Effects Of Land-use Conversion From Double-rice To Vegetable Cultivation On Net Ecosystem Carbon Budget And Greenhouse Gas Emissions

In recent years,the proportion of vegetables in diets is increasing with the improvement of people's living standards.Given the decline in profitability of traditional rice cultivation,the increasing demands and economic benefits from vegetables result in a considerable share of rice paddy fields conversion to vegetable production,becoming a prevailing and common agricultural practice in China.Although such land-use conversion gives high economic returns,some environmental problems are also emerged.Vegetable cultivation is generally characterized by higher nitrogen fertilizer inputs and much more intensive tillage in comparison with rice cultivation.Such land management change potentially alters soil physiochemical properties,biological processes and associated C and N cycles,with implications for soil fertility and greenhouse gas?GHG?emissions.However,little is known on net ecosystem carbon budget?NECB?and GHG balance in response to this land-use conversion,especially at different time scales after conversion.It is therefore necessary to critically evaluate the impacts of land-use conversion from rice to vegetable cultivation on NECB and GHG emissions,with respect to attaining high economic benefits while devising appropriate strategies for soil fertility maintenance and GHG mitigation.Six adjacent rice paddy plots,being under double-rice cultivation?early rice–late rice–winter fallow?for more than 100 years,were selected at the Changsha Research Station for Agricultural&Environmental Monitoring of the Chinese Academy of Sciences in Changsha City,Hunan Province,China.Three rice paddy plots were randomly chosen for drainage and conversion to vegetable production plots?Veg?after late rice harvest in October 2012,and 3 plots remained in rice production?Rice?.Each of the Rice and Veg plots were divided into two subplots:one for no nitrogen fertilization?Rice-N⁰ and Veg-N⁰?,and the other for conventional nitrogen fertilization?Rice-N⁺and Veg-N⁺?,with three replicates for each treatment.The components of NECB,fluxes of CO₂,CH₄ and N₂O,and related environmental factors from the rice paddy and converted vegetable fields were determined over the 4-year study period.The main results of this study are as follows:1.Rice paddy conversion to vegetable cultivation strengthened the impact of soil temperature on soil organic matter mineralization,thus enhancing soil C mineralization process.This land-use conversion led to substantial C losses(2.6 to 4.5 Mg C ha^-1yr^-1),due to reduced C input by 44%-52%and increased soil organic carbon mineralization by 46%-59%as compared to Rice.The magnitude of C losses from the converted vegetable field was highest in the first year,and showed a decreasing trend in the following years.Nitrogen fertilization shifted rice paddy from a slight C source in Rice-N⁰ to a significant C sink in Rice-N⁺,and alleviated the impact of rice conversion to vegetable cultivation on soil C losses via increased C inputs from higher crop productivity.2.CH₄ fluxes from Rice exhibited a distinct seasonal variation pattern,with peak values occurring during rice growing seasons.CH₄ emission during rice growing seasons contributed 94%-99%of the annual CH₄ emission from Rice over the 4 years.Soil ammonium content,soil temperature and moisture were the main factors regulating CH₄fluxes from Rice,together explaining 70%of the temporal variation in CH₄ fluxes over the study period.Annual CH₄ emissions from Rice were positively related to net primary production?NPP?across the 4 years,suggesting that NPP could partially explain the inter-annual variation in CH₄ emission from rice paddy.The increased NPP facilitated additional C input for CH₄ production and the pathway for CH₄ emission from rice paddy.3.Land-use conversion from rice to vegetable cultivation substantially reduced CH₄emission from Veg by 96%-97%as compared to Rice over the study period.Annual CH₄emissions from Veg were significantly higher in the first year relative to any later years,suggesting that land-use legacy has strong effect on CH₄ emission.Nitrogen fertilization had no impact on CH₄ emission from both Rice and Veg.4.Land-use conversion from rice to vegetable cultivation led to substantial N₂O emission,particularly in the first year,regardless of nitrogen fertilization.The variability in annual N₂O emission from Veg was closely linked to NPP variation.N₂O emission from Veg mainly occurred in summer seasons when soil temperature was above 20?,where the natural logarithm of the ratio of N₂O flux to NO₃^--N was significantly positively related to WFPS,suggesting that denitrification process was the dominant pathway for N₂O production.N₂O fluxes were significantly and positively related to soil heterotrophic respiration rates?R_h?in Veg-N⁺among the 4 years,and the dependence of N₂O fluxes on R_h was greater in the first year relative to the subsequent three years.These results suggested that soil organic matter and N mineralization contributed to N₂O emissions in the context of land-use conversion from rice to vegetable production.Therefore,it suggests that guidelines for N₂O emission factors from agricultural soils should be reevaluated,particularly for the effects of lowland conversion to upland cropping systems on SOM mineralization contribution to N₂O production.5.Land-use conversion from rice to vegetable cultivation significantly increased the global warming potential?GWP?of Veg by 116%-395%relative to Rice in the first year,primarily due to the increased C losses and N₂O emissions far outweighing the decreased CH₄ emission.In contrast,the GWPs of Rice were similar to those of Veg in the following years after conversion,because of the decreased CH₄ emission fully offsetting the increased C losses and N₂O emission.Nitrogen fertilization increased the contribution of CH₄ to,but had little effect on,the overall GWP of Rice.Nitrogen fertilization and land-use conversion interactively increased the GWP in the first year via increased N₂O emission,contributing to the largest GHG emission from Veg-N⁺.These results indicated that land-use conversion from rice to vegetable cultivation had significant impacts on the overall GWP only at the initial stage upon conversion.In summary,land-use conversion from rice to vegetable cultivation led to substantial C losses,with the magnitude of C losses from the converted vegetable field decreasing yearly.Nitrogen fertilization alleviated the impact of rice conversion to vegetable cultivation on C losses.This land-use conversion significantly increased N₂O while decreasing CH₄ emissions over the whole study period,but increased the overall GWP only in the first year upon conversion.The variability in annual CH₄ emission from Rice and N₂O emission from Veg was closely linked to NPP variation.Soil organic matter and N mineralization significantly contributed to N₂O emissions from Veg.This study suggests that soil C budget and GHG emissions in the first year upon conversion are the most important and therefore,should be considered when evaluating the environmental consequences of land-use conversion.This study also helps us develop effective options to alleviate the effects of land-use conversion on soil C losses and GHG emissions,and for sustainable agricultural production and GHG mitigation.