B.G. Ord

Microbial biomass and activity in soils amended with glucose

Abstract Four contrasting soils were amended with glucose at concentrations up to 10 mg g−1 soil. The soils were incubated at 22°C for 14 days and the biomass determined at various times by chloroform fumigation or substrate-induced respiration. The adenosine triphosphate (ATP) content or the amylase and dehydrogenase activities were also determined. The size of the increases in biomass, ATP content and the enzyme activities was generally related to the amount of glucose added. The initially higher ATP levels quickly declined, and apparent substrate conversion figures up to 84% indicated that substrate-induced respiration overestimated the biomass. There were generally no significant correlations between ATP, biomass or enzyme activities.

Changes in microbial biomass and activity in soils amended with phenolic acids

Abstract The phenolic acids p -hydroxybenzoic, ferulic, caffeic and vanillic acid, were added to soil of the Countesswells series that had been fallow or carried crops of potatoes, peas or barley for two consecutive years. Changes in phenolic acid concentration, the soil biomass, the respiration rate, and soil amylase activity were measured over 28 days. All the phenolic acids were sorbed by the soils which was generally in the order caffeic > ferulic = vanillic > hydroxybenzoic acid. The phenolic acids stimulated soil respiration and increased the biomass as determined by the substrate-induced respiration method. but the fumigation method of biomass assessment gave anomalous results. The soil amylase activity was initially increased by phenolic acid amendments but soon decreased, and after 7 days was less than in non-amended soil although activity had increased again after 28 days. The rates of respiration and the total phenolic acid concentrations were similar to unamended controls after 28 days. The immediate respiration response, measured 1–6 h after amendment, indicated that caffeic acid gave the largest initial response of the phenolic tested, this being 55–72% of that given by glucose. Soil from the potato plot showed the highest immediate response to the phenolic acid amendments measured as a proportion of the respiration response to glucose. The findings suggest that some crops stimulate the growth of phenolic-acid degrading organisms.

A novel method for characterisation and quantification of plant root exudates

A microcosm unit is described which readily allows manipulation of experimental conditions to enable the subsequent impact on root exudation release to be monitored with time. Festuca ovina and Plantago lanceolata seedlings were grown in this microcosm unit over a 34 day experimental period under conditions of high (3.75 mol m−3 N) or low (1.25 mol m−3 N) nitrate-nitrogen treatment. At the end of the experimental period the seedlings in the microcosms were labelled with [14C]-CO2 and the fate of the label within the plant and its release by the roots monitored. Total organic carbon (TOC) content of the collected exudate material was measured throughout the experimental period as well as during the 14C-chase period and comparison of plant C budgets using these two measurements is discussed. Nitrogen treatment as found to have a greater effect on exudate release by F. ovina than by P. lanceolata seedlings as indicated by both the total organic carbon and 14C results. The use and applications of the microcosm unit are discussed.

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.

Amelioration by Volutella ciliata of the phytotoxicity of vanillic acid towards the growth of Pisum sativum L

Abstract An apparatus is described for studying the interaction between soil fungi and the growth of higher plants in the presence of specific phytotoxins. At concentrations above 1 mm, caffeic and vanillic acids inhibited the growth of Pisum sativum cultured in nutrient solutions under axenic conditions. Soil fungi capable of using phenolic acids as energy sources were isolated from soil on kaolin aggregates into which a phenolic acid had been incorporated. Some of the fungi ( Volutella ciliata, Gliocladium roseum and a Penicillium sp.) were isolated on to agar plates and grown in nutrient solutions containing a specific phenolic acid. One of the fungal isolates, V. ciliata , was compared with an XAD-4 resin, for effectiveness in reducing a phytotoxic concentration of vanillic acid towards the growth of pea seedlings. Reducing the concentration of vanillic acid from 1 to 0.2 mm enhanced the growth of the main root and increased the number of laterals so that the root system resembled that of control plants without the phenolic acid. The precise effect depended on the age of the plant when the phytotoxic concentration of the vanillic acid was reduced. At a 1 mM concentration of NO 3 -N (10% of concentration in the usual nutrient solution), concentrations as low as 10 μm vanillic acid were phytotoxic towards the growth of pea seedlings, and this effect was also ameliorated by V. ciliata . The V. ciliata produced no phytotoxins per se . Reducing phytotoxic concentrations of vanillic acid during the first 3 days of culture was more effective than subsequent reduction, and after 8 days the plants did not recover subsequently.

