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Nitrous oxide (NO) affects climate change as a greenhouse gas and indirectly contributes to stratospheric ozone depletion. The main source of NO in soils is denitrification which requires high soil moisture, carbon and nitrate. Nitrification inhibitors can be used to mitigate emissions of NO from soils. In Portugal, fertilisers are often applied when soils are still relatively warm and moist conditions conducive to denitrification. A Portuguese arable soil was inhibited with dicyandiamide, a nitrification inhibitor and the effect on soil microbiological activity and composition was determined after 46 days. Soils were then incubated and received carbon and ammonium under high soil water conditions and mineral N and NO fluxes were measured during 22 days. We found that dicyandiamide decreased microbial populations and activity, but did not alter composition. Pre-conditioning of the soil with dicyandiamide was 80% more effective in reducing fluxes of NO than simultaneous application with fertiliser. [ABSTRACT FROM AUTHOR]

Much work has been done on gaseous emissions and leaching of nitrogenous compounds from whole soil profiles and also from soil surface measurements which are assumed to be mainly due to topsoil activity. In soils with an impervious clay subsoil, the boundary between the topsoil and subsoil may provide an interface for microbial activity, including N transformations. In this study, we investigated movement and transformations of two reactive N species (nitrate and urea) at the subsoil interface using a series of replicate, intact soil blocks, under two contrasting watering regimes. We measured fluxes in both liquid and gaseous phases and demonstrated that nitrate reaching the subsoil interface does not necessarily leach into water systems, but may denitrify immediately and could, therefore, add to atmospheric pollution through N2O production. On the other hand, ammonium reaching the subsoil interface either directly, or after mineralization, appears to be more mobile than expected and has the potential to pollute watercourses. [ABSTRACT FROM AUTHOR]

Nitrogen mineralization was measured in three permanent pastures – either fertilized or unfertilized grass, or a mixed grass-clover sward – which were further amended with either fertilizer or cattle dung over a summer growing season. Measurements were made at 4-weekly intervals from June to October. Rates of net mineralization were similar in each of the background treatments (overall mean 0.99±0.091 kg N ha–1 day–1) and did not change markedly during the experiment. From the second sampling (July) onwards, rates of mineralization in all the dung treatments were higher than in the control by a factor of up to 2. In the fertilizer-amended treatments, rates were also consistently (but not significantly) higher than in the control. However, the relatively small effect of fertilizer detected at each sampling had a significant cumulative effect by the end of the experiment. There was no interaction between the background and current treatments. Potential mineralization, measured by anaerobic incubation, increased in all the treatments over the period of the experiment, showing an accumulation of readily mineralizable residues. Total N mineralized and the N accumulated during the experiment were calculated and compared. This approach suggests that potential measurements could provide a good estimate of changes in soil N supply that would not be otherwise detectable in changes in soil total N in the short-term. [ABSTRACT FROM AUTHOR]

Net mineralization was measured in free-draining and poorly drained pasture soils using three different field incubation methodologies. Two involved the use of enclosed incubation vessels (jar or box) containing C2H2 as a nitrification inhibitor. The third method confined soil cores in situ in an open tube in the ground, with an anion-exchange resin at the base to retain leached NO3 – (resin-core technique, RCT). Measurements were made on three occasions on three free-draining pastures of different ages and contrasting organic matter contents. In general, rates of net mineralization increased with pasture age and organic matter content (range: 0.5–1.5 kg N ha–1 day–1) and similar rates were obtained between the three techniques for a particular pasture. Coefficients of variation (CVs) were generally high (range: 10.4–98.5%), but the enclosed incubation methods were rather less variable than the RCT and were considered overall to be the more reliable. The RCT did not include C2H2 and, therefore, newly formed NO3 – may have been lost through denitrification. In a poorly drained pasture soil, there were discrepancies between the two enclosed methods, especially when the soil water content approached field capacity. The interpretation of the incubation measurements in relation to the flux of N through the soil inorganic N pool is discussed and the drawbacks of the various methodologies are evaluated. [ABSTRACT FROM AUTHOR]

