Soil‐specific response functions of organic matter mineralization to the availability of labile carbon

Soil organic matter (SOM) mineralization processes are central to the functioning of soils in relation to feedbacks with atmospheric CO₂ concentration, to sustainable nutrient supply, to structural stability and in supporting biodiversity. Recognition that labile C‐inputs to soil (e.g. plant‐derived) can significantly affect mineralization of SOM (‘priming effects’) complicates prediction of environmental and land‐use change effects on SOM dynamics and soil C‐balance. The aim of this study is to construct response functions for SOM priming to labile C (glucose) addition rates, for four contrasting soils. Six rates of glucose (3 atm% ¹³C) addition (in the range 0–1 mg glucose g^?1 soil day^?1) were applied for 8 days. Soil CO₂ efflux was partitioned into SOM‐ and glucose‐derived components by isotopic mass balance, allowing quantification of SOM priming over time for each soil type. Priming effects resulting from pool substitution effects in the microbial biomass (‘apparent priming’) were accounted for by determining treatment effects on microbial biomass size and isotopic composition. In general, SOM priming increased with glucose addition rate, approaching maximum rates specific for each soil (up to 200%). Where glucose additions saturated microbial utilization capacity (>0.5 mg glucose g^?1 soil), priming was a soil‐specific function of glucose mineralization rate. At low to intermediate glucose addition rates, the magnitude (and direction) of priming effects was more variable. These results are consistent with the view that SOM priming is supported by the availability of labile C, that priming is not a ubiquitous function of all components of microbial communities and that soils differ in the extent to which labile C stimulates priming. That priming effects can be represented as response functions to labile C addition rates may be a means of their explicit representation in soil C‐models. However, these response functions are soil‐specific and may be affected by several interacting factors at lower addition rates.     

Functional analysis of the copy 1 of the fixNOQP operon of Ensifer meliloti under free‐living micro‐oxic and symbiotic conditions

Aim

In this work, phenotypic analyses of a Ensifer meliloti fixN1 mutant under free‐living and symbiotic conditions have been carried out.

Methods and Results

Ensifer meliloti fixN1 mutant showed a defect in growth as well as in TMPD‐dependent oxidase activity when cells were incubated under micro‐oxic conditions. Furthermore, haem c staining analyses of a fixN1 and a fixP1 mutant identified two membrane‐bound c‐type cytochromes of 27 and 32 kDa, present in microaerobically grown cells and in bacteroids, as the FixO and FixP components of the E. meliloti cbb₃ oxidase. Under symbiotic conditions, fixN1 mutant showed a clear nitrogen fixation defect in alfalfa plants that were grown in an N‐free nutrient solution during 3 weeks. However, in plants grown for a longer period, fixNOQP1 copy was not indispensable for symbiotic nitrogen fixation.

Conclusions

The copy 1 of the fixNOQP operon is involved in E. meliloti respiration and growth under micro‐oxic conditions as well as in the expression of the FixO and FixP components of the cbb₃ oxidase present in free‐living microaerobic cultures and in bacteroids. This copy is important for nitrogen fixation during the early steps of the symbiosis.

Significance and Impact of the Study

It is the first time that a functional analysis of the E. meliloti copy 1 of the fixNOQP operon is performed. In this work, the cytochromes c that constitute the cbb₃ oxidase operating in free‐living micro‐oxic cultures and in bacteroids of E. meliloti have been identified.     

Unimodal size scaling of phytoplankton growth and the size dependence of nutrient uptake and use

Phytoplankton size structure is key for the ecology and biogeochemistry of pelagic ecosystems, but the relationship between cell size and maximum growth rate (μ_max) is not yet well understood. We used cultures of 22 species of marine phytoplankton from five phyla, ranging from 0.1 to 10⁶ μm³ in cell volume (V_cell), to determine experimentally the size dependence of growth, metabolic rate, elemental stoichiometry and nutrient uptake. We show that both μ_max and carbon‐specific photosynthesis peak at intermediate cell sizes. Maximum nitrogen uptake rate (V_maxN) scales isometrically with V_cell, whereas nitrogen minimum quota scales as V_cell^0.84. Large cells thus possess high ability to take up nitrogen, relative to their requirements, and large storage capacity, but their growth is limited by the conversion of nutrients into biomass. Small species show similar volume‐specific V_maxN compared to their larger counterparts, but have higher nitrogen requirements. We suggest that the unimodal size scaling of phytoplankton growth arises from taxon‐independent, size‐related constraints in nutrient uptake, requirement and assimilation.     

Vegetation feedbacks of nutrient addition lead to a weaker carbon sink in an ombrotrophic bog

To study vegetation feedbacks of nutrient addition on carbon sequestration capacity, we investigated vegetation and ecosystem CO₂ exchange at Mer Bleue Bog, Canada in plots that had been fertilized with nitrogen (N) or with N plus phosphorus (P) and potassium (K) for 7–12 years. Gross photosynthesis, ecosystem respiration, and net CO₂ exchange were measured weekly during May–September 2011 using climate‐controlled chambers. A substrate‐induced respiration technique was used to determine the functional ability of the microbial community. The highest N and NPK additions were associated with 40% less net CO₂ uptake than the control. In the NPK additions, a diminished C sink potential was due to a 20–30% increase in ecosystem respiration, while gross photosynthesis rates did not change as greater vascular plant biomass compensated for the decrease in Sphagnum mosses. In the highest N‐only treatment, small reductions in gross photosynthesis and no change in ecosystem respiration led to the reduced C sink. Substrate‐induced microbial respiration was significantly higher in all levels of NPK additions compared with control. The temperature sensitivity of respiration in the plots was lower with increasing cumulative N load, suggesting more labile sources of respired CO₂. The weaker C sink potential could be explained by changes in nutrient availability, higher woody : foliar ratio, moss loss, and enhanced decomposition. Stronger responses to NPK fertilization than to N‐only fertilization for both shrub biomass production and decomposition suggest that the bog ecosystem is N‐P/K colimited rather than N‐limited. Negative effects of further N‐only deposition were indicated by delayed spring CO₂ uptake. In contrast to forests, increased wood formation and surface litter accumulation in bogs seem to reduce the C sink potential owing to the loss of peat‐forming Sphagnum.     

