Factors affecting ammonium capture by some Victorian lignites.

HighlightsVictorian lignites were assessed for their NH4+ retention capacity using adsorption isotherms and 15N tracing.NH4+ adsorption capacity of lignites increased (up to 3-fold) with pH, especially from pH 5 to 7.Biological immobilisation did not play a substantial role in the NH4+ retention capacity of the lignites.pH-dependent NH4+ adsorption was the dominant means by which lignite retained NH4+.

Greenhouse Gas Inventory Model for Biochar Additions to Soil.

Stabilizing the global climate within safe bounds will require greenhouse gas (GHG) emissions to reach net zero within a few decades. Achieving this is expected to require removal of CO2 from the atmosphere to offset some hard-to-eliminate emissions. There is, therefore, a clear need for GHG accounting protocols that quantify the mitigation impact of CO2 removal practices, such as biochar sequestration, that have the potential to be deployed at scale. Here, we have developed a GHG accounting methodology for biochar application to mineral soils using simple parameterizations and readily accessible activity data that can be applied at a range of scales including farm, supply chain, national, or global. The method is grounded in a comprehensive analysis of current empirical data, making it a robust method that can be used for many applications including national inventories and voluntary and compliance carbon markets, among others. We show that the carbon content of biochar varies with feedstock and production conditions from as low as 7% (gasification of biosolids) to 79% (pyrolysis of wood at above 600 °C). Of this initial carbon, 63-82% will remain unmineralized in soil after 100 years at the global mean annual cropland-temperature of 14.9 °C. With this method, researchers and managers can address the long-term sequestration of C through biochar that is blended with soils through assessments such as GHG inventories and life cycle analyses.

Divergent responses of soil organic carbon accumulation to 14 years of nitrogen addition in two typical subtropical forests.

Developing an understanding of the response of soil organic carbon (SOC) to N addition is critical to quantify and predict the terrestrial carbon uptake under increasing N deposition in the future. However, results from field studies on the response of SOC content and composition to N addition are highly variable across different ecosystems. The interpretation of SOC responses to N addition are often complicated by the differences in climate, soil substrate and other factors. To address this question, we measured SOC and its components in adjacent broadleaved and coniferous subtropical forests after 14 years of N addition. SOC in the top 50 cm increased by 2.1 kg m-2, 1.8 kg m-2 and 1.2 kg m-2 for low, medium and high rates of N addition in the broadleaved forest, but did not change significantly in the coniferous forest. Increased SOC in the broadleaved forest was contributed by the significant increases in particulate organic carbon (POC), humus organic carbon (HOC) in the 0-10 cm and 30-50 cm soil layers and resistant organic carbon (ROC) in the 0-10 cm soil layer. 13C nuclear magnetic resonance (NMR) spectra of coarse SOC revealed a decrease in easily decomposed carbon (C) and a shift in recalcitrant C. The increased SOC accumulation in the broadleaved forest was largely driven by altered rates of organic matter decomposition, rather than C inputs to soil. Land-history and low nutrient availability may have contributed to the lack of significant impact of N addition on SOC in the coniferous forest. Our results suggested the different controls of SOC accumulation and less sensitivity of SOC chemical composition at the molecular level to N addition in the two subtropical forest soils.

Impacts of land reclamation on tidal marsh 'blue carbon' stocks.

Tidal marsh ecosystems are among earth's most efficient natural organic carbon (C) sinks and provide myriad ecosystem services. However, approximately half have been 'reclaimed' - i.e. converted to other land uses - potentially turning them into sources of greenhouse gas emissions. In this study, we applied C stock measurements and paleoanalytical techniques to sediments from reclaimed and intact tidal marshes in southeast Australia. We aimed to assess the impacts of reclamation on: 1) the magnitude of existing sediment C stocks; 2) ongoing C sequestration and storage; and 3) C quality. Differences in sediment horizon depths (indicated by Itrax-XRF scanning) and ages (indicated by lead-210 and radiocarbon dating) suggest a physical loss of sediments following reclamation, as well as slowing of sediment accumulation rates. Sediments at one meter depth were between ~2000 and ~5300 years older in reclaimed cores compared to intact marsh cores. We estimate a 70% loss of sediment C in reclaimed sites (equal to 73 Mg C ha-1), relative to stocks in intact tidal marshes during a comparable time period. Following reclamation, sediment C was characterized by coarse particulate organic matter with lower alkyl-o-alkyl ratios and higher amounts of aromatic C, suggesting a lower extent of decomposition and therefore lower likelihood of being incorporated into long-term C stocks compared to that of intact tidal marshes. We conclude that reclamation of tidal marshes can diminish C stocks that have accumulated over millennial time scales, and these losses may go undetected if additional analyses are not employed in conjunction with C stock estimates.

Converting beach-cast seagrass wrack into biochar: A climate-friendly solution to a coastal problem.

Excessive accumulation of plant 'wrack' on beaches as a result of coastal development and beach modification (e.g. groin installation) is a global problem. This study investigated the potential for converting beach-cast seagrass wrack into biochar as a 'climate-friendly' disposal option for resource managers. Wrack samples from 11 seagrass species around Australia were initially screened for their biochar potential using pyrolysis techniques, and then two species - Posidonia australis and Zostera muelleri - underwent detailed analyses. Both species had high levels of refractory materials and high conversion efficiency (48-57%) of plant carbon into biochar carbon, which is comparable to high-quality terrestrial biochar products. P. australis wrack gave higher biochar yields than Z. muelleri consistent with its higher initial carbon content. According to 13C NMR, wrack predominantly comprised carbohydrates, protein, and lignin. Aryl carbon typical of pyrogenic materials dominated the spectrum of the thermally-altered organic materials. Overall, this study provides the first data on the feasibility of generating biochar from seagrass wrack, showing that biocharring offers a promising climate-friendly alternative to disposal of beach wrack in landfill by avoiding a portion of the greenhouse gas emissions that would otherwise occur if wrack was left to decompose.

