Although soil microorganisms play a central role in the soil processes that determine nutrient availability and productivity of forest ecosystems, we are only beginning to understand how microbial communities are shaped by environmental factors and how the structure and function of soil microbial communities in turn influence rates of key soil processes. Here we compare the structure and function of soil microbial communities in seven mature, undisturbed forest types across a range of regional climates in British Columbia and Alberta, and examine the variation in community composition within forest types. We collected the forest floor fermentation (F) and humus (H) layers and upper 10 cm of mineral soil at 3 sites in each of seven forest types (corresponding to seven Biogeoclimatic zones) in both spring and summer. Phospholipid fatty acid analysis was used to investigate the structure of soil microbial communities and total soil microbial biomass; potential activities of extra-cellular enzymes indicated the functional potential of the soil microbial community in each layer at each site. Multivariate analysis indicated that both structure and enzyme activities of soil microbial communities differed among the forest types, and significantly separated along the regional climate gradient, despite high local variation. Soil moisture and organic matter contents were most closely related to microbial community characteristics. Forests in the Ponderosa Pine and Mountain Hemlock zones were distinct from other forests and from each other when comparing potential enzyme activities and had the most extreme moisture and temperature values. Forest floors from the hot and dry Ponderosa Pine forests were associated with enzymes characteristic of water-stress and high concentrations of phenols and other recalcitrant compounds. The wet and cold Mountain Hemlock forests were associated with low enzyme activity. An influence of tree species was apparent at the three sites within the Coastal Western Hemlock zone; high bacterial:fungal biomass ratios were found under western redcedar (Thuja plicata) which also had high pH and base-cation levels, and under Douglas-fir (Pseudotsuga menziesii), which had high N availability. Potential activities enzymes differed among soil layers: potential activities of phenol oxidase and peroxidase were highest in mineral soil, whereas phosphatase, betaglucosidase, NAGase, sulfatase, xylosidase and cellobiohydrolase were highest in the forest floors.

Soil moisture is the major factor influencing microbial community structure and enzyme activities across seven biogeoclimatic zones in western Canada

Tree species influence on microbial communities in litter and soil: Current knowledge and research needs

Atmospheric nitrogen (N) deposition derived from fossil-fuel combustion, land clearing, and biomass burning is occurring over large geographical regions on nearly every continent. Greater ecosystem N availability can result in greater aboveground carbon (C) sequestration, but little is understood as to how soil C storage could be altered by N deposition. High concentrations of inorganic N accelerate the degradation of easily decom- posable litter and slow the decomposition of recalcitrant litter containing large amounts of lignin. This pattern has been attributed to stimulation or repression of different sets of microbial extracellular enzymes. We hypothesized that soil C cycling in forest ecosystems with markedly different litter chemistry and decomposition rates would respond to anthro- pogenic N deposition in a manner consistent with the biochemical composition of the dominant vegetation. Specifically, oak-dominated ecosystems with low litter quality should gain soil C, and sugar maple ecosystems with high litter quality should lose soil C in response to high levels of N deposition (80 kg N-ha-1-yr-1). Consistent with this hypothesis, we observed over a three-year period a significant loss of soil C (20%) from a sugar maple- dominated ecosystem and a significant gain (10%) in soil C in an oak-dominated ecosystem, a result that appears to be mediated by the regulation of the microbial extracellular enzyme phenol oxidase. Elevated N deposition resulted in changes in soil carbon that were ecosystem specific and resulted from the divergent regulatory control of microbial extracellular en- zymes by soil N availability.

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Nitrogen deposition modifies soil carbon storage through changes in microbial enzymatic activity

Elevated nitrogen (N) deposition in humid tropical regions may aggravate phosphorus (P) deficiency in forest on old weathered soil found in these regions. From January 2007 to August 2009, we studied the responses of soil microbial biomass and community composition to P addition (in two monthly portions at level of 15 g P m−2 yr−1) in three tropical forests in southern China. The forests were an old-growth forest and two disturbed forests (mixed species and pine dominated). The objective was to test the hypothesis that P addition would increase microbial biomass and change the composition of the microbial community, and that the old-growth forests would be more sensitive to P addition due to its higher soil N availability. Microbial biomass C (MBC) was estimated twice a year and the microbial community structure was quantified by phospholipid fatty acid (PLFA) analysis at the end of the experiment. Addition of P significantly increased the microbial biomass and altered the microbial community composition in the old-growth forest, suggesting that P availability is one of the limiting factors for microbial growth. This was also reflected by significant increases in soil respiration after P addition. In contrast, P addition had no effect on the microbial biomass and the microbial community composition in the pine forests. Also in the mixed forest, the microbial biomass did not significantly respond to P addition, but soil respiration and the ratio of fungal-to-bacteria was significantly increased.

