Agricultural soil management to reduce N2O emission

Abstract

N2O is a greenhouse gas with a radiative forcing 298 times that of CO2 and is the substance that contributes most to the destruction of the stratospheric ozone layer. Current global N2O emission from the earth surface is 18.5 Tg y−1. Soil N2O emission is 13 Tg y−1, in which agricultural soil N2O emission accounts for 7 Tg y−1. N2O is produced as byproduct during nitrification process and interim product during denitrification process, mainly produced by denitrification in soils with increasing fertilizer application, as evidenced in agricultural fields. N2O emission increased significantly during the practices of fertilizer application, compost application, and harvesting, and peaked at a soil temperature of 20 °C or higher and a water-filled pore space of 60-70%. N2O emissions increased due to a decrease in soil pH. Soil pH tended to increase with the application of compost, and N2O in the compost plot decreased over time compared to the chemical fertilizer plot. This is because the optimal pH of incomplete denitrifying bacteria causing nitrous oxide emission is lower than that of complete denitrifying bacteria. Therefore, it is recognized that the pH should be 6.5 or more to suppress the N2O. In order to make better handling of compost, compost pellet has been developed, because it was found that nitrogen addition into compost pellet reduces N2O emission. N2O emission is related to the amount of inorganic nitrogen in soil. N2O emission increases with organic matter decomposition in soil and nitrogen fertilizer application. In organic soil (peat), huge N2O emission of several 100 kgN ha−1 y−1 was found in vegetable fields in a tropical peat land. Organic matter decomposition increases with drop of ground water table level, enhancing N2O emission. An eight-year oil palm plantation study at a tropical peat land showed that NO3- N concentration decreased and N2O emissions decreased with the increase in oil palm yield. The yield of oil palm increased with the increase of water-filled pore space of the soil and N2O significantly decreased when the water-filled pore space became more than 70%. Therefore, following three managements are recommended to reduce N2O emission from agricultural soil: 1) Restrict excessive nitrogen application (apply nitrogen according to the amount of nitrogen taken up by crops); 2) Prevent excessive organic matter decomposition (maintain capillary water rise without excessively decreasing ground water table level in peatlands); 3) Prevent low pH of soil (apply compost to reduce chemical nitrogen fertilizer application).