Profile Distribution Characteristics Of Soil Organic Matter Quantity And Structure In Chernozem Under Different Land Use Types

Urban construction,the demand for food and other pressures have led to great changes in land use types,especially by the natural vegetation（forestland,grassland）into other land use types.It will significantly change the levels of quality and quantity of organic matter entered into each pedogenic horizon of soil,and then it will affect the decomposition of soil organic matter（SOM）and mineralization process.Thus,the quantity,composition and structural properties of SOM are affected.At present,studies on SOM composition and structure change mainly focus on natural vegetation reclamation and conversion of farmland to forestland and grassland（including forestland soil and farmland soil under different management measures）.However,there are few systematic studies on the mid-and long-term changes of SOM after reclamation of natural grassland,and many studies are conducted on soil tillage layer or soil profile of 0～30 cm.In addition,mechanical stratification（fixed depth）sampling method was adopted to ignore the influence of soil genesis on the distribution of organic matter in the profile,and there were few studies on the distribution characteristics and structural properties of organic matter in each soil horizon in 0～100 cm soil under different land use types.Therefore,it is of great significance to understand the cumulative effect and turnover mechanism of organic matter in soil profiles under different land use types to understand soil fertility,global carbon cycle and sustainable land use.Natural grassland,artificial woodland and cropland soil in Chernozem region were selected as the research objects in this study.On the basis of observing and describing the morphological characteristics of soil profile,different soil horizons（A,AB,B_k,BC and C）in0～100 cm soil were collected.Applying density grouping method and humus grouping method,the profile distribution characteristics of SOM quantity,physical components and chemical components in Chernozem under different land use types were studied.The SOM map features and molecular structure of soil in A,AB and B_k horizons were analyzed by ¹³C NMR（nuclear magnetic resonance）,which is of great significance for revealing the transformation modes of soil fertility,scientific and rational utilization of soil resources and establishing environmentally friendly land use patterns.The main results were as follows:（1）The profile distribution characteristics of organic carbon content and storage in Chernozem under different land use types were analyzed.The content and storage of soil organic carbon（SOC）in grassland were significantly higher than those in woodland and cropland,and the A-horizon aggregation in woodland soil was more severe than that in cropland and grassland.After the grassland was converted to woodland and was reclaimed to cropland,SOC content decreased by 10%and 16%respectively,and SOC storage decreased by12%and 15%respectively.From horizon A to horizon C,the SOC content and storage of woodland decreased sharply,while the SOC content and storage of cropland decreased slowly.The SOC content and storage of woodland and cropland were evolving to mature forest ecosystem and farmland ecosystem respectively.The upper soil was mainly affected by land use types,while the lower soil was affected by both land use types and soil properties.Rational land use management policies should be formulated to protect natural grassland from destruction,in order to reduce SOC loss and play an important role of carbon sequestration in grassland ecosystem in semi-arid areas.（2）The SOM density component properties of Chernozem under different land use types were studied.From horizon A to horizon C,light fraction organic carbon（LFOC）and heavy fraction organic carbon（HFOC）storage of grassland decreased gradually,LFOC and HFOC storage of woodland decreased sharply（concentrated in horizon A）,and LFOC and HFOC storage of cropland decreased slowly.However,LFOC storage decreased faster than HFOC storage under the three land use types.After the grassland was converted into woodland,the LFOC mass fraction of woodland in horizon A was significantly higher than that of grassland and cropland,but the LFOC concentration,content and storage of woodland in horizon A were lower than that of natural grassland.Although the SOC profile of woodland was evolving towards mature forest ecosystem,it had not reached a complete equilibrium state.After the grassland was reclaimed to cropland,the mass fraction,content,and storage of LFOC decreased,especially the content（37%）and storage（34%）of LFOC in cropland.Tillage reduces the number of plant residues entering the soil,while tillage destroys the soil structure and exposes SOC to facilitate microbial decomposition,thus changing the content and distribution ratio of LFOC.However,the proportion of HFOC in total organic carbon in cropland increased,indicating that tillage may make SOM tend to be stable and complicated.