Sources of nitrous oxide emissions from agriculturally managed peatlands.

The draining and fertilization of peatlands for agriculture is globally an important source of the greenhouse gas nitrous oxide (N2 O). Hitherto, the contribution of major sources to the N2 O emission-that is, fertilization and nitrogen (N) release from peat decomposition-has not yet been deciphered. This hampers the development of smart mitigation strategies, considering that rewetting to halt peat decomposition and reducing N fertilization are promising N2 O emission-reduction strategies. Here, we used machine learning techniques and global N2 O observational data to generalize the distribution of N2 O emissions from agriculturally managed peatlands, to distinguish the sources of N2 O emissions, and to compare mitigation options. N2 O emissions from agriculturally managed croplands were 401.0 (344.5-470.9) kt N year-1 , with 121.6 (88.6-163.3) kt N year-1 contributed by fertilizer N. On grasslands, 64.0 (54.6-74.7) kt N2 O-N year-1 were emitted, with 4.6 (3.7-5.7) kt N2 O-N year-1 stemming from fertilizer N. The fertilizer-induced N2 O emission factor ranged from 1.5% to 3.2%. Reducing the current fertilizer input by 20% could achieve a 10% N2 O emission reduction for croplands but only 3% for grasslands. Rewetting 1.9 Mha cropland and 0.26 Mha grassland would achieve the same N2 O emission reductions. Our results suggest that N2 O mitigation strategies for managed peatlands should be considered separately across land-use types and climatic zones. For croplands, particularly in the tropics, relevant N2 O mitigation potentials are achievable through both fertilizer N reduction and peatland rewetting. For grasslands, management schemes to halt peat degradation (e.g. rewetting) should be considered preferentially for mitigating N2 O and contributing to meeting climate goals.

Carbon sequestration in soils and climate change mitigation-Definitions and pitfalls.

The term carbon (C) sequestration has not just become a buzzword but is something of a siren's call to scientific communicators and media outlets. Carbon sequestration is the removal of C from the atmosphere and the storage, for example, in soil. It has the potential to partially compensate for anthropogenic greenhouse gas emissions and is, therefore, an important piece in the global climate change mitigation puzzle. However, the term C sequestration is often used misleadingly and, while likely unintentional, can lead to the perpetuation of biased conclusions and exaggerated expectations about its contribution to climate change mitigation efforts. Soils have considerable potential to take up C but many are also in a state of continuous loss. In such soils, measures to build up soil C may only lead to a reduction in C losses (C loss mitigation) rather than result in real C sequestration and negative emissions. In an examination of 100 recent peer-reviewed papers on topics surrounding soil C, only 4% were found to have used the term C sequestration correctly. Furthermore, 13% of the papers equated C sequestration with C stocks. The review, further, revealed that measures leading to C sequestration will not always result in climate change mitigation when non-CO2 greenhouse gases and leakage are taken into consideration. This paper highlights potential pitfalls when using the term C sequestration incorrectly and calls for accurate usage of this term going forward. Revised and new terms are suggested to distinguish clearly between C sequestration in soils, SOC loss mitigation, negative emissions, climate change mitigation, SOC storage, and SOC accrual to avoid miscommunication among scientists and stakeholder groups in future.

Carbon farming: Climate change mitigation via non-permanent carbon sinks.

The role of carbon farming in agriculture or forestry to mitigate climate change is currently under intensive scientific discussion along with the gradual but progressing evolution of the voluntary carbon market and its certification. An overarching issue is the question of the permanence of terrestrial carbon sinks. In this comment, I discuss the climate benefit of non-permanent carbon sinks in light of a recent publication stating that carbon certificates fall short of expectations for climate change mitigation because of their non-permanence. The beneficial effect of short-lived sinks is real and quantifiable, and this understanding is applicable within ex ante biophysical discounting, which has the potential to improve the trustworthiness of climate change mitigation via carbon farming.

Organochemical Characterization of Peat Reveals Decomposition of Specific Hemicellulose Structures as the Main Cause of Organic Matter Loss in the Acrotelm.

