Effect of climate change,CO2 trends,nitrogen addition,and land‐cover and management intensity changes on the carbon balance of European grasslands

Several lines of evidence point to European managed grassland ecosystems being a sink of carbon. In this study, we apply ORCHIDEE‐GM a process‐based carbon cycle model that describes specific management practices of pastures and the dynamics of carbon cycling in response to changes in climatic and biogeochemical drivers. The model is used to simulate changes in the carbon balance [i.e., net biome production (NBP)] of European grasslands over 1991–2010 on a 25 km × 25 km grid. The modeled average trend in NBP is 1.8–2.0 g C m^?2 yr^?2 during the past two decades. Attribution of this trend suggests management intensity as the dominant driver explaining NBP trends in the model (36–43% of the trend due to all drivers). A major change in grassland management intensity has occurred across Europe resulting from reduced livestock numbers. This change has ‘inadvertently’ enhanced soil C sequestration and reduced N₂O and CH₄ emissions by 1.2–1.5 Gt CO₂‐equivalent, offsetting more than 7% of greenhouse gas emissions in the whole European agricultural sector during the period 1991–2010. Land‐cover change, climate change and rising CO₂ also make positive and moderate contributions to the NBP trend (between 24% and 31% of the trend due to all drivers). Changes in nitrogen addition (including fertilization and atmospheric deposition) are found to have only marginal net effect on NBP trends. However, this may not reflect reality because our model has only a very simple parameterization of nitrogen effects on photosynthesis. The sum of NBP trends from each driver is larger than the trend obtained when all drivers are varied together, leaving a residual – nonattributed – term (22–26% of the trend due to all drivers) indicating negative interactions between drivers.     

Effects of land use change and management on the European cropland carbon balance

We model the carbon balance of European croplands between 1901 and 2000 in response to land use and management changes. The process‐based ORCHIDEE‐STICS model is applied here in a spatially explicit framework. We reconstructed land cover changes, together with an idealized history of agro‐technology. These management parameters include the treatment of straw and stubble residues, application of mineral fertilizers, improvement of cultivar species and tillage. The model is integrated for wheat and maize during the period 1901–2000 forced by climate each 1/2‐hour, and by atmospheric CO₂, land cover change and agro‐technology each year. Several tests are performed to identify the most sensitive agro‐technological parameters that control the net biome productivity (NBP) in the 1990s, with NBP equaling for croplands the soil C balance. The current NBP is a small sink of 0.16 t C ha^?1 yr^?1. The value of NBP per unit area reflects past and current management, and to a minor extent the shrinking areas of arable land consecutive to abandonment during the 20th Century. The uncertainty associated with NBP is large, with a 1‐sigma error of 0.18 t C ha^?1 yr^?1 obtained from a qualitative, but comprehensive budget of various error terms. The NBP uncertainty is dominated by unknown historical agro‐technology changes (47%) and model structure (27%), with error in climate forcing playing a minor role. A major improvement to the framework would consist in using a larger number of representative crops. The uncertainty of historical land‐use change derived from three different reconstructions, has a surprisingly small effect on NBP (0.01 t C ha^?1 yr^?1) because cropland area remained stable during the past 20 years in all the tested land use forcing datasets. Regional cross‐validation of modeled NBP against soil C inventory measurements shows that our results are consistent with observations, within the uncertainties of both inventories and model. Our estimation of cropland NBP is however likely to be biased towards a sink, given that inventory data from different regions consistently indicate a small source whereas we model a small sink.     

Large soil organic carbon increase due to improved agronomic management in the North China Plain from 1980s to 2010s

Agricultural soils are widely recognized to be capable of carbon sequestration that contributes to mitigating CO₂ emissions. To better understand soil organic carbon (SOC) stock dynamics and its driving and controlling factors corresponding with a period of rapid agronomic evolution from the 1980s to the 2010s in the North China Plain (NCP), we collected data from two region‐wide soil sampling campaigns (in the 1980s and 2010s) and conducted an analysis of the controlling factors using the random forest model. Between the 1980s and 2010s, environmental (i.e. soil salinity/fertility) and societal (i.e. policy/techniques) factors both contributed to adoption of new management practices (i.e. chemical fertilizer application/mechanization). Results of our work indicate that SOC stocks in the NCP croplands increased significantly, which also closely related to soil total nitrogen changes. Samples collected near the surface (0–20 cm) and deeper (20–40 cm) both increased by an average of 9.4 and 5.1 Mg C ha^?1, respectively, which are equivalent to increases of 73% and 56% compared with initial SOC stocks in the 1980s. The annual carbon sequestration amount in surface soils reached 10.9 Tg C year^?1, which contributed an estimated 43% of total carbon sequestration in all of China's cropland on just 27% of its area. Successful desalinization and the subsequent increases in carbon (C) inputs, induced by agricultural projects and policies intended to support crop production (i.e. reconstruction of low yield farmland, and agricultural subsidies), combined with improved cultivation practices (i.e. fertilization and straw return) since the early 1980s were the main drivers for the SOC stock increase. This study suggests that rehabilitation of NCP soils to reduce salinity and increase crop yields have also served as a pathway for substantial soil C sequestration.     

