近20年重庆市农牧生产体系氮素流动特征及驱动力分析

【目的】通过评价重庆地区农牧系统的氮素流动特征,明确氮素的主要损失途径及驱动因素,提出未来农牧生产中氮素管理措施,为实现重庆地区农牧系统氮养分的高效利用提供数据支撑。【方法】通过查阅文献及统计数据并结合调查研究,采用NUFER(nutrient flow in food chain,environment and resources use)评价方法,以重庆市农牧生产系统为主要研究对象,定量1996—2015年间农牧系统的氮素流动特征,明确其环境代价。在此基础上,探讨人类活动与农牧系统氮素流动特征变化之间的关系,并通过对2020年不同氮素管理情景分析,探索区域氮素的可持续利用途径。【结果】1996—2015年,重庆市农牧系统氮素输入总量增长19.2%,2015年达到1 006 Gg,其中化肥贡献57%以上;氮素输出总量增长16.5%,2015年达到844 Gg,环境损失为最大的氮素输出项,占输出量的61.7%左右,其中农田生产和畜禽养殖系统分别贡献45.1%和54.9%;农牧系统内氮素循环总量增长42.7%,2015年达到448 Gg,其中秸秆还田和畜禽粪尿还田氮量分别达到32.7和95.0 Gg,20年间分别增长55%和44%。1996—2015年重庆市蔬菜瓜果等经济作物种植比例从10%升至28%,粮食作物种植比例从82%降至62%,肉蛋奶等动物主产品输出量增加83%。农牧生产结构的变化与氮素利用和损失的变化呈现显著的线性正相关关系,但人均GDP(gross domestic product)的增长与氮素利用和损失的变化没有呈现明显的线性关系,表明影响系统氮素流动特征的主导因素应该是农牧生产结构变化,而经济发展水平的变化只是起到了刺激农牧产品消费的作用。通过情景分析,若在2020年实现《重庆市农业农村发展"十三五"规划》的农产品生产目标,保持目前的氮肥投入和氮素管理现状,将会增加18 Gg氮素损失,同时降低氮素利用率1.2%;通过提高系统内部氮素循环利用率,优化作物生产养分管理,可在实现生产目标的同时,分别减少氮肥投入和氮素损失量15%和4.2%,提高农牧系统氮素利用率1.3%以上。【结论】在近20年中,重庆市农牧生产系统氮素流动特征发生了很大变化,氮素输入强度和环境损失在不断增加,而农牧生产结构改变是其变化的主要驱动力。在重庆未来农牧生产中,减少畜禽生产系统氮素的直接排放,加大畜禽粪尿和秸秆循环利用力度,优化作物生产系统氮素管理措施和提高氮素利用效率,是实现农牧生产和生态环境的平衡发展的有效措施。

Nitrogen Flow of Crop-Livestock Production System and Its Driving Forces in Chongqing over the Past 20 Years

【Objective】 The objectives of this study are to evaluate the characteristics of nitrogen (N) flow in the crop-livestock system and analyze the main pathways of N loss and main drivers of the change in N flow in the crop-livestock system, put forward the effective N management measures, and to provide a powerful technical support for efficient N resource adoption in crop-livestock system in Chongqing.【Method】The crop-livestock system of Chongqing was defined as research case. A quantitative analysis of the N flow and loss and of their relationships with human activities from 1996 to 2015 was reported using the nutrient flow in food chain, environment and resources use model (NUFER). The data were mainly derived from literature, statistics and research reports. Pathways to a sustainable N use were explored through scenario analysis of different N managements in 2020.【Result】The total N input of crop-livestock system in Chongqing increased by 19.2% from 1996 to 2015. The amount of N input reached 1 006 Gg in 2015 and fertilizer consumption contributed 57% to it. The total N output of crop-livestock system in Chongqing increased by 16.5% from 1996 to 2015 and it has reached 844 Gg in 2015. Environmental loss was the largest part of N output in the crop-livestock system, which accounted for 61.7% of total output. Crop and animal production systems contributed 45.1% and 54.9% to the total N loss in 2015, respectively. The total amount of N cycle in the crop-livestock system increased by 42.7% and reached 448 Gg in 2015. The N amounts in manure and straw returning reached 32.7 and 95.0 Gg, respectively, which increased by 55% and 44% from 1996 to 2015. The cultivation proportion of vegetable and fruit to total sown areas increased from 10% to 28% from 1996 to 2015. Meanwhile, the percentage of cereals crops area dropped from 82% to 62%. In addiction，the export of main animal products increased by 83%, such as meat, milk and eggs. A significant liner relationship was observed between changes in N use and loss and changes in production structure, while the relationship between changes in N use and loss and changes in gross domestic production (GDP) was non-linear. It suggested that the dominant factor affecting N flow should be production structure, and GDP just played a role in stimulating consumption of crop and animal products. If the goal of agricultural production in the “13th Five-Year Plan for Agricultural Development in Chongqing Municipality” is fulfilled by 2020 with current N fertilizer inputs and N management practices, it will increase N loss by 18 Gg and reduce N use efficiency by 1.2% relative to the status of 2015. Scenario with increased the N cycling rate in the crop-livestock system and optimized nutrients management of crop production, indicated 15% and 4.2% reductions in N fertilizer input and N loss, respectively. It also would greatly increase N use efficiencies (NUE).【Conclusion】The characteristics of N flow in the crop-livestock system have been greatly changed over the past 20 years in Chongqing. Increased N input and total environmental loss were observed since 1996. The main driver for these changes was adjustment of production structure. Key measures for ensuring food production and minimizing environmental costs are (1) reducing direct emission of N from the livestock system to the environment, (2) enhancing the recycling of straw and manure, (3) optimizing N management and improving NUE in crop production system.