Barry Thornton

Effects of severity of defoliation on root functioning in grasses.

Grass shoots after defoliation can be supplied with the nitrogen required for regrowth by either root uptake or remobilization of stores. Whilst it is accepted that after a single defoliation inhibition of root uptake and remobilization from root occurs, it has not been established how the capability of roots to supply nitrogen by uptake and from storage is affected with differing severities of regular defoliation, as experienced by grazed swards. The objective was to examine this question using Agrostis castellana Boiss et Reut., Festuca rubra L., Lolium perenne L. and Poa trivialis L., grasses associated with sites of differing fertility, grown in sand culture and defoliated weekly at a height of either 4 or 8 cm. Nitrogen was supplied as NH4NO3 in a complete nutrient solution. The use of 15N as a tracer allowed the nitrogen supplied to the shoot by root uptake and remobilization to be discriminated over a 35 day period. An increased severity of defoliation resulted in decreased root mass, and increased nitrogen uptake per unit root weight for all species. Increased severity of defoliation did not affect uptake on a per plant basis for A. castellana, 0.54 mg N (plant)-1 (week)-1 and P. rubra, 0.40 mg N (plant)-1 (week)-1, whilst mg N (plant)-1(week)-1 decreased from 0.54 to 0.14, and 0.54 to 0.34 for L perenne and P. trivialis respectively. For plants clipped at 4 or 8 cm, over 88% and 77% respectively of uptake appeared in the shoot. Nitrogen was remobilized from roots to the shoot for A. castellana and F. rubra when clipped at 4 cm, and for A. castellana, L. perenne and P. trivialis when clipped at 8 cm. Uptake by roots was more important than remobilization from roots in supplying nitrogen to the shoot. The ability to maintain the supply of nitrogen by uptake and remobilization to the shoot with increased severity of defoliation was species dependent.

Are humus forms, mesofauna and microflora in subalpine forest soils sensitive to thermal conditions?

This study focuses on the biological and morphological development of humus profiles in forested Italian Alpine soils as a function of climate. Humus form description, systematic investigation of microannelid communities and polyphasic biochemical fingerprinting of soil microbial communities (denaturing gradient gel electrophoresis (DGGE) and phospholipid fatty acid analysis (PLFA)) were performed to compare sites differing in mean annual temperature due to different altitude and exposure. Although the soil biota showed complex responses, several differences in soil biological properties seem to be due to thermal differences. Although soil acidity also determines biological properties, it is not a state factor but rather influenced by them. The thickness of the organic layer and the acidification of the subjacent mineral horizon increased under cooler conditions (north-exposure; higher altitude), whereas the thickness of the A horizon inversely decreased. Species richness of microannelid assemblages was higher under warmer conditions (south-exposure; lower altitude) and the vertical distribution of microannelids shifted along the gradient to lower temperatures from predominant occurrence in the mineral soil to exclusive occurrence in the organic layer. Microbial biomass (total PLFA) was higher at the cooler sites; the prevalence of Gram-negative bacteria could be ascribed to their better adaptation to lower temperature, pH and nutrient contents. The δ13C signatures of the PLFA markers suggested a lower decomposition rate at the cooler sites, resulting in a lower respiratory loss and an accumulation of weakly decomposed organic material. DGGE data supported the PLFA results. Both parameters reflected the expected thermal sequence. This multidisciplinary case study provided indications of an association of climate, mesofauna and microbiota using the humus form as an overall link. More data are however needed and further investigations are encouraged.

Resource quality affects carbon cycling in deep-sea sediments

Deep-sea sediments cover ∼70% of Earths surface and represent the largest interface between the biological and geological cycles of carbon. Diatoms and zooplankton faecal pellets naturally transport organic material from the upper ocean down to the deep seabed, but how these qualitatively different substrates affect the fate of carbon in this permanently cold environment remains unknown. We added equal quantities of 13C-labelled diatoms and faecal pellets to a cold water (−0.7 °C) sediment community retrieved from 1080 m in the Faroe-Shetland Channel, Northeast Atlantic, and quantified carbon mineralization and uptake by the resident bacteria and macrofauna over a 6-day period. High-quality, diatom-derived carbon was mineralized >300% faster than that from low-quality faecal pellets, demonstrating that qualitative differences in organic matter drive major changes in the residence time of carbon at the deep seabed. Benthic bacteria dominated biological carbon processing in our experiments, yet showed no evidence of resource quality-limited growth; they displayed lower growth efficiencies when respiring diatoms. These effects were consistent in contrasting months. We contend that respiration and growth in the resident sediment microbial communities were substrate and temperature limited, respectively. Our study has important implications for how future changes in the biochemical makeup of exported organic matter will affect the balance between mineralization and sequestration of organic carbon in the largest ecosystem on Earth.

