Phosphorus availability and arbuscular mycorrhizal fungi limit soil C cycling and influence plant responses to elevated CO2 conditions.

Enhanced soil organic matter (SOM) decomposition and organic phosphorus (P) cycling may help sustain plant productivity under elevated CO2 (eCO2) and P-limiting conditions. P-acquisition by arbuscular mycorrhizal (AM) fungi and their impacts on SOM decomposition may become even more relevant in these conditions. Yet, experimental evidence of the interactive effect of AM fungi and P availability influencing altered SOM cycling under eCO2 is scarce and the mechanisms of this control are poorly understood. Here, we performed a pot experiment manipulating P availability, AM fungal presence and atmospheric CO2 levels and assessed their impacts on soil C cycling and plant growth. Plants were grown in chambers with a continuous 13C-input that allowed differentiation between plant- and SOM-derived fractions of respired CO2 (R), dissolved organic C (DOC) and microbial biomass (MBC) as relevant C pools in the soil C cycle. We hypothesised that under low P availability, increases in SOM cycling may support sustained plant growth under eCO2 and that AM fungi would intensify this effect. We found the impacts of CO2 enrichment and P availability on soil C cycling were generally independent of each other with higher root biomass and slight increases in soil C cycling under eCO2 occurring regardless of the P treatment. Contrary to our hypotheses, soil C cycling was enhanced with P addition suggesting that low P conditions were limiting soil C cycling. eCO2 conditions increased the fraction of SOM-derived DOC pointing to increased SOM decomposition with eCO2. Finally, AM fungi increased microbial biomass under eCO2 conditions and low-P without enhanced soil C cycling, probably due to competitive interactions with free-living microorganisms over nutrients. Our findings in this plant-soil system suggest that, contrary to what has been reported for N-limited systems, the impacts of eCO2 and P availability on soil C cycling are independent of each other.

Long-term irrigation in a drought-prone pine forest leads to vertical redistribution of C stocks and accelerates C cycling in the soil

<p>European forests are facing higher frequencies of extreme droughts, potentially impairing tree growth and ecosystem functioning. Drought limits the metabolic activity of plants and soil organisms, either directly or through reduced belowground carbon (C) allocation of recent assimilates, thus affecting C cycling in the plant-soil system. However, the net effect on belowground soil C storage is still unclear, as drought suppresses both C inputs from plants and outputs from soils. Moreover, understanding the underlying mechanisms is complicated due to long-term acclimation and adaptation of plant and soil organisms to water limitation.</p><p>We investigated the impact of repeated summer droughts in a Scots pine (<em>Pinus sylvestris</em> L.) forest on soil C storage and C cycling, taking advantage of a large-scale irrigation experiment  running since 2003 in a dry inner-Alpine valley in Switzerland (Pfynwald, Valais), which removed the “natural” water limitation. We assessed the responses of soil organic carbon (SOC) stocks and C fluxes by measuring litter fall and decomposition, fine root biomass and production, soil CO<sub>2</sub> effluxes, C-mineralization, and <sup>13</sup>C-labelled glucose utilization by soil microorganisms. </p><p>After 16 years of irrigation, the organic layers lost significant amounts of C (-1000 g m<sup>-2</sup>), despite a 50% increase in litter fall. This C loss was almost compensated by a C gain in the mineral soil (+870 g m<sup>-2</sup>) under irrigation. The decrease in C storage in the organic layers can be related to a three-fold increase in litter decomposition mainly through soil macrofauna as indicated by a litter-bag experiment. In parallel, the C gain in the mineral soil can be attributed mainly to increased incorporation of litter by soil fauna, together with greater C input from the rhizosphere (+70% fine root biomass for Scots pine in mineral soil). Furthermore, irrigation stimulated soil CO<sub>2</sub> efflux as well as microbial C-mineralization of organic and mineral soil, indicating enhanced soil C cycling. Addition of <sup>13</sup>C-enriched glucose to mineral soils revealed a stronger utilization of this easily available C substrate in the drought than in the irrigated soils, together with a negative priming of soil organic matter (SOM) decomposition shortly after substrate addition. These results suggest that the altered quantity and quality of C inputs under irrigation has increased the availability of easily degradable C in soil.</p><p>This study reveals that long-term summer irrigation in a drought-prone pine forest has strong impacts on multiple interlinked processes of the soil C cycle. The removal of water limitation strongly altered vertical soil C distribution, accelerated soil C cycling and altered the substrate use by soil organisms, but had only a small net effect on the whole-profile SOC stocks.</p>

