ABSTRACT
Afforestation is beneficial to improving soil carbon pools. However, due to the lack of deep databases, the variations in soil carbon and the combined effects of multiple factors after afforestation have yet to be adequately explored in >1 m deep soils, especially in areas with deep-rooted plants and thick vadose zones. This study examined the multivariate controls of soil organic carbon (SOC) and inorganic carbon (SIC) in 0-18 m deep under farmland, grassland, willow, and poplar in loess deposits. The novelty of this study is that the factors concurrently affecting deep soil carbon were investigated by multiwavelet coherence and structural equation models. On average, the SOC density (53.1 ± 5.0 kg m-2) was only 12% of SIC density (425.4 ± 13.8 kg m-2), with depth-dependent variations under different land use types. In the soil profiles, the variations in SOC were more obvious in the 0-6 m layer, while SIC variations were mainly observed in the 6-12 m layer. Compared with farmland (SOC: 17.0 kg m-2; SIC: 122.9 kg m-2), the plantation of deciduous poplar (SOC: 28.5 kg m-2; SIC: 144.2 kg m-2) increased the SOC and SIC density within the 0-6 m layer (p < 0.05), but grassland and evergreen willow impacted SOC and SIC density insignificantly. The wavelet coherence analysis showed that, at the large scale (>4 m), SOC and SIC intensities were affected by total nitrogen-magnetic susceptibility and magnetic susceptibility-water content, respectively. The structural equation model further identified that SOC density was directly controlled by total nitrogen (path coefficient = 0.64) and indirectly affected by magnetic susceptibility (path coefficient = 0.36). Further, SOC stimulated the SIC deposition by improving water conservation and electrical conductivity. This study provides new insights into afforestation-induced deep carbon cycles, which have crucial implications for forest management and enhancing ecosystem sustainability in arid regions.
Subject(s)
Carbon , Soil , Soil/chemistry , Carbon/analysisABSTRACT
Water quality is an increasing concern in the dry regions of the world as it affects and reduces the quantity of available water. Our objective was to investigate the sources, drivers, spatiotemporal patterns of nitratenitrogen (NO3-N) transport in the streamwater and groundwater in a dry and a wet season in seven large rivers located in the Loess Plateau of China (640,000 km2, 100 million population), which is a region with marked influence of human activities on streamflow and groundwater. We collected 510 streamwater and groundwater samples and found that NO3-N was significantly lower in the dry season (< 5.0 mg L-1) than the wet season (> 5.0 mg L-1). In the wet season, NO3-N was lower in the streamwater than groundwater; however, the spatial variation in the NO3-N was greater in streamwater, with higher concentrations in two rivers (Wei and Fen). The source characterization using stable isotopes of NO3 from the Wei River showed that chemical N fertilizers and soil organic N contributed ~ 75% of NO3 to streamwater and that soil organic N was the greatest contributor of NO3 to groundwater (~ 60%) than streamwater (< 40%). The spatial pattern of NO3-N was dominated by fertilizer application and varied seasonally with rainfall-runoff and streamflow-groundwater connectivity. Our results showed the complicated patterns and sources of NO3 pollution in streamwater and groundwater and highlight that more emphasis should be placed to prevent and restore the degraded water quality in the dry regions.
ABSTRACT
Investigation of stable isotope composition under different land use types is helpful for understanding soil water movement and hydrological effects of land use change. We collected soil samples in profiles ï¼ 15 m deep under four land use types (i.e. farmland, grassland, Salix cheilophila and Populus sp.) in the loess deposits of northern Shaanxi. We measured hydrogen and oxygen stable isotope composition of soil water to explore the mechanism of soil water movement and the impacts of land use types. The isotope compositions of soil water under four land use types were significantly different. The δD values of soil water under farmland, grassland, S. cheilophila and Populus sp. were -81.1--60.1, -91.2--61.0, -87.4--63.6 and -73.5--62.2, while the δ18O values were -11.2--7.6, -12.6--8.2, -11.5--8.1 and -9.9--7.7, respectively. The soil water stable isotopes fluctuated across the profiles. The soil water isotope compositions in the layers of 0-3 m changed sharply, with the δD values being -80.2--61.8, -75.9--65.5, -76.0--63.6 and -73.5--62.2, respectively. In the layers of 3-12 m, the isotope profiles of farmland and grassland were parabolic, whereas those of S. cheilophila and Populus sp. were relatively stable. Soil water isotope compositions in the layers deeper than 12 m were generally stable with the δD values of -80.8--71.5, -83.0--67.5, -87.4--76.0 and -67.5--64.3, respectively. Across the four land use types, soil water stable isotope compositions were not significantly different either in the shallow layers or in the deep soil layers, but their differences in the layers of 3-12 m were significant. Soil moisture was mainly recharged from precipitation with piston flow as the main form of soil water movement. Soil water under four land use types might be recharged by wet events of different intensities. Soil water under farmland and grassland could be recharged by wet events of small intensity, but that under S. cheilophila and Populus sp. may be mainly recharged by the rainstorm in summer and autumn.
