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Understanding how plant root traits and soil microbial processes influence soil erodibility

Final Report Summary - FIXSOIL (Understanding how plant root traits and soil microbial processes influence soil erodibility)

Marie Curie Intra-European Fellowships (IEF) 2013 Grant No. 626666
Fixsoil - Understanding how plant root traits and soil microbial processes influence soil erodibility
Scientific Coordinator 1: Alexia Stokes Scientific Coordinator 2: Robert Griffiths
Researcher: Dr. Luis Merino-Martín –
Host Institution: INRA, France & CEH, UK Period Covered: 1 May 2014 – 31 April 2015.
Project webpage:

Fixsoil project aimed to understand the role of roots and microorganisms (and their interrelationships) on soil aggregate stability. In the context of ecosystem services, the aim of this project was to investigate the ecosystem service of soil fixation provided by different crop / forest. This aim was achieved by determining how the function and role of different parts of a root system and the microbial communities provide the soil fixation service with regard to soil erosion.

The main research objectives of Fixsoil project were:
1. Study the influences of vegetation composition and structure on soil physical and chemical characteristics and their relationships with root traits and microbial communities.
2. Identify if the soil microbial communities mediate the effect of roots and land use on soil aggregate stability.

Regarding the research objective [1], field-sites in France and the UK representing natural forest, agro-forest and organic farmland in temperate oceanic and Mediterranean regions were selected. Soil cylinders (n=108) and samples for soil aggregate stability were collected at two soil depths (surface and 15-20 cm) in relation with rooting depths in the different field sites. The microbial community diversity from soil samples collected in the different field sites was assessed using molecular approach. Total DNA was extracted and bacterial and fungal community biodiversity was assessed using Next Generation Sequencing Illumina of 16S rRNA genes (bacteria) and the ITS region (fungi) to phylogenetically identify responsive taxa. Publicly available bioinformatics pipelines such as QIIME and PIPITS to demultiplex the resultant sequences were used, quality filter and normalise reads to a standardised number per sample. These sequences were then identified by reference to existing databases. We determined the aggregate stability using the well-established standard method, ISO/CD 10930, developed by Yves LeBissonais (collaborator of the project). Physical and chemical analyses on soils were performed to investigate whether intrinsic soil chemical properties are more determinant of soil aggregate stability than root/microbial traits.

Within the research objective [2], inoculation experiments were performed to test explicitly the role of different microbial communities in contributing to changes in soil aggregate stability. This inoculation of native microbe to test in what extent the changes in soil aggregate stability could be mediated by the variation in soil microbial communities is a technique recently developed. Following the assessment of the aggregating properties of soils under three different land management, reciprocal inoculations of microbial communities were compared in their influence on the aggregation properties of sterilised soils. Intact soil cores of 1.4 dm3 were collected in a culture and another set of soil cores were collected in Wytham woods. Another set of pots were also filled with a loamy soil to be used as a control treatment. The cores of each treatment were sterilized by gamma irradiation with a range between 40 and 60 kGy. The upper 1.5 centimetres of twenty-seven soil cores were inoculated with fresh soil collected from culture and woods and another set of were not inoculated. On each combination, we sowed seeds from two species: Brachypodium sylvaticum (fibrous root system) and Urtica dioica (tap/adventitious root system) and a third treatment with no growth of plants was used as a control. The same sequencing techniques as for the field samples were used for this experiment. Pots from this experiment were sampled and bulked in order to integrate the root spatial heterogeneity. We determined specific root length and density (using the WinRhizo software) and biomass in each pot. Measurements of Glomalin Related Soil Proteins (GRSP) were also performed using the colorimetric Bradford protein assay and ergosterol using liquid chromatography.

Concerning the research objective [1], aggregate stability was affected by land use. Furthermore, soils near trees or treelines showed higher aggregate stability than open areas. Notably, the effect of the proximity to trees on aggregate stability in the subalpine environment was affected by altitude. Land use and proximity to trees was also correlated to microbial diversity. Therefore, a correlational change on microbial communities and soil aggregate stability from field samples was found along the climatic gradient.

Regarding the inoculation experiment, soil inoculation was found as a useful technique for managing microbial diversity in pots. More importantly, the microbial inoculation had an effect on aggregate stability (only for the case of the control soil, fig. 1). The relationships between microbial communities’ inoculation and compounds such as ergosterol and GRSP were not significant.

The project will fully achieve its objectives when the scientific articles have been published. The potential impact of the Fixsoil project will be mainly on two areas: i) to the scientific community: the identification of close links between microbial communities and soil aggregate stability and having found an effect of microbial communities on aggregate stability will help profoundly to the understanding of relationships between soil structure and microbial communities at the rhizosphere; ii) to the society: the identification of land use (and specially agroforestry) and management strategies that preserve the stability of soils and their microbial biota will help managers and farmers on the decision making in order to benefit from both, production and preservation of microbial biodiversity and protection against erosion.