Control of plant pathogens by chemical means is frequently associated with harmful residues in edible goods and in many cases the production of chemical pesticides and their application causes major negative health and environmental impacts. Plants produce a wide variety of secondary plant metabolites, known as phytochemicals (PCH) that are involved in the interaction of plants/pests/diseases. Among PCH substances, glucosinolates (GLU) represent the most studied and important group of phytochemicals and are usually found in Brassica species. There is increasing evidence that bioactive (PCH–rich) green manures, and particularly GLU-containing plant materials, have the potential as control agents for major plant bacterial diseases, fungal diseases, plant parasitic nematodes, repel insects and mollusks. The effect seems to be regulated through the formation of biologically reactive isothiocyanates.
However the PCH-control mechanism for the pathogenic microorganisms is considered to be nonspecific, thus effects on beneficial soil organisms, e.g. bacteria and fungi, associated with organic carbon (C) transformation and nitrogen (N) cycle in soil may also be affected. Soil microorganisms play a key role in the C and nitrogen N cycling processes. Microbial processes are responsible for the production of carbon dioxide (CO2), methane (CH4), ammonia (NH3), nitrous (N2O) and nitric (NO) oxide emissions. N2O, CO2 and CH4 are considered the three major greenhouse gases (GHG) contributing to global warming, while N2O also affects the ozone layer. Nitric oxide and ammonia, although are not effective GHG at atmospheric concentrations, play an important role in atmospheric chemistry and are associated to acid rains also to undesired changes in oligotrophic ecosystems.
Although agronomic values of green manures are relatively well studied, the consequences of their use on environmental issues have not been evaluated in detail. Therefore, the broad aim of this project is to study how these amendments affect beneficial soil organisms and also how they subsequently affect the emissions of GHG from the soil. A second point is that there should be evaluations on the metabolites derived from the major compounds in bioactive plants. More specific objectives are:
(A) to evaluate the effects of PCH-rich plant materials (1) on composting and, at what extent, this transformation process can degrade PCH and affect the type of metabolites produced; (2) on mineralization of organic C and N added to soils, and; (3) on emission of C and N gaseous compounds of environmental significance;
(B) How the effects referred to in A may be explained or linked to the changes in the structure of soil microbial populations and enzymatic activities related to the C and N cycles.
To achieve the objectives the project was divided in five tasks: Task I. Production of the green manures to be used by other tasks and will study the composting process of PCH rich materials to evaluate if their composition affects the processing; Task 2. Evaluation of the effect of bioactive materials on the mineralization of organic C and N added to soils and on emissions of C and N gaseous components with environmental significance. The influence of temperature, soil water content and repeated applications of phytochemical-rich materials will be assessed; Tasks 3 Evaluation of PCH and their metabolites in plants, compost and soils in samples collected task 1 and 2; Tasks 4 and 5 will evaluate, respectively, microbial population structure and enzyme activities in samples from the incubation of soils with fresh plants and composted materials.
A multidisciplinary research team from UTAD and ISA will cooperate in all tasks and the most recent methodologies will be applied. Profiling of major PCH and metabolites will be done using standard HPLC-MS and GC-MS methods for volatile and non-volatile PCH that have previously been developed and optimized. Assessment of soil microorganisms population structure will be carried out applying molecular techniques, phospholipid fatty acid (PLFA) profiling and the Most Probable Number methods.
Expected key-results from this work include information regarding the effect of PCH-rich plant materials on:
-The composting process and, vice-versa, how composting may degrade or inactivate phytochmenicals and their metabolites;
- Mineralization of organic N and C added to soil;
-Emission of gases with important greenhouse impact or other environmental risks;
-Structure of soil microbial populations, namely with bacteria involved in N and C cycles and soil enzymatic activity and how microbes have been affected by PCH and consequently they affect N and C cycles.
The scientific results expected will provide new and relevant information on the effects of green manures in fundamental plant and soil processes. The results about compost effectiveness in controlling soil pathogens will be of commercial relevance for plant disease control in crops.