Insights into nitrogen dynamics and nitrate loss from agricultural soils based on long-term lysimeter observations and a 5-year isotope measurement campaign

Nitrogen is a fundamental plant nutrient and the most important fertilizer in modern agriculture. At the same time nitrate based nitrogen loss from agroecosystems becomes an increasing environmental problem in ground- and surface waters. The lysimeter station Brandis in Saxony, Germany, provides detailed observations of water and solute fluxes under representative agricultural landuse since 1981. Despite substantial efforts and success in regulation and assessment of fertilizer needs and the reduction of fertilization excess, the seepage water analysis reveals increasing or stagnating levels of nitrate concentration in groundwater recharge in a broad range of soil types. This apparent decoupling between input and output is evident in all soil types under investigation and raises some important questions concerning the nitrate loss in agricultural soils:

• Which part of the soil N-cycle contributes to the seepage water nitrate export?
• What are the main drivers of nitrate loss in agricultural soils?
• Can residence times of mineral fertilizer nitrogen be estimated?
• Will reduced fertilization excess lead to timely reductions in nitrate loss to the groundwater?

We investigated these questions with long-term solute balances and state-of-the-art isotope methods. Analysis of source δ ¹⁵N ratios in soil, atmospheric deposition and fertilizer in combination with a 5-year campaign of δ¹⁵N and δ¹⁸O analysis of seepage water nitrate allows a source identification with dual-isotope plots and mixing models. The results clearly show that the main source of nitrate loss with the seepage water is the soil organic matter pool in all investigated soils. Analysis of the long-term nitrogen balances and the soil samples show furthermore a substantial accumulation of fertilization excess within the upper meter of agricultural soils and indicate that the residence time of nitrogen in the lysimeters might be substantially longer than water residence times. Isotope analysis in combination with mixing model analysis suggest that the nitrate loss is mainly driven by nitrification of this nitrogen legacy in the post-harvest period. Thus, the results hold an explanation why the current regulation efforts have not yet led to the desired reductions in nitrogen loadings of seepage water fluxes. Furthermore, the apparent decoupling between nitrogen input in agricultural soils and the seepage water output makes a timely reduction of nitrate concentrations, by reductions in fertilization excess alone, in groundwater recharge unlikely.