Surplus Carbon Drives Allocation and Plant-Soil Interactions

Plant growth is usually constrained by the availability of nutrients, water, or temperature, rather than photosynthetic carbon (C) fixation. Under these conditions leaf growth is curtailed more than C fixation, and the surplus photosynthates are exported from the leaf. In plants limited by nitrogen (N) or phosphorus (P), photosynthates are converted into sugars and secondary metabolites. Some surplus C is translocated to roots and released as root exudates or transferred to root-associated microorganisms. Surplus C is also produced under low moisture availability, low temperature, and high atmospheric CO2 concentrations, with similar below-ground effects. Many interactions among above- and below-ground ecosystem components can be parsimoniously explained by the production, distribution, and release of surplus C under conditions that limit plant growth.Non peer reviewe

Evidence of a strong coupling between root exudation, C and N availability, and stimulated SOM decomposition caused by rhizosphere priming effects

Increased temperatures and concomitant changes in vegetation patterns are expected to dramatically alter the functioning of northern ecosystems over the next few decades. Predicting the ecosystem response to such a shift in climate and vegetation is complicated by the lack of knowledge about the links between aboveground biota and belowground process rates. Current models suggest that increasing temperatures and rising concentrations of atmospheric CO2 will be partly mitigated by elevated C sequestration in plant biomass and soil. However, empirical evidence does not always support this assumption, as elevated temperature and CO2 concentrations also accelerate the belowground C flux, in many cases extending to increased decomposition of soil organic matter (SOM) and ultimately resulting in decreased soil C stocks. The mechanism behind the increase has remained largely unknown, but it has been suggested that priming might be the causative agent. Here, we provide quantitative evidence of a strong coupling between root exudation, SOM decomposition, and release of plant available N caused by rhizosphere priming effects. As plants tend to increase belowground C allocation with increased temperatures and CO2 concentrations, priming effects need to be considered in our long-term analysis of soil C budgets in a changing environment. The extent of priming seems to be intimately linked to resource availability, as shifts in the stoichiometric nutrient demands of plants and microorganisms will lead to either cooperation (resulting in priming) or competition (no priming will occur). The findings lead us on the way to resolve the varying response of primary production, SOM decomposition, and release of plant available N to elevated temperatures, CO2 concentrations, and N availability

Spatial variability of soil fungal and bacterial abundance: Consequences for carbon turnover along a transition from a forested to clear-cut site

Predicted alterations in belowground plant-allocated C as a result of environmental change may cause compositional shifts in soil microbial communities, and it has been hypothesized that such alterations will influence C mitigation in forest ecosystems. In order to investigate to what degree living trees influence the abundance and activity of mycorrhizal fungi, saprotrophic fungi, and bacteria we used a geostatistical approach to examine natural abundance stable-isotope-ratios of soil microbial PLFAs, respired CO2, and different soil organic C pools in a 100 point grid extending from an area of retention trees into a clear-cut area. Labile C from trees was the major source of C for the fungal communities and influenced the composition of the microbial community and soil respiration rates up to ten meters into the clear-cut. When the input of labile plant C decreased, it appeared that microorganisms became increasingly dependent on recycled C released during microbial turnover, resulting in a decrease in soil respiration. Our findings demonstrate that plants not only influence the structure and function of soil microbial communities but also act as important regulators of belowground C flux, soil C sequestration and, ultimately, soil C stocks. (c) 2013 Elsevier Ltd. All rights reserved

Dispersed Variable-Retention Harvesting Mitigates N Losses on Harvested Sites in Conjunction With Changes in Soil Microbial Community Structure

