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Background and Aims: Crop residues are important for the redistribution of alkalinity within soils. A net increase in pH following residue addition to soil is typically reported. However, effects are inconsistent in the field due to confounding soil processes and agronomic practises. Methods: A column experiment investigated the effects of canola, chickpea and wheat residues, differing in alkalinity content and C:N ratio, on soil pH changes in a Podosol (Podzol; initial pH 4.5) and Tenosol (Cambisol; initial pH 6.2) under field conditions. Results: Residues (10 g dry matter kg soil; 0-10 cm) increased soil pH, and temporal changes in alkalinity depended on the residue and soil type. Alkalinity was generated via abiotic association reactions between H and added organic matter and via ammonification and decarboxylation processes during decomposition. Alkalinity from canola and chickpea residues moved down the soil profile (10-30 cm) and was attributed to nitrate immobilisation and organic anion decomposition by soil microbes. Conclusions: The application of residues to acid and moderately acid soils increased the pH of both topsoil and subsoils, which persisted over 26 months. Maximal increase of pH observed at 3 months was correlated with the concentration of excess cations in the residues. [ABSTRACT FROM AUTHOR]

The carbon chemistry of 10 profiles of peat soil has been described in detail using 13C nuclear magnetic resonance (NMR) spectroscopy. The changes with depth in the allocation of signal to different carbon functional groups were consistent with an increase in the extent of decomposition (EOD) of the organic material with depth. This increase in EOD with depth is typical of peat soils. Incubation experiments were carried out on peats spanning the range of EODs encountered, to investigate the effect upon mineralization of substrate quality (as defined by 13C NMR spectroscopy), water content and particle size. The confounding factors of depth, water content, bulk density, aeration and carbon content were eliminated by incubating ground peat material in a sand matrix. The size of the mineralizable carbon pool and the rate of carbon mineralization were both significantly affected by substrate quality, water content and particle size. Substrate quality had the greatest effect upon the size of the mineralizable carbon pool: as substrate quality decreased, so too did the size of the mineralizable carbon pool. Water content had the greatest effect upon the rate of carbon mineralization, which increased and then decreased as water content increased, with a maximum rate constant at a volumetric water content of 0.37 cm3 cm−3. [ABSTRACT FROM AUTHOR]

Carbon dioxide emissions and the mass loss of peat incubated in situ were measured in peat soils in the Australian Alps. The carbon chemistry of incubated peats was characterized using 13C nuclear magnetic resonance (NMR). In situ decomposition decreased as a function of increasing alkyl carbon content of the initial organic matter, providing direct evidence of the oft-cited link between substrate quality and decomposition rates. More mass loss occurred in the bog peat samples than in the dried peat samples. However, at the peat surface, the amount of CO2 emitted was not significantly different between bog peat and dried peat. Carbon inputs, and therefore the sink or source status, of these peats are yet to be quantified. [ABSTRACT FROM AUTHOR]

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]

