Nitrogen Dynamics Model Research Articles

Long-Term Nitrate Export Pattern from Hubbard Brook Watershed 6 Driven by Climatic Variation

From 1964 through 1994, the pattern of nitrate (NO3−) export from Watershed 6 at Hubbard Brook Experimental Forest (HBEF) in New Hampshire, U.S.A., exhibited 10 years of high export (1968–1977) followed by 12 years of low export (1978–1989), with four ‘spikes’ in 1970, 1973, 1976, and 1990. Disruptions of N cycling by soil freezing, insect defoliation, or drought have been suggested to explain this pattern. We developed a model of nitrogen dynamics demonstrating that most of the long-term pattern can be reproduced without explicit consideration of these events. Comparisons of simulated N fluxes between high and low export years suggested that inorganic N input to the soil, from both atmospheric N deposition and N mineralization, was significantly higher during periods of high streamflow NO3− flux than in low periods. Simulated inorganic N pools (ammonium and nitrate) and fluxes (nitrification, plant uptake, denitrification, and ammonia volatilization) were also significantly higher in these periods. By swapping the time sequences of inorganic N input between high and low export years, it was shown that N mineralization, not atmospheric N deposition, drives the simulated long-term pattern. Although simulated nitrification showed a stronger relationship with measured streamflow NO3− flux than did N mineralization, nitrification rate depended upon availability of soil ammonium supplied from N mineralization. Because N mineralization in the model varies only with soil temperature and moisture, we conclude that shifts in the interaction of these two variables over time produced the shifts in NO3− stream exports.

Modelling of nitrogen release from MBT waste

The “LaNDy” model (landfill nitrogen dynamics model) is a new mathematical tool for the evaluation of the long-term behaviour of nitrogen in mechanical-biologically pretreated (MBP) waste. LaNDy combines a hydraulic model based on RICHARD’s equation with one-dimensional heat flow in landfills, kinetics of biological degradation, gas diffusion, nitrification and denitrification. A suitable temperature-dependent N mineralisation sub-model was based on numerous data from the literature and own LSR-experiments. With the “nitrification modus” of the LaNDy model, kinetic data of nitrification, thermodynamic data of denitrification and diffusion characteristics of gaseous components (especially of oxygen and methane) are used as an additional input for the preliminary calculation of the long-time impact of nitrification and denitrification. Examples of predicted temperature distribution and leachate ammonium concentrations, using different landfill size, age of the landfill (10 to ≈100 a) and hydraulic conductivity of the MBP waste, are presented in this paper.

Cultivation and Soil Organic Matter Management in Low Input Cereal Production following the Ploughing out of Grass Leys

Nitrogen fertility of low input cereal systems following ploughing out of grass leys is investigated using the soil nitrogen dynamics model SOILN with its linked cereal growth model, supported by a set of experimental data for spring and winter barley cropping. Soil organic matter (OM) plays an important part in transfer of fertility from the fertility-building ley phase to the fertility exploitation arable phase of ley-arable rotations. One aspect investigated is a possible role of ‘protected organic matter’, a component of soil OM which generally mineralises at a slow rate, but which becomes ‘unprotected’ so it mineralises at a somewhat faster rate for a limited period following disturbance of the soil by ploughing. Measurements of CO2 emissions in the laboratory and field support the concept of protected OM. In simulations with the SOILN model, this is represented by a temporary increase in the value of the humus decomposition rate constant for a few weeks following ploughing. The assumption of raised humus decomposition rates after ploughing makes some improvements to fits of simulations with the linked models to measured grain and straw yields and N offtakes, as well as giving a more realistic decline in soil OM over successive years of cereal cropping. Predictive simulations demonstrate how, in the absence of mineral N fertiliser, cereal crop yields decline over the years following ploughing out of leys, but yields can be maintained by making applications of organic manure or slurry.

Modeling soil quality thresholds to ecosystem recovery at Fort Benning, GA, USA

The objective of this research was to use a simple model of soil carbon (C) and nitrogen (N) dynamics to predict nutrient thresholds to ecosystem recovery on degraded soils at Fort Benning, Georgia, in the southeastern USA. Artillery, wheeled, and tracked vehicle training at military installations can produce soil disturbance and potentially create barren, degraded soils. Ecosystem reclamation is an important component of natural resource management at military installations. Four factors were important to the development of thresholds to recovery of aboveground biomass on degraded soils: (1) initial amounts of aboveground biomass, (2) initial soil C stocks (i.e., soil quality), (3) relative recovery rates of biomass, and (4) soil sand content. Forests and old fields on soils with varying sand content had different predicted thresholds for ecosystem recovery. Soil C stocks at barren sites on Fort Benning were generally below predicted thresholds to 100% recovery of desired future ecosystem conditions defined on the basis of aboveground biomass. Predicted thresholds to ecosystem recovery were less on soils with more than 70% sand content. The lower thresholds for old field and forest recovery on more sandy soils were apparently due to higher relative rates of net soil N mineralization. Calculations with the model indicated that a combination of desired future conditions, initial levels of soil quality (defined by soil C stocks), and the rate of biomass accumulation determine the predicted success of ecosystem recovery on disturbed soils.

