Erosion and deposition controlling redistribution and biodegradation of nitrogen fractions along a Mollisol agricultural landscape

Purpose Soil erosion controls nitrogen (N) bioavailability and immobilization in soil, thereby affecting soil fertility and ecological risk. In eroding landscapes, however, emphasis has been placed on soil carbon (C) dynamics, largely neglecting the mechanisms controlling the distribution and bioavailability of N in topsoil versus subsoil. Here, we examined how erosion changes the size of dissolved N pools, mobilization of aggregate-associated total N (ATN), and the biodegradation of soil N in topsoil (0-20 cm) versus subsoil (80-100 cm) along an eroding agricultural landscape. Materials and methods Soil samples collected from three representative topographic sites, up-slope (non-erosion), mid-slope (erosion), and down-slope (deposition), were fractionated and incubated to investigate N pool transformation and bioavailability. Results and discussion The results showed that 75.9 to 96.3% of the dissolved total N (DTN) occurred in the organic form. At depths of 0–80 cm, the up-slope site had significantly higher DON contents than the mid-slope and down-slope sites. The macroaggregate-associated N in the down-slope increased by 1.44 to 2.40 times compared with the mid-slope for topsoil and subsoil. The highest C/N ratios in all aggregate fractions were observed at depositional sites, indicating that particulate organic matter is preferentially transported. Nitrification was dominant in N mineralization and eroding topsoil had significantly higher nitrification levels than non-eroding and depositional soil. Conclusions Our findings suggest that erosion significantly reduced the size of dissolved N pools, accelerated ATN mobilization in eroding sites, and increased N bioavailability in the eroding sites. Therefore, we highlighted a thorough 1-m profile understanding and assessed N pools along the Mollisol eroding landscape.