The 15N isotope dilution method was combined with a field incubation technique to provide simultaneous measurements of gross and net rates of N turnover in three long-term swards: unfertilized (Z) or receiving N either from N fixation as clover (C), or as 200 kg fertilizer N ha–1 year–1 (F). Uniform N enrichment of soil microplots was achieved with a multi-point soil injector to measure mineralization/immobilization turnover and nitrification over a 4-day incubation. Net rates of mineralization ranged between 0.6 and 2.9 μg N g–1 day–1 and in all three treatments were approximately half the gross rates. Nitrification rates (gross) were between 1.0 and 1.6 μg N g–1 day–1. In the F treatment, the turnover of NH4 +-N and NO3 –-N pools was on a 2- and 4-day cycle, respectively, whereas in the N-limited treatments (C and Z) turnover rates were faster, with the NO3 –-N pools turning over twice as fast as the NH4 +-N pools. Therefore, available N was recycled more efficiently in the C and Z treatments, whereas in the F treatment a higher N pool size was maintained which would be more vulnerable to leakage. A large proportion of the added 15N was recovered in the soil microbial biomass (SMB), which represented a 4–5 times larger sink for N than the plant biomass. Although the C treatment had a significantly lower SMB than the grass-only treatments, there were no differences in microbial activity. Gross rates of nitrification increased along the gradient of N input intensity (i.e. Z2H2) tended to increase microbial immobilization, but did not influence plant N uptake. In this study, the value of combining different techniques to verify net rates was demonstrated and the improved methodology for 15N labelling of soil enabled measurements to be obtained from relatively undisturbed soil under natural field conditions. [ABSTRACT FROM AUTHOR]

This study evaluated the effect of cultivation and reseeding on the distribution and fate of soil mineral N (SMN), soluble organic N (SON) and potentially mineralisable N (PMN) in the soil profile of two long-term grasslands in the UK. Cultivation and reseeding significantly increased the total soluble N concentration (SMN plus SON) of the soil profile (0–90 cm), with over 50 mg SON kg–1 observed. By contrast, the PMN pool was unaffected by cultivation and declined with increasing soil depth. The flush in SON and SMN observed in both soils disappeared within 1 year following cultivation. The fate of SON appeared to be dependent on soil type, with considerably more movement to deeper layers apparent in the profile of a silty clay loam (30% clay) than in a clay loam (49% clay). Mineralisation and/or immobilisation of SON in the topsoil probably accounted for the changes observed in the SON content of the clay loam. SON is an important N pool in grassland soils and cultivation has a significant impact on its release. Measurements of SON should therefore be included in studies of N cycling in agricultural cropping systems, so that full account may be taken of its potential as a source or sink of mobile N. [ABSTRACT FROM AUTHOR]

Gross rates of soil processes and microbial activity were measured in two grazed permanent pasture soils which had recently been amended with N fertilizer or dung. 15N studies of rates of soil organic matter turnover showed gross N mineralization was higher, and gross N immobilization was lower, in a long-term fertilized soil than in a soil which had never received fertilizer N. Net mineralization was also found to be higher in the fertilized soil: a consequence of the difference between the opposing N turnover processes of N mineralization and immobilization. In both soils without amendments the soil microbial biomass contents were similar, but biomass activity (specific respiration) was higher in the fertilized soil. Short-term manipulation of fertilizer N input, i.e. adding N to unfertilized soil, or witholding N from previously fertilized soil, for one growing season, did not affect gross mineralization, immobilization or biomass size and activity. Amendments of dung had little effect on gross mineralization, but there was an increase in immobilization in both soils. Total biomass also increased under dung in the unfertilized soil, but specific respiration was reduced, suggesting changes in the composition of the biomass. Dung had a direct effect on the microbial biomass by temporarily increasing available soil C. Prolonged input of fertilizer N increases soil C indirectly as a result of enhanced plant growth, the effect of which may not become evident within one seasonal cycle. [ABSTRACT FROM AUTHOR]

A study was conducted to determine mineralization rates in the field and in different soil layers under three grassland managements (viz. a reseeded sward, a permanent sward with a conventional N management, and a long-term grass sward with 0 N (0-N) input). Potential mineralization rates of soil particles (sand, silt and clay) and macro-organic matter fractions of different sizes (i.e. 0.2–0.5, 0.5–2.0 and >2 mm) were also determined in the laboratory. In the reseeded plots, net mineralization was unchanged down to 40 cm depth. In the undisturbed conventional-N swards, mineralization rates were substantially higher in the top layer (0–10 cm) than in the deeper layers. In plots which had received no fertilizer N, mineralization was consistently low in all the layers. There was more macro-organic matter (MOM) in the 0-N plots (equivalent to 23 g kg–1 soil for 0–40 cm) than in the two fertilized plots (i.e . conventional-N and reseeded) which contained similar amounts (ca. 15 g kg–1 soil). C and N contents of separated soil particles did not differ amongst the treatments, but there were large differences with depth. Potential mineralization in the bulk soil was greatest in the 0–10 cm layers and gradually decreased with depth in all the treatments. Separated sand particles had negligible rates of potential mineralization and the clay component had the highest rates in the subsurface layers (10–40 cm). MOMs had high potential rate of mineralization in the surface layer and decreased with soil depth, but there was no clear pattern in the differences between different size fractions. [ABSTRACT FROM AUTHOR]

