Allan Sim

Rhizodeposition and C-partitioning of Lolium perenne in axenic culture affected by nitrogen supply and defoliation

This study investigated the effects of N-supply and partial defoliation on C-partitioning, root morphology and soluble rhizodeposition, for Lolium perenne grown in axenic sand culture systems percolated with nutrient solution. Plants were grown for 36 d in nutrient solutions with differing N concentrations (4 mM or 0.02 mM NH4+NO3-), and effects of repeated defoliation to 4 cm were determined. The ‘low N’ supply reduced (P < 0.05) dry matter accumulation, with proportionately increased partitioning to the root systems. Root morphology was also altered at ‘low N’, with development of a finer root system, manifest as increased (P < 0.05) specific root length. Concurrent with these effects on growth of L. perenne, ‘low N’ increased (P < 0.05) exudation of C-compounds from roots on a per g root basis. Defoliation was found to increase exudation (P < 0.05) of soluble compounds for periods of 3-5 d following each cut, at both N-supply rates. The effects of N-supply and defoliation are of importance in understanding the coupling of plant productivity to nutrient cycling in soils with differing N availabilities and for grassland systems which are subject to grazing.

How does nitrogen availability alter rhizodeposition in Lolium multiflorum Lam. during vegetative growth

The objective of this work was to determine if the impact of nitrogen (N) on the release of organic carbon (C) into the soil by roots (rhizodeposition) correlated with the effect of this nutrient on some variables of plant growth. Lolium multiflorum Lam. was grown at two levels of N supply, either in sterile sand percolated with nutrient solution or in non-sterile soil. The axenic sand systems allowed continuous quantification of rhizodeposition and accurate analysis of root morphology whilst the soil microcosms allowed the study of 14C labelled C flows in physico-chemical and biological conditions relevant to natural soils. In the axenic sand cultures, enhanced N supply strongly increased the plant biomass, the plant N content and the shoot to root ratio. N supply altered the root morphology by increasing the root surface area and the density of apices, both being significantly positively correlated with the rate of organic C release by plant roots before sampling. This observation is consistent with the production of mucilage by root tips and with mechanisms of root exudation reported previously in the literature, i.e. the passive diffusion of roots solutes along the root with increased rate behind the root apex. We proposed a model of root net exudation, based on the number of root apices and on root soluble C that explained 60% of the variability in the rate of C release from roots at harvest. The effects of N on plant growth were less marked in soil, probably related to the relatively high supply of N from non-fertiliser soil-sources. N fertilization increased the shoot N concentration of the plants and the shoot to root ratio. Increased N supply decreased the partitioning of 14C to roots. In parallel, N fertilisation increased the root soluble 14C and the 14C recovered in the soil per unit of root biomass, suggesting a stimulation of root exudation by N supply. However, due to the high concentration of N in our unfertilised plants, this stimulation was assumed to be very weak because no significant effect of N was observed on the microbial C and on the bacterial abundance in the rhizosphere. Considering the difficulties in evaluating rhizodeposition in non sterile soil, it is suggested that the root soluble C, the root surface area and the root apex density are additional relevant variables that should be useful to measure along with the variables that are commonly determined when investigating how plant functioning impacts on the release of C by roots (i.e soil C, C of the microbial biomass, rhizosphere respiration).

Uptake and transport of phosphorus by Agrostis capillaris seedlings from rapidly hydrolysed organic sources extracted from32 P-labelled bacterial cultures

Cultures of the soil bacterium Serratia liquifaciens grimesii were grown with32 P labelled phosphate, to produce a uniformly32 P labelled source of microbial P. Extracts of the bacteria were prepared by sonication, dialysis and filtration to provide a clear sterile solution which was characterised in terms of dissolved organic and condensed P (DOP and DCP) and molecular weight range. The extract was used as a source of P to Agrostis capillaris L. seedlings in nutrient solution from which orthophosphate was omitted. In a time course experiment, root surface phosphatase activity increased as soon as extract was added to the root medium, DOP was rapidly hydrolysed and orthophosphate concentration increased rapidly. These processes were complete within about 8 h, after which phosphatase activity fell to its original level, and the plants absorbed molybdate reactive P from the nutrient solution so that it reached its original concentration over 48 h. DCP concentrations did not change significantly throughout the experiment. This work clearly demonstrated that DOP but not DCP, as a component of a bacterial extract produced by a relatively straightforward method, was quickly hydrolysed and the P made available for plant uptake.

