The impact of relict organic materials on the denitrification capacity in the unsaturated-saturated zone continuum of three volcanic profiles.

The denitrification capacity of wetlands, riparian zones, and aquifers in glacial outwash areas is well documented, but little or no information exists for volcanic profiles, particularly those containing relict organic matter contained in or on top of paleosols (old soils buried by volcanic deposits) below the groundwater table. Relict carbon contained in these layers could provide the necessary electrons to fuel heterotrophic denitrification. To the best of our knowledge, this is the first study investigating the denitrification capacity in both the unsaturated and saturated zone of volcanic profiles. Samples from three profile types with differing organic matter distribution were amended with N-enriched nitrate (NO-) and incubated in the laboratory under anaerobic conditions. Dinitrogen (N) dominated the N gas fluxes; averaged across all samples, it accounted for 96% of the total N (nitrous oxide [NO] and N) gas fluxes. Dinitrogen fluxes were generally highest in the A horizon samples (4.1-6.2 nmol N g h), but substantial fluxes were also observed in some paleosol layers (up to 0.72 nmol N g h). A significant correlation ( < 0.001) was found between the concentration of extractable dissolved organic carbon and the total N gas flux produced in samples from below the A horizon, suggesting that heterotrophic denitrification was the dominant NO attenuation process in this study. Extrapolation of lab-derived denitrification capacities to field conditions suggests that the denitrification capacity of profiles containing relict soil organic matter in the saturated zone exceeds the estimated N leaching from the root zone.

The influence of unsaturated zone drainage status on denitrification and the redox succession in shallow groundwater.

Since nitrate is a major agricultural freshwater contaminant, denitrification is the environmentally most important step in the ecological succession of redox processes that can occur in groundwater. Understanding where and to what extent denitrification occurs would enable spatially differentiated land management and regulation. We investigated in a dairy farming catchment in the North Island of New Zealand the influence of the unsaturated zone's drainage status on the redox succession in shallow groundwater along a well transect spanning drainage conditions from well drained to very poorly drained. Groundwater samples were analysed for a variety of parameters including nitrate, tritium, dinitrogen, argon, methane and nitrous oxide. The redox classification based on measured redox-sensitive parameters broadly matched the a priori assessed drainage status of the overlying unsaturated zone. Only the groundwater underlying the well-drained soil was oxic and reflected the N losses from the intensive pastoral land use, with nitrate nitrogen concentrations up to 9.6 mg L-1. All other sites had mildly to strongly reduced groundwater and concomitantly decreasing or low nitrate concentrations, even at the water table. The tritium-derived mean residence time (MRT) estimates for the oxic groundwater (12 and 14 y) were within the range found in mildly reduced groundwater from the imperfectly drained sites (6-24 y), with the exception of one sample from below an aquitard (105 y). In contrast, the strongly reduced groundwater observed at the poorly and very poorly drained sites was relatively immobile (55 to >110 y). Denitrification was confirmed by the nitrate dual isotope signatures, and by the occurrence of excess dinitrogen, and likely occurred in both the unsaturated and saturated zones. A coherent sampling scheme throughout the unsaturated zone - saturated zone continuum should be used in future studies to allow ascertaining the exact location of denitrification activity in vertical profiles.

Vertical stratification of redox conditions, denitrification and recharge in shallow groundwater on a volcanic hillslope containing relict organic matter.

