摘要
CO2和N2O等温室气体引起气候变暖和臭氧层破坏是当今全球性的环境问题。农田土壤CO2和N2O排放及其对环境的影响越来越受到人们的重视。目前农林复合系统已被IPCC列为温室气体减排措施,但有关土壤CO2和N2O排放的研究主要集中在森林、草地及农田生态系统,农林复合系统土壤CO2和N2O排放的研究不多。本文采用静态箱-气相色谱法,以豆科紫穗槐和禾木科香根草绿篱复合系统为研究对象,研究了不同绿篱刈割枝叶管理方式下冬小麦—夏玉米轮作全生育期土壤CO2和N2O排放特征,分析评价了影响排放的主要因子,并初步估算了绿篱复合种植、作物单作及绿篱单作各系统的碳平衡,旨在进一步认识等高绿篱—农业复合系统生态环境效应,为合理估算我国陆地生态系统土壤CO2和N2O生成量、编制我国温室气体排放清单及制定相关管理政策提供科学依据。经过为期1年的研究,取得的主要结论如下：
(1)绿篱复合系统及作物、绿篱单作系统土壤CO2和N2O排放通量均呈明显的季节变化规律,表现为夏季排放量最高、春秋季次之、冬季最低。各处理土壤C02排放通量的变化范围为3.77～213.88 mg m-2 h-1,排放总量介于3417.59～6435.91kg C hm-2a-1之间；土壤N2O排放通量的变化范围为-3.84～92.11μg m-2h-1,排放总量介于0.49～1.33 kg N hm-2a-1之间。
(2)绿篱刈割枝叶还田显著增加了土壤CO2和N2O排放。冬小麦和夏玉米生育期,绿篱复合系统土壤CO2排放通量大小顺序均为：枝叶翻施还田>表施还田>移出小区,且紫穗槐复合系统的排放量高于香根草复合系统。冬小麦生育期,绿篱枝叶表施还田方式下的土壤N2O排放通量大于翻施还田；夏玉米生育期,枝叶翻施还田的排放通量要高于表施还田。农田转变为紫穗槐林地和香根草草地也显著增加了土壤CO2和N2O排放。
(3)土壤水热因子对土壤CO2和N2O排放的影响具有明显地季节性差异。冬小麦生育期土壤CO2和N2O排放通量与土壤温度呈显著或极显著正相关,土壤水分的影响居于次要地位；夏玉米生育期土壤CO2和N2O排放则主要受土壤水分的影响。
(4)土壤CO2平均排放通量与土壤有机碳、MBC均值和全氮含量呈显著正相关；但同一处理不同观测时期的CO2排放通量与对应的MBC之间的相关性较低。绿篱复合系统土壤N2O排放通量与土壤全氮和NO3--N含量呈显著正相关,与NH4+-N含量呈负相关,与土壤有机碳、C／N比及MBN的相关性较低；绿篱单作系统土壤无机氮与N2O排放通量无明显相关关系。
(5)土壤CO2和N2O排放与植物生长密切相关。冬小麦生育期,土壤CO2排放主要受小麦生长的影响,夏玉米生育期则受玉米和绿篱生长的共同作用。土壤N2O排放通量与作物生物量呈显著线性相关,同时绿篱种类显著影响了土壤N2O的排放。
(6)绿篱复合系统及作物、绿篱单作系统表层土壤CO2排放通量与土壤N2O排放通量呈显著或极显著的正相关关系。
(7)生态系统碳平衡计算结果表明：各系统碳汇源的强度范围介于-493.28～2380.23 kg C hm-2之间,冬小麦和夏玉米生育期的NPP/Rs范围分别为0.70～2.40和0.85～2.83。作物单作系统表现为大气CO2的“源”。绿篱进入农田改变了系统碳的“汇源”特征。香根草复合系统在冬小麦生育期为大气碳的“源”,夏玉米生育期则表现为大气碳的“汇”；紫穗槐复合系统在冬小麦和夏玉米生育期均为大气碳的“汇”,且其汇的强度大于香根草复合系统。农田转变为绿篱样地后有利于大气碳的固定,且香根草单作系统的碳汇强度大于紫穗槐单作系统。
CO2 and N2O has been paid great attention due to their substantial contribution to global warming and ozone depletion. It has suggested that agricultural soil is an important source of CO2 and N2O. Nowadays, many researchers have focused on demonstrating the soil CO2 and N2O fluxes of forests, grassland and farmland ecosystems, few report was found on trace CO2 and N2O fluxes from contour hedgerow intercropping system, one of the agroforestry patterns. We examined the effects of hedge prunings returning to field on the fluxes of soil N2O and CO2 under wheat-maize rotation in Amorpha fruticosa and Vetiveria zizanioides intercropping systems on a loamy clay soil, at Xianning, Hubei province, China. Soil CO2 and N2O fluxes were determined using the closed chamber-gas chromatography method. Some potential factors such as soil temperature, soil water, soil carbon, soil nitrogen, soil microbial biomass and plant growth were also measured during winter wheat-summer maize growth period in 2008-2009. The main results were as follows:
(1) Soil CO2 and N2O fluxes from different plots had obvious seasonal variation, with the highest in summer, higher in spring and autumn, and the lowest in winter. With the annual flux from 3417.59 to 6435.91 kg C hm-2 a-1, soil CO2 flux under different systems ranged from 3.77 to 213.88 mg m-2h-1. Soil N2O flux under different systems ranged from-3.84 to 92.11μg m-2 h-1, with the annual flux from 0.49 to 1.33 kg N hm-2 a-1.
