Short-term changes of the δ15N natural abundance of aboveground biomass and topsoil in temperate grassland – Effects of prolonged summer drought

Abstract

Analysis of isotopic signatures of nitrogen (15N) is a widely used method for identifying and tracking pathways of N losses in agriculture. It is not yet clear what drives the development of the isotopic signal over time after change of management in grasslands. In a three-year field experiment in cut grassland on a sandy soil in northwest Germany, we investigated the response of isotopic signals in aboveground biomass and topsoil to different fertilizer management regimes: cattle slurry (org.) and calcium ammonium nitrate (CAN, syn.) were applied at different rates (0 kg N ha−1; 120 kg N ha−1 org.; 240 kg N ha−1 org.; 240 kg N ha−1 org./syn.; 360 kg N ha−1 org./syn.). Samples of aboveground biomass and topsoil were taken at the end of vegetation period every year. For aboveground biomass, the effects of fertilizer treatment on the isotopic signal were already evident in the second year and δ15N values ranged from 0.933‰ to 8.049‰. In the topsoil, the δ15N values ranged from 3.670‰ to 5.322‰ and a differentiation in δ15N values among fertilizer treatments could be proven for the third year of the study. We conclude that isotopic signals in aboveground biomass and topsoil can already be detected after a period of 2–3 years. Drought conditions can boost the detection of isotopic signals as they favor NH3 losses and lead to a reduced sink function of biomass and, consequently, large N surpluses.

Keywords

• Stable isotopes
• Natural 15N abundance
• Organic N fertilizer application
• N balance