Search

The voluntary carbon market for agricultural soil carbon sequestration is accelerating at a rapid pace with over a dozen companies and marketplaces having recently announced carbon crediting programs. These programs aim to bring verified carbon credits into the market using published measurement, reporting, and verification protocols. Given the varied approaches to measuring and accounting for changes in soil carbon represented among these different protocols, there is significant uncertainty whether a credit generated in one market has any equivalency to a credit generated in another program. We see a critical need for scientists to play an active role in helping guide protocol development and to conduct relevant research. To that end, we identify important areas where confusion about protocols and their implementation hamper progress on this front, and highlight key areas for improved communication and transparency between market stakeholders and the research community. [ABSTRACT FROM AUTHOR]

Spectroscopy is a powerful means of increasing the availability of soil data necessary for understanding carbon cycling in a changing world. Here, we develop a calibration transfer methodology to appropriately apply an existing mid infrared (MIR) spectral library with analyte data on the distribution of soil organic carbon (SOC) into particulate (POC), mineral-associated (MAOC), and pyrogenic (PyC) forms to nearly 8000 soil samples collected in the Great Plains ecoregion of the United States. We then use this SOC fraction database in combination with a machine learning-based predictive soil mapping approach to explore the controls on the distribution of fractions through soil profiles and across the region. The relative abundance of each fraction had unique depth distribution profiles with POC fraction dropping exponentially with depth, the MAOC fraction having a broad distribution with a maxima at 35–50 cm, and the PyC fraction showed a slight subsurface maxima (10–20 cm) and then a steady decline with increasing depth. Within the Great Plains ecoregion, clay content was a strong control on the total amount and relative proportion of each fraction in both the surface and subsoil horizons. Sandy soils and soils in cool semi-arid regions contained significantly more POC relative to the MAOC and PyC fractions. Cultivated soils had significantly less SOC than grassland soils with losses following a predictable pattern: POC > MAOC ≫ PyC. This SOC fraction database and resulting maps can now form the basis for improved representation of SOC dynamics in biogeochemical models. [ABSTRACT FROM AUTHOR]

While soil organic carbon (C) is the foundation of productive and healthy ecosystems, the impact of the ecology of microorganisms on C-cycling remains unknown. We manipulated the diversity, applied here as species richness, of the microbial community present in similar soils on two contrasting land-covers—an adjacent pasture and forest—and observed the transformations of plant detritus and soil organic matter (SOM) using stable isotope (13C) tracing coupled with a novel nuclear magnetic resonance (NMR) experiment. The amount of detritus-C degraded was not affected by the microbial diversity (p > 0.05), however the fate of detritus- and SOM-C across the diversity gradient was complex and land cover-dependent. For example, in the pasture soil, higher diversity led to lower CO2 production (p = 0.001), a trend driven solely by SOM-C mineralization. There was no relationship between diversity and detritus-C mineralization or production of new mineral-associations after one year (p > 0.05). In contrast, in the forest soil higher diversity resulted in increased detritus-C (p = 0.01) and SOM-C (p = 0.0008) mineralization and decreased mineral-associated organic matter formation (p = 0.02). In both land cover types, retention efficiency—a measure that integrates both microbial physiology and the ability of the ecosystem to retain C—explained C loss and transformation trends. Overall, this demonstrates that the trajectory of C gained and lost is altered by land management-induced changes to microbial communities, soil structure, and chemical characteristics underlying SOM persistence. [ABSTRACT FROM AUTHOR]

Changes in soil carbon stocks following forest harvest can be an important component of ecosystem and landscape-scale C budgets in systems managed for bioenergy or carbon-trading markets. However, these changes are characterized less often and with less certainty than easier-to-measure aboveground stocks. We sampled soils prior to the whole-tree harvest of Watershed 5 at the Hubbard Brook Experimental Forest in 1983, and again in years 3, 8, and 15 following harvest. The repeated measures of total soil C in this stand show no net change in the O horizon over 15 years, though mixing with the mineral soil reduced observed O horizon C in disturbed areas in post-harvest years 3 and 8. Mineral soil C decreased by 15% (20 Mg ha−1) relative to pre-harvest levels by year 8, with no recovery in soil C stocks by year 15. Proportional changes in N stocks were similar. The loss of mineral soil C offset two-thirds of the C accumulation in aboveground biomass over the same 15 years, leading to near-zero net C accumulation post-harvest, after also accounting for the decomposition of slash and roots. If this result is broadly representative, and the extent of forest harvesting is expanded to meet demand for bioenergy or to manage ecosystem carbon sequestration, then it will take substantially longer than previously assumed to offset harvest- or bioenergy-related carbon dioxide emissions with carbon uptake during forest regrowth. [ABSTRACT FROM AUTHOR]

