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

Abstract

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 ¹⁵N 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 ¹⁵N label also allows the quantification of nitrous oxide (N₂O) and dinitrogen (N₂) losses by applying the ¹⁵N gas flux method (¹⁵NGF).

Erosion induced topsoil dilution significantly reduced gross N turnover and consequently N₂O and N₂ emissions in both treatments with and without plants. The oxidation of ammonium (NH₄⁺) to nitrate (NO₃⁻) was by far the dominating N pathway across all investigated topsoils, followed by N_org mineralization > NH₄⁺ immobilization. However, more than 90 % of total N₂O losses derived from the NO₃⁻ pool, with coupled nitrification-denitrification assumed to be the dominant process at water contents of ∼40 % water-filled pore space (WFPS). Although maize more than doubled N₂O + N₂ 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.