| Nitrogen(nitrogen,N)is an indispensable mineral nutrient element in plant growth,known as "life element",and is also a major limiting source of productivity in many terrestrial ecosystems.At present,there are many researches on the change of soil N component by vegetation restoration,but most of them focus on the effect of vegetation restoration on a single soil N component.There are still few comprehensive studies on the effect of vegetation restoration on various N components,especially;there are few reports on the effects of subtropical vegetation restoration on soil soluble N components.The effect of vegetation restoration on soil N pool composition and availability is still unclear.Therefore,we adopted the method of space instead of time to select four adjacent plant communities with almost the same habitat conditions in the subtropical hilly area of Hunan Province.The four vegetation communities were composed of Loropetalumchinense+Vaccinium bracteatum+Rhododendron simsii scrub-grass-land(LVR),L.chinense+Cunninghamia lanceolata+Quercus fabri shrubbery(LCQ),Pinus massoniana+Lithocarpusglaber+ L.chinense coniferous-broad leaved mixed forest(PLL),L.glaber+Cleyera japonica+Cyclobalanopsis glauca evergreen broad-leaved forest(LAG)to represent the secondary forest successional sequence in this region.Study the change of soil soluble N component content,density and soil physical and chemical properties,and micro.And indoor aerobic culture method is used to study different vegetation restoration stages of seasonal change of soil N mineralization characteristics,revealing the forest soil effective N libraries and their conversion rate of the response mechanism of vegetation restoration,for scientific evaluation of the effect of subtropical forest vegetation restoration and reconstruction,as well as the scientific measures for sustainable forest management and provide a scientific basis for soil N library management.The main results are as follows:(1)Vegetation restoration significantly affected soil physical and chemical properties and microbial biomass.With vegetation restoration,soil bulk density and pH value decreased,but there was no significant difference among different restoration stages.The contents of clay fraction(<2 μm),organic carbon(SOC),total nitrogen(TN),total phosphorus(TP),microbial biomass carbon(BC),nitrogen(BN),phosphorus(BP)in each soil layer increased significantly with vegetation restoration,and compared with LVR,The contents of clay fraction<2 μm in LCQ,PLL and LAG soil layers increased by 2.36%65.68%,SOC content by 30.47%-127.41%,TN content by 97.44%-216.67%,TP content by 10.00%-108.33%,BC content by 31.61%-74.96%.BN is 54.27%-113.55%,BP is 12.90%85.05%.(2)The content and density of soluble N component in soil were significantly affected by vegetation restoration.The content of soluble organic nitrogen(SON)and ammonium nitrogen(NH4+-N)increased significantly with vegetation restoration.Compared with LVR,the contents of SON in 0-20 cm soil depth in LAG,PLL and LCQ increased by 35.23%,26.75%and 18.34%,respectively,and NH4+-N contents increased by 85.51%,45.58%and 37.10%,respectively.Nitrate nitrogen(NO3--N)content in all soil layers decreased first and then increased with vegetation restoration,and LAG was the highest,while LCQ was the lowest.SON content was the highest and NO3--N content was the lowest in each vegetation restoration stage.With vegetation restoration,the percentage of SON content in TN content increased,while the percentage of NH4+-N and NO3--N decreased.The density of SON,NH4+-N and NO3--N in 0-20 cm soil layer increased with vegetation restoration.SON density was the highest,followed by NH4+-N,and NO3--N density was the lowest.Compared with LVR,SON density of LAG,PLL and LCQ 0-20 cm soil layer increased by 267.20%,98.40%and 86.30%,NH4+-N increased by 17.7%,7.9%and 11.6%,and NO3--N density of LAG increased by 13.16%.The growth rate of SON and NH4+-N density was"fast-slow-fast",while that of NO3--N density was "slow-slow-fast".The effects of soil physicochemical properties and microbial biomass on SON and NH4+-N were greater than those on NO3--N.The content of NH4+-N in 0-20 cm soil layer at each vegetation restoration stage was higher in non-growing season than in growing season,and LAG,PLL and LCQ seasons had significant differences.The NO3--N content in non-growing season was lower than that in growing season,but the difference between LVR and LAG seasons was significant.SON content in non-growing season was lower than that in growing season,and there was significant difference among seasons in each vegetation restoration stage.(3)Soil N mineralization was significantly affected by vegetation restoration.In both growing season and non-growing season,the net ammoniation,net nitrification and net mineralization of soil increased significantly with vegetation restoration.Compared with LVR,the net ammoniation amount of LCQ,PLL and LAG in 0-20 cm soil layer increased by 25.00%-67.86%,the net nitrification amount increased by 357.41%-1219.44%,and the net mineralization amount increased by 212.50%-726.04%in non-growing season.In the growing season,it increased by 57.67%-122.75%,67.47%-379.26%,64.61%-304.33%,respectively.Nitrification was the main process of soil N mineralization in different vegetation restoration stages,and the net nitrification accounted for 56.25%-89.85%of the net mineralization,which increased with vegetation restoration.The net ammonification rate,net nitrification rate and net mineralization rate were positively correlated with annual litter amount,clay content<2 μm,SOC,TN,TP content,BC,BN and BP.The seasonal variation of net ammonium,net nitrification and net mineralization in 0-20 cm soil layer at each vegetation restoration stage showed that the growing season was significantly higher than the non-growing season. |
