Missing Sink Research Articles

Global mass of buoyant marine plastics dominated by large long-lived debris

The fate of plastics that enter the ocean is a longstanding puzzle. Recent estimates of the oceanic input of plastic are one to two orders of magnitude larger than the amount measured floating at the surface. This discrepancy could be due to overestimation of input estimates, processes removing plastic from the surface ocean or fragmentation and degradation. Here we present a 3D global marine mass budget of buoyant plastics that resolves this discrepancy. We assimilate observational data from different marine reservoirs, including coastlines, the ocean surface, and the deep ocean, into a numerical model, considering particle sizes of 0.1–1,600.0 mm. We find that larger plastics (>25 mm) contribute to more than 95% of the initially buoyant marine plastic mass: 3,100 out of 3,200 kilotonnes for the year 2020. Our model estimates an ocean plastic input of about 500 kilotonnes per year, less than previous estimates. Together, our estimated total amount and annual input of buoyant marine plastic litter suggest there is no missing sink of marine plastic pollution. The results support higher residence times of plastics in the marine environment compared with previous model studies, in line with observational evidence. Long-lived plastic pollution in the world’s oceans, which our model suggests is continuing to increase, could negatively impact ecosystems without countermeasures and prevention strategies.

Experimental chemical budgets of OH, HO2, and RO2 radicals in rural air in western Germany during the JULIAC campaign 2019

Abstract. Photochemical processes in ambient air were studied using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient air was continuously drawn into the chamber through a 50 m high inlet line and passed through the chamber for 1 month in each season throughout 2019. The residence time of the air inside the chamber was about 1 h. As the research center is surrounded by a mixed deciduous forest and is located close to the city Jülich, the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements of hydroxyl (OH), hydroperoxyl (HO2), and organic peroxy (RO2) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (kOH, the inverse of the OH lifetime) and a comprehensive set of trace gas concentrations and aerosol properties allowed for the investigation of the seasonal and diurnal variation of radical production and destruction pathways. In spring and summer periods, median OH concentrations reached 6 × 106 cm−3 at noon, and median concentrations of both HO2 and RO2 radicals were 3 × 108 cm−3. The measured OH reactivity was between 4 and 18 s−1 in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (Ox= NO2 + O3) determined from NO2 and O3 concentration measurements. The chemical budgets of radicals were analyzed for the spring and summer seasons, when peroxy radical concentrations were above the detection limit. For most conditions, the concentrations of radicals were mainly sustained by the regeneration of OH via reactions of HO2 and RO2 radicals with nitric oxide (NO). The median diurnal profiles of the total radical production and destruction rates showed maxima between 3 and 6 ppbv h−1 for OH, HO2, and RO2. Total ROX (OH, HO2, and RO2) initiation and termination rates were below 3 ppbv h−1. The highest OH radical turnover rate of 13 ppbv h−1 was observed during a high-temperature (max. 40 ∘C) period in August. In this period, the highest HO2, RO2, and ROX turnover rates were around 11, 10, and 4 ppbv h−1, respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO2 production and destruction rates were balanced, but unexplained RO2 and ROX production reactions with median rates of 2 and 0.4 ppbv h−1, respectively, were required to balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy radical reaction rates with NO were highly uncertain due to the low peroxy radical concentrations close to the limit of NO interferences in the HO2 and RO2 measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO2 were found with maximum rates of 3.0 and 2.0 ppbv h−1, respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO2 to OH conversion reaction (up to 2 ppbv h−1) and a missing primary radical source (0.5–1.4 ppbv h−1). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO2 isomerization reactions from isoprene, HO2 uptake on aerosol, and RO2 production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study.

Limitations in representation of physical processes prevent successful simulation of PM<sub>2.5</sub> during KORUS-AQ

