Deep accumulation of soluble organic nitrogen after land-use conversion from woodlands to orchards in a subtropical hilly region

Deep nitrate accumulation in typical black soil critical zones of Northeast China

Deep nitrate accumulation below 1 m has been observed in various soil regions, yet remains undocumented in the black soil (mainly Phaeozems and Chernozems) region. Climatic and edaphic factors likely influence deep nitrate accumulation on a large scale, although existing studies primarily focus on individual sites. In order to evaluate the distribution and controlling factors of deep nitrate in the black soil region, inorganic nitrogen forms and regolith properties of nine boreholes spanning humid, semi-humid, and semi-arid areas in Fujin, Hailun, and Lindian in northeast China were analyzed down to a depth of 10 m. The results revealed significant nitrate accumulation in Lindian, peaking at 11.03 mg N kg-1 at a depth of 3 m underground. Nitrate storage from the land surface to a depth of 10 m in Lindian ranged from 459.65 kg N ha-1 to 1072.88 kg N ha-1, with over 70 % of nitrate stored below 1 m. Nitrate accounted for 97.74 % of the total N stock in Lindian. Ammonium accumulation has been observed at a deeper depth in Hailun, with no nitrate accumulation detected in Hainlun and Fujin. Regolith properties such as clay, silt, sand, and pH playing a crucial role in reshaping the vertical pattern of nitrate. The presence of nitrate pools at greater depths in intensively managed black soil regions should be taken into account for the sustainable utilization of soil resources and the mitigation of groundwater pollution risks.

Five-year vegetation conversion from pasture to C3 and C4 plants affects dynamics of SOC and TN and their natural stable C and N isotopes via mediating C input and N leaching

Understanding the effects of land-use change on stock and composition of soil organic carbon (SOC) and nitrogen (N) is pivotal for sustainable agriculture and climate change adaption. However, previous studies have often overlooked the specific vegetation type in land-use changes. Therefore, a five-year lysimeter block experiment was conducted, involving non-vegetation, eulalia (C4 plant), and clover (C3 plant) to investigate the impacts of vegetation conversion from pasture on SOC and N dynamics and their natural stable isotopes. Non-vegetation caused 26.21 % and 25.88 % decreases in SOC and total N (TN) contents. Five-year eulalia and clover cultivation maintained stable SOC content, with clover exhibiting higher soil TN content. Eulalia-derived soil C was 1.64-7.58 g C kg-1 and SOC loss in eulalia treatment was 1.86-7.90 g C kg-1. Soil δ13C in eulalia increased at a rate of 0.90 ‰ year-1, significantly surpassing clover and non-vegetation treatments. Conversely, soil δ15N decreased over time, showing insignificant difference among all treatments. Eulalia exhibited significantly higher dry weight and δ13C but lower TN content compared with clover. However, no significant differences were observed in total C and δ15N between the two vegetation treatments. Non-vegetation exhibited higher dissolved organic C concentration than two vegetation treatments in 2017, decreasing over time. Dissolved TN and nitrate concentrations in leachate followed the order clover> non-vegetation> eulalia, with nitrate being the predominant form of N leaching from leachate. Our findings reveal that vegetation conversion affects soil C and N contents, and alters their natural isotopes as well as the leaching of labile soluble nutrients. Notably, non-vegetation consistently reduced SOC and TN contents, whereas eulalia cultivation maintained SOC content, improved C/N ratio and δ13C, and reduced N leaching compared with clover cultivation. These results highlight the potential of eulalia as a candidate plant for enhancing C sequestration and reducing N leaching in cold regions of Japan.