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Background and aims: Northern peatlands are large repositories of carbon. Peatland vascular plant community composition has been functionally associated to a set of biogeochemical processes such as carbon cycling. Yet, we do not fully understand to what extent vascular plant functional types (PFTs) affect the quality of dissolved organic matter, and if there is any feedback on soil microbial activity. Methods: Using a longer-term plant removal experiment in a boreo-nemoral peatland in Southern Sweden, we relate the dominance of different vascular plant functional types (i.e. ericoids and graminoids) to the chemistry of the dissolved organic matter (DOM) and microbial enzymatic activities (fluorescein diacetate hydrolysis, FDA). Results: Our results show that PFTs modifies the composition of DOM moieties, with a decrease of low molecular weight organic compounds after vascular plant removal. The decrease of enzymatic activity by up to 68 % in the plant removal plots suggests a reduction in DOM mineralization in the absence of vascular plants. Conclusions: Our results show that plant-derived low molecular organic compounds enhance peatland microbial activity, and suggest that an increase of vascular plant cover in response to climate change can potentially destabilize the OM in peatlands, leading to increased carbon losses. [ABSTRACT FROM AUTHOR]

Peatlands are important sinks of atmospheric carbon (C) that, in response to climate warming, are undergoing dynamic vegetation succession. Here we examined the hypothesis that the uptake of nutrients by different plant growth forms (PGFs) is one key mechanism driving changes in species abundance in peatlands. Along an altitude gradient representing a natural climate experiment, we compared the variability of the stable C isotope composition (δC) and stable nitrogen (N) isotope composition (δN) in current-year leaves of two major PGFs, i.e. ericoids and graminoids. The climate gradient was associated with a gradient of vascular plant cover, which was parallelled by different concentrations of organic and inorganic N as well as the fungal/bacterial ratio in peat. In both PGFs the C natural abundance showed a marginal spatial decrease with altitude and a temporal decrease with progression of the growing season. Our data highlight a primary physical control of foliar δC signature, which is independent from the PGFs. Natural abundance of foliar N did not show any seasonal pattern and only in the ericoids showed depletion at lower elevation. This decreasing δN pattern was primarily controlled by the higher relative availability of organic versus inorganic N and, only for the ericoids, by an increased proportion of fungi to bacteria in soil. Our space-for-time approach demonstrates that a change in abundance of PGFs is associated with a different strategy of nutrient acquisition (i.e. transfer via mycorrhizal symbiosis versus direct fine-root uptake), which could likely promote observed and predicted dwarf shrub expansion under climate change. [ABSTRACT FROM AUTHOR]

[Display omitted] • The role of temporary grasslands as negative emission technology was assessed. • A new approach to account for bias in bulk density is developed. • C-deficit in croplands decreases linearly with the increase of temporary grasslands. • SOC storage potential in top and subsoil are equivalent. • SOC storage potential corresponds to 1 year of GHG emissions from the same region. Atmospheric C sequestration in agricultural soils is viewed as one of the most promising negative emission technologies currently available. Nonetheless, it remains unclear how strongly soil organic carbon (SOC) stocks respond to agricultural practices, especially for subsoil. Here, we assess the SOC storage potential in croplands and how the presence of temporary grasslands (TG) in the crop rotation affects SOC stocks. We developed a new approach to correct for bias in bulk density (BD) induced by sampling conditions and land-use effects with a data-driven model to predict the BD of fine soil (<2 mm) for reference condition. Using 54 permanent grassland and cropland sites with various proportions of TG from a monitoring network in Switzerland, we showed that SOC stock differences down to 50-cm depth between cropland and permanent grasslands (maximum: 3.0 ± 0.8 kg C m−2) depend on the TG proportion in the crop rotation, regardless of clay content and pH. An increase of the TG proportion by 10% would induce a SOC gain of 0.40 ± 0.13 kg C m−2. The responses of topsoil (0–20 cm) and subsoil (20–50 cm) SOC stocks to TG proportion were linear and equivalent. The effect of TG on SOC storage would have been underestimated by 58% without accounting for subsoil stocks response and by 16% without BD corrections. The conversion of all croplands to permanent grasslands in the study region would potentially store a quantity of SOC equivalent to the anthropogenic greenhouse gas emissions generated by the same region during one year. Although the potential of agricultural soils as negative emission technology is relatively modest compared to former expectations, the findings demonstrate the potential to manage SOC and its associated ecosystem services at large scales and down to deep soil layers. [ABSTRACT FROM AUTHOR]

