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Soil acidity can have a potent influence on soil microbial composition and function, but it remains unclear if this influence is a direct response to the concentration of [H.sup.+] or an indirect response caused by the influence of pH on other soil chemical properties. We hypothesized that the composition and function of soil microorganisms would shift in response to decreases in the availability of inorganic P caused by mobilized AL We tested this hypothesis at Dysart Woods, a mosaic of old growth and second growth mixed, mesophytic forest in eastern Ohio. We evaluated microbial community composition (phospholipid fatty acid [PLFA] analysis) and function (extracellular enzyme activity) in the context of P availability and common metrics of soil fertility. We observed a threshold response where available organic P ([P.sub.o], i.e., bicarbonate-extracted, molybdate-unreactive P) dominated (~80%) the availability of total available P when mobilized Al was present (>0.1 [cmol.sub.c] [kg.sup.-1]), but when Al was immobilized, available inorganic [P.sub.i], i.e., bicarbonate-extracted, molybdate-reactive P) content was significantly greater (~50%). Available [P.sub.o] significantly correlated (r = 0.54) with acid phosphatase activity. Using nonmetric multidimensional sealing, we observed that PLFA composition ordination was significant along a gradient of soil pH and available [P.sub.o], Because the composition and function of the microbial community changes due to available [P.sub.o], results suggest that microbial communities in acidic forest soils are functionally P limited. The availability of [P.sub.o] did not influence the microbial biomass, however, indicating that the microbial community compensates for the apparent P limitation by altering its composition. doi: 10.2136/sssaj2009.0426

We quantified and compared carbon (C) pools at a Missouri Ozark experimental forest 8 years after different harvest treatments. Total C pools were 182, 170, and 130 Mg C * [ha.sup.-1] for the control (no-harvest management; NHM), singletree, uneven-age management (UAM), and clearcut even-age management (EAM) stands, respectively. Harvesting reduced the live tree C pool by 31% in the UAM, 93% in EAM stands, and increased the coarse woody debris (CWD) C pool by 50% in UAM and 176% for EAM, compared with NHM stands. UAM significantly (p = 0.02) increased the mineral soil C pool by 14%, whereas EAM had no effect. More interestingly, the distribution of C among various components (i.e., live, dead wood, CWD, litter, and soil) ranged from 0.7% to 29% on NHM stands and from 0.1% to 43% on EAM stands. Soil nitrogen (N) (%) was significantly correlated with soil C (%) in the UAM stands, whereas soil temperature was negatively related to live tree C. Soil N (%) and canopy cover were significantly correlated with live tree and soil C (%) pools at EAM stands. Our results revealed that the largest C pool in these forests was living trees. The soil and CWD C pool sizes suggest the importance of dynamics of decaying harvest debris, which influences N retention. Nous avons quantifie et compart; les reservoirs de carbone (C) dans une foret experimentale situtie dans les monts Ozark au Missouri huit ans apres que differents traitements de recolte eurent ete appliques. La quantite totale de carbone dans les differents reservoirs de carbone atteignait respectivement 182, 170 et 130 Mg C * [ha.sup.-1] dans le traitement temoin (amenagement sans recolte), avec un amenagement inequienne par pied d'arbre (AIPA) et avec un amenagement equienne et une coupe a blanc (AECB). La recolte a reduit le reservoir de C des arbres vivants de 31% dans le systeme AIPA et de 93% dans le systeme AECB et augmente le reservoir de C dans les debris ligneux grossiers (DLG) de 31% dans le systeme AIPA et de 176% dans le systeme AECB comparativement aux peuplements non recoltes. Le systeme AIPA a significativement (p = 0,02) augmente le reservoir de C du sol mineral de 14% tandis que le systeme AECB n'a eu aucun effet. Il etait plus interessant de constater que la distribution de C parmi les diverses composantes (c.-a-d. le bois vivant, le bois mort, les DLG, la litiere et le sol) variait de 0,7% a 29% dans les peuplements non coupes et de 0,1% a 43% dans les peuplements coupes a blanc. L'azote (N) (%) dans le sol etait significativement correle avec le C (%) dans le sol dans le systeme AIPA. Nos resultats revelent que le plus important reservoir de C dans ces forets est constitue des arbres vivants. La dimension des reservoirs de C du sol et des DLG montre l'importance de la dynamique de la decomposition des dechets de coupe qui influence la retention de N. [Traduit par la Redaction], Introduction Carbon (C) pools in terrestrial ecosystems are receiving increasing attention partly because terrestrial ecosystems have some potential to store C and, thus, help offset increases in atmospheric carbon dioxide [...]

