We appraise the present geographical extent and inherent knowledge limits, following two decades of research on elevated CO2 responses in plant communities, and ask whether such research has answered ...the key question in quantifying the limits of compensatory CO2 uptake in the major biomes. Our synthesis of all ecosystem-scale (between 10m2 and 3000m2 total experimental plot area) elevated CO2 (eCO2) experiments in natural ecosystems conducted worldwide since 1987 (n=151) demonstrates that the locations of these eCO2 experiments have been spatially biased, targeting primarily the temperate ecosystems of northern America and Europe. We consider the consequences, suggesting fundamentally that this limits the capacity of the research to understand how the world's major plant communities will respond to eCO2. Most notably, our synthesis shows that this research lacks understanding of impacts on tropical forests and boreal regions, which are potentially the most significant biomes for C sink and storage activity, respectively. Using a meta-analysis of the available data across all biomes, we show equivocal increases in net primary productivity (NPP) from eCO2 studies, suggesting that global validation is needed, especially in the most important biomes for C processing. Further, our meta-analysis identifies that few research programs have addressed eCO2 effects on below-ground C storage, such that at the global scale, no overall responses are discernable. Given the disparity highlighted in the distribution of eCO2 experiments globally, we suggest opportunities for newly-industrialized or developing nations to become involved in further research, particularly as these countries host some of the most important regions for tropical or sub-tropical forest systems. Modeling approaches that thus far have attempted to understand the biological response to eCO2 are constrained with respect to collective predictions, suggesting that further work is needed, which will link models to in situ eCO2 experiments, in order to understand how the world's most important regions for terrestrial C uptake and storage will respond to a future eCO2 atmosphere.
•Limits to understanding following two decades' elevated CO2 (eCO2) research are examined.•A bias in the global distribution of plant community eCO2 experiments is demonstrated.•The implications via global metrics for carbon dynamics are considered.•Whether limited experimental lengths and soil carbon analyses have been inadequate is examined.•New collaborative frameworks for experiments in carbon dense regions are proposed.
Storage of soil samples prior to metagenomic analysis presents a problem. If field sites are remote or if samples are collected by third parties, transport to analytical laboratories may take several ...days or even weeks. The bulk of such samples and requirement for later homogenisation precludes the convenient use of a stabilisation buffer, so samples are usually cooled or frozen during transit. There has been limited testing of the most appropriate storage methods for later study of soil organisms by eDNA approaches. Here we tested a range of storage methods on two contrasting soils, comparing these methods to the control of freezing at -80 °C, followed by freeze-drying. To our knowledge, this is the first study to examine the effect of storage conditions on eukaryote DNA in soil, including both viable organisms (fungi) and DNA contained within dying/dead tissues (plants). For fungi, the best storage regimes (closest to the control) were storage at 4 °C (for up to 14 d) or active air-drying at room temperature. The worst treatments involved initial freezing, followed by thawing which led to significant later spoilage. The key spoilage organisms were identified as
Metarhizium carneum
and
Mortierella
spp., with a general increase in saprotrophic fungi and reduced abundances of mycorrhizal/biotrophic fungi. Plant data showed a similar pattern, but with greater variability in community structure, especially in the freeze-thaw treatments, probably due to stochastic variation in substrates for fungal decomposition, algal proliferation and some seed germination. In the absence of freeze drying facilities, samples should be shipped refrigerated, but not frozen if there is any risk of thawing.