Abstract. High levels of fine particulate matter (PM2.5) pollution in East Asia often exceed local air quality standards. Observations from the Korea–United States Air Quality (KORUS-AQ) field campaign in May and June 2016 showed that development of extreme pollution (haze) occurred through a combination of long-range transport and favorable meteorological conditions that enhanced local production of PM2.5. Atmospheric models often have difficulty simulating PM2.5 chemical composition during haze, which is of concern for the development of successful control measures. We use observations from KORUS-AQ to examine the ability of the GEOS-Chem chemical transport model to simulate PM2.5 composition throughout the campaign and identify the mechanisms driving the pollution event. At the surface, the model underestimates sulfate by −64 % but overestimates nitrate by +36 %. The largest underestimate in sulfate occurs during the pollution event, for which models typically struggle to generate elevated sulfate concentrations due to missing heterogeneous chemistry in aerosol liquid water in the polluted boundary layer. Hourly surface observations show that the model nitrate bias is driven by an overestimation of the nighttime peak. In the model, nitrate formation is limited by the supply of nitric acid, which is biased by +100 % against aircraft observations. We hypothesize that this is due to a large missing sink, which we implement here as a factor of 5 increase in dry deposition. We show that the resulting increased deposition velocity is consistent with observations of total nitrate as a function of photochemical age. The model does not account for factors such as the urban heat island effect or the heterogeneity of the built-up urban landscape, resulting in insufficient model turbulence and surface area over the study area that likely results in insufficient dry deposition. Other species such as NH3 could be similarly affected but were not measured during the campaign. Nighttime production of nitrate is driven by NO2 hydrolysis in the model, while observations show that unexpectedly elevated nighttime ozone (not present in the model) should result in N2O5 hydrolysis as the primary pathway. The model is unable to represent nighttime ozone due to an overly rapid collapse of the afternoon mixed layer and excessive titration by NO. We attribute this to missing nighttime heating driving deeper nocturnal mixing that would be expected to occur in a city like Seoul. This urban heating is not considered in air quality models run at large enough scales to treat both local chemistry and long-range transport. Key model failures in simulating nitrate, mainly overestimated daytime nitric acid, incorrect representation of nighttime chemistry, and an overly shallow and insufficiently turbulent nighttime mixed layer, exacerbate the model's inability to simulate the buildup of PM2.5 during haze pollution. To address the underestimate in sulfate most evident during the haze event, heterogeneous aerosol uptake of SO2 is added to the model, which previously only considered aqueous production of sulfate from SO2 in cloud water. Implementing a simple parameterization of this chemistry improves the model abundance of sulfate but degrades the SO2 simulation, implying that emissions are underestimated. We find that improving model simulations of sulfate has direct relevance to determining local vs. transboundary contributions to PM2.5. During the haze pollution event, the inclusion of heterogeneous aerosol uptake of SO2 decreases the fraction of PM2.5 attributable to long-range transport from 66 % to 54 %. Locally produced sulfate increased from 1 % to 25 % of locally produced PM2.5, implying that local emissions controls could have a larger effect than previously thought. However, this additional uptake of SO2 is coupled to the model nitrate prediction, which affects the aerosol liquid water abundance and chemistry driving sulfate–nitrate–ammonium partitioning. An additional simulation of the haze pollution with heterogeneous uptake of SO2 to aerosol and simple improvements to the model nitrate simulation results in 30 % less sulfate due to 40 % less nitrate and aerosol water, and this results in an underestimate of sulfate during the haze event. Future studies need to better consider the impact of model physical processes such as dry deposition and nighttime boundary layer mixing on the simulation of nitrate and the effect of improved nitrate simulations on the overall simulation of secondary inorganic aerosol (sulfate + nitrate + ammonium) in East Asia. Foreign emissions are rapidly changing, increasing the need to understand the impact of local emissions on PM2.5 in South Korea to ensure continued air quality improvements.

Estuaries as Filters for Riverine Microplastics: Simulations in a Large, Coastal-Plain Estuary

Public awareness of microplastics and their widespread presence throughout most bodies of water are increasingly documented. The accumulation of microplastics in the ocean, however, appears to be far less than their riverine inputs, suggesting that there is a “missing sink” of plastics in the ocean. Estuaries have long been recognized as filters for riverine material in marine biogeochemical budgets. Here we use a model of estuarine microplastic transport to test the hypothesis that the Chesapeake Bay, a large coastal-plain estuary in eastern North America, is a potentially large filter, or “sink,” of riverine microplastics. The 1-year composite simulation, which tracks an equal number of buoyant and sinking 5-mm diameter particles, shows that 94% of riverine microplastics are beached, with only 5% exported from the Bay, and 1% remaining in the water column. We evaluate the robustness of this finding by conducting additional simulations in a tributary of the Bay for different years, particle densities, particle sizes, turbulent dissipation rates, and shoreline characteristics. The resulting microplastic transport and fate were sensitive to interannual variability over a decadal (2010–2019) analysis, with greater export out of the Bay during high streamflow years. Particle size was found to be unimportant while particle density – specifically if a particle was buoyant or not – was found to significantly influence overall fate and mean duration in the water column. Positively buoyant microplastics are more mobile due to being in the seaward branch of the residual estuarine circulation while negatively buoyant microplastics are transported a lesser distance due to being in the landward branch, and therefore tend to deposit on coastlines close to their river sources, which may help guide sampling campaigns. Half of all riverine microplastics that beach do so within 7–13 days, while those that leave the bay do so within 26 days. Despite microplastic distributions being sensitive to some modeling choices (e.g., particle density and shoreline hardening), in all scenarios most of riverine plastics do not make it to the ocean, suggesting that estuaries may serve as a filter for riverine microplastics.