Atmospheric N[O.sup.-.sub.3] deposition has the potential to disrupt litter decomposition in temperate forests by suppressing enzymes responsible for lignin degradation. A reduction in phenol oxidase activity could potentially trigger an increase in soluble phenolic compounds, which in turn are known to decrease the activity of cellulolytic enzymes like [beta]-glucosidase. Our study investigated whether the inhibition of lignin degradation by experimental N[O.sup.-.sub.3] deposition could increase soluble phenolics in soil, suppress [beta]-glucosidase activity, and potentially explain a greater export of dissolved organic C (DOC) from northern hardwood ecosystems. We found no evidence that the suppression of phenol oxidase by N[O.sup.-.sub.3] additions increased soluble phenolics in mineral soil, nor did we find a strong inverse relationship between soluble phenolics and [beta]-glucosidase activity. It appears that reductions in mineral soil lignolytic activity induced by experimental deposition are not responsible for greater DOC export from soil.

On a global scale, human activity has increased the atmospheric input of N[O.sup.-.sub.3] to many terrestrial ecosystems. Anthropogonic N[O.sup.-.sub.3] may be a potent modifier of ecosystem function, especially in temperate forests that are sometimes N limited. However, the impact of chronic N deposition on soil microorganisms is still poorly understood. Nitrate entering Lake States forests is rapidly assimilated by the microbial community and it is subsequently released as N[H.sup.+.sub.4]. Because high levels of N[H.sup.+.sub.4] inhibit the activity of lignin-degrading soil fungi, we reasoned that chronic N additions could alter the composition and function of heterotrophic microbial communities in soil, and hence the ecosystem-level processes they mediate. We tested our hypothesis in four northern hardwood ecosystems in northern Michigan, which received experimental N additions (30 kg N[O.sup.-.sub.3]-N [ha.sup.-1] [yr.sup.-1]) during the past 8 yr. We quantified microbial community function by measuring the activity of extracellular enzymes involved in plant fitter degradation and described microbial community composition using phospholipid fatty acid (PLFA) analysis. Chronic N additions significantly suppressed [beta]-glucusidase activity by 24% in mineral soil and suppressed phenol oxidase activity by 35% in surface litter. We found no evidence that chronic N additions altered microbial community composition; N[O.sup.-.sub.3] addition did not alter the relative abundance of bacterial, actinomycetal, fungal, or protozoan PLFAs. However, N[O.sup.-.sub.3] additions significantly reduced microbial biomass by 18% relative to the control treatment. Results indicate that N additions broadly suppressed all microbial groups, not just the activity and abundance of lignin-degrading fungi.

Photosynthesis by leaves and acquisition of water and minerals by roots are required for plant growth, which is a key component of many ecosystem functions. Although the role of leaf functional traits in photosynthesis is generally well understood, the relationship of root functional traits to nutrient uptake is not. In particular, predictions of nutrient acquisition strategies from specific root traits are often vague. Roots of nearly all plants cooperate with mycorrhizal fungi in nutrient acquisition. Most tree species form symbioses with either arbuscular mycorrhizal (AM) or ectomycorrhizal (EM) fungi. Nutrients are distributed heterogeneously in the soil, and nutrient-rich "hotspots" can be a key source for plants. Thus, predicting the foraging strategies that enable mycorrhizal root systems to exploit these hotspots can be critical to the understanding of plant nutrition and ecosystem carbon and nutrient cycling. Here, we show that in 13 sympatric temperate tree species, when nutrient availability is patchy, thinner root species alter their foraging to exploit patches, whereas thicker root species do not. Moreover, there appear to be two distinct pathways by which thinner root tree species enhance foraging in nutrient-rich patches: AM trees produce more roots, whereas EM trees produce more mycorrhizal fungal hyphae. Our results indicate that strategies of nutrient foraging are complementary among tree species with contrasting mycorrhiza types and root morphologies, and that predictable relationships between below-ground traits and nutrient acquisition emerge only when both roots and mycorrhizal fungi are considered together. [ABSTRACT FROM AUTHOR]

