Decomposition Characteristics Of Maize Straw And Its Effect On Soil Biological And Physical Properties In The Mollisol Region

Straw returning is one of the effective measures for sustainable utilization of farmland,which can not only reduce environmental pollution caused by burning straw,but also increase soil organic matter and prevent soil structure degradation.However,the effects of straw returning technology on soil quality are different in different regions,which depend on local climate,soil properties and different straw returning technologies.The mollisol region is located in the cold temperate zone and high latitude of Northeast China,and the average annual temperature is not high,which limits the comprehensive promotion of straw returning to a certain extent.In order to establish a suitable straw returning method for different accumulated temperate zones in high-latitude mollisol area of China,this paper selected Nenjiang,Hailun and Harbin as the test sites to systematically study the site,the extent of straw-soil mixture,straw size,returning amount and returning depth on the decomposing process of maize straw and soil improvement(microbial community structure,chemical composition of soil organic matter and soil physical properties).The main progress of the paper is as follows:(1)Straw-soil mixture significantly improved the decomposition rate constant and accelerated the formation of stable substances.The traditional nylon mesh bag method was to place the straw residues in the nylon mesh bags,and the straw cannot be mixed with the soil outside the mesh bags.The traditional net bag method was improved in this study.The straw and soil were fully mixed and placed in nylon net bags.The two methods were used to compare the decomposition process of straw,and the effects of straw returning depths,straw returning amounts and sizes and sites on the decomposition of straw were discussed.The results showed that the decomposition rate constant of straw-soil mixture increased by 18-160% compared with straw only treatment.There was no significant effect on the decomposition rate constant of straw by the depth of straw returning and the test site.During the decomposition process,the relative proportions of O-alkyl and anomeric C decreased,while alkyl C,aromatic C,carbonyl C and Nalkyl/methoxyl C groups increased.Straw-soil mixture increased the relative proportion of alkyl C,aromatic C and NCH functional groups.In addition,the mass remaining of different straw returning amounts and sizes decreased to 8%-30% in the first four months of decomposition,and then remained basically unchanged in Nenjiang and Hailun areas,while slightly decreased in Harbin areas.(2)Soil microbial community structure were different in three locations.Straw returning significantly increased soil microbial biomass,changed the composition of microbial community,and promoted the humification process of soil organic matter.After straw was returned to the field,grampositive bacteria significantly increased.The ratio of fungi/bacteria decreased with the increase of the decomposition time in both Harbin and Hailun sites,while the ratio of fungi/bacteria increased in Nenjiang.The ratio of gram-positive bacteria to gram-negative bacteria increased with increasing of the decomposition time.After the straw was returned to the field,the chemical compositio n of soil organic matter changed: the relative proportion of alkyl C increased,while the carbonyl C and aromatic C–O groups decreased.At the same time,the decomposition index of O–alkyl C /alkyl C decreased,and the degree of humification of soil organic matter deepened.With the extension of straw returning time,N–alkyl/methoxyl C,O–alkyl C(OCH)and Carbonyl C functional groups of soil organic matter were significantly correlated with the microbial community structure succession process.(3)The stability and tensile strength of soil aggregates increased after the straw were returned to the field.The two indicators reached the maximum in the 3 months of decomposition time at Hailun site,while 1 or 2 months at Nenjiang and Harbin sites.And the stability and tensile strength of soil aggregates significantly increased in the first maize growing season(0–3 months of straw decomposition time)at the three sites,but the effect was significantly reduced in the second year(12–17 months of straw decomposition time),and there were still effects.The change magnitude of these indices were the greatest at Hailun site,straw returning caused large changes in organic carbon content in large aggregates and free light group,and organic carbon was mainly stored in mesoaggregate,microaggregate and heavy fractions.In summary,straw decomposition was mainly affected by the contact between straw and soil for high-latitude mollisol areas.The straw mixed with soil can accelerate selectively to retain the aromatic C and alkyl C and the formation of soil organic matter.Meanwhile,the process of straw transformation into soil organic matter was protected by the structure of microbial community,and promoted the formation of aggregate and the combination of clay minerals.Straw can be converted into soil organic matter by microorganisms,which can sequester organic carbon,improve soil properties,maintain soil fertility and realize sustainable development of farmland production.