Question A better understanding of the response of Sphagnum mosses and associated vascular plants to climate warming is relevant for predicting the carbon balance of peatlands in a warmer world. Open-top chambers ( OTCs) have been used to investigate the effect on soil biogeochemical processes in peatlands, but little information is available on the effects of OTCs on microclimate conditions and the associated response of the plant community. We aimed to understand how simulated warming and differences in soil moisture affect plant species cover. Location A Sphagnum-dominated peatlands in French Jura. Methods We used OTCs to measure the effect of a near-ground temperature increase (+1.5 °C on average) on vegetation dynamics over five growing seasons (2008-2012) in a Sphagnum-dominated peatland, in two adjacent microhabitats with different hydrological conditions - wet and dry. Microclimatic conditions and plant species abundance were monitored at peak biomass in years 1, 2, 3 and 5 and monthly during the plant growing season in year 5. Results The response to warming differed between vascular plants and bryophytes, as well as among species within these groups, and also varied in relation to soil moisture. Andromeda polifolia abundance responded positively to warming, while Vaccinium oxycoccus responded negatively, and Eriophorum vaginatum showed a high resistance. Conclusion Depth of rooting of vascular plants appeared to control the response in plant abundance, while moss abundance depended on various other interacting factors, such as shading by the vascular plant community, precipitation and soil moisture. [ABSTRACT FROM AUTHOR]

Background and Aims: Alterations in snow cover driven by climate change may impact ecosystem functioning, including biogeochemistry and soil (microbial) processes. We elucidated the effects of snow cover manipulation (SCM) on above-and belowground processes in a temperate peatland. Methods: In a Swiss mountain-peatland we manipulated snow cover (addition, removal and control), and assessed the effects on Andromeda polifolia root enzyme activity, soil microbial community structure, and leaf tissue and soil biogeochemistry. Results: Reduced snow cover produced warmer soils in our experiment while increased snow cover kept soil temperatures close-to-freezing. SCM had a major influence on the microbial community, and prolonged 'close-to-freezing' temperatures caused a shift in microbial communities toward fungal dominance. Soil temperature largely explained soil microbial structure, while other descriptors such as root enzyme activity and pore-water chemistry interacted less with the soil microbial communities. Conclusions: We envisage that SCM-driven changes in the microbial community composition could lead to substantial changes in trophic fluxes and associated ecosystem processes. Hence, we need to improve our understanding on the impact of frost and freeze-thaw cycles on the microbial food web and its implications for peatland ecosystem processes in a changing climate; in particular for the fate of the sequestered carbon. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mineral-associated organic C deficits in croplands and grasslands were estimated. • The relationship between soil C-saturation and soil physical quality was assessed. • Challenges to the C-saturation concept and the SOC:clay ratio were highlighted. • C-saturation in grassland was positively linked to SOC accrual history. • SOC accrual is more beneficial for soil physical quality in croplands. Reliable estimations of soil organic carbon (SOC) deficits in agroecosystems are crucial in evaluating the atmospheric C sequestration potential of agricultural soils and supporting management decisions. Nonetheless, the co-benefit on soil quality resulting from SOC accrual is rarely considered. Here, we assessed SOC saturation and soil physical quality in permanent grasslands (PG) and croplands (CR) by applying the C-saturation concept and the SOC:clay ratio as an indicator of soil physical quality to a set of long-term monitoring sites in western Switzerland. For this goal, we produced a new relationship between the silt + clay (SC) particles and the C stored in the mineral-associated fraction (MAOM C) and we assessed the assumption that grasslands can be used as C-saturated reference sites. The saturation in PG was not coincidental as it depended on the C accrual history. Hence, PG with the lowest MAOM C have not reached their C-saturation level and present a potential SOC storage under optimal management. The MAOM C saturation in CR was low (62 ± 4%) and corresponded to a deficit of −8.8 ± 1.2 mg C g−1 soil as compared to the current level in PG. The saturation was mainly affected by the proportion of temporary grassland in the crop rotation. The relative distribution of C between MAOM (∼80%) and the fine and coarse particulate organic matter (POM) was not affected by land-use types. The MAOM C saturation in this study (MAOM C = 0.372 × SC + 4.23) was similar to that reported in the litterature, but discrepancies appeared when the silt and clay contents were considered separately. SC was by far the main factor explaining MAOM C amount in PG (semi-partial R2: 0.66). In contrast to other studies, the C content of MAOM in PG (43 mg C g−1 SC) was not related to the SC content, suggesting a fixed maximal value in C-saturated soils. Nonetheless, MAOM C saturation may be underestimated as the least saturated PG might still accumulate MAOM C. Finally, the SOC:clay ratio was correlated with MAOM C saturation level in CR, but not in PG suggesting that targeting SOC accrual in CR optimizes the benefits between soil C storage and soil quality. [ABSTRACT FROM AUTHOR]

