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High nitrification rates which convert ammonium (NH4+) to the mobile ions NO2− and NO3− are of high ecological significance because they increase the potential for N losses via leaching and denitrification. Nitrification can be performed by chemoautotrophic or heterotrophic organisms and heterotrophic nitrifiers can oxidise either mineral (NH4+) or organic N. Selective nitrification inhibitors and 15N tracer studies have been used in an attempt to separate heterotrophic and autotrophic nitrification. In a laboratory study we determined the effect of cattle slurry on the oxidation of mineral NH4+-N and organic-N by labelling the NH4+ or NO3− pools separately or both together with 15N. The size and enrichment of the mineral N pools were determined at intervals. To calculate gross N transformation rates a 15N tracing model was developed. This model consists of the three N-pools NH4+, NO3− and organic N. Sub-models for decomposition of degradable carbon in the soil and the slurry were added to the model and linked to the N transformation rates. The model was set up in the software ModelMaker which contains non-linear optimization routines to determine model parameters. The application of cattle slurry increased the rate of nitrifcation by a factor of 20 compared with the control. The size and enrichment of the mineral N pools provided evidence that nitrification was due to the conversion of NH4+ to NO3− and not the conversion of organic N to NO3−. There was evidence that slurry-enhanced oxidation of NH4+ to NO3− was due to a combination of autotrophic and heterotrophic transformations. Slurry application increased the mineralisation rate by approximately a factor of two compared with the control and the rate of immobilisation of NH4+ by approximately a factor of three. [Copyright &y& Elsevier]

Denitrifying enzyme activity (DEA) and flux rates of nitrous oxide (N2O) and dinitrogen (N2) were studied in DEA assays on soils treated with 3,4-dimethylpyrazole phosphate (DMPP). Nitrous oxide and N2 fluxes were quantified by 15N gas-flux method with no additional enzymatic inhibitors, thus, overcoming problems associated with the use of chloramphenicol and acetylene. The nitrification inhibitor DMPP did not affect DEA or the measured gas emissions even when applied in concentrations 14 times higher than the recommended concentration. [Copyright &y& Elsevier]

A laboratory incubation study was conducted with nine UK soils to determine the effect of soil physical and chemical properties, and temperature, on the efficacy the nitrification inhibitor dicyandiamide (DCD). Nitrogen was applied to soil as ammonium chloride at a rate of 100 μg N g −1 dry soil, and incubated at 60% water-filled-pore-space at either 5, 15 or 25 °C. The ammonium ( NH 4 + ) pool was enriched with 15 N to 60 atom% excess and DCD was applied at a rate of 10 μg g −1 dry soil. The concentrations and enrichments of the NH 4 + and nitrate ( NO 3 − ) pools, along with nitrous oxide (N 2 O) flux measurements, were determined regularly for 60 days after N application. Gross soil N transformation rates were quantified with a 15 N tracing model. The persistence of DCD was strongly related ( P < 0.001) to temperature with the measured half-life across all soils of 89, 37 and 18 days at 5, 15, and 25 °C, respectively. There was wide variation in the half-life of DCD among soils; which was predominantly associated with the soil oxalate extractable Fe concentration. Greater ( P < 0.001) inhibition in autotrophic nitrification by DCD occurred at 5 and 15 °C compared to 25 °C. Across all soils and temperatures DCD increased the rate of mineralisation of recalcitrant organic-N and the rate of adsorption of free NH 4 + , however, effects varied between soils. DCD did not have a significant effect on the rate of oxidation of recalcitrant organic-N to NO 3 − or on any of the immobilisation processes or mineralisation of labile N to NH 4 + . The efficacy of DCD in inhibiting net NO 3 − production best correlated with soil Cu ( r = −0.82), % clay ( r = −0.71), total N ( r = −0.66) and LOI ( r = −0.61). Stepwise multiple regression showed that Cu, oxalate extractable Fe and oxalate extractable Al explained 85.0% of the variation in the percentage inhibition of net NO 3 − production by DCD. The inhibitor also reduced cumulative N 2 O emissions, with reductions negatively correlated with a range of soil properties associated with organic matter. We provide evidence that the interaction between temperature, soil clay content and soil organic matter governs the efficacy of DCD. The grassland soils had higher native total N concentrations than the arable soils, hence the inhibition of net NO 3 − production by DCD was lower and this resulted in an overall inhibition in N 2 O emissions of 58% and 81% for grassland and arable soils respectively. [ABSTRACT FROM AUTHOR]

