CH4 and N2O emissions increased following the conversion of aquaculture ponds to rice monoculture and rice–shrimp coculture fields in southeast China

In recent years, numerous aquaculture ponds in southeast China have been transformed into rice paddies or rice-shrimp fields. This shift in land use can potentially alter the biogeochemical cycling of carbon and nitrogen, thereby influencing CH₄ and N₂O emissions. However, the exact impacts and factors driving these changes remain unclear. Herein, a two-year field experiment was conducted to evaluate and compare CH₄ and N₂O emissions from shrimp ponds (SP), alongside reclaimed rice monoculture (RM) and rice-shrimp coculture (RS) fields that were converted from shrimp ponds. The findings showed that converting aquacultural wetlands to RM significantly increased annual emissions, with CH₄ rising dramatically from 103 to 490 kg/(ha·yr) (a 375.7% increase) and N₂O increasing from 4.22 to 7.39 kg/(ha·yr) (a 75.1% increase). However, further converting RM into RS notably reduced annual emissions, with CH₄ decreasing from 490 to 189 kg/(ha·yr) and N₂O from 7.39 to 4.32 kg/(ha·yr), corresponding to reductions of 61.4% and 41.5%, respectively. This agricultural land use change significantly impacted the reliance of CH₄ and N₂O fluxes on both biotic and abiotic variables across the three wetland systems, stemming from diverse agricultural practices. Furthermore, the scaled global warming potential (SGWP) and net ecosystem economic profit (NEEP)-SGWP of RM (24.1 t CO₂-eq/(ha·yr) and 125 kg CO₂-eq per $/(ha·yr)) were obviously higher than those of RS (9.66 t CO₂-eq/(ha·yr) and 4.76 kg CO₂-eq $/(ha·yr)) and SP (5.78 t CO₂-eq/(ha·yr) and 1.1 kg CO₂-eq per $/(ha·yr)), respectively. The results highlight that the conversion of aquaculture SP to RM and further to RS coculture can drastically reduce greenhouse gas emissions while enhancing economic benefits, thereby addressing environmental and profitability issues arising from the reclamation of SP.