Question: Do soil water content and/or soil nitrogen (N) content and/or soil phosphorus (P) content affect the biomass of Vaccinium myrtillus and V. vitis-idaea in a sub-alpine heath? Location: Dolomites, northern Italy, 1800 m a.s.l. Methods: We determined above-ground and below-ground biomass of the shrubs at three sites, each on a different substrate type. At each site, we determined soil N- and P- contents. We also determined leaf water potential (ψ1), N- and P-concentrations in plant tissues and litter, as well as δ13C and δ15N in mature leaves. Results: V. myrtillus biomass was highest at the silicate site, V. vitis-idaea biomass was highest at the carbonate site. Both shrubs had low biomass at the peat site, possibly due to a toxic effect of waterlogging in wet soils. For both species, pre-dawn ψ1 indicated optimal hydration and midday ψ1 did not show any sign of water stress. Water use efficiency (WUE) did not differ among sites for any species. Whole-plant nutrient concentrations showed that, with increasing biomass, N was diluted in V. myrtillus tissues while P was diluted in V. vitis- idaea tissues. Foliar N-concentration was higher overall for V. myrtillus. Foliar P-concentration in V. myrtillus peaked at the silicate site. Foliar N : P ratios suggested that V. myrtillus was primarily P-limited and V. vitis-idaea primarily N-limited. Conclusions: Water content affected the distribution of the two shrubs in a similar way, higher P-availability in the soil enhanced V. myrtillus rather than V. vitis-idaea. [ABSTRACT FROM AUTHOR]

Optimizing the use of phosphorus (P) fertilizers is essential considering the environmental issues linked to over fertilization and the limited rock phosphate reserve. Diagnosing the crop P status by calculating a phosphorus nutrition index (PNI) based on the plant critical P concentration (P c) could help determine the minimum amount of P fertilizer to ensure maximum crop yield. This study investigated (i) the effect of P fertilization on winter wheat (Triticum hybernum L.) shoot biomass and grain yield after several decades of different annual P fertilization rates, (ii) the stability over several site-years of the P c expressed either as a function of shoot biomass or shoot nitrogen (N) concentration, and (iii) the possibility of using the PNI or the shoot N-to-P c ratio as nutritional indicators to diagnose P deficiency and predict the expected response to P fertilization. Shoot biomass and its P and N concentrations were measured weekly at five site-years along with grain yield after several decades of P fertilization treatments that ranged between 0 and 5/3 of the theoretical P crop uptake. The P fertilization did not affect the grain yield, but it generally increased shoot biomass especially at the CD 27- -37 developmental stages. The P c expressed either as a function of shoot biomass or shoot N concentration differed among site-years and this was attributed to differences in crop N status. When N was not deficient, we developed a P c dilution curve as a function of shoot biomass (SB) (4.56 × SB−0.279) along with a linear relationship (P c = 1.10 + 0.061 × N) and a power function (P c = 0.34 × N0.632) between P c and shoot N concentration. The N-to-P c ratio was related to shoot biomass accumulation according to a power function, but the relationship also differed among site-years. The relative shoot biomass responded positively to the PNI up to different thresholds for limiting and non-limiting N conditions. The relative grain yield, however, was not related to the PNI. The PNI, based on the P c dilution curve, has potential as a nutritional indicator to diagnose P deficiency and the expected response to P fertilization, but more research is needed to clarify the effect of N deficiencies on the P c dilution curve. [ABSTRACT FROM AUTHOR]

