| The subalpine and alpine region, as affected by seasonal freezing and thawing, is a sensitive area to climate change. As an indisputable fact, global warming can alter the pattern of seasonal freezing and thawing, and directly and indirectly influence the processes of above-and below-ground ecology, and in turn give strong effects on the terrestrial ecosystem process. Soil microorganism not only plays their roles as driving force in material cycling and energy flow, but also acts as a sensitive bio-indicator to environmental change. The ongoing winter warming under climate change scenarios is suggested to have great effects on the pattern of seasonal snow cover and seasonal freeze-thaw and significantly change the structure and function of soil microorganism, subsequent have certain consequences for biogeochemical processes in the high latitudinal and altitudinal ecosystems. As yet, more attentions have concerned about the effect of experimental warming on the structure and function of soil microbial community during the growing season in the high latitudinal regions, and little information is available on the microbial response to experimental winter warming. Therefore, it is difficult for us to deeply understand the ecological process in the high-frigid ecosystem and predict the potential change of soil process under climate change scenarios.The subalpine and alpine forest of western Sichuan located in the eastern edge of Qinghai-Tibet plateau and the upper reaches of the Yangtze River is a principal part of the second biggest forest region (southwest forest region) in China, which play important roles in conserving water, nursing biodiversity and indicating regional climate change. Hence, in this study, we conducted two experiments consisting of a field investigation and an experimental warming to assess the effects of global warming on the biogeochemical processes in the high latitudinal and altitudinal ecosystems.(1) From August2009to August2010, the diversity of soil microorganism and their biochemical properties were simultaneously measured in primary fir (Abies faxoniana) forest (PF), fir and birch (Betula albosinensis) mixed forest (MF) and secondary fir forest (SF) which are three representative forests in the study region, employing the method of field sampling in combination with laboratory analysis in different critical periods, especially to the winter.(2) From May2010to August2011, untouched soil cores that sampled from the primary fir forest were respectively laid in three different altitudes (3600,3300and3000) to simulate experimental winter warming with increase amplitude of air temperature0℃(3600),2℃(3300) and4℃(3000). The soil microbial properties, soil enzyme activities, and soil carbon and nutrients content were simultaneously measured during the winter and growing season.1. Season freeze-thaw significantly influenced the dynamic of soil microbial biomass in the subalpine and alpine forest. The freezing process decreased the microbial biomass C (MBC) and microbial biomass N (MBN) in both the soil organic layer (OL) and mineral soil layer (MS), but increased the microbial biomass P (MBP) in the two layers. In contrast, the thawing process increased the MBC, MBN and MBP in the two soil layers. The MBC/MBN, MBC/MBP and MBN/MBP showed a decrease and then increase trend as the soil sampled time. The lowest values of MBC/MBP and MBN/MBP were observed in the deeply frost stage (except MBC/MBP in the MF), and the lowest values of MBC/MBN were observed in the frozen stage.2. As the soil freezing and thawing processes, the bacterial and fungal rDNA copy numbers showed a decrease and then increase trend in the OL and MS, and the lowest values were observed in the soil freezing stage. The archaeal rDNA copy numbers in the OL also showed a similar change as the bacteria and fungi, but that in the MS showed an increase and then decrease trend in the PF and SF, and the highest values were observed in the soil freezing stage. Moreover, the bacterial rDNA copy numbers in the OL were higher than that in the MS. However, the fungal rDNA copy numbers in the OL were lower than that in the MS from growing season to deeply frozen stage, and the archaea rDNA copy numbers in the OL also were lower than that in the MS in the soil freezing stage and deeply frozen stage. In addition, season freeze-thaw also obviously affected the diversity of bacteria, archaea, and fungi in both the OL and MS. For the bacteria, the freezing process decreased the diversity index (H’) and increased the dominance index (C’) in the two soil layer, but followed thawing process increased the diversity index (H’) and decreased the dominance index (C). Although similar dynamic of the diversity indices were observed