G. J. D. Kirk

Carbon losses from all soils across England and Wales 1978-2003

More than twice as much carbon is held in soils as in vegetation or the atmosphere, and changes in soil carbon content can have a large effect on the global carbon budget. The possibility that climate change is being reinforced by increased carbon dioxide emissions from soils owing to rising temperature is the subject of a continuing debate. But evidence for the suggested feedback mechanism has to date come solely from small-scale laboratory and field experiments and modelling studies. Here we use data from the National Soil Inventory of England and Wales obtained between 1978 and 2003 to show that carbon was lost from soils across England and Wales over the survey period at a mean rate of 0.6% yr-1 (relative to the existing soil carbon content). We find that the relative rate of carbon loss increased with soil carbon content and was more than 2% yr-1 in soils with carbon contents greater than 100 g kg-1. The relationship between rate of carbon loss and carbon content is irrespective of land use, suggesting a link to climate change. Our findings indicate that losses of soil carbon in England and Wales—and by inference in other temperate regions—are likely to have been offsetting absorption of carbon by terrestrial sinks.

The biogeochemistry of submerged soils

Preface.Acknowledgements.1. Introduction.1.1 Global Extent of Submerged Soils and Wetlands.1.2 Biogeochemical Characteristics.1.3 Types of Submerged Soil.1.3.1 Organic Soils.1.3.2 Mineral Soils.1.3.3 Relation between Soils and Landform.2. Transport Processes in Submerged Soils.2.1 Mass Flow.2.2 Diffusion.2.2.1 Diffusion Coefficients in Soil.2.2.2 Propagation of pH Changes Through Soil.2.3 Ebullition.2.4 Mixing by Soil Animals.3. Interchange of Solutes between Solid, Liquid and Gas Phases.A. WATER.3.1 Composition of the Water.3.1.1 Acid and Bases.3.1.2 Speciation.3.1.3 Equilibrium Calculations.3.2 pH Buffer Capacity.3.3 Equilibrium with the Gas Phase.3.3.1 Floodwater CO2 Dynamics.3.4 Gas Transport Across the Air-Water Interface.3.4.1 CO2 Transfer Across the Air-Water Interface.B. SOIL.3.5 The Solid Surfaces in Soils.3.6 The Solid Surfaces in Submerged Soils.3.6.1 Organic Matter in Submerged Soils.3.7 Solid-Solution Interactions.3.7.1 Adsorption.3.7.2 Precipitation.3.7.3 Co-Precipitation in Solid Solutions.3.7.4 Inhibition of Precipitation.3.7.5 Equations for Solid-Solution Interactions.4. Reduction and Oxidation.4.1 Thermodynamics and Kinetics of Redox Reactions.4.1.1 Electron Activities and Free Energy Changes.4.1.2 Redox Potentials.4.1.3 Relation between pe and Concentration of Redox Couples.4.1.4 pe-pH Diagrams.4.1.5 Energetics of Reactions Mediated by Microbes.4.2 Redox Conditions