No abstract

In many areas of the Loess Plateau, groundwater is too deep to extract, making meteoric water (snow and rain) the only viable water resource. Here we traced the rainwater and water vapor sources using the δ2H and δ18O signature of precipitation in the northern mountainous region of Yuzhong on the Loess Plateau. The local meteoric water line in 2016 and 2017 was defined as δ2H = 6.8 (±0.3)∙δ18O + 4.4 (±2.0) and δ2H = 7.1 (±0.2)∙δ18O + 1.5 (±1.6), respectively. The temperature and precipitation amount are considered to be the main factor controlling the δ2H and δ18O variation of precipitation, and consequently, relationships were first explored between δ18O and local surface air temperature and precipitation amount by linear regression analysis. The temperature effect was significant in the wet seasons but was irrelevant in the dry seasons on daily and seasonal scales. The amount effect was significant in the wet seasons on a daily scale but irrelevant in the dry seasons. However, based on the data of the Global Network of Isotopes in Precipitation (GNIP) (1985–1987, 1996–1999) of Lanzhou weather station, the amount effects were absent at seasonal scales and were not useful to discriminate either wetter or drier seasons or even wetter or drier decades. Over the whole year, the resulting air mass trajectories were consistent with the main sources of water vapor were from the Atlantic Ocean via westerlies and from the Arctic region, with 46%, 64%, and 40% of water vapor coming from the westerlies, and 54%, 36%, and 60% water vapor from the north in spring, autumn and winter, respectively. In the summer, however, the southeast monsoon (21%) was also an important water vapor source in the Loess Plateau. Concluding, using the δ2H and δ18O signatures of precipitation water, we disentangled and quantified the seasonal wind directions that are important for the prediction of water resources for local and regional land use.

Legume pasture species have been widely used to re‐establish vegetation cover and control soil erosion in semiarid degraded ecosystems. Nevertheless, the long‐term effects of revegetation on aboveground biomass (AB) and the soil properties under different topographies remain unclear. We conducted a 16‐year in situ experiment to evaluate soil properties and biomass dynamics under fallow, sweet clover (Melilotus officinalis L.) and alfalfa (Medicago sativa L.), in northeast‐facing, southeast‐facing, and horizontal landscapes from 2003 to 2018. After 16 years of revegetation, soil organic carbon (SOC), soil total nitrogen (TN), soil total phosphorus (TP) concentrations were higher in alfalfa fields in the northeast‐ and southeast‐facing landscapes and greater in sweet clover fields in the horizontal landscape alongside soil profiles. In the 0–20 cm, the SOC, TN, and TP concentrations in alfalfa fields increased at rates of 0.322, 0.034, and 0.010 g kg−1 yr−1 in the northeast‐facing landscape and at rates of 0.189, 0.022, and 0.011 g kg−1 yr−1 in the southeast‐facing landscape. The SOC, TN, and TP concentrations in sweet clover fields increased at rates of 0.129, 0.023, and 0.009 g kg−1 yr−1 in the horizontal. Alfalfa introduction is recommended for northeast‐facing landscapes whereas alfalfa and fallow are recommended in southeast‐facing landscapes, introduce sweet clover is recommended in a horizontal landscape to improve the soil nutrient concentrations at mid‐long time. These results suggest that topography influences vegetation restoration by affecting plant growth and soil nutrient and should be considered during the revegetation process to ensure revegetation success and sustainable land use.