The ratio of isotopes of carbon (13C:12C or δ13C) and nitrogen (15N:14N or δ15N) are common indicators of the flow and storage of organic matter in coastal wetland research. Effective use of these indicators requires quantification and understanding of: (1) the variability of isotope signatures of potential organic matter source materials; and (2) the influence of organic matter decomposition on isotopic signatures. While it is well-established that organic matter characteristics change during the decomposition process, there has been little direct quantification of any concurrent shifts in isotope signatures for coastal detritus. In this study, we addressed this by quantifying: (1) shifts in sample composition using solid-state 13C Nuclear Magnetic Resonance (NMR) spectroscopy; and (2) shifts in δ13C and δ15N signatures of coastal plant tissues from field litterbag experiments. We observed significant shifts in 13C NMR spectra across the course of deployment for all four plant tissues assessed (leaves of mangrove Avicennia marina; branchlets of supratidal tree Casuarina glauca; leaf wrack and roots/rhizomes of the seagrass Zostera muelleri), driven largely by the preferential loss of labile constituents and concentration of more resistant macromolecules, such as lignin and leaf waxes. While there were shifts in isotope ratios for all species, these varied in direction and magnitude among species, tissue type and isotopes. This included δ13C enrichments of up to 3.1‰ and 2.4‰ in leaves of A. marina, and branchlets of C. glauca, respectively, but δ13C depletions of up to 4.0‰ for Z. muelleri. Shifts in δ15N varied among species and tissue types, with few clear temporal patterns. Partial least squares regression analyses showed that some tissue isotope signatures can be reliably predicted on the basis of sample composition (13C NMR spectra), however, multiple inter- and intra-species variations preclude a simple explanation of isotopic signature shifts on the basis of plant-material molecular shifts alone. Further, we cannot preclude the potential influence of microbe-associated organic matter on sample composition or isotopic signatures. Our findings emphasise the importance of considering decomposition effects on stable isotope signatures in blue carbon ecosystems. Isotope approaches will remain a valuable tool in coastal ecosystem research, but require robust experimental approaches (including appropriate use of decomposed end-members or fractionation correction factors; quantification of microbial organic matter) and quantification of decomposition dynamics for specific plant tissues and environmental settings. [ABSTRACT FROM AUTHOR]

A correction to the article "Prospects and problems of simple linear models for estimating symbiotic N2 fixation by crop and pasture legumes," by M. J. Unkovich, J. Baldock, and M. B. Peoples, that was published in 2011 issue is presented.

There is ample evidence to suggest that liming can regulate soil organic carbon (SOC) pools either directly through influencing the solubility of SOC or indirectly by altering total organic C input as crop residue and SOC loss via change in microbial activity. The aim of this study was to determine the long-term impact of lime application on the quantity and quality of SOC in acid soils. Soils were collected at depths of 0-10, 10-20, 20-30, 30-40, and 40-50 cm from four long-term lime trials with various lime rates (0-25 t ha), lime histories (5-35 years), and soil textures (clay content 5-36 %). Surface application of lime was effective in ameliorating both topsoil and subsoil acidities at sites with low clay content. Liming decreased dissolved organic C (DOC) at 0-30 cm but increased its aromaticity. Total SOC at 0-10 cm decreased or remained unchanged following long-term liming, depending on the rates of lime application and crop management. Changes in the contents of particulate organic C (POC) and humic organic C (HOC) predicted by mid-infrared spectroscopy (MIR) and partial least squares regression (PLSR) showed a similar trend to total SOC at all sites. Lime application had no significant effect on SOC below 10-cm layers and on the MIR-predicted resistant organic C (ROC) fraction. Solid-state C nuclear magnetic resonance (NMR) spectra indicated that the alkyl C content and alkyl/O-alkyl C ratio were lower in the limed than unlimed plots. Liming possibly had a marked effect on regulating the decomposition and preservation of certain C compounds. The apparent accumulation of alkyl C in the unlimed soil could indicate the potential ability of acid soils to store SOC. [ABSTRACT FROM AUTHOR]

Sorption of organic carbon (OC) to phyllosilicate clays and hydrous iron oxides retards its mineralization, thus contributing to stabilization of organic carbon in soils. The degree of protection varies with the nature of the minerals present. In a previous study, we reported that when the amount of OC exceeds the sorption capacity of minerals, the rate of OC mineralization is determined primarily by mineral surface area. Here, we report on OC mineralization under conditions of less OC loading and with OC pre-sorbed to the mineral surfaces. The same suite of minerals (kaolinite, illite and smectite with and without goethite coating and illite coated with haematite, goethite and ferrihydrite) was used as in the previous study. The stability of sorbed OC decreased in the order kaolinite>illite>smectite amongst the uncoated clays. Goethite coating of kaolinite and smectite increased the stability of sorbedOMagainst microbial decomposition, while the stability of illite-associated OC did not change with goethite coating. For illite coated with different hydrous iron oxides, only ferrihydrite increased the stability of sorbed OC against microbial decomposition. These differences reflect closely the differences in the strength and reversibility of OC sorption, as measured in previous batch sorption experiments on these systems, rather than reflecting differences in mineral surface area. These findings demonstrate that at relatively small loadings of OC, the degree of protection of sorbed OC provided by mineral surfaces is controlled primarily by the strength of organo-mineral associations. [ABSTRACT FROM AUTHOR]

