Spatiotemporal Heterogeneity Of Soil Nitrogen On A Mollisol Slope In Northeast China

Mollisol regions is the key commodity grain basket of China.Because of the unreasonable farming practices in the time of less than hundred years,soil erosion was serious and soil nutrients heterogeneity was compicated.Thus,it is difficult to manage the soil nutrients with high accuracy in the farmland.In addition,due to the driving forces of soil moisture and temperature changes in different seasons and the influence from different agricultural management methods?fertilization,tillage,planting patterns,crop varieties,etc.?.This made the spatial and temporal distribution of soil nitrogen?N?is more complicated.The high heterogeneity of N in farmland soils may lead to excess or deficiency of fertilizers,and?1?Over-application of N fertilizer aggravates environmental pollution;?2?Insufficient N fertilizer application reduces crop yield and quality.In the past decades,the N heterogeneity on the large-scale in autumn were mainly focused on,and only few reports were about the spatial-temporal heterogeneity of N during crop growth.As well as,the spatiotemporal dynamics of soil N at this stage directly related to the crop growth process and the final crop yield.Mollisol soil is located in the high latitudes?northeast China?.It is a long time of freezing-thawing process during"autumn-winter"and"winter-spring"alternations each year?more than six months?.Soil structure,soil erosion,N transformation and migration were changed by freeze-thaw action,and then it also changed the spatial distribution of soil N during the freezing-thawing process.Because the N-spatial distribution pattern of the soil in autumn or spring was mostly used to guide the precision fertilization,and adjust the fertilization.Which increased the uncertainty during the precision fertilization when ignoring the differences between autumn and the following spring.For this reason,the spatiotemporal heterogeneity of soil nitrogen during crop growth stage and freeze-thaw cycles were both considered.The spatiotemporal heterogeneity of soil N and its main driving mechanism were systematically analyzed on a mollisol slope in Northeast China,and the results can provide theoretical basis and technical support for N management in Mollisols regions.The experiment was conducted at the Xiang Yang Experimental Station of Northeast Agricultural University,and it is located in the middle of the typical mollisol region in the northeast China.The spatiotemporal dynamics of soil N under treatments of continuous corn with residue?CC+?,continuous corn without residue?CC-?,corn and soybean rotation with residue?CSR+?and corn and soybean rotation without residue?CSR-?on continuous slope sites?summit slope,shoulder slope,back slope,foot slope,and toe slope?were focused on a Mollisol slope in two years to?during crop growth stage and freeze-thaw process?.In the growing season of the crops,the soil samples were collected by using the grid sampling method.Soil ammonium?NH₄⁺-N?,nitrate nitrogen?NO₃^--N?and total available nitrogen?AN,NH₄⁺-N+NO₃^--N?,and the related soil physical and chemical indicators and crop growth indicators were measured.Before freezing?Autumn?,slope positions were covered by the device which was air-permeable,precipitation-preventing and erosion-preventing and was named as"covered membranes".After thawing,soil samples were collected by using the grid method in different layers?0-5 cm,5-10 cm,and 10-20 cm?,and total nitrogen?TN?and alkaline hydrolysis nitrogen?AHN?were measured.Both of classical statistics and geostatistical analysis methods were used to analyze the spatiotemporal heterogeneity of soil N under different tillage treatments and in different slope positions after freezing-thawing process.Combined with the soil physicochemical factors,crop growth traits to analying the main driving mechanism of the spatiotemporal heterogeneity of soil N systematically.The main research results and conclusions are as follows:The spatial heterogeneity of N and its main driving mechanism during the crop growing stages:?1?Soil NH₄⁺-N were 10.47-22.01 mg