No missing sink

Plastic Pollution Estimates of the flux of microplastics from rivers, in the context of the mass of plastic that has been observed in the ocean, have made it appear that a large, unidentified sink of plastics must exist there. Weiss et al. show that there may not be a missing sink after all. By reformulating how mass fluxes are calculated from observations of particle numbers, they demonstrate that those mass fluxes were overestimated by two to three orders of magnitude. This explains why the residence time of plastics in the ocean seemed so puzzlingly short and implies that ocean plastics may persist and degrade over longer periods than previously thought. Science , abe0290, this issue p. [107][1] [1]: /lookup/doi/10.1126/science.abe0290

The missing ocean plastic sink: Gone with the rivers.

Plastic floating at the ocean surface, estimated at tens to hundreds of thousands of metric tons, represents only a small fraction of the estimated several million metric tons annually discharged by rivers. Such an imbalance promoted the search for a missing plastic sink that could explain the rapid removal of river-sourced plastics from the ocean surface. On the basis of an in-depth statistical reanalysis of updated data on microplastics-a size fraction for which both ocean and river sampling rely on equal techniques-we demonstrate that current river flux assessments are overestimated by two to three orders of magnitude. Accordingly, the average residence time of microplastics at the ocean surface rises from a few days to several years, strongly reducing the theoretical need for a missing sink.

Joint CO2 Mole Fraction and Flux Analysis Confirms Missing Processes in CASA Terrestrial Carbon Uptake Over North America

AbstractTerrestrial biosphere models (TBMs) play a key role in the detection and attribution of carbon cycle processes at local to global scales and in projections of the coupled carbon‐climate system. TBM evaluation commonly involves direct comparison to eddy‐covariance flux measurements. We use atmospheric CO2 mole fraction ([CO2]) measured in situ from aircraft and tower, in addition to flux‐measurements from summer 2016 to evaluate the Carnegie‐Ames‐Stanford‐Approach (CASA) TBM. WRF‐Chem is used to simulate [CO2] using biogenic CO2 fluxes from a CASA parameter‐based ensemble and CarbonTracker version 2017 (CT2017) in addition to transport and CO2 boundary condition ensembles. The resulting “super ensemble” of modeled [CO2] demonstrates that the biosphere introduces the majority of uncertainty to the simulations. Both aircraft and tower [CO2] data show that the CASA ensemble net ecosystem exchange (NEE) of CO2 is biased high (NEE too positive) and identify the maximum light use efficiency Emax a key parameter that drives the spread of the CASA ensemble in summer 2016. These findings are verified with flux‐measurements. The direct comparison of the CASA flux ensemble with flux‐measurements confirms missing sink processes in CASA. Separating the daytime and nighttime flux, we discover that the underestimated net uptake results from missing sink processes that result in overestimation of respiration. NEE biases are smaller in the CT2017 posterior biogenic fluxes, which assimilate observed [CO2]. Flux tower analyses reveal an unrealistic overestimation of nighttime respiration in CT2017 which we attribute to limited flexibility in the inversion strategy.

Variation characteristics and influencing mechanism of CO2 flux from lakes in the Badain Jaran Desert: A case study of Yindeer Lake

Carbon sequestration levels in lakes are often underestimated despite being vital for the comprehensive analysis of CO2 budget and significantly influencing inland lakesin arid areas. In this study, the eddy covariance (EC) technique was used to measure CO2 flux for six consecutive years during 2012–2017 in a small salt-lake (Yindeer Lake) replenishedby groundwater in southeastern Badain Jaran Desert, China. The results indicate that: (1) Yindeer Lake was a net sink of CO2, which led to the estimate that the lakes in the Badain Jaran Desert can absorb approximately 1.91 × 103 t of CO2 per year andhelp slow down the rate of global warming. (2) The effects of meteorological factors on the CO2 flux of the lake vary with timescales. (3) Inorganic processes, in which pH is the main influence mechanism, control CO2 absorption and releases in Yindeer Lake. Thus, the study results provide a new guide for solving the “missing sink” problem.

