The Initial Responses Of Simulated N Deposition In Litter Decomposition Of Intercropping System Alnus Formosana- Lolium Multiflorum Compound Mode

As one of the most outstanding problems in the current global environmental change, the continuous increase of nitrogen deposition has already had an important impact on the forest ecological system and the process of the decomposition of litter. Meanwhile, the speed of the litter decomposition, one of the important links of material circulation and energy flow in forest ecological system, plays an important role in the forest soil fertility maintaining and ecosystem stability. Whereas, the effect of the input of exogenous nitrogen on litter decomposition process is still controversial due to the different ecological environments, tree species, litter properties, and the decomposition stages. Thus, its mechanism remains to be further studied. Furthermore, the construction of forest-grass complex model is an effective solution for the sustainable development of artificial forests. As one of the tree species returning farmland to forest in China, Alnus formosana is of great importance for its highly ecological and economic values. However, little is known about the response of nitrogen deposition, which is derived of litter decomposition in intercropping system Alnus formosana and Lolium multiflorum compound mode.By positioning simulated nitrogen deposition, the current study aimed to investigate the initial responses of simulated N deposition in litter decomposition of intercropping system Alnus formosana and Lolium multiflorum compound mode. According to the model of the intercropping system Alnus formosana and Lolium multiflorum compound mode (mode A) and the controls (mode B, without Lolium multiflorum lam), the litter decompositions were grouped into the aboveground litter (leafy shoot, fruit and mixtures) and the underground litter (root of Aluns, root of grass and mixtures). Then, the two groups were treated with different concentration of Nitrogen (N) including 0,5,10, and 20 g·N·m-2·a-1 within each mode, respectively. Thus, the response of exogenous N supplement in the complex model of a single grass species & mixed litter decomposition process and its mechanisms were revealed through simulated nitrogen deposition process. After simulated nitrogen deposition processing of litter decomposition la, the main results are as follows:(1) During the one-year study, the rate of several litter decompositions in Alnus formosana abandoned farmland was in line with the Olson negative exponential model. There were differences among the different types of litter decomposition rate, grass roots decomposed fastest, then leaf and root, and branch and fruit were the lowest. Different types of litter components’hybrid decomposition had positively interactive effects, and significantly promoted the decomposition of litter matrix. The construction of Alnus formosana-lolium multiflorum compound mode significantly speeded up the leaf, branch, and fine root decomposition (p<0.05). Simulated nitrogen deposition had promoted the litter mass loss in early stage of the decomposition, and restrained the late stage of B pattern litter composition. The restraint of different nitrogen deposition level on litter decomposition varied from litter types and promoted the litter composition of A pattern. The promotion effect on litter of overground reached significant level; the leaf was the most significant (p<0.05). However, its promotion effect on underground litter was limited, reaching significant level only in the first 40 days. Implantation of lolium multiflorum significantly reduced the half-life of the litter components (T50%) and turnover period (T95%) of Alnus formosana-lolium multiflorum compound mode, and improved the turnover rate of material in ecological system. In the whole study period, the decomposition rate of litter components was significantly correlated with two or more of litter C, N, P, cellulose, lignin, C/N, and lignin/N quality index. The C content contributed 82.6% of the litter decomposition rate. In simulated nitrogen deposition environment, the effect of the litter quality on the litter decomposition rate was disrupted via exogenous N supplement.(2) The process of litter decomposition in Alnus formosana returned farmland indicated net carbon mineralization, and the quality of carbon residue rate conformed to the negative exponential model. Simulated nitrogen deposition significantly reduced the aboveground litter (leaf, fruit, and branch) and the mixture carbon mineralization. However, there were not any significant changes found in carbon decomposition and release for underground litter. The construction of the intercropping system of Alnus formosana-Lolium multiflorum compound mode significantly promoted carbon mineralization in the process of litter decomposition, and effectively neutralized the inhibition of carbon mineralization in the process of litter decomposition. Within la, there were 1-2 processes of Nitrogen enrichments in the components of the litter, and net nitrogen release was observed in the later stage. The dynamics of P indicated that (1) release-enrichment-release for aboveground litter, (2) net nitrogen release in underground litter, and (3) net release for K. In summary, different levels of nitrogen deposition significantly affected the release of N, P, and K, which suggested promotion during the early stage while inhibition in the late stage. Moreover, the implantation of Lolium multiflorum further promoted the release of N, P, and K.(3) Within a period of one-year study, there were markedly seasonal variations existed in six kinds of soil enzymes from Taiwan alder abandoned lands. In general, the activity of polyphenol oxidase, peroxidase, and urease appeared its peak in spring, respectively; cellulase activity peak in summer; acid phosphatase activity peak in fall; and invertase activity peak in winter. Exogenous nitrogen changed the activity of soil enzyme of Taiwan alder farmland; meanwhile, great differences existed because of different nitrogen deposition on soil enzyme activity. Effects of nitrogen deposition on soil enzyme activities had cumulative effects. In the simulated nitrogen deposition environment, exogenous nitrogen input increased soil cellulase and invertase activity, the higher nitrogen levels, the stronger enzyme activity. During the late stage of the simulated nitrogen deposition environment, the invertase activity was significantly inhibited; the activity of urease and acid phosphatase was increased in low N levels while inhibited in high N levels. Exogenous nitrogen input inhibited activity of polyphenol oxidase and peroxidase, and the time and extent of inhibition sustained with higher nitrogen levels. During one-year study, the activity of six kinds of soil enzymes were positively correlated with the accumulation of litter decomposition rates with different N supplemental levels in the simulated nitrogen deposition environment. After planting perennial ryegrass, the soil enzyme activity was obviously improved, which is of great benefit to the use of soil nutrient cycling.(4) Litter decomposition nutrients are the important origins of soil nutrients. Nitrogen deposition increased the contents of TOC, TN, AN, TP, and AP, and reduced C/N and C/P in soil. TOC、N and P had significantly positive relationship with nitrogen deposition, meanwhile, the increasing of nitrogen deposition may enhance the soil fertility and plant growth, thus increased carbon fixation. After one-year nitrogen deposition simulation research, the contents of TN and TP in soil of Alnus formosana-Lolium perenne compound mode were lower than that in soil of CK. However, TOC, AN, AP in soil of Alnus formosana-Lolium perenne compound pattern were higher than that in soil of CK. Although planting perennial ryegrass increased the absorption does of soil nutrient such as N and P, the fast turnover of litter (mainly underground root) improved soil enzyme activities and the effectiveness of N and P, which is conducive to soil nutrient utilization and circulation, as well as absorption and storage of carbon sequestration of system.Above all, the effect of nitrogen deposition simulation on Alnus formosana plantation without artificial grass was that, promoted litters decomposition at earlier stage, and suppressed litters decomposition at later stage. The inhibitory effect of nitrogen deposition level on litter decomposition was distinct as litter types, which was mainly caused by the suppressed decomposition enzyme activities at later stage. Planting with perennial ryegrass significantly increased the soil enzyme activity, improved the soil environment, promoted the absorption of exogenous N supplement, relieved the inhibitory effect of nitrogen deposition on litter decomposition, and promoted the litter decomposition rate significantly. Therefore, constructing grass and compound ecosystem in high nitrogen deposition area could relieve the inhibitory effect of nitrogen deposition on litter decomposition, promote the matter turnover in ecosystem, and improve the carbon sequestration of system. Furthermore, it is of theoretical and practical importance for plantation ecosystem to maintain planted forestland and increase atmospheric nitrogen deposition continously.