Co-variation Of Vegetation And Soil Quality During Ecosystem Restoration In The Hilly Region Of The Semiarid Loess Plateau

The hilly region of the semiarid Loess Plateau locates the ecosystem fragile zone innorthwest china, and is one of the most severe ecosystem degraded areas. Serious soil erosionand water loss, vegetation destruction, and frequent arid and sand blown by wind in recentyears which led to ecosystem instability. It was not only a threat to the sustainabledevelopment for the region, but also maybe impact other nearby areas and east regions. Soilquality restoration and soil management is the key for vegetation construction andeco-environment sustainable development in the semiarid Loess Plateau. Aimed at therequirement of eco-environment construction and the hotspot in soil and environment sciences inthe Loess Plateau, taking on the soil and vegetation by three vegetation restoration measurements(abandoned land, establishing seeded grassland and shrub) in typical hilly region of the semiaridLoess Plateau (Lanzhou region) as research object, through field experiment and lab test,combining with historical data collection and field investigation, soil quality evolvementmechanism in the process of ecosystem restoration was studied in this project. The vegetationtraits and soil quality indicators changes were identified in different vegetation restoration ways,and using some statistical analysis tools (Duncan's multiple range comparison, simplecorrelation and principal components analysis) to analyses quantificational soil quality factorsfor opening out the relationship between vegetation development and soil quality factors.Finally recommendation for vegetation restoration and soil quality improvement was proposed.The goal was to provide scientific reference for vegetation rehabilitation and soil quality care inthe semiarid Loess Plateau, and enrich the theory and method for soil quality management. Themain results were as follows:1. Co-variation of vegetation and soil quality in different age alfalfa fields:(1) The vegetation traits of different old alfalfa fields: the yields of alfalfa increased withthe prolong of planted ages from 3- to 9-year-old, but the speed of alfalfal yield increasing wasfaster during the period of time from 3- to 5-year-old, and the yield was relatively stability in9-year-old alfalfa field, the largest yield was 406 g hay/m^-2, alfalfal yield would decrease after9-year-old with the degradation of alfalfal. The evegetation coverage and plant height trends weresimilar with the yield, with the largest values 85.5% and 52.8cm, respectively. Alfalfa severedecay and lots of them dead after 20 year, the yield observably decreased, the coverage and plantheight were very low. The richness of plant species increased with the growing years. There weremore species richness in 25-year-old alfalfal field, it was 22.(2) The soil quality of alfalfa field indicated distinct different during long-termdevelopment. Soil organic carbon (SOC), total N and available P notable decline from 3- to9-year-old alfalfa fields at 0-60cm depth, the lowest values were 7.95g kg^-1, 1.20g kg(-1) and7.12 mg kg^-1, respectively. The three properties values durative increased after 15-year-old.The pH values gradually decreased with the alfalfa older, it was 7.78 in 25-year-old alfalfafield. Microbial activity indexes (microbial biomass carbon (MBC), microbial biomassnitrogen (MBN), and basal respiration (BR)) continuously declined from 3- to 9-year-old after planted, and the lowest values were found in 9-year-old alfalfa field, they were 88.5.0μg Cg^-1, 16.3μg N g^-1, 0.356 CO₂-Cμg g^-1 h^-1 at 0-20cm depth (spring). They increased after9-year-old, and these values in 25-year-old alfalfa field were 254.6.0μg C g^-1, 52.2μg N g^-1and 0.55 CO₂-Cμg g^-1 h^-1, respectively, at 0-20cm depth (autumn). Soil qCO₂ has reversechange trend with microbial biomass in different old alfalfa fields, the lowest value wasindicated in 25-year-old alfalfa field, and it was 2.25μg CO₂-C kg^-1 MBC h^-1 at 0-20cm depth(autumn), the largest value was 4.02μg CO₂-C kg^-1 MBC h^-1 at 0-20cm depth (spring) in9-year-old alfalfa field.Soil hydrolytic enzyme activities (urease, invertase, neutral phosphatase and cellulolyticenzyme activity) gradually decreased from 3- to 9-year-old alfalfa field with the year gradient,and the change trend was reverse after 9-year-old. Soil oxidation-reduction enzyme activitiesincluded catalase and Polyphenoloxidase activity, catalase activity change was similar with thechange of hydrolytic enzyme activities, but Polyphenoloxidase activity seemed contrary trendwith catalase activity.