This thesis reports a series of experiments performed to investigate the effects of soil pH on mineralisation processes in agricultural soils. The experiments utilised a short term soil incubation assay which measured the mineralisation of a mixture of 15 uniformly 14C labelled amino acids to 14CO2• Amino acids were chosen to represent the labile pool of dissolved organic matter and their mineralisation provides a means of comparing the activity of the soil microbial population performing mineralisation of nitrogenous substrates.
An examination of mineralisation in five profiles of acidic soils (chapter 4) showed that the rate of amino acid mineralisation decreased with depth and was correlated significantly and linearly with total C and total N within each profile. The profiles examined contained high levels of nitrate suggesting the presence of significant populations of acid tolerant nitrifying bacteria. A strong positive correlation between basal respiration rate and the rate of amino acid mineralisation was demonstrated within most profiles.
A study investigating the impact of varying intensities of sheep grazing from three upland regions of the UK on soil mineralisation processes was undertaken (chapter 5). This showed that the soil microbial biomass was maximal at low to intermediate levels of grazing across the three regions and declined as the legacy of grazing was reduced through the long term removal of sheep. High intensity of grazing tended to reduce the phenotypic evenness (a component of diversity) of the microbial community. Net mineralisation rates were highest in mid-successional and lightly grazed treatments in all regions and were generally lowest at the extremes of grazing influence. However, the rate of amino acid mineralisation was generally lowest in the short-term ungrazed and lightly grazed treatments and were fastest at the highest grazing pressure in all regions, supporting the model that heavily grazed grassland favours fast nutrient cycles dominated by labile substrates. Multiple regression of
data from all sites showed that the impact of grazing on the activity of the SMB that actively mineralises nitrogenous substrates appears to function primarily through its effect on soil pH.
A field study was conducted to investigate the feasibility of decreasing nitrate
leaching in cereal and grass plots by acidification of soil (chapter 6). The results from porous ceramic cup extracted soil water showed that nitrate concentrations in drainage water were greater in cereal plots than in grass plots. Soil acidification lowered nitrate concentrations in drainage water, substantially over winter and spring in grass plots and in cereal plots the effect was minimal during winter but became more substantial in spring. Data indicates that soil acidification decreased nitrification rates, causing the ammonium pool to accumulate. Soil acidification also lowered levels of dissolved organic nitrogen in soil water, usually to a greater degree in grass than in cereal plots. It was concluded that it may be possible to use careful soil pH management as a tool to control nitrate leaching without compromising the quality of drainage water, this may be more effective on grassland than on arable crops. Long term experimental plots from Rothamsted Experimental Station, Woburn
Experimental Farm and the Scottish Agricultural College Craibstone Estate were
sampled to investigate the effect of soil pH on a range of microbially mediated soil mineralisation characteristics and processes (chapter 7). The results showed that soil pH did not significantly affect indigenous mineral nitrogen levels at the time of sampling and had little consistent effect on levels of soluble organic nitrogen or carbon across soil types. Soil pH also did not show any great influence on net ammonification, net nitrification or net total mineralisation in a 30 d aerobic incubation. Soil pH was positively correlated with soil microbial biomass carbon and nitrogen and soil basal respiration in each soil type. The proportions of organic C and N that were biomass C and N were positively correlated to soil pH, indicating an increase in availability of the C and N present in the soil with rising pH. Glucose-C and urea-C mineralisation rates were fastest at intermediate points of the pH range studied in each soil type. Arginine-C mineralisation was positively and linearly related to soil pH.
Results of chapters 5, 6 and 7 showed that soil pH had a significant impact on the rate of amino acid mineralisation. Acidity increased the proportion of added amino acid-C used in respiration and decreased the proportion used in biomass, implying an acidity-induced stress on the microbial population. Soil microbial biomass C or N, basal respiration rates, total soil C and N and dissolved organic C and N were shown not to adequately predict the rate of amino acid mineralisation over the range of soil types studied. Suggestions for further investigations into the soil pH effects on characteristics of organic substrates and how they may determine carbon and nitrogen cycling rates are made.