Nitrogen legacies in agricultural soils? Evidence from long-term lysimeter balances and isotope analyses

Nitrogen is a fundamental plant nutrient and important fertilizer in modern agriculture, while nitrate-based nitrogen losses from agroecosystems become an increasing problem in ground- and surface waters. To reduce the emission of nitrate from agricultural soil, substantial efforts were made in regulation and assessment of plant specific fertilizer needs to reduce fertilization excess. Unfortunately, those efforts have not yet led to a considerable reduction of nitrate loadings in ground and seepage water under agricultural land use. This lag of response is often explained with residence and transport times in groundwater and a potential contribution of nitrogen legacies accumulated in soils.

Since 1980 the water and solute balances of different soils under agricultural land use are investigated at the lysimeter station Brandis (Saxony, Germany). Additionally, historic marking experiments with ¹⁵N enriched fertilizers were performed on some of the investigated soils. The combination of the long-term nitrogen balances together with an ¹⁵N isotope measurement campaign clearly show, that in a broad range of soils:

• a substantial amount of the historic fertilizer excess has accumulated in the soils
• historic fertilizations with ¹⁵N enriched fertilizer are still visible in the top soils after 40 years of agricultural landuse
• nitrogen residence times are independent of water transport times
• nitrate loss from soil organic matter pools is a major source of nitrate lost by seepage water

Our results clearly show, that substantial nitrogen legacies from fertilization excess can accumulate in a broad range of soil types. It becomes evident that considering and understanding the dynamics of this biochemical nitrogen legacy in agricultural soils is key to explain the lag of response in water quality observations in ground- and surface waters.