Grassland renovation increases N2O emission from a volcanic grassland soil in Nasu, Japan

To investigate the effects of renovation (ploughing and resowing) on nitrous oxide (N2O) emissions from grassland soil, we measured N2O fluxes from renovated and unrenovated (control) grassland plots. On 22 August in both 2005 and 2006 we harvested the sward, ploughed the surface soil and then mixed roots and stubble into the surface soil with a rotovator. Next, we compacted the soil surface with a land roller, spread fertilizer at 40 kg N ha(-1) on the soil surface and sowed orchardgrass (Dactylis glomerata L., Natsumidori). In the control plot, we just harvested the sward and spread fertilizer. We determined N2O fluxes for 2 months after the renovation using a vented closed chamber. During the first 2 weeks, the renovated plot produced much more N2O than the control plot, suggesting that N was quickly mineralized from the incorporated roots and stubble. Even after 2 weeks, however, large N2O emissions from the renovated plot were recorded after rainfall, when the soil surface was warmed by sunshine and the soil temperature rose 2.7-3.0 degrees C more than that of the control plot. In 2005, during the 67-day period from 19 August to 26 October, the renovated and control plots emitted 5.3 +/- 1.4 and 2.8 +/- 0.7 kg N2O-N ha(-1), with maximum fluxes of 3,659 and 1,322 mu g N2O-N m(-2) h(-1), respectively. In 2006, during the 65-day period from 21 August to 26 October, the renovated and control plots emitted 2.1 +/- 0.6 and 0.96 +/- 0.42 kg N2O-N ha(-1), with maximum fluxes of 706 and 175 mu g N2O-N m(-2) h(-1), respectively. The cumulative N2O emissions from plots in 2005 were greater than those in 2006, presumably because rainfall just after renovation was greater in 2005 than in 2006. These results suggest that incorporated roots and stubble may enlarge the anaerobic microsites in the soil in its decomposing process and increase the N2O production derived from the residues and the fertilizer. In addition, rainfall and soil moisture and temperature conditions during and after renovation may control the cumulative N2O emission.