氮素管理的指标

用什么指标(indicators或indexes)来衡量不同尺度或生产体系氮素管理的优劣,是提高氮素管理水平的核心问题。近年来,国际上在氮素管理方面提出了许多新概念、方法和指标,以客观评价和改进氮素管理水平,为生产者和政策制定者提供评判标准。本文系统介绍了国际上氮素管理指标的概念、含义、计算方法及对结果的解析,包括:(1)氮素投入、收益与环境效应的关系;(2)氮素收支(budget)、氮素平衡(balance)和氮素盈余(surplus);(3)氮素利用率(Nitrogen Use Efficiency,NUE);(4)氮素利用率与氮素输入、输出及盈余的关系;(5)旱地作物收获后土体硝态氮允许残留量(Residual nitrate-N)。在此基础上,汇总了我国在不同尺度、不同土壤-作物体系氮素研究结果,初步建立了我国典型农田的氮素管理指标,为实现优良的氮素管理提供科学依据。

Indexes of Nitrogen Management

Indexes of Nitrogen Management JU Xiaotang1 GU Baojing2 (1 College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China) (2 Department of Land Management, Zhejiang University, Hangzhou 310058, China) Purpose: To assess the nitrogen (N) management in different production systems or under different scales, choosing which indicators or indexes is crucial question to be answered for improving the N management. Recently, many new concepts, methods and indicators have been proposed for the N management internationally, to objectively evaluate and improve the N management level, and to provide criteria for producers and policy makers. Although these concepts and methods have been applied to the N studies and management in China in some cases, the understanding of these concepts and methods, and how to make the calculation and interpret the results still lacks systemic knowledge. No indicators thus have been developed to guide the policy making and practical application for the N management in agricultural systems in China. Therefore, this paper introduces the concepts, definitions, calculations and result analysis of international N management. On the basis of this introduction, we attempt to integrate the results from N studies under different scales and in different soil-crop systems, summed up the N management indicators for typical crop systems, and provide a scientific basis for achieving the better N management. Method: This study conducted literature searches on the N management indicators, summarized the concept and calculation methods of main N management indicators, including N budget, N balance, N surplus, and N use efficiency (NUE), and allowable residual nitrate in upland crops soil after harvest, and analyzed the relationship between these metrics. Results: (1) The environmental cost is low if the N input-output of a production system within a reasonable limit under a certain technical and management level; the environmental cost increases rapidly with the increase of input after the net benefit reaching the maximum level; N inputs should be maintained within this limit. (2) N balance is an important indicator of N management and policy making, which is defined as the relationship between detail N inputs and outputs of a given system at a given time period; the excessive (surplus) or deficit (deficiency) N is calculated as the difference between N input and output on a per area basis; the concepts of N budget and N balance is different, and N budget is to calculate the detailed variations of N input, output, loss and system accumulation; N balance is divided into three categories: farm gate, soil surface, and soil system; A tiered approach is recommended to calculate the N input, output and balance from farm scale to country scale; N surplus indicators should be determined based on soil-climatic conditions, the type of ecological agriculture and environmental capacity. (3) Within a given boundary, N use efficiency (NUE) is defined as the ratio of input and output of N product: NUE = N products output / N input. (4) International community recommended to report N output and surplus simultaneously when reporting the NUE, and the relationship analysis of NUE, N output and N surplus can help to improve the N management and achieve a better goal; when NUE<1, N output is lower than input, the N that is not removed accumulates in the soil or enters the environment; when NUE>1, N removal is larger than N input, representing soil N depletion that would result in decline of soil fertility; the recommended NUE internationally is ranged from 0.5 to 0.9; harvested N is around 80 N kg hm-2 a-1, and the N surplus is also around 80 N kg hm-2 a-1; NUE> 0.9 indicates soil N depletion, and NUE <0.5 indicates low N us efficiency. (5) Soil inorganic N (mainly nitrate) in the root layer before sowing is often used as an effective indicator of soil N supply under rain-fed conditions, and residual nitrate in the root layer after harvest has been considered as a measure of leaching potential to assess the reasonableness of N management. (6) Most N balance studies are on field scale in China, and 80 and 100 kg N hm-2 a-1 N surplus could be the reference indexes of current production and management according to long-term accumulation of rich research data for the North China Plain wheat - maize rotation system and the Yangtze River Plain rice-wheat rotation system, respectively; the abovementioned reference index is based on a yearly rotation period, and the N surpluses should be set to 40, 40 and 60 kg N hm-2 if allocated to the major crops of wheat, corn, and rice in a single crop season; the allowable residual nitrate in the 0-1 m root layer should be lower than 100 kg N hm-2 after the harvest of wheat and corn in the North China. Conclusion: The good N management should control the N surplus and NUE within the target range, maximize the N recycling and minimize the diffusion of N to the environment. Developing and applying the index system of N management is aim to evaluate and optimize the N management and achieve the targeted productivity, environmental protection, soil fertility improvement, and sustainable use and succession of the environment. These indicators can measure the existing management and measures, and as a benchmark for future improvements. The policy-makers can use these indicators to formulate corresponding policies, and implement in the practices.