Vegetation restoration plays an important role in soil carbon storage, with the relatively lacking information in either the deep soil or the soil inorganic processes. The study aims to investigate the effect of returning farmland to the forest on deep soil carbon in the hilly and gully region of the Chinese Loess Plateau. Four sampling sites were selected: PO (Platycladus orientalis (Linn.) Franco forest; oriental arborvitae), PT (Pinus tabulaeformis Carr. Forest; Chinese red pine), AO (apple orchard) and FL (farmland, as a control treatment). Soil organic carbon (SOC) and soil inorganic carbon (SIC) content were analysed in 20-m soil profiles, along with the responsible factors. The mean SOC content slightly increased in the 1-5 m layer in the order FL < PO =AO < PT (p < 0.05) and in the 5-10 m layer in the order FL < PO < PT < AO, but no differences were found in the 0–1 m layer. Compared with FL, the SOC storages of PO, PT and AO increased by 6.71%, 16.28% and 2.50%, respectively. SIC content was relatively uniform throughout the profile, regardless of land-use types. The soil profiles in the study area presented vast SIC storage that was 8.8–10.2 times higher than SOC storage. After the 36-yrs implementation of the “Grain for Green” Project, SOC in 20 m soil profiles insignificantly increased as a cost of water depletion while SIC did not change. PT is the preferred tree species for soil carbon sequestration on afforested farmland in this region.

The source and hydrochemical makeup of a stream reflects the connectivity between rainfall, groundwater, the stream, and is reflected to water quantity and quality of the catchment. However, in a semi-arid, thick, loess covered catchment, temporal variation of stream source and event associated behaviors are lesser known. Thus, the isotopic and chemical hydrograph in a widely distributed, deep loess, semi-arid catchment of the northern Chinese Loess Plateau were characterized to determine the source and hydrochemical behaviors of the stream during intra-rainfall events. Rainfall and streamflow were sampled during six hydrologic events coupled with measurements of stream baseflow and groundwater. The deuterium isotope (2H), major ions (Cl-, SO42-, NO3-, Ca2+, K+, Mg2+, and Na+) were evaluated in water samples obtained during rainfall events. Temporal variation of 2H and Cl- measured in the groundwater and stream baseflow prior to rainfall was similar; however, the isotope compositions of the streamflow fluctuated significantly and responded quickly to rainfall events, likely due to an infiltration excess, overland dominated surface runoff during torrential rainfall events. Time source separation using 2H demonstrated greater than 72% on average, the stream composition was event water during torrential rainfall events, with the proportion increasing with rainfall intensity. Solute concentrations in the stream had loglinear relationships with stream discharge, with an outling anomaly during an intra-rainfall event on Oct. 24, 2015. Stream Cl- behaved nonconservative during rainfall events, temporal variation of Cl- indicated a flush and washout at the onset of small rainfall events, a dilution but still high concentration pattern in high discharge and old water dominated in regression flow period. This study indicated that streamflow responded to rainfall events quickly and composition was dominated by overland flow. Stream isotope and hydrochemistry controlled by infiltration excess, overland flow indicated that stored water in the thick, loess covered areas were less connected with stream runoff. Solute transport may threaten water quality in the area, requiring further analysis of the performance of the eco-restoration project.