Grassland biodiversity restoration increases resistance of carbon fluxes to drought

Evidence suggests that the restoration of plant diversity in grasslands not only brings benefits for biodiversity conservation, but also the delivery of ecosystem services. While biodiversity-function experiments show that greater plant diversity increases resistance of plant productivity to climate extremes, it is not known whether real-world management options for grassland restoration likewise stabilize ecosystem responses to extreme climate events. We used a long-term (23 year) field experiment in northern England to test the hypothesis that management aimed at biodiversity restoration increases the resistance and recovery of ecosystem carbon (C) fluxes to short-term summer drought. This was tested by measuring plant, soil and microbial responses to a simulated drought in experimental grassland plots where fertilizer application and seed addition have been managed to enhance plant species diversity. The cessation of fertilizer application brought about small increases in plant species richness. Additionally, cessation of fertilizer application reduced overall plant productivity and promoted hemi-parasitic plants at the expense of grasses and forbs. Resistance of CO 2 fluxes to drought, measured as ecosystem respiration, was greater in non-fertilized plots, as lower plant biomass reduced water demand, likely aided by proportionally more hemi-parasitic plants further reducing plant biomass. Additionally, legumes increased under drought, thereby contributing to overall resistance of plant productivity. Recovery of soil microbial C and nitrogen was more rapid after rewetting than soil microbial community composition, irrespective of restoration treatment, suggesting high resilience of soil microbial communities to drought. Synthesis and applications. This study shows that while grassland diversity restoration management increases the resistance of carbon fluxes to drought, it also reduces agricultural yields, revealing a trade-off for land managers. Furthermore legumes, promoted through long-term restoration treatments, can help to maintain plant community productivity under drought by increasing their biomass. As such, grassland management strategies not only have consequences for ecosystem processes, but also the capacity to withstand extreme weather events

Bomb-¹⁴C analysis of ecosystem respiration reveals that peatland vegetation facilitates release of old carbon

The largest terrestrial-to-atmosphere carbon flux is respired CO<sub>2</sub>. However, the partitioning of soil and plant sources, understanding of contributory mechanisms, and their response to climate change are uncertain. A plant removal experiment was established within a peatland located in the UK uplands to quantify respiration derived from recently fixed plant carbon and that derived from decomposition of soil organic matter, using natural abundance <sup>13</sup>C and bomb-<sup>14</sup>C as tracers. Soil and plant respiration sources were found respectively to contribute ~ 36% and between 41-54% of the total ecosystem CO<sub>2</sub> flux. Respired CO<sub>2</sub> produced in the clipped (‘soil’) plots had a mean age of ~ 15 years since fixation from the atmosphere, whereas the <sup>14</sup>C content of ecosystem CO<sub>2</sub> was statistically indistinguishable from the contemporary atmosphere. Results of carbon mass balance modelling showed that, in addition to respiration from bulk soil and plant respired CO<sub>2</sub>, a third, much older source of CO<sub>2</sub> existed. This source, which we suggest is CO<sub>2</sub> derived from the catotelm constituted between ~ 10 and 23% of total ecosystem respiration and had a mean radiocarbon age of between several hundred to ~ 2000 years before present (BP). These findings show that plant-mediated transport of CO<sub>2</sub> produced in the catotelm may form a considerable component of peatland ecosystem respiration. The implication of this discovery is that current assumptions in terrestrial carbon models need to be re-evaluated to consider the climate sensitivity of this third source of peatland CO<sub>2</sub>

Dominant native and non-native graminoids differ in key leaf traits irrespective of nutrient availability

Aim: Nutrient enrichment is associated with plant invasions and biodiversity loss. Functional trait advantages may predict the ascendancy of invasive plants following nutrient enrichment but this is rarely tested. Here, we investigate (a) whether dominant native and non-native plants differ in important morphological and physiological leaf traits, (b) how their traits respond to nutrient addition, and (c) whether responses are consistent across functional groups. Location: Australia, Europe, North America and South Africa. Time period: 2007–2014. Major taxa studied: Graminoids and forbs. Methods: We focused on two types of leaf traits connected to resource acquisition: morphological features relating to light-foraging surfaces and investment in tissue (specific leaf area, SLA) and physiological features relating to internal leaf chemistry as the basis for producing and utilizing photosynthate. We measured these traits on 503 leaves from 151 dominant species across 27 grasslands on four continents. We used an identical nutrient addition treatment of nitrogen (N), phosphorus (P) and potassium (K) at all sites. Sites represented a broad range of grasslands that varied widely in climatic and edaphic conditions. Results: We found evidence that non-native graminoids invest in leaves with higher nutrient concentrations than native graminoids, particularly at sites where native and non-native species both dominate. We found little evidence that native and non-native forbs differed in the measured leaf traits. These results were consistent in natural soil fertility levels and nutrient-enriched conditions, with dominant species responding similarly to nutrient addition regardless of whether they were native or non-native. Main conclusions: Our work identifies the inherent physiological trait advantages that can be used to predict non-native graminoid establishment, potentially because of higher efficiency at taking up crucial nutrients into their leaves. Most importantly, these inherent advantages are already present at natural soil fertility levels and are maintained following nutrient enrichment