(2) As expected, hedge prunings returning to field enhanced the release of soil CO2 and N2O. Significant difference in soil CO2 flux was observed among different hedge species prunings management practice, following the order of Incorporated-pruning> Surface-applied pruning> Removal of pruning, and the effect of A.fruticosa system was more obvious. The soil N2O flux of Incorporated-pruning was higher than Surface-applied pruning in winter wheat growing season, whereas the opposite trend was found during summer maize growth period. Furthermore, conversion from cropland to A.fruticosa forestland and/or Vetiver grassland might increase the release of soil CO2 and N2O to what extent.
(3) The influence of soil temperature and moisture on soil CO2 and N2O fluxes is significantly different with seasonal variation. During winter wheat growth period, soil CO2 and N2O fluxes were strongly exponentially correlated with soil temperature among all treatments, and the influence of soil moisture was of minor status. However, soil CO2 and N2O fluxes were significantly affected by soil moisture in summer maize growing season.
(4) Correlation analysis indicated that the average of CO2 flux was significantly positively correlated with soil organic carbon, average microbial biomass carbon and total nitrogen, but the relationship between soil microbial biomass carbon and soil CO2 flux under different growth stages was lower. Under hedgerow intercropping systems, the total nitrogen and soil NO3--N were significantly positive correlative with production of soil N2O flux, the opposite trend was observed for soil NH4+-N. However, no clear relationship existed between soil inorganic nitrogen and N2O flux in A.fruticosa forestland and/or Vetiver grassland. No significant dependence of N2O flux on the soil organic carbon, soil C/N ratio and soil microbial biomass nitrogen was observed among treatments.
(5) Correlation analysis showed that soil CO2 flux was mainly affected by wheat growth during winter wheat growth period. However, it was commonly influenced by the growth of maize and hedges in summer maize season. Soil N2O flux was well correlated with wheat biomass and maize biomass. It indicated that crop growth was one of the important factors influencing soil CO2 and N2O fluxes. In addition, different hedge species also remarkably influenced soil N2O flux.
(6) In A.fruticosa and/or Vetiver intercropping system, monoculture crop and monoculture hedge systems, there were significant linear relationships between soil CO2 flux and soil N2O flux.
(7) Analysis of carbon budget showed that the intensity of carbon sequestration or source ranged from-493.28 to 2380.23 kg C hm-2 in different systems. The ratio of NPP/Rs for winter wheat and summer maize growing season ranged from 0.70 to 2.40 and from 0.85 to 2.83, respectively. Hedger intervening farmland system changed its feature of carbon sink and source. Vetiver intercropping system acted as carbon source of the atmosphere CO2 over winter wheat growing season, whereas the opposite trend during summer maize growth stage. A.fruticosa intercropping system, which carbon sink strength was bigger than Vetiver intercropping system, was a carbon pool of the atmosphere CO2 throughout the year. Furthermore, the land use conversion from cropland to A.fruticosa forestland and/or Vetiver grassland might increase the carbon sequestration to a great extent, and the carbon pool was greater in Vetiver grassland than that in A.fruticosa forestland.
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