In semi-arid regions, soil phosphorus (P) dynamics in cereal-legume intercropping are not yet fully elucidated, particularly in relation to integrated application of fertilizers. To this aim, we investigate the effects of different fertilizers on various P fractions in relation to the rhizosphere-microbial processes in a cowpea/maize intercropping system. Field experiments were conducted during two consecutive years (2016–2017) in a split-plot design by establishing cowpea/maize alone or intercropped onto the main plot, while the sub-plot was treated with four types of fertilization, i.e. no fertilizer addition (control), organic amendment (compost), mineral fertilizers (NPK) and multi-nutrient enriched compost (NPKEC). Our results showed that NPKEC fertilizer increased NaHCO 3 -P i by 69% in maize, 62% in cowpea and 93% in intercropped plots compared to control plots. Similarly, a significant increase in the NaHCO 3 -P o fraction was also recorded with NPKEC treatment in all cropping systems. In case of moderately labile P, NPKEC fertilizer caused the highest increase of NaOH-P o (12.87 ± 0.50 mg P kg−1 soil) and NaOH-P i (22.29 ± 0.83 mg P kg−1 soil) fractions in intercropped plots. Except for intercropping, NPK application caused an increase in the non-available P fraction (HCl-P i), while the use of NPKEC decreased the HCl-P i concentration in all cropping systems, suggesting stronger merits both for intercropping and NPKEC. Surprisingly, maize exhibited substantially higher phosphatases activity compared to cowpea in monoculture amended with compost, implying distinct crop strategies for adaptation under low P conditions. Based on the multi-factor analysis, the close association of NaHCO 3 –P with P solubilizing bacteria, root carboxylates and pH indicated that rhizosphere processes are the strongest predictors of immediately available P. Since alkaline phosphatase (ALP) is a P-degrading enzyme of microbial origin, rhizosphere related ALP association may have originated from root-associated microflora promoting P mobilization. Furthermore, the strong association of microbial biomass P (MBP) and acid phosphates (ACP) with NaOH–P fraction indicated moderately available P cycle in soil was mainly driven by microbial-related processes. Factor analysis map and two-way ANOVA confirmed that fertilization regime had a stronger effect on all tested variables compared to cropping system. Altogether, our results suggest that a combination of microbial-rhizosphere processes controls the dynamics of P fertility in semi-arid soils. In the broader context of improving soil P fertility, it is highly recommended the use of environmentally sustainable sources of fertilizer, such as NPKEC, which can enhance the competitive performance of legume-cereal intercropping under semi-arid agroecosystems. Image 1 • Multi-nutrient compost (NPKEC) was tested for P fertility under cowpea-maize cropping. • In all cropping systems, NPKEC increased labile inorganic P ~62–93% than control. • Intercropping with NPKEC reduced non-available inorganic P by 29% over NPK fertilizers. • Type of fertilization steers intercropping related rhizosphere-microbial P processes. • Multi-nutrient compost enhanced competitiveness of intercrops in semi-arid agroecosystem. [ABSTRACT FROM AUTHOR]

In northern peatlands, reduction of Sphagnum dominance in favour of vascular vegetation is likely to influence biogeochemical processes. Such vegetation changes occur as the water table lowers and temperatures rise. To test which of these factors has a significant influence on peatland vegetation, we conducted a 3-year manipulative field experiment in Linje mire (northern Poland). We manipulated the peatland water table level (wet, intermediate and dry; on average the depth of the water table was 17.4, 21.2 and 25.3 cm respectively), and we used open-top chambers (OTCs) to create warmer conditions (on average increase of 1.2°C in OTC plots compared to control plots). Peat drying through water table lowering at this local scale had a larger effect than OTC warming treatment per see on Sphagnum mosses and vascular plants. In particular, ericoid shrubs increased with a lower water table level, while Sphagnum decreased. Microclimatic measurements at the plot scale indicated that both water-level and temperature, represented by heating degree days (HDDs), can have significant effects on the vegetation. In a large-scale complementary vegetation gradient survey replicated in three peatlands positioned along a transitional oceanic-continental and temperate-boreal (subarctic) gradient (France-Poland-Western Siberia), an increase in ericoid shrubs was marked by an increase in phenols in peat pore water, resulting from higher phenol concentrations in vascular plant biomass. Our results suggest a shift in functioning from a mineral-N-driven to a fungi-mediated organic-N nutrient acquisition with shrub encroachment. Both ericoid shrub encroachment and higher mean annual temperature in the three sites triggered greater vascular plant biomass and consequently the dominance of decomposers (especially fungi), which led to a feeding community dominated by nematodes. This contributed to lower enzymatic multifunctionality. Our findings illustrate mechanisms by which plants influence ecosystem responses to climate change, through their effect on microbial trophic interactions., (© 2023 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Peatlands are important sinks for atmospheric carbon (C), yet the role of plant functional types (PFTs) for C sequestration under climatic perturbations is still unclear. A plant-removal experiment was used to study the importance of vascular PFTs for the net ecosystem CO2 exchange (NEE) during (i.e., resistance) and after (i.e., recovery) an experimental drought. The removal of PFTs caused a decrease of NEE, but the rate differed between microhabitats (i.e., hummocks and lawns) and the type of PFTs. Ericoid removal had a large effect on NEE in hummocks, while the graminoids played a major role in the lawns. The removal of PFTs did not affect the resistance or the recovery after the experimental drought. We argue that the response of Sphagnum mosses (the only PFT present in all treatments) to drought is dominant over that of coexisting PFTs. However, we observed that the moment in time when the system switched from C sink to C source during the drought was controlled by the vascular PFTs. In the light of climate change, the shifts in species composition or even the loss of certain PFTs are expected to strongly affect the future C dynamics in response to environmental stress.