Distribution of soil invertase in relation to the root systems of Picea sitchensis (Bong.) carr. and Acer pseudoplatanus L. during development of young plants

A method is described for sampling rhizosphere soil under newly establishedPicea sitchensis andAcer pseudoplatanus. The technique involves taking soil samples to a depth of 150 mm at 100 mm intervals along transects, each 45° from its neighbour, radiating from the base of the stem. Invertase activities were measured in the soil samples and compared to their activities in fallow and rhizosphere soils. When the field soil was dry, the tree root systems were carefully excavated to retain as many fine roots as possible. The distribution of the soil invertase was matched to the spatial distribution of the roots showing the precise position of the rhizosphere relative to the initial ‘blind’ soil sampling. Statistics were applied to derive equations for calculating the percentage enzyme activity relative to that found in rhizosphere soil at various locations radiating from the base of the stem. This information was subsequently applied to soil sampled under trees of the same age as those excavated to give a non-destructive method for sampling rhizosphere soil routinely from under a large number of trees.

Influence of phenolic acids on the sodium, calcium and chloride contents of Pisum sativum under axenic conditions

Abstract We have reported (Vaughan and Ord, 1990, 1991) that phenolic acids, commonly found in soil solutions (Whitehead, 1964; Hartley and Whitehead, 1985), can have dramatic effects on the fresh weights and morphology of pea roots. These phenolic acids can also have substantial effects on the uptake of nutrients. Thus, for example, it has been shown (Kobza and Einhellig, 1987) that 0.5 mM ferulic acid can lower the amounts of P, K and Mg in Sorghum bicolor L. and Vigna sinensis (cowpeas) grown in syringic and protocatechuic acids contained less N, P, K and Fe than the untreated controls (Al Saadawi et al., 1986). In none of these studies were axenic conditions used to eliminate complexities which may arise from the presence of micro-organisms using the phenolic acids as carbon sources (Vaughan et al., 1988). Similarly, relatively few attempts have been made to elucidate the mode of action of the phenolic acids on nutrient uptake (Balke, 1985).

A simple technique for producing 13C or 14C-labelled fronds of Lemna gibba and their use in soil incubation investigations

The duckweed Lemna gibba required light and a suitable energy source such as sucrose, glucose or fructose, for maximum growth in culture. The requirement for light was relatively unimportant and the plants grew well in a photon flux density of only 52 μmol m-2s-1 PAR. The uptake and incorporation of uniformly labelled 14C-glucose into fronds was related only to the concentration of the sugar.When incubated with soil, labelled L. gibba behaved in a manner similar to that of labelled ryegrass roots which had been produced by a more elaborate technique using a 14CO2 labelled atmosphere. During incubation with soil for 224 days the L. gibba material (specific activity 6133 Bq mg-1 d. wt) lost 64% of its radioactivity as 14CO2 and ryegrass (specific activity 6634 Bq mg-1 d. wt) lost 49%. Alkaline extracted humic and fulvic acids from soil had specific activities for the L. gibba incubation of 3409 and 407 Bq mg-1 solid and for ryegrass roots of 4609 and 546 Bq mg-1 solid respectively. The production of 13C or 14C-labelled L. gibba can be undertaken using only simple equipment producing material the specific radioactivity of which can be controlled by adjusting the activity of the sugar energy source.