Substrate limitations to microbial activity in taiga forest floors

A combination of laboratory and field experiments showed substantial differences in microbial activity, substrate processing, and N cycling in forest floor samples from different Alaskan boreal forest ecosystems. In soils from black spruce (Picea mariana [Mill.] B.S.P.) communities, low organic matter quality (e.g. low %N, high C:N, high lignin, high lignin:N, low pH, low extractable inorganic N) and cold soils were associated with low rates of soil respiration, microbial turnover and gross microbial N uptake in both laboratory and field measurements. Soils from aspen (Populus tremuloides Michx.) communities had attributes of low organic-matter quality (high lignin and high lignin:N) but also attributes favorable to decomposition (high pH, high %N, high soil temperature) and exhibited much higher relative microbial activity in the field than in the laboratory, probably because warmer field conditions or other processes that occurred only in the field (e.g. root exudation) offset the effects of low organic matter quality. Field soils from birch (Betula papyrifera Marsh.) communities on warm sites also exhibited higher in situ rates of microbial activity than expected from their performance in the laboratory. Microbial activity was more important than microbial biomass in explaining community differences in soil respiration and N cycling. Addition of labile carbon (C) and nitrogen (N) substrates to soils in the field and in the laboratory permitted microbial resource limitations to be evaluated. Microbial response to added N was greatest when labile C was abundant. Microbial demand for available soil N was greatest in soils with the highest organic C concentrations and the lowest rates of N mineralization. These observations support the conventional concept that microbial activity responds to a balanced supply of C and N. However, microbial respiration responded more strongly to sucrose (field) and cellobiose (laboratory) than to cellulose addition, indicating that the degree of defined C limitation depends on the nature of the substrate added and the ability of microbial populations to utilize it. Respiration and N immobilization responded more strongly to substrate additions than did microbial biomass, suggesting that the nature of resource limitation depends on the particular microbial parameter considered. The response of microbial respiration to added C and N also depended on the quality of native soil organic matter.