Analysis On The Spatial And Temporal Variations Of Soil Nitrogen And Its Influencing Factors In The Core Area Of Chengdu Plain In Recent 35 Years

The contents and dynamic variation of soil nitrogen affecte the quality of croplands,integrated nitrogen management in agriculture practices and global carbon and nitrogen cycle directly.Therefore,accurately grasp spatio-temporal dynamics of soil nitrogen is widely concerned and explored.Based on the 166 typical soil profile data gathered during the 2nd National Soil Survey in 1980 s and the measured data of 187 soil profile samples collected in 2016 and 2017,regression equation and soil depth functions were done between soil organic carbon and bulk density to estimate the density and storage of soil total nitrogen(STN).The analysis of variance and regressionwere used to analyze the effects of different soil-forming factors on the temporal and spatial variation of soil nitrogen in the core area of Chengdu plain in the recent 35 years.Results indicated that:(1)In the 35 years,the STN content and soil alkali-hydrolyzable nitrogen(SAN)content increased dramatically in topsoil(0～20 cm),the mean content of STN was 1.67g·kg-1 in 1980 s and 1.91 g·kg-1 in 2010 s,increased by 14.37%,and the mean content of SAN was 128.33 mg·kg-1 and 138.01 mg·kg-1 in the 1980 s and 2010 s respectively,increased by 7.54%.There were kinds of decreases in the STN of the 20～100 cm soil layer between 5.17% and 16.88%.Four parent materials in this study,the highest extent of increase was Q3 old alluvium in the STN(32.28%)and SAN(19.79%)in topsoil.The STN content of Q3 old alluvium and Q4 Gray-brown alluvium saw an opposite trend,the former went up by 3.96% while the latter fell 13.01% in the 20～40 cm soil layer.The STN content in 40～100 cm experienced a downward trend which range of decline were between 1.92% and 23.81%.In the two soil types,the STN and SAN contents of surface stagnic anthrosols layer(0～20 cm)raised by 14.20% and 7.10%,respectively,while alluvic primosols increased by 34.29% and 24.56%,respectively.The STN level in all types fell down among 5.17% and 41.07% in 20～100 cm soil layer.In the perspective of soil sub-groups,the STN and SAN contents of other 4 groups experienced an upward trend between 4.85% and 34.29% except that gleyed paddy soil showed a decline(4.15%and 3.31%).The nitrogen content of all soil genus in percolated yellow mud soil went up considerably between 19.48% and 82.46%.By contrast,the content showed a most obvious decrease between 10.81% and 31.93% in gleyed shioda.(2)The STN density and nitrogen storage showed an increase tendency in the study area during the 35 years.The mean STN density and nitrogen storage were 1.24 kg·m-2and 3.835 Tg in 0～100 cm depth in 1980 s,where as in 2010 s the mean was 1.27 kg·m-2and 3.928 Tg,respectively,both increased by 2.42%.Every soil layer among 0～100 cm depth,their range of nitrogen density and storage fluctuated b etween-9.09% and 9.30%.In all soil layers,Q4 Gray-brown alluvium and Q4 Purple alluvium have the highest level of STN density while Q4 Gray alluvium and Q4 Purple alluvium respectively reached the highest and lowest point in nitrogen storage.The densit y and storage of STN raised generally in 0～100 cm layers,which growth rate were Q4 Purple alluvium(5.34%)> Q3 old alluvium(4.89%)> Q4 Gray-brown alluvium(2.98%)> Q4 Gray alluvium(1.64%).40～60 cm depth saw an downward in the density and storage of STN in stagnic anthrosols(10.56%)as well as 0～100 cm depth saw a similar trend in alluvic primosols(24.30%).In the 0～100 cm depth,the density and storage of STN in all sub-groups increased between 0.19% and 12.28% except that Gleyi-Stagnic Antrosols and alluvic primosols declined by 4.18% and 24.30%.The biggest scale of raise in all soil layers of STN density and storage was in the percolated yellow mud soil(17.76%～75.53%),and the tendency of decrease happened in percolated yellow mud soil(1.36%～16.26%).(3)The soil nitrogen content spatial variability in 0～40 cm layers in the study area were affected jointly by structural and random factors,but tended to be mainly affected by structural factors.During the 35 years,spatial and temporal variation of soil nitrogen in the study area affected differentially by parent materials(4.0～11.2%),soil genius(3.8%～12.3%),fertilizer application(1.2%～11.9%)and so on.Parent materials have more significant influenceon the spatial distribution of the STN and SAN,while soil types have less influence in the 0～20 cm layer.Subgroups and soil genius have notable effects on the spatial changes of the STN and SAN in all soil layers.The influence of land use was more significant on the STN and SAN spatial variability in 0～20 cm layer,finally,the effect of fertilization on the spatial distribution of soil nitrogen content ranged from 2.3% to 3.1%.(4)The spatial distribution of STN,STN density,SAN content were generally consistent in 1980 s and 2010 s.Soil nitrogen showed a gradual increase from north to south in 1980 s while the trend from the middle of study area to northeast and southwest were decrease in 2010 s,scilicet,the content was comparatively lower in the south of the Qingbai River and north of the Jinma River.The soil nitrogen spatial variation mainly distributed in the northeast(Pengzhou-Xindu area),Midwest(Dujiangyan-Chongzhou area),Southwest(Dayi)of the study area in the 35 years,which showed an increasing trend.By contrast,the central east(Pixian-Wenjiang),southeast(the junction of Qionglai and Xinjin)of study area and the east of Chongzhou showed a downward trend.