Probabilistic modeling of nitrogen and carbon dynamics in water-limited ecosystems

The nitrogen and carbon dynamics of water-limited ecosystems are significantly controlled by the soil water content, which in turn depends on soil properties, climate, and vegetation characteristics. Because of its impact on soil aeration, microorganism environmental stress, and ion transport within the pore spaces, the soil water content controls the activity of microbial biomass with important effects on the rates of decomposition, mineralization, nitrification, and denitrification. Mineral nitrogen is mainly lost in the leaching and plant uptake processes, which are both controlled by the soil water content. To assess both the long-term and the short-term impact of soil moisture dynamics on the soil nitrogen and carbon budgets, models of the N and C cycles need to operate at daily resolutions (or higher). On the other hand, long-term projections require a stochastic modeling of the climate forcing to generate long replicates of the climate signal as well as to assess the system response to climate change. This paper reviews a modeling framework developed by the authors [Proc. R. Soc. Lond. A 455 (1999a) 3789; Adv. Water. Res. 26 (2003) 45; Adv. Water Resour. 26 (2003) 59; Sci. J. 5 (2003) 781] for the process-based analysis of soil moisture, nitrogen, and carbon dynamics, presenting a synthesis of the main results of those investigations.

Ecosystem modelling of the cycling of marine dimethylsulfide: a review of current approaches and of the potential for extrapolation to global scales

There is rising interest from oceanic and atmospheric scientists in the potential role of dimethylsulphide (DMS) in regulating global climate. The increased availability of field observations of DMS and related compounds (DMS(P)) and of their transformation rates in the ocean has stimulated the development of ecosystem models of marine sulfur cycling. The models cover a wide range of complexity levels and spatial/temporal scales, from zero-dimensional local simulations spanning a few days to regional/global simulations driven by ocean general circulation models. The degree of complexity required to model DMS(P) dynamics, particularly the differentiation into phyto plankton species or groups, remains an important open question. First attempts to drive these models with vertically resolved turbulence models suggest interesting interactions between DMS(P) dynamics and fine-scale ocean mixing that can modify fluxes of DMS to the atmosphere. Recent models also bring into focus the strong affinities between the cycling of DMS(P) and that of dissolved organic carbon in the surface ocean. Formal parameter estimation techniques, which are increasingly used in ecosystem modelling of carbon and nitrogen dynamics, should play a stronger role in the development of DMS sulfur modelling. Extrapolation of DMS cycling and fluxes to the global scale presently relies largely on empirical approach. A semiempirical approach, based on a simple ecosystem model, is shown to reproduce gross features of the global distribution of DMS in the surface ocean. This shows promise for the continuing development of ecosystem models for global modelling of marine sulfur fluxes to the atmosphere.

Modelling environmental impacts of deposition of excreted nitrogen by grazing dairy cows

The soil nitrogen (N) and carbon dynamics model SOILN (which has interactive links to a grass growth model), and the dual-porosity contaminant transport model MACRO, have been used to study environmental pollution arising from grazing dairy cows. The models had been calibrated and tested in previous studies related to livestock agriculture. Information about N contents and other characteristics of urine and faeces excreted by dairy cows was assembled from literature sources. Watercourse pollution by nitrate and ammonium was the main environmental impact considered. Denitrified nitrogen losses were also estimated as an indicator of nitrous oxide pollution of air. Higher levels of nitrate pollution in tile drains (which feed into watercourses) were shown to arise under grazing compared to fields receiving slurry and cut for silage. Much of this raised nitrogenous pollution arises late in the grazing season. High levels of nitrate pollution could be attributed to various factors, including the fact that cows tend to congregate in certain areas of a field at a localised stocking rate much higher than the overall stocking rate, and due to deposition of N at times when grass cannot utilise it as a plant nutrient. The fact that urine and faeces patches are concentrated over a small proportion of the field area did not give an increase in overall loss when this was considered along with field areas receiving no excretions. Rapid transport through soil macropores of ammonium from urine led to high pollution loads during grazing on wet soil. In contrast to leaching, simulated N losses by denitrification were at a low level, and appeared to show little variation with factors which had a large effect on leaching losses. Overall, the forms of pollution most damaging to the environment due to spatially non-uniform excretion by grazing animals, appeared to be leached ammonium from urine transported by macropore flow, and leached nitrate exacerbated both due to cows congregating and due to deposition at times of low plant N uptake.