The retention of agricultural residues in cropping systems to maintain soil fertility is also important for the redistribution of alkalinity. In systems that adopt minimum or no-tillage practices residue incorporation into the soil may occur slowly and the contribution of soluble and insoluble residue fractions to pH change may vary temporally and spatially. In this study we examined the contribution of whole, water soluble (70°C for 1 hour for two cycles) and insoluble fractions of canola, chickpea and wheat residues (added at 10 g kg−1 soil) to pH change in a Podosol (Podzol; initial pH 4.5) and a Tenosol (Cambisol; initial pH 6.2) over a 59-day incubation period. Whole residues increased pH in both soils, with the magnitude of the pH increase (chickpea > canola > wheat) being related to alkalinity content (concentration of excess cations) of the residue. Temporal release of alkalinity was only observed for the larger alkalinity content canola and chickpea residues and the change in pH was greater than during the initial period (approximately 4 hours; T0). Increases in pH were attributed to the decarboxylation of organic anions and the association of H+ with organic anions and other negatively charged chemical functional groups. The relative contribution of these processes depended on the residue and the initial soil pH. Our results show that 40-62% of the alkalinity of canola and chickpea residues resided in the soluble fraction. Furthermore, pH increases caused by soluble fractions may be transient if these contain large N concentrations. Soil properties that influence inorganic N dynamics such as inhibition of nitrification at acid pH will be important in determining the subsequent direction and magnitude of pH change. [ABSTRACT FROM AUTHOR]

In low-rainfall cropping systems, understanding the water balance, and in particular the storage of soil water in the rooting zone for use by crops, is considered critical for devising risk management strategies for grain-based farming. Crop-soil modeling remains a cost-effective option for understanding the interactions between rainfall, soil, and crop growth, from which management options can be derived. The objective of this study was to assess the error in the prediction of soil water content at key decision points in the season against continuous, multi-layer soil water measurements made with frequency domain reflectometry (FDR) probes in long-term experiments in the Mallee region of South Australia and New South Wales. Field estimates of the crop lower limit or drained upper limit were found to be more reliable than laboratory-based estimates, despite the fact that plant-available water capacity (PAWC) did not substantially differ between the methods. Using the Agricultural Production Systems sIMulator (APSIM) to simulate plant-available water over three-year rotations, predicted soil water was within 7 mm (PAWC 64 to 99 mm) of the measured data across all sowing events and rotations. Simulated (n = 46) wheat grain production resulted in a root mean square error (RMSE) of 492 kg ha-1, which is only marginally smaller than that of other field studies that derived soil water limits with less detailed methods. This study shows that using field-derived data of soil water limits and soil-specific settings for parameterization of other properties that determine soil evaporation and water redistribution enables APSIM to be widely applied for managing climate risk in low-rainfall environments. [ABSTRACT FROM AUTHOR]

