Impact of different soil erosion levels on N transformation processes and gaseous N losses: An incubation study

Soil erosion is a key driver of soil redistribution, often causing nutrient losses from agricultural fields and contributing to nutrient overload in natural ecosystems. The removal of topsoil leads to truncated soil profiles on shoulder slopes, in which the plough can incorporate deeper soil material. This can change soil properties and, thus, alter biogeochemical cycling. Despite the increased interest in understanding SOM turnover in eroded topsoils, studies on N cycling in this context are rare and often focus only on isolated aspects of the N cycle (Berhe et al., 2018).

We designed a short-term mesocosm experiment, combining different ¹⁵N-tracing techniques, to quantify almost all N transformation processes in topsoils mixed with different amounts of subsoil to simulate three erosion levels. Nitrogen transformation pathways were simulated using the numerical model Ntrace (Rütting & Müller, 2007), considering N uptake by maize (Zea mays) at early development stages. The ¹⁵N labelling also allows the quantification of N₂O and N₂ losses, originating either from the soil NO₃-N or NH₄-N pool. N₂O losses were determined automatically by a gas chromatograph and N₂ by isotope ratio mass spectrometry by applying the ¹⁵N gas flux method in N₂-depleted atmosphere (Kemmann et al., 2021).

The incorporation of subsoil material resulted in decreased C_org and N_tot contents with increasing erosion levels, leading to reduced nitrogen turnover and, consequently, lower N₂O and N₂ emissions in both maize-planted and unplanted treatments. Autotrophic nitrification was the dominating process across all erosion levels. Nevertheless, most N₂O and N₂ emissions originated from coupled nitrification-denitrification, even at water contents <40 % WFPS. Surprisingly, the growth of maize plants increased N₂O and N₂ emissions more than twice at early growth stages. However, the overall effect of the erosion level was considerably greater than the effect of plant presence. Our study contributes to a more comprehensive understanding of N cycling in agricultural soils of hilly landscapes, which is essential for enhancing nitrogen fertilizer use efficiencies and reducing N pollution.

References

Berhe, A. A., Barnes, R. T., Six, J., & Marín-Spiotta, E. (2018). https://doi.org/10.1146/annurev-earth-082517-010018

Kemmann, B., Wöhl, L., Fuß, R., Schrader, S., Well, R., & Ruf, T. (2021). https://doi.org/10.1111/gcbb.12879

Rütting, T., & Müller, C. (2007). https://doi.org/10.1016/j.soilbio.2007.04.006