Dynamics of soil carbon following destruction of tropical rainforest and the subsequent establishment of Imperata grassland in Indonesian Borneo using stable carbon isotopes

Southeast Asia has the highest rate of tropical rainforest deforestation worldwide, and large deforested areas have been replaced ultimately by the highly invasive grass Imperata cylindrica. However, information on the carbon (C) budget with such land transition is very scarce. This study presents the dynamics of soil C following rainforest destruction and the subsequent establishment of Imperata grassland in the lowland humid tropics of Indonesian Borneo using stable C isotopes. To evaluate the relative contribution of organic matter originating from primary forest (C₃) and grasslands (C₄), we compared soil C stock and natural ¹³C abundance from six sites to a depth of 100 cm using samples with a wide range of soil textures. Twelve years after the first soil sampling in the grasslands, we re‐sampled to examine temporal changes in soil organic matter. The grassland topsoil (0–5 cm) is an active layer with rapid decomposition and incorporation of fresh C (mean residence time: 7.5 year) and a substantial proportion of the stable C pool (37%). The decline in forest‐derived C was slight, even at 5–10 cm depths, and subsoil (20–100 cm depth) forest‐derived C did not change along the forest‐to‐grassland chronosequence. Grassland‐derived C stock increased significantly in the subsurface and subsoils (5–100 cm). Simulation indicated that total soil C stock (0–100 cm) increased by 18.6 Mg ha^?1 from initial primary forest (58.0 Mg ha^?1) to a new equilibrium state of the grassland (76.6 Mg ha^?1) after 30–50 years of grassland establishment. This research indicates that the soil did not function as a CO₂ source when the deforested area was replaced by Imperata grassland on the Ultisols of the Asian humid tropics. Instead, increased soil C stocks offset CO₂ emissions, with the C offset accounting for 6.6–7.4% of the loss of biomass C stock.     

The greenhouse gas balance of European grasslands

The greenhouse gas (GHG) balance of European grasslands (EU‐28 plus Norway and Switzerland), including CO₂, CH₄ and N₂O, is estimated using the new process‐based biogeochemical model ORCHIDEE‐GM over the period 1961–2010. The model includes the following: (1) a mechanistic representation of the spatial distribution of management practice; (2) management intensity, going from intensively to extensively managed; (3) gridded simulation of the carbon balance at ecosystem and farm scale; and (4) gridded simulation of N₂O and CH₄ emissions by fertilized grassland soils and livestock. The external drivers of the model are changing animal numbers, nitrogen fertilization and deposition, land‐use change, and variable CO₂ and climate. The carbon balance of European grassland (NBP) is estimated to be a net sink of 15 ± 7 g C m^?2 year^?1 during 1961–2010, equivalent to a 50‐year continental cumulative soil carbon sequestration of 1.0 ± 0.4 Pg C. At the farm scale, which includes both ecosystem CO₂ fluxes and CO₂ emissions from the digestion of harvested forage, the net C balance is roughly halved, down to a small sink, or nearly neutral flux of 8 g C m^?2 year^?1. Adding CH₄ and N₂O emissions to net ecosystem exchange to define the ecosystem‐scale GHG balance, we found that grasslands remain a net GHG sink of 19 ± 10 g C‐CO₂ equiv. m^?2 year^?1, because the CO₂ sink offsets N₂O and grazing animal CH₄ emissions. However, when considering the farm scale, the GHG balance (NGB) becomes a net GHG source of ?50 g C‐CO₂ equiv. m^?2 year^?1. ORCHIDEE‐GM simulated an increase in European grassland NBP during the last five decades. This enhanced NBP reflects the combination of a positive trend of net primary production due to CO₂, climate and nitrogen fertilization and the diminishing requirement for grass forage due to the Europe‐wide reduction in livestock numbers.     

