Homogeneous soil slurries were employed for testing the regulating factors for aerobic denitrification. The presence of nitrite, at a relatively high concentration, was a strong inducer of aerobic nitrous oxide production, during which no dinitrogen evolution was measured. High concentrations of nitrite also appeared to inhibit reduction of nitrous oxide under more suitable denitrifying conditions (low oxygen), which resulted in a high denitrifier nitrous oxide-to-dinitrogen ratio. In contrast, dinitrogen production was efficient in near-anoxic slurries when nitrate or low concentrations of nitrite were present. Nitrous oxide and dinitrogen production in soil slurries exhibited various responses with the addition of different carbon compounds. Simple sugars (glucose and mannitol) induced the lowest nitrous oxide production whereas more complex substrates (glutamic acid and butyrate) induced more nitrous oxide under oxic conditions. In addition, no dinitrogen production occurred when slurries were incubated with more than 2% oxygen in the headspace, except when supplemented with butyrate. In addition to soil slurries a culture-based approach was adopted to investigate whether bacterial cells extracted from soil exhibited any aerobic denitrification activity. During the respiratory depletion of oxygen these extracted cells only initiated denitrification when oxygen concentration fell below 10 &'956;M, and once anoxic denitrification was highly efficient with little intermediates accumulating. However, anoxic cells, containing a fully functional denitrification pathway, appeared to sustain denitrification where re-exposed to oxygen. The resulting denitrification was highly perturbed in that nitrous oxide was the dominant product. The results suggest that aerobic denitrification is a possibility in soils and that nitrite might be a replacement. Dynamic changes in oxygen could lead to higher soil nitrous oxide production following an anoxic phase.

Soils are important life-supporting systems to human society and their long-term sustainability is high on both the scientific and political agenda. However soil functioning remains poorly understood because of the difficulty in studying complex biogeochemical interactions, which mostly occur belowground. This PhD study aims to improve the understanding of plant-soil-microbial interactions in grassland ecosystems. First, it was addressed how key microbial functions (i.e. extracellular enzyme activities) might respond to common management practices (e.g. nutrient fertilization) and to land use change (i.e. conversion from arable to grassland). Second potential linkages between enzyme activities and critical ecosystem processes such as soil carbon (C) and nitrogen (N) sequestration were investigated.

Vascular plants comprise 20-30% lignin, constituting a considerable organic input to soils. Lignin is not necessarily preserved in soils, but the fate of its decomposition products in the wider environment is not well understood. Therefore, the overarching hypothesis tested herein was that a significant proportion of lignin is solubilised and lost from soils by transport in water. Solid phase extraction was used to extract lignin phenols from dissolved organic matter (DOM) from water outlets adjacent to major land use types (grazed grassland, deciduous woodland, and moorland) and compared to the lignin phenols from representative vegetation types, animal dungs and soils from each land use type. The phenols were identified and quantified using thermally assisted hydrolysis and methylation using tetramethylammonium hydroxide. Leachates from lysimeters treated with four vegetation types (grass, buttercup, ash, and oak) were sampled in a 22 month chronosequence, showing that some of the dominant phenols detected in the vegetation were also dominant in the respective DOM. A proportional relationship between increasing temperature and loss of representative lignin phenols in DOM was observed. Comparison of the dominant phenols in vegetation, soil and water sampled from field sites suggested specific lignin phenols could be used as biomarkers for different land uses. The concentrations of organic carbon-normalised total lignin phenols in the soils were similar to those in water, indicating that a considerable proportion of lignin in soils is lost via leaching. There was no significant difference in losses of lignin phenols between each land use type. Application of different rates of dissolved lignin to lysimeters indicated that the amount of water added was a dominant driver of transport through soil over 16 days, and that molecular structure also influenced transport rates of individual phenols. The impact of this research is that climate change (increased precipitation and warming) may significantly affect the loss of lignin by increased solubilisation and leaching from soils.

Better understanding of the interaction between the soil physical properties determining water and nitrate availability and the root proliferation and gene expression components of nutrient acquisition could contribute to food security, but may have been limited by experimental systems. A sand rhizotron system was developed to investigate Arabidopsis (Arabidopsis thaliana) root responses to altered water and nitrate supply as manipulated by soil physical properties. When this system was compared to agar, root disparities were explained by differences in hydraulic properties, highlighting the importance of the soil physical component. The sand rhizotron system was adopted to quantify root proliferation and gene expression responses to altered water and nitrate availability in wild-type and selected mutant seedlings. In the sand rhizotron system, primary root length and lateral root density were oppositely regulated by water availability, but similarly independent of nitrate supply. The expression of the nitrate transporter AtNRT2.1 and the aquaporin AtPIP2.2 was coordinated across all treatments. Their concentration-dependent hydraulic regulation was confirmed for AtNRT2.1 by in situ imaging of a Green Fluorescent Protein reporter line. AtNAR2.1 and AtNRT2.1 expression demonstrated independent responses to water and nitrate availability despite the requirement of AtNAR2.1 for AtNRT2.1 uptake function. Root proliferation responses to water availability under high (10.0 mM) nitrate were lost in the atnar2.1 mutant and coincided with altered hormone-associated gene (AtEIN2, AtABI4 and AtIPT5) expression. Root proliferation and AtNAR2.1 responses to water availability under high (10.0 mM) nitrate required AtPIP2.2. The coordination of root proliferation and gene expression responses to altered water and nitrate availability is proposed, that includes novel roles for AtNRT2.1, AtNAR2.1 and AtPIP2.2.

No description available.

No description available.