‘Blue carbon’ ecosystems—seagrasses, tidal marshes, and mangroves—serve as dense carbon sinks important for reducing atmospheric greenhouse gas concentrations, yet only recently have stock estimates emerged. We sampled 96 blue carbon ecosystems across the Victorian coastline (southeast Australia) to quantify total sediment stocks, variability across spatial scales, and estimate emissions associated with historical ecosystem loss. Mean sediment organic carbon (Corg) stock (±SE) to a depth of 30 cm was not significantly different between tidal marshes (87.1 ± 4.90 Mg Corg ha−1) and mangroves (65.6 ± 4.17 Mg Corg ha−1), but was significantly lower in seagrasses (24.3 ± 1.82 Mg Corg ha−1). Location (defined as an individual meadow, marsh, or forest) had a stronger relationship with Corg stock than catchment region, suggesting local-scale conditions drive variability of stocks more than regional-scale processes. We estimate over 2.90 million ± 199,000 Mg Corg in the top 30 cm of blue carbon sediments in Victoria (53% in tidal marshes, 36% in seagrasses, and 11% in mangroves) and sequestration rates of 22,700 ± 5510 Mg Corg year−1 (valued at over $AUD1 million ± 245,000 year−1 based on the average price of $AUD12.14 Mg CO2 eq−1 at Australian Emissions Reduction Fund auctions). We estimate ecosystem loss since European settlement may equate to emissions as high as 4.83 million ± 358,000 Mg CO2 equivalents (assuming 90% remineralization of stocks), 98% of which was associated with tidal marsh loss, and what would have been sequestering 9360 ± 2500 Mg Corg year−1. This study is among the first to present a comprehensive comparison of sediment stocks across and within coastal blue carbon ecosystems. We estimate substantial and valuable carbon stocks associated with these ecosystems that have suffered considerable losses in the past and need protection into the future to maintain their role as carbon sinks. [ABSTRACT FROM AUTHOR]

There is an increasing recognition that sorption and precipitation reactions between the dissolved phase of organic matter and reactive minerals and metals found in soils are an important carbon stabilization mechanism. We explored the relative importance of this sorption mechanism with pedological shifts in soil properties by conducting a dissolved organic matter leaching/sorption experiment using intact soil cores from a substrate/age gradient in Hawai'i. In the subsurface horizons, sorption of dissolved organic carbon was often positively correlated short-range ordered (SRO) mineral content, with sorption rates approaching 100% when SRO minerals dominated. Dissolved organic matter sorption in the presence of SRO minerals was highly selective towards aromatic compounds, consistent with prior nuclear magnetic resonance spectroscopy conducted on carbon found in SRO rich mineral soil. In subsurface horizons where SRO content was low (youngest and oldest sites in the chronosequence), sorption was also found to be high but much less selective, more reversible and more degradable indicating that different stabilization mechanisms are operative. These experimental results provide further evidence for a direct mechanism by which volcanic soils are able to store disproportionately high amount of soil organic matter via retention of aromatic acids. [ABSTRACT FROM AUTHOR]