High concentrations of plastic hidden beneath the surface of the Atlantic Ocean

Concern over plastic pollution of the marine environment is severe. The mass-imbalance between the plastic litter supplied to and observed in the ocean currently suggests a missing sink. However, here we show that the ocean interior conceals high loads of small-sized plastic debris which can balance and even exceed the estimated plastic inputs into the ocean since 1950. The combined mass of just the three most-littered plastics (polyethylene, polypropylene, and polystyrene) of 32–651 µm size-class suspended in the top 200 m of the Atlantic Ocean is 11.6–21.1 Million Tonnes. Considering that plastics of other sizes and polymer types will be found in the deeper ocean and in the sediments, our results indicate that both inputs and stocks of ocean plastics are much higher than determined previously. It is thus critical to assess these terms across all size categories and polymer groups to determine the fate and danger of plastic contamination.

Early diagenesis and accumulation of redox-sensitive elements in East Siberian Arctic Shelves

The Arctic shelf makes up half of the area within the Arctic Ocean. It plays an important role in the global carbon cycle, whilst its part in the recycling process of redox-sensitive elements (RSEs) is still not clear. This study has selected nine sediment cores, 30 to 40 cm in length, covering a wide area over the East Siberian Arctic Shelves (ESAS) and analyzed a suite of RSEs (S, Mn, Mo, U, V, Cd) in the sediments, along with Al, Fe, total organic carbon (TOC), grain size, and sedimentation rate. The redox environment in surface sediments was clearly divided based on the S and Mn contents of the cores, ranging from being more oxidized on the western East Siberian Sea shelf, to more reduced on the Chukchi Sea shelf. TOC also showed an increasing trend eastward in the study region. Statistically significant correlations were found between Cd and S, V and Fe, Mo and Mn, and U and TOC, implying their close relationships during early diagenetic processes at the sediment-water interface. Across the study area, authigenic accumulation occurred for all RSEs, except Mn. The estimated authigenic accumulation fluxes for Mo, U, V, and Cd over the entire ESAS are 0.21 ± 0.09, 0.086 ± 0.043, 11.0 ± 9.2 and 0.019 ± 0.014 × 108 mol y−1, respectively, which are comparable to values reported in suboxic marginal sediments for Mo, U, and Cd, but exceptionally high for V. Although the sampling resolution was coarse and there are uncertainties in the sedimentation rates and detrital background values of V, we propose the authigenic accumulation flux for V on the broad and shallow ESAS may be a significant but missing sink term for the global V cycle.

The missing sink - quantification, categorisation and sourcing of beached macro-debris in the Scottish Orkney Islands

Around the coastline of the UK, macro-debris has been observed in average densities of over 700 items per metre. Systematic beach-cleans were conducted at 35 sites around the Scottish Orkney Islands, in order to quantify and categorise the level of marine debris found there. Litter was collected from 100 m transects and categorised by its material, broad source (terrestrial or marine) and potential sector source. Variation between sites, and the relative contribution of pre-determined environmental variables in influencing said variation, were analysed using the “capscale” function for a canonical analysis of principle coordinates (CAP). 513 items/m were observed, (77% plastic), with “String/cord (<1cm diameter)” being the most abundant and widely distributed litter type. 47% of macro-debris was attributed to the fishing sector and < 10% to leisure, living and tourism-associated activities. Conversely, the unique regional hydrodynamics must be examined further, before the source of any given item can be categorically assigned.

Oxidation of Isoprene by Neutral Iron Oxide Nanoclusters in the Gas Phase

Understanding the heterogeneous oxidation of isoprene is crucial to discover the missing sink of the atmospheric isoprene. Iron oxide nanoparticles dominate the most abundant type of anthropogenic ...