Peatlands store carbon in the form of dead organic residues. Climate change and human impact impose risks on the sustainability of the peatlands carbon balance due to increased peat decomposition. Here, we investigated molecular changes in the upper peat layers (0-40 cm), inferred from high-resolution vertical depth profiles, from a boreal peatland using two-dimensional 1H-13C nuclear magnetic resonance (NMR) spectroscopy, and comparison to δ13C, δ15N, and carbon and nitrogen content. Effects of hydrological conditions were investigated at respective sites: natural moist, drainage ditch, and natural dry. The molecular characterization revealed preferential degradation of specific side-chain linkages of xylan-type hemicelluloses within 0-14 cm at all sites, indicating organic matter losses up to 25%. In contrast, the xylan backbone, galactomannan-type hemicelluloses, and cellulose were more resistant to degradation and accumulated at the natural moist and drainage site. δ13C, δ15N, and carbon and nitrogen content did not correlate with specific hemicellulose structures but reflected changes in total carbohydrates. Our analysis provides novel insights into peat carbohydrate decomposition and indicates substantial organic matter losses in the acrotelm due to the degradation of specific hemicellulose structures. This suggests that variations in hemicellulose content and structure influence peat stability, which may have important implications with respect to climate change.

Achievable agricultural soil carbon sequestration across Europe from country-specific estimates.

The role of soils in the global carbon cycle and in reducing GHG emissions from agriculture has been increasingly acknowledged. The '4 per 1000' (4p1000) initiative has become a prominent action plan for climate change mitigation and achieve food security through an annual increase in soil organic carbon (SOC) stocks by 0.4%, (i.e. 4‰ per year). However, the feasibility of the 4p1000 scenario and, more generally, the capacity of individual countries to implement soil carbon sequestration (SCS) measures remain highly uncertain. Here, we evaluated country-specific SCS potentials of agricultural land for 24 countries in Europe. Based on a detailed survey of available literature, we estimate that between 0.1% and 27% of the agricultural greenhouse gas (GHG) emissions can potentially be compensated by SCS annually within the next decades. Measures varied widely across countries, indicating differences in country-specific environmental conditions and agricultural practices. None of the countries' SCS potential reached the aspirational goal of the 4p1000 initiative, suggesting that in order to achieve this goal, a wider range of measures and implementation pathways need to be explored. Yet, SCS potentials exceeded those from previous pan-European modelling scenarios, underpinning the general need to include national/regional knowledge and expertise to improve estimates of SCS potentials. The complexity of the chosen SCS measurement approaches between countries ranked from tier 1 to tier 3 and included the effect of different controlling factors, suggesting that methodological improvements and standardization of SCS accounting are urgently required. Standardization should include the assessment of key controlling factors such as realistic areas, technical and practical feasibility, trade-offs with other GHG and climate change. Our analysis suggests that country-specific knowledge and SCS estimates together with improved data sharing and harmonization are crucial to better quantify the role of soils in offsetting anthropogenic GHG emissions at global level.

Belowground plant allocation regulates rice methane emissions from degraded peat soils.

Carbon-rich peat soils have been drained and used extensively for agriculture throughout human history, leading to significant losses of their soil carbon. One solution for rewetting degraded peat is wet crop cultivation. Crops such as rice, which can grow in water-saturated conditions, could enable agricultural production to be maintained whilst reducing CO2 and N2O emissions from peat. However, wet rice cultivation can release considerable methane (CH4). Water table and soil management strategies may enhance rice yield and minimize CH4 emissions, but they also influence plant biomass allocation strategies. It remains unclear how water and soil management influences rice allocation strategies and how changing plant allocation and associated traits, particularly belowground, influence CH4-related processes. We examined belowground biomass (BGB), aboveground biomass (AGB), belowground:aboveground ratio (BGB:ABG), and a range of root traits (root length, root diameter, root volume, root area, and specific root length) under different soil and water treatments; and evaluated plant trait linkages to CH4. Rice (Oryza sativa L.) was grown for six months in field mesocosms under high (saturated) or low water table treatments, and in either degraded peat soil or degraded peat covered with mineral soil. We found that BGB and BGB:AGB were lowest in water saturated conditions where mineral soil had been added to the peat, and highest in low-water table peat soils. Furthermore, CH4 and BGB were positively related, with BGB explaining 60% of the variation in CH4 but only under low water table conditions. Our results suggest that a mix of low water table and mineral soil addition could minimize belowground plant allocation in rice, which could further lower CH4 likely because root-derived carbon is a key substrate for methanogenesis. Minimizing root allocation, in conjunction with water and soil management, could be explored as a strategy for lowering CH4 emissions from wet rice cultivation in degraded peatlands.