Diel patterns of autotrophic and heterotrophic respiration among phenological stages

Improved understanding of the links between aboveground production and allocation of photosynthate to belowground processes and the temporal variation in those links is needed to interpret observations of belowground carbon cycling processes. Here, we show that combining a trenching manipulation with high‐frequency soil respiration measurements in a temperate hardwood forest permitted identification of the temporally variable influence of roots on diel and seasonal patterns of soil respiration. The presence of roots in an untrenched plot caused larger daily amplitude and a 2–3 h delay in peak soil CO₂ efflux relative to a root‐free trenched plot. These effects cannot be explained by differences in soil temperature, and they were significant only when a canopy was present during the growing season. This experiment demonstrated that canopy processes affect soil CO₂ efflux rates and patterns at hourly and seasonal time scales, and it provides evidence that root and microbial processes respond differently to environmental factors.     

Ethylene – and oxygen signalling – drive plant survival during flooding

Flooding is a widely occurring environmental stress both for natural and cultivated plant species. The primary problems associated with flooding arise due to restricted gas diffusion underwater. This hampers gas exchange needed for the critical processes of photosynthesis and respiration. Plant acclimation to flooding includes the adaptation of a suite of traits that helps alleviate or avoid these stressful conditions and improves or restores exchange of O₂ and CO₂. The manifestation of these traits is, however, reliant on the timely perception of signals that convey the underwater status. Flooding‐associated reduced gas diffusion imposes a drastic change in the internal gas composition within submerged plant organs. One of the earliest changes is an increase in the levels of the gaseous plant hormone ethylene. Depending on the species, organ, flooding conditions and time of the day, plants will also subsequently experience a reduction in oxygen levels. This review provides a comprehensive overview on the roles of ethylene and oxygen as critical signals of flooding stress. It includes a discussion of the dynamics of these gases in plants when underwater, their interaction, current knowledge of their perception mechanisms and the resulting downstream changes that mediate important acclimative processes that allow endurance and survival under flooded conditions.     

Contrasting responses of heterotrophic and autotrophic respiration to experimental warming in a winter annual‐dominated prairie

Understanding how soil respiration (Rs) and its source components respond to climate warming is crucial to improve model prediction of climate‐carbon (C) feedback. We conducted a manipulation experiment by warming and clipping in a prairie dominated by invasive winter annual Bromus japonicas in Southern Great Plains, USA. Infrared radiators were used to simulate climate warming by 3 °C and clipping was used to mimic yearly hay mowing. Heterotrophic respiration (Rh) was measured inside deep collars (70 cm deep) that excluded root growth, while total soil respiration (Rs) was measured inside surface collars (2–3 cm deep). Autotrophic respiration (Ra) was calculated by subtracting Rh from Rs. During 3 years of experiment from January 2010 to December 2012, warming had no significant effect on Rs. The neutral response of Rs to warming was due to compensatory effects of warming on Rh and Ra. Warming significantly (P < 0.05) stimulated Rh but decreased Ra. Clipping only marginally (P < 0.1) increased Ra in 2010 but had no effect on Rh. There were no significant interactive effects of warming and clipping on Rs or its components. Warming stimulated annual Rh by 22.0%, but decreased annual Ra by 29.0% across the 3 years. The decreased Ra was primarily associated with the warming‐induced decline of the winter annual productivity. Across the 3 years, warming increased Rh/Rs by 29.1% but clipping did not affect Rh/Rs. Our study highlights that climate warming may have contrasting effects on Rh and Ra in association with responses of plant productivity to warming.     

Soil nitrous oxide emissions following crop residue addition: a meta‐analysis

Annual production of crop residues has reached nearly 4 billion metric tons globally. Retention of this large amount of residues on agricultural land can be beneficial to soil C sequestration. Such potential impacts, however, may be offset if residue retention substantially increases soil emissions of N₂O, a potent greenhouse gas and ozone depletion substance. Residue effects on soil N₂O emissions have gained considerable attention since early 1990s; yet, it is still a great challenge to predict the magnitude and direction of soil N₂O emissions following residue amendment. Here, we used a meta‐analysis to assess residue impacts on soil N₂O emissions in relation to soil and residue attributes, i.e., soil pH, soil texture, soil water content, residue C and N input, and residue C : N ratio. Residue effects were negatively associated with C : N ratios, but generally residue amendment could not reduce soil N₂O emissions, even for C : N ratios well above ca. 30, the threshold for net N immobilization. Residue effects were also comparable to, if not greater than, those of synthetic N fertilizers. In addition, residue effects on soil N₂O emissions were positively related to the amounts of residue C input as well as residue effects on soil CO₂ respiration. Furthermore, most significant and stimulatory effects occurred at 60–90% soil water‐filled pore space and soil pH 7.1–7.8. Stimulatory effects were also present for all soil textures except sand or clay content ≤10%. However, inhibitory effects were found for soils with >90% water‐filled pore space. Altogether, our meta‐analysis suggests that crop residues played roles beyond N supply for N₂O production. Perhaps, by stimulating microbial respiration, crop residues enhanced oxygen depletion and therefore promoted anaerobic conditions for denitrification and N₂O production. Our meta‐analysis highlights the necessity to connect the quantity and quality of crop residues with soil properties for predicting soil N₂O emissions.