| In this study, we take the typical plants litter on grassland ecosystem in C hinese loess hilly- gully region as the object, using field simulation(Natural condition) and laboratory incubation(remove the effects of environmental factors) methods to study the litter decomposition. According to monitor the litter dynamic tendency of dry matter weight, and calculate the weight losing rate, to describe plant lit ter decomposition process and the decomposition rate variation. Determinating the contents of C, N, P, lignin, cellulose and tannin, calculating the ratios of C/N, C/P and the other stoichiometry to research the inflections among the litter substrate quality, soil properties and decomposition rate. In order to reveal typical plant litter dynamic decomposition process o n grassland ecosystem in Chinese loess hilly- gully region, establish litter decomposition process model, provide the theory basis for the material and energy benign circulation promoting, carry out in-depth scientific support on structure, function and dynamics variation of grassland ecosystem. The main conclusions are as following:(1) In litter decomposition process, the dynamic changes of litter organic carbon shown the trend of fluctuation. In field simulated experiment, litter organic carbon released in spring, summer and autumn, and accumulated in winter. Along with the litter deco mposition, the effect of releasing and accumulation changed slowly. In different plant, the lossing rate was fastest in Caragana korshinskii(Ca.K), the index of NAI decreased by 56%, and it was slowest in Thymus mongolicus(Th.M), the index of NAI just released 37 %.In different organ, the losing rate of leaves were higher than roots and steams, the large aperture litter bags were more conducive for litter organic C decomposition. Litter organic C was in the process of releasing, it was also the process of exogenous carbon supply and re-release. In early decomposition stage, litter total N emergence a great leaching release, and then the element migration basically follows: enrichment—release volatility cycle model. It has no obvious seasonal variation. In different plant, the litter losing rate was slowest in Stipa bungeana(St.B: In field simulated experiment, the index of N AI just released 39 %). Litter total P in early decomposition stage with a quite rapid releasing, in the first decomposition cycle(0-375 days), litter total P completed the leaching in lager scale and than enriched in the latter period. In different plant, Agropyron cristatum(Ag.C) and Stipa bungeana(St.B) was fastest in total P decomposition(NAI: 85-86 %), and Thymus mongolicus with the slowest releasing rate(NAI: 85-86 %). The losing rates of leaves and steams were greater than roots(expect Stipa bungeana), there were no significant difference between lager and small aperture litter bags. In the whole decomposition process, lignin, cellulose and tannin decomposition to follow: enrichment—release volatility cycle model. Litter cellulose and lignin were accumulated in spring, summer and autumn, and released in winter. In different, Agropyron cristatum(Ag.C) were faster in lignin and cellulose decomposition, and the amount of Caragana korshinskii(Ca.K) tannin was largest(NAI: 76-85 %).(2) Litter losing rate in different typical plant were shown ascendant trend in the whole process. The rate of Agropyron cristatum(Ag.C) and Artemisia vestita(Ar.V) were maintained upward trends. Decomposition appears obvious stages in Stipa bungeana(St.B) and Thymus mongolicus(Th.M) litter, they are in the alternations of slow and fast. For Artemisia grialdii(Ar.G) and Caragana korshinskii(Ca.K), the substantial increasing were shown in the early stage, and then kept the rate slow. In different typical plant, the decomposition rates were faster in Agropyron cristatum and Caragana korshinskii, and lowest in Artemisia grialdii. According to estimate the time that litter decomposition requied in different stages by process model, we discovered that the time Agropyron cristatum(Ag.C : 2003 days—5.49 years) decomposition needed was shortest, Caragana korshinskii(Ca.K : 2673 days—7.32 years) secondly, and Artemisia grialdii(Ar.G) was longest(14675 days—40.21 years.). As the research shown, litter decomposition rate were significant related with total N(positive) and C/N ratio(negative), correlated with N/P(positive) and lignin/N(negative). The higher initial N concentration and N/P, the lower C/N ratio are the direct causes that made Caragana korshinskii(Ca.K) with rapid decomposition. For Artemisia grialdii(Ar.G), although with higher initial N concentration, C/N, C/P, lignin/N and strong inhibition by lignin in later phase which lead to the litter decomposition was fast decomposed in the early stage, and gradually slowdown in later. Litter initial N content is the dominant factor that effect litter decomposition in the early decomposition stage. But in later phase, litter decomposition is controlled by the lignin. In different organ, the decomposition rates of leave and steam are faster than root, large-aperture lens decomposition bag is more conducive for litter decomposition(except Caragana korshinskii).