Model based scenario studies to optimize the regional nitrogen balance and reduce leaching of nitrate and sulfate of an agriculturally used water catchment

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Oxidation of pyrite by nitrate (autotrophic denitrification) was identified as the main cause for sulfate increase in drinking water wells in an agriculturally used watershed, located in the north of Lower Saxony (Germany). Nitrate, which inducts this microbial catalyzed process, is drained into ground water predominantly from agricultural fertilization. The increase of sulfate in the ground water can only be stopped by reducing nitrate leaching into the ground water. To analyze the negative influence of agricultural fertilization on the quality of ground water different fertilization strategies were deducted for an investigated area of 890 ha. Calculated on the basis of nutrient balance of soil surface, the current average nitrogen balance in the investigated area amounts to 91 kg N ha-1 a−1. Farm-gate balance of nutrients is a strong indicator for assessing potential nutrient losses caused by leaching. This indicator shows comparable accuracy to the calculated nutrient balance of soil surface which demands, however, much more data input for calculations. Nitrate concentrations in seepage water in 2 m depth layer of the soil from agricultural fields were simulated with the model HERMES for the whole investigated area (agricultural land + forest). The nitrate concentration in seepage water was calculated for the whole area on the basis of farm-gate nutrient balance as an annual average, which amounts to 14.0 mg NO3–N l−1 (62 mg NO3 l−1). In order to keep the nitrate concentration of the ground water below the threshold value for drinking water (EU-water directive: 11.3 mg NO3–N l−1 (50 mg NO3 l−1) and to limit pyrite oxidation, different scenarios with simulation studies to optimize fertilization measures were developed. Only those scenarios which assured reduction of an average nitrate concentration in the drainage water below 11.3 mg NO3–N l−1 (50 mg NO3 l−1) without profit cuts for the farms were analyzed.