Nitrogen and phosphorus were added experimentally in a bog in the southern Alps. It was hypothesized that alleviating nutrient limitation will increase vascular plant cover. As a consequence, more carbon will be fixed through higher rates of net ecosystem CO(2) exchange (NEE). The vascular cover did increase at the expense of Sphagnum mosses. However, such vegetation changes were largely independent of the treatment and were probably triggered by an exceptional heatwave in summer 2003. Contrary to the tested hypothesis, NEE was unaffected by the nutrient treatments but was strongly influenced by temperature and water-table depth. In particular, ecosystem respiration in the hot summer of 2003 increased dramatically, presumably owing to enhanced heterotrophic respiration in an increased oxic peat layer. At the end of the experiment, the Sphagnum cover decreased significantly in the nitrogen-fertilized treatment at hummock microhabitats. In the long term, this will imply a proportionally greater accumulation of vascular litter, more easily decomposable than the recalcitrant Sphagnum litter. As a result, rates of carbon fixation may decrease because of stimulated respiration.

Peat bogs have historically represented exceptional carbon (C) sinks because of their extremely low decomposition rates and consequent accumulation of plant remnants as peat. Among the factors favoring that peat accumulation, a major role is played by the chemical quality of plant litter itself, which is poor in nutrients and characterized by polyphenols with a strong inhibitory effect on microbial breakdown. Because bogs receive their nutrient supply solely from atmospheric deposition, the global increase of atmospheric nitrogen (N) inputs as a consequence of human activities could potentially alter the litter chemistry with important, but still unknown, effects on their C balance. Here we present data showing the decomposition rates of recently formed litter peat samples collected in nine European countries under a natural gradient of atmospheric N deposition from approximately 0.2 to 2 g.m(-2).yr(-1). We found that enhanced decomposition rates for material accumulated under higher atmospheric N supplies resulted in higher carbon dioxide (CO2) emissions and dissolved organic carbon release. The increased N availability favored microbial decomposition (i) by removing N constraints on microbial metabolism and (ii) through a chemical amelioration of litter peat quality with a positive feedback on microbial enzymatic activity. Although some uncertainty remains about whether decay-resistant Sphagnum will continue to dominate litter peat, our data indicate that, even without such changes, increased N deposition poses a serious risk to our valuable peatland C sinks.

•  We studied the effects of increasing levels of atmospheric nitrogen (N) deposition on nutrient limitation of ombrotrophic Sphagnum plants. •  Fifteen mires in 11 European countries were selected across a natural gradient of bulk atmospheric N deposition from 0.1 to 2 g/m 2  year -1 . Nutritional constraints were assessed based on nutrient ratios of N, phosphorus (P), and potassium (K) in Sphagnum plants collected in hummocks (i.e. relatively drier microhabitats) and in lawns (i.e. relatively wetter microhabitats). •  Nutrient ratios in Sphagnum plants increased steeply at low atmospheric N input, but above a threshold of N deposition of c. 1 g/m 2  year -1 the N : P and N : K ratios tended to saturation. Increasing atmospheric N deposition was also accompanied by a reduced retention of Ca and Mg in Sphagnum plants and a decreased stem volumetric density in hummock Sphagnum plants. •  We suggest a critical load of N deposition in Europe of 1 g/m 2  year -1 above which Sphagnum plants change from being N-limited to be K + P colimited, at N : P > 30 and N : K > 3.

Two subalpine dwarf-shrub heath communities with differing levels of soil nutrient availability were subjected to a 3-year experimental manipulation, including nutrient addition or removal of one of the two co-dominant species from each community. The main objective of our study was to assess the relative importance of interspecific competition versus nutrient limitation in relation to soil fertility. We also aimed to investigate if and to what extent current-year shoot size, leaf-based rates of net photosynthesis and foliar nutrient status accounted for the observed changes in the aboveground biomass of the shrubs. At the end of the experiment, neighbour removal increased the aboveground biomass of all shrubs, especially in the more fertile community, while fertilization did not. We concluded that: (1) competition is more effective than nutrient limitation in structuring the vegetation of subalpine heathlands; and (2) competition intensity is stronger in the more fertile community. The observed patterns of variations in aboveground biomass were not consistently related to net photosynthetic rates, size of individual shoots and foliar nutrient status. Hence, we also concluded that the growth response of dwarf shrubs to altered environmental conditions is primarily determined by developmental plasticity.