Negative Priming Effect on Organic Matter Mineralisation in NE Atlantic Slope Sediments

The priming effect (PE) is a complex phenomenon which describes a modification (acceleration or retardation) in the mineralisation rate of refractory organic matter (OM) following inputs of labile material. PEs are well-studied in terrestrial ecosystems owing to their potential importance in the evolution of soil carbon stocks but have been largely ignored in aquatic systems despite the fact that the prerequisite for their occurrence, i.e. the co-existence of labile and refractory OM, is also true for sediments. We conducted stable isotope tracer experiments in continental margin sediments from the NE Atlantic (550–950 m) to study PE occurrence and intensity in relation to labile OM input. Sediment slurries were treated with increasing quantities of the 13C-labelled diatom Thalassiosira rotula and PE was quantified after 7, 14 and 21 days. There was a stepwise effect of diatom quantity on its mineralisation although mineralisation efficiency dropped with increasing substrate amounts. The addition of diatomaceous OM yielded a negative PE (i.e. retardation of existing sediment OM mineralisation) at the end of the experiment regardless of diatom quantity. Negative PE is often the result of preferential utilisation of the newly deposited labile material by the microbial community (“preferential substrate utilization”, PSU) which is usually observed at excessive substrate additions. The fact that PSU and the associated negative PE occurred even at low substrate levels in this study could be attributed to limited amounts of OM subject to priming in our study area (∼0.2% organic carbon [OC]) which seems to be an exception among continental slopes (typically >0.5%OC). We postulate that PEs will normally be positive in continental slope sediments and that their intensity will be a direct function of sediment OC content. More experiments with varying supply of substrate targeting C-poor vs. C-rich sediments are needed to confirm these hypotheses.

Medium-term fertilization of grassland plant communities masks plant species-linked effects on soil microbial community structure

According to the singular hypothesis of plant diversity, different plant species are expected to make unique contributions to ecosystem functioning. Hence, individual species would support distinct microbial communities. It was hypothesized that microbial community dynamics in the respective rhizospheres of, two floristically divergent species, Agrostis capillaris and Prunella vulgaris that were dominant in a temperate, upland grassland in northern Greece, would support distinct microbial communities, in agreement to the singular hypothesis. Phospholipid lipid fatty acid (PLFA) profiles of the rhizosphere soil microbial community were obtained from the grassland which had been subjected to factorial nitrogen (N) and phosphorus (P) fertilization over five plant growth seasons. The soil cores analyzed were centered on stands of the two co-occurring target plant species, sampled from five blocks in all four factorial N and P fertilization combinations. Distinct PLFA clustering patterns following principle component analysis of PLFA concentrations revealed that, in the absence of P fertilization, soils under the two plant species supported divergent microbial communities. In the P fertilized plots, however, no such distinction could be observed. Results reveal that nutrient fertilization may mask the ability of plant species to shape their own rhizosphere microbial community.

The role of N-remobilisation and the uptake of NH4+ and NO3- by Lolium perenne L. in laminae growth following defoliation under field conditions

Several studies have previously shown that shoot removal of forage species, either by cutting or herbivore grazing, results in a large decline in N uptake (60%) and/or N2 fixation (80%). The source of N used for initial shoot growth following defoliation relies mainly on mobilisation of N reserves from tissues remaining after defoliation. To date, most studies investigating N-mobilisation have been conducted, with isolated plants grown in controlled conditions. The objectives of this study were for Lolium perenne L., grown in a dense canopy in field conditions, to determine: 1) the contribution of N-mobilisation, NH4+ uptake and NO3- uptake to growing shoots after defoliation, and 2) the contribution of the high (HATS) and low (LATS) affinity transport systems to the total plant uptake of NH4+ and NO3-. During the first seven days following defoliation, decreases in biomass and N-content of roots (34% and 47%, respectively) and to a lesser extent stubble (18% and 43%, respectively) were observed, concomitant with mobilisation of N to shoots. The proportion and origin of N used by shoots (derived from reserves or uptake) was similar to data reported for isolated plants. Both HATS and LATS contributed to the total root uptake of NH4+ and NO3-. The Vmax of both the NH4+ and NO3- HATS increased as a function of time after defoliation, and both HATS systems were saturated by substrate concentrations in the soil at all times. The capacity of the LATS was reduced as soil NO3- and NH4+ concentrations decreased following defoliation. Data from 15N uptake by field-grown plants, and uptake rates of NH4+ and NO3- estimated by excised root bioassays, were significantly correlated, though uptake was over-estimated by the later method. The results are discussed in terms of putative mechanisms for regulating N uptake following severe defoliation.

