Erosional effects on distribution and bioavailability of soil nitrogen fractions in the Belgian Loess Belt

Soil erosion strongly influences the transport and fate of carbon (C) and nitrogen (N) in hillslope soils. However, in dynamic landscapes, erosional effects on soil N cycling and primary controls on N bioavailability are not well understood: particularly with respect to differences between topsoil and subsoil. Here we aim to explore the influence of erosion on (i) spatial distributions of soil N fractions and (ii) controls on N bioavailability in eroding vs. depositional sites within the Belgian Loess Belt. Soil samples were fractionated by aggregate size and density. In addition, intact soil samples were incubated to determine the influence of oxygen status (0, 5, and 20%) and labile organic matter on mineralization and nitrification of N in the context of erosion. The results showed that the deposition of eroded upslope soil materials led to N enrichment throughout entire soil profiles. Across both eroding and depositional sites, more than 93% of the total N was associated with minerals. Increased macro-aggregate- and mineral-associated N at the depositional site indicated that aggregation and N stabilized by minerals contribute to N enrichment in the depositional soils. Inorganic N, mostly NO₃⁻-N, was also larger at the depositional site. Oxygen concentrations were positively related to net N nitrification and mineralization rates regardless of geomorphic position. Glucose addition significantly reduced net N mineralization and nitrification rates. In conclusion, our results indicate that soil erosion might not only lead to spatial variations of N pools but also potentially affect the transformation and bioavailability of N along eroding hillslopes. Future research should consider the fate of different N species in eroding landscapes and consequences for both carbon sequestration and N leaching.