kg^-1,20.34-24.58 mg kg^-1 and 23.04-27.26 mg kg^-1 in the seedling stage,growth stage,mature stage,respectively.The content of NO₃^--N in soil were6.07-16.84 mg kg^-1,4.93-20.94 mg kg^-1,and 10.97-22.85 mg kg^-1 at seedling stage,growing stage,and in mature stage,respectively.?2?During the crop growing period,AN in CSR-treated soil was lower than that in CC-treated soil?less than 3.6%?.Straw returning caused the N competation between microorganisms and crop,and resulted that AN under the treatments with straws was relatively lower at mature stage.?3?During crop growth period,due to the influence from topography,rainfall,and soil erosion,AN was higher on the summit slope(18.57⁶3.70 mg kg^-1)and bottom slope(16.17⁷1.75mg kg^-1),while it was lower on the back slope(16.12⁴4.08 mg kg^-1).?4?During crop growth period,soil N was affected by both structural factors?precipitation,temperature,soil erosion and crop growth process?and random factors?fertilization,weeding,etc.?,the dynamic changes of soil AN were similar between different treatments,but was differences between the years,and the predominent trend is upward.?5?In the crop growth period,soil AN with soybean planted was less than corn planted?less5.2%?in CSR systems,and AN content corn planted in CC systems was higher than corn planted in CSR systems.Straw returning?CC+and CSR+?obviously reduced soil AN content.?6?AN was mainly affected by NO₃^--N when no fertilizer applied at seedling stage;When N fertilizer was applied,the changes of AN is simlar to NH₄⁺-N and NO₃^--N and AN were affected by both of them.During this period,soil AN was mainly affected by many factors such as soil erosion,fertilization,and soil microbes.?7?In the crop growth period,the spatiotemporal heterogeneity of AN was determined by the soil NH₄⁺-N,NO₃^--N content,which was mainly influenced by many factors such as soil erosion,low fertilizer,soil microbes and crop growth.?8?In autumn,the spatiotemporal heterogeneity of AN was mainly determined by the content of NH₄⁺-N when no N fertilization.The spatiotemporal heterogeneity of AN was mainly determined by both the contents of NH₄⁺-N and NO₃^--N when N fertilizer was applied.The spatial distribution of AN during this period was mainly affected by many factors such as soil erosion,soil sediment,following fertilization,soil microbes and crop growth.The spatial heterogeneity of N and its main driving mechanism during the freeze-thaw process:?1?The content of TN were 0.65 to 2.39g kg^-1 in before freezing,and were 0.66-1.30g kg^-1after freezing-thawing process.TN with cover?membranes covered?was higher than that without cover(0.57⁰.86 g kg^-1).Soil AHN content was 80.3-239.4 mg kg^-1 in before freezing,and was between 91.9¹81.5 mg kg^-1 in after freezing-thawing process.AHN with cover at 63.8³54.7 mg kg^-1.?2?After freezing-thawing process,when neglected the effect of slope positions,the soil TN content generally showed a decreasing trend.With the increasing of soil depth,the content of AHN was higher?8.5-35.21%?when the soil layer was near the soil surface in the summit and the bottom slope due to the dynamics of soil moisture.?3?After freezing-thawing process,all treatments without cover,the content of N on the back slope was the lowest(TN:0.42⁰.77 g kg^-1;AHN:44.33¹21.33 mg kg^-1),while was higher on the summit slope(TN:0.68¹.70 g kg^-1;AHN:67.67¹91.33 mg kg^-1)and bottom slope(TN:0.34⁰.97 g kg^-1;AHN:70.00²98.67 mg kg^-1).?4?By influencing of different straw amendment,the soil AHN in the treatments without cover in soil layer showed a significant downward trend in 2015-2016,while showed an increase trend in 2016-2017.?5?TN was generally higher in the treatment with cover than that without cover?20.7%?.This was mainly influenced by soil erosion,and the water movement driven by evaporation and freeze-thawprocess.Due to the influence of the multiple factors,AHN dynamics were inconsistent in different years.AHN content with cover was higher than that without covered in same layer of2015-2016.The AHN with cover was lower than that without cover,and its content was lower than that before freezing-thawing process in 2016-2017.