Long‐term impacts of manure amendments on carbon and greenhouse gas dynamics of rangelands

Livestock manure is applied to rangelands as an organic fertilizer to stimulate forage production, but the long‐term impacts of this practice on soil carbon (C) and greenhouse gas (GHG) dynamics are poorly known. We collected soil samples from manured and nonmanured fields on commercial dairies and found that manure amendments increased soil C stocks by 19.0 ± 7.3 Mg C ha^?1 and N stocks by 1.94 ± 0.63 Mg N ha^?1 compared to nonmanured fields (0–20 cm depth). Long‐term historical (1700–present) and future (present–2100) impacts of management on soil C and N dynamics, net primary productivity (NPP), and GHG emissions were modeled with DayCent. Modeled total soil C and N stocks increased with the onset of dairying. Nitrous oxide (N₂O) emissions also increased by ~2 kg N₂O‐N ha^?1 yr^?1. These emissions were proportional to total N additions and offset 75–100% of soil C sequestration. All fields were small net methane (CH₄) sinks, averaging ?4.7 ± 1.2 kg CH₄‐C ha^?1 yr^?1. Overall, manured fields were net GHG sinks between 1954 and 2011 (?0.74 ± 0.73 Mg CO₂ e ha^?1 yr^?1, CO₂e are carbon dioxide equivalents), whereas nonmanured fields varied around zero. Future soil C pools stabilized 40–60 years faster in manured fields than nonmanured fields, at which point manured fields were significantly larger sources than nonmanured fields (1.45 ± 0.52 Mg CO₂e ha^?1 yr^?1 and 0.51 ± 0.60 Mg CO₂e ha^?1 yr^?1, respectively). Modeling also revealed a large background loss of soil C from the passive soil pool associated with the shift from perennial to annual grasses, equivalent to 29.4 ± 1.47 Tg CO₂e in California between 1820 and 2011. Manure applications increased NPP and soil C storage, but plant community changes and GHG emissions decreased, and eventually eliminated, the net climate benefit of this practice.     

Baseline map of organic carbon in Australian soil to support national carbon accounting and monitoring under climate change

We can effectively monitor soil condition—and develop sound policies to offset the emissions of greenhouse gases—only with accurate data from which to define baselines. Currently, estimates of soil organic C for countries or continents are either unavailable or largely uncertain because they are derived from sparse data, with large gaps over many areas of the Earth. Here, we derive spatially explicit estimates, and their uncertainty, of the distribution and stock of organic C in the soil of Australia. We assembled and harmonized data from several sources to produce the most comprehensive set of data on the current stock of organic C in soil of the continent. Using them, we have produced a fine spatial resolution baseline map of organic C at the continental scale. We describe how we made it by combining the bootstrap, a decision tree with piecewise regression on environmental variables and geostatistical modelling of residuals. Values of stock were predicted at the nodes of a 3‐arc‐sec (approximately 90 m) grid and mapped together with their uncertainties. We then calculated baselines of soil organic C storage over the whole of Australia, its states and territories, and regions that define bioclimatic zones, vegetation classes and land use. The average amount of organic C in Australian topsoil is estimated to be 29.7 t ha^?1 with 95% confidence limits of 22.6 and 37.9 t ha^?1. The total stock of organic C in the 0–30 cm layer of soil for the continent is 24.97 Gt with 95% confidence limits of 19.04 and 31.83 Gt. This represents approximately 3.5% of the total stock in the upper 30 cm of soil worldwide. Australia occupies 5.2% of the global land area, so the total organic C stock of Australian soil makes an important contribution to the global carbon cycle, and it provides a significant potential for sequestration. As the most reliable approximation of the stock of organic C in Australian soil in 2010, our estimates have important applications. They could support Australia's National Carbon Accounting System, help guide the formulation of policy around carbon offset schemes, improve Australia's carbon balances, serve to direct future sampling for inventory, guide the design of monitoring networks and provide a benchmark against which to assess the impact of changes in land cover, land management and climate on the stock of C in Australia. In this way, these estimates would help us to develop strategies to adapt and mitigate the effects of climate change.     

