The effect of reduced tillage on nitrogen dynamics in silt loam soils

Crop rotations in Western Europe contain crops that seem not suitable for reduced tillage (RT) agriculture because they often include beets and potatoes, resulting in a high disturbance of the soil at the formation of the ridges and at harvest. Therefore, the short- and long-term effects of RT on the stratification and stock of total nitrogen (TN) in the 0–40 cm depth layer and the nitrogen (N) mineralization in the upper 15 cm depth layer of silt loam soils was evaluated. For doing so, 10 fields at seven locations representing the important types of RT systems in Belgium, applied for a different number of years, and eight fields under conventional tillage (CT) with comparable soil type and crop rotation were selected. Despite the presence of root and tuber crops in these rotations, the stratification of the percentage of TN and of the carbon to nitrogen ratio was more pronounced under RT than CT fields. The TN stock in the RT fields worked with a cultivator or soil loosener was comparable to CT fields, even after 20 years. No trend could be found in the change in TN stock of reduced tillage by direct drilling compared to CT. The N mineralization rate in undisturbed soil cores under controlled conditions in the laboratory was on average 0.20 ± 0.08 mg N kg−1 dry soil day−1 for RT fields compared to an average of 0.13 ± 0.05 mg N kg−1 dry soil day−1 for CT fields. This increase in N mineralization rate was correlated with a higher microbial biomass nitrogen content.