Spatiotemporal Variation And Enhancement Potential Of Cropland Soil Organic Matter And Main Nutrients Content In Jiangsu Province

Fertility of soil is the base for high and stable cereals yield.Soil fertility is the result of the interaction of five major soil factors such as the soil parent material,climate,biology,topography and time,and of the fertilization,planting system and straw utilization.It is the premise of optimizing soil management and realizing soil fertility improvement via deep investigation of the factors affecting soil fertilizy and exploring the affecting law of these factors.Jiangsu is an important grain-producing area in China,and the constraint of cultivated land resources is prominent;therefore,analysis of the spatial and temporal variation of the main indexes of cropland soil fertility and their influencing factors,and studies and exploration of specific measures to improve soil fertility are of great significance to promote the efficient utilization of cropland soil resources and to implement nutrient reduction and efficient utilization meanwhile.Based on the data from long-term location monitoring points Jiangsu basic cropland from 2008 to 2015,classic statistical method,geostatistic analysis and kriging interpolation method were adopted to study the spatial and temporal variation characteristics of important indexes of soil fertility including contents of organic matter,total nitrogen,available phosphorus,and available potassium.Multifactor analysis of variance was used to investigate the significant influencing factors and their functional rules on spatiotemporal variations of soil organic matter,total nitrogen,available phosphorus and available potassium,and on the basis,the effects of fertilizer application and straw returned to cropland soil on the improving potential of the contents of soil organic matter,total nitrogen,available phosphorus,available potassium were analyzed by using stepwise regression model.The main results are as follows:1.From 2008 to 2015,the dynamic changes of soil organic matter and nutrient contents in cropland in Jiangsu province were as follows:the average contents of soil organic matter and total nitrogen increased obviously,while the amounts of available phosphorus and available potassium showed fluctuation trends.Compared to 2008,the increased values of soil organic matter and total nitrogen amounts in 2015 were 7.8%and5.2%,respectively,and no significant difference of the variations of available phosphorus and available potassium was observed.In terms of different farming areas,average annual increase of organic matter in coastal farming areas was highest,and increased value was least in Ning-Zhen-Yang hilly farming areas.The annual increase of available phosphorus was highest in Xuhuai and Yanjiang agricultural areas,while the annual change value was negative in coastal and Taihu agricultural areas.The average annual increase of available potassium was highest in Xuhuai agricultural area,while Lixia river and coastal agricultural area showed negative values.No significant difference was observed for the annual variation of total nitrogen in different agricultural areas.2.From 2008 to 2015,spatial distribution characteristics of soil organic matter and nutrients showed similar trends.The spatial variation of soil organic matter,total nitrogen and available potassium showed obvious regional characteristics,while the regional characteristics of available phosphorus were not obvious.Similar distribution of soil organic matter and total nitrogen was observed,with highest value in Lixia river agricultural area followed by Taihu agricultural area and lowest value in coastal agricultural area was observed.Highest value of available potassium content was observed in coastal and Lixia river areas,while farming areas along the Yangtze river showed the lowest value.No obvious spatial distribution characteristic for available phosphorus was observed.Soil organic matter and available potassium showed moderate spatial autocorrelation,while total nitrogen and available phosphorus had weak spatial autocorrelation.Compared with 2008,the spatial autocorrelation between soil organic matter and available potassium decreased in 2015,indicating that the influence of human and nature factors on organic matter and available potassium was increasing and decreasing.3.From 2008 to 2015,the contents of soil organic matter and nutrients were significantly affected by the input of fertilizers.The natural factors of soil parent material,climate and other natural factors had an interaction effect with the input of fertilizers on specific nutrients.The variations of soil organic matter and total nitrogen content were significantly affected by the initial amount.(1)Variation of soil organic matter content showed positive and negative correlations with nitrogen fertilizer application content and its initial amounts,respectively.The soil organic matter content increased with the middle and low input of nitrogen fertilizer(N?414.0 kg·ha-1·yr-1)in combination with high input of potassium(K2O>147.0 kg·ha-1·yr1.(2)The change of total nitrogen content revealed positive and negative correlations with application content and its initial amounts,respectively,and climate was a significant factor with the low input of nitrogen fertilizer(N?312.0 kg·ha-1·yr-1)or with the super high input(N>517.5 kg·ha·-1·yr-1),while under medium and high inputs of nitrogen fertilizer,climate was not a significant factor.