(3) Litter labile C and N decomposition were beneficial for soil C and N supplement, soil organic C and total N were increased in different degree, but it was just small increasing. However, soil total P, nitrate N and ammounium N were gradually consumed in the decomposition process. In different plant, the ratio of C /N shown the same trends that was decreased at first, then increased and re-decreased. Agropyron cristatum(Ag.C) and Caragana korshinskii(Ca.K) with the obvious releasings, the ratio was decreaed 3.34 that compared with original value. The ratios of soil C/P and N/P showed an overall increasing trend, it was obviously in Thymus mongolicus(Th.M: soil C/P ratio), the ratio was twice higher than original one. In the decomposition process, the dynamic changes of soil microbial biomass C was shown as the normal distribution, they were increased in different degree, except Agropyron cristatum(Ag.C). Soil microbial biomass N showed a n fluctuation changes, the trends with different change in latter decomposition, Caragana korshinskii(Ca.K), Agropyron cristatum(Ag.C) and Stipa bungeana(St.B) were continue rised, the other were declined in different extent. Soil microbial biomass P were increased first, and then decline in the decomposition process, they were increased in different degree. Among the classic plant, the changes of Thymus mongolicus(Th.M) and Agropyron cristatum(Ag.C) were obviously. Soil microbial biomass C/N ratio showed a flucturation trend that was increased firstly and then decreased. However, soil microbial biomass N/P were maintained an upward trend. In different plant, the ratio of Cmic/Nmic was decreased greatly in Caragana korshinskii(Ca.K), Thymus mongolicus(Th.M) and Agropyron cristatum(Ag.C), and the ratios of Nmic/Pmic was increased largely in Stipa bungeana(St.B). Caragana korshinskii(Ca.K), as the nitrogen fixing plant, the characteristics of higher N contentration, lower C/N ratio and strong abilities of biological nitrogen fixation made the decomposition rate of it is faster, the nitrogen returning is more than the other plant. Caragana korshinskii(Ca.K) is the tycial plant on soil N contribution.(4) Soil invertase activity showed a trend of decreasing after first increased, soil urease activity showed as the changes of descending. The variations of Caragana korshinskii(Ca.K) was maximum, and Artemisia grialdii(Ar.G) was minmum. Litter decomposition process would stimulate soil invertase activity and promote carbohydrates conversion, which make soil invertase activity increased in a certain period, but it would decreased gradually follows by nutrient releasing. Soil urease activity reflects soil nitrogen supplement, Caragana korshinskii(Ca.K) decomposition is the most obvious plant with nitrogen supplyment. Soil protease activity reflect how many available nitrogen source that soil have. In different plant, litter decomposition of Caragana korshinskii(Ca.K), Agropyron cristatum(Ag.C) and Stipa bungeana(St.B) with higher soil protease activity that provides more effective nitrogen sources to soil microorganism. When soil cellobiase enzymolysis those litter with higher cellulose, such as Stipa bungeana, Agropyron cristatum and Thymus mongolicus, carbon source cannot satisfy soil microorganism requirement, the cellobiase activity would decreased fast.(5) The correlations between litter losing rate and soil environmental factors and climatic factors shown, precipitation, sunshine duration are the important factors that affect litter decomposition. Soil temperature, air temperature and humidity have different de gree influence on litter decomposition. Litter decomposition are mainly depends on rainfall leaching and physical fragmentation. Long time sunshine, intensity light and amount of rainfall accelerate litter direct leaching and physical fragmentation process. And synergistic effect with temperature that affect soil microbial groups and activity which speed up litter decomposition. Litter losing rate was correlated with soil temperature, air temperature and humidity in negative way. The influences of soil temp erature and air humidity on roots and steams were more effective than on leaves. Soil temperature was significant related with the root sof Agropyron cristatum(Ag.C in lager and small aperture bags:-0.7625 and-0.7455) and the steams of Artemisia grialdii(Ar.G in lager and small aperture bags:-0.7382 and-0.6886).(6) After a series of screening and contrasting, we used the Logarithmic equation that comply with the laws of nature and the Boltzmann equation that with the S curve, and comply with the phase characteristics to regress litter decomposition process in field simulation and laboratory incubation, respectively. Using litter decomposition model to simulated and forecasted typical plant litter decomposition process, we discovered litter decomposition process is the process of dry matter quality losing. When the dry matter loss to the limit value, litter decomposition tends to stagnate or with very slow rate. In different typical plant litter, due to the Agropyron cristatum with the persistent decomposition and fast litter losing rate which make it with the most complete decomposition. In field simulation, leaf and steam decomposed more completely than root, the litter in large-aperture lens decomposition bag is decomposed faster than in little-aperture lens, but litter in little-aperture lens were decomposed completely. |
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