• Chemical and thermal analyses of fractions from 75 soils were cross-correlated. • Observed strong correlations between broad thermal regions and chemical types. • EGA data generated stronger correlations with NMR than did DSC. • Demineralization treatments resulted in substantial differences in correlations. • Bioenergetics may be better approach than chemical composition for SOM reactivity. While many advanced analytical methods have been applied to soil organic matter (SOM), its highly complex and heterogeneous chemical composition still eludes complete characterization. Analytical thermal analysis has been proposed as a relatively rapid, inexpensive method for SOM characterization that requires no pre-treatment, but is challenging due to a lack of direct information about chemical composition. The goal of this study was to inform the interpretation of coupled differential scanning calorimetry and evolved gas analyses (DSC, CO2-EGA) using spectral correlations with solid phase 13C NMR data. We used a subset of soils collected as part of the Australian National Soil Carbon Research Program (SCaRP), which were physically fractionated and characterized using conventional analytical methods. Correlating the well-understood NMR spectra with the less-understood DSC and CO2-EGA thermograms provided some indications of which chemical compounds combust at which temperatures. Overall, the EGA data generated stronger correlations compared to correlations with DSC data, which was attributable to greater variability in DSC data due thermal reactions associated with minerals. Direct comparison of NMR and thermal data for the mineral associated organic matter in the fine (<50 µm) fraction was not possible due to the need to demineralize samples prior to NMR analyses. Thermal analyses showed substantial differences in samples pre- and post-HF pretreatment, and the NMR data for HF treated samples showed scattered and weak correlation patterns with DSC and EGA data for untreated samples. While precise chemical compositions cannot be gleaned directly from thermal analyses results, thermal approaches provide an avenue of investigation into SOM reactivity based on bioenergetics that may be a quantitative representation of SOM persistence. [ABSTRACT FROM AUTHOR]

An accurate measurement of soil pH buffer capacity (pHBC) is essential for estimating lime requirement and predicting soil acidification in farming systems. The soil pHBC is commonly determined by acid-base titration (1:5 soil : solution); however, no standardized equilibration time, type of acid and alkali or concentration of electrolyte has been recommended. This study aimed to establish a standard procedure that is relatively method-independent and reflects the actual soil pH buffering ability. Nineteen soils with a wide range of pH, clay and organic carbon contents were used. Measuring pHBC in a 0.01 m CaCl2 suspension minimized the effects of both ionic strength and cation valence after addition of different types and rates of alkalis and acids. The time required to reach near equilibrium (1 hour to 28 days) was greatly expedited by sonication pretreatment. For soils with a large carbon content, biological reactions alone could elevate soil pHBC and addition of biocides was essential during pHBC measurement. When compared with two commonly used methods, 1:1 soil :water extraction and field moist incubation, the 1:5 CaCl2 method was less affected by ionic strength and biological activity, and more suitable for use as a standard method.We recommend using 0.01m CaCl2 at a soil-to-solution ratio of 1:5, with addition of HCl and NaOH or Ca(OH)2, chloroform and sonication for pHBC determination. [ABSTRACT FROM AUTHOR]

Ecosystem-, biome-, and global-level oxidative ratio (OR) estimates are used to understand ecosystem gas exchange processes, and assess the sizes of the terrestrial biosphere and ocean carbon sinks, and the mechanisms controlling them. We have developed analytical methods to measure the OR of terrestrial carbon stocks from biomass chemistry that give accurate and precise estimates of ecosystem atmosphere-biosphere gas flux OR. Here we apply these techniques to agricultural ecosystems to address two uncertainties: (1) whether changing crop species distribution can change ecosystem OR, and (2) whether nutrient status can change ecosystem OR. Analyzing the top three crops in the U.S. (soybean, corn, wheat), we showed differences between the OR of net primary production (OR) of legumes (1.109 ± 0.003) versus grass crops (corn: 1.028 ± 0.001; wheat: 1.033 ± 0.001). Our preliminary estimate of the average U.S. agriculture OR is 1.058 in 2010, and extrapolations indicate that the U.S. agricultural OR has increased from 1.040 in 1930, likely due to shifts in agricultural land use (e.g. oat acreage decreases while soybean acreage increases over time). We also observed that increasing nitrogen fertilization rate led to only a small increase in OR. Taken together, our results indicate that it is possible for biome-level OR to vary over time and space. The largest driver of OR variation in this study was variation in crop distribution, which changed OR by an order of magnitude more than variations in nitrogen fertilizer application. [ABSTRACT FROM AUTHOR]

