New field technique to determine in-situ gross nitrification rates on an intact 4 m 2 scale on arable land

Nitrification is one major part of the terrestrial nitrogen cycle and is responsible for the N supply to microbes and plants. Furthermore, it opens N-loss pathways. Quantifying actual gross rates of nitrification is of growing interest due to the risk of nitrate-N leaching into groundwater. Gross nitrification measurements are often conducted either in disturbed soils or in small intact soil cores. Both approaches can have methodological issues. Our study presents a newly developed technique at an intact 2 × 2 m 2 field scale that was tested extensively on agricultural (sandy) soils. The irrigation technique allowed for a uniform distribution of 15 NO 3 − using a tracer solution. It further enabled a calculation of gross nitrification rates directly in the field. The gross nitrification rates within the 4 m 2 plots were highly variable. Individual plots showed gross nitrification rates between 3.9 and 17.9 µmol kg −1 soil d −1 . At the chosen meter scale, the dependency of the nitrification rate on environmental and soil parameters could be observed. Nitrification was influenced by the mean soil temperature during field incubation. Nitrification rates normalized for temperature (20 °C) showed a negative linear correlation with the C/N ratio of the plots (r 2 = 0.78).