Simultaneous measurement of net nitrogen mineralization and denitrification rates in soil using nitrification inhibitor 3,5-dimethylpyrazole

It is important to be able to quantify the different pathways in the soil nitrogen cycle to be able to understand the environmental performance of agricultural management practices. Two of the most important processes are soil nitrogen mineralization (the conversion of nitrogen from organic to plant available mineral form) and denitrification (the loss of mineral nitrogen to the atmosphere as a gas). Soil nitrogen mineralization is usually measured by incubating a soil and measuring the increase in mineral nitrogen over time. While this works for low and medium soil water content, it does not work at high soil water content as the mineral nitrogen may be lost from the soil by denitrification. In addition, current methods of measuring soil denitrification rates are quite time consuming and expensive, and are difficult to use for routine analyses. In this study we demonstrate a novel approach that allows both soil nitrogen mineralization and denitrification to be measured using a simple assay. The assay uses a nitrification inhibitor (3,5-dimethylpyrazole or DMP). The inhibitor blocks the mineral nitrogen produced by mineralization from being lost by denitrification so that both processes can be measured simultaneously. The assay provides a practical means to quantify the rates of nitrogen mineralization and denitrification simultaneously over a wide range of soil water contents. The assay can be readily scaled up to routinely test multiple soils in an efficient manner, has limited material costs, and is also relatively simple to perform.