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The oxidative ratio (OR) of the net ecosystem carbon balance is the ratio of net O2 and CO2 fluxes resulting from photosynthesis, respiration, decomposition, and other lateral and vertical carbon flows. The OR of the terrestrial biosphere must be well characterized to accurately estimate the terrestrial CO2 sink using atmospheric measurements of changing O2 and CO2 levels. To estimate the OR of the terrestrial biosphere, measurements are needed of changes in the OR of aboveground and belowground carbon pools associated with decadal timescale disturbances (e.g., land use change and fire). The OR of aboveground pools can be measured using conventional approaches including elemental analysis. However, measuring the OR of soil carbon pools is technically challenging, and few soil OR data are available. In this paper we test three solid-state nuclear magnetic resonance (NMR) techniques for measuring soil OR, all based on measurements of the closely related parameter, organic carbon oxidation state (Cox). Two of the three techniques make use of a molecular mixing model which converts NMR spectra into concentrations of a standard suite of biological molecules of known Cox. The third technique assigns Cox values to each peak in the NMR spectrum. We assess error associated with each technique using pure chemical compounds and plant biomass standards whose Cox and OR values can be directly measured by elemental analyses. The most accurate technique, direct polarization solid-state 13C NMR with the molecular mixing model, agrees with elemental analyses to ±0.036 Cox units (±0.009 OR units). Using this technique, we show a large natural variability in soil Cox and OR values. Soil Cox values have a mean of −0.26 and a range from −0.45 to 0.30, corresponding to OR values of 1.08 ± 0.06 and a range from 0.96 to 1.22. We also estimate the OR of the carbon flux from a boreal forest fire. Analysis of soils from nearby intact soil profiles imply that soil carbon losses associated with the fire had an OR of 1.091 (±0.003). Fire appears to be a major factor driving the soil C pool to higher oxidation states and lower OR values. Episodic fluxes caused by disturbances like fire may have substantially different ORs from ecosystem respiration fluxes and therefore should be better quantified to reduce uncertainties associated with our understanding of the global atmospheric carbon budget.

Abstract 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 [...]

Measurement of dispersible clay is important for the diagnosis of structural stability problems in soil. However, clay dispersibility is known to change with water content and time. The purpose of the present study was to determine how incubation of sodic soil under different water content regimes influences clay dispersibility. Two topsoils (depth 0-0.1 m), one sodic [exchangeable sodium percentage (ESP) 9.7] and the other non-sodic (ESP 3.8), were collected from an experimental pasture at Kyabram, Victoria, and 2 soils, a sodic topsoil (depth 0-0.1 m, ESP 6.9) and the corresponding subsoil (depth 0.2-0.3 m, ESP 25.7), were collected from a cropped field at Two Wells, South Australia. The soils were incubated for 264 days in a split-plot design. The main treatments were soil type and incubation water content: continuously air-dry, continuously wet (-50 kPa), or with wet/dry cycles. The subtreatment was water content at analysis: air dry or wet (-50 kPa). Clay dispersion was greater when measured on wet soils than dry soils, irrespective of water contents during the prior incubation. Electrical conductivity increased, and sodium adsorption ratio (SAR), pH, and organic carbon content decreased as a function of the time for which the soils were wet. In the Kyabram soils that were wet when analysed, easily dispersible clay content increased with SAR. Decreases in moderately dispersible clay under the wetting/drying regime were not related to electrolyte composition, and were attributed to particle rearrangement and cementation. The decreases in clay dispersibility with time occurred despite net losses of carbohydrate and aliphatic materials. An implication of the work is that the decomposition of soil organic matter, even in the absence of fresh additions, may reduce clay dispersion in sodic soils by altering electrolyte concentration and composition. Additional keywords: sodium adsorption ratio, organic matter, pH, water content., Introduction The tendency of clay to disperse is an important property of soil, and it has been suggested that values of dispersible clay should be collected routinely for assessing the [...]

Symbiotic dinitrogen (N2) fixation of crop and pasture legumes is a critical component of agricultural systems, but its measurement is expensive and labour intensive. Simple models which can provide approximations based on crop or pasture dry matter production would be useful for agrononomists and those interested in regional nitrogen (N) cycle fluxes. We investigate meta analysis of published data on legume shoot dry matter production, shoot %N and legume %N fixed (%Ndfa) and look for relationships among these, as a possible way of providing useful approximations of N2 fixation. We restricted our analysis to Australian studies where we have ready access to the primary data and where cultivars, management and climate are more constrained compared to a universal dataset. Regression analysis between shoot dry matter and amounts of shoot N2 fixed were strong for all crop and pasture legumes with significant differences in slope and intercept values being obtained between pastures and crops, and between chickpea ( Cicer arietinium) and all other crop and pasture legumes. Annual pasture legumes showed the strongest linear relationship between N2 fixation and shoot dry matter and had the greatest slope (20.2–24.3 kg N2 fixed/t), compared to 18.7 kg N2 fixed/t for the perennial pasture legume lucerne (alfalfa, Medicago sativa), and between 10.7 to 23.0 kg N2/t for crop legumes, depending upon species. It was recognised that the use of such shoot-based relationships would underestimate the total amounts of N2 fixed since the contributions of fixed N present in, or derived from, roots and nodules are not included. Furthermore there needs to be careful consideration of the validity of an intercept term, which might reflect suppression of N2 fixation at low dry matter and high soil mineral N availability, or possibly the use of non-linear regression. For chickpea crops grown in north-eastern Australia, multiple regression indicated that N2 fixation was much more closely correlated with %Ndfa than dry matter production. Evidence presented also indicated that %Ndfa of other crops and lucerne in this region may similarly be influenced by soil mineral N. The regression approach presented provides a statistical basis to approximate N2 fixation in the first instance. This work highlights some of the dangers of fitting single regressions to aggregated datasets and using these to approximate symbiotic N2 fixation. The analysis indicates that where pasture legumes are grown in mixtures with non-legumes, and driven to high dependence on N2 fixation, simple linear regressions may be quite useful, provided that possible differences between species are investigated as the slopes of the regressions between these can be quite different. For crop legumes, where low dependence on N2 fixation can occur at higher mineral N availability, there is a need to carefully consider the intercept term, obtain estimates of mineral N availability, and/or resort to non-linear models. The gross generalisations presented in scatter plots cannot be reliably applied any more specifically, even within the datasets from which they were generated, and in some cases even within legume species between regions. They cannot substitute for direct measurement where any certainty is required under a particular set of defined conditions. [ABSTRACT FROM AUTHOR]