Biochemical suitability of crop residues for cellulosic ethanol: disincentives to nitrogen fertilization in corn agriculture.

Concerns about energy security and climate change have increased biofuel demand, particularly ethanol produced from cellulosic feedstocks (e.g., food crop residues). A central challenge to cropping for cellulosic ethanol is the potential environmental damage from increased fertilizer use. Previous analyses have assumed that cropping for carbohydrate in residue will require the same amount of fertilizer as cropping for grain. Using (13)C nuclear magnetic resonance, we show that increases in biomass in response to fertilization are not uniform across biochemical classes (carbohydrate, protein, lipid, lignin) or tissues (leaf and stem, grain, reproductive support). Although corn grain responds vigorously and nonlinearly, corn residue shows only modest increases in carbohydrate yields in response to high levels of fertilization (25% increase with 202 kg N ha(-1)). Lignin yields in the residue increased almost twice as much as carbohydrate yields in response to nitrogen, implying that residue feedstock quality declines as more fertilizer is applied. Fertilization also increases the decomposability of corn residue, implying that soil carbon sequestration becomes less efficient with increased fertilizer. Our results suggest that even when corn is grown for grain, benefits of fertilization decline rapidly after the ecosystem's N demands are met. Heavy application of fertilizer yields minimal grain benefits and almost no benefits in residue carbohydrates, while degrading the cellulosic ethanol feedstock quality and soil carbon sequestration capacity.

Importance of mechanisms and processes of the stabilisation of soil organic matter for modelling carbon turnover.

This paper reviews current knowledge of soil organic carbon (SOC) dynamics with respect to physical protection, soil moisture and temperature, and recalcitrant carbon fractions (such as charcoal) in predominantly agricultural soils. These factors are discussed within the framework of current soil organic matter models. The importance of soil structure in the stabilisation of organic residues through physical protection has been documented previously in various studies. In addition, changes in soil structure associated with tillage can significantly affect soil organic matter decomposition rates. The concept of physical protection has been incorporated into several soil carbon models as a function of soil texture. While soil texture can affect the soil's capacity for aggregation and adsorption, factors such as soil moisture and temperature may further enhance or reduce the extent of physical protection. While adsorption and aggregation can slow decomposition processes, it is unlikely that these processes are solely responsible for the high mean residence times measured in biologically active surface soils. Accordingly, chemical recalcitrance appears to be the only mechanism by which soil organic carbon can be protected for long periods of time.

Determination of T1rhoH relaxation rates in charred and uncharred wood and consequences for NMR quantitation.

The performance of three different techniques for determining proton rotating frame relaxation rates (T1rhoH) in charred and uncharred woods is compared. The variable contact time (VCT) experiment is shown to over-estimate T1rhoH. particularly for the charred samples, due to the presence of slowly cross-polarizing 13C nuclei. The variable spin (VSL) or delayed contact experiment is shown to overcome these problems; however, care is needed in the analysis to ensure rapidly relaxing components are not overlooked. T1rhoH is shown to be non-uniform for both charred and uncharred wood samples; a rapidly relaxing component (T1rhoH = 0.46-1.07 ms) and a slowly relaxing component (T1rhoH = 3.58-7.49) is detected in each sample. T1rhoH for each component generally decreases with heating temperature (degree of charring) and the proportion of rapidly relaxing component increases. Direct T1rhoH determination (via 1H detection) shows that all samples contain an even faster relaxing component (0.09-0.24 ms) that is virtually undetectable by the indirect (VCT and VSL) techniques. A new method for correcting for T1rhoH signal losses in spin counting experiments is developed to deal with the rapidly relaxing component detected in the VSL experiment. Implementation of this correction increased the proportion of potential 13C CPMAS NMR signal that can be accounted for by up to 50% for the charred samples. An even greater proportion of potential signal can be accounted for if the very rapidly relaxing component detected in the direct T1rhoH determination is included; however, it must be kept in mind that this experiment also detects 1H pools which may not be involved in 1H-13C cross-polarization.

Impact of remote protonation on 13C CPMAS NMR quantitation of charred and uncharred wood.

The impact of inefficient cross polarization (long TCH values), caused by long 13C-1H internuclear distances, on 13C CPMAS NMR spectra of charred and uncharred woods is determined by simultaneously fitting data from complementary variable spin lock and variable contact time experiments. As expected, the impact is minimal for uncharred woods, but is very significant for the charred woods. Quantification of the decrease in CPMAS signal intensity caused by both inefficient cross polarization and rapid T1rhoH relaxation is achieved using an advanced spin counting methodology, for which the term "spin accounting" is proposed. 13C CPMAS NMR observabilities determined using the spin accounting methodology were close to 100% for the uncharred samples, and 69-82% for the charred samples. This represents a large improvement on the 30-40% observabilities determined using other spin counting techniques. Furthermore, it is shown that remote protonation and rapid T1rhoH relaxation are roughly equally responsible for the low signal intensity of standard (I ms contact time) 13C CPMAS spectra of charcoal.