(3) Simple correlation analyses showed that organic C was positively correlated with alfalfaplanting years (r=0.648), but total N was significantly and positively correlated with alfalfa old(r=0.801^*), and available P was inversely correlated with alfalfa old (r=-0.033). SOC and TotalN were significantly and positively correlated with urease activity (r=0.952^** and r=0.854^*,respectively), and invertase activity (r=0.95^**and r=0.89^*, respectively), and catalase activity(r=0.846^*and r=0.868^*, respectively), and neutral phosphatase activity (r=0.958^**and r=0.843^*,respectively). However, the inverse correlation was found between SOC and total N andPolyphenoloxidase activity (r=-0.944^**and r=-0.905^*, respectively). MBC and MBN wassignificantly and positively correlated with SOC (r=0.875^* and r=0.916^*, respectively), and totalN (r=0.904^* and r=0.892^*, respectively), and alfalfa old (r=0.615 and r=0.694, respectively).2. Co-variation of vegetation and soil quality in different age abandoned lands:(1) Annual and biannual plants (Setaria viridis L., Corispermum declinatum Steph. exStev. vat. tylocarpum (Hance) Tsien et C. G. Ma. and Elsholtzia ciliate (Thunb.) Hyland) werethe dominant species during primary differental ages (from 2- to 7-year-old); while someperennials (such as Leymus secalinus (Georgi) Tzvel, Heteropappus altaicus (Willd.) Novopokrand Thermopsis lanceolata) became dominant species in abandoned lands from 7- to11-year-old. Artemisia frigida Willd. replaced the T. lanceolata and became one of dominantspecies from 11- to 20-year-old; and Stipa bungeana Trin., Stipa breviflora Griseb. and L.secalinus (Georgi) Tzvel developed into new dominant species during higher successional stage(from 20- to 43-year-old). However, S. bungeana Trin. and Artemisia gmelinii Web. ex slechm.were steady dominant community in native grassland. The vegetation coverage enhanced withthe abandoned lands' old, the coverage was 20% in 2-year-old abandoned land, and it was50% and 55% coverage was found in 20- and 43- year-old abandoned land.(2) SOC indicated continuously increasing trend with the abandoned development (p＜0.05).Total N and available P showed decreasing trend from 2- to 20-year-old abandoned lands (p＜0.05),but they increased from 20- to 43-year-old abandoned lands. The values of soil bulk density, pHand C/N gradually enhanced from 2- to 11-year-old, but they decreased after 11 year sinceabandoned. MBC values showed significant difference among different old abandoned lands(p＜0.05). MBC indicated decreasing trend from 2- to 7-year-old abandoned lands along timegradient, and it continuously increased after 7 year since abandoned. There were more MBC in 43-year-old abandoned land and native grassland. However, MBN values had not obvious changetrend from 2- to 11-year-old abandoned lands, MBN continuously increased after 11 year sinceabandoned. MBC/SOC ratios were higher from 20- to 43-year-old abandoned lands than thosefrom 2- to 20-year-old. MBC/MBN showed similar trend with MBC/SOC ratios. Basal respirationvalues had significant difference among abandoned lands (p＜0.05), they continuously increasedwith the years of abandoned in successional sere. However, soil qCO₂ indicated inverse trend withbasal respiration, the range of soil qCO₂ values from 5.78 to 9.92 mg CO₂-C (g MBC)^-1 h^-1.Catalase, cellulolytic enzyme, invertase activity and neutral phosphatase activity graduallyenhanced along time gradient (p＜0.05), but polyphenoloxidase activity had no distinct trend.Statistic and analyses results showed that MBC at 0-20cm depth in all differental ages wassignificantly and positively correlated with SOC(r=0.988^**) and total N (r=0.937^**), but wassignificantly and inversely with soil pH(r=-0.935^**). MBN was also significantly andpositively correlated with SOC(r=0.98^**) and total N (r=0.934^**). Six enzyme activities wassignificantly and positively correlated with SOC (r≥0.216, p≤0.041) and total N (r≥0.304^*),MBC, MBN (r≥0.235^*).3. Co-variation of vegetation and soil quality in different age Caragana korshinskii Kom.shrub lands:Soil pH and bulk gradually declined with development of shrub, but SOC and total Nvalues was inverse (p＜0.05). The trend of change NH₄⁺-N and NO₃^--N in shrub lands weresimilar with SOC ((p＜0.05). Microbial activities (MBC, MBN, MBC/MBN ratio, MBC/SOC ratio,BR) indicated distinctly enhance trend along the time gradient (p＜0.05). However, soil qCO₂showed inverse trend with microbial activity change. Catalase, cellulolytic enzyme, invertaseactivity and neutral phosphatase activity gradually enhanced with the shrub growing years(p＜0.05). The difference of soil quality between shrub canopy and alley indicated soil qualityunder C. korshinskii Kom. canopy was better than alley, some soil quality properties values(SOC, total N, MBC, MBN, BR, et al.) under shrub canopy were larger than alley, it mean C.korshinskii Kom. canopy had "islands of fertility" effect.4. Soil microbial activities and enzyme activities factors showed difference in differentsampling seasons, most of them had higher values in summer and autumn than spring. On soilprofile, the values of most of them decreased with the depth. The soil moisture analyses indicatedthe similar rule for three vegetation restoration measures, i.e., soil moisture values decreased withplants growing years or abandoned years increasing, and it resulted in dry soil layer developingwithin different depth range.5. The compare of three vegetation restoration ways affected on soil quality results indicated:shrub establishment and abandoned lands were better for soil quality restoration than developingalfalfa fields during 15 years. The increment of SOC, total N, microbial activities and enzymeactivities in shrub lands and abandoned lands were obvious higher than alfalfa fields, and alfalfaconsumed more soil nutrients and soil water in the process of alfalfa production, which led to soilquality decline. So developing shrub land and abandoned lands was in favor for degradedecosystem restoration in hilly regions of semiarid Loess Plateau. It was also important for seededgrassland to improve management level and technology in forage production in order to avoid soilquality worse.