in the OL for the archaea, reverse change trend was observed in the MS. Moreover, the diversity of fungi showed increase trend as seasonal freeze-thaw.3. Season freeze-thaw greatly influenced the extractable inorganic N (ammonium and nitrate), soil urease activity, and soil ammonia-oxidizing microorganism. The freezing process significantly decreased the soil urease activity, the quantity of the soil ammonia-oxidizing bacteria and soil ammonia-oxidizing archaea. After that, these indices significantly increased. The ammonium and extractable N showed a decrease and then increase trend in the MF and SF as affected by seasonal freeze-thaw. However, the nitrate showed an increase and then decrease trend. Moreover, the concentration of ammonium was significantly higher than that of nitrate.4. As compared with air temperature increased0℃(NT), air temperature increased2℃(T2) obviously decreased the monthly mean soil temperature in both the OL and MS, but air temperature increased4℃(T4) significantly increased the monthly mean soil temperature in both the OL and MS. The freeze-thaw cycles in the OL and MS were decreased by11and8in the T2treatment, respectively, and these were decreased by1and3in the T4treatment. The soil temperature showed obvious fluctuation during the freezing and thawing period, and the amplitude of fluctuation in the T4was higher than that in the NT and T2. During soil freezing stage, the mean soil temperature of NT, T2, and T4in the OL was-0.17±1.21℃,-1.17±0.90℃-0.65±0.97℃, respectively, and that in the MS was-0.06±0.99℃,-0.52±0.76℃,-0.08±0.72℃, respectively. During soil thawing stage, the mean soil temperature of NT, T2, and T4in the OL2.04±2.11℃,0.35±0.98℃,0.04±0.57℃, respectively, and that in the MS was1.87±1.89℃、-0.11±0.27℃、0.8±1.33℃, respectively.5. As compared with the NT, the MBC, MBN, and MBP in the OL were decreased with the increase of simulated warming amplitude, while MBN in the MS was increased with the increase of simulated warming amplitude. The ratio of MBC/MBN in OL was increased in growing season, but decreased in non-growing season. The ratio of MBC/MBN in MS was decreased. The ratio of MBC/MBP and MBN/MBP in OL and MS were both increased, only except for thawing period. Moreover, Bacterial, archaeal and fungal rDNA numbers were both increased in compared with control plot after annual warming treatment. The archaeal and fungal rDNA numbers has a positive feedback to the increase of simulated warming amplitude. The relative number of archaea and fungal was both increased by warming treatment. In addition, soil bacterial and archaeal community structure was sensitive to warming treatment, while fungal community was more consistent. The diversity of bacteria in OL and diversity of archaeal and fungal in OL and MS were both increased by warming treatment.6. The warming decreased the activity of urease in all studied periods. But the effect of warming to invertase activity was depended on different periods. Warming increased the numbers of ammonia-oxidizing bacteria, especially during thawing period. But the numbers of ammonia-oxidizing archaea in OL was decreased by warming, while ammonia-oxidizing archaea in MS was change a little. In addition, the Log ratio of AOA to AOB in OL was decreased by warming, while this ratio in MS did not change with obviously tendency.7. After annual different warming treatment, total organic carbon (TC) content was decreased by seasonal freeze-thaw. The TC content in OL was increased by warming in the end of studied period. Dissolved organic carbon (DOC) content was decreased by seasonal freeze-thaw. Total nitrogen (TN) content was decreased by seasonal freeze-thaw. Dissolved organic nitrogen (DON) content was decreased by seasonal freeze-thaw. The DON content was mainly increased by warming. NH4+-N content of OL was mainly increased by seasonal freeze-thaw, which in MS was decreased. The NH4+-N content was mainly decreased by warming. The NO3--N content was decreased by seasonal freeze thaw. The NO3--N content was mainly increased by warming. The mineral nitrogen content was decreased by seasonal freeze thaw. But the mineral nitrogen content was mainly increased by warming. But the effect of warming on TC, DOC, TN, NH4+-N, was depended on different periods. In addition, the soil nitrogen net mineralization rate and the amount of soil mineralized nitrogen was increased by warming amplitude n different treatment of OL and MS. However, the soil inorganic nitrogen was obviously immobilized in growing season, but it was minerlization in non-growing (freeze-thaw) season. The soil inorganic nitrogen in MS was mainly immobilized in all studied periods (except for T4treatment). |
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