Soil changes occurring under A. mangium were studied by comparing soil chemical properties between A. mangium plantations and adjacent non- A. mangium areas, both on a regional and a local scale. Soil pH, exchangeable base cations (especially Ca), and base saturation decreased under A. mangium while exchangeable acidity and Al increased. Translocation of base cations down the soil profile was shown by a decrease in their concentrations in the upper parts of the soil profile and an increase at depths. This appears to be related to an increase in organic C and N content in the lower soil horizons. Solute chemistry and fluxes in precipitation, throughfall, stemflow and in the forest floor, at 30 and 60 cm soil depth were studied. Rates of litter production, litter decomposition and N-mineralization were also quantified. Canopy interception of precipitation was high (20%) resulting in retention of most of the solutes. These, however, were later released in considerable amounts from the forest floor. Dissolved organic C showed the highest flux from the forest floor. Litter production was high under A. mangium at 12 t ha-1 yr-1 while leaf litter decomposition rate was relatively low at 0.45 yr-1 suggesting that forest floor as a significant source of organic acids. While considerable NO3-N flux from the forest floor was also seen, concentration of NO3-N was highest in the surface mineral soil. Soil nitrification rate was estimated as 17.7 g g-1 30 d-1 for the 0-15 cm depth. In a laboratory soil leaching experiment, interactions of throughfall and stemflow solutions with A. mangium litter caused significant changes in the soil chemical properties. Retention of DOC in the soil plus the continual increase in leachate pH suggest enhanced weathering. Increase in soil pH, exchangeable bases and base saturation was observed suggesting that in the short term A. mangium results in 'ameliorative' effects on the soils, but may be deleterious in the long-term especially on highly weathered soils.

No description available.

The nature and distribution of phosphorus in soils has been reviewed with particular attention to organic forms. Chromatographic studies by various workers indicate that a major part of the organic P in many soils consists of inositol phosphates (I.P.). Several stereoisomers of inositol (hexahydroxycyclohexane) have been identified but the chief one found in soils is the myo-form and the whole range of esters from lP. to lPg has been detectedbut the ones most abundant in soils are IP5+. A major problem in research on the subject is to extract the various forms without degradation of the more labile constituents and then to isolate them free of contaminating materials. It was found that when soils are extracted with boiling formic acid or with formic acid/water or formic acid/acetylacetone solutions most of the soil P remained undissolved in the residue due to absorption or precipitation by polyvalent metal ions. . A brief boiling with 6m HC1 dissolved most of inorganic and the organic P from a clay loam, with full recovery of added lPg biit the P in a sandy loam was only partially dissolved. The latter soil contained acid soluble titanium minerals and Til,+ precipitates lPg completely from 6m HC1. A short boiling with 6M HCl/0.5 HP solution containing 0.16m T1CI4. dissolved most of the inorganic P from both soils while the unhydrolysed organic P was mostly retained in the soil residues., being precipitated by Ti. Inclusion of HP to form TiFg2- with half the Ti prevented precipitation of inorganic P by Zr. Hie organic P was dissolved from the soil residues by shaking with 3M HC1 + cupferron to precipitate Ti and it was then recovered by precipitation with Ba from an ethanolic/ethanolamine solution at pH 9 Chromatographic studies confirmed that most of this material was IP5 + lPg with, small . amounts of lower esters, and some unidentified organic P from the sand;/- loam. The procedure evolved is' simple to operate and should prove useful for further detailed stxidies of the organic P in soils.

Ten extracting solutions were used to study the effects of pH, solvent/soil ratio and temperature on the amounts of P and E extracted from 51 soils, 38 collected from experimental plots in England and Wales and the remainder from Scotland and Greece, representing wide ranges in texture, parent materials and soil characteristics. The amounts of P extracted by 0.5M HOAc and NaOAc mixtures buffered to 5 equidistant steps within the pH range from 4.00 - 5°25 were determined. The data for 31 soils at seven solvent/soil ratios obtained by plotting pH against P extracted, show that the solubility of P falls off strongly as the pH is raised, above pH 4"00o On average, at solvent/soil ratios 3d25 and 200, the P extracted decreased by 36$ and 45$ respectively for each unit rise in pHc No clear pH dependent relation for the K-extraction was obtained. The effects of solvent/soil ratio were studied over the range 3125 to 200. Generally the amounts of P and K extracted increased as the ratio increased. The magnitude of the ratio effect on the P-extracted was generally in the order HOAc > acetate buffer solutions > NaHCO. , being substantially greater in the calcareous soils, except forthe NaHCO. extraction0 Por K-extraction the ratio effect was about the samefor HOAc and acetate buffer solutions, greater for NaHCOj, especially at high ratios, and much smaller for the NH. OAc solution, The replacing ability ofHOAc solution was about 6. of that of NH. OAc at ratio 40, of acetate buffersolutions about 43$ at ratio 5> and that of NaHCO. solution about 65$ at ratio20, but more than 10C$ at ratios > 100. The effect of temperature on the extraction of P with acetate buffer and HOAc solutions was complex and its direction and magnitude depended on the nature of the soil and the solvent/soil ratio. NaHCO. extraction was very sensitive to changes in temperature, increasingby about 3$ per °C risec Temperature effect on the extraction of K by acetate buffer and HOAc solution had the same positive magnitude, about 1--$ per °C increase, but it was generally very small for NH. OAc extraction. The soils were cropped in the greenhouse with ryegrass in two separate experiments with different levels of phosphorus and potassium fertilizers to provide P/K response curves. P and K uptakes and 'P1 and 'K' values equivalent to the a-values as defined by Dean, were used as plant parameters for correlation with chemical methods of assessing soil P and Ko The NaHCO. amethod was leastaffected by soil type and yielded the highest coefficients of correlation. The 'P' and 'K' values appeared promising for evaluating the P and K status of soils in long-term experiments, differentiating between different P and K levels.