The magnitude of the carbon flux between soil and atmosphere has prompted efforts to better understand the controls over the fate of carbon in plant litter that re-enters the atmosphere as carbon dioxide or is sequestered as soil organic carbon (SOC). It remains unresolved if the long-term fate of litter carbon is driven by biochemical properties of litter or by soil properties that reduce the ability of soil organisms to decompose litter-derived carbon. The prominent role that reactive soil minerals play in stabilising SOC have hindered investigation into the single role of litter quality on long-term SOC stability. Here we investigated the independent effects of litter quality on soil carbon stabilisation across a 460,000 year sand dune chronosequence characterised by a pronounced nutrient and litter quality gradient with minimum presence of interfering soil minerals. Using a steady state turnover model to interpret radiocarbon activity in soils collected ≈40 years apart, we show that the turnover time of SOC in the A horizon averaged 22 years (ranging from 16 to 27 years) across the chronosequence. This finding strongly contrasts other chronosequences where SOC turnover rates range from 60 to 726 years in concert with changing abundance and composition of soil minerals. Our study demonstrates that the long-term stability of SOC in surface horizons may be largely determined by interaction with soil minerals and that litter quality per se does not govern carbon stabilisation. [ABSTRACT FROM AUTHOR]

Adsorption of dissolved organic compounds onto mineral surfaces is increasingly recognized as a significant, if not dominant, carbon stabilisation mechanism in many soils. By utilising carbon-13 enriched dissolved organic carbon (DOC) source materials in a repeated leaching-sorption-incubation study, we show here that the biochemical composition of mineral-retained organic matter (OM) is similar across four different classes of clay minerals but the quantity and stability of this OM is both a function of source material and clay mineralogy. Three to eight times as much carbon was retained on a mass basis when the same amount of DOC derived from eucalyptus versus wheat litter was applied, and the retained wheat-derived OM was up to 2.4 times more degradable than that of the eucalyptus source. For both litter types, carbon retention across the clay types was not significantly different; whereas, the stability of the retained OM was different but depended on which litter extract had been applied. The wheat-derived DOC was more stable when retained by allophane and oxides than by illite and smectite. Solid-state C NMR spectroscopic results indicated that despite large compositional differences in both source litter and resultant DOC, the composition of the mineral-retained OM was similar across clay classes with lignin-derived aromatic and carboxylic compounds dominating. Differences in the amount of carbon retained were related to differences in the proportions of aromatic, phenolic and carboxylic C in the DOC produced from the two litter sources. Differences in the stability across the clay classes were correlated with the abundance of metals and short-range ordered minerals. These results suggest that whenever reactive mineral surfaces and metals are present in a soil, a similar form of relatively unaltered litter derived OM can be adsorbed but that the longer term stability of sorbed OM, and thus in situ composition, will be a function of the mineralogy (reactivity) of the specific minerals involved in the binding process. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) is a critical phase in terrestrial carbon and nutrient cycling forming the basis of many ecosystem functions, yet the primary drivers controlling its flux from organic horizons and resultant chemical composition remain only partially understood. We studied dissolved organic matter production and chemistry from organic soil horizons across a 4.1 My old well-constrained chronosequence in Hawaii. Controlled soil column irrigation and leaching experiments were conducted on field moist organic soil horizons to quantify microbial activity, DOM production and chemistry. Both microbial activity (defined as CO production per unit substrate C) and DOM production were found to be lowest in the youngest (0.3 ky) and oldest (4.1 My) sites of the chronosequence, where nutrients (N and P respectively) were most limiting. By contrast, DOM production and microbial activity was greatest at the intermediate-aged (20-350 ky) sites where nutrients were least limiting, unrelated to the mass of organic matter found in the organic horizons. While differences in production rates were found, C NMR spectroscopic results indicated that there was a convergence of chemistry from the solid to the dissolved phase at all sites. In particular, all DOM samples were found to have a high proportion of aromatic acids. With supporting data from a diverse range of ecosystems, we postulate that chemical homogenization of DOM relative to source material is a common feature of many ecosystems due to two microbially mediated processes: (1) similar extracellular enzymatic oxidation conferring solubility to a subset of degradation products; and (2) the rapid selective consumption of the more labile organic compounds in the soil solution. [ABSTRACT FROM AUTHOR]

