Impacts of natural factors and farming practices on greenhouse gas emissions in the North China Plain: A meta-analysis

Requirements for mitigation of the continued increase in greenhouse gas (GHG) emissions are much needed for the North China Plain (NCP). We conducted a meta-analysis of 76 published studies of 24 sites in the NCP to examine the effects of natural conditions and farming practices on GHG emissions in that region. We found that N₂O was the main component of the area-scaled total GHG balance, and the CH₄ contribution was <5%. Precipitation, temperature, soil pH, and texture had no significant impacts on annual GHG emissions, because of limited variation of these factors in the NCP. The N₂O emissions increased exponentially with mineral fertilizer N application rate, with y = 0.2389e^0.0058x for wheat season and y = 0.365e^0.0071x for maize season. Emission factors were estimated at 0.37% for wheat and 0.90% for maize at conventional fertilizer N application rates. The agronomic optimal N rates (241 and 185 kg N ha⁻¹ for wheat and maize, respectively) exhibited great potential for reducing N₂O emissions, by 0.39 (29%) and 1.71 (56%) kg N₂O-N ha⁻¹ season⁻¹ for the wheat and maize seasons, respectively. Mixed application of organic manure with reduced mineral fertilizer N could reduce annual N₂O emissions by 16% relative to mineral N application alone while maintaining a high crop yield. Compared with conventional tillage, no-tillage significantly reduced N₂O emissions by ~30% in the wheat season, whereas it increased those emissions by ~10% in the maize season. This may have resulted from the lower soil temperature in winter and increased soil moisture in summer under no-tillage practice. Straw incorporation significantly increased annual N₂O emissions, by 26% relative to straw removal. Our analysis indicates that these farming practices could be further tested to mitigate GHG emission and maintain high crop yields in the NCP.