| Unreasonable land uses and managements have led to serious soil degradation on theLoess Plateau, which is a threat to the land resources. Therefore, improving soil quality orspeeding up the vegetation restoration has become an important goal of ecological restorationon the Loess Plateau. This study selects different land use patterns (natural grassland, orchard,farmland, abandoned land) and different revegetation modes (Robinia pseudoacacia,Caragana korshinskii, mixed forests and abandoned land) in the Zhifanggou catchment atAnsai, Shaanxi Province. The site is typical of the Loess Plateau’s hilly region. Soil sampleswere collected from the0～10cm,10～30cm and30～60cm depths and analyzed todetermine soil N content (nitrate-N, ammonium-N, organic N, mineralizable N, microbialbiomass N, soluble organic N) and microbial activity (phospholipids fatty acid (PLFA),Microbial biomass C (Cmic), Microbial biomass N (Nmic) and Microbial biomass P (Pmic),soil basic respiration, etc), in order to detect the change of soil N, microbial activity and theirrelationships. The main conclusions are as follows:(1) In different land use patterns, soil total N was highest in the natural grassland, soilnitrate-N, ammonium-N, organic N, mineralizable N and soluble organic N in naturalgrassland were significantly greater than the farmland, orchard and abandoned land; soil Cmic,Nmic, Pmic and microorganism entropy were high overall in the natural grassland. Soil Nmicwas significantly higher in the orchard and farmland than in the natural grassland andabandoned land. Soil Cmic, Nmic, Pmic were lowest in the abandoned land. The change ofsoil total phospholipid fatty acid (PLFA), that is soil total living cells biomass, was as follows:natural grassland> farmland> orchard> abandoned land. Soil total microorganism biomassand activities were highest in the natural grassland. Overall, the natural grassland was a goodway to improve comprehensive soil quality compared with other land uses.(2) In the treatment of natural abandoned soil, soil total N significantly increased as theabandoned age increased: in the soil depth of0～10cm,10～30cm, Soil total N of8-yr-abandoned land were approximately2~3times to that of1,2-yr-abandoned land. Theresponse of N content to abandoned-year was great different. Five different forms of N(except for mineralizable N) were significantly higher (p<0.05) in the8-yr-abandoned than in the1,2-yr-abandoned land. Furthermore, soil total phospholipids fatty acid (Total PLFA),microbial biomass, microorganism quotient and basic respiration were also significantlyhigher (p<0.05) in the8-yr-abandoned than in the1,2-yr-abandoned land. To sum up,long-term abandoned treatment was an effective measure to maintain and improve soilfertility.(3) During the process of artificial Robinia pseudoacacia plantation, the change of soil Nwas as follows: Soil nitrate-N, ammonium-N, total N and organic N content graduallyincreased as plantation age increased but decreased as soil depth increased. Total soil N in the0～10cm depth increased from0.40g kg-1in the10-yr-old plantation to0.87g kg-1in the30-yr-old plantation. This was an increase on the soil N fertility scale from level-7(﹤0.05%)to level-5(0.076～0.1%). Soil Cmic and Pmic both increased as plantation age increased anddecreased as soil depth increased. Furthermore, the microorganism quotient increasedsignificantly as plantation age increased. The soil metabolic quotient of all three soil depthsdecreased significantly as plantation age increased. The R. pseudoacacia plantations alsoimproved some other physical and chemical soil properties: Organic C and available P, whichwere sensitive to changes of soil microorganism, increased significantly as plantation ageincreased. The R. pseudoacacia increased the amount of carbon available to soilmicroorganisms and also improved the soil carbon utilization. So the Cmic and Pmic weresignificantly increased. The change of soil Pmic was consistent with the main microbialgroups. Moreover, the increase in microbial activity increased the content of soilmineralizable N, nitrate-N, soluble organic N and overall soil fertility.(4) In different artificial forests plantation modes: the comprehensive improvement ofsoil physical and chemical properties was: R. pseudoacacia> Caragana korshinskii>2-mixed forest (Chinese Pinus+Amorpha fruticosa)>3-mixed forest (Robiniapseudoacacia+Prunus davidiana+Prunus sibirica). The changes of Soil N were as follows:Five different forms of N content (except for soluble organic N) were the lowest in the5-yr-old3-mixed forest,2-mixed forest had good N accumulation effect: Soil ammonium-Nand organic N were almost the same in20-yr-old2-mixed forest and30-yr-old C. korshinskii.Soil ammonium-N, nitrate-N, organic N, mineralizable N and microbial biomass N followedthe same order of R. pseudoacacia> C. korshinskii>2-mixed forest>3-mixed forest, whichwas the same with the change of soil physical and chemical properties. Overall, Soil Ncontent, physical and chemical properties, microorganism biomass and microbial activitywere all best in R. pseudoacacia, the plantation of R. pseudoacacia is a good way to improvesoil comprehensive quality.(5) Different slopes (shady, sunny, half-shady and Hilltop) were chosen, in order to study the effect of space variation to C. Korshinskii soil properties. The results showed that thedifference of slope had significantly effects (p<0.05) to six nitrogen forms (nitrate-N, organicN, mineralizable N, microbial biomass nitrogen (Nmic), soluble organic N, ammonium-N).Soil nitrogen contents, basic respiration and microorganism entropy were significantly higheron the shady and half-shady slopes than on the sunny slope and hilltop (p<0.05), Pmic on thehilltop was significantly higher than other sample sites (p<0.05).(6) In this study,28different phospholipids fatty acids were detected:10:02-OH,12:0,12:02-OH,12:03-OH,13:0,14:0,14:02-OH,14:03-OH, i15:0, a15:0,15:0,16:02-OH,i16:0,16:1ω7c,16:0, i17:0, cy17:0,17:0,10Me17:0,18:2ω6c,9c,18:1ω9c,18:1ω9t,18:0,10Me18:0,18:1ω7, cy19:0,10Me19:0,20:0. In different revegetation patterns, soil totalPLFA, the relative abundance of bacteria (aerobes, anaerobes and gram-positive bacteria (G+))and actinomycetes increased with increasing R. pseudoacacia plantation age, while soil fungirelative abundance decreased; in different plantation modes, the soil total PLFA, the relativeabundance of bacteria (aerobes, anaerobes and G+) was all significantly higher than C.korshinskii,2-mixed and3-mixed forest; soil total PLFA was highest on the sunny slope butthe relative abundance of bacteria and actinomycetes on the shady slope was significanthigher than other sites. From the distribution features of the microbial groups, in the differentland uses and various revegetation modes, the bacteria had the biggest percentage in totalphospholipid fatty acid, followed by fungi and actinomycetes, and unknown bacteria the least(protozoa was not detected). In terms of main bacteria distribution features, the G+had thelargest percentage of total PLFA, followed by aerobic bacteria. |
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