in Soils.4.2.1 Changes with Depth in the Soil.4.2.2 Changes with Time.4.2.3 Calculated Changes in pe, pH and Fe During Soil Reduction.4.2.4 Measurement of Redox Potential in Soil.4.3 Transformations of Nutrient Elements Accompanying Changes in Redox.4.3.1 Transformations of Carbon.4.3.2 Transformations of Nitrogen.4.3.3 Transformations of Sulfur.4.3.4 Transformations of Phosphorus.4.4 Oxidation of Reduced Soil.4.4.1 Kinetics of Fe2+ Oxidation.4.4.2 Simultaneous Diffusion and Oxidation in Soil.5. Biological Processes in the Soil and Floodwater.5.1 Microbiological Processes.5.1.1 Processes Involved in Sequential Reduction.5.1.2 Nitrate Reduction.5.1.3 Iron and Manganese Reduction.5.1.4 Sulfate Reduction.5.1.5 Methanogenesis.5.1.6 Aerobic Processes.5.2 Macrobiological Processes.5.2.1 Net Primary Production and Decomposition.5.2.2 The Floodwater-Soil System.5.2.3 Floodwater Properties.5.2.4 Floodwater Flora.5.2.5 Fauna.5.3 Is Biodiversity Important?6. Processes in Roots and the Rhizosphere.6.1 Effects of Anoxia and Anaerobicity on Plant Roots.6.1.1 Adaptations to Anoxia.6.1.2 Armstrong and Becketts Model of Root Aeration.6.2 Architecture of Wetland Plant Root Systems.6.2.1 Model of Root Aeration versus Nutrient Absorption.6.2.2 Root Surface Required for Nutrient Absorption.6.3 Nutrient Absorption Properties of Wetland Plant Roots.6.3.1 Ion Transport in Roots.6.3.2 Ion Transport in Wetland Roots.6.4 Root-Induced Changes in the Soil.6.4.1 Oxygenation of the Rhizosphere.6.4.2 The pH Profile Across the Rhizosphere.6.5 Consequences of Root-induced Changes.6.5.1 Nitrification-Denitrification in the Rhizosphere.6.5.2 Solubilization of Phosphate.6.5.3 Solubilization of Zinc.6.5.4 Immobilization of Cations.6.6 Conclusions.7. Nutrients, Toxins and Pollutants.7.1 Nutrient and Acidity Balances.7.1.1 Nutrient Balances in Ricefields.7.1.2 Acidity Balances in Ricefields.7.1.3 Peat Bogs.7.1.4 Riparian Wetlands.7.1.5 Tidal Wetlands.7.2 Toxins.7.2.1 Acidity.7.2.2 Iron Toxicity.7.2.3 Organic Acids.7.2.4 Salinity.7.3 Trace Elements.7.3.1 Global Cycling of Trace Elements.7.3.2 Transport Through Soil and into Plant Roots.7.3.1 Mobilities of Individual Trace Elements.8. Trace Gases.8.1 Methane.8.1.1 Global Budget.8.1.2 Processes Governing Methane Emissions from Rice.8.1.3 Modelling Methane Emission.8.1.4 Estimating Emissions at the Regional Scale.8.1.5 Possibilities For Decreasing Emissions.8.2 Nitrogen Oxides.8.2.1 Global Budget.8.2.2 Processes Governing Nitrous and Nitric Oxide Emissions from Rice.8.2.3 Differences between Rice Production Systems.8.3 Ammonia.8.3.1 Global Budget.8.3.2 Processes Governing Ammonia Emissions from Rice.8.4 Sulfur Compounds.8.4.1 Global Budget.8.4.2 Emissions from Ricefields.8.5 Carbon Sequestration.References.Index.

