Lorna A. Dawson

Impact of root herbivory by insect larvae on soil microbial communities

Bentgrass (Agrostis capillaris) and clover (Trifolium repens) were grown as pure swards and mixtures in pots containing soil from the NERC Soil Biodiversity field site located in Scotland. Six weeks after plant establishment leatherjacket larvae (Tipula paludosa) were added at field density to half the pots and the impacts of their feeding on plant shoot and root biomass and soil microbial communities was determined after 10 days. Plate counts and community level physiological profiles (CLPP) were used to characterise the microbial communities. Larval herbivory had a significant negative effect on shoot growth of both grass and clover and root biomass of grass. In mixed swards, larvae preferentially fed on clover. Soil microbial community structure was altered in the presence of larvae with populations of pseudomonads being significantly increased. These community differences may be attributed to increased quantity and qualitative changes in carbon flux to the soil as a result of root herbivory, as indicated by differences in the CLPPs of microbial communities in the presence and absence of larvae. This was mainly due to increased utilisation of some sugars, carboxylic and amino acids in the presence of larvae.

Microbial DNA profiling by multiplex terminal restriction fragment length polymorphism for forensic comparison of soil and the influence of sample condition

Aims:  To evaluate: (i) the impact of air‐drying on bacterial, archaeal and fungal soil DNA profiles and (ii) the potential use of multiplex‐terminal restriction fragment length polymorphism (M‐TRFLP) as a tool for forensic comparison of soil.

Depth distribution of cherry (Prunus avium L.) tree roots as influenced by grass root competition

We investigated the effect of competition from grass roots (as controlled by herbicide application) on the depth distribution of white roots in cherry trees, grown with varying rates and frequency of application of N in an agroforestry system. Statistical summaries of distribution, namely mean and skewness, produced a concise and interpretable analysis of the data.There was a large increase in tree root numbers in the surface horizons after the herbicide had reduced grass root competition. Where the surrounding grass had not been reduced by herbicide, the average depth of tree roots increased with time, contrasting with a marked shift in the mode of distribution of root numbers to shallow depths when grass competition was removed. These findings are important in the understanding of plant root competition and for prescribing best practise for tree establishment in agroforestry systems.

Influence of pasture management (nitrogen and lime addition and insecticide treatment) on soil organisms and pasture root system dynamics in the field

At an upland field site in Scotland on an established Festuca-Agrostis pasture, the effects of soil amendment on root dynamics, using nitrogen and lime and the regular application of insecticide, were studied over a period of 1 year. The most common insect root herbivore at the site was Tipula paludosa, and the application of insecticide (chlorpyrifos) reduced numbers of all insect larvae of all species. Root biomass, root appearance, root disappearance and root density were all reduced by insecticide. This reduced rooting could reflect reduced root replacement, due to the reduction in root herbivory in insecticide-treated plots or could be a direct affect of insecticide application on the roots. Root appearance, root disappearance and C and N input to the soil were increased by treatment with nitrogen and lime, while root survival time was reduced. The nitrogen and lime treatment also increased bacterial numbers in the soil and enhanced their potential C utilization. An altered rooting density and longevity was brought about by the two soil treatments, which could have both direct and indirect effects on the soil biota.

Responses of plant traits of four grasses from contrasting habitats to defoliation and N supply

The objective of the study was to identify specific plant traits determining adaptation of grass species to defoliation and N availability, and thus having a major impact on species dynamics, primary productivity, and on nutrient cycling in grassland ecosystems. It was specifically examined whether the response of species to defoliation is related to their plasticity in leaf growth and in leaf growth zone components, and whether the response of species to nitrogen is related to their plasticity in root morphology and subsequent N acquisition, and to N losses through leaf senescence. The study was conducted on L. perenne and D. glomerata, two grazing tolerant species from fertile habitats, and on F. arundinacea and F. rubra, two less grazing tolerant species from less fertile habitats. Plants were subjected to repeated defoliation at three cutting heights under both high N and low N supply. Biomass allocation, leaf elongation, characteristics of the leaf growth zone (height and relative growth rate), and root morphology, N uptake and N losses through leaf senescence were evaluated. Under high N supply, L. perenne and D. glomerata showed the greatest tolerance to defoliation, due to a large plasticity in the height of the leaf growth zone and due to compensatory growth, either within the leaf growth zone or between growing leaves. Under low N supply, F. rubra was the only species with the ability to develop a more branched root system and a greater length of tertiary roots than under high N. As a consequence, under low N supply F. rubra had a higher specific N uptake and a higher growth rate than the other species. This slow growing species also showed a higher nitrogen allocation to dead leaves and subsequently a higher potential N loss to leaf litter.