Climate change factors such as elevated atmospheric carbon dioxide (CO2) and ozone (O3) can exert significant impacts on soil microbes and the ecosystem level processes they mediate. However, the underlying mechanisms by which soil microbes respond to these environmental changes remain poorly understood. The prevailing hypothesis, which states that CO2- or O3-induced changes in carbon (C) availability dominate microbial responses, is primarily based on results from nitrogen (N)-limiting forests and grasslands. It remains largely unexplored how soil microbes respond to elevated CO2 and O3 in N-rich or N-aggrading systems, which severely hinders our ability to predict the long-term soil C dynamics in agroecosystems. Using a long-term field study conducted in a no-till wheat-soybean rotation system with open-top chambers, we showed that elevated CO2 but not O3 had a potent influence on soil microbes. Elevated CO2 (1.56ambient) significantly increased, while O3 (1.46ambient) reduced, aboveground (and presumably belowground) plant residue C and N inputs to soil. However, only elevated CO2 significantly affected soil microbial biomass, activities (namely heterotrophic respiration) and community composition. The enhancement of microbial biomass and activities by elevated CO2 largely occurred in the third and fourth years of the experiment and coincided with increased soil N availability, likely due to CO2- stimulation of symbiotic N2 fixation in soybean. Fungal biomass and the fungi:bacteria ratio decreased under both ambient and elevated CO2 by the third year and also coincided with increased soil N availability; but they were significantly higher under elevated than ambient CO2. These results suggest that more attention should be directed towards assessing the impact of N availability on microbial activities and decomposition in projections of soil organic C balance in N-rich systems under future CO2 scenarios. [ABSTRACT FROM AUTHOR]

Over the last two and half decades, strong evidence showed that the terrestrial ecosystems are acting as a net sink for atmospheric carbon. However the spatial and temporal patterns of variation in the sink are not well known. In this study, we examined latitudinal patterns of interannual variability (IAV) in net ecosystem exchange (NEE) of CO2 based on 163 site-years of eddy covariance data, from 39 northern-hemisphere research sites located at latitudes ranging from ∼29°N to ∼64°N. We computed the standard deviation of annual NEE integrals at individual sites to represent absolute interannual variability (AIAV), and the corresponding coefficient of variation as a measure of relative interannual variability (RIAV). Our results showed decreased trends of annual NEE with increasing latitude for both deciduous broadleaf forests and evergreen needleleaf forests. Gross primary production (GPP) explained a significant proportion of the spatial variation of NEE across evergreen needleleaf forests, whereas, across deciduous broadleaf forests, it is ecosystem respiration (Re). In addition, AIAV in GPP and Re increased significantly with latitude in deciduous broadleaf forests, but AIAV in GPP decreased significantly with latitude in evergreen needleleaf forests. Furthermore, RIAV in NEE, GPP, and Re appeared to increase significantly with latitude in deciduous broadleaf forests, but not in evergreen needleleaf forests. Correlation analyses showed air temperature was the primary environmental factor that determined RIAV of NEE in deciduous broadleaf forest across the North American sites, and none of the chosen climatic factors could explain RIAV of NEE in evergreen needleleaf forests. Mean annual NEE significantly increased with latitude in grasslands. Precipitation was dominant environmental factor for the spatial variation of magnitude and IAV in GPP and Re in grasslands. [ABSTRACT FROM AUTHOR]

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