As an alternative to clear-cutting, variable-retention harvesting is now standard forest management practice on the coast of British Columbia and in temperate forests globally, due to the benefits associated with maintaining mature forest species and forest structural diversity. Although there is some evidence that variable-retention harvesting, particularly single-tree (dispersed) retention will mitigate the impacts of clear-cutting on soil microbial communities and nutrient cycling, findings have been inconsistent. We examined microbial community structure (phospholipid-fatty acid), and nutrient availability (PRSTM probes) in a large (aggregated) retention patch and over three harvesting treatments: dispersed retention, clear-cut and clear-cut edge 2 years after harvest. Unlike previous studies, we did not observe elevated nitrate in the harvested areas, instead ammonium was elevated. Availability of N and other nutrients were surprisingly similar between the dispersed-retention treatment and the retention patch. The microbial community, however, was different in the clear-cut and dispersed-retention treatments, mostly due to significantly lower abundance of fungi combined with an increase in bacteria, specifically Gram-negative bacteria. This was accompanied by lower δ13CPDB value of the Gram-negative PLFA's in these treatments, suggesting the decline in mycorrhizal fungal abundance may have allowed the dominant Gram-negative bacteria to access more of the recently photosynthesized C. This shift in the microbial community composition in the dispersed-retention treatment did not appear to have a major impact on microbial functioning and nutrient availability, indicating that this harvesting practice is more effective at maintaining generic microbial functions/processes. However, as Mn levels were twice as high in the retention patch compared to the harvested treatments, indicating the other “narrow” processes (i.e., those performed by a small number of specialized microorganisms), such as lignin degradation, catalyzed by Mn peroxidase, which concomitantly removes Mn from solution, may be more sensitive to harvesting regimes. The effect of harvesting on such narrow nutrient cycling processes requires further investigation

Links between methane oxidation rates and methanotroph community composition in a pine forest soil

The main gap in our knowledge about what determines the rate of CH4 oxidation in forest soils is the biology of the microorganisms involved, the identity of which remains unclear. In this study, we used stable-isotope probing (SIP) following 13CH4 incorporation into phospholipid fatty acids (PLFAs) and DNA/RNA, and sequencing of methane mono-oxygenase (pmoA) genes, to identify the influence of variation in community composition on CH4 oxidation rates. The rates of 13C incorporation into PLFAs differed between horizons, with low 13C incorporation in the organic soil and relatively high 13C incorporation into the two mineral horizons. The microbial community composition of the methanotrophs incorporating the 13C label also differed between horizons, and statistical analyses suggested that the methanotroph community composition was a major cause of variation in CH4 oxidation rates. Both PLFA and pmoA-based data indicated that CH4 oxidizers in this soil belong to the uncultivated ‘upland soil cluster α’. CH4 oxidation potential exhibited the opposite pattern to 13C incorporation, suggesting that CH4 oxidation potential assays may correlate poorly with in situ oxidation rates. The DNA/RNA-SIP assay was not successful, most likely due to insufficient 13C-incorporation into DNA/RNA. The limitations of the technique are briefly discussed

Links between methanotroph community composition and CH4 oxidation in a pine forest soil

The main gap in our knowledge about what determines the rate of CH4 oxidation in forest soils is the biology of the microorganisms involved, the identity of which remains unclear. In this study, we used stable-isotope probing (SIP) following (CH4)-C-13 incorporation into phospholipid fatty acids (PLFAs) and DNA/RNA, and sequencing of methane mono-oxygenase (pmoA) genes, to identify the influence of variation in community composition on CH4 oxidation rates. The rates of C-13 incorporation into PLFAs differed between horizons, with low C-13 incorporation in the organic soil and relatively high C-13 incorporation into the two mineral horizons. The microbial community composition of the methanotrophs incorporating the C-13 label also differed between horizons, and statistical analyses suggested that the methanotroph community composition was a major cause of variation in CH4 oxidation rates. Both PLFA and pmoA-based data indicated that CH4 oxidizers in this soil belong to the uncultivated 'upland soil cluster alpha'. CH4 oxidation potential exhibited the opposite pattern to C-13 incorporation, suggesting that CH4 oxidation potential assays may correlate poorly with in situ oxidation rates. The DNA/RNA-SIP assay was not successful, most likely due to insufficient C-13-incorporation into DNA/RNA. The limitations of the technique are briefly discussed