Influence of root herbivory on growth response and carbon assimilation by white clover plants

Abstract One of the most significant pests of white clover (Trifolium repens L.) is the clover root weevil (Sitona lepidus L. Coleoptera; Curculionidae). The adult of this species feeds on the leaves of the plant. However, root feeding by the larvae is more insidious. An experiment is described that was designed to test the hypothesis that root nodule herbivory by early instar larvae of S. lepidus reduces the assimilation of C by plants of white clover. White clover plants were grown (± larvae) in individual growth chambers and the net carbon (C) assimilation by each plant was estimated by monitoring CO2 flux in the chambers. White clover plants which had been infested with larvae had a significantly (P

Use of near-infrared reflectance spectroscopy to predict the percentage of dead versus living grass roots.

We tested the potential of near-infrared reflectance spectroscopy (NIRS) to predict the percentage of dead versus living roots of five grass species grown in monocultures under field conditions. Root death was induced after total severance of aboveground vegetation. Root samples were collected immediately after this treatment to obtain predominantly live roots (L), and then one (D1) and two months (D2) to obtain dead roots. NIRS spectra of L samples were different from D1 and D2 samples for four of the five species. The percentage of live and dead roots and root C and N were significantly predicted by NIRS. Validation of live and dead root percentage calibration was achieved with an error of prediction of 15%. These results show the potential of NIRS to predict the percentage of dead and live roots under field conditions and open up new opportunities in estimating more accurately below-ground net primary production of grasslands.

A Hierarchical Model for Compositional Data Analysis

This article introduces a hierarchical model for compositional analysis. Our approach models both source and mixture data simultaneously, and accounts for several different types of variation: these include measurement error on both the mixture and source data; variability in the sample from the source distributions; and variability in the mixing proportions themselves, generally of main interest. The method is an improvement on some existing methods in that estimates of mixing proportions (including their interval estimates) are sure to lie in the range [0, 1]; in addition, it is shown that our model can help in situations where identification of appropriate source data is difficult, especially when we extend our model to include a covariate. We first study the likelihood surface of a base model for a simple example, and then include prior distributions to create a Bayesian model that allows analysis of more complex situations via Markov chain Monte Carlo sampling from the likelihood. Application of the model is illustrated with two examples using real data: one concerning chemical markers in plants, and another on water chemistry.

Root feeding behaviour of Tipula paludosa (Meig.) (Diptera: Tipulidae) on Lolium perenne (L.) and Trifolium repens (L.)

The larvae of the cranefly, Tipula paludosa (Meig.), graze on plant root systems and are a prevalent pest of upland grassland. Using a slant board system, their effect on two pasture species, Trifolium repens L. and Lolium perenne L. were studied. The larvae fed voraciously on the main root axes of T. repens, causing a 15% reduction in primary axis root length. In contrast, there was no overall effect of grazing on root length of L. perenne. However, the proportion of the root system of L. perenne present as laterals was reduced. The plant N content of T. repens was reduced by grazing, possibly due to leakage of N-containing compounds or selectivity of N-rich tissue by the grazing invertebrate. Larvae were larger when fed on T. repens, and their faeces had a higher concentration of bacteria than when fed on L. perenne.

Impact of defoliation on the distribution of 15N-labelled synthetic sheep urine between shoots and roots of Agrostis capillarisand soil N pools

An experiment was carried out to test the hypothesis that defoliation of a semi-natural grassland species influenced the fate of the urea nitrogen (N) from sheep urine. The distribution of the 15N added as urea in a synthetic sheep urine (SSU) at 50 g N m−2, to Agrostis capillarisand fallow soil was followed over a period of 56 days in the plant and soil in a glasshouse microplot experiment. The grass was subject to three regimes; regular twice weekly defoliation, a single defoliation (both to 40 mm), and uncut. Regular cutting increased the total N concentration of shoots and reduced the biomass of roots, though 15N recovery in the shoots was not appreciably different from the uncut grass. A single defoliation immediately before SSU addition decreased 15N recovery. More 15N was recovered in the surface 50 mm of soil beneath the regularly cut than in the single and uncut treatments. The soil mineral N pool was the largest sink for SSU 15N. Microbial N increased threefold with the addition of SSU, but effects of the grass treatments on the proportion of SSU 15N in the microbial biomass could not be detected. It was concluded that regular defoliation of grass compared with no cutting could lead to greater losses of NO3− by leaching and denitrification.