The success of shifting cultivation systems developed by subsistence farmers testifies to the resilience of the "natural" soil-plant ecosystems to recover from the offtake of nutrients in crops and loss of soil struc­ ture. By contrast, the development of intensive cropping systems requires large inputs especially of nitrogen, together with phosphorus, sulphur and other essential elements in order to maintain the nutrient levels needed for abundant crop yields. As Dr. Cooke ably pointed out in his introductory lecture, the dis­ coveries and experiments of the 19th century encouraged farmers in temperate zones to rely greatly on chemical fertilizers supplements. However, the work of Charles Da{win on soil mixing by earthworms and the discovery by Hellriegel and Wilfarth in 1886 that the nodules on legume roots contain colonies of symbiotic bacteria able to "capture" atmospheric nitrogen molecules to the benefit of the host plant heralded a growing realization of the importance of soil biota in fertility studies. Biological fixation of nitrogen has been the theme of many meetings and publi~ations hitherto but at this Conference, convened on the delightful campus of Reading University, attention was mainly focussed on other biological processes in soil fertility. These Proceedings record the dominant themes and include six keynote addresses delivered at plenary sessions and seven introductory lectures to paper reading sessions by invited individuals plus 22 of the proferred papers, in six sections as tabled in the contents list.

Inhalt

Section 1: Cycling of nitrogen in soil-plant systems.- 1.1 Cycling of nitrogen in modern agricultural systems.- 1.2 Nitrogen relationships in intensively managed temperate grasslands.- 1.3 Components of the nitrogen cycle measured for cropped and grassland soil-plant systems.- 1.4 The uptake of soil and fertilizer-nitrogen by barley growing under Scottish climatic conditions.- 1.5 Leaching losses of nitrogen from a clay soil under grass and cereal crops in Finland.- 1.6 Tillage effects on nitrogen uptake by maize from fine textured soils in the northwestern Corn Belt, USA.- Section 2: Microbial and animal effects on soil fertility.- 2.1 The role of microorganisms in mediating and facilitating the uptake of plant nutrients from soil.- 2.2 Earthworms and soil fertility.- 2.3 Effect of lime, phosphorus and mycorrhizal fungi on growth, nodulation and nitrogen fixation by white clover (Trifolium repens) grown in UK hill soils.- 2.4 Effect of liming on spore germination, germ tube growth and root colonization by vesicular-arbuscular mycorrhizal fungi.- Section 3: Measurements of microbial populations and biomass: their roles in soil processes.- 3.1 Biological and biochemical analysis of soils.- 3.2 Extractable N and P in relation to microbial biomass in UK acid organic soils.- 3.3 Mineralization dynamics in fallow dryland wheat plots, Colorado.- 3.4 Dynamics of soil microbial biomass N under zero and shallow tillage for spring wheat, using 15N urea.- 3.5 Mineralization of C and N from microbial biomass in paddy soil.- 3.6 Influence of plant roots on C and P metabolism in soil.- 3.7 Response of soil microflora to moorland reclamation for improved agriculture.- Section 4: Anaerobic processes in soils and gaseous losses of nitrogen.- 4.1 Anaerobic processes in soil.- 4.2 Denitrification in field soils.- 4.3 Total denitrification and the ratio between N2O, and N2 during the growth of spring barley.- 4.4 Nitrite: a key compound in N loss processes under acid conditions?.- Section 5: Dynamics and models of organic carbon, nitrogen and phosphorus transformations in cultivated soils.- 5.1 The biological transformation of P in soil.- 5.2 Modelling C and N turnover through the microbial biomass in soil.- 5.3 Dynamics of organic matter in soils.- 5.4 Concepts of soil organic matter transformations in relation to organo-mineral particle size fractions.- 5.5 A simple method for calculating decomposition and accumulation of 'young' soil organic matter.- Section 6: Interaction effects of organisms, organic matter and management on soil structure.- 6.1 Interactions between biological processes, cultivation and soil structure.- 6.2 Soil organic matter and structural stability: mechanisms and implications for management.- 6.3 Influence of soil type, crop and air drying on residual carbohydrate content and aggregate stability after treatment with periodate and tetraborate.- 6.4 Changes in soil structure and biological activity on some meadow hay plots at Cockle Park, Northumberland.- 6.5 The effects on topsoil of long-term storage in stockpiles.- Section 7: Effects of noxious materials on biological processes in soils.- 7.1. Effects of pesticides and heavy metals on biological processes in soil.- 7.2 Microbial populations in trifluralin-treated soil.- 7.3 Chemical disinfestation and metabolic integrity of soil.- Index of key words.