The European carbon balance. Part 2: croplands

We estimated the long‐term carbon balance [net biome production (NBP)] of European (EU‐25) croplands and its component fluxes, over the last two decades. Net primary production (NPP) estimates, from different data sources ranged between 490 and 846 gC m^?2 yr^?1, and mostly reflect uncertainties in allocation, and in cropland area when using yield statistics. Inventories of soil C change over arable lands may be the most reliable source of information on NBP, but inventories lack full and harmonized coverage of EU‐25. From a compilation of inventories we infer a mean loss of soil C amounting to 17 g m^?2 yr^?1. In addition, three process‐based models, driven by historical climate and evolving agricultural technology, estimate a small sink of 15 g C m^?2 yr^?1 or a small source of 7.6 g C m^?2 yr^?1. Neither the soil C inventory data, nor the process model results support the previous European‐scale NBP estimate by Janssens and colleagues of a large soil C loss of 90 ± 50 gC m^?2 yr^?1. Discrepancy between measured and modeled NBP is caused by erosion which is not inventoried, and the burning of harvest residues which is not modeled. When correcting the inventory NBP for the erosion flux, and the modeled NBP for agricultural fire losses, the discrepancy is reduced, and cropland NBP ranges between ?8.3 ± 13 and ?13 ± 33 g C m^?2 yr^?1 from the mean of the models and inventories, respectively. The mean nitrous oxide (N₂O) flux estimates ranges between 32 and 37 g C Eq m^?2 yr^?1, which nearly doubles the CO₂ losses. European croplands act as small CH₄ sink of 3.3 g C Eq m^?2 yr^?1. Considering ecosystem CO₂, N₂O and CH₄ fluxes provides for the net greenhouse gas balance a net source of 42–47 g C Eq m^?2 yr^?1. Intensifying agriculture in Eastern Europe to the same level Western Europe amounts is expected to result in a near doubling of the N₂O emissions in Eastern Europe. N₂O emissions will then become the main source of concern for the impact of European agriculture on climate.     

Evaluation of terrestrial carbon cycle models for their response to climate variability and to CO2 trends

The purpose of this study was to evaluate 10 process‐based terrestrial biosphere models that were used for the IPCC fifth Assessment Report. The simulated gross primary productivity (GPP) is compared with flux‐tower‐based estimates by Jung et al. [Journal of Geophysical Research 116 (2011) G00J07] (JU11). The net primary productivity (NPP) apparent sensitivity to climate variability and atmospheric CO₂ trends is diagnosed from each model output, using statistical functions. The temperature sensitivity is compared against ecosystem field warming experiments results. The CO₂ sensitivity of NPP is compared to the results from four Free‐Air CO₂ Enrichment (FACE) experiments. The simulated global net biome productivity (NBP) is compared with the residual land sink (RLS) of the global carbon budget from Friedlingstein et al. [Nature Geoscience 3 (2010) 811] (FR10). We found that models produce a higher GPP (133 ± 15 Pg C yr^?1) than JU11 (118 ± 6 Pg C yr^?1). In response to rising atmospheric CO₂ concentration, modeled NPP increases on average by 16% (5–20%) per 100 ppm, a slightly larger apparent sensitivity of NPP to CO₂ than that measured at the FACE experiment locations (13% per 100 ppm). Global NBP differs markedly among individual models, although the mean value of 2.0 ± 0.8 Pg C yr^?1 is remarkably close to the mean value of RLS (2.1 ± 1.2 Pg C yr^?1). The interannual variability in modeled NBP is significantly correlated with that of RLS for the period 1980–2009. Both model‐to‐model and interannual variation in model GPP is larger than that in model NBP due to the strong coupling causing a positive correlation between ecosystem respiration and GPP in the model. The average linear regression slope of global NBP vs. temperature across the 10 models is ?3.0 ± 1.5 Pg C yr^?1 °C^?1, within the uncertainty of what derived from RLS (?3.9 ± 1.1 Pg C yr^?1 °C^?1). However, 9 of 10 models overestimate the regression slope of NBP vs. precipitation, compared with the slope of the observed RLS vs. precipitation. With most models lacking processes that control GPP and NBP in addition to CO₂ and climate, the agreement between modeled and observation‐based GPP and NBP can be fortuitous. Carbon–nitrogen interactions (only separable in one model) significantly influence the simulated response of carbon cycle to temperature and atmospheric CO₂ concentration, suggesting that nutrients limitations should be included in the next generation of terrestrial biosphere models.     