Ntrace is a family of tools for the analysis of 15N tracing data sets, to quantify the simultaneous gross nitrogen (N) transformation rates in terrestrial ecosystems. Starting with the original publication of Ntrace in 2007, a number of specialised tools have been developed that cover a range of applications such as nitrite and gaseous N dynamics. While the capability of the tool has been extended to cover a range of applications, the underlying parameter optimisation algorithm, based on the Metropolis MCMC algorithm, has remained unchanged. However, in more computational demanding applications, this algorithm turned out to be very time consuming, and in some cases it was not even able to identify unambiguous parameter sets. The aim of this study was, to test a new global optimisation algorithm (GlobalSearch), by re-analysing some of the published datasets. In the evaluation, the speed of finding a suitable solution but in particular the accuracy of the new algorithm to obtain an adequate fit to the observations was taken into account. A new uncertainty calculation for the optimised parameters, based on the Levenberg-Marquardt method, has also been implemented. The new Ntrace tool was able to find an adequate fit for all tested datasets, in often a fraction of the time compared to the original MCMC algorithm. For all test cases the accuracy of the fit was at least as good as with the original model. This is partly due to a new algorithm's option to vary the initial pool size. In summary, the new Ntrace tool is quicker in finding suitable parameters for 15N tracing data sets. The fit to measured data is often improved over the original tool. Thus, we recommend the use of the new Ntrace tool for analysing 15N tracing data. The new tool is programmed in MATLAB and can easily be adapted to new applications. • A new parameter optimisation method was tested to analyse complex 15N tracing data. • GlobalSearch (MATLAB) was compared to the original MCMC method in Ntrace. • The new algorithm is quicker and more accurate in finding suitable parameters. • New functions for initial pool sizes and parameter uncertainties were implemented. • The advanced Ntrace tool is recommended for analysing complex 15N tracing data. [ABSTRACT FROM AUTHOR]

Facing rising atmospheric CO 2 concentrations, subsoils may play an important role in the global carbon (C) cycle due to the presence of unsaturated mineral surfaces. Further, macroaggregation is considered a crucial process influencing C sequestration. However, analyses on subsoil aggregation and C retention processes under long-term elevated CO 2 (eCO 2 ) are lacking. In this study we investigated the long-term effect of +20% above ambient CO 2 concentration (corresponds to conditions reached 2035–2045) in a temperate grassland ecosystem at the Giessen Free Air CO 2 Enrichment (Gi-FACE), Germany. A depth-dependent response of macroaggregation to eCO 2 was observed: While in subsoil (15–45 cm depth) macroaggregation increased under eCO 2 , no CO 2 induced change in macroaggregation was detected in topsoil (0–15 cm). Increased macroaggregation in subsoil coincided with higher SOC content of large macroaggregates (LM). Mean residence time (MRT) of SOC in aggregate-size classes were not different among each other under eCO 2 . However, macroaggregates and bulk soil differed in their MRT between soil depths. Despite increased macroaggregation and an estimated high SOC sequestration potential in subsoil we could not observe an increase in SOC content of bulk soil. [ABSTRACT FROM AUTHOR]

A laboratory and a field study were conducted on a permanent grassland soil in Northern Ireland to investigate the effects of urea in combination with N process inhibitors such as the urease inhibitor N-(butyl) thiophosphoric triamide (NBPT) and/or the nitrification inhibitor dicyandiamide (DCD) on soil N dynamics. Urea enriched with n-butyl 15 N to 60 atom % was applied to soil at a rate of 100 μg N g −1 dry soil in the laboratory and 100 kg N ha −1 in the field. A numerical 15 N tracing model was used to quantify several simultaneously occurring gross N transformation rates in both studies. The changes in soil nitrate (NO 3 − ) and ammonium (NH 4 + ) concentrations and 15 N enrichment over a 25-day period as well as the concentration and 15 N enrichment of plant N at harvest were used to model soil gross N transformations. The results showed that the effect of N process inhibitors varied firstly between laboratory and field studies and secondly whether the inhibitors were applied individually or in combination. Overall DCD had a greater effect on the major soil N transformations than NBPT; reducing oxidation of NH 4 + , total nitrification, net NO 3 − produced, total mineralisation and the net adsorption of NH 4 + at both laboratory and field scale. The effect of DCD was similar for these transformations whether applied alone or co-applied with NBPT. In contrast NBPT had no significant effect on oxidation of NH 4 + , total nitrification, total mineralisation or total immobilisation compared to urea in the field, while the effect on these transformations in the laboratory was significant. The contrasting effects of inhibitors on gross N transformations between laboratory and field may relate to the differences in experimental conditions, e.g. soil preparation, environmental conditions and the contribution of plant biomass. To obtain a more realistic assessment of gross soil N transformations in situ , it is essential that laboratory experiments are supplemented with field studies. [ABSTRACT FROM AUTHOR]

