| It is widely recognized that leaf litter decomposition and the accompanying release of nutrients and CO2and formation of SOM are fundamental processes in ecosystem nutrient cycling, C flux, and humus formation. Thus, the study of the processes and the drive kinetics of litter decomposition in subtropical forests in China is important to understanding these ecosystem functioning.With the continuing increase in human activities in recent decades, such as urbanization and industrialization, which leading to a substantial increasing consumption of energy and thus emissions, there are induced climate and environment changes in global scale, such as the increased global temperature, the accelerated rates of acid rain and atmospheric N deposition. The global change may have multiple effects on forest ecosystems. Thus, their effects on ecological functioning of forest ecosystems, such as the drive kinetics of litter decomposition, have stimulated considerable interest, specifically in terms of the global nutrition cycle and its potential contribution to the rates of the ongoing global change in the coming decades.Two dominant litter types were chosen from Zijin Mountain in China:Quercus acutissima leaves from a broad-leaved forest and Pinus massoniana needles from a coniferous forest. The litter samples were incubated in microcosms with their original forest soil and treated with high temperature (10℃warmer, and control:20℃), acid rain (low pH:4.5, high pH:5.0, control:pH5.6), gradient nitrogen deposition (low nitrogen:1g N m-2yr-1, medium nitrogen:2g N m-2yr-1, high nitrogen:3g N m-2yr-1, the form of nitrogen fertilization:NH4NO3, control:water), and different forms of N deposition (the form of nitrogen fertilization:NH4+, NO3-, CO(NH2)2, mixed N (a mix of all three equally), the level of nitrogen fertilization:3g N m-2yr-1, control: water), respectively. During the incubation, chemical composition (i.e., lignin, total carbohydrate, and nitrogen), litter mass losses, soil pH values, and the activities of degradative enzymes were determined. This study was carried out to assess the drive kinetics of litter decomposition under high temperature, acid rain, gradient N deposition, and different forms of N deposition in a subtropical broad-leaved forest (Quercus acutissima) and a coniferous forest (Pinus massoniana) in Zijin Mountain in China:(1) to determine the differences in litter decomposition between broadleaf forest leaves and coniferous forest needles in response to simulated high temperature, acid rain, gradient N deposition, and different forms of N deposition, respectively,(2) to assess the effects of simulated high temperature, acid rain, gradient N deposition, and different forms of N deposition on litter decomposition and related soil enzyme activities, respectively, and (3) to identify the major degradative enzyme contributors which drive litter decomposition under simulated high temperature, acid rain, gradient N deposition, and different forms of N deposition, respectively.The results of this study are as follows:(1) High temperature accelerates decomposition rates of the two litter types, and there was no significant difference in the temperature sensitivity of litter decomposition between broadleaf forest leaves and coniferous forest needles. High temperature also enhances soil enzyme activities (i.e., catalase, cellulase, invertase, polyphenol oxidase, nitrate reductase, urease, and acid phosphatase) in the two forest types, and the temperature sensitivity of polyphenol oxidase was significantly higher than those of the other soil enzymes (i.e., catalase, cellulase, invertase, nitrate reductase, urease, and acid phosphatase). Meanwhile, the temperature sensitivity of nitrate reductase was significantly higher in the coniferous forest soil than in the broadleaf forest soil, while there was no significant difference in the temperature sensitivity of the other soil enzymes (i.e., catalase, cellulase, invertase, polyphenol oxidase, urease, and acid phosphatase) between broadleaf forest and coniferous forest. Catalase and polyphenol oxidase were primarily responsible for litter decomposition in the broad-leaved forest, while catalase was primarily responsible for litter decomposition in the coniferous forest. High temperature tended to accelerate lignin decomposition rates but restrain cellulose decomposition rates for broadleaf forest leaves, while the opposite results were found for coniferous forest needles. The high temperature-accelerated litter decomposition rates may be partly attributed to the accelerated soil enzyme activities. As a long-term consequence, the high temperature-induced acceleration of litter decomposition rates in forest ecosystems may induce C stored belowground to be transferred to the atmosphere as greenhouse gas CO2, which may alter the balance between C uptake and release, and then alter the global C cycle.