Effects of phosphorus addition on soil microbial biomass and community composition in three forest types in tropical China

Phospholipid ester-linked fatty acid (PLFA) profiles were used to evaluate soil microbial community composition for 9 land use types in two coastal valleys in California. These included irrigated and non-irrigated agricultural sites, non-native annual grasslands and relict, never-tilled or old field perennial grasslands. All 42 sites were on loams or sandy loams of similar soil taxa derived from granitic and alluvial material. We hypothesized that land use history and its associated management inputs and practices may produce a unique soil environment, for which microbes with specific environmental requirements may be selected and supported. We investigated the relationship between soil physical and chemical characteristics, management factors, and vegetation type with microbial community composition. Higher values of total soil C, N, and microbial biomass (total PLFA) and lower values of soil pH occurred in the grassland than cultivated soils. The correspondence analysis (CA) of the PLFA profiles and the canonical correspondence analysis (CCA) of PLFA profiles, soil characteristics, and site and management factors showed distinct groupings for land use types. A given land use type could thus be identified by soil microbial community composition as well as similar soil characteristics and management factors. Differences in soil microbial community composition were highly associated with total PLFA, a measure of soil microbial biomass, suggesting that labile soil organic matter affects microbial composition. Management inputs, such as fertilizer, herbicide, and irrigation, also were associated with the distinctive microbial community composition of the different cultivated land use types.

Soil microbial community composition and land use history in cultivated and grassland ecosystems of coastal California.

northern Wisconsin, Christensen (1969) found evidence for such a relationship, wherein communities of soil The composition and diversity of biotic communities are controlled microfungi were correlated with the occurrence of forest by the availability of growth-limiting resources. Resource availability for microbial populations in soil is controlled by the amount and types vegetation (also see Christensen et al., 1962; Tresner et of organic compounds entering soil from plant litter. Because plant al., 1954). Notwithstanding these observations, we have communities differ in the amount and type of substrates entering a limited understanding of the manner in which the soil, we reasoned that the composition and function of soil microbial broader soil microbial community varies with plant comcommunities should differ with the dominant vegetation. We tested munity composition at a landscape-level scale. this idea by studying two sugar maple (Acer saccharum Marsh.)- Temporal patterns of root and leaf litter production, dominated and one oak (Quercus spp.)-dominated forest ecosystems combined with variation in the chemical composition in northern Lower Michigan that differ in rates of soil N cycling. We of these tissues, could influence the composition and used phospholipid fatty acid (PLFA) analysis to gain insight into function of soil microbial communities. Root litter typimicrobial community composition, and we used a subset of Biolog cally contains more N and lignin than leaf litter (Aber GN substrates found in root exudate to assess the metabolic capabilities soil microbial communities. Although microbial biomass did not et al., 1990; Vogt et al., 1986). The fact that roots and differ among ecosystems, principal components analysis of bacterial, leaves differ chemically could lead to changes in microactinomycetal, and fungal PLFAs clearly separated the microbial com- bial community composition and function, depending munities of the three ecosystems. Similarly, principal components on temporal variation in the proportion of leaf vs. root analysis separated microbial communities by differences in growth on litter entering soil. In sugar maple‐dominated forests, carbohydrates, organic acids, and amino acids. Discrimination among the greatest addition of root litter to soil occurs when microbial communities in the three ecosystems by PLFAs and sub- fine-root mortality peaks in early autumn (Hendrick and strate use occurred in spring, summer, and fall, but the individual Pregitzer, 1992), but substantial inputs also can occur PLFAs and substrates contributing to discrimination changed during throughout the growing season. Unlike the continuous the growing season. Our results indicate that floristically and edaphiinput of dead fine roots to mineral soil, the majority of cally distinct forest ecosystems also differ in microbial community composition and substrate use. This pattern was consistent across aboveground litter is deposited in late autumn. Seasonal the growing season and repeatedly occurred across relatively large variation in above- and belowground litter production land areas. could influence microbial community composition and function, depending on the types of substrates available for microbial metabolism.

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Rachel T. Myers

Papers

Landscape-Level Patterns of Microbial Community Composition and Substrate Use in Upland Forest Ecosystems