Search

Long-term changes in soil organic carbon (SOC) resulting from management change are documented for many experimental situations, and corresponding trends in the field have been observed by national survey. Since these changes are relevant to atmospheric carbon balance a practical measure to confirm the impact of recent management decisions at any location, without resorting to repeated sampling, is highly attractive but none has previously been tested. This study assessed intra-aggregate C to fulfil the role, based on a temporary deviation from its predictable contribution to total SOC under stable management. A total of 166 surface soil samples (0-15 cm) were analyzed for intra-aggregate C using an established physical fractionation protocol or compatible scaled-up procedure. Soils were arable (or ley-arable) managed by conventional or minimum-tillage, or permanent grassland, and assigned 'stable' or 'changing' status on the basis of a verbal account of management history. Log-normal populations of intra-aggregate C were compared for soils of stable and changing status using F-tests. Intra-aggregate C shows promise as an indicator of changing SOC in arable soils up to 30% clay content, particularly soils < 20% clay. A larger dataset is required to establish its utility in grassland soils. It is not certain that intra-aggregate C is capable of confirming direction of change or trajectory (endpoint), and functions to indicate change, rather than confirm stable status. Supplementary information on the history of soil use and management is therefore essential in the interpretation of such measurements.

Knowledge of the stocks and the potential range of soil organic carbon (SOC) in various land–soil combinations is an important precursor to policies aimed at linking, for example, management of SOC to greenhouse gas emission controls. To investigate the factors controlling the percentage of SOC (%SOC) of soils in England and Wales, we made a multiple regression analysis of data for the 2448 arable and ley-arable sites in the 1980 England and Wales National Soil Inventory (NSI). Clay content, average annual precipitation and depth of topsoil explained 25.5% of the variation in %SOC, when calcareous and peaty soils and those susceptible to flooding were excluded. Using ‘robust’ statistics, ‘indicative SOC management ranges’ were estimated for different physiotopes, that is, landscape units for which the environmental factors governing %SOC are similar, namely soil clay content and precipitation. These ranges describe the expected %SOC range for an arable soil in a given physiotope. They have potential to support approximate targets for the %SOC of arable soils and for estimating upper and lower limits for sequestered soil carbon in arable systems.

Current models of soil organic matter (SOM) turnover tend to invoke pools that are defined by their contrasting first-order reactivity constants but which cannot be directly measured. New models may be based around fractions defined by procedures that can be used to isolate them experimentally. The drawback of such fractions is that they may display properties that are not sufficiently distinct or which vary in time or space. In this study the properties of two fractions from soils of contrasting geographical origin and under different nutrient management were examined using 13 C nuclear magnetic resonance (NMR) spectroscopy. The fractions were free SOM (FR-SOM, discrete organic particles located between stable aggregates), and intra-aggregate SOM (IA-SOM, discrete organic particles within stable aggregates). The composition of both fractions was highly consistent across soil types and environments, but the fractions differed significantly in the proportion of C present in five of the seven functional C groups identified by NMR (P

We used thermal analysis to compare the proportions of active and more stable components in soil organic matter (SOM) fractions and whole soil under contrasting agricultural land-uses. The fractions (free light, intra-aggregate, and organomineral fractions) were isolated using density fractionation. Exothermic weight loss between 300 and 350°C was attributed to a relatively labile portion comprising carboxyl and aliphatic C, and that between 400 and 450°C to the decomposition of material rich in aromatic components. Under arable cultivation, free light SOM showed much greater weight loss in the first exothermic range than intra-aggregate SOM. In soil receiving very small inputs of organic matter (a long-term bare fallow) the free light and intra-aggregate fractions displayed similar characteristics and resembled the intra-aggregate fraction from the arable soil. The difference between the free light and intra-aggregate fractions was also small for the grassland soil but the fractions resembled the free light fraction from the arable soil. Small total weight loss for whole soil and organomineral fractions demonstrated the value of physical fractionation techniques in establishing the effect of land-use on SOM with greater precision than is possible whole (unfractionated) soil.

Methane production in soil is a microbiological process that occurs under strict anaerobic conditions. A laboratory incubation study was conducted using 10 wetland rice (Oryza sativa L.) soils from the Philippines to establish soil characteristics that govern the redox characteristics and CH 4 production of soils upon wetting. Labile soil organic matter in the Ap horizon was quantified as enrichment of C and N and non-clay-protected C and N fractions. The C/N ratio of the enriched fraction controlled the rate of reduction upon flooding, whereas the buffering capacity, measured as extractable Fe 3+ content, controlled the reduction capacity for the soils. Methane production was influenced both by the reduction characteristics of the soils and labile organic substrates.

Monitoring and modeling the dynamics of N in flooded rice fields require reliable methods for both sampling and analysis. Destructive soil sampling and centrifugation and in situ sampling of soil solution using microporous polymer tubes (rhizon soil solution samplers; RSSS) were used to study the dynamics and variability of NH⁺₄ in two field experiments. Both methods gave similar results in unfertilized treatments, whereas significant differences were measured in treatments with N application. The RSSS were suited for measuring point dynamics or microvariation of nutrients in the root zone, whereas soil-core sampling was a better method to obtain quantitative N data integrated across a particular depth range. We observed a highly significant linear relationship between the logs of soil exchangeable and solution NH⁺₄ only until panicle initiation (PI). Microvariability accounted for a large proportion of the within-field variation and was significantly reduced by bulking of soil cores. Coefficients of variation were higher in the fertilized field compared with an unfertilized field and were changing throughout the growth period of rice, with the highest variability between PI and flowering. Sampling to obtain representative mean values at the plot or field scale can be optimized with regard to temporal changes in the spatial distribution patterns of NH⁺₄. Before transplanting (TP) and at the end of the growth period, less samples are needed and can be taken as composite samples of the whole puddled soil layer. Between TP and flowering, more samples are needed and should be taken in several depth ranges.