The Threshold Of Soil Organic Matter Content On Arable Mollisols Productivity

The effect of SOM threshold on the productivitywas observed by a positive correlation between soil organic matter(SOM)content and crop productivity under certain conditions.It is important to quantify the threshold and to reveal its formation mechanism for the basic farmland protection and degraded soil restoration.In this study,black soil(Mollisols)with great variation in SOM located in the Northeast China was studied.Five soils with different SOM amounts were collected and moved to Hailun city of Heilongjiang province,and an experiment called the spatial displacement experiment was conducted.With this method we can separate the effect of climate and SOM.(1)to quantify the threshold value in which the SOM influenced the crop productivity;(2)to determine the soil physical,chemical and biological mechanisms that will affect the threshold;(3)to study the effective group of SOM that may play big role in affecting the threshold value.Field trial includes two treatments,SOM and N fertilization,with five levels and three replicates.The gradient of SOM,assigned as A1,A2,A3,A4,A5,were 23.54 g kg-1,58.69 g kg-1,71.23 g kg-1,8.146 g kg-1 and 103.57 g kg-1 respectively.Nitrogen fertilization gradient,B1,B2,B3,B4 and B5 were 0 kg hm-2 N,10 kg hm-2,30 kg hm-2,70 kg hm-2,and 150 kg hm-2 respectively.Indicated crop was wheat.Soil samples were collected for SOM and NPK content analysis each year at seedling stage and after harvest.Crop yield also was recorded each year.There is a choice of measurement of soil bulk density,moisture content,temperature,infiltration rate,pH value,soil aggregates,soil particulate organic carbon,ammonia nitrogen,nitrate nitrogen,mineralization rate,microbial biomass carbon,nitrogen and carbon in plant in the whole growth period.Through 3 years of experimental observation,the following results were obtained:(1)Under the condition of no fertilization,the yield and SOM content of wheat showed a single peak curve relationship,which means there was threshold of SOM in affecting the crop productivity.Under no fertilization,namely soil productivity,SOM content and the yield of wheat present threshold of 72.40 g kg-1.When SOM is lower than the content,wheat yield increased with the increase of SOM;above the content,wheat yield reduced with the increase of SOM.In the application of 70 kg hm-2 N conditions,namely productivity of farmland,there also showed the threshold SOM content to wheat yield,as 76.04 g kg-1,with same change trend as no fertilization treatment.(2)Soil bulk density decreased at low SOM content level and then increased with the increasing of the SOM.Contrary to the trend of wheat yield and SOM content relationship,soil bulk density in A4 has the minimum of 0.87g cm-3.A5,the highest in SOM has a higher soil bulk density then A4 with 5.75%,but the difference was not significant.Soil water content(SWC)is increased with the increase of SOM content,the highest value in seedling stage and booting stage appeared in A5 treatment,0.12 percent higher than A4 and 7.53%than A1 in seedling and 1.26 percent and 12.77 percent higher than A4 and A1 in booting stage separately.A4 treatment has the highest SWC value in milk ripe stage,wax ripe stage and mature stage,wthile the lowest one was A1 treatment.Except milk stage,there was significant difference in SWC among each SOM levels,while the SWC was not significant between high levels of organic matter with different N levels.Field soil water holding capacity increases with the increase of SOM content,the biggest one is A5 treatment,1.8 percent,higher than A4 with no significant difference.Saturated water content exist single peak curve with SOM,the maximum value appeared in A4 treatment,6.40%higher than A5 treatment with significant difference among SOM levels.Soil temperature decreases with the increase of SOM content,but the performance of different stage is slightly different,topsoil temperature in A1 treatment in May and June were the highest but lowest in A5.The variation in temperature in May is 1.05?,0.73? in June.The highest value was in A3 in July and August,and variation was 0.56? in July and 0.66? in August.N application amount also had a significant effect on soil temperature,low fertility level(B1)and high fertility level(B4)was lower than that of medium fertility level(B2 and B3).Water stable aggregates(>250?m)and SOM content showed a single peak curve relationship,the maximum level was of A3,and A5 was lower than A3 by 7.67%,than A4 by 6.65%,A3 and A5 were significantly different.(3)Soil chemical properties analysis showed that total carbon(TC)content iin five kinds of soil were changed after move to the new place.TC in A1 rose slightly,has no change in A2,and decreased in A3,A4 and A5.During the growth period,the soil TN content in the soil was higher than that before sowing,and the interannual variation was decreased(2011-2012)and then increased(2012-2013).TP in 2011 and 2012 showed a decrease in the year,In 2013 was essentially flat.The content of TP in three years showed a decreasing trend at each SOM level;TK content in 2011 and 2012 is higher than before sowing,and in 2013 is lower than the content before sowing soil available nutrient in different growth period were different.They were higher in seedling stage and harvest stage than that in other periods.Among the available nutrient,soil available nitrogen(AN)showed a single peak curve with SOM content,the highest level appeared in A4,and were higher than A5 almost in all tests with 0.21%to 3.49%.Application of N fertilizer could increase the content of AN.There is also a single peak curve relationship between the available P(AP)and SOM,AP were lower in A1 and A5,while higher in A2,A3 and A4.Adequate amount of nitrogen fertilizer can increase AP content,and too much nitrogen will reduce AP content.There is no correlation between SOM and available potassium(AK),while AK in A4 topped in each period,with 36.45%-66.8%higher than A5,though the variation was lessen with the increase of N fertilizer level.The cation exchange capacity(CEC)increased with the increase of SOM content,which was significantly correlated with SOM(r = 0.964**).There was no correlation between SOM and soil pH,while there is a moderate negative correlation between N fertilizer application and soil pH value.The large amount of N fertilizer application would cause soil acidification,while the content of ammonium nitrogen decreased with the increase of SOM.The nitrate nitrogen content showed a single peak curve relationship with the SOM,and the content of nitrate nitrogen was highest at A4 level during the seedling stage and harvest stage.With the increase of N fertilizer application level,ammonia nitrogen and nitrate nitrogen was increased.Soil particulate organic matter(POM)in the size fraction from 2000?m to 250 ?m and from 250?m to 53?m was higher in A4,accounting for 30.72%and 8.65%,showed single peak curve relationship with SOM.A5 were highest in SOM and N in the two fraction sizes,with averaged SOM content at 126.51g kg-1 and 141.06g kg-1 and averaged N content at 0.57%and 0.65%respectively,and was significantly correlated with soil SOM.(4)Soil biological properties analysis showed that the microbial biomass C and N increase with SOM content increased,the highest value appeared in A5,and the lowest in A1,the variation is 62.18%for C and 232.56%for N.Microbial biomass N,increase with the amount of N fertilizer application.There is little effect on C and N contents in wheat seeds of SOM content.And also the relationship between C content and SOM is weak(r?0.121),while N content and SOM moderately correlated(r=0.770).N fertilizer level can significantly improve C and N content in wheat grain.C content and N fertilizer had a significant positive correlation(r=0.941*)and N content and N fertilization levels has highly significant positive correlation r=0.9870**.Stem C content has a moderately correlation(0.774)with SOM,showing a single peak curve.The highest C value appeared in A4,and increases with the increase of N fertilizer rate(r=0.910*),highest value appeared in B5.Stem N content increased with the increase of SOM and N fertilizer level too,the highest were in A5 and B5,with correlation index r=0.854 for SOM and r=0.969**for N fertilizer levels.This study quantitatively revealed the SOM threshold value for wheat yield.The highly correlated indicators included:saturated water content,soil water content,soil temperature,soil bulk density,soil aggregates,soil infiltration rate,nitrate nitrogen,POM content.