Modelling climate,CO2 and management impacts on soil carbon in semi-arid agroecosystems

In agroecosystems, there is likely to be a strong interaction between global change and management that will determine whether soil will be a source or sink for atmospheric C. We conducted a simulation study of changes in soil C as a function of climate and CO₂ change, for a suite of different management systems, at four locations representing a climate sequence in the central Great Plains of the US.Climate, CO₂ and management interactions were analyzed for three agroecosystems: a conventional winter wheat-summer fallow rotation, a wheat-corn-fallow rotation and continuous cropping with wheat. Model analyses included soil C responses to changes in the amount and distribution of precipitation and responses to changes in temperature, precipitation and CO₂ as projected by a general circulation model for a 2 × CO₂ scenario.Overall, differences between management systems at all the sites were greater than those induced by perturbations of climate and/or CO₂. Crop residue production was increased by CO₂ enrichment and by a changed climate. Where the frequency of summer fallowing was reduced (wheat-corn-fallow) or eliminated (continuous wheat), soil C increased under all conditions, particularly with increased (640 L L^–1) CO₂. For wheat-fallow management, the model predicted declines in soil C under both ambient conditions and with climate change alone. Increased CO₂ with wheat-fallow management yielded small gains in soil C at three of the sites and reduced losses at the fourth site.Our results illustrate the importance of considering the role of management in determining potential responses of agroecosystems to global change. Changes in climate will determine changes in management as farmers strive to maximize profitability. Therefore, changes in soil C may be a complex function of climate driving management and management driving soil C levels and not be a simple direct effect of either climate or management.     

Differential effects of warming and nitrogen fertilization on soil respiration and microbial dynamics in switchgrass croplands

The mechanistic understanding of warming and nitrogen (N) fertilization, alone or in combination, on microbially mediated decomposition is limited. In this study, soil samples were collected from previously harvested switchgrass (Panicum virgatum L.) plots that had been treated with high N fertilizer (HN: 67 kg N ha^?1) and those that had received no N fertilizer (NN) over a 3‐year period. The samples were incubated for 180 days at 15 °C and 20 °C, during which heterotrophic respiration, δ¹³C of CO₂, microbial biomass (MB), specific soil respiration rate (R_s: respiration per unit of microbial biomass), and exoenzyme activities were quantified at 10 different collections time. Employing switchgrass tissues (referred to as litter) with naturally abundant ¹³C allowed us to partition CO₂ respiration derived from soil and amended litter. Cumulative soil respiration increased significantly by 16.4% and 4.2% under warming and N fertilization, respectively. Respiration derived from soil was elevated significantly with warming, while oxidase, the agent for recalcitrant soil substrate decomposition, was not significantly affected by warming. Warming, however, significantly enhanced MB and R_s indicating a decrease in microbial growth efficiency (MGE). On the contrary, respiration derived from amended litter was elevated with N fertilization, which was consistent with the significantly elevated hydrolase. N fertilization, however, had little effect on MB and R_s, suggesting little change in microbial physiology. Temperature and N fertilization showed minimal interactive effects likely due to little differences in soil N availability between NN and HN samples, which is partly attributable to switchgrass biomass N accumulation (equivalent to ~53% of fertilizer N). Overall, the differential individual effects of warming and N fertilization may be driven by physiological adaptation and stimulated exoenzyme kinetics, respectively. The study shed insights on distinct microbial acquisition of different substrates under global temperature increase and N enrichment.     

Living roots magnify the response of soil organic carbon decomposition to temperature in temperate grassland

Increasing atmospheric carbon dioxide (CO₂) concentration is both a strong driver of primary productivity and widely believed to be the principal cause of recent increases in global temperature. Soils are the largest store of the world's terrestrial C. Consequently, many investigations have attempted to mechanistically understand how microbial mineralisation of soil organic carbon (SOC) to CO₂ will be affected by projected increases in temperature. Most have attempted this in the absence of plants as the flux of CO₂ from root and rhizomicrobial respiration in intact plant‐soil systems confounds interpretation of measurements. We compared the effect of a small increase in temperature on respiration from soils without recent plant C with the effect on intact grass swards. We found that for 48 weeks, before acclimation occurred, an experimental 3 °C increase in sward temperature gave rise to a 50% increase in below ground respiration (ca. 0.4 kg C m^?2; Q₁₀ = 3.5), whereas mineralisation of older SOC without plants increased with a Q₁₀ of only 1.7 when subject to increases in ambient soil temperature. Subsequent ¹⁴C dating of respired CO₂ indicated that the presence of plants in swards more than doubled the effect of warming on the rate of mineralisation of SOC with an estimated mean C age of ca. 8 years or older relative to incubated soils without recent plant inputs. These results not only illustrate the formidable complexity of mechanisms controlling C fluxes in soils but also suggest that the dual biological and physical effects of CO₂ on primary productivity and global temperature have the potential to synergistically increase the mineralisation of existing soil C.     

