Quantifying Optimum Nitrogen Rates And Nitrogen Losses For Rice Production In Southern China Based On Statistical Model

As a significant achievement in developing grain production, China has managed to feed 22 percent of the world’s population, using only 7 percent of the world’s arable land. Application of chemical fertilizer, especially nitrogen (N) fertilizer, is one of the major contributing factors. However, in recent year, the production has to face the strong challenge from over-application of chemical fertilizers, diminishing crop return and low nutrient use efficiency. On the other hand, overuse of N fertilizer has already induced a continuum of environmental hazard, as acidification in major cropland, pollution in water bodies and increases in greenhouse gas emission, by harming off site ecosystem and threatening to human health. Cutting nitrogen fertilizer use to an optimum extent would keep crop production and reduce environmental risks from N overuse.Rice is China’s staple food.69% of the total rice harvest area and 68% of the national rice production are contributed by the provinces in southern China, including Shanghai, Jiangsu, Zhejiang, Anhui, Fujian, Jiangxi, Hunan, Hubei, Guangdong and Guangxi. However, the regional N application accounts for 74% of the country’s consumption to rice paddies. Quantifying economic and ecological optimum nitrogen rates, therefore evaluating the impacts on production and environmental benefits, would be contributed to improving performances on high efficiency, low energy consumption and low pollution.The objectives of this study are 1) to quantify N application rates from the perspective of maximum yield, economic and ecological optima on regional scale in southern China during the period of 1999 to 2008, and to evaluate rice yields and N loss under scenarios of current N application and reduced N supply; 2) to quantitatively address the fate of fertilizer nitrogen loss, so as to assess the mitigation of environmental problems induced by N cutting.To approach the objectives above, model approach was employed in this study. By searching publications, we extracted data to parameterize the models of rice yield, N uptake and loss associated with fertilizer N rate. Based on the equivalence between marginal cost and marginal benefit, we estimated the fertilizer nitrogen rate for maximum yields, economic and ecological optima, so as to discuss the effects of different N application scenarios on rice yield, fertilizer N uptake by plants, fertilizer N residual in soil and loss to environment. Adopting the emission factor method, we estimated the fertilizer losses as the formats of NH3, N2O, NO, runoff and leaching, so as to evaluate its impacts on environmental pressure under different N scenario.Results of this study are presented as follows:During the periods of 1999 to 2008, the annual fertilizer N application rates to rice paddy was 198 kg ha-1, on average, in southern China. The rates were 172kg ha-1 yr-1, 203kg ha-1 yr-1, and 313kg ha-1 yr-1 for the rice categories of the double indica rice, the single indica rice and single japonica rice, respectively. There was no significant difference in annual fertilizer N rate for the same rice category on provincial scale. The regional fertilizer N consumption was declined between 1999 and 2003 for the harvest area decrease rapidly in Shanghai and Zhejiang. After that, the total inputs kept on the level of 2003 with a slight fluctuation with N rate. The regional N use efficiency (including aboveground and root biomass) was 44%, N loss rate was 38% and N residual rate was 18%, with regional disparity. In terms of the N losses rate at a single province scale,47% and 46% were performed in Jiangsu and Shanghai, respectively, which was greater than other provinces. The rate of Anhui, Fujian, Jiangxi, Hubei and Hunan were marked lower than the regional level.Using the functions, the nitrogen fertilizer rate for maximum yield, economically optima and ecologically optima in southern China were estimated to be 126-236 kg ha-104-192 kg ha-1, and 69-115 kg ha-1’respectively, depending on rice subspecies, varieties and cropping systems. We identified higher N accumulation capacity and yield potential in hybrid indica than in conventional indica, and in single rice than in double rice.Overuse of N to rice paddy was widespread in research area during 1999-2008. Considering the management practices out of mind rather than N, the provincial N application rate on current level was higher than that estimated under the maximum yield scenario, but the yield was lower slightly. The scenarios of economic and ecological optimum N rate indicated than reducing N use by 38%-57% would decrease rice yield by 5.0%-10.9%, N loss rate by 4.8%-18.4%, but an increase in N use efficiency by 5.6%-6.7%. In southern China, the fertilizer N loss in the form of NH3, N2O, NO, leaching and runoff accounted for 18.0%,0.4%,0.04%,0.5% and 4.4% of N inputs, respectively on regional scale, during the period of 1999-2008. Jiangsu and Hunan Province were the top 2 ranked, but the former was caused by great N rate and the later by large harvest area. Under the N scenarios of maximum yield, economic optima and ecological optima, the reductions of fertilizer N losses were ranged between 25% and 60%。The amount of N-induced N2O emission was 24.3 Gg N2O-N annually through direct and indirect pathways from paddy field in southern China, in which 64.7% was emitted directly, and 29.4% was from the way of NH3 valuation indirectly. Jiangsu, Guangdong and Guangxi hold great GHG mitigation cased by N2O emission.In the view of fertilizer N utilization and environmental benefit, we quantified ecological optimum N rates and estimated the rates on regional scale based on the equitation between marginal cost and marginal benefit. This method was departed from those N applied for economic benefits previously; considering the different N scenarios, we evaluated fertilizer N application currently and provide N rate for economic and ecological optimum N rates; we estimated the amount of N loss in different pathways and assessed the environmental pressure mitigation under fertilizer N reduction on regional scale. This study considered fertilizer N application, N uptake by plant, N loss to environment and yield production comprehensively, thereby providing information for sensible N management to mitigate N-induced environmental problems.