There is limited understanding of the relationship between carbon (C) chemistry and microbial community structure during decomposition of shoot and root residues and how plant age affects this. In this study, residues of young wheat shoots and roots, mature wheat shoots and roots or a 1:1 mix of mature shoot + root (MSR) were added to sand inoculated with a diverse microbial community. Respiration was measured over 60 days. On days 0, 15, 30 and 60, total C and nitrogen were measured, residue C chemistry was determined by 13C-NMR (nuclear magnetic resonance) spectroscopy and microbial community structure was assessed by phospholipid fatty acid (PLFA) analyses. Cumulative respiration was least in young roots and did not differ among the other residue types. In MSR, decomposition was similar to that of shoots and roots alone; shoot material appeared to be preferentially decomposed. The decomposition rate of all residues combined was not related to C chemistry. However, mineralized C (Cmin) was negatively correlated with the percentage of (aryl + O-aryl)-C in mature but not in young residues. Mineralized C of roots was positively correlated with the percentage of (di-O-alkyl + O-alkyl)-C, whereas this was not the case for shoots. Microbial community structure was influenced by time, plant organ and plant age. There was no general relationship between microbial community structure and C chemistry of the residues. [ABSTRACT FROM AUTHOR]

There is still much uncertainty as to how wildfire affects the accumulation of burn residues (such as black carbon (BC)) in the soil, and the corresponding changes in soil organic carbon (SOC) composition in boreal forests. We investigated SOC and BC composition in black spruce forests on different landscape positions in Alaska, USA. Mean BC stocks in surface mineral soils (0.34 ± 0.09 kg C m) were higher than in organic soils (0.17 ± 0.07 kg C m), as determined at four sites by three different C Nuclear Magnetic Resonance Spectroscopy-based techniques. Aromatic carbon, protein, BC, and the alkyl:O-alkyl carbon ratio were higher in mineral soil than in organic soil horizons. There was no trend between mineral soil BC stocks and fire frequencies estimated from lake sediment records at four sites, and soil BC was relatively modern (<54–400 years, based on mean ΔC ranging from 95.1 to −54.7‰). A more extensive analysis (90 soil profiles) of mineral soil BC revealed that interactions among landscape position, organic layer depth, and bulk density explained most of the variance in soil BC across sites, with less soil BC occurring in relatively cold forests with deeper organic layers. We suggest that shallower organic layer depths and higher bulk densities found in warmer boreal forests are more favorable for BC production in wildfire, and more BC is integrated with mineral soil than organic horizons. Soil BC content likely reflected more recent burning conditions influenced by topography, and implications of this for SOC composition (e.g., aromaticity and protein content) are discussed. [ABSTRACT FROM AUTHOR]

Australia's tidal marshes have suffered significant losses but their recently recognised importance in CO2 sequestration is creating opportunities for their protection and restoration. We compiled all available data on soil organic carbon (OC) storage in Australia's tidal marshes (323 cores). OC stocks in the surface 1 m averaged 165.41 (SE 6.96) Mg OC ha−1 (range 14-963 Mg OC ha−1). The mean OC accumulation rate was 0.55 ± 0.02 Mg OC ha−1 yr−1. Geomorphology was the most important predictor of OC stocks, with fluvial sites having twice the stock of OC as seaward sites. Australia's 1.4 million hectares of tidal marshes contain an estimated 212 million tonnes of OC in the surface 1 m, with a potential CO2-equivalent value of $USD7.19 billion. Annual sequestration is 0.75 Tg OC yr−1, with a CO2-equivalent value of $USD28.02 million per annum. This study provides the most comprehensive estimates of tidal marsh blue carbon in Australia, and illustrates their importance in climate change mitigation and adaptation, acting as CO2 sinks and buffering the impacts of rising sea level. We outline potential further development of carbon offset schemes to restore the sequestration capacity and other ecosystem services provided by Australia tidal marshes. [ABSTRACT FROM AUTHOR]