2) Though the increments were low, the content and storage of SOC and TN increased significantly after 13-year grassland restoration, and further more significantly higher storage of SOC and TN may need over 20-year grassland restoration. There were highest SOC and TN contents in the 40-100 cm soil layers in the 24-year restored grassland. The establishment of perennial species and its life characteristics exert great influence on soil erosion control and biomass input, and thus soil carbon and nitrogen accumulation.(3) During the process of grassland restoration, there were significantly positive relationship between the content of SOC and TN, between the storage of total TC and TN. The results indicated that the management of soil N may play a significant role in improving soil C storage considering the fact that the soil has low N content in the Loess Plateau of China.(4) The field observation showed that, compared to cropland, restored grassland totally decreased soil CO2 efflux. Cropland and restored 24-year grassland had significantly higher soil CO2 efflux. For the cropland, higher soil CO2 flux may be attributed to tillage practices which strength the process of soil organic carbon decomposition; for the 24-year restored grassland, higher soil CO2 flux may be attributed to higher mean soil temperature and moisture. The Q10 among different sites varied little at the time scale of growing seasons and total measuring seasons.(5) The lab soil incubation experiment showed that soil heterotrophic respiration(Rh) increase with the increase of soil temperature and moisture. The Rh rate of 0-5 cm soil listed in the order Cropland<24yr<5yr<13yr<32yr;The Rh rate of 20-40 cm soil listed in the order 32yr<13yr<5yr<24yr<Cropland. The results indicated that, there may be different mechanism to explain soil organic matter decomposition for the upper soil layers and the lower soil layers. The Q10 of 0-5 cm soil and 20-40 cm soil Rh showed little variation among soil moisture treatments and grassland restoration.(6) The soil incubation experiment showed that Pearson correlation coefficient between Rh and soil properties such as SOC, TN, C/N, NO3-, SMC and SMN was 0.88, 0.93, 0.68, 0.97, 0.79, and 0.96 respectively. There were significantly positive relationships between NO3- and Rh(P<0.05), SMN and Rh(P<0.05). The changes of soil nitrogen composition during grass restoration may exert great influence on Rh.(7)The observation of soil CO2 flux in the lab and field showed that cropland can be a carbon source, while resored grasslands carbon sinks. The life characteritics of successional grass species play a significant role in the ecological CO2 flux.