| Soil DON has high activity and it is easy to migrate though only account for a small part of soil N,and it is the main part of dissolved N in soil and water.DON is an important intermediate product of soil N transformation in forest ecosystems.It is directly related to the utilization efficiency and migration intensity of soil N,and plays a vital role in maintaining the N turnover and nutrient balance in terrestrial and water ecosystems.Understanding the dynamics,transformation,and ecological functions of soil DON in forest ecosystems has important theoretical and practical significance.The transformation of soil N is a key process in the ecosystem during the succession of forest.The sources and fate of soil DON are extensive,and are affected by multiple biological and abiotic factors.In previous studies,more attention had been paid to the characteristics and differences of soil DON between different regions and different forest types,little was paid to forest soils at different restoration or succession stages in subtropical regions.Changes in composition and structure of plant communities during the succession of subtropical forests profoundly affect the soil N cycling in the forest ecosystems.There is an urgent need to study the size of soil DON pool in subtropical forest,the contribution and influence of litter and soil enzymes on soil DON,the migration and transformation of DON in soil,and the contribution and influence of exogenous N on DON.Answering these scientific questions are of great significance to vegetation restoration and forest ecosystem management in subtropical forest ecosystems.In this study,three forest soils at different succession stages in the subtropical area of Dashanchong Forest Park(early coniferous forest,mid-deciduous broad-leaved forest,and late evergreen broad-leaved forest)were used as the research object.Litter and soils were collected in the field,the characteristics of litter quantity,nutrient content,water holding capacity and seasonal change were studied,and the characteristics of soil DON,soil microorganisms and enzyme activity were studied.The 15N stable isotope technology was used to study the transformation of forest soil DON and the contribution and influence of litter on soil DON using a combination of indoor simulation and field positioning research.Quantified the contribution of litter N to soil DON using undisturbed soil column in subtropical typical forest during the long-term decomposition of labelled litter in the field.The main results were as follows.1.The amount and composition of forest litter with different stages showed unique seasonal changes,and the water holding characteristics of litter also had seasonal changes.Saturated water holding capacity,saturated water holding rate,and the coefficients of water affinity of litter in deciduous broad-leaved forest were the highest,which was significantly higher than that in coniferous forest and evergreen broad-leaved forest.However,the half-saturation time,water absorption rate A value and B value were largest in coniferous forest litter.Saturated water holding capacity of litter was significantly positively correlated with water affinity and saturated water holding rate.Saturated water holding rate of litter was significantly negatively correlated with litter C content and C/N ratio(p<0.01),and was extremely significantly positively correlated with litter N content(p<0.01).2.The DON and enzyme activities in forest soils at different succession stages were significantly different.As the succession stage raised,the concentration of DON was increased relatively and was easier to be transferred,transformed,and utilized.Soil enzymes were an important driving factor for soil nutrient turnover,and the enzyme activities of glucosaminide(NAG),glucosidase(BG)and xylopyranoside(BX)in two types of broad-leaved forest soils were significantly higher than coniferous soil,and nutrient turnover were also faster.The RDA analysis showed that soil SON was positively correlated with the above three enzyme activities,thus soil DON was closely related to enzyme activity.3.Forest soils at different succession stages had different transport and utilization modes of labelled litter-derived DON.Evergreen broad-leaved forest soil can continuously transport litter-derived DON as a source;deciduous broad-leaved forest soil with high soil microorganisms and enzyme activities not only promoted the formation of soil DON,but also accelerated its transformation and mineralization;coniferous forest soil was more conservative in the using of litter-derived DON,not only litter-derived DON entered the soil slowly,but also had a smaller amount.Forest litter decomposition accelerated the consumption of soil inorganic N in the short term(2 weeks),supplemented soil DON,and accelerated soil DON consumption in the long term;the contribution of litter N to soil DON was only a small part,but this part of soil DON was still accumulated;the litter-derived inorganic N was consumed in a large amount,and was difficult to retain in soil in the later period.The addition of leaves and branches of litter had slightly different effects on soil N,but they all significantly affected the concentration of DON and its dynamic change in three forest soils;compared with leaf litter,the release speed of DON and mineralized N from branch litter was slower.The formation,transportation,and transformation of soil DON play an important role in reflecting the N cycle of forest soil at different succession stages,the utilization strategy of litter N,and the effectiveness of N;4.The one-year-labelled litter in-situ soil column experiment showed that,only 1.8%of soil DON came from labelled litter N and 98.2%from indigenous soil N under the decomposition of two-years of fallen litter during one year.Litter retention accelerated the reduction of soil DON in the first 2 months,while maintained a high DON concentration thereafter.The proportions of soil DON and TN derived from litter were significantly higher in 0-5 cm layer than the bottom soil(0-5 cm,10-20 cm),indicating that litter N mainly concentrated in the surface layer.5.The indoor simulation of the influence of litter DON input on the loss of DON in leachate showed that,the continuous input of DON from litter directly caused the leaching loss of soil DON,and accelerated soil nitrification in a short period,and more soil N was lost in the form of NO3--N.With the continuous increased of leaching times,the proportion of DON in the N component of the leachate continued to increase.The input of exogenous DON was the main body of DON in leachate.In summary,the size and transformation of soil DON changed significantly with the changes of forest succession stages.DON was closely related to forest litter,soil physical and chemical properties,soil microorganisms and enzyme activities.Forest litter was one of the most important sources of soil DON,while it contributed little to soil DON in the short term,even accelerated the migration and transformation of DON,and maintained a high concentration of soil DON in the long-term.The input of litter DON affected the original DON in the soil and directly caused the leaching loss of soil DON.DON is an important characterization that reflects the soil habitat and nutrient status,and plays a very important role in it. |
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