Jeffrey A. Baldock

Chemical composition and bioavailability of thermally altered Pinus resinosa (Red pine) wood.

Abstract The residues remaining after incomplete combustion of vegetation (char) can contribute significantly to the carbon content of soils. Char C is considered biologically inert relative to other forms of organic C in soils; however, the degree of biological inertness is likely to be a function of the extent that the combustion residues were altered during thermal treatment. The relationship between changes in chemical composition and biological inertness of char C created in the laboratory by heating Pinus resinosa sapwood to temperatures between 70 and 350xa0°C was quantified. Heating at each temperature was maintained until the mass of the residual char material stabilised (±2%). Chemical composition of the chars was assessed by elemental analysis, solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). The susceptibility of the heated sapwood to biological oxidation was quantified in a 120-day laboratory incubation. Thermal treatment at temperatures ⩾200xa0°C induced significant variations in chemical composition. Changes in elemental contents and molar elemental ratios were consistent with an initial dehydration and the formation of unsaturated structures. The NMR and DRIFT data indicated that the changes in the chemical composition with increasing heating temperature included a conversion of O-alkyl C to aryl and O-aryl furan-like structures, consistent with results from work examining the chemical changes associated with thermal treatment of cellulose, the major component of wood. The chemical changes significantly reduced the ability of a microbial inoculum derived from decomposing Pinus resinosa wood to mineralise carbon contained in the charred samples. The C mineralisation rate constants decreased by an order of magnitude for wood heated to ⩾200xa0°C.

Aspects of the chemical structure of soil organic materials as revealed by solid-state13C NMR spectroscopy

Solid-state cross-polarisation/magic-angle-spinning3C nuclear magnetic resonance (CP/MAS13C NMR) spectroscopy was used to characterise semi-quantitatively the organic materials contained in particle size and density fractions isolated from five different mineral soils: two Mollisols, two Oxisols and an Andosol. The acquired spectra were analysed to determine the relative proportion of carboxyl, aromatic, O-alkyl and alkyl carbon contained in each fraction. Although similar types of carbon were present in all of the fractions analysed, an influence of both soil type and particle size was evident.The chemical structure of the organic materials contained in the particle size fractions isolated from the Andosol was similar; however, for the Mollisols and Oxisols, the content of O-alkyl, aromatic and alkyl carbon was greatest in the coarse, intermediate and fine fractions, respectively. The compositional differences noted in progressing from the coarser to finer particle size fractions in the Mollisols and Oxisols were consistent with the changes noted in other studies where CP/MAS13C NMR was used to monitor the decomposition of natural organic materials. Changes in the C:N ratio of the particle size fractions supported the proposal that the extent of decomposition of the organic materials contained in the fine fractions was greater than that contained in the coarse fractions. The increased content of aromatic and alkyl carbon in the intermediate size fractions could be explained completely by a selective preservation mechanism; however, the further accumulation of alkyl carbon in the clay fractions appeared to result from both a selective preservation and anin situ synthesis.The largest compositional differences noted for the entire organic fraction of the five soils were observed between soil orders. The differences within orders were smaller. The Mollisols and the Andosol were both dominated by O-alkyl carbon but the Andosol had a lower alkyl carbon content. The Oxisols were dominated by both O-alkyl and alkyl carbon.A model describing the oxidative decomposition of plant materials in mineral soils is proposed and used to explain the influence of soil order and particle size on the chemical composition of soil organic matter in terms of its extent of decomposition and bioavailability.

Assessing the extent of decomposition of natural organic materials using solid-state 13C NMR spectroscopy

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has become an important tool for examining the chemical structure of natural organic materials and the chemical changes associated with decomposition. In this paper, solid-state 13C NMR data pertaining to changes in the chemical composition of a diverse range of natural organic materials, including wood, peat, composts, forest litter layers, and organic materials in surface layers of mineral soils, were reviewed with the objective of deriving an index of the extent of decomposition of such organic materials based on changes in chemical composition. Chemical changes associated with the decomposition of wood varied considerably and were dependent on a strong interaction between the species of wood examined and the species composition of the microbial decomposer community, making the derivation of a single general index applicable to wood decomposition unlikely. For the remaining forms of natural organic residues, decomposition was almost always associated with an increased content of alkyl C and a decreased content of O-alkyl C. The concomitant increase and decrease in alkyl and O-alkyl C contents, respectively, suggested that the ratio of alkyl to O-alkyl carbon (A/O-A ratio) may provide a sensitive index of the extent of decomposition. Contrary to the traditional view that humic substances with an aromatic core accumulate as decomposition proceeds, changes in the aromatic region were variable and suggested a relationship with the activity of lignin-degrading fungi. The A/O-A ratio did appear to provide a sensitive index of extent of decomposition provided that its use was restricted to situations where the organic materials were derived from a common starting material. In addition, the potential for adsorption of highly decomposable materials on mineral soil surfaces and the impacts which such an adsorption may have on bioavailability required consideration when the A/O-A ratio was used to assess the extent of decomposition of organic materials found in mineral soils.