A meta-analysis of soil biodiversity impacts on the carbon cycle

Loss of biodiversity impacts ecosystem functions, such as carbon (C) cycling. Soils are the largest terrestrial C reservoir, containing more C globally than the biotic and atmospheric pools together. As such, soil C cycling, and the processes controlling it, has the potential to affect atmospheric CO2 concentrations and subsequent climate change. Despite the growing evidence of links between plant diversity and soil C cycling, there is a dearth of information on whether similar relationships exist between soil biodiversity and C cycling. This knowledge gap occurs even though there has been increased recognition that soil communities display high levels of both taxonomic and functional diversity and are key drivers of fluxes of C between the atmosphere and terrestrial ecosystems. Here, we used meta-analysis and regression analysis to quantitatively assess how soil biodiversity affects soil C cycling pools and processes (i.e., soil C respiration, litter decomposition, and plant biomass). We compared the response of process variables to changes in diversity both within and across groups of soil organisms that differed in body size, a grouping that typically correlates with ecological function. When studies that manipulated both within- and across-body size group diversity were included in the meta-analysis, loss of diversity significantly reduced soil C respiration (−27.5%) and plant tissue decomposition (−18%) but did not affect above- or belowground plant biomass. The loss of within-group diversity significantly reduced soil C respiration, while loss of across-group diversity did not. Decomposition was negatively affected both by loss of within-group and across-group diversity. Furthermore, loss of microbial diversity strongly reduced soil C respiration (−41%). In contrast, plant tissue decomposition was negatively affected by loss of soil faunal diversity but was unaffected by loss of microbial diversity. Taken together, our findings show that loss of soil biodiversity strongly impacts on soil C cycling processes, and highlight the importance of diversity across groups of organisms (e.g., primary consumers and secondary decomposers) for maintaining full functionality of C cycle processes. However, our understanding of the complex relationships between soil biodiversity and C cycling processes is currently limited by the sheer number of methodological concerns associated with these studies, which can greatly overestimate or underestimate the impact of soil biodiversity on soil C cycling, challenging extrapolation to natural field settings. Future studies should attempt to further elucidate the relative importance of taxonomic diversity (species numbers) versus functional diversity.

A meta-analysis of soil biodiversity impacts on the carbon cycle

Loss of biodiversity can impact ecosystem functioning, such as altering carbon (C) cycling rates. Soils are the largest terrestrial C reservoir, containing more C globally than the biotic and atmospheric pools together. As such, soil C cycling, and the processes controlling it, have the potential to affect atmospheric CO2 concentrations and subsequent climate change. Despite the growing evidence of links between plant diversity and soil C cycling, there is a dearth of information on whether similar relationships exist between biodiversity of soil organisms (microbes and soil fauna) and C cycling. This is despite increasing recognition that soil communities display high levels of both taxonomic and functional diversity and are key drivers of fluxes of C between the atmosphere and terrestrial ecosystems. Here, we used meta-analysis and regression analysis to quantitatively assess how soil biodiversity affects soil C cycling pools and processes (i.e., soil C respiration, litter decomposition, and plant biomass). We compared the response of pool amd process variables to changes in biodiversity both within and across trophic groups of organisms. Overall, loss of soil diversity significantly reduced soil C respiration (−27.5%) and plant tissue decomposition (−18%), but did not affect above- and belowground plant biomass. Detailed analyses showed that loss of within-group biodiversity significantly reduced soil C respiration, while loss of across-group diversity did not. Decomposition was negatively affected by losses of both within-group and across-group diversity. Further, loss of microbial diversity strongly reduced soil C respiration (−41%). In contrast, plant tissue decomposition was negatively affected by loss of soil faunal diversity, but was unaffected by loss of microbial diversity. Taken together, our findings show that loss of soil biodiversity can strongly affect soil C cycling processes, and highlight the importance of diversity across organismal groups for maintaining full C cycling functionality. However, our understanding of the complex relationships between soil biodiversity and C cycling processes is currently limited by the sheer number of methodological concerns associated with these studies, which can greatly overestimate or underestimate the impact of soil biodiversity on soil C cycling. These limitations present challenges to extrapolation to natural field settings. Future studies should attempt to further elucidate the relative importance of taxonomic diversity vs. functional diversity.