Uncertainty in soil carbon accounting due to unrecognized soil erosion

The movement of soil organic carbon (SOC) during erosion and deposition events represents a major perturbation to the terrestrial carbon cycle. Despite the recognized impact soil redistribution can have on the carbon cycle, few major carbon accounting models currently allow for soil mass flux. Here, we modified a commonly used SOC model to include a soil redistribution term and then applied it to scenarios which explore the implications of unrecognized erosion and deposition for SOC accounting. We show that models that assume a static landscape may be calibrated incorrectly as erosion of SOC is hidden within the decay constants. This implicit inclusion of erosion then limits the predictive capacity of these models when applied to sites with different soil redistribution histories. Decay constants were found to be 15–50% slower when an erosion rate of 15 t soil ha^?1 yr^?1 was explicitly included in the SOC model calibration. Static models cannot account for SOC change resulting from agricultural management practices focused on reducing erosion rates. Without accounting for soil redistribution, a soil sampling scheme which uses a fixed depth to support model development can create large errors in actual and relative changes in SOC stocks. When modest levels of erosion were ignored, the combined uncertainty in carbon sequestration rates was 0.3–1.0 t CO₂ ha^?1 yr^?1. This range is similar to expected sequestration rates for many management options aimed at increasing SOC levels. It is evident from these analyses that explicit recognition of soil redistribution is critical to the success of a carbon monitoring or trading scheme which seeks to credit agricultural activities.     

Elevated atmospheric carbon dioxide increases soil carbon

The general lack of significant changes in mineral soil C stocks during CO₂‐enrichment experiments has cast doubt on predictions that increased soil C can partially offset rising atmospheric CO₂ concentrations. Here, we show, through meta‐analysis techniques, that these experiments collectively exhibited a 5.6% increase in soil C over 2–9 years, at a median rate of 19 g C m^?2 yr^?1. We also measured C accrual in deciduous forest and grassland soils, at rates exceeding 40 g C m^?2 yr^?1 for 5–8 years, because both systems responded to CO₂ enrichment with large increases in root production. Even though native C stocks were relatively large, over half of the accrued C at both sites was incorporated into microaggregates, which protect C and increase its longevity. Our data, in combination with the meta‐analysis, demonstrate the potential for mineral soils in diverse temperate ecosystems to store additional C in response to CO₂ enrichment.     

Conversion from forests to pastures in the Colombian Amazon leads to contrasting soil carbon dynamics depending on land management practices

Strategies to mitigate climate change by reducing deforestation and forest degradation (e.g. REDD+) require country‐ or region‐specific information on temporal changes in forest carbon (C) pools to develop accurate emission factors. The soil C pool is one of the most important C reservoirs, but is rarely included in national forest reference emission levels due to a lack of data. Here, we present the soil organic C (SOC) dynamics along 20 years of forest‐to‐pasture conversion in two subregions with different management practices during pasture establishment in the Colombian Amazon: high‐grazing intensity (HG) and low‐grazing intensity (LG) subregions. We determined the pattern of SOC change resulting from the conversion from forest (C3 plants) to pasture (C4 plants) by analysing total SOC stocks and the natural abundance of the stable isotopes ¹³C along two 20‐year chronosequences identified in each subregion. We also analysed soil N stocks and the natural abundance of ¹⁵N during pasture establishment. In general, total SOC stocks at 30 cm depth in the forest were similar for both subregions, with an average of 47.1 ± 1.8 Mg C ha^?1 in HG and 48.7 ± 3.1 Mg C ha^?1 in LG. However, 20 years after forest‐to‐pasture conversion SOC in HG decreased by 20%, whereas in LG SOC increased by 41%. This net SOC decrease in HG was due to a larger reduction in C3‐derived input and to a comparatively smaller increase in C4‐derived C input. In LG both C3‐ and C4‐derived C input increased along the chronosequence. N stocks were generally similar in both subregions and soil N stock changes during pasture establishment were correlated with SOC changes. These results emphasize the importance of management practices involving low‐grazing intensity in cattle activities to preserve SOC stocks and to reduce C emissions after land‐cover change from forest to pasture in the Colombian Amazon.     

