Paper Title
Long-Term Effect of a Conventional Organic-Synthetic Fertilizer Cultivation System and an Organic Fertilizer System Including Legumes on Nitrogen Turnover Processes

Abstract
Soil nitrogen (N) transformation processes are crucial for plant productivity and N loss from croplands. A scant body literature has indicated that application of organic or combined organic-synthetic fertilizer increases the mineralization-immobilization turnover, improving plant N supply and reducing N loss potential. However, the long-term impact of completely substituting synthetic fertilizers and integrating legumes in cultivation systems on gross soil N transformation rates, is rarely explored. To evaluate differences in the dynamics of soil N turnover we determined gross mineralization, nitrification and immobilization rates at two directly adjacent fields (CF, OF) during a green rye-maize rotation. CF, has been managed conventionally for the past 12 years, receiving manure and solid digestate with additions of synthetic fertilizer and without legumes in the crop rotation. OF, received exclusively dairy manure and had a share of more than 20% legumes in the crop rotation. Gross rates were determined using the "isotopic pool dilution" (IPD), with gross ammonium (NH4+) and nitrate (NO3¯) immobilization rates being additionally determined using the "reformed difference method" (RDM). For green rye and maize season in OF, the average gross N turnover rates of mineralization (m), nitrification (n), and NH4+ and NO3¯ immobilization (RDM) amounted to (1.66 ± 0.55; 1.31 ± 0.28; 1.24 ± 0.09; 0.63 ± 0.09), respectively, for soil depths of 7–17 cm. In contrast, higher gross rates of m (2.59 ± 0.90), n (1.97 ± 0.57), NH4+ (2.28 ± 0.10) and NO3¯ (1.33 ± 0.38) immobilization were observed in CF, because of the higher total organic carbon (34%) in the depth of 0-30 cm. The relative difference of NO3¯ immobilization between CF and OF was most pronounced, indicating a lower vulnerability of N losses through the pathway of denitrification and N leaching. The findings emphasize the importance of humus management in agricultural systems for soil gross N turnover processes. The advantages of mixing organic and synthetic fertilizers on CF soil buffer functions and long-term N storage are well established. Regular long-term observations of the gross N turnover processes are essential for a complete comprehension of the connections between C and N cycling in agricultural soils and a more effective nutrient management. Keywords - Organic farming, conventional farming, Isotopic pool dilution, Reformed difference method, Gross N turnover, Net N turnover, Green rye, maize, legumes.