17. Long-Term Measurements to Quantify the Impact of Arable Management Practices on Deep Seepage and Nitrate Leaching

Long-term soil hydrological measurements were used to quantify deep seepage and nitrate leaching in situ under undisturbed soil conditions. Deep seepage rates and nitrate losses from arable land managed under various farming regimes (integrated, integrated with irrigation, ecologic and low input) and tillage systems (plough and no-till) were quantified in the Pleistocene region of Northeast Germany from 1994 to 2005. Soil water content and tension measurements down to 3 m depth and soil water sampling were used to determine deep seepage dynamics and loss of nitrogen by leaching. As dependent on the management system, the nitrate concentration varied between 40 and 150 mg l⁻¹. In connection with annual deep seepage rates between 100 and 200 mm during the study period, the annual nitrogen loss varied between 14 and 41 kg ha⁻¹. Differences in nitrogen loss observed between the farming systems were low, but yields increased and nitrogen losses decreased as a result of irrigation throughout the variants. No-till treatment resulted in reduced nitrate leaching (18 kg ha⁻¹) as compared with the tillage system with plough and tooth cultivator (27 kg ha⁻¹). The method of quantifying deep seepage rates based on soil hydrological measurements was tested in comparison with lysimeter discharge measurements. Willmott’s index of agreement was d = 0.97 revealing the validity of this simplified approach. Results underline the hypothesis of the well balanced, slow and continuous progression of the soil water content below the root zone. The suitability of long-term soil hydrological in situ measurements for quantifying arable management effects on ecological processes – deep seepage dynamics and solute leaching – was confirmed.