Impact of land use/cover changes on carbon storage in a river valley in arid areas of Northwest China

Soil carbon pools could become a CO_2 source or sink, depending on the directions of land use/cover changes. A slight change of soil carbon will inevitably affect the atmospheric CO_2 concentration and consequently the climate. Based on the data from 127 soil sample sites, 48 vegetation survey plots, and Landsat TM images, we analyzed the land use/cover changes, estimated soil organic carbon（SOC） storage and vegetation carbon storage of grassland, and discussed the impact of grassland changes on carbon storage during 2000 to 2013 in the Ili River Valley of Northwest China. The results indicate that the areal extents of forestland, shrubland, moderate-coverage grassland（MCG）, and the waterbody（including glaciers） decreased while the areal extents of high-coverage grassland（HCG）,low-coverage grassland（LCG）, residential and industrial land, and cultivated land increased. The grassland SOC density in 0–100 cm depth varied with the coverage in a descending order of HCG〉MCG〉LCG.The regional grassland SOC storage in the depth of 0–100 cm in 2013 increased by 0.25×1011 kg compared with that in 2000. The regional vegetation carbon storage（S_（rvc）） of grassland was 5.27×10~9 kg in2013 and decreased by 15.7% compared to that in 2000. The vegetation carbon reserves of the under-ground parts of vegetation（S_（ruvb）） in 2013 was 0.68×10~9 kg and increased by approximately 19.01%compared to that in 2000. This research can improve our understanding about the impact of land use/cover changes on the carbon storage in arid areas of Northwest China.

Managed grassland alters soil N dynamics and N2O emissions in temperate steppe

Reclamation of degraded grasslands as managed grasslands has been increasingly accelerated in recent years in China. Land use change affects soil nitrogen（N） dynamics and nitrous oxide（N2O） emissions. However, it remains unclear how large-scale grassland reclamation will impact the grassland ecosystem as a whole. Here, we investigated the effects of the conversion from native to managed grasslands on soil N dynamics and N2O emissions by field experiments in Hulunber in northern China. Soil（0-10 cm）, nitrate（NO3-）,ammonium（NH4＋）, and microbial N were measured in plots in a temperate steppe（Leymus chinensis grassland） and two managed grasslands（Medicago sativa and Bromus inermis grasslands） in 2011 and 2012. The results showed conversion of L. chinensis grassland to M.sativa or B. inermis grasslands decreased concentrations of NO3--N, but did not change NH4-N . Soil microbial N was slightly decreased by the conversion of L. chinensis grassland to M.sativa, but increased by the conversion to B. inermis. The conversion of L. chinensis grassland to M. sativa（i.e., a legume grass） increased N2O emissions by 26.2%, while the conversion to the B. inermis（i.e., a non-legume grass） reduced N2O emissions by 33.1%. The conversion from native to managed grasslands caused large created variations in soil NO3-＋-N and NH4-N concentrations. Net N mineralization rates did not change significantly in growing season or vegetation type, but to net nitrification rate. These results provide evidence on how reclamation may impact the grassland ecosystem in terms of N dynamics and N2O emissions.