Short-term N and C dynamics in a grassland soil

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN038723 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Interactions between climate warming and land management regulate greenhouse gas fluxes in a temperate grassland ecosystem

Greenhouse gas (GHG) fluxes from grasslands are affected by climate warming and agricultural management practices including nitrogen (N) fertiliser application and grazing. However, the interactive effects of these factors are poorly resolved in field studies. We used a factorial in situ experiment - combining warming, N-fertiliser and above-ground cutting treatments - to explore their individual and interactive effects on plant-soil properties and GHG fluxes in a temperate UK grassland over two years. Our results showed no interactive treatment effects on plant productivity despite individual effects of N-fertiliser and warming on above- and below-ground biomass. There were, however, interactive treatment effects on GHG fluxes that varied across the two years. In year 1, warming and N-fertiliser increased CO2 and reduced N2O fluxes. N-fertilised also interacted with above-ground biomass (AGB) removal increasing N2O fluxes in year one and reducing CO2 fluxes in year two. The grassland was consistently a sink of CH4; N-fertilised increased the sink by 45% (year 1), AGB removal and warming reduced CH4 consumption by 44% and 43%, respectively (year 2). The majority of the variance in CO2 fluxes was explained by above-ground metrics (grassland productivity and leaf dry matter content), with microclimate (air and soil temperature and soil moisture) and below-ground (root N content) metrics also significant. Soil chemistry (soil mineral N and net mineralisation rate), below-ground (specific root length) and microclimate (soil moisture) metrics explained 49% and 24% of the variance in N2O and CH4 fluxes, respectively. Overall, our work demonstrates the importance of interactions between climate and management as determinants of short-term grassland GHG fluxes. These results show that reduced cutting combined with lower inorganic N-fertilisers would constrain grassland C and N cycling and GHG fluxes in warmer climatic conditions. This has implications for strategic grassland management decisions to mitigate GHG fluxes in a warming world

UK land use and soil carbon sequestration

This review explores the role of land use and land use change as a determinant of the soil's ability to sequester and store carbon in the UK. Over 95 percent of the UK land carbon stock is located in soils which are subjected to a range of land uses and global changes. Land use change can result in rapid soil loss of carbon from peatlands, grasslands, plantation forest and native woodland. Soil carbon accumulates more slowly (decadal) but gains can be made when croplands are converted to grasslands, plantation forest or native woodland. The need for land for food production and renewable forms of energy could have considerable influence on UK soil carbon storage in the future. There is a need to recognise the risk of soil carbon losses occurring when land use change to increase carbon storage is offset by compensatory land use conversions elsewhere that result in net carbon release. The protection of peatland and other organic soil carbon stocks, and the management of cropland, grassland and forest soils to increase carbon sequestration, will be crucial to the maintenance of the UK carbon balance. It will be necessary to develop policy to balance trade-offs between soil carbon gains with other land use priorities. These include the sustainable production of food, bio-energy and fibre crops and livestock, water quality and hydrology, greenhouse gas emission control and waste management, all of which are underpinned by the soil

A regulatory role for phenol oxidase during decomposition in peatlands

Unique peatland properties, such as their ability to preserve intact ancient human remains (bog bodies) and to store globally significant quantities of atmospheric CO2, can be attributed to their low rates of enzymic decomposition. Peatland soils are normally devoid of molecular oxygen in all, but the uppermost layer, and thus enzymes such as phenol oxidase, which require molecular oxygen for their activity, are rarely active. Interestingly, even the activities of enzymes such as hydrolases that have no oxygen requirement, are also extremely limited in peatlands. Here, we show that those low hydrolase activities can be indirectly attributed to oxygen constraints on phenol oxidase. On addition of oxygen, phenol oxidase activity increased 7-fold, P<0.05, a response that allowed phenolic depletion in the peatland soil. Phenolic materials are highly inhibitory to enzymes and their lower abundance allowed higher hydrolase activities (β-glucosidase 26%, P<0.05, phosphatase 18%, P<0.05, sulphatase 47%, P<0.01, xylosidase 16%, P<0.05 and chitinase 22%, P<0.05). Thus, oxygen constraints upon phenol oxidase activity promote conditions that inhibit decomposition. This mechanism has important implications for preservation of archaeological organic materials, sequestration of atmospheric CO2 and potentially in the preservation of food and treatment of water pollution