Plant exposure to enhanced UV-B radiation typically induces changes in leaf secondary metabolite profiles which will be inherited in litter, affecting litter breakdown and the carbon (C) dynamics of ...sensitive plant communities. A key enzyme in the decomposition process is phenol oxidase which is influenced by litter quality and, hence, a decomposition bioindicator. Here we investigated dwarf shrub litter decomposition following experimental community exposure to enhanced UV-B over two decades in the Swedish sub-Arctic. We examined the hypothesis that foliar UV-B exposure would alter litter quality to elevate phenol oxidase activity. This was tested in the field by measuring phenol oxidase activity in freshly collected mixed-community litter from under our experimental vegetation. A laboratory mesocosm was next used in a decomposition assay to investigate individual species responses over eight weeks, with an emphasis on the quality of leachate outputs from decomposing litter (from Empetrum hermaphroditum, Vaccinium vitis-idaea, Vaccinium uliginosum). In the assay bi-weekly collections of leachate were analysed for phenol oxidase activity, together with total phenolics and dissolved organic C (DOC). At the end of the assay litter mass loss and respired C were also determined. The initial assessment on field mixed-community litter found no enhanced UV-B treatment (henceforth: ‘UV-B treatment’) effect on phenol oxidase activity. However, in the controlled laboratory mesocosm assay, significant species-specific effects of the UV-B treatment were evident, with increased phenol oxidase activity in V. vitis-idaea leachate (P<0.001) and a significant reduction (P=0.05) in respired C. Leachate DOC release from the UV-B treatment was greater in both Vaccinium species and the effect on V. uliginosum was significant (P<0.05). The UV-B treatment had no effect on the total phenolic concentration of litter or leachates for any species, but there were significant differences in leachate total phenolics, both over time and between species. Also the initial phenolic concentration in leachates from the decomposing litter of E. hermaphroditum was greater than both Vaccinum species. Results suggest a species specific role for UV-B in influencing enzyme function and decomposition, dependent on individual traits. This has implications for decomposition dynamics in this system and more widely. Our study highlights the value of using a laboratory assay to gain a mechanistic understanding the species level impacts of a global change factor (UV-B) on decomposition, which are otherwise obscured by community-level responses and difficult to determine under field conditions.
Stellera chamaejasme L. is the most problematic weed in China's grasslands. Its root exudates affect co-occurring plants and thus may also affect soil fungi. Soils (0-20 cm depth) on two adjacent ...sites, one invaded the other uninvaded, were compared for a range of physiochemical parameters and by DNA sequencing of fungal communities. At the invaded site, relationships between S. chamaejasme abundance, soil physiochemical factors, and fungal communities were further investigated to determine whether these relationships corroborated conclusions on the basis of site differences that could be translated into functional variation. Results showed that the invaded soils had lower N, P, organic matter, fungal alpha diversity, and relative abundance of arbuscular mycorrhizal fungi (AMF), but greater abundance of pathogenic fungi. Organic matter and P were the edaphic factors most strongly linked to site differences in total fungal communities. Within the invaded site, organic matter rather than S. chamaejasme cover was closely linked to total fungal composition. However, on this site, a number of fungal species that had various ecological functions and that differentiated the two sites were related to S. chamaejasme cover. This study indicates that lower fertility soils may be more susceptible to invasion by S. chamaejasme. Although the influence of S. chamaejasme on total fungal community composition was limited, there was evidence of effects on particular fungal species. Further research is needed to determine whether these effects influence S. chamaejasme invasiveness.
(1) Elevated atmospheric CO2 (eCO2) may affect organic inputs to woodland soils with potential consequences for C dynamics and associated aggregation; (2) The Bangor Free Air Concentration Enrichment ...experiment compared ambient (330 ppmv) and elevated (550 ppmv) CO2 regimes over four growing seasons (2005–2008) under Alnus glutinosa, Betula pendula and Fagus sylvatica. Litter from the experiment (autumn 2008) and Lumbricus terrestris were added to mesocosm soils. Microbial properties and aggregate stability were investigated in soil and earthworm casts. Soils taken from the field experiment in spring 2009 were also investigated; (3) eCO2 litter had lower N and higher C:N ratios. F. sylvatica and B. pendula litter had lower N and P than A. glutinosa; F. sylvatica had higher cellulose. In mesocosms, eCO2 litter decreased respiration, mineralization constant (respired C:total organic C) and soluble carbon in soil but not earthworm casts; microbial‐C and fungal hyphal length differed by species (A. glutinosa = B. pendula > F. sylvatica) not CO2 regime. eCO2 increased respiration in field aggregates but increased stability only under F. sylvatica; (4) Lower litter quality under eCO2 may restrict its initial decomposition, affecting C stabilization in aggregates. Later resistant materials may support microbial activity and increase aggregate stability. In woodland, C and soil aggregation dynamics may alter under eCO2, but outcomes may be influenced by tree species and earthworm activity.
