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

Erosion is a key driver of topsoil removal in agriculture, resulting in nutrient losses and leaving truncated soil profiles on shoulder slopes, where subsoil material can be incorporated into topsoil by the plough, changing soil properties and, thus, altering biogeochemical cycling and associated nitrogen (N) losses. To date, the effects of topsoil erosion on N cycling have rarely been investigated. We conducted a short-term mesocosm experiment, integrating 15 N tracing techniques to quantify N transformation processes, focusing on N availability and N losses in three artificially eroded agricultural topsoils. Nitrogen transformation pathways were simulated using the numerical model Ntrace , considering N uptake by maize ( Zea mays L.) at early development stages. The 15 N label also allows the quantification of nitrous oxide (N 2 O) and dinitrogen (N 2 ) losses by applying the 15 N gas flux method ( 15 NGF). Topsoil erosion significantly reduced gross N turnover and consequently N 2 O and N 2 emissions in both treatments with and without plants. The oxidation of ammonium (NH 4 + ) to nitrate (NO 3 - ) was by far the dominating N pathway across all investigated topsoils, followed by N org mineralization > NH 4 + immobilization. However, more than 90% of total N 2 O losses derived from the NO 3 - pool, with coupled nitrification-denitrification assumed to be the dominant process at water contents of ∼40 % WFPS. Although maize more than doubled N 2 O+N 2 emissions in some treatments, the overall effect of topsoil dilution on gaseous N losses was considerably greater, independent of maize presence. Our study contributes to a more comprehensive understanding of N cycling in erosion-affected agricultural soils, which is essential for enhancing N fertilizer use efficiencies and reducing N pollution. • Topsoil erosion decreases gross N transformation rates by limiting substrate • Erosion impact on N transformation processes was not affected by young maize • N 2 O and N 2 losses following fertilization decrease with increasing erosion level