Legumes increase grassland productivity with no effect on nitrous oxide emissions

Aims: Grasslands are important agricultural production systems, where ecosystem functioning is affected by land management practices. Grass-legume mixtures are commonly cultivated to increase grassland productivity while reducing the need for nitrogen (N) fertiliser. However, little is known about the effect of this increase in productivity on greenhouse gas (GHG) emissions in grass-legume mixtures. The aim of this study was to investigate interactions between the proportion of legumes in grass-legume mixtures and N-fertiliser addition on productivity and GHG emissions. We tested the hypotheses that an increase in the relative proportion of legumes would increase plant productivity and decrease GHG emissions, and the magnitude of these effects would be reduced by N-fertiliser addition. Methods: This was tested in a controlled environment mesocosm experiment with one grass and one legume species grown in mixtures in different proportions, with or without N-fertiliser. The effects on N cycling processes were assessed by measurement of above- and below-ground biomass, shoot N uptake, soil physico-chemical properties and GHG emissions. Results: Above-ground productivity and shoot N uptake were greater in legume-grass mixtures compared to grass or legume monocultures, in fertilised and unfertilised soils. However, we found no effect of legume proportion on N 2O emissions, total soil N or mineral-N in fertilised or unfertilised soils. Conclusions: This study shows that the inclusion of legumes in grass-legume mixtures positively affected productivity, however N cycle were in the short-term unaffected and mainly affected by nitrogen fertilisation. Legumes can be used in grassland management strategies to mitigate climate change by reducing crop demand for N-fertilisers

Microbial “hotspots” of organic matter decomposition in temperate peatlands are driven by local spatial heterogeneity in abiotic conditions and not by vegetation structure

Climate change is triggering rapid shifts in plant communities and alterations in soil abiotic conditions in peatlands, with cascading effects on belowground decomposers and ecosystem C turnover. However, elucidating the dominant causal relationships between plant communities, soil biota and C fluxes in these vulnerable ecosystems requires a better understanding of the spatio-temporal variability of abiotic and biotic drivers. In this study we investigated the effects of biotic (plant functional types, PFTs) and abiotic factors (soil temperature and soil moisture) in determining dynamic patterns of soil microbial community structure and C cycling. Four representative temperate peatland habitats were selected based on their peat forming vegetation – an Atlantic wet heathland, two active blanket bogs with herbaceous plants (Molinia caerulea and Eriophorum angustifolium), and a transition mire dominated by Sphagnum mosses located along an altitudinal gradient to include the natural variations in soil temperature and water content regimes. We found that peat microbial communities were more strongly linked to local abiotic conditions than to the dominant above-ground vegetation. Aerobic conditions and warmer temperatures accelerated fungal driven decomposition and CO2 emissions under shrubs, whereas decreases in Gram−negative bacteria promoted increased C losses under Molinia. These findings suggest that small spatial differences in abiotic conditions can create local “hotspots” of organic matter decomposition. We propose that temperate peatlands should be considered as ‘ecosystem sentinels’ for climate change, acting as early-warning indicators of climate-carbon feedbacks

Spatial variability of bomb C-14 in an upland peat bog

As part of a study investigating the carbon balance of a blanket bog, we made an assessment of the spatial variation of radiocarbon concentrations in the surface layers of a small area of peatland in the north of England. The peat depth at which bomb-14C content was the highest varied considerably between cores sampled from across the site. At several sampling locations, 14C levels &gt;100% Modern were confined to the surface 8 cm, whereas bomb 14C was evident at 1 site, located only meters away, to a depth of at least 1216 cm. Using the layer where 14C levels first exceeded 100% Modern as a chronological reference layer, we estimated the carbon accumulation rate over the last 50 yr for the surface peat at each site (range ~20 to ~125 g C m2 yr1). Our results show that although carbon accumulation over the last 50 yr was similar across the site, variation in the depth to which bomb 14C was evident implied considerable variation in the vertical peat growth rate