Stable isotopes (δ13C, δ15N) and biomarkers as indicators of the hydrological regime of fens in a European east-west transect.

Peatland degradation is tightly connected to hydrological changes and microbial metabolism. To better understand these metabolism processes, more information is needed on how microbial communities and substrate cycling are affected by changing hydrological regimes. These activities should be imprinted in stable isotope bulk values (δ 15N, δ 13C) due to specific isotopic fractionation by different microbial communities, their metabolic pathways and nutrient sources. We hypothesize that stable isotope values and microbial abundance are correlated and act as indicators of different hydrological regimes. We sampled an East-West transect across European fens in 14 areas and conducted a stable isotope (δ 13C, δ 15N) and membrane fatty acid (mFA) analysis. Within each area an undrained, drained and rewetted site was selected. Rewetted sites were separated based on when rewetting occurred. We found differences in the upper layers of all sites in microbial-derived mFAs and stable isotope values corresponding to hydrological regimes. The highest and lowest quantities of microbial-derived mFAs were measured in undrained and drained sites, respectively. Fungal-derived mFAs were especially lower in drained sites. Simultaneously, δ15N stable isotope values were highest in drained sites. In addition, stable isotope values and microbial-derived mFAs showed distinct depth trends. In undrained sites stable isotopes values slightly increased with depth. In drained sites, δ15N values decreased downwards, whereas δ13C values increased. Overall microbial-derived mFAs decreased with depth. These patterns presumably result from anoxic conditions and high peat recalcitrance in the deeper layers. In sites with short time of rewetting, the microbial-derived mFAs and stable isotope values were similar to values of drained sites, while with increasing rewetting time values shifted to those of undrained sites. We conclude that biomarkers indicate that stable isotope values reflect specific microbial metabolic processes, which differ with hydrological regimes, and thus could indicate both drainage and rewetting in fens.

δ15N natural abundance in permafrost soil indicates impact of fire on nitrogen cycle.

The impact of fire on the nitrogen (N) cycle of natural ecosystems is arguable. Here we report and interpret an observation from boreal ecosystems in the Lena River basin, Sakha Republic (Yakutia), Russian Federation. Different types of permafrost soil (0-30 cm depth) were sampled along transects (60-150 m length) from the forest edge towards the centre of four separate thermokarst depressions under grassland. The average values of δ(15)N were remarkably similar within three transects, but differed systematically between them. Three findings point towards fire being the cause of the observed pattern. First, the spatial extent of systematic differences in soil δ(15)N coincides with the extent of typical fire scars in the region. Second, soil enrichment in (15)N is larger in the proximity of settlements, where fire is generally more frequent than in more remote places. Third, there is a significant positive correlation between δ(15)N values and the ratio of black C to total N. These findings point towards fire having a marked impact on soil δ(15)N and, accordingly, on the N cycle of this cold and dry ecosystem.

Soil carbon loss from drained agricultural peatland after coverage with mineral soil.

Drainage for agriculture has turned peatlands from a net sink to a net source of carbon (C). In order to reduce the environmental footprint of agricultural peatland drainage, and to counteract soil subsidence, mineral soil coverage is becoming an increasingly used practice in Switzerland. To explore the effect of mineral soil coverage on soil C loss and the source of CO2 from peatland drained for agriculture, we utilized the radiocarbon signature (F14C) of soil C and emitted CO2 in the field. The experiment, located in the Swiss Rhine Valley, was carried out on two adjacent drained organic soils, either without mineral soil cover (reference 'Ref'), or covered with mineral soil (thickness ~ 40 cm) (coverage 'Cov') 13 years ago. Drainage already commenced 130 years ago and the site was managed as meadow since the 1970ies. Drainage induced 41-75 kg C m-2 loss, which is equivalent to annual C loss rates of 0.49-0.58 kg C m-2 yr-1 and 0.31-0.63 kg C m-2 yr-1 for Cov and Ref, respectively. Mineral soil coverage had no significant effect on the amount of heterotrophic respiration, however, at Cov, the radiocarbon signature of heterotrophic CO2 was significantly (p<0.01) younger than at Ref, indicating that mineral soil coverage moved the source of decomposition of soil organic carbon (SOC) from a higher share of old peat towards a higher share of relatively younger material. In summary, our study lends support to the hypothesis that mineral soil coverage might reduce the decomposition of old peat underneath, and may therefore be a promising peatland management technique for the future use of drained peatland for agriculture.