Inorganic carbon sequestration and its mechanism of coastal saline-alkali wetlands in Jiaozhou Bay, China

The problem of “missing sink” in the carbon cycle has not been properly explained. To reveal the effects of coastal saline-alkali wetlands on global inorganic carbon sequestration, soil samples with different salinity and alkalinity were collected in Jiaozhou Bay coastal wetlands, and the main work were: split the soil CO2 flux through inactivation treatment; simulate the CO2 absorption process of saline alkali soils; and analyze the mechanism of the above process through stable isotopic labeling. The inorganic carbon sequestration process is common in coastal saline-alkali soil. The increasing salinity and alkalinity of soil can improve its CO2 uptake capacity, and the soil alkalinity showed greater effects on the capacity than soil salinity. The leaching/absorption of atmosphere CO2 and the dissolution/precipitation of carbonate system are confirmed as the two mechanisms of inorganic carbon sequestration. The former mechanism dominates the sequestration process in coastal wetlands. We conclude that the abundant water sources and the high alkalinity of soils contribute to the advantages of carbon sequestration processes in coastal saline-alkali wetlands.

Atmospheric Acetaldehyde: Importance of Air-Sea Exchange and a Missing Source in the Remote Troposphere.

We report airborne measurements of acetaldehyde (CH3CHO) during the first and second deployments of the National Aeronautics and Space Administration (NASA) Atmospheric Tomography Mission (ATom). The budget of CH3CHO is examined using the Community Atmospheric Model with chemistry (CAM-chem), with a newly-developed online air-sea exchange module. The upper limit of the global ocean net emission of CH3CHO is estimated to be 34 Tg a-1 (42 Tg a-1 if considering bubble-mediated transfer), and the ocean impacts on tropospheric CH3CHO are mostly confined to the marine boundary layer. Our analysis suggests that there is an unaccounted CH3CHO source in the remote troposphere and that organic aerosols can only provide a fraction of this missing source. We propose that peroxyacetic acid (PAA) is an ideal indicator of the rapid CH3CHO production in the remote troposphere. The higher-than-expected CH3CHO measurements represent a missing sink of hydroxyl radicals (and halogen radical) in current chemistry-climate models.

Nutrient retention during ecosystem succession: a revised conceptual model

We propose an important revision to a previously published conceptual model of nutrient retention during terrestrial ecosystem succession, which predicted that ecosystem losses of limiting nutrients such as nitrogen (N) should increase as rates of biomass accumulation slow during late stages of succession. This revision explicitly recognizes that mineral soil horizons (the layers of subsoil beneath the organic‐rich surface layers) can behave as an “N bank”, serving as a source of N for growing forests early in secondary succession and as a sink for N later in succession as plant biomass accumulation slows. If this soil N sink were present, mature forests would continue to retain N for decades following the cessation of biomass accumulation. This conceptual model is consistent with long‐term data collected from the Hubbard Brook Experimental Forest in the US state of New Hampshire, where existing N budgets that exclude the mineral soil horizon indicate both a missing source early in succession and a missing sink as the forest has matured.

Spatiotemporal distribution and national measurement of the global carbonate carbon sink

The magnitudes, spatial distributions and contributions to global carbon budget of the global carbonate carbon sink (CCS) still remain uncertain, allowing the problem of national measurement of CCS remain unresolved which will directly influence the fairness of global carbon markets and emission trading. Here, based on high spatiotemporal resolution ecological, meteorological raster data and chemical field monitoring data, combining highly reliable machine learning algorithm with the thermodynamic dissolution equilibrium model, we estimated the new CCS of 0.89 ± 0.23 petagrams of carbon per year (Pg C yr−1), amounting to 74.50% of global net forest sink and accounting for 28.75% of terrestrial sinks or 46.81% of the missing sink. Our measurement for 142 nations of CCS showed that Russia, Canada, China and the USA contribute over half of the global CCS. We also presented the first global fluxes maps of the CCS with spatial resolution of 0.05°, exhibiting two peaks in equatorial regions (10°S to 10°N) and low latitudes (10°N to 35°N) in Northern Hemisphere. By contrast, there are no peaks in Southern Hemisphere. The greatest average carbon sink flux (CCSF), i.e., 2.12 tC ha−1 yr−1, for 2000 to 2014 was contributed by tropical rainforest climate near the equator, and the smallest average CCSF was presented in tropical arid zones, showing a magnitude of 0.26 tC ha−1 yr−1. This research estimated the magnitudes, spatial distributions, variations and contributions to the global carbon budget of the CCS in a higher spatiotemporal representativeness and expandability way, which, via multiple mechanisms, introduced an important sink in the terrestrial carbon sink system and the global missing sink and that can help us further reveal and support our understanding of global rock weathering carbon sequestration, terrestrial carbon sink system and global carbon cycle dynamics which make our understanding of global change more comprehensive.

