Technologies for increasing carbon storage in soil to mitigate climate change

Means to enhance storage of carbon in soil or avoid its loss from soil are discussed and examined from the viewpoint of policy. In particular, technologies that have until now received little attention are assessed. The main means by which soil carbon might be increased are first listed. These are the following: (i) increasing the rate of input of organic matter; (ii) decreasing the rate of its decomposition by biological or chemical means; (iii) increasing the rate of its stabilization by physico-chemical protection within aggregates and organo-mineral complexes; and (iv) increasing the depth or more correctly the total soil volume sequestering carbon at maximum rate. Immediate gains in carbon storage might be made by switching to more perennial crops, especially grasses that, as a result of breeding, are able to put more carbon into soil. In the longer term, targets for research such as understanding the role of enzymes in carbon turnover and the exploitation of the capacity in subsoils are suggested. Increased fixation of CO2 as inorganic carbonate in soils by application of silicate wastes may have some role

A simple reactive-transport model of calcite precipitation in soils and other porous media

Calcite formation in soils and other porous media generally occurs around a localised source of reactants, such as a plant root or soil macro-pore, and the rate depends on the transport of reactants to and from the precipitation zone as well as the kinetics of the precipitation reaction itself. However most studies are made in well mixed systems, in which such transport limitations are largely removed. We developed a mathematical model of calcite precipitation near a source of base in soil, allowing for transport limitations and precipitation kinetics. We tested the model against experimentally-determined rates of calcite precipitation and reactant concentration–distance profiles in columns of soil in contact with a layer of HCO3−-saturated exchange resin. The model parameter values were determined independently. The agreement between observed and predicted results was satisfactory given experimental limitations, indicating that the model correctly describes the important processes. A sensitivity analysis showed that all model parameters are important, indicating a simpler treatment would be inadequate. The sensitivity analysis showed that the amount of calcite precipitated and the spread of the precipitation zone were sensitive to parameters controlling rates of reactant transport (soil moisture content, salt content, pH, pH buffer power and CO2 pressure), as well as to the precipitation rate constant. We illustrate practical applications of the model with two examples: pH changes and CaCO3 precipitation in the soil around a plant root, and around a soil macro-pore containing a source of base such as urea

The solution of convection–diffusion equations for solute transport to plant roots

To help develop models of solute uptake that are computationally efficient and suitable for up-scaling to whole root systems, we provide three alternative analytical solutions of equations for transport to absorbing roots by convection and diffusion, and we compare their performance with a numerical solution over the range of plant and soil conditions found in practice. We point out an important pitfall in commonly used methods to solve convection–diffusion equations numerically and show how it can be avoided. We identify a simple analytical solution that is valid for all realistic combinations of parameter values, and show that for most purposes it performs as well as a complete, but more complex, analytical solution allowing fully for both convection and diffusion. We conclude that this solution is suitable for use in general solute uptake models

Sur la prediction du devenir du cesium radioactif dans le sol de prairie.

International audienc

Phosphate sorption-desorption behaviour and phosphorus release characteristics of three contrasting lowland rice soils of Cambodia

Understanding the P sorption and release characteristics of different soils can help in determining fertiliser P requirements for the growth of rice (Oryza sativa L.). Glasshouse and laboratory experiments were undertaken to observe the long-term release characteristics of P from added fertiliser for the early growth of rice, and also to determine P sorption-desorption behaviour of three contrasting lowland soils from Cambodia: Prateah Lang (Plinthustalf), Koktrap (Plinthaquult) and Toul Samroung (Endoaqualf). In the pot experiment, rice was treated with four P rates (0, 5, 10 and 20 mg/kg soil) and grown over five successive cropping cycles, each of six to eight weeks. Phosphorus sorption desorption isotherms were constructed by equilibrating with 0, 10, 20 and 40 mg P/1 in 0.01 M CaCl2 solution at 25 oC. On the sandy Prateah Lang (PL) and clayey Toul Samroung (TS) soils, addition of 10 mg P/kg soil was adequate in the first crop for maximum tiller number, plant height, total dry matter, P concentration, and total P uptake. By contrast, about 20 mg P/kg was needed for the maximum growth and total P uptake on the clayey acid Koktrap (KT) soil. After two crops, plant growth progressively declined at all P levels, but the decrease in yields and total P uptake on the clayey TS and KT soils was slower than for plants grown on the sandy PL soil. Resin-P extractable was the smallest P fraction compared to other major soil-P (NaOH-Pi, NaOH-Po and Residual-P) pools in all soil groups, but recovery from the Resin-P pool was higher in the sandy PL soil than in the clayey TS and KT soils. The declining amounts recovered from all the extractable soil P fractions, especially major soil P (NaOH-Pi and Po and Residual-P) pools with succeeding rice crops grown on all the soils could be attributed to continued reactions of the added P fertiliser by soils in addition to plant P uptake during each plant-growing cycle. The clayey KT and TS soils sorbed five-fold more P than the sandy PL soil in oxidized conditions. Phosphorus desorption was initially greatest from the sandy PL soil: but with increasing numbers of soil extractions, the release of sorbed P declined faster than in the clayey KT and TS soils. The cumulative desorbed P was greater from the clayey KT and TS soil than from the sandy PL soil. The greater P sorbed by the clayey soils should ensure a longer duration of the residual P effect