Effects of defoliation and atmospheric CO2 depletion on nitrate acquisition, and exudation of organic compounds by roots of Festuca rubra

The aim of this study was to investigate the physiological basis of increased root exudation from Festuca rubra, in response to defoliation. The hypothesis, that assimilate supply to roots is a key determinant of the response of root exudation to defoliation was tested by imposing CO2-deplete (< 50 μmol mol−1) atmospheres to F. rubra. This was done as a non-destructive means of preventing supply of new assimilate to roots of intact and defoliated plants. F. rubra was grown in axenic sand systems, with defoliation and CO2-depletion treatments applied to plants at 14 and 35 days after planting. Root exudation and NO3− uptake were quantified throughout, and post-treatment uptake and allocation of N were determined from the distribution of 15N label, supplied as 15NO3−. Defoliation of F. rubra resulted in significantly (P <0.01) increased root exudation, CO2-depletion did not result in increased exudation from plants of either age. When treatments were applied to F. rubra after 14 days, defoliation and CO2-depletion each reduced NO3− uptake significantly (P <0.05). However, in older plants, uptake of NO3− was less sensitive to defoliation and CO2-depletion. The results indicate that increased root exudation following defoliation is not related directly to reduced assimilate supply to roots. This was evident from the lack of effect of CO2-depletion on root exudation, and the absence of correlation between root-C efflux and the rate of NO3− uptake. The physiological basis of increased exudation following defoliation remains uncertain, but may be dependent on physical damage, either directly or as a consequence of systemic responses to wounding.

Measuring the 13C content of soil‐respired CO2 using a novel open chamber system

Carbon dioxide respired by soils comes from both autotrophic and heterotrophic respiration. 13C has proved useful in differentiating between these two sources, but requires the collection and analysis of CO2 efflux from the soil. We have developed a novel, open chamber system which allows for the accurate and precise quantification of the delta13C of soil-respired CO2. The chamber was tested using online analyses, by configuring a GasBench II and continuous flow isotope ratio mass spectrometer, to measure the delta13C of the chamber air every 120 s. CO2 of known delta13C value was passed through a column of sand and, using the chamber, the CO2 concentration stabilized rapidly, but 60 min was required before the delta13C value was stable and identical to the cylinder gas (-33.3 per thousand). Changing the chamber CO2 concentration between 200 and 900 micromol.mol(-1) did not affect the measured delta13C of the efflux. Measuring the delta13C of the CO2 efflux from soil cores in the laboratory gave a spread of +/-2 per thousand, attributed to heterogeneity in the soil organic matter and roots. Lateral air movement through dry sand led to a change in the delta13C of the surface efflux of up to 8 per thousand. The chamber was used to measure small transient changes (+/-2 per thousand) in the delta13C of soil-respired CO2 from a peaty podzol after gradual heating from 12 to 35 degrees C over 12 h. Finally, soil-respired CO2 was partitioned in a labelling study and the contribution of autotrophic and heterotrophic respiration to the total efflux determined. Potential applications for the chamber in the study of soil respiration are discussed.

13C PLFAs: a key to open the soil microbial black box?

BackgroundPhospholipid fatty acid (PLFA) analysis is an effective non-culture-based technique for providing information on the living soil microbial community. The coupling of 13C tracers with PLFA analysis can indicate the response of microbial populations to environmental change and has been widely used to trace C flux in soil-plant systems.ScopeBased on studies applying 13C PLFA analysis, the current technological status, current applications and future opportunities are discussed and evaluated. First we describe some aspects of the labelling and analytical methodology. The approaches to study the incorporation of 13C substrate and rhizodeposition C into soil microbial communities are compared. We continue with the application of 13C-labelling to study soil microbial communities, including the utilization of soil mineralisation products, the C flux from plants into the soil microbial pool, the biodegradation of pollutants and on the application to a specific microbial group, i.e. methanotrophs. Additionally, some perspectives on the limitations of the 13C PLFA method and future research avenues are noted.ConclusionsAlthough including some limitations and complications, the 13C PLFA method provides an excellent tool for understanding the relationship between microbial populations and soil biogeochemical cycling, thus providing a key to open the soil microbial black box.

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.