The spatial-temporal distribution of nitrogen and N2O emission from soil and sediment in agricultural watershed of Tuojia River
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 watcr,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-1 and (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 S1＜S4＜S3＜S2 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,p＜0.01) 、NO3--N (r=0.80,p＜0.01) and temperature (r=0.57,p＜0.01),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.
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