Volatile organic compounds (VOCs) in photochemically aged air from the eastern and western Mediterranean

Abstract. During the summertime CYPHEX campaign (CYprus PHotochemical EXperiment 2014) in the eastern Mediterranean, multiple volatile organic compounds (VOCs) were measured from a 650 m hilltop site in western Cyprus (34° 57′ N/32° 23′ E). Periodic shifts in the northerly Etesian winds resulted in the site being alternately impacted by photochemically processed emissions from western (Spain, France, Italy) and eastern (Turkey, Greece) Europe. Furthermore, the site was situated within the residual layer/free troposphere during some nights which were characterized by high ozone and low relative humidity levels. In this study we examine the temporal variation of VOCs at the site. The sparse Mediterranean scrub vegetation generated diel cycles in the reactive biogenic hydrocarbon isoprene, from very low values at night to a diurnal median level of 80–100 pptv. In contrast, the oxygenated volatile organic compounds (OVOCs) methanol and acetone exhibited weak diel cycles and were approximately an order of magnitude higher in mixing ratio (ca. 2.5–3 ppbv median level by day, range: ca. 1–8 ppbv) than the locally emitted isoprene and aromatic compounds such as benzene and toluene. Acetic acid was present at mixing ratios between 0.05 and 4 ppbv with a median level of ca. 1.2 ppbv during the daytime. When data points directly affected by the residual layer/free troposphere were excluded, the acid followed a pronounced diel cycle, which was influenced by various local effects including photochemical production and loss, direct emission, dry deposition and scavenging from advecting air in fog banks. The Lagrangian model FLEXPART was used to determine transport patterns and photochemical processing times (between 12 h and several days) of air masses originating from eastern and western Europe. Ozone and many OVOC levels were ∼ 20 and ∼ 30–60 % higher, respectively, in air arriving from the east. Using the FLEXPART calculated transport time, the contribution of photochemical processing, sea surface contact and dilution was estimated. Methanol and acetone decreased with residence time in the marine boundary layer (MBL) with loss rate constants of 0.74 and 0.53 day−1 from eastern Europe and 0.70 and 0.34 day−1 from western Europe, respectively. Simulations using the EMAC model underestimate these loss rates. The missing sink in the calculation is most probably an oceanic uptake enhanced by microbial consumption of methanol and acetone, although the temporal and spatial variability in the source strength on the continents might play a role as well. Correlations between acetone and methanol were weaker in western air masses (r2 = 0.68), but were stronger in air masses measured after the shorter transport time from the east (r2 = 0.73).

Pyrogenic organic matter produced during wildfires can act as a carbon sink – a reply to Billings &amp; Schlesinger (2015)

In response to our paper ‘Pyrogenic organic matter production from wildfires: a missing sink in the global carbon cycle’ (Santín et al., 2015), Billings & Schlesinger (2015) argue that pyrogenic organic matter (PyOM) formation is not a missing C sink. Firstly, they state that ‘formation of PyOM does not remove CO2 from the atmosphere’. Indeed, PyOM formation does not directly take CO2 from the atmosphere, but, as Billings & Schlesinger (2015) explain, fire transforms one type of fixed C (biomass) into another (PyOM). This transformation is critical here, as the charred C has lower degradation rates than the ‘uncharred’ biomass C (e.g. Maestrini et al., 2014). The pyrogenic transformations that occur during charring of biomass do not constitute an immediate removal of C from the atmosphere, but they do delay the emission of that C to the atmosphere (compared to the uncharred precursors of PyOM). Taking a wider temporal perspective, this delay leads to a net additional transfer of C from atmosphere to land compared to a situation without PyOM production. This mechanism is defined as ‘C sequestration’ (Powlson et al., 2011) and is the same concept on which the production of biochar (i.e. anthropogenic PyOM) and its application to soils for offsetting C emissions is based on (e.g. Woolf et al., 2010). C sinks are thus not only processes withdrawing directly CO2 from the atmosphere as argued by Billings & Schlesinger (e.g. increases of net photosynthesis or C capture & storage; Scott et al., 2013), but also those mechanisms decreasing the net flux of C between land and atmosphere (e.g. soil C erosion and burial under certain conditions; Billings et al., 2010), with PyOM formation being one of them (Lal, 2008). Secondly, Billings & Schlesinger (2015) argue that, to resolve the C budget (i.e. current discrepancies between the magnitude of accounted sources and sinks of atmospheric CO2), we must identify processes that have increased their rate of CO2 capture in the last few decades. The identification of new net land C sinks would indeed explain the current discrepancy observed. However, addressing errors and omissions in the different terms of the global C balance can also help to reduce this discrepancy (Houghton, 2012). PyOM production is currently not considered in the global C cycle and, therefore, our argument that it is a missing component (Santín et al., 2015) is valid. Inclusion of PyOM will not close the C budget by itself, but it will contribute to its further understanding. Elucidating more fully the global C balance is of course challenging, but very important due to its direct influence on the global climate. Efforts from the scientific community need to be directed not only towards improving C accounting itself, but also to its correct use in climate policy (e.g. Searchinger et al., 2009). Scientific discussion is essential to achieve these aims, and we welcome the opportunity provided here to contribute to this process.

