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 (CO), main exchanges of non-CO carbon, and methane (CH) and nitrous oxide (NO) fluxes were continuously monitored throughout two year-round crop cycles (Y1 and Y2: 1 and 2 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 CO exchange and non-CO carbon exchanges by fertilization, seeding, and harvest) and greenhouse gas budgets (GHGBs: adding CH and NO fluxes to the NECBs based on CO equivalents) showed that the cotton cropping system persistently functioned as an intensive carbon (-1527 and -974 kg C ha yr) and greenhouse gas (GHG) source (5618 and 3591 kg CO-eq ha yr) because of the large CO emissions during the long fallow periods (5748 and 5160 kg CO ha 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 yr in Y2). Although high irrigation water and chemical fertilizer inputs stimulated NO emissions, the wheat-maize cropping system still behaved as an important GHG sink (-4257 kg CO-eq ha yr 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 CO-eq ha yr). 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.