Global Climate Change Effects on Soil Microbial Biomass Stoichiometry in Alpine Ecosystems

Abstract

Alpine ecosystems are sensitive to global climate change-factors, which directly or indirectly affect the soil microbial biomass stoichiometry. In this paper, we have compared the soil microbial biomass stoichiometry ratios of alpine ecosystems using the global average values. In the comparison, the responses and mechanisms of soil microbial biomass stoichiometry to nitrogen deposition, altered precipitation, warming, and elevated atmospheric carbon dioxide (CO2) concentration in the alpine ecosystem were considered. The alpine ecosystem has a higher soil microbial-biomass-carbon-to-nitrogen ratio (MBC:MBN) than the global average. In contrast, the soil microbial-biomass-nitrogen-to-phosphorus (MBN:MBP) and carbon-to-phosphorus ratios (MBC:MBP) varied considerably in different types of alpine ecosystems. When compared with the global average values of these ratios, no uniform pattern was found. In response to the increase in nitrogen (N) deposition, on the one hand, microbes will adopt strategies to regulate extracellular enzyme synthesis and excrete excess elements to maintain stoichiometric balance. On the other hand, microbes may also alter their stoichiometry by storing excess N in their bodies to adapt to the increased N in the environment. Thus, a decrease in MBC:MBN and an increase in MBN:MBP are observed. In addition, N deposition directly and indirectly affects the soil fungal-to-bacterial ratio (F:B), which in turn changes the soil microbial biomass stoichiometry. For warming, there is no clear pattern in the response of soil microbial biomass stoichiometry in alpine ecosystems. The results show diverse decreasing, increasing, and unchanging patterns. Under reduced precipitation, microbial communities in alpine ecosystems typically shift to a fungal dominance. The latter community supports a greater carbon-to-nitrogen ratio (C:N) and thus an increased soil MBC:MBN. However, increased precipitation enhances N effectiveness and exacerbates the leaching of dissolved organic carbon (DOC) and phosphorus (P) from alpine ecosystem soils. As a result, a decrease in the soil MBC:MBN and an increase in the soil MBN:MBP are evident. Elevated atmospheric CO2 usually has little effect on the soil MBC:MBN in alpine ecosystems, mainly because of two reasons. These are: (i) N is the main limiting factor in alpine ecosystems, and (ii) alpine ecosystems accumulate higher soil organic carbon (SOC) and microbes and preferentially decompose “old” carbon (C) stocks. The response of soil microbial stoichiometry to global climate change factors in alpine ecosystems is diverse, and the impact pathways are complex. Future studies need to focus on the combined effects of multiple global climate change factors on microbial stoichiometry and the mechanism of microbial stoichiometric balance.