Opportunities for increased nitrogen-use efficiency from improved lowland rice germplasm

Understanding of the mechanisms governing the efficient use of N by rice plants—both its acquisition and internal use—is reviewed. Acquisition efficiency is considered in terms of root properties influencing the absorption and assimilation of NH+4 and other N species, and their regulation; root-induced changes in the rhizosphere affecting N mineralization, transformation and transport; and root-associated biological N2 fixation. Efficiency of internal use is considered in terms of the translocation, distribution and remobilization of absorbed N in different plant organs, flag leaf N import/export and leaf senescence patterns, and the efficiency with which N is used in CO2 fixation. Evidence for genetic variation in both acquisition efficiency and internal-use efficiency is given for plants under N-sufficient and N-limited conditions. The possibility of incorporating in rice the machinery for N2 fixation is discussed.

Phosphate solubilization by organic anion excretion from rice (Oryza sativa L.) growing in aerobic soil

A mathematical model of P solubilization by organic anion excretion from roots is described and used to account for P solubilization by rice (Oryza sativa L.) plants growing in aerobic soil. In previous experiments with rice in an aerobic, highly-weathered, P-deficient soil, we found that the plants were able to solubilize P from an alkali-soluble pool and thereby increase their P uptake. The solubilization could not be explained by pH changes nor by the release of phosphatases. In subsequent experiments we found excretion of citrate from rice roots into nutrient solutions, and the synthesis and excretion of citrate tended to increase under P starvation. The model allows for the diffusion of the organic anion away from a root, its decomposition by soil microbes, its reaction with the soil in solubilizing P, and diffusion of the solubilized P back towards the root as well as away from it. We calculated the rate of citrate excretion from rice roots growing in soil based on measured steady-state citrate concentrations in the rhizosphere and calculated rates of decomposition. Calculations using these and other model parameters obtained independently showed that the observed solubilization and increased P uptake by rice growing in soil could be accounted for. A sensitivity analysis of the model is given.

Opportunities to improve phosphorus efficiency and soil fertility in rainfed lowland and upland rice ecosystems

Abstract The use of improved germplasm and management to increase rice production in rainfed lowland and upland systems is discussed. Understanding of the mechanisms conferring P efficiency, particularly external efficiency through root-induced changes in the rhizosphere, is reviewed together with evidence for genetic variation in P efficiency in upland and lowland rice germplasm. The following areas for improving resource and input management are considered: P management under alternately wet and dry soil conditions; P fertilizer formulations for banding in highly-weathered soils; cumulative responses to P fertilizer, including interactions with biological N fixation by legumes; amelioration of subsoil acidity, especially by leaching down the effects of surface-applied lime; and the management of soil spatial variability. Areas for future research are given.

Progress in selected areas of rhizosphere research on P acquisition

Large reserves of P have accumulated in soils of developed countries because additions of P fertiliser to sustain agricultural production have exceeded crop removal. By contrast, in many developing countries in the tropics and subtropics, soil P reserves are gravely low and large additions are required before maintenance requirements begin to decline. In addition, the cost of P fertiliser will increase as the currently accessible deposits of high-grade phosphate rock (PR) diminish. Developing plants that efficiently tap soil P reserves and low grade PR is therefore a priority for agricultural research. For the 50th anniversary of the New Zealand Soil Science Society, this paper reviews research on P efficiency in plants, conducted by staff, students, and research associates of Massey University, in the context of other research into plant mechanisms that enhance P uptake, including effects of root geometry, mycorrhizal associations, and root-induced changes in the soil. Techniques for fractionation of soil P are highlighted.

Evidence for the mechanisms of zinc uptake by rice using isotope fractionation.

In an earlier study, we found that rice (Oryza sativa) grown in nutrient solution well-supplied with Zn preferentially took up light (64)Zn over (66)Zn, probably as a result of kinetic fractionation in membrane transport processes. Here, we measure isotope fractionation by rice in a submerged Zn-deficient soil with and without Zn fertilizer. We grew the same genotype as in the nutrient solution study plus low-Zn tolerant and intolerant lines from a recombinant inbred population. In contrast to the nutrient solution, in soil with Zn fertilizer we found little or heavy isotopic enrichment in the plants relative to plant-available Zn in the soil, and in soil without Zn fertilizer we found consistently heavy enrichment, particularly in the low-Zn tolerant line. These observations are only explicable by complexation of Zn by a complexing agent released from the roots and uptake of the complexed Zn by specific root transporters. We show with a mathematical model that, for realistic rates of secretion of the phytosiderophore deoxymugineic acid (DMA) by rice, and realistic parameters for the Zn-solubilizing effect of DMA in soil, solubilization and uptake by this mechanism is necessary and sufficient to account for the measured Zn uptake and the differences between genotypes.

