Laboratory Incubations Reveal Potential N2o Emission Link to Decomposed CO2 and N Input.

Agricultural upland soil is either a potential source or sink of greenhouse gases (GHG) depend on the management practices of cropping and fertilization systems. While mitigation GHG through increased sequestration of carbon in soils by organic amendments applicationd, the trade-off with N₂O emission is an issue to concern. 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. We used a laboratory incubation approach to measure CO₂ and N₂O emissions from two Andisols managed under conventional fertilizer (CF) and long-term straw compost (SC) applications in central Japan. After 15 years of straw compost applying, the soil carbon content was 5.8% which was greater than the conventional fertilizer management (4.2%). The aim of this study was to assess the effects of long-term straw compost applying on soil organic matter decomposition (CO₂ emission) and N₂O emission in response to temperature and fertilizer-N addition.

Soil samples were incubated with and without fertilizer-N addition for 90 days after a 10-day pre-incubation period. Incubation was carried out at 15 ^oC, 25 ^oC, and 35^oC with soil water contents all adjusted to pF 1.5. For all treatments SC showed greater CO₂ and N₂O emissions per gram dried soil base. For example without fertilizer-N addition at 25 ^oC, the 90-day accumulated respired CO₂ was 2.3 times higher in SC than CF while N₂O emission was 3.8 times greater in SC than CF. We found potentially (i.e. without fertilizer-N addition) N₂O emission was significantly positive related to respired CO₂ due to more nitrogen mineralization following more decomposition of organic matter. Adding fertilizer-N did not change the total CO₂ emission but resulted in much greater N₂O emissions than without N addition. However, the timing of peak in N₂O emission was different between SC and CF. CF showed the peak of N₂O emission in the first week for all treatment temperatures, while N₂O emitted more slowing in SC. The results indicate long-term straw compost applying could have changed the soil carbon quality that may change the magnitude of soil N₂O emission in response to fertilizer-N addition.