Modelling the inorganic nitrogen behaviour in a small Mediterranean forested catchment, Fuirosos (Catalonia)

The aim of this work was to couple a nitrogen (N) sub-model to already existent hydrological lumped (LU4-N) and semi-distributed (LU4-R-N and SD4-R-N) conceptual models, to improve our understanding of the factors and processes controlling nitrogen cycling and losses in Mediterranean catchments. The N model adopted provides a simplified conceptualization of the soil nitrogen cycle considering mineralization, nitrification, immobilization, denitrification, plant uptake, and ammonium adsorption/desorption. It also includes nitrification and denitrification in the shallow perched aquifer. We included a soil moisture threshold for all the considered soil biological processes. The results suggested that all the nitrogen processes were highly influenced by the rain episodes and that soil microbial processes occurred in pulses stimulated by soil moisture increasing after rain. Our simulation highlighted the riparian zone as a possible source of nitrate, especially after the summer drought period, but it can also act as an important sink of nitrate due to denitrification, in particular during the wettest period of the year. The riparian zone was a key element to simulate the catchment nitrate behaviour. The lumped LU4-N model (which does not include the riparian zone) could not be validated, while both the semi-distributed LU4-R-N and SD4-R-N model (which include the riparian zone) gave satisfactory results for the calibration process and acceptable results for the temporal validation process.

Amino-acid cycling drives nitrogen fixation in the legume - Rhizobium symbiosis

The biological reduction of atmospheric N-2 to ammonium (nitrogen fixation) provides about 65% of the biosphere's available nitrogen. Most of this ammonium is contributed by legume rhizobia symbioses(1), which are initiated by the infection of legume hosts by bacteria (rhizobia), resulting in formation of root nodules. Within the nodules, rhizobia are found as bacteroids, which perform the nitrogen fixation: to do this, they obtain sources of carbon and energy from the plant, in the form of dicarboxylic acids(2,3). It has been thought that, in return, bacteroids simply provide the plant with ammonium. But here we show that a more complex amino-acid cycle is essential for symbiotic nitrogen fixation by Rhizobium in pea nodules. The plant provides amino acids to the bacteroids, enabling them to shut down their ammonium assimilation. In return, bacteroids act like plant organelles to cycle amino acids back to the plant for asparagine synthesis. The mutual dependence of this exchange prevents the symbiosis being dominated by the plant, and provides a selective pressure for the evolution of mutualism.

The effects of a changing pollution climate on throughfall deposition and cycling in a forested area in southern England

This study compares two sets of measurements of the composition of bulk precipitation and throughfall at a site in southern England with a 20-year gap between them. During this time, SO2 emissions from the UK fell by 82%, NOx emissions by 35% and NH3 emissions by 7%. These reductions were partly reflected in bulk precipitation, with deposition reductions of 56% in SO4,38% in NO3, 32% in NH4, and 73% in H+. In throughfall under Scots pine, the effects were more dramatic, with an 89% reduction in SO4 deposition and a 98% reduction in H+ deposition. The mean pH under these trees increased from 2.85 to 4.30. Nitrate and ammonium deposition in throughfall increased slightly, however. In the earlier period, the Scots pines were unable to neutralise the high flux of acidity associated with sulphur deposition, even though this was not a highly polluted part of the UK, and deciduous trees (oak and birch) were only able to neutralise it in summer when the leaves were present. In the later period, the sulphur flux had reduced to the point where the acidity could be neutralised by all species — the neutralisation mechanism is thus likely to be largely leaching of base cations and buffering substances from the foliage. The high fluxes are partly due to the fact that these are 60–80 year old trees growing in an open forest structure. The increase in NO3 and NH4 in throughfall in spite of decreased deposition seems likely due to a decrease in foliar uptake, perhaps due to the increasing nitrogen saturation of the catchment soils. These changes may increase the rate of soil microbial activity as nitrogen increases and acidity declines, with consequent effects on water quality of the catchment drainage stream.

Water quality trends in the Windrush catchment: Nitrogen speciation and sediment interactions

For the predominantly agricultural River Windrush catchment, spatial variations in concentrations of nitrogen species and suspended sediment were strongly related to geology and land use. Temporal patterns of NO3- and NO2- concentrations during the three year study were highly correlated with seasonal variations in baseflow. Suspended sediment concentrations were mainly controlled by storm discharge. Variations in total ammonium concentrations reflected both flow controls. Suspended sediment effects total ammonium and organic nitrogen transport to the aquatic system, and in-stream cycling processes. Organic nitrogen did not display consistent seasonal variations, but concentrations occasionally exceeding those of NO3-. Overall, NO3- and organic nitrogen were the most important at 60% and -40%, of total nitrogen load, respectively. Future assessments of agriculture impact on river water quality should consider the total nitrogen load, and not solely that of NO3-.

Landscape, regional and global estimates of nitrogen flux from land to sea: errors and uncertainties

Regional to global scale modelling of N flux from land to ocean has progressed to date through the development of simple empirical models representing bulk N flux rates from large watersheds, regions, or continents on the basis of a limited selection of model parameters. Watershed scale N flux modelling has developed a range of physically-based approaches ranging from models where N flux rates are predicted through a physical representation of the processes involved, through to catchment scale models which provide a simplified representation of true systems behaviour. Generally, these watershed scale models describe within their structure the dominant process controls on N flux at the catchment or watershed scale, and take into account variations in the extent to which these processes control N flux rates as a function of landscape sensitivity to N cycling and export. This paper addresses the nature of the errors and uncertainties inherent in existing regional to global scale models, and the nature of error propagation associated with upscaling from small catchment to regional scale through a suite of spatial aggregation and conceptual lumping experiments conducted on a validated watershed scale model, the export coefficient model. Results from the analysis support the findings of other researchers developing macroscale models in allied research fields. Conclusions from the study confirm that reliable and accurate regional scale N flux modelling needs to take account of the heterogeneity of landscapes and the impact that this has on N cycling processes within homogenous landscape units.