黄土高原干旱半干旱区退耕还草对N2O和NO排放的影响

Soil-atmosphere exchanges of nitrogen oxides (N2O and NO) respond to land use change, one of the major human activities that are deeply concerned contributing to global warming and regional atmospheric pollution. "Grain for green" project has been conducted in Loess Plateau during the past two decades, in which huge amounts of degraded arable soils were restored to forest and grassland. However，previous researches on biological consequences of this massive campaign of land use change have been focusing on restoring succession of soil and vegetation rather than soil emissions of N2O and NO. How and to which extent the restoration age and method impact on N2O and NO emissions remain unclear in this temperate arid and semi-arid region. This proposal would measure in situ annual N2O and NO emissions from rain-fed winter wheat fields, restored native grassland and seeded forage grassland (alfalfa) on retired cropland with the same and/or similar soil management history, together with related climate, soil and vegetation dynamics, in Loess Plateau during a three-year period. To interpret the field data, studies of soil incubation experiment with 15N isotope dilution technique for detecting gross mineralization and nitrification and acetylene block technique for detecting denitrification, and mathematic model simulation as well, would also be carried out. The aims are to (1) identify spatial and temporal variations in N2O and NO emissions under different land use and underlying mechanisms, (2) quantify the effect of age and method of grassland restoration on N2O and NO emissions and related enviromental factors, and subsquently (3) assess the biological consequences of grassland restoration in Loess Plateau as one of human efforts to reclaim degraded soil and vegetation.

土地利用变化是驱动土壤氮素痕量气体N2O和NO排放发生改变的重要人为因素，对全球增暖和大气污染产生深刻的影响。"退耕还林还草"工程已在黄土高原推广近20年，关于退耕还林还草生态功能的研究大多以土壤和植被恢复为主，很少涉及N2O和NO排放。不同退耕年限和退耕方式在多大程度上影响N2O和NO排放仍然是全球变化研究中亟需回答的科学问题。本项目拟以黄土高原干旱半干旱区冬小麦田和具有相同或相似管理背景且退耕1-3、5-7和10-12年的撂荒草地和人工种植苜蓿草地为研究对象，连续三年观测N2O和NO排放通量及气候、土壤和植被的周年动态变化，结合土壤氮素总矿化、硝化和反硝化速率的测定以及数学模型模拟，揭示不同土地利用方式下N2O和NO排放的变化规律及发生机制，定量评估不同退耕年限和还草方式对N2O和NO排放及相关环境变量的影响，从氮素气体排放的角度探讨退耕还草在黄土高原土壤和植被恢复过程中的生态功能。

氧化亚氮（N2O）不仅是重要的温室气体，还是破坏平流层臭氧的光化学反应底物。一氧化氮（NO）具有高度的光化学活性，是对流层臭氧浓度升高的主要前体物；NO在大气中的最终产物为硝酸盐，是大气颗粒物质和酸雨的主要成分。大气中约62%的N2O排放来自于土壤；NO主要排放源为工业和汽车尾气排放，但土壤释放的NO对区域大气环境质量仍有显著影响。土壤N2O和NO主要由微生物硝化、反硝化过程产生，微生物活动受气候条件、土壤性质和田间管理措施的共同影响，造成N2O和NO排放的时空异质性，给准确计算土壤气体排放量及评估管理措施的减排效率带来极大困难。本项目的主要目标是揭示黄土区典型农田和退耕草地N2O和NO排放特征及其主要影响因子，定量分析有机肥替代化肥、退耕年限和还草方式对气体排放及相关环境变量的影响。利用静态暗箱法对农田和草地土壤N2O和NO排放通量进行三年（2015-2018）观测。农田试验采用随机区组试验设计，设置对照、全化肥和化肥加牛粪处理，草地试验设置撂荒和人工苜蓿草地。农田N2O和NO排放高峰主要出现在施肥和灌溉后；当土壤温度高于20℃时，施肥处理NO/N2O比和土壤孔隙含水量（WFPS）呈现极显著负相关关系（r2>0.50, P<0.01），说明温度和土壤水分含量可调节硝化、反硝化作用的相对重要性。施肥处理N2O和NO年排放量变化范围分别为2.15-2.9和0.34-1.46 kg N ha-1，施用牛粪处理N2O排放较全化肥处理偏高但差异不显著，施用牛粪处理NO排放显著高于全化肥处理。综合分析各处理长期（25-50年）土壤有机碳（SOC）含量和N2O排放的变化，表明有机肥替代化肥可能不是减少温室气体排放的有效途径；建议通过降低反硝化过程产生的N2O排放来应对SOC固定和N2O排放之间的消长关系。NOE模型能够有效模拟农田土壤反硝化速率和N2O排放，但模型应用于不同气候区时，必须考虑土壤温度对反硝化速率的影响。退耕草地N2O和NO年排放量极低（变化范围0.01-0.3 kg N ha-1），与施肥农田处理的结果相比几乎可以忽略不计，主要原因是土壤氮底物浓度低且缺乏季节、年际间变化，限制了微生物活动和气体排放。