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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]

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 benefits of sequestering carbon are many, including improved crop productivity, reductions in greenhouse gases, and financial gains through the sale of carbon credits. Achieving better understanding of the sequestration process has motivated many deterministic models of soil carbon dynamics, but none of these models address uncertainty in a comprehensive manner. Uncertainty arises in many ways-around the model inputs, parameters, and dynamics, and subsequently model predictions. In this paper, these uncertainties are addressed in concert by incorporating a physical-statistical model for carbon dynamics within a Bayesian hierarchical modelling framework. This comprehensive approach to accounting for uncertainty in soil carbon modelling has not been attempted previously. This paper demonstrates proof-of-concept based on a one-pool model and identifies requirements for extension to multi-pool carbon modelling. Our model is based on the soil carbon dynamics in Tarlee, South Australia. We specify the model conditionally through its parameters, soil carbon input and decay processes and observations of those processes. We use a particle marginal Metropolis-Hastings approach specified using the LibBi modelling language. We highlight how samples from the posterior distribution can be used to summarise our knowledge about model parameters, to estimate the probabilities of sequestering carbon and to forecast changes in carbon stocks under crop rotations not represented explicitly in the original field trials. Copyright © 2014 John Wiley & Sons, Ltd. [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]

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]

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]

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]