Organic farming and soil carbon sequestration: what do we really know about the benefits?

Organic farming is believed to improve soil fertility by enhancing soil organic matter (SOM) contents. An important co-benefit would be the sequestration of carbon from atmospheric CO2. Such a positive effect has been suggested based on data from field experiments though many studies were not designed to address the issue of carbon sequestration. The aim of our study was to examine published data in order to identify possible flaws such as missing a proper baseline, carbon mass measurements, or lack of a clear distinction between conventional and organic farming practices, thereby attributing effects of specific practices to organic farming, which are not uniquely organic. A total of 68 data sets were analyzed from 32 peer-reviewed publications aiming to compare conventional with organic farming. The analysis revealed that after conversion, soil C content (SOC) in organic systems increased annually by 2.2% on average, whereas in conventional systems SOC did not change significantly. The majority of publications reported SOC concentrations rather than amounts thus neglecting possible changes in soil bulk density. 34 out of 68 data sets missed a true control with well-defined starting conditions. In 37 out of 50 cases, the amount of organic fertilizer in the organic system exceeded that applied in the compared conventional system, and in half of the cases crop rotations differed between systems. In the few studies where crop rotation and organic fertilization were comparable in both systems no consistent difference in SOC was found. From this data analysis, we conclude that the claim for beneficial effects of organic farming on SOC is premature and that reported advantages of organic farming for SOC are largely determined by higher and often disproportionate application of organic fertilizer compared to conventional farming.

Soil organic matter stoichiometry as indicator for peatland degradation.

Peatlands accumulate organic matter (OM) under anaerobic conditions. After drainage for forestry or agriculture, microbial respiration and peat oxidation induce OM losses and change the stoichiometry of the remaining organic material. Here, we (i) evaluate whether land use (cropland CL, grassland GL, forest FL, natural peatland NL) is associated with different peat stoichiometry, (ii) study how peat stoichiometry changes with OM content and (iii) infer the fate of nitrogen upon soil degradation. Organic C and soil N were measured for 1310 samples from 48 sites in Switzerland, and H and O for 1165. The soil OM content and C/N ratio were most sensitive to land use and are hence best suited as indicators for peatland degradation. OM contents (CL < GL < FL < NL), H/C, O/C, C/N ratios, and OM oxidation states were significantly different between land use types in top- and subsoils. With decreasing bulk OM content, C was relatively depleted while H and particularly N were higher. The data suggest very high N mobilization rates from strongly decomposed peat in agricultural topsoil. A comparison to peat C and N from mostly intact peatlands of the Northern hemisphere reveals that agriculture and, to a lesser extent, forestry induce a progressed state of soil degradation.

Publisher Correction: Soil organic matter stoichiometry as indicator for peatland degradation.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Designing biochar properties through the blending of biomass feedstock with metals: Impact on oxyanions adsorption behavior.

Metal-blending of biomass prior to pyrolysis is investigated in this work as a tool to modify biochar physico-chemical properties and its behavior as adsorbent. Six different compounds were used for metal-blending: AlCl3, Cu(OH)2, FeSO4, KCl, MgCl2 and Mg(OH)2. Pyrolysis experiments were performed at 400 and 700 °C and the characterization of biochar properties included: elemental composition, thermal stability, surface area and pore size distribution, Zeta potential, redox potential, chemical structure (with nuclear magnetic resonance) and adsorption behavior of arsenate, phosphate and nitrate. Metalblending strongly affected biochars' surface charge and redox potential. Moreover, it increased biochars' microporosity (per mass of organic carbon). For most biochars, mesoporosity was also increased. The adsorption behavior was enhanced for all metal-blended biochars, although with significant differences across species: Mg(OH)2-blended biochar produced at 400 °C showed the highest phosphate adsorption capacity (Langmuir Qmax approx. 250 mg g-1), while AlCl3-blended biochar produced also at 400 °C showed the highest arsenate adsorption (Langmuir Qmax approx. 14 mg g-1). Significant differences were present, even for the same biochar, with respect to the investigated oxyanions. This indicates that biochar properties need to be optimized for each application, but also that this optimization can be achieved with tools such as metal-blending. These results constitute a significant contribution towards the production of designer biochars.

