The addition of biochar and chitin to the soil can control the bacterial community around plant roots, according to a doctorate research by ILVO-UGent researcher Caroline De Tender.
Through genomics techniques, she examined the control capacity of these two soil additives to more micro-organisms that are growth promoting on the one hand and, on the other hand, make the plant and the plant environment more resistant to pathogens. The results for biochar and chitin proved convincing in the cultivation of strawberry and lettuce respectively. Chitin was even found to suppress the survival of the Salmonella enterica bacterium when it occurs on the leaves of lettuce. An infection with that bacterium can cause diarrhea in consumers.
Substrate additives and rhizosphere
In order to prevent diseases and to obtain a good yield, pesticides and fertilizers are often used in growing crops. Various alternatives, such as the environmentally-friendly substrate additives biochar and chitin, are examined. Biochar is made by heating and degradation of biomass under limited oxygen supply, but chitin is an important building block of cell walls of fungi and exoskeleton of insects and shellfish such as crab and shrimp. Both products are thus obtained by upgrading of what is considered to be waste products. For her research, Caroline De Tender added both substances to the soil or substrate in the cultivation of strawberry or lettuce.
Both biochar and chitin are able to change the composition of the microbial community of the rhizosphere, the narrow zone of soil around the roots, to such an extent that a higher number of plant growth promoters and biocontrol organisms are installing themselves. To that end, Caroline De Tender examined the microbiome of the rhizosphere in relation to plant growth, disease development and the survival of pathogens on the plant.
Biochar for strawberry, chitin for lettuce
Biochar has an effect on the properties of the soil and the substrate, both physically and chemically. For example, research showed that the evaporation of water decreased and that biochar added additional potassium and phosphorus to the growth medium. In addition the administration of biochar resulted in a change in the bacterial community of the strawberry rhizosphere, with a doubling to quadrupling the percentage of growth promoters and biocontrol organisms. The fungus community of the strawberry rhizosphere remained unchanged. Biochar caused better growth of the strawberry plants (a doubling in dry weight of the plant), provided a doubling of the fruit yield and increased disease resistance of the plant against fungal rot caused by fungus Botrytis cinerea. All of these effects can be attributed to the change in soil composition and the change in the bacterial community of the rhizosphere.
The addition of chitin to field soil for lettuce caused a change in both the bacterial and fungal community of the rhizosphere. Both growth promoters as well as degradation of chitin were stimulated, so that up to 13 times more of these microorganisms could be observed. Stimulation of chitin degradation in the soil causes harmful fungi, insect larvae and nematodes in the soil to be fought. Also more chitin degradation products end up in the soil, and they are known to increase the resilience of the plants to attackers. This was confirmed in the tests: In fact for lettuce an improved growth (a 25 percent increase in plant growth) and a decreased survival of Salmonella enterica on the leaves were established. The use of chitin in soil can thus be an interesting strategy to reduce the risk of outbreaks of diarrhea due to Salmonella infection.
Genomics as the basis of the research
The use of high-throughput sequencing techniques made it possible to study the taxonomic composition of microbial communities and the way they function in the rhizosphere. In addition, focus was mainly on amplicon sequencing (metabar coding) and shotgun metagenomics. Genomics opens up a range of new research opportunities in such kind of research. Complex compositions of microbial populations, and their change under the influence of an external factor, can be mapped out. ILVO launched an internal genomics platform 4 years ago, in which up till now about 20 different research questions and domains are clustered. Researcher Caroline De Tender herself also conducted for her doctorate also genomics studies of a totally different bacterial community, namely the bacteria that settle on plastic waste at sea (more information about this second part of the research).