Ecosystem productivity is independent of some soil properties at equilibrium

Long runs of a mechanistic model of forest carbon (C) and nitrogen (N) dynamics (Edinburgh Forest Model) suggest that, when in a steady state, ecosystem productivity may be insensitive to the specific rate of N mineralization of soil organic matter (SOM). At equilibrium, productivity and other vegetation properties are determined primarily by climate. This is so because, given time, modelled ecosystems tend to generate amounts of SOM that are able to supply N at rates which do not greatly limit plant growth. When specific N mineralization rates are low, large amounts of SOM accumulate, whereas when specific N mineralization rates are high, small amounts of SOM accumulate. However, it may take several millenia for equilibrium conditions to be reached following disturbance (particularly following degrading disturbance) and during that time N mineralization rates determine the speed of progress toward equilibrium.

Inorganic Nitrogen Losses from a Forested Ecosystem in Responseto Physical, Chemical, Biotic,and Climatic Perturbations

Nitrate leaching to streams is a sensitive indicator of the biogeochemical status of forest ecosystems. Two primary theories predicting long-term (decadal) changes in nitrate loss rates (N saturation theory and the nutrient retention hypothesis) both predict increasing dissolved inorganic nitrogen (DIN) losses for watershed 6 (W6), the biogeochemical reference watershed at the Hubbard Brook Experimental Forest (HBEF). Measured values, however, have declined substantially since measurements began in the mid-1960s. Are these theories wrong, or are there other important controls on DIN losses at the annual to decadal time scale that have obscured the tendency toward higher losses over time? We tested the individual and combined effects of several forms of disturbance on DIN loss rates from northern hardwood forests by comparing predictions from a relatively simple model of forest carbon, nitrogen, and water dynamics (PnET-CN) with the long-term record of annual DIN loss from W6 at HBEF. Perturbations tested include interannual climate variation, changes in atmospheric chemistry (CO2, O3, N deposition), and physical and biotic disturbances (two harvests, a hurricane salvage, and a defoliation event). No single disturbance caused changes in DIN losses to mimic measured values. Only when run with all of the disturbances combined did the model-predicted pattern of interannual change in DIN loss approach the measured record. Single-disturbance simulations allow an estimation of the role of each in the total pattern of DIN loss. We conclude that DIN losses from W6 were elevated in the 1960s by a combination of recovery from extreme drought and a significant defoliation event. N deposition alone, in the absence of other disturbances, would have increased DIN losses by 0.35 g N m−2y−1. These findings indicate that predictions of DIN losses must take into account the full spectrum of disturbance events and changes in environmental conditions impacting the systems examined.

An organic matter and nitrogen dynamics model for the ecological analysis of integrated aquaculture/agriculture systems: II. Model evaluation and application

The performance of a model developed to simulate organic matter and nitrogen dynamics in integrated aquaculture/agriculture systems was evaluated using sensitivity analysis and model verification procedures with data from three sites. The model sensitivity analysis results were used to identify parameters that required high accuracy in measurement, and to suggest areas of future research in integrated aquaculture/agriculture systems. Model sensitivity analysis results showed that research on stocking density, sediment processes and water management practices was required, in order to improve the overall understanding of the functioning of the integrated aquaculture/agriculture system. Results from model verification runs showed that the model performance was satisfactory with respect to fish growth, crop growth, pond and terrestrial nitrogen and organic matter simulations. However, the modeling of phytoplankton biomass was less satisfactory and the results suggested the need for more site-specific calibration of exogenous factors and the consideration of phytoplankton species composition in simulating grazing preferences by fish. Application of the model to investigate the effects of different cycling pathways on nitrogen retention and productivity showed that the recycling of plant wastes to aquaculture ponds had a major effect in reducing system nitrogen losses and increasing system productivity. Sediment organic matter processes were identified as the major determinants of system nitrogen retention, as measured by the nitrogen retention index. The modeling study also showed that the number of linkages between the aquaculture and agriculture components was not directly related to nitrogen retention and productivity in the system. The results of the modeling study suggest that the system nitrogen status may be more important than the number of pathways in determining the number and type of cycling pathways that should be incorporated in integrated aquaculture/agriculture system.

