Substrate limitations to microbial activity in taiga forest floors

A combination of laboratory and field experiments showed substantial differ ences in microbial activity, substrate processing, and N cycling in forest floor samples from different Alaskan boreal forest ecosystems. In soils fro m black spruce (Picea mariana [Mill.] B.S.P.) communities. low organic matt er 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 s oil respiration, microbial turnover and gross microbial N uptake in both la boratory and field measurements. Soils from aspen (Populus tremuloides Mich x.) communities had attributes of low organic-matter quality thigh lignin a nd high lignin:N) but also attributes favorable to decomposition thigh pH. high %N. high soil temperature) and exhibited much higher relative microbia l activity in the field than in the laboratory, probably because warmer fie ld conditions or other processes that occurred only in the held (e.g. root exudation) offset the effects of low organic matter quality. Field soils fr om birch (Betula papyrifera Marsh.) communities on warm sites also exhibite d higher in situ rates of microbial activity than expected from their perfo rmance in the laboratory. Microbial activity was more important than microb ial biomass in explaining community differences in soil respiration and N c ycling. Addition of labile carbon (C) and nitrogen (N) substrates to soils in the f ield and in the laboratory permitted microbial resource limitations to be e valuated. Microbial response to added N was greatest when labile C was abun dant. 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 r esponded more strongly to sucrose (field) and cellobiose (laboratory) than to cellulose addition, indicating that the degree of defined C limitation d epends on the nature of the substrate added and the ability of microbial po pulations to utilize it. Respiration and N immobilization responded more st rongly to substrate additions than did microbial biomass, suggesting that t he nature of resource limitation depends on the particular microbial parame ter considered. The response of microbial respiration to added C and N also depended on the quality of native soil organic matter. (C) 2001 Elsevier S cience Ltd. All rights reserved.