利用紫色土养分循环长期定位研究平台,通过静态箱-气袋采气-化学发光氮氧化物分析仪观测了紫色土夏玉米-冬小麦轮作周期的NO周年排放特征。结果表明,施肥促发紫色土NO的峰值排放,玉米与小麦季的土壤NO排放峰值均出现在施肥后20d内。常规夏玉米-冬小麦轮作条件下NO年累积排放通量为0.450 kg N·hm-2,其中玉米季的NO累积排放通量为0.200 kg N·hm-2,排放系数为0.13%,小麦季的NO累积排放通量为0.250 kg N·hm-2,排放系数为0.20%。玉米季的NO排放速率与土壤表层温度呈指数响应关系,小麦季与温度关系不明显。土壤湿度对于玉米季常规施肥条件下NO的排放有显著的响应关系。种植作物的土壤较裸地的NO排放通量都有不同程度的降低。
采用原状土柱-乙炔抑制培养法研究了施肥对紫色土玉米生长季土壤N2O排放通量和反硝化作用的影响。结果表明:玉米季施肥显著增加土壤N2O排放和反硝化损失,同时,各施肥处理间N2O排放与反硝化损失量差异显著。猪厩肥、猪厩肥配施氮磷钾肥、氮肥、氮磷钾肥和秸秆配施氮磷钾肥等处理的土壤N2O排放量分别为3.01、2.86、2.51、2.19和1.88 kg hm-2,分别占当季氮肥施用量的1.63%、1.53%、1.30%、1.09%和0.88%,反硝化损失量分别为6.74、6.11、5.23、4.69和4.12 kg hm-2,分别占当季氮肥施用量的3.97%、3.55%、2.97%、2.61%和2.23%,不施肥土壤的N2O排放量和反硝化损失量仅为0.56和0.78 kg hm-2。施肥是紫色土玉米生长前期(2周内)土壤N2O排放和反硝化速率出现高峰的主要驱动因子,土壤铵态氮和硝态氮含量是影响土壤N2O排放、土壤硝化和反硝化作用的限制因子,土壤含水量是重要影响因子,降雨是主要促发因素。土壤N2O排放量与反硝化损失量的比值介于0.45～0.72之间,土壤反硝化损失量极显著高于土壤N2O排放量,说明土壤反硝化作用是紫色土玉米生长季氮肥损失的重要途径。
采用静态暗箱-气相色谱法对紫色土坡耕地常规施肥处理(CL)、坡耕地不施肥处理(CL-CK)和退耕15、30年的桤柏混交林地(FL15、FL30)的土壤N2O排放通量进行为期1年的观测,同时测定土壤温度、土壤湿度、土壤无机氮含量等。结果表明,观测期内CL、CL-CK、FL15与FL30的N2O平均释放速率分别是25.6、6.60、1.20、4.35μg N2O-N·m-2·h-1,CL小麦季N2O平均释放速率是18.0μg N2O-N·m-2·h-1,玉米季35.2μg N2O-N·m-2·h-1,CL土壤N2O排放速率显著高于CL-CK、FL15和FL30(P<0.01),且CL-CK高于FL15、FL30(P<0.01),FL30高于FL15(P<0.01)。CL、CL-CK、FL15和FL30全年的N2O排放量分别为1.01、0.400、0.050、0.310 kg N2O-N·hm-2。比较CL以及CL-CK的N2O排放总量,停止施氮的措施对土壤N2O排放的减排潜力达到0.610 kg N2O-N·hm-2。与CL-CK相比,FL15、FL30土壤N2O释放量分别减少0.350、0.090 kg N2O-N·hm-2,主要原因是退耕后土壤碳氮比升高,土壤无机养分、温度以及湿度等发生变化。
Quantification of soil spatial and temporal variability at watershed scale is important in ecological modeling,precision agriculture,and natural resources management.The spatio-temporal variations of soil nitrogen under different land uses in a small watershed(12.10 km 2) in the hilly area of purple soil at the upper reaches of the Yangtze River in southwestern China were investigated by using conventional statistics,geostatistics,and a geographical information system in order to provide information for land management and control of environmental issues.A total of 552 soil samples(0 to 15 cm) from 276 sites within the watershed were collected in April and August of 2011,and analyzed for soil total nitrogen(STN) and nitrate nitrogen(NO3-N).We compared spatial variations of STN and NO3-N under different land uses as well as the temporal variations in April(dry season) and August(rainy season).Results showed that STN contents were deeply affected by land-use types;median STN values ranged from 0.94 to 1.27 g.kg-1,and STN contents decreased in the following order:paddy field > forestland > sloping cropland.No significant difference was found for STN contents between April and August under the same land use.However,NO3N contents were 23.26,10.58,and 26.19 mg.kg-1 in April,and 1.34,8.51,and 3.00 mg.kg-1 in August for the paddy field,sloping cropland and forestland,respectively.Nugget ratios for STN indicated moderate spatial dependence in the paddy field and sloping cropland,and a strong spatial dependence in forestland.The processes of nitrogen movement,transformation,absorption of plant were deeply influenced by land use types;as a result,great changes of soil nitrogen levels at spatial and temporal scales were demonstrated in the studied watershed.