Natural denitrification in groundwater systems has been recognised as an ecosystem service that reduces the impact of agriculturally-derived nitrate inputs to surface waters. Identification of this ecosystem service within the landscape would permit spatially differentiated land management and legislation. However, spatial variation in groundwater redox conditions poses a significant challenge to such a concept. To gain understanding of the small-scale mosaic of biogeochemical and hydrological controls on denitrification, we established a well field consisting of 11 multilevel well (MLW) clusters on a hillslope containing relict organic matter buried by volcanic deposits 1.8 ka before present. Based on site-specific redox classification thresholds, vertical redox gradients and denitrification potentials were detected at 7 of the 11 sites. Palaeosols or woody debris, which had previously been identified in laboratory experiments as resident electron donors fuelling denitrification, were visually recognisable at 4 of the 7 MLW sites with vertical redox gradients. Moderately enhanced groundwater dissolved organic carbon (DOC) concentrations occurred where resident electron donors were evident. DOC concentrations were lower where anoxic and nitrate-depleted groundwater was found but with an absence of resident electron donors. In these instances, it was assumed that nitrate reduction had occurred somewhere upgradient of the sampled well screen along the lateral groundwater flow path, with the proximate electron donor (DOC) largely consumed in the process, since no evidence was found for denitrification being fuelled by inorganic electron donors. Due to high variability in the isotopic signature of nitrate in oxidised groundwater, the nitrate dual isotope method did not yield firm evidence for denitrification. However, realistic vertical patterns were obtained using the excess N2 method. Tritium-based age dating revealed that oxic conditions were restricted to young groundwater (mean residence time ≤ 3 y), while anoxic conditions were observed across a wider age range (3-25 y).

Coronary arterial surgery in the elderly: its effect in the relief of angina.

Financial and other constraints, such as operative risk, may prevent older patients being considered for coronary arterial bypass grafting. Grafting was performed in 315 elderly patients (244 males, 71 females, age 65-79, mean 69 years) between 1981-1986. All patients had limiting angina, 38% had rest pain, 90% were housebound and 80% had triple-vessel disease. Impairment of left ventricular function was absent in 46%, mild in 20%, moderate in 23% and severe in 10% of patients. Grafts (saphenous vein or internal mammary artery) were inserted into 3 vessels (52%), 4 vessels (42%), 5 vessels (6%), 6 vessels (0.5%). Death during surgery occurred in 1.6% and a further 3.5% of patients died later during the same admission (70% of deaths were among the 33% with preoperative moderate or severe left ventricular impairment). Surgical complications included myocardial infarction (8%), cerebrovascular accident (1%), transient cerebral vascular ischaemia (5%), chest infection (10%) and wound infection (4%). Median stay on the intensive care unit was 1 day and median total hospital stay 12 days. 299 patients therefore survived to leave hospital and follow-up data are available for 217 (72%) of these. 96% were subjectively improved by surgery, 88% being free of angina on no antianginal drugs a median of 72 weeks (range 8-307) and a further 8% not limited by angina on medical therapy a median of 85 weeks (range 9-302) after surgery. We conclude that coronary arterial surgery is an effective treatment for angina in the elderly. This will have consequences for future resource allocation if the elderly are not to be denied effective therapy because of financial rather than clinical restraints.

Restrictive cardiomyopathy and constrictive pericarditis: non-invasive distinction by digitised M mode echocardiography.

It is difficult to distinguish between restrictive cardiomyopathy and constrictive pericarditis on the basis of clinical findings and simple investigation. Cardiac catheterisation has been the reference standard for diagnosis but even this does not always permit an accurate distinction. A Summagraphics digitiser and Prime 750 computer system were used to digitise the echocardiograms of 15 patients with restrictive cardiomyopathy, 10 with constrictive pericarditis and a group of 20 age and sex matched normal subjects of similar age and sex distribution. Compared with controls, patients with restrictive cardiomyopathy showed a significant reduction in the following variables (a) decreased fractional shortening, (b) decreased peak left ventricular filling and emptying rates, (c) decreased percentage posterior wall thickening, and (d) decreased peak left ventricular posterior wall thickening and thinning rates. Whereas patients with constrictive pericarditis only had significantly reduced peak left ventricular filling and posterior wall thinning rates and significantly increased posterior wall thinning rate. When patients with restrictive cardiomyopathy were compared with those with constrictive pericarditis the significant differences were: (a) decreased peak left ventricular emptying rate, (b) decreased percentage posterior wall thickening, and (c) decreased peak left ventricular posterior wall thickening and thinning rates. Digitisation of M mode echocardiograms, with particular attention to posterior wall function, may be a useful adjunct to cardiac catheterisation in distinguishing restrictive cardiomyopathy from constrictive pericarditis.