ot Soil ecology KEN KILLHAM Department of Plant and Soil Science, University of Aberdeen with electron micrographs by R A L P H FOSTER, CSIRO Division of Soils, South Australia CAMBRIDGE UNIVERSITY PRESS
Contents Foreword Introduction xv xvii 1 The soil environment Introduction The soil as a source of energy and nutrients to the biota Soil minerals as a source of nutrients Soil organic matter as a source of nutrients Living biomass as a source of nutrients Structural aspects of the soil habitat Soil water Water stress Soil water and nutrient supply to the soil biota Soil atmosphere The soil atmosphere as a source of carbon and nutrients to the soil biota Soil redox potential Soil ph Soil temperature Soil light Soil microsites Conclusions 1 1 2 3 6 7 10 12 16 16 19 20 24 28 31 32 33 2 The soil biota 34 Introduction 34 Plant roots 34 IX
x Contents Soil microbes - bacteria, actinomycetes, fungi and algae 40 Introduction 40 Bacteria 41 Actinomycetes 46 Fungi 47 Algae 52 Protozoa (microfauna) 54 Slime moulds 57 Larger soil fauna (meso- and macrofauna) 57 Introduction 57 Oligochaetes - earthworms and enchytraeid worms 58 Nematodes 59 Arthropods 60 Molluscs 60 Soil viruses 61 3 Ecological interactions between the soil biota 62 Introduction 62 Plant/microbial interactions 62 The legume/r/iizobi'wm association 62 The mycorrhizal association - a plant root/fungus interaction 66 The rhizosphere - a zone of interaction between plant roots and the soil biota 71 Animal/microbial interactions 75 Animal/animal interactions 76 Plant/animal interactions 77 Microbe/microbe interactions 78 Complex interactions 79 Soil biological community dynamics 79 Root population dynamics 79 Microbial population dynamics 80 Competition between organisms in soil 83 Prey/predator interactions and microbial population dynamics in soils 86 Population dynamics of soil animals 87
Contents xi 4 The ecology of soil nutrient cycling 89 Introduction 89 The carbon cycle 89 Introduction - the global carbon cycle 89 C-inputs to the soil carbon cycle 90 The turnover of carbon in soils 92 Forms of soil organic carbon 93 Decomposition of different forms of organic carbon in soil 95 Rates of carbon turnover 100 Factors affecting the rate of carbon turnover in soil 102 Decomposition of organic carbon in soil - production of stable soil organic matter 103 Decomposer communities in the soil 105 The interaction between the carbon cycle and other soil nutrient cycles 106 The nitrogen cycle 108 Introduction 108 N-inputs to the soil/plant system 109 Immobilisation/mineralisation reactions and plant N-uptake 114 Nitrification 124 Denitrification 131 Dissimilatory reduction of nitrate to ammonium 136 Ammonia volatilisation 136 The use of 15 N in the study of soil N-cycling 138 The sulphur cycle 141 Introduction 141 S-inputs to the soil/plant system 142 Sulphur mineralisation/immobilisation reactions in soil 143 Oxidation of inorganic S-compounds in soil 145 Reduction of oxidised sulphur compounds in soil 148 Volatilisation of sulphur from soil 149 5 Ecology of extreme soil environments - soil water stress 151 Introduction 151 Soil water/salt stress and the plant 152 The plant root and soil water stress 154 Mycorrhizal response to soil water stress 157
xii Contents Mechanisms of mycorrhizal mediation of plant drought tolerance 158 Phosphorus nutrition and mycorrhizal response to water stress 159 Water stress and the mycorrhizal infection process 160 Water stress and soil microbes 160 Water stress and soil animals 164 Conclusions 166 6 Ecology of polluted soils 167 Introduction 167 Ecology of soils polluted from acid deposition 167 Introduction 167 Effects of acid deposition on soil processes 167 Factors controlling the extent of acid deposition effects 168 Acid deposition and soil ecology - mechanisms of effects 170 Changes in the soil biota due to acid deposition 172 Acid deposition and effects in the rhizosphere 173 Acid rain and soil animals 174 Conclusions 174 Ecology of soils polluted with radionuclides 175 Introduction 175 Dynamics of radionuclides in soil 175 Cycling of radionuclides by the soil biota 177 Modelling of radionuclide movement in the soil/plant system 181 Conclusions 181 7 Manipulation of soil ecology - 'soil biotechnology' 182 Introduction 182 Biofertilisation by rhizobial inoculation 182 Potential for inoculation - temperate and tropical agriculture 182 Rhizobial effectiveness and inoculum production 183 Factors affecting inoculum success 184 Improvement of rhizobial inocula 185 Control of the soil biota 186 Pesticidal control 186 Biological control 196
Contents xiii Biofertilisation by mycorrhizal fungal inoculation 198 Potential benefits of mycorrhizal fungal inoculation 198 Ectomycorrhizas 200 Endomycorrhizas 201 Factors affecting the competitiveness of mycorrhizal fungal inocula 202 Conclusions 203 Genetically modified plants and microbes for use in the environment 203 Introduction 203 Microbial symbionts such as N 2 -fixing rhizobia and mycorrhizal fungi 204 Genetic enhancement of the effectiveness of microbial inocula for antagonistic control of soil-borne plant pathogens 205 Cloning of genes involved in the bacterial production of insecticidal toxins 206 Genetic modification of microbial inocula for the production of plant growth regulators 207 Genetic modification of microbial inocula for the detoxification of contaminated soil - bioremediation 207 Soil ecological effects of the use of genetically modified plants and microbes 208 The fate of recombined DNA in soil 210 Conclusions 211 References 212 Further reading 222 Index 231