The biochemical and elemental compositions of marine plankton: A NMR perspective

The traditional Redfield–Ketchum–Richards (1963) equation for the production (or respiration) of ‘‘average marine plankton’’ 106 CO2 þ 16 HNO3 þ H3PO4 þ 122 H2O ¼ð CH2OÞ106ðNH3Þ16ðH3PO4 Þþ 138 O2 has long been a useful guideline for establishing the ratios and reaction extents of the bioactive elements in ocean systems. The empirical formula on the right of the above equation for marine plankton biomass adequately represents the C/N/P of mixed marine plankton collected in towed nets, but includes an impossibly elevated hydrogen content and a questionably high level of organic oxygen. An elevated estimate of oxygen content is particularly critical because it would lead to an underestimate of the amount of O2 required for complete respiration of plankton biomass. Although direct biochemical measurements have been used previously to constrain the compositions, and hence the reaction stoichiometries, of marine plankton and their remains, such analyses can be prone to error and analytical bias. To cast a new light on the chemical composition of marine plankton, we determined the major functional group distribution of organic carbon in mixed plankton tows from five contrasting ocean sites using cross-polarization, magic-angle spinning carbon-13 nuclear magnetic resonance (CP/MAS 13 C NMR). Using a mixing model that relates NMR spectral data to biochemical composition, we estimate an average major biochemical composition (weight basis) for these plankton samples of 65% protein, 19% lipid and 16% carbohydrate. This biochemical composition corresponds to an average elemental formula for plankton biomass of C106H177O37N17S0.4, whose complete oxidation requires 154 moles of O2. Although preliminary, this 13 C NMR-based estimate indicates elemental compositions and respiratory oxygen demands that are widely different from those indicated by the RKR composition (C106H260O106N16 and 138 O2, respectively) and those determined in many previous field studies. D 2002 Published by Elsevier Science B.V.

Concentration and composition of dissolved organic carbon in streams in relation to catchment soil properties

Two adjacent catchments in the Otway Ranges of Victoria, Australia (Redwater and Clearwater) produce water with markedly different concentrations of dissolved organic carbon (DOC) during summer. Water from Redwater Creek had a DOC concentration of 32 mg L−1, while water from Clearwater Creek had a DOC concentration of 3.8 mg L−1. Examination of the catchments revealed that while climate, topography, vegetation and land use were similar, the soils were different. The objective of this study was to examine the relationship between the concentration and chemical composition of DOC in stream waters and the nature of soils in the two catchments. Soil mapping determined that clayey soils formed on Cretaceous sediments (Cretaceous soils) occurred throughout both catchments, but that Redwater Catchment also contained a large area (39%) of sandy soils formed on Tertiary sediments (Tertiary soils). The concentration of DOC in forest floor leachate was high in both the Tertiary and Cretaceous areas; however, the concentration of DOC in water draining areas dominated by Tertiary soils was greater than that in water draining areas dominated by Cretaceous soils. Laboratory experiments showed that the Cretaceous soils had higher adsorption capacities for forest floor leachate DOC than the Tertiary soils. The difference in DOC concentrations of the streams was therefore attributed to the difference in adsorption capacity of catchment soils for DOC. Adsorption capacities of the soils were found to be a function of their clay contents and specific surface areas.Solid-state3C nuclear magnetic resonance spectroscopy and pyrolysis-mass spectrometry were used to determine the chemical structure of DOC found in streams and forest floor leachate samples and that remaining in solution after interaction with soil. Chemistry of DOC in forest floor leachate was similar before and after interaction with soil, indicating no preferential adsorption of a particular type of carbon. Thus, differences between the chemical structure of stream DOC and forest floor leachate DOC could be attributed to microbial modifications during its movement through soils and into the streams, rather than losses by adsorption.

Linking soils and streams: sources and chemistry of dissolved organic matter in a small coastal watershed.

[1]xa0To understand the hydrologic and biogeochemical controls on the age and recalcitrance of dissolved organic matter (DOM) found in stream waters, we combined hydrometric monitoring along a topographic gradient from ridge to channel with isotopic (13C and 14C) and spectroscopic (UV and 13C nuclear magnetic resonance) analyses of soil and stream water samples in a small coastal watershed in California. With increasing discharge, dissolved organic carbon concentrations increased from 2.2 to 10.9 mg C L−1, Δ14C values increased from −125 to +120‰, δ13C values decreased from −24 to −29‰, C:N ratios increased from 6.5 to 15.4, and specific UV adsorption increased from 1.4 to 3.8 L mg C−1 m−1. These changes in DOM composition are consistent with a shift in source from old and recalcitrant soil organic matter (OM) sources found in deep soil horizons to young and relatively fresh OM sources found in the surface horizons. Results from this study suggest upland soils of the watershed become DOM production limited as indicated by a seasonal depletion and chemical shift in soil DOM, whereas highly productive soils in the hollow act as a near-infinite DOM source. Hydrologic connectivity of this DOM-rich riparian source region to the stream ultimately constrains DOM export, and the stream DOM composition reflects the combined influence of soil biogeochemical cycling of OM and hydrologic routing of water through the landscape.

