Effects of Organic Matter Applications On Carbon Sequestration and GHG Emissions in An Upland Andisol in Japan.

Soil carbon sequestration in agricultural lands has been deemed a sustainable option to mitigate rising atmospheric CO₂ levels through improved management practices such as organic matter applications. Long-term organic matter applications not only increase soil organic carbon but also change organic matter quality such as increasing labile carbon proportion. This may lead to changes in nitrogen mineralization and so as the emission of N₂O. Therefore, while mitigation GHG through increased sequestration of carbon in soils by organic matter applications, the trade-off with N₂O emission is an issue to concern. We measured CO₂ and N₂O emissions from an upland Andisol managed under and long-term rice straw compost (RSC) application in central Japan. The purpose of this study was to quantify potential tradeoffs in GHGs mitigation of organic matter application while compared to the conventional fertilizer (CF) management.

The study site is located at central Japan and the RSC field has been continually applied for rice straw compost (72.5 Mg F.W./ha, about 5.1 Mg C/ha and 275 kg N/ha) in late July since 1995. The sweet corn grew from May to July with 250 kgN/ha of chemical fertilizer addition. After RSC applying, sorghum was planted as green manure from August to November, and winter wheat was planted as green manure from November to April. All green manures and corn residue were incorporated into the soil. The GHG fluxes were measured everyday (9:00-9:30, July 2011 to July 2013) using two offline automatic gas sampling systems attached to 9 automatic chambers. Each 3 chambers used for RSC, RSC_blank (no RSC and residue incorporating) and CF treatments.

Following the RSC application and green manure incorporation, soil carbon content has increased to 5.8% which was greater than the CF management (4.2%). RSC plot showed a slight higher CO₂ emission (including organic matter decomposition and root respiration) than CF plot (403.2 and 326.0 kg C m^-2 period^-1, respectively). Meanwhile, N₂O emission from RSC plot was significantly higher than CF plot, e.g. about 2 to 3 times higher during July to October (56.1 and 23.0 mg N m^-2 period^-1, respectively). N₂O emission from RSC seemed negligible; however, long-term RSC application could have changeg the magnitude of soil N₂O emission in response to rainfall events. Significantly greater N₂O emitted accompanying corn residue and green manure incorporation, and also resulted in large variation in annual N₂O emission. This study highlights the importance of taking annual variability into account for quantifying organic matter managements on potential tradeoffs in soil carbon sequestration and other GHG emissions.