| Litter, the link between soil and plant, is an important part of nutrient return to soil which has important influence in material cycle of soil ecosystem. Research on the effect of litter decomposition on soil carbon, nitrogen, and soil microorganisms, aimed at the quantity and quality of the sources of soil carbon and nitrogen during revegetation, could further explore soil carbon sequestration and nitrogen circle during revegetation. Based on the Guyuan station of Institute of Soil and Water Conservation, three plant litters were considered as experimental material. We sampled leaf and root of Stipa bungeana Trin.(St.B), Artemisia sacrorum Ledeb.(Ar.S), Thymus mongolicus(Th.M). in the study area, and simulated the litter decomposition by nylon bag method and laboratory culture, and simulated different stage of litter decomposition by different litter adding ways. The variable characteristics of soil carbon, nitrogen, amd microorganism during litter decomposition have been analyzed, and the variable characteristics could explain the effect of soil microorganism on soil carbon and nitrogen, which could provide the scientific basis for soil carbon sequestration, the increasing in soil ecological functional stability after revegetation and the subsequent management of vegetation. The main conclusions are congregated as the following:1. Soil carbon and nitrogen have been greatly affected by water and temperature condition during litter decomposition in the field simulation. Soil organic carbon and nitrate nitrogen at 0-5 cm and 5-15 soil layers increased with the decomposition of leaf and root litter, also soil total nitrogen at 0-5 cm soil layer increased. However litter decomposition had no significant impact in soil ammonium nitrogen. After 741 d litter decomposition, the concentrations of soil organic carbon in the surface soil under St.B, Ar.S, Th.M leaf litter decomposition increased by 1.00, 6.61 and 1.11 g kg-1; the concentrations of soil organic carbon in the surface soil under St.B, Ar.S, Th.M root litter decomposition increased by 1.02, 1.35 and 6.60 g kg-1; the concentrations of soil total nitrogen in the surface soil under St.B, Ar.S, Th.M leaf litter decomposition increased by 0.06, 0.33, 0.18 g kg-1. The influence of litter decomposition on soil carbon and nitrogen increase in this order: Ar.S>Th.M>St.B. Soil organic carbon under Th.M-root litter decomposition was higher than other litter decompositions, and soil total nitrogen under St.B-root litter decomposition was higher than other litter decompositopns. In the addition, in the process of litter decomposition, the increasing in soil nitrogen was observed in the early stage, while soil organic carbon sequestrated in the later stage.2. Both the decomposition of leaf litter and root litter enhanced soil microbial biomass carbon, nitrogen, soil invertase activity, soil urease, and soil nitrate reductase in the field simulation. Apart from soil invertase activity, other soil microbial properties were observed more sensitive than soil carbon and nitrogen to litter decomposition. The rate of change in soil microbial biomass carbon and nitrogen under leaf litter decreased as: St.B>Ar.S>Th.M, under root litter decreased in this order: Th.M>St.B>Ar.S. After 741 d litter decomposition, the rates of change in soil microbial biomass under St.B, Ar.S, Th.M leaf decomposition were 463.7 %, 659.7 % and 361.8 %; the rates of change in soil microbial biomass were 519.0, 461.9, and 676.7 % under St.B, Ar.S, Th.M root decomposition. The increasing in soil urease and nitrate reductase was observed in the later stage, the rate of change in soil nitrate reductase was always higher than 800 %. There were significant differences in soil enzyme activity with different plant litters. Soil invertase activity under Th.M-leaf litter was higher than other leaf litters, and soil invertase activity under Ar.S and Th.M-root litter were significantly higher than St.B-root litters. Soil urease activity under Th.M-leaf litter was significantly higher than other leaf litters, and showed no significantly different under different root litter. Soil nitrate reductase under St.B and Ar.S leaf litter were significantly higher than Th.M-leaf litter, and was higher under Th.M-root litter than other litters. The dynamic of soil microbial biomass and soil enzyme activity showed that more sensitive soil enzyme activity lagged behind soil microbial biomass.3. Soil organic carbon and nitrogen showed different variable characteristics under three leaf litter adding ways. There were no significant differences in soil organic carbon, nitrogen, nitrate nitrogen under surface mulch with leaf litter. Soil ammonium nitrogen increased under surface mulch with leaf litter in the later stage, and increased as Th.M>St.B>Ar.S. Soil organic carbon increased by 37.9 and 9.0 % mixed with bulk St.B and Th.M-leaf litter, and showed the variable characteristics of first increasing, then decreasing, and secondary increasing. Soil total nitrogen increased by 12.4 %, 28.0 %, 12.5 % after mixed with bulk St.B, Ar.S and Th.M-leaf litter. Under mixed with bulk St.B, Ar.S leaf litter, soil total nitrogen showed the variable characteristics of first increasing, then decreasing, and secondary increasing, whereas under mixed with bulk Th.M-leaf litter, soil