Abstract: A 15N tracing study was carried out to identify microbial and abiotic nitrogen (N) transformations in a south Chilean Nothofagus betuloides forest soil which is characterized by low N inputs and absence of human disturbance. Gross N transformation rates were quantified with a 15N tracing model in combination with a Markov chain Monte Carlo sampling algorithm for parameter estimation. The 15N tracing model included five different N pools (ammonium (NH4 +), nitrate (NO3 −), labile (Nlab) and recalcitrant (Nrec) soil organic matter and adsorbed NH4 +), and ten gross N transformation rates. The N dynamics in the N. betuloides ecosystem are characterized by low net but high gross mineralization rates. Mineralization in this soil was dominated by turnover of Nlab, while immobilization of NH4 + predominantly entered the Nrec pool. A fast exchange between the NH4 + and the adsorbed NH4 + pool was observed, possibly via physical adsorption on and release from clay lattices, providing an effective buffer for NH4 +. Moreover, high NH4 + immobilization rates into the Nrec pool ensure a sustained ecosystem productivity. Nitrate, the most mobile form of N in the system, is characterized by a slow turnover and was produced in roughly equal amounts from NH4 + oxidation and organic N oxidation. More than 86% of the NO3 − produced was immediately consumed again. This study showed for the first time that dissimilatory nitrate reduction to ammonium (DNRA) was almost exclusively (>99%) responsible for NO3 − consumption. DNRA rather than NO3 − immobilization ensures that NO3 − is transformed into another available N form. DNRA may therefore be a widespread N retention mechanism in ecosystems that are N-limited and receive high rainfalls. [Copyright &y& Elsevier]

A laboratory incubation study was conducted on a temperate grassland soil to quantify the main mineral nitrogen (N) transformation rates and pathways via a15N tracing approach. Soil samples were taken from a long-term phosphorus (P) trial to investigate the effects on gross N-transformations under high and low phosphorus amendment. The soils were incubated over a 2-week period and treated with ammonium-nitrate (NH 4 NO 3) which was applied to the soil both with and without a glucose amendment and labelled with 15N either on the ammonium (NH 4 +) or nitrate (NO 3 −) moiety at 50% atom enrichment. The results showed immobilisation to greatly outweigh mineralisation and that NO 3 − was predominantly produced via heterotrophic nitrification. Individual pathways for NO 3 − production were quantified including oxidation of NH 4 +, recalcitrant and labile organic N. Oxidation of labile organic N to NO 3 −, a newly considered pathway, accounted for between 63 and 83% of total NO 3 − production across the various treatments and P levels. This process was significantly higher in the low-P rather than the high-P soils (p < 0.05), highlighting the effect of soil P on the microbial community. • The relative abundance of C, N and P significantly influences N-transformations. • A novel nitrification pathway of oxidation of labile organic N to NO 3 − was found. • This pathway of accounted for between 63 and 83% of total NO 3 − production. • This novel pathway was significantly higher in the low-P soils when not C-limited. [ABSTRACT FROM AUTHOR]

This research investigated the effect of long-term phosphorus (P) addition relative to carbon (C) availability on nitrous oxide (N 2 O) emissions from an ungrazed grassland soil via two incubation experiments. No significant effect of soil P on N 2 O was found under C-limited conditions, while under added-C, cumulative N 2 O was significantly higher from low-P (p < 0.05) rather than high-P soils. CO 2 was not significantly different between P-levels. This highlights the influence of soil P on N 2 O emissions under non C-limiting conditions. This is one of the first studies demonstrating available-C moderating the effect of P on N 2 O emissions from temperate grassland soils. • Phosphorus effects on N 2 O emissions from soils are poorly understood. • Carbon limitation masks effects of phosphorus on nitrous oxide emissions. • When soil not C-limited, N 2 O reduced at higher P concentrations. • Manipulation of C:N:P ratio can reduce N 2 O emissions from grassland. [ABSTRACT FROM AUTHOR]