Tolerance of rice germplasm to zinc deficiency

A database is described containing the results of screening trials for tolerance to zinc (Zn) deficiency and other soil stresses made by IRRI and collaborators over the past 25 years. The data are scores based on visual symptoms in field and greenhouse screenings and yield data from field trials. The database includes search and retrieval functions. It can be downloaded from ftp://ftp.cgiar.org/icis. The data are used to explore differences in tolerance and relations with other plant and environmental variables. It is concluded that there is useful variation in tolerance to Zn deficiency in the rice germplasm that could be exploited in breeding programs. There appears to be no yield cost associated with tolerance, and tolerant genotypes often also have tolerance to salinity and P deficiency for reasons that are not apparent. Tolerance can be identified satisfactorily by scoring for visual symptoms in early growth stages in well-managed Zn-deficient soils. This simple procedure lends itself to large-scale screening, for example of populations for gene mapping.

Measurement of zinc stable isotope ratios in biogeochemical matrices by double-spike MC-ICPMS and determination of the isotope ratio pool available for plants from soil

AbstractAnalysis of naturally occurring isotopic variations is a promising tool for investigating Zn transport and cycling in geological and biological settings. Here, we present the recently installed double-spike (DS) technique at the MAGIC laboratories at Imperial College London. The procedure improves on previous published DS methods in terms of ease of measurement and precisions obtained. The analytical method involves addition of a 64Zn–67Zn double-spike to the samples prior to digestion, separation of Zn from the sample matrix by ion exchange chromatography, and isotopic analysis by multiple-collector inductively coupled plasma mass spectrometry. The accuracy and reproducibility of the method were validated by analyses of several in-house and international elemental reference materials. Multiple analyses of pure Zn standard solutions consistently yielded a reproducibility of about ±0.05‰ (2 SD) for δ66Zn, and comparable precisions were obtained for analyses of geological and biological materials. Highly fractionated Zn standards analyzed by DS and standard sample bracketing yield slightly varying results, which probably originate from repetitive fractionation events during manufacture of the standards. However, the δ66Zn values (all reported relative to JMC Lyon Zn) for two less fractionated in-house Zn standard solutions, Imperial Zn (0.10 ± 0.08‰: 2 SD) and London Zn (0.08 ± 0.04‰), are within uncertainties to data reported with different mass spectrometric techniques and instruments. Two standard reference materials, blend ore BCR 027 and ryegrass BCR 281, were also measured, and the δ66Zn were found to be 0.25 ± 0.06‰ (2 SD) and 0.40 ± 0.09‰, respectively. Taken together, these standard measurements ascertain that the double-spike methodology is suitable for accurate and precise Zn isotope analyses of a wide range of natural samples. The newly installed technique was consequently applied to soil samples and soil leachates to investigate the isotopic signature of plant available Zn. We find that the isotopic composition is heavier than the residual, indicating the presence of loosely bound Zn deposited by atmospheric pollution, which is readily available to plants. FigureZinc isotope ratio pools of bulk soil and the associated acid leach (estimated plant available pool) as measured by double-spike MC-ICPMS. δxZnLyon-JMC=(Rsample/RJMC-Lyon -1)x103, where Rsample and RJMC-Lyon denote the xZn/64Zn isotope ratio of the sample and standard (JMC-Lyon), respectively, and where x denotes either 66 or 68.

Plant-mediated processess to acquire nutrients: nitrogen uptake by rice plants

The ways in which root–soil interactions can control nutrient acquisition by plants is illustrated by reference to the N nutrition of rice. Model calculations and experiments are used to assess how uptake is affected by root properties and N transport through the soil. Measurements of the kinetics of N absorption and assimilation and their regulation, and of interactions between NH4+ and NO3− nutrition, are described. It is shown that uptake of N from the soil–-as opposed to N in ricefield floodwater which can be absorbed very rapidly but is otherwise lost by gaseous emission–-will often be limited by root uptake properties. Rice roots are particularly efficient in absorbing and assimilating NO3−, and NH4+ absorption and assimilation are stimulated by NO3−. The uptake of NO3− formed in the rice rhizosphere by root-released O2 may be more important than previously thought, with beneficial consequences for rice growth. Other root-induced changes in the rice rhizosphere and their consequences are discussed.