Labile, recalcitrant, microbial carbon and nitrogen and the microbial community composition at two Abies faxoniana forest elevations under elevated temperatures

We investigated the interactions of altitude and artificial warming on the soil microbial community structure in a subalpine Abies faxoniana forest in southwestern China after four years of warming. Open top chambers (OTCs) at two elevations (3000 m and 3500 m) were established, and their soil microbial characteristics, organic carbon (C) and nitrogen (N) were measured. The microbial community structure was quantified by phospholipid fatty acid (PLFA) analysis. A two-step sulfuric acid hydrolysis was used to quantify the labile and recalcitrant C fractions in the soil organic matter. The results showed that bacterial PLFAs and gram-negative bacterial PLFAs increased and the fungal PLFAs and the fungi/bacteria ratio decreased with warming at the high altitude. By contrast, the warming effects on those parameters at low altitude were small. The higher proportion of labile easily decomposable soil C may explain the different responses of the microbial community composition at the two altitudes. An RDA analysis confirmed that the variations in the soil community structure were significantly associated with soil organic matter properties such as the sizes of the soil labile N pool (LP-N), the recalcitrant N pool (RP-N), and the labile C pool as well as dissolved organic C (DOC) and dissolved organic N concentrations (DON). Our results also showed that labile C and N pools increased with the altitude, but the microbial biomass C as measured with chloroform fumigation techniques decreased. Warming increased only the recalcitrant C pools at the high altitude. Given the longer mean residence time for recalcitrant C and the much greater size of this soil organic carbon pool, the results indicated that a rise in temperature in our case increased soil C pools at higher altitudes, at least during the early stages of experimental soil warming. Warming could also cause changes in the composition of the microbial community and enzyme activities, consequently leading to functional changes in soil ecosystem processes at the high altitude.