total nitrogen showed an variable characteristics of first increasing and then decreasing. Soil organic carbon, total nitrogen, nitrate nitrogen greatly increased when crushing leaf litter added. Soil organic carbon increased by 9.0-50.1 %, total nitrogen inceased by 50.1-62.6 %, and soil nitrate nitrogen increased by 238.0-377.9 %. Soil ammonium was restrained when crushing leaf litter added, and showed a wave-like decreasing trend.4. Soil microbial biomass carbon slightly increased in the middle stage under surface mulch with leaf litter. Soil invertase and urease activity remain stable under surface mulch with leaf litter, and the rate of change in soil nitrate reductase was more than 200 % under surface mulch. When bulk leaf litter added, soil microbial biomass carbon increased in this order: Ar.S>St.B>Th.M. Soil urease remained stable, whereas soil invertase and nitrate reductase increased when bulk leaf litter added. After 112 d decomposition, when added bulk St.B, Ar.S and Th.M-leaf litter soil invertase activity were 22.3、23.0、49.3 mg g-1 24h-1 higher than contral soil, and soil nitrate reductase activity were 2.36、1.90、1.13 μg g-1 24h-1 higher than contral soil. When crushing leaf litter added, soil microbial biomass carbon decreased with time. After 112 d decomposition, soil microbial biomass carbon were >430 % higher than contral soil. When St.B-leaf litter add, SMB-C was higher than other leaf litter. We found no significant difference in soil invertase actibity when crushing leaf litter added. When crushing Th.M and St.B leaf litter added, soil invertase actibity was lower than contral soil. Only Th.M leaf litter could enhanced soil urease activity, whereas St.B and Ar.S leaf litter reduced soil urease actibity, this decreasing would be related to the decreasing of soil ammonium.5. Soil microbial communities showed different variable characteristics under different litter adding ways. Under surface mulch with leaf litter, the increasing in soil bacteria and fungus was observed in the early and middle stage, in 28-56 d decomposition soil microbes readjusted to the environment. Also in this stage the change of soil microbial community structure resulted from the increasing of soil fungus and G+ bacteria. The increasing of soil G+ bacteria, actinomycete and fungus were all observed in early stage(7-28 d) mixed with bulk St.B-leaf litter. Soil bacteria increased after 28 days, and soil fungus increased after 56 days when mixed with bulk Ar.S and Th.M-leaf litters. However when mixed with Ar.S-leaf litter the increasing of soil fungus and bacteria resulted in simple soil microbial community, when mixed with Th.M-leaf litter it resulted in more diversified microbial community. When mixed with crushing litter, soil G+ bacteria and fungus were higher than other adding ways. Soil microbial biomass carbon decreased as Th.M>Ar.S>St.B, and there were no significant differences within soil microbial community structures when mixed with St.B and Th.M-crushing litter. Substantial changes in soil microbial community structures were observed when mixed with crushing litter, soil fungus played an important role in litter decomposition. Soil total PLFAs, bacterial PLFAs, G+ bacterial PLFAs and G– bacterial PLFAs showed the trend of decreasing, increasing, and then decreasing, whereas soil fungus decreased with decomposition time.6. Transformation of carbon and nitrogen between litter and soil during leaf litter decomposition could explained that the increasing in soil carbon and nitrogen during vegetation resulted from the litter decomposition. The results of principal component analysis further showed that the decomposition of leaf and root litter could significantly influenced soil environment, especially soil microbial properties and nitrate nitrogen which could reflected the differences of different leaf and root litters, leaf litters in this order: Ar.S>Th.M>St.B, root litter increased as Th.M>St.B>Ar.S. Soils were observed in different characteristics among three litter adding ways. Surface mulch with leaf litter had a little impact on soil microorganisms in the early stage; when mixed with leaf litter soil microorganisms were the most sensitive property, three leaf-litter decreased as: Ar.S>Th.M>St.B; when mixed with crushing leaf-litter, the adding of Ar.S and Th.M leaf litters had greater impact on soil properties.The above conclusions indicate that both the decomposition of leaf and root litter could promote soil organic carbon sequestration, however required a long time. Soil total nitrogen slightly increased, while soil nitrate nitrogen significantly increased with litter decomposition, however if the increasing in soil nitrate nitrogen would result in the loss of soil nitrogen should been studied further. The improvement of soil microbial biomass and enzyme activity induced by litter decomposition indicate that litter plays an important part on soil ecological functions. Therefore, in the subsequent management of grassland vegetation restoration, litter should should been avoid of human influence and been protected to ensure the stability of litter layer, which could promote soil organic carbon sequestration and maintain soil ecological functions during vegetration restoration. |
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