Greenhouse gas exchange and comprehensive global warming potential under different wheat-maize rotation patterns

Studies on the emissions of greenhouse gases and global warming potential (GWP) under different tillage systems have benefited scientific research on the effects of agricultural management on mitigating greenhouse gas emission and reducing global warming. Such studies have also laid the theoretical basis for establishing measures to reduce global greenhouse gas emissions. Long term tillage and straw return to soil experiments were set up in 2001 at the Luancheng Agro-ecosystem Experimental Station (LAES) of Chinese Academy of Sciences. The experiments included 5 treatments — no-tillage with whole maize residue mulching (M1), no-tillage with chopped maize residue mulching (M2), rotary tillage with chopped maize residue incorporation (X), mouldboard ploughing with chopped maize residue incorporation (F) and mouldboard ploughing with maize residue remove (CK, representing conventional tillage method). The experiment monitored N₂O, CO₂ and CH4 fluxes in wheat-maize rotation fields using the static chamber method / gas chromatography technique for the period from October 2008 to September 2010. Total greenhouse gas emissions and GWP were also estimated. Meanwhile, during the experimental period, the amount of fuel consumed by farm machines and power consumed during irrigation and fertilizer application were recorded and transformed to carbon equivalent using a transformation coefficient. In the study, crop yield and aboveground biomasses were measured and carbon sequestration calculated. The total GWP under the 5 tillage treatments were estimated based on the identified parameters of greenhouse effect. The results indicated that wheat-maize rotation fields served as the source of N₂O and CO₂, and also the sink of CH₄. In M1, M2, X, F and CK treatments, total N₂O emissions from soil were 2.06 kg(N₂O-N)?hm2?a1, 2.28 kg(N₂O-N).hm^-2.a^-1, 2.54 kg(N₂O-N).hm^-2.a^-1, 3.87 kg(N₂O-N).hm^-2.a^-1 and 2.29 kg(N₂O-N).hm^-2.a^-1; total CO₂ emissions from soil of 6 904 kg(CO₂-C).hm^-2.a^-1, 7 351 kg(CO₂-C).hm^-2.a^-1, 8 873 kg(CO₂-C).hm^-2.a^-1, 9 065 kg(CO₂-C).hm^-2.a^-1 and 7 425 kg(CO₂-C).hm^-2.a^-1; and total CH₄ sink of 2.50 kg(CH₄-C).hm^-2.a^-1, 1.77 kg(CH₄-C).hm^-2.a^-1, 1.33 kg(CH₄-C).hm^-2.a^-1, 1.38 kg(CH₄-C).hm^-2.a^-1 and 1.57 kg(CH₄-C).hm^-2.a^-1, respectively. GWPs in M1 and M2 treatments were negative, which indicated that farmland ecosystems under no-tillage with straw served as carbon sink, with annual carbon retention of 947–1 070 kg(C).hm^-2 after subtracting directly or indirectly carbon equivalent emitted from the system. GWPs for other treatments were positive, with GWPs for CK, F and X of 3 364 kg(C).hm^-2, 989 kg(C).hm^-2 and 343 kg(C).hm^-2, respectively. This suggested that for wheat-maize rotation system in the North China, chopped crop residue incorporation with rotary tillage was optimal tillage practice with relatively lower greenhouse effects and higher grain yield.