Modelling low molecular weight organic acid dynamics in forest soils

Low molecular weight organic acids such as citrate and oxalate have been hypothesized to play a key role in rhizosphere ecology and pedogenesis. A mathematical site-specific model, DYNLOW, was constructed to describe the temporal and spatial dynamics of these organic acids in coniferous forest soils using the modelling software STELLA^®. Experimentally derived values for biodegradation, adsorption, and daily values of soil temperature, moisture and hydrological flow were used to parameterize the model. The model describes the dynamics and downward movement of oxalate and citrate through the horizons (O, AE, E, Bhs, Bs) of three podzolic soil profiles in Sweden. After calibration, the model predicted average soil solution organic acid concentrations ranging from <1 to 90 μM, which was in agreement with experimental measurements (<1 to 116 μM). The model results indicated that microbial degradation of organic acids was in quantitative terms the biggest process regulating soil solution concentrations. Primary production rates of organic acid in the soil were predicted to be high (<1 to 1250 nmol g⁻¹ soil d⁻¹) in comparison to the amount present at steady state in the soil solution pool (<0.1 to 240 nmol g⁻¹ soil). The downward transfer of organic acids between soil horizons due to mass flow was predicted to be a small flux (<0.1 to 3% of the total loss) compared to that lost by microbial biodegradation. The model predicted that the amount of basal soil respiration that could be attributable to the microbial turnover of organic acids was on average 19±22% of the basal CO₂ production across all sites and horizons for citrate and 7±7% for oxalate. The model results are discussed in the context of pedogenesis, forest soil respiration and organic matter production.     

Soil acidification used as a management strategy to reduce nitrate losses from agricultural land

pH is known to be a primary regulator of nutrient cycling in soil. Increasing soil acidity in agricultural systems has the potential to slow down N cycling and reduce N losses from leaching thereby enhancing sustainability and reducing pollution. We conducted a field experiment to investigate the impact of acidity on N leaching in arable and grassland agricultural systems. The results showed that nitrate (NO₃⁻) concentrations in soil water were greater under arable than under grassland. Soil acidification significantly lowered NO₃⁻ concentrations in soil water over winter and spring under grassland, whilst in cereal plots a similar effect was only observed in spring. Our results suggest that soil acidification decreased nitrification causing an accumulation of NH₄⁺ which was not subject to leaching. Dissolved organic nitrogen (DON) concentrations in soil water were significantly greater under arable than grassland. Soil acidification lowered concentrations of DON in soil water, usually to a greater extent in grassland than in arable plots. It was concluded that it may be possible to use careful soil pH management as a tool to control NO₃⁻ leaching without compromising the quality of drainage water, and that this may be more effective on grassland than on arable crops.     

Free amino sugar reactions in soil in relation to soil carbon and nitrogen cycling

Amino sugars represent a major constituent of microbial cell walls and hydrolyzed soil organic matter. Despite their potential importance in soil nitrogen cycling, comparatively little is known about their dynamics in soil. The aim of this study was therefore to quantify the behaviour of glucosamine in two contrasting grassland soil profiles. Our results show that both free amino sugars and amino acids represented only a small proportion of dissolved organic N and C pool in soil. Based upon our findings we hypothesize that the low concentrations of free amino sugars found in soils is due to rapid removal from the soil solution rather than slow rates of production. Further, we showed that glucosamine removal from solution was a predominantly biotic process and that its half-life in soil solution ranged from 1 to 3 h. The rates of turnover were similar to those of glucose at low substrate concentrations, however, at higher glucosamine concentrations its microbial use was much less than for glucose. We hypothesized that this was due to the lack of expression of a low affinity transport systems in the microbial community. Glucosamine was only weakly sorbed to the soil's solid phase (K_d=6.4±1.0) and our results suggest that this did not limit its bioavailability in soil. Here we showed that glucosamine addition to soil resulted in rapid N mineralization and subsequent NO₃⁻ production. In contrast to some previous reports, our results suggest that free amino sugars turn over rapidly in soil and provide a suitable substrate for both microbial respiration and new biomass formation.     

Microbial response time to sugar and amino acid additions to soil

Soil microbial respiration is derived predominantly from the turnover of carbohydrates and proteins in soil. In most agricultural ecosystems, these C compounds enter soil mainly from rhizodeposition (root exudation and turnover). Our aim was to determine how long it takes for the microbial population to reach their maximum mineralization potential after the addition of low-molecular-weight (MW) rhizodeposits to the soil. We added sugar in the form of glucose and amino acids in the form of glycine to an arable, grazed grassland, Eucalyptus forest and boreal forest soil and monitored CO₂ efflux over a 6-h period. Artificial rainwater amended (zero C addition) or unamended soils were used as controls. The Michaelis-Menten substrate utilization profiles showed vastly different patterns of microbial mineralization capacity and substrate affinity between the soils. However, in all soils we showed that activation of the soil microbial community to C addition occurred almost instantaneously (?60 s) with the average time taken to reach half maximal CO₂ production being 14±8 min for glucose and 10±8 min for glycine. After reaching their maximal mineralization potential, the rate of CO₂ evolution remained constant for the remainder of the experiment. Our results showed that while substrate uptake and mineralization within the soil microbial biomass was activated quickly, subsequent adaptation and upregulation of its C processing capacity did not occur at least in the short term. The fast rate of microbial activation and substrate use we partially attribute to the large degree of functional redundancy that exists within the soil microbial community for processing rhizodeposits.     