(2) Acid rain reduces litter decomposition rates of the two little types, and the effects of acid rain on litter decomposition rates of coniferous forest needles were higher than on those of broadleaf forest leaves. Simulated acid rain also reduces the activities of cellulase, invertase, nitrate reductase, acid phosphatase, alkaline phosphatase, polyphenol oxidase, and urease, while it enhances catalase activities in most cases during litter decomposition. Catalase and polyphenol oxidase were primarily responsible for litter decomposition in acid treatments, and catalase was primarily responsible for litter decomposition in control treatment in the broad-leaved forest. Nitrate reductase and urease were primarily responsible for litter decomposition in acid treatments, and invertase was primarily responsible for litter decomposition in control treatment in the coniferous forest. According to the results of this study, soil C in subtropical forests would accumulate as a long-term consequence of continued acid rain. This may presumably alter the balance of ecosystem C flux, nutrients cycling, and humus formation, which may, in turn, have multiple effects on forest ecosystems.(3) Medium-N and high-N fertilization significantly accelerates litter decomposition rates of broadleaf forest leaves, while only high-N fertilization significantly accelerates litter decomposition rates of coniferous forest needles, suggesting that the temperature sensitivities of litter decomposition for broadleaf forest leaves were significantly higher than that for coniferous forest needles under N deposition. N fertilization enhances the activities of catalase, cellulase, invertase, acid phosphatase, polyphenol oxidase, and urease, while it reduces nitrate reductase activities in most cases during litter decomposition. Cellulase and nitrate reductase were primarily responsible for litter decomposition in the broad-leaved forest, while catalase, cellulase, and acid phosphatase were primarily responsible for litter decomposition in the coniferous forest under no N fertilization; catalase, cellulase and acid phosphatase were primarily responsible for litter decomposition in the broad-leaved forest, while catalase, cellulase, invertase, and nitrate reductase were primarily responsible for litter decomposition in the coniferous forest under N fertilization. The accelerated litter decomposition rates under N fertilization may cause an increase of the greenhouse gas CO2in the atmosphere. Elevated atmospheric CO2concentration might also lead to a warmer climate, which may enhances the rates of litter decomposition in forest ecosystems. Thus, anthropogenic N deposition can lead to related changes in climatic factors, which, in turn, generally have multiple effects on forest ecosystems.(4) All four forms of N fertilization [NH4+, NO3-, CO(NH2)2, and a mix of all three] significantly accelerates litter decomposition rates in the broadleaf forest, while only the mixed N and CO(NH2)2fertilization significantly accelerates litter decomposition rates in the coniferous forest, suggesting that the temperature sensitivities of litter decomposition for broadleaf forest leaves were significantly higher than that for coniferous forest needles under N deposition. Litter decomposition rates under the mixed N fertilization were significantly higher than those under any single form of N fertilization. All forms of N fertilization enhance soil enzyme activities (i.e., catalase, cellulase, invertase, polyphenol oxidase, nitrate reductase, urease, and acid phosphatase) during litter decomposition in the two forest types. Soil enzyme activities under the mixed N fertilization were higher than those under any single form of N fertilization. Catalase, nitrate reductase, and polyphenol oxidase activities under inorganic N fertilization were higher than those under organic N fertilization, while cellulase, invertase, acid phosphatase, and urease activities under organic N fertilization were higher than those under inorganic N fertilization, suggesting that some groups of soil microorganisms that secrete exoenzymes for litter decomposition may prefer either organic N or inorganic N. The accelerated litter decomposition rates under N fertilization may cause an increase in N availability for micro-decomposer metabolism, greater N uptake for plants, and possibly a greater amount of C sequestered in plant biomass as a long-term consequence of continued N deposition, following this, the activities of micro-decomposer metabolism, the amount of litter fall and the quality of litter may all increase, which, in turn, could enhances litter decomposition rates in forest ecosystems and initiate a so-called feed-forward cycle. |
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