Review on greenhouse gas emission and reduction in wheat-maize double cropping system in the North China Plain

The winter-wheat and summer-maize double cropping system in the North China Plain (NCP) is the classic intensive crop production pattern with high water demand and nitrogen fertilizer inputs. The carbon (C) emission quantities are higher than the carbon sequestration quantities in the cropping system. C was being lost from the intensive wheat-maize double cropping system in the NCP at a rate of 77 g(C)·m^-2·a^-1 when harvest removals are considered, even though crop residue C is input into the soil since 30 years ago. High nitrogen (N) fertilizer application rate>400 g(N)·hm^-2·a^-1 results in the increase of C emissions directly. Yield-scaled N₂O emission is lowest at N application rate of 136 g(N)·hm^-2·a^-1. And it is found that maximal crop yield is achieved at a N application rate of 317 g(N)·hm^-2·a^-1, which is 20% less than current practice. More than 90% of the annual cumulative greenhouse gas (GHG) fluxes originated at soil depths shallower than 90 cm. The subsoil (>90 cm) is not a major source or sink of GHG, but it acts as a 'reservoir'. Considering the synthetic greenhouse effect, some measures of greenhouse gas reductions were put forward in papers such as reductions of fertilizer input and water supply and improving farming system (tillage reduction or zero tillage). Furthermore C reduction needs to be in step with C sequestration. In the future, studies on greenhouse gas emissions in NCP require to be further strengthened in the following aspects:1) in-situ continuous online monitoring of canopy scale greenhouse gases, and using stable isotope techniques to track their sources and proportions; 2) in soil profile, using stable isotope techniques to study the sources and proportions of greenhouse gases, and exploring the responding mechanism between greenhouse gas production/consumption in soil profile and their emissions at soil surface is fairly crucial; 3) using models to estimate greenhouse gas emissions of soil-atmosphere continuum.