UV‐B radiation and elevated CO2 may impact rhizosphere processes through altered below‐ground plant resource allocation and root exudation, changes that may have implications for nutrient ...acquisition. As nutrients limit plant growth in many habitats, their supply may dictate plant response under elevated CO2. This study investigated UV‐B exposure and elevated CO2 effects, including interactions, on plant growth, tissue chemistry and rooting responses relating to P acquisition. The sub‐arctic grass Calamagrostis purpurea was subjected to UV‐B (0 or 3.04 kJ m−2day−1) and CO2 (ambient 380 or 650 ppmv) treatments in a factorial glasshouse experiment, with sparingly soluble P (0 or 0.152 mg P per plant as FePO4) a further factor. It was hypothesized that UV‐B exposure and elevated CO2 would change plant resource allocation, with CO2 mitigating adverse responses to UV‐B exposure and aiding P uptake. Plant biomass and morphology, tissue composition and rhizosphere leachate properties were measured. UV‐B directly affected chemical composition of shoots and interacted with CO2 to give a greater root biomass. Elevated CO2 altered the composition of both shoots and roots and increased shoot biomass and secondary root length, while leachate pH decreased. Below‐ground responses to CO2 did not affect P acquisition although P limitation progressively reduced leachate pH and increased secondary root length. Although direct plant growth, foliar composition and below‐ground nutrient acquisition responses were dominated by CO2 treatments, UV‐B modified these CO2 responses significantly. These interactions have implications for plant responses to future atmospheric conditions.
► Less decomposable residues were more effective in promoting aggregation. ► Less decomposable residues favoured aggregate fungi and reduced soluble C. ► Earthworms improved aggregate stability with ...decomposable residues. ► Earthworms changed aggregate composition, especially for more labile residues. ► Earthworms enhanced clay–mineral associations in aggregates.
Earthworm activity and the composition of organic inputs are known to affect soil aggregate dynamics. How earthworm interacts with different organic residues, and how these interactions vary with time is still unclear. Earthworm processing of residues may modify their effectiveness as stabilising agents through changes in residue composition and disposition with regard to mineral particles. A mesocosm (2L) experiment was conducted to assess how Lumbricus terrestris L. and residues (grass-clover hay or wheat straw at 20gkg−1) affected aggregate stability (measured by wet sieving) over 40, 80 and 100 days; several microbial and organic carbon indices likely to affect aggregation were also measured to indicate temporal changes in stabilisation mechanisms. FT-IR analyses investigated relationships between kaolinite and specific organic fractions to indicate the degree of residue-mineral mixing in aggregates. In the absence of earthworms, straw was more effective than hay in promoting stability (+21% vs +13%), especially later in the incubations. Hay increased microbial biomass-C (MBC +73 vs +58mgkg−1) and K2SO4 extractable-C (KSE-C +44 vs −9mgkg−1) more than straw, but the converse was true for hyphal length (+6 vs +7.9mg−1). Earthworms reduced or eliminated these differences. Earthworms increased aggregation at 40 days (+17%), although aggregation at 100 days was not affected. Earthworms changed the organic composition of aggregates (e.g. a relative decrease in polysaccharides) and may have promoted adsorption of otherwise labile fractions onto clays. Soon after residue inputs, aggregation was more closely associated with MBC and KSE-C, whereas with time aggregation became more closely related to hyphal length. The extent and persistence of benefits associated with organic inputs of differing quality will be influenced by earthworms, with earthworm mediated incorporation of residues into aggregates particularly important for the effectiveness of readily decomposable residues.
