Effect Of Vegetation Restoration On Soil Nitrogen Mineralization And Enzyme Activities In Mountainous Area Of Southern Ningxia, Northwest China

Unreasonable land uses and managements leads to serious soil degradation inmountainous area of southern Ningxia, which is a threat to the land resources and ecologicalenvironment. Revegetation was an important way for improving soil structure and ecologicalrestoration in this area. However, the effect of vegetation restoration on the dynamics of soilnitrogen components and related enzyme activity during soil in-situ mineralization was notclear yet. Therefore, In order to study the effect of different revegetation pattern on thedynamics of soil nitrogen component and enzyme activities, soil mineral nitrogen (NH4-N,NO3-N, NO2-N) and net mineralization rate, organic N and its components (acid hydrolysis N,hydrolysis ammonia N, amino N, amino sugar N, unknown N and acid unhydrolysis N) andmicrobial biomass N, soluble organic N, four enzymes (Protease, Urease, Nitrate reductase,Arginine deaminase) activities was investigated under three different grassland (naturalgrassland, artificial turf and abandoned land), three different woodland (caragana, hickory,apricot) and three land use patterns (shrubbery, grass intercropping, artificial turf) inmountainous area of southern Ningxia， Northwest China, using the Close-Top TubeIncubation method. To reveal the effects of different vegetation types, land use and seasonalvariation on soil nitrogen transformation and related enzyme activities. The mainconclusions of this study showed as follows.(1)Soil basic physical and chemical properties changed significantly with incubationtime and season variation in different soils. The contents of organic carbon, C/N were highestat incubation initial period (0d,spring), while it decreased with incubation time and the arrivedthe lowest at120d (August, summer). Soil bulk density not changed with incubation time.Study area soil is alkaline, pH were about8.5. Soil organic carbon content changed indifferent soils as follow: natural grassland>natural restoration grassland>artificial turf,caragana>wild apricot> hickory, shrub land> grass intercropping> artificial grass. Due tohuman disturbance, C/N and soil bulk density were significantly higher in natural grasslandthan other restoration vegetations.(2)Inorganic nitrogen content of different soil changed significantly with incubation time and seasonal variation, and its content were lowest in120d (August). Soil mineralization,nitrification and ammonification rates were lowest in June-August period. This was becausesoil moisture content was low during this period, In addition, one of the reasons was retainingmore nitrogen for vegetation growth, There was a significant positive correlation among thesix N fractions. Different soil, nitrogen content and net mineralization rate, net nitrificationrates, net ammonification rate were showed natural grassland>natural restoration grassland>artificial turf, caragana>Prunus>apricot, shrub land>grass intercropping>Artificial grass. Itmeans that natural grassland and caragana shrub land can improve the transformation of soilnitrogen and improve the nutritional status of Loess Hilly areas.(3)Organic nitrogen in different soils were relatively stable in the whole incubationprocess. Microbial biomass nitrogen and soluble organic nitrogen showed significantdifferences in different incubation time, and its content were lowest in120d (August). Totalnitrogen content of the acid hydrolysis solution (acid hydrolysis N) showed lower atincubation initial period and then increased, finally remains unchanged, and its content waslowest at120d(August), hightest at360d (next April). Acid unhydrolysis N showed theopposite trend, and its contents was highest in the initial incubation period, was lowest in360d (April), the left organic nitrogen components has different variation for differentvegetation. Each organic nitrogen components and organic carbon, total nitrogen wereshowed a significant positive correlation. Organic nitrogen, microbial biomass nitrogen,soluble organic nitrogen and organic nitrogen components were showed naturalgrassland>Natural restore grassland>artificial grass, Caragana>wild apricot>hickory, shrubland>grass intercropping> artificial turf(4)Protease, urease, nitrate reductase and arginine deaminase activities showedsignificant differences in different incubation time. Except protease, other three enzymeactivities showed rise in incubation initial period and then decreased, and its activity werehighest at360d, and were lowest in60d. Protease activity was highest in natural grassland at240d, and were highest at60d in the natural recovery grassland, grass intercropping andshrubbery, and were highest at180d in artificial grass and three kinds of woodland soils, Thismay refelected that protease activity were relatively sensitive to changes of soil andvegetation and hydrothermal conditions. Activities of four enzymes were showed naturalgrassland> Natural restore grassland> artificial grass, they were highest in Caragana shrubland, followed by hickory, apricot minimum, shrub land> grass intercropping> artificial grass.Under different vegetation, soil enzymes activities showed a significant positive correlation,with organic matter, various forms of nitrogen.