Estimating organic carbon in the soils of Europe for policy support

The estimation of soil carbon content is of pressing concern for soil protection and in mitigation strategies for global warming. This paper describes the methodology developed and the results obtained in a study aimed at estimating organic carbon contents (%) in topsoils across Europe. The information presented in map form provides policy-makers with estimates of current topsoil organic carbon contents for developing strategies for soil protection at regional level. Such baseline data are also of importance in global change modelling and may be used to estimate regional differences in soil organic carbon (SOC) stocks and projected changes therein, as required for example under the Kyoto Protocol to the United Nations Framework Convention on Climate Change, after having taken into account regional differences in bulk density. The study uses a novel approach combining a rule-based system with detailed thematic spatial data layers to arrive at a much-improved result over either method, using advanced methods for spatial data processing. The rule-based system is provided by the pedo-transfer rules, which were developed for use with the European Soil Database. The strong effects of vegetation and land use on SOC have been taken into account in the calculations, and the influence of temperature on organic carbon contents has been considered in the form of a heuristic function. Processing of all thematic data was performed on harmonized spatial data layers in raster format with a 1 km × 1 km grid spacing. This resolution is regarded as appropriate for planning effective soil protection measures at the European level. The approach is thought to be transferable to other regions of the world that are facing similar questions, provided adequate data are available for these regions. However, there will always be an element of uncertainty in estimating or determining the spatial distribution of organic carbon contents of soils.     

樟疫多聚半乳糖醛酸酶Pcpg 1、Pcpg 2和Pcpg 4基因的克隆、测序及其遗传转化

研究优化了樟疫(Phytophthora cinnamomi)多聚半乳糖醛酸酶Pcpg1、Pcpg2和Pcpg4基因克隆的PCR条件，并对其进行了克隆、测序和遗传转化。克隆获得了3个基因，其大小均为1011bp；通过构建表达载体和遗传转化获得了3个基因的转基因菌系；3个基因均能指导合成相应的PG酶，其中转化Pcpg2基因的酵母菌(Saccharomyces cewvisiae)所分泌的多聚半乳糖醛酸酶(PG)酶活性最强，Pcpg4和对照Anpg(Aspergillus niger polygalacturonase)1次之，而Pcpg1基因所指导合成的PG酶无活性。Western blotting表明，所克隆的3个基因所指导合成的PG酶均有不同程度的糖基化。     

Sub-obliterative effects of mine-tailing on marine infaunal benthos

The effect of mine-tailing on benthic productivity in a threshold inlet was explored by studies on the polychaete Ammotrypane aulogaster Rathke, 1843. Ammotrypane was chosen as an indicator species of potentially reduced production in the area fringing total benthic obliteration due to its abundance (up to 1017 m^?2) at the time of the surveys. Densities, weights and lengths were inversely related to greater tailing deposition. These observations indicate that under light tailing there is some loss in benthic production. More detailed time-series studies are now required to measure the extent of the loss.     

Evaluation of pheromone release from commercial mating disruption dispensers

Pome fruit growers and crop consultants have expressed concerns about the seasonal release performance of commercial codling moth mating disruption dispenser products. Because of these concerns, we developed a laboratory flow-through volatile collection system (VCS) for measuring the volatile release of the codling moth sex pheromone, codlemone, from commercially available hand-applied dispensers. Under controlled air-flow and temperature conditions, the released vapor was trapped onto a polyurethane foam adsorbent followed by solvent extraction, solvent reduction, and GC/MS determination. Method recovery and breakthrough validations were performed to demonstrate system reliability before determining codlemone release from commercial dispensers field-aged over 140 days. The volatile collection was carried out in a consistent manner among five dispenser types most commonly used by growers, so that direct comparison of performance could be made. The comparison showed differences in the amount of pheromone released and in the patterns of release throughout the season between dispenser types. The variation in release performance demonstrates the need for routine evaluation of commercially marketed mating disruption dispensers. We believe that the simple and cost-effective volatile collection system can assist pheromone dispenser manufacturers in determining seasonal dispenser performance before new products are introduced into the commercial market and in rapidly verifying dispenser release when field-aged dispenser efficacy is in question.     

Dissolved organic nitrogen uptake by plants—an important N uptake pathway?

The direct uptake of dissolved organic nitrogen (DON) by plants has the potential to be a primary Factor in ecosystem functioning and vegetation succession particularly in N-limiting environments. Clear experimental evidence to support this view, however, is still lacking. Further, many of the experimental approaches used to assess whether DON is important may be compromised due to the use of inappropriate methods for comparing and quantifying plant available inorganic and organic soil N pools. In addition, experiments aimed at quantifying plant DON capture using dual-labelled (¹⁵N, ¹³C) organic N tracers often do not consider important aspects such as isotope pool dilution, differences in organic and inorganic N pool turnover times, bi-directional DON flows at the soil-root interface, and the differential fate of the ¹⁵N and ¹³C in the tracer compounds. Based upon experimental evidence, we hypothesize that DON uptake from the soil may not contribute largely to N acquisition by plants but may instead be primarily involved in the recapture of DON previously lost during root exudation. We conclude that while root uptake of amino acids in intact form has been shown, evidence demonstrating this as a major plant N acquisition pathway is still lacking.