Sustained effects of atmospheric [CO2] and nitrogen availability on forest soil CO2 efflux

Soil CO₂ efflux (F_soil) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO₂] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long‐term effects of these factors on F_soil are less clear. Expanding on previous studies at the Duke Free‐Air CO₂ Enrichment (FACE) site, we quantified the effects of elevated [CO₂] and N fertilization on F_soil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual F_soil increased under elevated [CO₂] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO₂] reduced F_soil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity, but declined after productivity saturated. Despite treatment‐induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Interannually, low soil water content decreased annual F_soil from potential values – estimated based on temperature alone assuming nonlimiting soil water content – by ca. 0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO₂]. Variability in soil N availability among plots accounted for the spatial variability in F_soil, showing a decrease of ca. 114 g C m^?2 yr^?1 per 1 g m^?2 increase in soil N availability, with consistently higher F_soil in elevated [CO₂] plots ca. 127 g C per 100 ppm [CO₂] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO₂] and N fertilization on F_soil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production.     

Short‐term CO2‐C emissions from soil prior to sugarcane (Saccharum spp.) replanting in southern Brazil

New management strategies should be identified to increase the potential of bioenergy crops to minimize climate change. This study quantified the impact of sugarcane (Saccharum spp.) harvest systems, straw and soil management on carbon dioxide (CO₂) fluxes prior to crop replanting carried out on February 2010 in southern Brazil. The soil studied was classified as Haplustult (USDA Soil Taxonomy). Three sugarcane harvest systems were considered: burned (BH) and green harvest with straw maintained on (GH SM) or removed from (GH SR) the soil surface. Our hypothesis is that intensive tillage and the management of sugarcane crop straw could lead to higher CO₂ emissions from soil. We measured CO₂ emissions in no‐till (NT) conditions and after conventional tillage (CT), and with or without dolomite and agricultural gypsum applications. Soil CO₂ emissions were measured with a Li Cor chamber (Model Li‐8100). Water content of soil and soil temperature readings were first taken 24 h after tillage, over the next 25 days after tillage with 18 measurement days. The removal of sugarcane straw from the soil surface resulted in the rapid reduction of water content of soil (6% in volume) followed by a 64% increase in soil CO₂‐C emissions, supporting our hypothesis. Additional soil CO₂‐C emissions caused by removal of crop straw were 253 kg CO₂‐C ha^?1, which is as high as CO₂‐C losses induced by tillage. Dolomite and agricultural gypsum applications did not always increase CO₂ emissions, especially when applied on soil surface with crop straw and tilled. The conversion from burned to green harvest systems can improve the soil C sequestration rate in sugarcane crops when combined with reduced tillage and straw maintenance on soil surface. The effect of straw removal and related CO₂ emission for electricity generation should be considered in further studies from sugarcane areas.     

Net soil carbon input under ambient and elevated CO2 concentrations: isotopic evidence after 4 years

Elevation of atmospheric CO₂ concentration is predicted to increase net primary production, which could lead to additional C sequestration in terrestrial ecosystems. Soil C input was determined under ambient and Free Atmospheric Carbon dioxide Enrichment (FACE) conditions for Lolium perenne L. and Trifolium repens L. grown for four years in a sandy‐loam soil. The ¹³C content of the soil organic matter C had been increased by 5‰ compared to the native soil by prior cropping to corn (Zea mays) for > 20 years. Both species received low or high amounts of N fertilizer in separate plots. The total accumulated above‐ground biomass produced by L. perenne during the 4‐year period was strongly dependent on the amount of N fertilizer applied but did not respond to increased CO₂. In contrast, the total accumulated above‐ground biomass of T. repens doubled under elevated CO₂ but remained independent of N fertilizer rate. The C:N ratio of above‐ground biomass for both species increased under elevated CO₂ whereas only the C:N ratio of L. perenne roots increased under elevated CO₂. Root biomass of L. perenne doubled under elevated CO₂ and again under high N fertilization. Total soil C was unaffected by CO₂ treatment but dependent on species. After 4 years and for both crops, the fraction of new C (F‐value) under ambient conditions was higher (P= 0.076) than under FACE conditions: 0.43 vs. 0.38. Soil under L. perenne showed an increase in total soil organic matter whereas N fertilization or elevated CO₂ had no effect on total soil organic matter content for both systems. The net amount of C sequestered in 4 years was unaffected by the CO₂ concentration (overall average of 8.5 g C kg^?1 soil). There was a significant species effect and more new C was sequestered under highly fertilized L. perenne. The amount of new C sequestered in the soil was primarily dependent on plant species and the response of root biomass to CO₂ and N fertilization. Therefore, in this FACE study net soil C sequestration was largely depended on how the species responded to N rather than to elevated CO₂.     

