脱甲河农业流域土壤沉积物氮素时空分布与N_2O释放

The spatial-temporal distribution of nitrogen and N_2O emission from soil and sediment in agricultural watershed of Tuojia River

为研究脱甲河农业小流域氮素输出特性,运用流动注射仪法和顶空平衡-气相色谱法于2015年4月—2016年1月对流域内4级河段(S1、S2、S3和S4)稻田-岸坡-河底沉积物土壤铵态氮(NH_4^+-N)、硝态氮(NO_3^--N)及水体溶存氧化亚氮(N_2O)浓度进行了连续10个月的监测,并利用双层扩散模型法对水系N_2O排放通量进行了估算.结果表明:脱甲河流域稻田-岸坡-河底沉积物NH_4^+-N含量逐渐升高,NO_3^--N含量逐渐降低,其中,岸坡及河底沉积物土壤中的氮主要以NH_4^+-N形式为主,均值分别为(7.38±0.62)mg·kg-1和(16.49±1.70)mg·kg^(-1);稻田土壤和脱甲河水体中的氮主要以NO_3^--N为主,均值分别为(7.40±0.81)mg·kg^(-1)和(1.55±0.03)mg·L^(-1).水体溶存N_2O浓度范围在0.005~7.37μmol·L^(-1)之间,均值为(0.54±0.05)μmol·L^(-1);扩散通量在-1.11~1811.29μg·m^(-2)·h^(-1)之间,均值为(130.10±12.04)μg·m^(-2)·h^(-1),每年向大气输出的N_2O量为11.40 kg·hm-2.其中,在早稻生长初期和早晚稻收割、栽种交替时段N_2O输出量达到高峰.空间上,N_2O扩散通量表现为S1<S4<S3<S2,S1级河段显著低于其他3级河段(p<0.01).相关分析表明,脱甲河表层水体N_2O扩散通量与NH_4^+-N(r=0.87,p<0.01)、NO_3^--N(r=0.80,p<0.01)和水温(r=0.57,p<0.01)呈显著正相关,流域内稻田-岸坡-河底沉积物及水体NH_4^+-N和NO_3^--N浓度间相关性不显著.脱甲河农业小流域氮素流失主要包括稻田-岸坡-河底沉积物中铵态氮、硝态氮及水体中N_2O,在水稻栽种期间出现高峰,存在较大氮素流失风险,因此,开展农业小流域氮素流失研究对区域氮素周转及农业生产活动具有重要的指导意义.

To investigate the nitrogen loading of agricultural watershed in Tuojia River,10 months＇ continuous survey（ from April 2015 to January 2016）was conducted to observe concentration of ammonium nitrogen（ NH4＋-N） and nitrate nitrogen（ NO3--N） in sediment,river water,slope soil and paddy soil,dissolved nitrous oxide（ N2O） concentration and flux are also employed in 4 different reaches of Tuojia River（ S1,S2,S3 and S4） by using the headspace balance method and diffusion model method. Results show that NH4＋-N concentration gradually increased,while NO3--N concentration decreased in paddy soil,slope soil and sediment. NH4＋-N was the primary form of nitrogen in soil of slope and sediment in Tuojia River watershed,with an average of（7.38±0.62） mg·kg（-1） and（ 16.49 ± 1.70） mg·kg（-1） respectively,while the nitrogen loading in soil of paddy and river water was mainly presented by NO3--N with an average of（ 7.40 ± 0. 81） mg·kg-1and（ 1. 55 ± 0. 03） mg·L（-1）. The dissolved N2O concentration was 0. 005 7. 37（ 0. 54 ± 0. 05）μmol·L（-1）,and N2O flux was-1.11 to 1811.29（ 130.10 ± 12. 04） μg·m（-2）·h（-1）,in the four reaches of Tuojia River. The gross emission of N2O from Tuojia River was estimated with the value of 11.40 kg·hm（-2）. During the early growth stage and the alternation of harvest and transplant of rice production,N2O flux reached a peak. The spatial distribution of N2O fluxes follows the sequence of S1S4S3S2 and N2O flux of S1 reach was significantly lower than that of the other three reaches. Results of correlation analysis show that the N2O flux positively correlated with the NH4＋-N（ r = 0.87,p0.01） 、NO3--N（r= 0.80,p0.01） and temperature（r= 0.57,p0.01）,NH4＋-N and NO3--N concentration of river water showed a weak correlation relationship with the soil of paddy,slope and river sediment. The results indicate that the comparatively high nitrogen loading suggested potential risks of nitrogen losses.Investigation of nitrogen loss in small agricultural watershed is very important both for the regional nutrition cycling and sustainable agricultural production.