Abstract:

The services provided by croplands have been underestimated. Besides providing food and fiber, croplands provide many other functions and services, such as nutrient transformation, pollination, biological control, beautiful scenery, recreation, and so on. In this study, gas regulation services provided by fertilized and unfertilized rice (Oryza sativa) paddy ecosystems and their values were investigated based on field experiments conducted on the Wusi Farm in Fengxian County, Shanghai in 2002. Paddy fields regulated two types of gases: O₂ emissions and greenhouse gases (GHGs), including CO₂ uptake and CH₄ and N₂O emissions.
At elongation stage, booting stage, panicle initiation stage and maturation stage, the biomass of fertilized and unfertilized paddy fields were determined and O₂ emissions calculated using the photosynthesis formula based on plant biomass. The maximum O₂ emissions from fertilized and unfertilized paddy fields were 770.15 and 506.62 kg·hm^-2·d^-1, respectively. Compared to the price of industrial O₂ and the cost of afforestation in China, their values were calculated as 190.72 and 289.93 yuan·hm^-2·d^-1.
On August 15, 18, 21 and 25, and September 2, 8, 14 and 22, the gases from paddy fields were sampled and their fluxes determined. The results showed that the fluxes of CH₄ from fertilized and unfertilized paddy fields ranged from 4.86 mg·m^-2·h^-1 to 14.96 mg·m^-2·h^-1 and from 3.34 mg·m^-2·h^-1 to 50.67 mg·m^-2·h^-1, respectively. For N₂O, they ranged from 0.008 6 mg·m^-2·h^-1 to 0.056 6 mg·m^-2·h^-1 and from 0.000 6 mg·m^-2·h^-1 to 0.014 9 mg·m^-2·h^-1, and for CO₂ from 347.63 mg·m^-2·h^-1 to 730.20 mg·m^-2·h^-1 and from 205.06 mg·m^-2·h^-1 to 679.51 mg·m^-2·h^-1, in fertilized and unfertilized paddy fields, respectively. Using a CO₂ equivalent, the integrated GHGs fluxes were computed as -1.42-39.71 kg·hm^-2·d^-1 CO₂-C (fertilized) and -50.56-25.60 kg·hm^-2·d^-1 CO₂-C (unfertilized). Based on the average cost of the Sweden carbon tax and afforestation in China, the economic value of GHGs regulation was calculated. The results indicated that the valuation of CO₂ absorption by fertilized paddy fields was not much more than that by unfertilized ones. Second, urea amendment aggravated the negative values of N₂O emissions from paddy fields. Third, the negative values of CH₄ emission from paddy fields were mitigated by urea amendment. And fourth, the valuation of GHGs regulation by fertilized fields was promoted by urea amendments. In this study, we were trying to provide a fair method to evaluate the values of croplands to support an effective way to recognize the benefits derived from croplands, especially by paddy fields, and to promote the sustainability of cropland ecosystems.

Key words: Gas regulation, Paddy fields, Ecosystem services, Value assessment, Urea amendment