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.

Soil P resources, plant growth and rooting characteristics in nutrient poor upland grasslands

A field study was undertaken to establish the demand for P by mixed herbage, manipulated by cutting regimes, and the extent to which orthophosphate alone in soil solution could meet this demand from three cambisols derived from different parent materials. Differences in soil types were sufficient to produce significantly different rooting patterns at each site. Yields for 7-and 10-cm treatments generally exceeded those for swards cut to 2-and 4-cm. The highest yields were from plots cut once at the end of the season, or when herbage was cut in June and October only. Yields fell in the second season by an average of 30%. Two cuts in the season resulted in almost twice the P uptake compared with other treatments, leading to the view that a “silage” cut stimulated root growth. Rooting was deepest in Tarves Association soil (Dystric cambisol), densest in Insch Association soil (Eutric cambisol) and intermediate in Foudland Association soil (Dystric cambisol) but herbage yield at each site was similar. Whole season mean P and N content in roots ranged from 1.0 to 3.4 and from 8.1 to 27.9 mg g−1 dry weight, respectively. The lowest values were in once cut herbage and were half those in herbage cut in June and October only. Data for the total P resources of the soils, extractable P, and shoot and root P at each site are presented together with data for P in soil solution (principally organic) from an associated soil solution study. There was a disparity between daily uptake and orthophosphate in soil solution. These findings suggested that it was probable that soluble organic forms of P are important for P nutrition in these nutrient poor soils, and could account for the excess of observed P uptake (from soils low in P) over that predicted by mechanistic mathematical models.

Browsing affects intra-ring carbon allocation in species with contrasting wood anatomy

Current knowledge on tree carbon (C) allocation to wood is particularly scarce in plants subjected to disturbance factors, such as browsing, which affects forest regeneration worldwide and has an impact on the C balance of trees. Furthermore, quantifying the degree to which tree rings are formed from freshly assimilated vs. stored carbohydrates is highly relevant for our understanding of tree C allocation. We used (13)C labelling to quantify seasonal allocation of stored C to wood formation in two species with contrasting wood anatomy: Betula pubescens Ehrh. (diffuse-porous) and Quercus petraea [Matt.] Liebl. (ring-porous). Clipping treatments (66% shoot removal, and unclipped) were applied to analyse the effect of browsing on C allocation into tree rings, plus the effects on tree growth, architecture, ring width and non-structural carbohydrates (NSCs). The relative contribution of stored C to wood formation was greater in the ring-porous (55-70%) than in the diffuse-porous species (35-60%), although each species followed different seasonal trends. Clipping did not cause a significant depletion of C stores in either species. Nonetheless, a significant increase in the proportion of stored C allocated to earlywood growth was observed in clipped birches, and this could be explained through changes in tree architecture after clipping. The size of C pools across tree species seems to be important in determining the variability of seasonal C allocation patterns to wood and their sensibility to disturbances such as browsing. Our results indicate that the observed changes in C allocation to earlywood in birch were not related to variations in the amount or concentration of NSC stores, but to changes in the seasonal availability of recently assimilated C caused by modifications in tree architecture after browsing.

Uptake of ammonium and nitrate ions from acid mist applied to Sitka spruce [Picea sitchensis (Bong.) Carr.] grafts over the course of one growing season

AbstractCloned Sitka spruce grafts were subjected to NH4NO3 mist (1 mm day-1 equivalent) acidified with H2SO4, for 4 days a week, from April until November. Three N treatments at pH 5.0 used 0.01 mol m-315NH415NH3, 1.6 mol m-315NH4 NO3 or 1.6 mol m-3 NH415NO3. At pH 2.5, the treatments were15NH415NO3,15NH4NO3 and NH415NO3 all at 1.6 mol m-3. At the end of the growing season,15N was found in every part of the trees, even when NH4NO3 was supplied at 0.01 mol m-3. It was shown that both

Labile and recalcitrant plant fractions are utilised by distinct microbial communities in soil: Independent of the presence of roots and mycorrhizal fungi

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