Solid-state CP/MAS 13C N.M.R. analysis of bacterial and fungal cultures isolated from a soil incubated with glucose

Bacteria and fungi were isolated from a sample of the Meadows fine sandy loam, an Alfisol, and selectively cultured in nutrient solutions at 20°C for 5 days. The bacteria and fungi were collected, washed with deionized water, freeze dried and analysed using conventional and dipolar dephased solid state CP/MAS 13C n.m.r. spectroscopy. To obtain a quantitative estimate of the chemical composition of the bacterial and fungal carbon, a recycle delay of 3.0 s was required to allow complete relaxation between pulses, and the acquired signal intensities had to be corrected for the amount of signal relaxation which occurred during the contact time (i.e. T1pH effects). The bacterial materials contained more alkyl and carboxyl carbon but less O-alkyl and acetal carbon than the fungal materials. Comparison of the composition of the bacterial and fungal carbon with that of the native and residual substrate carbon contained in the clay and light fraction of a sample of Meadows fine sandy loam incubated with 13C-glucose indicated that the soil microbial population was dominated by fungi.

Soil Security: Solving the Global Soil Crisis

Soil degradation is a critical and growing global problem. As the world population increases, pressure on soil also increases and the natural capital of soil faces continuing decline. International policy makers have recognized this and a range of initiatives to address it have emerged over recent years. However, a gap remains between what the science tells us about soil and its role in underpinning ecological and human sustainable development, and existing policy instruments for sustainable development. Functioning soil is necessary for ecosystem service delivery, climate change abatement, food and fiber production and fresh water storage. Yet key policy instruments and initiatives for sustainable development have under-recognized the role of soil in addressing major challenges including food and water security, biodiversity loss, climate change and energy sustainability. Soil science has not been sufficiently translated to policy for sustainable development. Two underlying reasons for this are explored and the new concept of soil security is proposed to bridge the science–policy divide. Soil security is explored as a conceptual framework that could be used as the basis for a soil policy framework with soil carbon as an exemplar indicator.

Accounting for soil carbon sequestration in national inventories: a soil scientist's perspective.

As nations debate whether and how best to include the agricultural sector in greenhouse gas pollution reduction schemes, the role of soil organic carbon as a potential large carbon sink has been thrust onto center stage. Results from most agricultural field trials indicate a relative increase in soil carbon stocks with the adoption of various improved management practices. However, the few available studies with time series data suggest that this relative gain is often due to a reduction or cessation of soil carbon losses rather than an actual increase in stocks. On the basis of this observation, we argue here that stock change data from agricultural field trials may have limited predictive power when the state of the soil carbon system is unknown and that current IPCC (Intergovernmental Panel on Climate Change) accounting methodologies developed from the trial results may not properly credit these management activities. In particular, the use of response ratios is inconsistent with the current scientific understanding of carbon cycling in soils and response ratios will overestimate the net?net sequestration of soil carbon if the baseline is not at steady state.

Dispersed clay and organic matter in soil: their nature and associations

Clay dispersion in soil results in structural instability and management problems. The aim of this study was to determine whether or not the easily dispersed colloidal materials differ in their properties from colloidal materials that do not disperse easily. Soil samples from nthe topsoil of sodic and non-sodic variants of an Alfisol under irrigated pasture (Kyabram, Victoria, Australia), and from the topsoil and subsoil of a sodic Alfisol under cultivation (Two Wells, South Australia) were fractionated into easily dispersed, moderately dispersed, and difficult to disperse clay, and silt, sand, and light fractions. As a proportion of total clay, easily dispersed clay content was greatest in the subsoil, and least in the Kyabram topsoils. In the topsoils, easily dispersed clay had larger particle size and lower cation exchange capacity than difficult to disperse clay, suggesting that high surface area and charge lead to increased inter-particle interactions and lower dispersibility. Easily dispersed clay had lower organic C ncontents than difficult to disperse clay. Organic matter was examined by 13C nuclear magnetic resonance, and the spectra were interpreted using major groups of biomolecules as model components. In all soils, organic matter in the easily dispersed clay fraction contained a high proportion of amino acids, suggesting that amino acids or proteins acted as dispersants. Difficult to disperse clay contained a high proportion of aliphatic materials in the topsoils, nand carbohydrate in the subsoil, suggesting that these materials acted as water-stable glues. Selectivity for Na (KG) was negatively correlated with organic C content in the clay fractions. In the Kyabram soils, KG was greater in easily dispersed clay than in di±cult to disperse clay. In Two Wells soil, clay with high KG appeared to have already moved out of the topsoil, into the subsoil. This work showed that variability in the nature of organic matter and clay particles has an important influence on clay dispersion in sodic and non-sodic soils.