Lustgasavgång från åkermark vid reglering av grundvattennivån

In a global perspective, the efficiency of nitrogen fertiliser use in agriculture is low. In Sweden the crop uptake of applied nitrogen fertiliser is about 70%. The largest losses of nitrogen from arable land are due to leaching, soil erosion and gaseous emissions. Controlled drainage is a method that aims to improve water and nutrient use efficiency. The technique is simple and involves the use of weirs in the collector drains and thus damming the outlet of the drainage system, in order to reduce the outflow. Controlled drainage has been shown to have large effects on the quantity and quality of drain outflow by reducing the transport of nitrogen and phosphorus. A stated disadvantage with the system is that the emission of laughing gas (N2O) from arable land might increase. The aims of this literature review are to synthesise results from previous studies on N2O emission from arable land, illuminate the effects of different management options on levels of N2O emission, elucidate lack of knowledge and need of additional research and, finally, estimate expected N2O emission and reduced nitrogen leaching if controlled drainage are to be introduced in Sweden. Production and emission of N2O depend on the water content in soil, available nitrogen and carbon, pH, temperature, diffusion rate and gas exchange with the atmosphere. Different management options affect N2O emission, directly and indirectly, by affecting the nitrogen content in soil and the gas exchange with the atmosphere. The N2O emission rates varies depending on cultivated crop, fertilisation management, such as timing, rate, type and combinations of different types as well as with different soil tillage, drainage and irrigation techniques. Management options that aim at reducing N2O emission should first of all be to create optimal growing conditions for the crop. The only way to reduce nitrogen losses from arable land is by an efficient use of applied nitrogen. It is possible to decrease N2O emission by adjusting management options, such as fertilisation and irrigation, to actual crop and soil type, timing of soil tillage and, when using controlled drainage adjusting the groundwater level to for the season optimal levels. In general, there is a lack of knowledge of how different cropping systems affect N2O emission and more research is needed on the relation between N2O emission and harvest. It is as yet not possible to estimate the effects of controlled drainage on direct and indirect N2O emission, due to lack of knowledge of the capacity of the soil to store, transport and reduce produced N2O. Results from previous research show that controlled drainage can reduce nutrient leaching. The reduction of nitrogen loads can be large if controlled drainage is introduced on suitable land of the coastal areas of Southern Sweden. With less nitrogen leaching it is possible to reduce nitrogen fertilisation rates and thereby reduce direct and indirect N2O emission. In this study, the effects of controlled drainage have been calculated only on indirect N2O emissions due to lack of measured data. The background emissions are assumed to redouble. Estimated increase in indirect N2O emission, if controlled drainage is to be introduced, represents a value of 1% of total N2O emission in Sweden per year