Distribution of nitrous oxide emissions from managed organic soils under different land uses estimated by the peat C/N ratio to improve national GHG inventories.

Nitrous oxide (N2O) contributes substantially to greenhouse gas (GHG) emissions in the agricultural and land-use sectors. Owing to the high effort needed for measuring N2O emissions and the resulting lack of sufficient field measurements to apply at country-wide scale, soil-borne N2O emissions are often estimated by applying published IPCC default emission factors. To examine the data reported in the national GHG inventory, the current study utilizes a large data set of soil C/N ratios to predict N2O emissions and their distribution from drained organic soils in Switzerland. Calculated emission rates increase in the order of forest

Restricting the nonlinearity parameter in soil greenhouse gas flux calculation for more reliable flux estimates.

The static chamber approach is often used for greenhouse gas (GHG) flux measurements, whereby the flux is deduced from the increase of species concentration after closing the chamber. Since this increase changes diffusion gradients between chamber air and soil air, a nonlinear increase is expected. Lateral gas flow and leakages also contribute to non linearity. Several models have been suggested to account for this non linearity, the most recent being the Hutchinson-Mosier regression model (hmr). However, the practical application of these models is challenging because the researcher needs to decide for each flux whether a nonlinear fit is appropriate or exaggerates flux estimates due to measurement artifacts. In the latter case, a flux estimate from the linear model is a more robust solution and introduces less arbitrary uncertainty to the data. We present the new, dynamic and reproducible flux calculation scheme, kappa.max, for an improved trade-off between bias and uncertainty (i.e. accuracy and precision). We develop a tool to simulate, visualise and optimise the flux calculation scheme for any specific static N2O chamber measurement system. The decision procedure and visualisation tools are implemented in a package for the R software. Finally, we demonstrate with this approach the performance of the applied flux calculation scheme for a measured flux dataset to estimate the actual bias and uncertainty. The kappa.max method effectively improved the decision between linear and nonlinear flux estimates reducing the bias at a minimal cost of uncertainty.

Overestimation of Crop Root Biomass in Field Experiments Due to Extraneous Organic Matter.

Root biomass is one of the most relevant root parameters for studies of plant response to environmental change, soil carbon modeling or estimations of soil carbon sequestration. A major source of error in root biomass quantification of agricultural crops in the field is the presence of extraneous organic matter in soil: dead roots from previous crops, weed roots, incorporated above ground plant residues and organic soil amendments, or remnants of soil fauna. Using the isotopic difference between recent maize root biomass and predominantly C3-derived extraneous organic matter, we determined the proportions of maize root biomass carbon of total carbon in root samples from the Swiss long-term field trial "DOK." We additionally evaluated the effects of agricultural management (bio-organic and conventional), sampling depth (0-0.25, 0.25-0.5, 0.5-0.75 m) and position (within and between maize rows), and root size class (coarse and fine roots) as defined by sieve mesh size (2 and 0.5 mm) on those proportions, and quantified the success rate of manual exclusion of extraneous organic matter from root samples. Only 60% of the root mass that we retrieved from field soil cores was actual maize root biomass from the current season. While the proportions of maize root biomass carbon were not affected by agricultural management, they increased consistently with soil depth, were higher within than between maize rows, and were higher in coarse (>2 mm) than in fine (≤2 and >0.5) root samples. The success rate of manual exclusion of extraneous organic matter from root samples was related to agricultural management and, at best, about 60%. We assume that the composition of extraneous organic matter is strongly influenced by agricultural management and soil depth and governs the effect size of the investigated factors. Extraneous organic matter may result in severe overestimation of recovered root biomass and has, therefore, large implications for soil carbon modeling and estimations of the climate change mitigation potential of soils.