In low-lying land, the impact of agriculture on flooding has focussed on soil compaction, whilst in the uplands there has been more interest in the influence of afforestation. The potential effect of ...acidification of previously limed upland grassland soils on this risk has been overlooked. The marginal economics of upland farms has led to inadequate lime application on these grasslands. In Wales, UK, agronomic improvement of upland acid grasslands with liming was widespread in the last century. The extent and topographical distribution of this land use in Wales was estimated and these characteristics were mapped in four catchments studied in more detail. Then 41 sites on improved pastures within the catchments were sampled, where lime had not been applied for periods of between two and 30 years; unimproved acid pastures adjacent to five of these sites were also sampled. Soil pH, organic matter, infiltration rates and earthworm populations were recorded. Grasslands at risk of acidification without maintenance liming were estimated to cover almost 20 % of upland Wales. The majority of these grasslands were located on steeper slopes (gradients >7o) where any reduction in infiltration would promote surface runoff and limit rainwater retention. The extent of these pastures varied markedly between the four study catchments. There was a 6-fold reduction in infiltration rates between high and low pH soils, and this trend was correlated with reductions in anecic earthworm abundance. The vertical burrows of these earthworms are important for infiltration and no such earthworms were present in the most acidic soils. Recently limed soils had infiltration rates similar to those of unimproved acid pastures. Soil acidification has the potential to exacerbate flood risk but further research is needed to assess the extent of any impact. Modelling of catchment specific flood risk should include the extent of upland soil acidification as an additional land use factor.
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•Liming of improved upland pastures in the UK has declined.•Most such pastures are on steeper slopes prone to runoff.•Without liming soil acidification reduces earthworm abundance and infiltration.•Reduced infiltration can promote surface runoff on slopes and enhance flood risk.•Catchment targeted support for maintenance liming may mitigate flood risk.
The use of duckweed species to remediate nutrient-rich wastewater has grown as a field of research and in industry; however, the need to dilute wastewater to the low ammoniacal-N concentrations ...tolerated by duckweed represents a barrier to commercially implementing these systems in agriculture. This study investigated the potential for acidifying anaerobically digested cattle slurry (digestate), shifting the NH4+:NH3 equilibrium towards the less toxic ionised form, thus allowing the growth of Lemna minor on less dilute wastewater. First, a study was conducted to identify the ammoniacal-N concentrations tolerated by L. minor and to confirm the positive effect of lower pH on growth in high nutrient solutions using modified Hutner's solutions at two pH levels (8.2 and 6.5). In Hutner's solution at a pH of 8.2, L. minor growth was highest at the lowest ammoniacal-N concentration of 10 mg L−1 and decreased with increasing concentrations. At a pH of 6.5, L. minor growth remained unaffected with increasing ammoniacal-N up to a concentration of 250 mg L−1. L. minor was then grown in digestate concentrations ranging from 5% (65 mg L−1 ammoniacal-N) to 30% (350 mg L−1 ammoniacal-N), based on its growth in Hutner's solutions. It was hypothesised, that growth would decrease as the digestate concentration increased at pH 8.2, and that acidifying digestate to pH 6.5 would alleviate this effect. On unamended digestate (pH 8.2), L. minor growth was prevented even in the most dilute treatment (5%); however, on acidified digestate (pH 6.5), growth rates remained positive and significantly higher than the unamended controls up to the 20% dilution (239.3 mg L−1 ammoniacal-N). Higher growth rates in the Hutner's solutions compared to digestate, particularly at pH 8.2 where no growth was recorded in digestate, suggest the presence of additional inhibitory factors in complex, high-nutrient wastewaters, and potentially sub-optimal concentrations of some of the nutrients provided in Hutner's solution. Nevertheless, correlation matrix analysis of digestate chemical properties highlighted the importance of acidification, with a strong negative correlation between pH and L. minor growth rate. For the first time, we demonstrate that by lowering pH, L. minor could be grown on dilutions of nutrient-rich agricultural wastewater that were otherwise toxic, and which make it feasible as a nutrient removal method. These findings could have important implications for implementing duckweed-based remediation systems in agriculture, increasing water- and land use efficiency, and thus, their commercial viability.
•L. minor growth was strongly influenced by media pH and ammoniacal-N concentration.•Acidification enabled growth on otherwise toxic wastewater concentrations.•Less dilution results in higher water use efficiency.
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