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Objective: Acute hypoxemic respiratory failure (AHRF) is a common reason for emergency pediatric intensive care. An objective assessment of disease severity from acute physiological parameters would be of value in clinical practice and in the design of clinical trials. We hypothesised that there was a difference in the best early respiratory indices in those who died compared with those who survived. Design: A prospective observational study of 118 consecutive AHRF admissions with data analysis incorporating all blood gases. Setting: A pediatric intensive care unit in a national children’s hospital. Interventions: None. Results: Mortality was 26/118,22% (95% confidence interval 18–26%). There were no significant differences in the best alveolar-arterial oxygen tension gradient (A-aDO2, torr), oxygenation index (OI), ventilation index (VI), or PaO2/FIO2 during the first 2 days of intensive care between the survivors and non-survivors. Only the mean airway pressure (MAP, cm H2O) used for supportive care was significantly different on days 0 and 1 ( p≤0.05) with higher pressure being used in non-survivors. Multiple logistic regression analysis did not identify any gas exchange or ventilator parameter independently associated with mortality. Rather, all deaths were associated with coincident pathology or multi-organ system failure, or perceived treatment futility due to pre-existing diagnoses instead of unsupportable respiratory failure. When using previously published predictors of outcome (VI>40 and OI>40; A-aDO2>450 for 24 h; A-aDO2>470 or MAP>23; or A-aDO2>420) the risk of mortality was overestimated significantly in the current population. Conclusion: The original hypothesis was refuted. It appears that the outcome of AHRF in present day pediatric critical care is principally related to the severity of associated pathology and now no longer solely to the severity of respiratory failure. Further studies in larger series are needed to confirm these findings. [ABSTRACT FROM AUTHOR]

An improved method is described for incubating intact soil cores in the field, which permits concurrent measurement of net mineralization, nitrification, denitrification and leaching. Cores were enclosed in PVC tubes with minimal disturbance to the physical state or to the natural cycles of wetting/drying, soil temperature and aeration during an incubation lasting 4–5 days. An example of the application of the method is given in which soils with contrasting drainage characteristics were compared. Over a 64-day experimental period, 58% of the mineralized nitrogen (N) in a freely drained soil was nitrified and 36% of the nitrate-N (NO3 –-N) was denitrified. In a poorly drained soil, 72% of the mineralized N was nitrified and 63% of the NO3 –-N was denitrified. In both soil types, 18% of the remaining NO3 –-N was leached. Rates of nitrification were significantly correlated with net mineralization ( r2=0.41 and 0.52) and also closely correlated with denitrification ( r2=0.67 and 0.68) in the freely and poorly drained soils, respectively. Independent measurements of these processes, using alternative techniques (for the same period), compared favourably with measurements obtained with the improved incubation method. Adoption of this method has a number of advantages with respect to field net N mineralization, and also allows interpretation of the impact this may have on other N transformation processes. [ABSTRACT FROM AUTHOR]

Soil microbial biomass (SMB) activity was investigated in a long-term experiment in which grazed swards received annual inputs of 200 N kg ha–1. SMB total C and total N, specific respiration, ammonification and nitrification were examined over a 10 week period, following the first and the second seasonal applications of N. Whilst there was no effect on biomass C and N, additions of N appeared to increase biomass activity. Nitrification was weakly correlated with ammonification ( r2=0.413) and the latter was stimulated by the addition of N ( P<0.05), suggesting a ‘priming’ effect. [ABSTRACT FROM AUTHOR]

"This study evaluated the suitability of a soil core incubation technique (with acetylene added to inhibit loss of N by denitrification; CIT) and a resin-core incubation technique (RCT) for measurement of net N mineralisation under arable cropping conditions. A conventional N balance (BAL) approach to the measurement of N mineralisation was used for comparison. In a sandy soil during winter 1996/97, CIT estimates of net N mineralisation were approximately 3 times greater than RCT and BAL estimates, which were in close agreement. Soil disturbance(with the consequent exposure of physically protected organic matter) did not enhance the rate of net N mineralisation measured by CIT onthe sandy, low-organic-matter soil studied, although an increase in soil aeration may have enhanced rates above those measured by RCT andBAL. Overall, RCT was considered to be the more favourable techniquefor estimation of net N mineralisation. It also provided a measure of nitrate leaching which was comparable to that obtained by porous ceramic water samplers. However, separate estimates of the likely loss of N by denitrification should be obtained with soils which are particularly vulnerable (eg. poor aeration and high clay or water content). Spatial variability was a particular problem with all three techniques which can be overcome by taking a large number of soil cores to increase sample replication. [ABSTRACT FROM AUTHOR]