Investigating the biochar effects on C‐mineralization and sequestration of carbon in soil compared with conventional amendments using the stable isotope (δ13C) approach

Biomass‐derived black carbon (biochar) is considered to be an effective tool to mitigate global warming by long‐term C‐sequestration in soil and to influence C‐mineralization via priming effects. However, the underlying mechanism of biochar (BC) priming relative to conventional biowaste (BW) amendments remains uncertain. Here, we used a stable carbon isotope (δ¹³C) approach to estimate the possible biochar effects on native soil C‐mineralization compared with various BW additions and potential carbon sequestration. The results show that immediately after application, BC suppresses and then increases C‐mineralization, causing a loss of 0.14–7.17 mg‐CO₂–C g^?1‐C compared to the control (0.24–1.86 mg‐CO₂–C g^?1‐C) over 1–120 days. Negative priming was observed for BC compared to various BW amendments (?10.22 to ?23.56 mg‐CO₂–C g^?1‐soil‐C); however, it was trivially positive relative to that of the control (8.64 mg‐CO₂–C g^?1‐soil‐C). Furthermore, according to the residual carbon and δ¹³C signature of postexperimental soil carbon, BC‐C significantly increased (P < 0.05) the soil carbon stock by carbon sequestration in soil compared with various biowaste amendments. The results of cumulative CO₂–C emissions, relative priming effects, and carbon storage indicate that BC reduces C‐mineralization, resulting in greater C‐sequestration compared with other BW amendments, and the magnitude of this effect initially increases and then decreases and stabilizes over time, possibly due to the presence of recalcitrant‐C (4.92 mg‐C g^?1‐soil) in BC, the reduced microbial activity, and the sorption of labile organic carbon (OC) onto BC particles.     

Modifications of the carbon and nitrogen allocations in the plant (Triticum aestivum L.) soil system in response to increased atmospheric CO2 concentration

The aim of this work was to examine the response of wheat plants to a doubling of the atmospheric CO₂ concentration on: (1) carbon and nitrogen partitioning in the plant; (2) carbon release by the roots; and (3) the subsequent N uptake by the plants. The experiment was performed in controlled laboratory conditions by exposing fast-growing spring wheat plants, during 28 days, to a ¹⁴CO₂ concentration of 350 or 700 L L^–1 at two levels of soil nitrogen fertilization. Doubling CO₂ availability increased total plant production by 34% for both N treatment. In the N-fertilized soil, the CO₂ enrichment resulted in an increase in dry mass production of 41% in the shoots and 23% in the roots; without N fertilization this figure was 33% and 37%, respectively. In the N-fertilized soil, the CO₂ increase enhanced the total N uptake by 14% and lowered the N concentration in the shoots by 23%. The N concentration in the roots was unchanged. In the N-fertilized soil, doubling CO₂ availability increased N uptake by 32% but did not change the N concentrations, in either shoots or roots. The CO₂ enrichment increased total root-derived carbon by 12% with N fertilization, and by 24% without N fertilization. Between 85 and 90% of the total root derived-¹⁴C came from respiration, leaving only 10 to 15% in the soil as organic ¹⁴C. However, when total root-derived ¹⁴C was expressed as a function of root dry weight, these differences were only slightly significant. Thus, it appears that the enhanced carbon release from the living roots in response to increased atmospheric CO₂, is not due to a modification of the activity of the roots, but is a result of the increased size of the root system. The increase of root dry mass also resulted in a stimulation of the soil N mineralization related to the doubling atmospheric CO₂ concentration. The discussion is focused on the interactions between the carbon and nitrogen allocation, especially to the root system, and the implications for the acquisition of nutrients by plants in response to CO₂ increase.Abbreviations N soil fertilization without nitrogen - N soil fertilization with nitrogen