The Production of Plant Growth Regulators in Soil through Interactions between Earthworms, Microorganisms and Organic Materials

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A. PROJECT SUMMARY There is considerable evidence that earthworms can have major influences on the growth of plants in natural and managed ecosystems. It is well-established that the key roles of earthworms in soils are: the fragmentation and decomposition of organic matter, the promotion of microbial activity and the release and transformation of plant nutrients. However, the contributions of earthworms to improvements in plant growth often greatly exceed the potential that they have to improve soil structure and nutrient availability. There is very good research evidence that interactions between earthworms and microorganisms may influence plant growth through various biological factors that are not fertility-mediated, e.g. by producing substances that promote plant growth similar in effect to or even possibly including auxins, gibberellins or cytokinins. Such a possibility, if occurring widely enough to affect the growth of plants significantly, has important implications for both the growth of plants in .natural ecosystems as well as crop production in managed ecosystems, and would have-important ecological and economic implications for the role of the soil biota in overall plant productivity. During the last 2-3 years it has become increasingly clearly demonstrated that soil microorganisms can produce significant amounts of plant growth regulators (PGRs). This raises the question whether earthworms can influence the production of PGRs indirectly, through increasing microbial activity and as a consequence affect plant growth significantly, independent of their infiuences on nutrient availability. This project focuses on establishing whether earthworms can stimulate the overall production of plant growth-regulating substances in soil, what form they take, whether they occur in amounts sufficient to influence plant growth significantly, and how long they persistS. The three main objectives in this project are: To determine, using laboratory plant bioassays, whether plant grow th-influencing substances (PGIs), other than nutrients, are produced through in the soil through the activities of earthworms in organic matter breakdown. To isolate, characterize, identify and quantify, using physico-chemical methods, some specific plant growth-regulating compounds (PGRs) produced through the activities of earthworms in organic matter breakdown. To determine whether plant growth can be influenced significantly by plant growth regulators (PGRs) produced in the soil through the activities of earthworms in organic matter breakdown. The project will attempt to confirm and quantify the production of PGRs by interactions between two species of earthworm and microorganisms on two kinds of plant organic matter in laboratory cultures. It will also assess the production of PGRs in the soils of two long-term field experiments with manipulated earthworm populations. The influence of PGR's in aqueous extracts from earthworm cultures and from earthworm-manipulated field soils on growth of vegetables and other plants will be integrated .