An organic matter and nitrogen dynamics model for the ecological analysis of integrated aquaculture/agriculture systems: I. model development and calibration

A dynamic and mechanistic mass balance model (Integrated Aquaculture/Agriculture System model, or IAAS) for predicting nitrogen and organic matter outputs from aquaculture ponds and their subsequent recycling in conventional agriculture practices has been developed and calibrated using data from Honduras, Thailand and Malawi. The model, developed using Stella™ modeling software (High Performance Systems, Inc. Hanover, New Hampshire), simulates individual fish growth, organic matter, nitrogen (organic, total ammonia and nitrate), dissolved oxygen and phytoplankton, crop growth, soil nitrogen concentrations and soil water balance. Processes included in the model are fish growth, crop biomass growth, allochthonous and autochthonous organic matter production, organic matter decomposition, nitrogen input, nitrogen mineralization, nitrification, denitrification, diffusion, uptake and leaching. The model has been calibrated using literature and observed parameter values from experiments. The calibration procedure involves running the model using inputs from observed data, comparing the model output to observed data, and making appropriate adjustments to parameter values until a general fit between the model and observed values is observed. The structure of the model allows users to modify parameter values to suit different simulation scenarios via a user interface display that also includes graphs and tables for model output.

AN ANALYTICAL MODEL OF SOIL ORGANIC CARBON DYNAMICS BASED ON A SIMPLE "HOCKEY STICK" FUNCTION

We developed a simple analytical model for carbon and nitrogen dynamics in soils. Description of the soil C and N dynamics focus on the total carbon residue function, R, defined as the fraction of original plant carbon input remaining in the soil as a function of time, and nitrogen concentration function, N, defined as the N concentration of the carbon residue as a function of R. A simple two-parameter function, R = Exp(−(kt)α), was used for total carbon residue function. N concentration of carbon residue is derived to be a linear function of R. We tested our model with data reported in the literature. The model parameters (k and α) were calculated from independently determined gross turnover time and 14C age of soil organic carbon. Our model accurately predicts C and N dynamics in response to fertilization and manure application for time periods ranging from a few months to more than 150 years.

Modelling the effect of the spatial distribution of agricultural practices on nitrogen fluxes in rural catchments

An integrated, hydrology and nitrogen dynamics model was developed to study the spatial interactions between soil and groundwater that can affect the nitrogen delivery to streamwater in rural catchments. The hydrological model TNT is based on TOPMODEL hypotheses but is it fully distributed according to a regular square grid. A subsurface flow component was distinguished to account for the supply of groundwater and nitrate to downslope soils. The crop growth and nitrogen biotransformations were simulated using an existing generic crop model, STICS. Both models are process-based, but kept as simple as possible. The integrated model was applied to theoretical catchments to analyze the combined effects of geomorphology and crop distribution on the whole catchment nitrogen budget. The catchments differed both in the slope profile and in the pattern of water pathways. The results suggest that placing crops acting as nitrogen sinks downslope potentially polluting crops could reduce significantly the streamwater contamination by nitrate. This effect is the highest for catchments with parallel water pathways and a wide concave bottomland. Nitrogen uptake by sink crops was quantitatively more important than denitrification to reduce nitrogen output. It is concluded that this model, although still in development, may prove an interesting working tool to investigate the effect of the landscape structure on nutrient budgets in ecosystems.

Analytical Modeling of Nitrogen Dynamics in Soils and Ground Water

An analytical model, known as RISK-N, is developed to simulate nitrogen cycling in soils, and nitrate transport and fate in soils and ground water. The soil is separated into upper-root, lower-root, and intermediate-vadose zones, each with uniform properties. Transport in each soil zone is modeled on the basis of complete mixing. Transport in the aquifer, however, is modeled using a two-dimensional advection-dispersion transport equation. A simulation is made with a hypothetical corn plot, using meteorological, soil, hydrologic, and hydrogeological data for the South Platte River region of northeastern Colorado. Sensitivity analyses are performed on model parameters. This study shows that the RISK-N model is capable of simulating nitrate leaching rates, as well as ground-water concentrations, that are consistent with those obtained by numerical models, while requiring fewer input variables.