Pyrogenic organic matter production from wildfires: a missing sink in the global carbon cycle.

Wildfires release substantial quantities of carbon (C) into the atmosphere but they also convert part of the burnt biomass into pyrogenic organic matter (PyOM). This is richer in C and, overall, more resistant to environmental degradation than the original biomass, and, therefore, PyOM production is an efficient mechanism for C sequestration. The magnitude of this C sink, however, remains poorly quantified, and current production estimates, which suggest that ∽1-5% of the C affected by fire is converted to PyOM, are based on incomplete inventories. Here, we quantify, for the first time, the complete range of PyOM components found in-situ immediately after a typical boreal forest fire. We utilized an experimental high-intensity crown fire in a jack pine forest (Pinus banksiana) and carried out a detailed pre- and postfire inventory and quantification of all fuel components, and the PyOM (i.e., all visually charred, blackened materials) produced in each of them. Our results show that, overall, 27.6% of the C affected by fire was retained in PyOM (4.8 ± 0.8 t C ha−1), rather than emitted to the atmosphere (12.6 ± 4.5 t C ha−1). The conversion rates varied substantially between fuel components. For down wood and bark, over half of the C affected was converted to PyOM, whereas for forest floor it was only one quarter, and less than a tenth for needles. If the overall conversion rate found here were applicable to boreal wildfire in general, it would translate into a PyOM production of ∽100 Tg C yr−1 by wildfire in the global boreal regions, more than five times the amount estimated previously. Our findings suggest that PyOM production from boreal wildfires, and potentially also from other fire-prone ecosystems, may have been underestimated and that its quantitative importance as a C sink warrants its inclusion in the global C budget estimates.

Significant Role for Microbial Autotrophy in the Sequestration of Soil Carbon

Soils were incubated for 80 days in a continuously labeled (14)CO(2) atmosphere to measure the amount of labeled C incorporated into the microbial biomass. Microbial assimilation of (14)C differed between soils and accounted for 0.12% to 0.59% of soil organic carbon (SOC). Assuming a terrestrial area of 1.4 × 10(8) km(2), this represents a potential global sequestration of 0.6 to 4.9 Pg C year(-1). Estimated global C sequestration rates suggest a "missing sink" for carbon of between 2 and 3 Pg C year(-1). To determine whether (14)CO(2) incorporation was mediated by autotrophic microorganisms, the diversity and abundance of CO(2)-fixing bacteria and algae were investigated using clone library sequencing, terminal restriction fragment length polymorphism (T-RFLP), and quantitative PCR (qPCR) of the ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) gene (cbbL). Phylogenetic analysis showed that the dominant cbbL-containing bacteria were Azospirillum lipoferum, Rhodopseudomonas palustris, Bradyrhizobium japonicum, Ralstonia eutropha, and cbbL-containing chromophytic algae of the genera Xanthophyta and Bacillariophyta. Multivariate analyses of T-RFLP profiles revealed significant differences in cbbL-containing microbial communities between soils. Differences in cbbL gene diversity were shown to be correlated with differences in SOC content. Bacterial and algal cbbL gene abundances were between 10(6) and 10(8) and 10(3) to 10(5) copies g(-1) soil, respectively. Bacterial cbbL abundance was shown to be positively correlated with RubisCO activity (r = 0.853; P < 0.05), and both cbbL abundance and RubisCO activity were significantly related to the synthesis rates of [(14)C]SOC (r = 0.967 and 0.946, respectively; P < 0.01). These data offer new insights into the importance of microbial autotrophy in terrestrial C cycling.