（3）The characteristics and influencing factors of humus composition in Chernozem under different land use types were analyzed.From horizon A to horizon C,the contents of humus composition in grassland,woodland and cropland decreased gradually with the soil horizon deepens,and the E₄ value of HA all decreased in the three land use types,while the E₄/E₆ ratio all increased.The proportion of humic acid（HA）and fulvic acid（FA）in total soil organic carbon increased gradually with the deepening of soil horizon.However,the proportion of humin（HM）in total organic carbon decreased with the deepening of soil horizon.The content of HA,FA and HM decreased after grassland was converted to woodland and was reclaimed to cropland.After natural grassland was converted to woodland,the light density,aroma and molecular complexity of HA in soil humus increased slightly.After the natural grassland was reclaimed to cropland,the light density,aroma and molecular complexity of HA in soil humus were significantly enhanced.The differences of HA content in soil profiles under different land use types occurred in every soil horizon,while the differences of FA content mainly occurred in horizon A and below horizon B_k,indicating that the impact mechanisms of land use types on HA and FA were different.And FA,which is easily decomposed,was susceptible to land use changes in the surface horizon.The HA and FA contents of grassland were higher than those of woodland and cropland,but the HA/FA ratio was generally lower than that of woodland and cropland,for FA had high chemical activity and was easy to be mineralized,and its content decreased rapidly after reclamation.The contents of total nitrogen and total phosphorus were positively correlated with humus components（p<0.01）,and p H value and soil particles had different effects on humus components.（4）The profile distribution characteristics of combined forms humus content in different land use types were clarified.From horizon A to horizon C,the content of loosely combined humus decreased sharply in woodland,gradually in cropland and slowly in grassland.The stably combined humus of grassland increased first and then decreased,while the stably combined humus of woodland and cropland continued to decrease.The tightly combined humus content of grassland,woodland and cropand continued to decrease.After grassland was converted to woodland and was reclaimed to cropland,causing loosely combined humus,stably combined humus,tightly combined humus content all decreased.Loosely combined humus fell 16%and 28%respectively,stably combined humus fell 17%and 15%respectively,tightly combined humus fell 2%and 6%respectively,loosely combined humus decreased more than stably combined humus and tightly combined humus.After natural grassland was converted to woodland and was reclaimed to cropland,FA in loosely combined humus and stably combined humus were more easily affected than HA,the HA/FA ratio of loosely combined humus and stably combined humus increased,indicating that the degree of soil humus was enhanced.（5）The molecular structure characters of organic matter in Chernozem under different land use types were revealed and quantified.From horizon A to horizon B_k,the dominant functional groups of grassland and woodland SOC were alkyl carbon（24%～36%）,while the dominant functional groups of cropland SOC were both alkyl carbon（11%～32%）and aromatic carbon（23%～36%）.From horizon A to horizon B_k,the proportion of SOM alkoxy carbon decreased（except overall increase in grassland）,the content of aromatic carbon and carboxyl carbon increased,and the stability was enhanced.After the grassland was converted to woodland and was reclaimed to cropland,the proportion of alkyl-oxygen carbon decreased,the proportion of aromatic carbon increased,the proportion of alkyl-carbon increased in woodland and while decreased in cropland.The activity of alkoxo carbon and aromatic carbon was different,and the alkoxo carbon,which was easy to decompose,decreased rapidly after grassland reclamation,so the ratio of functional groups of soil organic matter varied greatly under different land use type.The ratio of alkyl carbon to alkoxo carbon increased overall,aromatic degree increased,aliphatic carbon/aromatic carbon index decreased,hydrophobic carbon/hydrophilic carbon index increased after grassland was converted to woodland and was reclaimed to cropland.The results indicated that after natural grassland was converted into woodland and was reclaimed to cropland,soil humus with simple structure tended to be decomposed by mineralization under disturbance,resulting in an increase in the proportion of complex and stable humus.This study revealed that the SOC content and storage in Chernozem grassland were abundant,and the natural grassland should be protected and rationally utilized to give full play to the carbon sequestration benefit of grassland.After the grassland was converted into woodland,the profile distribution characteristics of SOM in woodland developed towards the mature forest ecosystem,and SOC"surface aggregation"was prominent.However,the woodland was still a carbon volatilization process with single tree species,lack of conservation,and insufficient SOM stability.After reclaimed grassland to cropland,cropland became carbon source,which was strongly influenced by cultivation and fertilization.It was not only environmentally unfriendly,but also destroyed the sustainable use of land resources.In the future,cropland use should strengthen carbon supply.