Net ecosystem carbon and greenhouse gas budgets in fiber and cereal cropping systems

To assess the contributions of fiber and cereal production on climate change, the net ecosystem exchange of carbon dioxide (CO₂), main exchanges of non-CO₂ carbon, and methane (CH₄) and nitrous oxide (N₂O) fluxes were continuously monitored throughout two year-round crop cycles (Y1 and Y2: 1^st and 2^nd year-round crop cycles, respectively) using eddy covariance, biometric observation, and static chamber methods in typical cotton and wheat-maize rotational cropping systems in China. The evaluation of net ecosystem carbon budgets (NECBs: considering net ecosystem CO₂ exchange and non-CO₂ carbon exchanges by fertilization, seeding, and harvest) and greenhouse gas budgets (GHGBs: adding CH₄ and N₂O fluxes to the NECBs based on CO₂ equivalents) showed that the cotton cropping system persistently functioned as an intensive carbon (-1527 and -974 kg C ha^-1 yr^-1) and greenhouse gas (GHG) source (5618 and 3591 kg CO₂-eq ha^-1 yr^-1) because of the large CO₂ emissions during the long fallow periods (5748 and 5160 kg CO₂ ha^-1 in Y1 and Y2, respectively). The wheat-maize cropping system had high net ecosystem production (NEP) and low harvest index and therefore, served as a notable carbon sink (1461 kg C ha^-1 yr^-1 in Y2). Although high irrigation water and chemical fertilizer inputs stimulated N₂O emissions, the wheat-maize cropping system still behaved as an important GHG sink (-4257 kg CO₂-eq ha^-1 yr^-1 in Y2) because of the tremendous net carbon sequestration. However, in Y1 incidental wind damage lowered the NEP and turned the wheat-maize cropping system into a GHG source (2144 kg CO₂-eq ha^-1 yr^-1). The NEP, NECBs, and GHGBs of the double cropping system generally exceeded those of the single cropping system. The traditional rotation between double and single cropping systems should be restored to maintain soil carbon storage and alleviate the radiative forcing effects of cotton production.