Estimation of nitrogen losses via denitrification from a heavy clay soil under grass

Denitrification is an important pathway for nitrogen (N) loss from agricultural soils, but measured field data on denitrification in clay soils are scarce. The objective of the present research was to obtain and compare three different approaches to estimate the annual N loss via denitrification from a heavy clay soil. Using the acetylene inhibition technique we measured denitrification monthly at 20 °C in intact soil cores of a clay soil under intensively managed grass over a period of 2 years. This approach resulted in estimates of N loss via denitrification of 127 kg N ha-1 year-1 in 2003 and 143 kg N ha-1 year-1 in 2004. We also measured potential denitrification. These data were used in combination with independently calibrated 'reduction functions' to correct for suboptimal soil nitrate contents, water filled pore space or soil temperature. This approach resulted in estimates of 143 and 325 kg N ha-1 year-1 for 2003 and 2004, respectively. These measurements furthermore indicated that 75% of the denitrification occurred in the upper 20 cm of the soil. Estimates of denitrification based on field balances, including measurements of leaching losses to ground and surface waters, were 152 kg N ha-1 year-1 for 2003 and 5 kg N ha-1 year-1 for 2004, when storage of nitrogen was assumed negligible. While all three different estimation methods have considerable uncertainty, they invariably lead to the conclusion that N losses via denitrification in this intensively managed clay soil are high (more than 79 kg N ha-1 year-1). Measurement of the actual denitrification gave the most consistent and least uncertain estimates and accordingly we concluded that direct measurement of the actual denitrification should be preferred despite it's theoretical shortcomings. This implies that about 25% of the N applied to the field as fertilizer and manure is lost to the environment. In this heavy clay soil 90% of this loss proceeds via denitrification, and 10% proceeds via leaching and drainage.