Understanding the role of the soil microbial and invertebrate communities in plant-soil interactions

Lead Research Organisation: Rothamsted Research
Department Name: Unlisted


The major route of carbon (C) into soils is through plant material, either as plant root exudates or decomposing organic matter. The quality and quantity of C entering below-ground ecosystems are generally controlled through agricultural management practices driving changes in the above-ground botanical composition. Soil food webs are based on three different C pools - root exudates, litter, and recalcitrant soil organic matter. It is thought that these create 3 distinct energy channels in the soil food web and are thus critical in controlling soil community compositional dynamics and also many of those soil functions considered desirable in healthy soils. Although the relative size of the 'living' soil C pool (the microbial biomass) is small compared to the total C pool, it is the former that is important in facilitating the cycling of nutrients in the soil. We will be testing how changes in the quality and quantity of C entering the soil are driven by changes in plant species composition and how such changes impact on soil communities and the functions that they perform. We will study:
1) the mechanism of C movement from the plant into soil food webs, both the relative importance of exudation and litter inputs and how management (including root herbivory) influences this;
2) the relative importance of the different feeding (energy) channels in the soil and how they interact with soil functions;
3) the relative importance of biotic and abiotic modifiers of C turnover and how it changes with scale;
4) how management and soil type influence rhizosphere community selection by plants.
Description 1.If soil communities rely on plant-derived carbon, is biodiversity lost when this primary source is removed? Soil microbial and mesofaunal communities at the Rothamsted Highfield site were compared under a mixed grass sward, arable rotation and a section maintained as a bare-fallow for the past 50 years by regular tillage. Organic matter reserves have been degraded and microbial and mesofaunal numbers and mite diversity have declined in this unique bare-fallow site, where fresh carbon inputs have been drastically reduced. However, it supports a species-rich metabolically active bacterial community of similar diversity to that in soil maintained as grass sward. Thus bacterial diversity (but not abundance) is apparently independent of plant inputs.

2. The diminished and structurally altered microbial community of

the bare-fallow soil was not impaired in its utilisation of tracer

additions of soluble plant-derived substrates. Therefore, whilst the

availability of these substrates may affect the relative abundances

of microbial populations (e.g. r- and K-strategists), exclusion of

plant sources of these substrates did not result in reduced physiological capability to utilise them. However, our results indicate

that the utilisation of insoluble plant-derived substrates was impaired in the bare-fallow soil. This likely reflects that insoluble

plant fractions include complex compounds that are specific to

plant-inputs (e.g. cellulose and lignin) and that require particular

enzymes for their utilisation. Therefore, long-term exclusion of

plant-inputs may have reduced the diversity of organisms potentially capable of utilising insoluble plant-derived substrates, as this physiological trait would not confer a competitive advantage in bare-fallow soil.

3. In an follow-on study, where we significantly altered the long term management of the plots above, we found that changing management can rapidly affect both the mesofaunal community composition and abundance. This suggests that it is possible to re-establish mesofaunal communities and their function through the differential planting.

4. Understanding trophic linkages within the soil food web (SFW) is hampered by its opacity, diversity and limited niche specialisation. Here, we define the structure of the SFW in two habitats (grassland versus woodland) on the same soil type. Testing the hypothesis that land management would alter the SFW in these habitats. Stable isotope ratios of carbon (C) and nitrogen (N) from all invertebrates were used as a proxy for trophic niche. Differences between taxon stable isotope ratios and community-wide metrics highlighted habitats with similar taxa had separate different SFWs, using different basal resources, either driven by root or litter derived resources. Overall, we conclude that plant type can act as a top-down driver of community functioning, and that differing land management can alter impact on the whole SFW.

5. Arbuscular mycorrhizal fungi (AMF) are known to reduce the growth of generalist root-feeding insects, but whether the same is true for a specialist insect is unknown.We found that a specialist root feeder was less affected by the presence of AMF than are generalist species. However, AMF enable a plant to tolerate the effects of root loss, and this is dependent on the number of mycorrhizal species in the root system.
Exploitation Route The findings have been used by us and others in subsequent grant applications
Sectors Agriculture, Food and Drink,Environment
Description The findings from this project have been used in subsequent grant applications. The techniques developed have been used by researchers in other laboratories.
First Year Of Impact 2011
Sector Agriculture, Food and Drink,Environment
Impact Types Economic,Policy & public services
Description Journey to the Centre of the Earth: the first 23cm: Royal Society's annual Summer Science Exhibition 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Millions of individual organisms and many thousands of species can be found in just one teaspoon of soil. Scientists from Rothamsted Research are using new methods to study this huge complexity in soil, to understand how it works and what previously undiscovered organisms it may contain.
Year(s) Of Engagement Activity 2010
URL http://www.rothamsted.ac.uk/journey-centre-earth-first-23cm