(3)Variations of available phosphorus contents showed a positive relationship with phosphate fertilizer application amount.Crop system was a significant influence factor of available phosphorus content under the ultralow input(P2O5?69.0 kg·ha-1·yr-1),while the value was not significant when the application amount is higher than 69.0 kg·ha-1·yr-1.(4)Soil available potassium content contents showed a positive relationship with potassium fertilizer application amount.Under the low input of potassium(K2O?82.5 kg·ha-1·yr-1),soil parent material and climate change were a significant influence factors,and under low potassium fertilizer input and above level,soil parent material and climate impact were not influence factors.4.The spatial variation of soil organic matter is significantly affected by soil parent material and the planting system.Soil parent material,climate,landform and other natural factors were significant factors affecting the spatial variation of specific nutrients,and they interacted with cropping systems under certain conditions.(1)The spatial variation of organic matter was influenced by cropping system,soil parent material and their interaction.Lagoonal facies sediment parent material showed generally higher soil organic matter content,no significant difference of soil organic matter content was observed in different planting systems,and the soil organic matter content in rice and wheat rotation system is higher in other parent materials.(2)The space variation of total nitrogen contents is significantly affected by planting system and the interaction effect of planting system and climate.The total nitrogen content in rice and wheat rotation and vegetable soils was higher.Significant difference of the total nitrogen in rice and wheat rotation soil was observed in different climatic zones,but the influence of climatic types in other cropping systems was not significant.(3)The spatial variation of available phosphorus is influenced by cropping system,climate and their interaction.The available phosphorus contents in vegetable soils were highest among different planting systems.For the same cropping system,the available phosphorus content of soil in different climate zones varied significantly.(4)The spatial variation of available potassium in soil was affected by landform,and the interaction of cropping system and climate.The content of available potassium in Lixia river shallow plain area and the coastal plain area in northern Jiangsu province was the highest.The content of available potassium in vegetable soil but not in other cropping systems varied significantly in different climatic zones.As time goes on,the influence of planting system on the spatial variation of soil organic matter,total nitrogen and available potassium showed an increasing trend,while the influence on available phosphorus decreased.The influence of parent material of soil on organic matter and of landform on available potassium both decreased.5.The effects of straw returning and chemical fertilizer input on soil organic matter and nutrient contents in paddy and wheat rotation fields were further studied.Results showed that straw returning amount was positively correlated with the change of soil organic matter,total nitrogen and available potassium contents.With a reduced nitrogen fertilizer application of 120.3 kg·ha-1·a,increasing straw returning to the field with 898.7 kg·ha-1·yr-1 induced a stable change rate of soil organic matter,indicating that in a suitable proportion of amounts of nitrogen fertilizer reduction and straw returned to the filed,straw can substitute nitrogen fertilizer for stabilizing soil organic matter content.The effect of straw returned to the filed on the increase of total nitrogen and available potassium content was much less than that of the application of chemical fertilizers,and had no significant effect on the increase of available phosphorus.From 2016,increasing the returning rate of straw to the field up 50%,and reducing nitrogen input amount to 32.5%and 27.0%in Lixia river and Taihu lake agriculture areas,respectively could still achieve the goal of soil organic matter and total nitrogen improvement;the agricultural areas of Xuhuai,Lixia river,and Taihu lake and so on with high soil available phosphorus content,the input of phosphate fertilizer can be controlled;and the agricultural areas along the coast,along the river and in Ning-Zhen-Yang hilly need to further increase the application amount of potassium fertilizer.In summary:(1)from 2008 to 2015,the organic matter and total nitrogen content of cropland soil in Jiangsu increased significantly,while the contents of available phosphorus and available potassium fluctuated from year to year,and the annual average change of some agricultural areas is negative,which needs to be paid attention to;(2)no significant change in the spatial distribution pattern of soil organic matter and nutrients in cropland in the whole province during the past 7 years was observed;(3)there is a universal interaction effect among the factors that affected the spatial and temporal variation of soil organic matter and nutrients,indicating that the improvement of soil fertility must be planned from a multi-factor perspective;(4)in areas with high soil organic matter and total nitrogen contents,the goal of fertility improvement can be achieved by controlling nitrogen input and improving straw returning to the field,and according to the regional variation characteristics of available phosphate and available potassium,input of phosphate fertilizer and potassium fertilizer can be increased or decreased to achieve the goal of synergetic soil fertility improvement and environmental friendliness.