Temporal And Spatial Distribution Characteristics Of Soil Organic Matter And Total Nitrogen On Typical Mollisols Farmland

The Mollisols region is an important grain base in China.Soil erosion has seriously restrained the sustainable utilization of slope farmland in this region.In recent years,most studies focused on soil erosion characteristics,processes and mechanisms in this region.However,it was difficult to set the control plot in field scale and obtain long term experiment data,and little information was available on the effects of soil and water loss and soil and water conservation measures on the soil physical and chemical properties in the field scale,especially for the soil organic matter content(SOM).The content of SOM in thirteen downslope farmland of the typical Mollisols region,with slope gradient ranged from 1° to 7°,was measured in 2004 and 2016.The descending rate of SOM was quantified.Besides,based on the systematic investigation and measurement before and after 10 years of contour tillage in a 1.4 hm2 typical sloping farmland,the variation of SOM before and after contour tillage was studied.The recovery effect of contour tillage on SOM was revealed.Results were as follows:(1)The variation rate of SOM and TN in the slope farmland under conventional tillage was quantitatively revealed.Compared with 2004,the content of SOM and TN decreased by 6.72%and 25.34% in 2016.The annual average decrease rate was 0.56% and 2.11% for SOM and TN,respectively during the 12 years.But the C/N ratio increased by 27.96%,the annual increment rate reached to 2.27%.The effects of different slope factors(slope type,slope length and slope gradient)on the decrement of SOM and TN were different.For the slope type,the content of SOM and TN decreased in the order of convex slope,concave slope,and straight slope.For slope length,there was no obvious rule in the decrement of SOM with the increase of slope length range form 0 to 500 m,but the decrement of TN and the increment of C/N ratio decreased with the increase of slope length ranged form 100 to 500 m.For slope gradient,the decrement of SOM and TN was decreased in the order of 2° ~ 5°,5° ~ 7°,and 1° ~ 2°.For slope position,the decrement of SOM was decreased in the order of middle slope,foot slope,and upper slope,but the decrement of TN was decreased in the order of foot slope,upper slope,and middle slope.(2)The background value of 137 Cs in this study was 1513.5 Bq·m-2.The average erosion rate was 2482 t·km-2·a-1.The upper slope and middle slope was the erosion areas,the erosion rate was 2600 t·km-2·a-1 and 2958 t·km-2·a-1,respectively;but the foot slope was the depositionarea,the erosion rate was-569 t·km-2·a-1.The decrement of SOM reached the maximum in the middle slope with seriously soil erosion.(3)The content of SOM of degraded farmland was improved after 10 years contour tillage,but the content of TN decreased.The content of SOM increased by 2.61 g·kg-1(8.4%),the annual average increment rate was 0.70%,the maximum increment appears in the west of foot slope with most severely soil erosion,which increased by 25.1%.In the space,the content of SOM decreased by 3.7% at the west of middle-upper slope.The content of TN decreased by 0.04 g·kg-1(2.8%),the annual average increment rate was 0.23%.TN content increased by 18.2% in some regions of the upper slope and foot slope,but decreased in other regions,the decrement reached maximum in the middle-upper slope(26.6%).(4)Compared with classic statistical method,the effect of soil and water conservation was better revealed by geo-statistics method.The variation of soil properties before and after contour tillage was qualified.The effect of soil and water conservation on spatial heterogeneity of soil properties was quantitatively revealed.The space variation before and after contour tillage could be directly reflected by the spatial distribution map,which could reveal the effect of soil and water conservation in the space.Therefore,in order to improve the soil fertility of degraded farmland,water and soil conservation measures are not enough,more nitrogen fertilizer should also be added properly.This study presents important references for soil and water conservation in the Mollisols slope farmland of Northeast China.