Transformation And Stabilization Characteristics Of Straw Residue Inputs In Different Fertility Level Soils

Soil organic carbon?C?in farmland determines soil fertility levels,it is related with the sustainable agricultural development,global carbon sequestration,greenhouse gas emission and climate change.The soils in Northeast China as one of the three largest Black soil regions in the world,the long-term unreasonable tillage and utilization have led to the depletion of soil fertility,deterioration of physical and chemical properties,and degradation of ecological functions.The protection for Black soil region has been paid great attention by the ministry of national agriculture.Increasing the application of manure fertilizer and returning straw to the field is reasonable.The effective measures to maximize the utilization of agricultural wastes can realize soil C storage increasement.However,it is not clear for the threshold value of straw input levels added into different fertility soil,and how straw C addition affects the decomposition of native organic C.The increase of C input will continue to increase SOC content,but most of the researches have small SOC content range.Therefore,this study set different straw input levels to achieve a enough larger range of SOC content,and expect to observe the response of different fractions to SOC contents.In addition,different fertility levels affecting soil C sequestration rate,the specific stabilization mechanism and the transformation process of organic C between different fractions are unknown.Therefore,based on the long-term Phaeozem soil?FAO?and Luvisol soil?FAO?Experiment Station in Northeast China,soil samples were collected from the plots with two treatments:low fertility level soils without fertilizer and high fertility level soils applied with organic manure.We carried on this research with application of ¹³C-labeled maize straw to soil at the rates of 0%,1%,3%,5%,10%?0 g,1 g,3 g,5 g,10 g per 100 g dried soil?,using an in-situ carborundum tube method in low and high fertility level Phaeozem and Luvisol soils at experimental sites for 360 days.Then,the distribution of maize straw C in different aggregates were determined and different protective mechanisms were quantitatively analyzed by wet sieving and density fractionation method.The objects of this study were to explore the stabilization mechanism of SOC,and further to improve the potential of SOC sequestration by straw returning to the different fertility level soils.The main results showed that:?1?After 360-day of incubation,the straw C in the soil decreased to 22.8%-27.2%of the initial value with different fertility levels soils.0-60 days is the key period of straw carbon decomposition,which is also the most active period for soil organic carbon or organic carbon in aggregate.?2?The new organic C was enough to supplement the decomposition of native SOC when 5%and 10%of maize straw were added.?3?The content of straw C in the soil macroaggregate fractions was higher than that in microaggregate fractions,indicating that the straw C was prior stabilized in the macroaggregate fractions.?4?There was a nonlinear relationship between the total SOC content and the silt and clay fraction when more than 5%maize straw input added to the high fertility level soil.High levels of maize straw added could promote the soil aggregate fractions organic C content increasing and to protect the stability of native SOC.?5?According to the variation characteristics of fraction organic C with different protection mechanisms,we found that the organic C fraction of unprotected mechanism showed increasing with the maize straw addition.The phy-chemical protected silt and clay fraction was occluded in the microaggregate fraction and the organic C content did not change significantly during the whole incubation period.?6?Analysis of variance showed that the total SOC contents of Phaeozem and Luvisol soil were significantly affected by straw addition amount,and culture time?P<0.001?.The contents of organic C macro-and micro-aggregate fractions in Phaeozem and Luvisol were significantly affected by the amount of straw addition and incubation time.?7?Correlation analysis showed that there was no significant correlation between the silt and clay fraction with the light fraction and macroaggregates.In addition,the microaggregate fractions showed significant positive correlation with each fraction?except for the light fraction?,indicating that the microaggregate fraction had not reached the stable and were susceptible to the changes of other fractionsThese results indicated that the distribution and stabilization of straw carbon in soils were jointly restricted by its addition amount and fertility level.Maize straw addition increased SOC and macroaggregate fraction C content.SOC was mainly stored in macroaggregate fraction and has played an important role in the stabilization of external organic C in soil such as straw returning.The increase of organic C content of different aggregates in low fertility soil was significantly greater than that in high fertility soil,providing a positive feedback effect for the improvement of soil fertility.The decomposing and fixing processes of straw C and external C input to soils are needed to be further explored and studied in the future.