The Structure Of Soil Organic Carbon And Enzyme Activity Under Different Land Types

An understanding of the effects of different land uses on soil organic carbon structure and enzyme activities is crucial for the improvement of soil quality and carbon sequestration capacity.Four land types were selected（wetland,grassland,cropland and shrubland）.Soil samples were respectively collected at 0～10cm,10-20cm and 20-30cm.Through the laboratory analysis of soil active organic carbon and nitrogen fractions,¹³C CPMAS NMR analysis of organic carbon structure,and soil incubation the enzyme activity（β-glucosidase,cellobiohydrolase,N-acetyl-glucosaminidase,leucine aminopeptidase,glycine aminopeptidase,acid phosphatase,alkaline phosphatase and phenol oxidase）were determined.The results indicated:（1）Wetland sites had the highest soil active organic carbon and nitrogen fractions under four land types.Soil dissolved organic carbon and nitrogen,nitrate nitrogen and microbial biomass nitrogen contents were significantly decreased when wetland sites were degraded into grassland sites and reclaimed into cropland sites.After cropland sites restoration to shrubland sites,soil easy oxidizable carbon were significantly increased while ammonia nitrogen and microbial biomass nitrogen contents were significantly reduced.（2）By ¹³C CPMAS NMR analysis,the alkoxy and alkyl carbon of soil organic carbon structure were relatively higher under different land types.The polyhydroxy carbon in the alkoxy carbon,aromatic C-0 in the aromatic carbon and COO/N-C=O group carbon in the carboxyl carbon were relatively higher.In comparison to wetland sites,the grassland sites showed slightly lower alkyl and aromatic carbon,and higher alkoxy carbon and carboxyl carbon.Compared to the wetland sites,the aliphatic carbon/aromatic carbon was increased,whereas the hydrophobic carbon/hydrophilic carbon was decreased at grassland sites.After wetland reclamation,alkoxy and alkyl carbon were declined while the aromatic carbon and carboxyl carbon were increased at cropland soil 0～10cm depth.In addition,cropland sites had higher alkyl/alkoxy carbon and hydrophobic/hydrophilic carbon.However,in the 20～30cm soil depth,wetland sites had lower the alkyl/alkoxy carbon and hydrophobic/hydrophilic carbon.Cropland converted into shrubland soil,aromatic carbon and carboxyl carbon were decreased in the 0～10cm soil depth.In all,wetland sites had the highest soil stability of organic carbon structure.When wetland sites were degraded into grassland sites and reclaimed into cropland sites,the degree decomposition of soil organic carbon increased and soil stability of organic carbon structure decreased.（3）Wetlands degradation into grasslands and reclamation into croplands,the phenol oxidase activity was respectively increased by 1 and 1.7 times.However,the activities of β-glucosidase and cellobiohydrolase were speedly reduced.Compared to wetland soil,they respectively decreased by 26.09%and 17.84%at grassland sites,and decreased by 45.64%and 38.12%at cropland sites.At shrubland sites cellobiohydrolase and acid phosphatase activities were higher,whereas aminopeptidase and alkaline phosphatase were lower compared to the cropland sites.In whole,the wetlands degradation into grasslands and reclamation into croplands,the phenol oxidase activity was significantly increased,whereas hydrolase activity was significantly decreased.In sum,it is very necessary to wetland protection and cropland restoration in order to improve soil quality.