Nitrous Oxide Emissions and Methane Uptake from Organic and Conventionally Managed Arable Crop Rotations on Farms in Northwest Germany

Land-use extensification by shifting from conventional to organic arable farming is often discussed as a measure for reducing greenhouse gas (GHG) emissions from agricultural land. Doubts about the benefits arise when emissions are calculated per product unit, particularly where high yields are possible under conventional management. Among the non-CO2 GHG emissions, nitrous oxide (N2O) is the main contributor from arable land and is controlled by soil type, environmental conditions and management. In order to investigate how land-use change from conventional to organic farming would perform under highly productive site conditions in northwest Germany, and how this would affect the important greenhouse gases N2O and methane (CH4), an on-farm field research was conducted over two experimental years. Two site-specific organic crop rotations, (i) with 25% legumes (grass + clover-winter wheat-winter rye-oats) and (ii) with 40% legumes (grass + clover-winter wheat-winter rye-spring field peas-winter rye), were compared with (iii) a conventional arable rotation (winter oilseed rape-winter wheat-winter wheat-sugar beet-winter wheat) and two reference systems, (iv) extensive grassland and (v) a beech forest), which were chosen as the baseline. The results showed that organic farming had lower N2O emissions of 0.7 N2O-N ha(-1) year(-1) than the conventional rotation, with 2.1 kg N2O-N ha(-1) year(-1) (p < 0.05), but higher emissions than the extensive grassland (0.3 kg N2O ha(-1) year(-1)) and beech forest (0.4 kg N2O ha(-1) year(-1)). CH4 emissions were a negligible part of total GHG emissions (as CO2 equivalents) in the two arable systems, and considerable uptake of CH4 from the forest soils showed this was a GHG sink in the first experimental year. Organic systems produced up to 40% lower crop yields, but the emissions per product unit in rotation (iii) was not superior to (ii) during the two experimental years. Thus, arable organic farming showed the ability to produce agricultural commodities with low N2O emissions per unit area, and no differences in product-related emissions compared with conventional farming. Conventional and organic systems both showed potential for further mitigation of N2O emissions by controlling the field level nitrogen surplus to a minimum, and by the optimized timing of the removal of the grass-clover ley phase.