For soil carbon to be effectively sequestered beyond a timescale of a few decades, this carbon must become incorporated into passive reservoirs or greater depths, yet the actual mechanisms by which this occurs is at best poorly known. In this study, we quantified the magnitude of dissolved organic carbon (DOC) leaching and subsequent retention in soils of a coniferous forest and a coastal prairie ecosystem. Despite small annual losses of DOC relative to respiratory losses, DOC leaching plays a significant role in transporting C from surface horizons and stabilizing it within the mineral soil. We found that DOC movement into the mineral soil constitutes 22% of the annual C inputs below 40 cm in a coniferous forest, whereas only 2% of the C inputs below 20 cm in a prairie soil could be accounted for by this process. In line with these C input estimates, we calculated advective transport velocities of 1.05 and 0.45 mm year−1 for the forested and prairie sites, respectively. Radiocarbon measurements of field-collected DOC interpreted with a basic transport-turnover model indicated that DOC which was transported and subsequently absorbed had a mean residence time of 90–150 years. Given these residence times, the process of DOC movement and retention is responsible for 20% of the total mineral soil C stock to 1 m in the forest soil and 9% in the prairie soil. These results provide quantitative data confirming differences in C cycles in forests and grasslands, and suggest the need for incorporating a better mechanistic understanding of soil C transport, storage and turnover processes into both local and regional C cycle models. [ABSTRACT FROM AUTHOR]

For soil carbon to be effectively sequestered beyond a timescale of a few decades, this carbon must become incorporated into passive reservoirs or greater depths, yet the actual mechanisms by which this occurs is at best poorly known. In this study, we quantified the magnitude of dissolved organic carbon (DOC) leaching and subsequent retention in soils of a coniferous forest and a coastal prairie ecosystem. Despite small annual losses of DOC relative to respiratory losses, DOC leaching plays a significant role in transporting C from surface horizons and stabilizing it within the mineral soil. We found that DOC movement into the mineral soil constitutes 22% of the annual C inputs below 40 cm in a coniferous forest, whereas only 2% of the C inputs below 20 cm in a prairie soil could be accounted for by this process. In line with these C input estimates, we calculated advective transport velocities of 1.05 and 0.45 mm year−1 for the forested and prairie sites, respectively. Radiocarbon measurements of field-collected DOC interpreted with a basic transport-turnover model indicated that DOC which was transported and subsequently absorbed had a mean residence time of 90–150 years. Given these residence times, the process of DOC movement and retention is responsible for 20% of the total mineral soil C stock to 1 m in the forest soil and 9% in the prairie soil. These results provide quantitative data confirming differences in C cycles in forests and grasslands, and suggest the need for incorporating a better mechanistic understanding of soil C transport, storage and turnover processes into both local and regional C cycle models. [ABSTRACT FROM AUTHOR]

In this study, we examined changes in isotopic (13C and 14C) and spectroscopic (UV and 13C NMR) properties of dissolved organic carbon (DOC) in relation to soil organic matter (SOM) to elucidate the sources and sinks of DOC as water percolates through the soils of two contrasting upland coastal California ecosystems—a redwood forest and a coastal prairie. Despite differences in the distribution of C stocks and litter chemistry at these two sites, we found similar shifts in DOC chemistry with soil depth. DOC concentrations dropped rapidly with increasing depth, with an accompanying decrease in the C:N ratio, an increase in the δ13C value and an decrease in specific UV adsorption. In the grassland soil, Δ14C values declined from current atmospheric values (+70‰) in the surface horizon to −75‰ at 100 cm. In the redwood soil, the Δ14C value of 111‰ in O horizon leachates was indicative of OM with a residence time of 8–10 yrs, with a decrease in Δ14C values to −80‰ at 100 cm. Solid-state CP/MAS 13C NMR spectra were generally most similar to highly humified OM, with a general decrease in the relative abundance of aromatic compounds and an increase in the alkyl C/O-alkyl C ratio with increasing depth. All of these trends are consistent with the shifts in SOM properties with increasing depth, which are interpreted to mean a shift from fresh plant material to older, highly altered OM. In this Mediterranean climate, we found distinct seasonal shifts in the quantity and composition of DOC found in soil solution during the winter rainy period that was also consistent with a shift from recent labile substrates to older, highly altered OM. These results fit in with a growing body of literature suggesting that the source of much of the DOC within mineral soils is the local soil OM, and the 14C data, in particular, indicate that DOC at depth is not simply the fraction of surficial leachates that have not been adsorbed or decomposed. Rather, exchange reactions with a portion of the more stabilized SOM pool exert the strongest control on both the concentration and composition of DOC found in these soils. [ABSTRACT FROM AUTHOR]