A model of carbon, nitrogen and phosphorus dynamics and their interactions in river marginal wetlands

A model describing carbon, nitrogen and phosphorus dynamics and their interactions in riverine wetlands was developed to: (1) investigate the nature of the interactions between the C, N and P cycles; (2) study the role of processes in the vegetation and in the soil; (3) study the way in which these processes and interactions are governed by major controlling variables (temperature and wetness of the soil); (4) quantitatively assess nutrient-related functions in river marginal wetlands; and (5) simulate the effects of management and other human influences in (or outside) the wetland on these functions. The model distinguishes several plant and soil compartments and allows for grazing by large herbivores. One of the main features of the model is a factor called ‘mode’, which influences most processes rates. This describes soil redox potential, water table and soil oxygen content. Sensitivity analysis showed that the most sensitive areas in the model coincide with the main interactions between C and other nutrient cycles. These are plant growth, as influenced by nutrient content of the plant, and decomposition of soil organic matter, which is coupled with nitrogen and phosphorus mineralisation and immobilisation. The model was applied to a slope at Kismeldon Meadows, Devon, southwestern England, and showed a good fit to field data.

Simulation of nitrogen uptake, fixation and leaching in a grass/white clover mixture

To represent nitrogen cycling in a low input grass/legume pasture system, a previously developed, weather‐driven grass/white clover growth model has been adapted to become the crop growth component of the soil nitrogen dynamics model SOILN. This provides a means of simulating nitrogen uptake by a grass/white clover crop, an important component of the overall nitrogen balance in low‐input grassland systems.Crop growth is represented by a photosynthesis equation adapted to take account of competition between the two crops for resources of light, water and nitrogen in the soil. Water shortage is represented by linked simulations with the soil water and heat model SOIL, and nitrogen shortage by links with the SOILN model. Nitrogen fixation has been introduced according to an equation for potential fixation reduced by environmental factors, particularly temperature. Transfer of nitrogen‐rich clover plant material to the soil nitrogen pools of SOILN (from where it becomes available as a nutrient for grass) is also represented. The model is tested by comparing simulated cut crop yields and nitrogen content of cut material with measured data from perennial ryegrass/white clover at a test site. Soil nitrogen processes in the model are tested by comparing simulated and measured nitrate in drainflows. Apart from some discrepancies between simulated and measured results attributable to the inherent instability of a mixed crop system, agreement is reasonable by the standards of biological system models, indicating that the combined model gives a realistic representation of carbon and nitrogen processes in grassland with a grass, legume mixed crop.

Simulation of biomass, carbon and nitrogen accumulation in grass to link with a soil nitrogen dynamics model

In order to represent nitrogen and carbon cycling in the soil–plant–atmosphere continuum, a previously developed weather‐driven grass growth model has been adapted to become the crop growth component of the soil nitrogen dynamics model SOILN. This provides a means of simulating nitrogen uptake by the grass crop, an important component of the overall nitrogen balance in grassland.Grass growth is represented by a photosynthesis equation adjusted to take account of respiration as well as constraints due to lack of water and nitrogen in the soil. Water shortage is represented by linked simulations with the soil water and heat model SOIL, and nitrogen shortage by links with the SOILN model. Accumulated biomass and the nitrogen component of biomass are allocated to leaf, stem and root pools, and flows from live biomass pools to those representing above‐ and below‐ground senescent material are also represented. The model is tested by comparing simulated cut grass yields and nitrogen contents of cut material with measured data at a test site. Soil nitrogen processes in the model are tested by comparing simulated and measured nitrate in drainflows. Agreement is reasonable, indicating that the combined model gives a␣realistic representation of carbon and nitrogen processes in grassland.The use of the combined model in a predictive manner has been demonstrated in a comparison of nitrogen balances with a number of alternative slurry and mineral nitrogen fertilizer application scenarios.

Parameter selection and testing the soil nitrogen dynamics model SOILN

Abstract. The SOILN model with a crop growth submodel for grass and cereal crops and the associated soil water and heat model SOIL were selected out of a number of similar models to simulate nitrogen cycling in a soil/crop system. The main parameter values required by the model were selected on the basis of a combination of field experiments and literature sources. Experimental data measured on grassland at Dumfries in the West of Scotland and on arable land at Bush Estate near Edinburgh were used to test the model. Simulated biomass yields and nitrogen contents of harvested biomass were in reasonable agreement with measured values for both grass and cereal crops. There were similar trends in accumulated leached nitrate between the simulations and experiments at the sites. Any discrepancy between simulated and measured nitrate leached appeared to correspond to similar discrepancies between simulated and measured water flow. The comparison between simulated and experimental results suggests that the model with the selected parameter values can simulate nitrogen and carbon cycling both in grassland and in arable land, and make convincing predictions about the effects of varying soil, crop, fertilizer and manure management practices. A basic sensitivity analysis carried out on the parameters determining the biological and biochemical processes showed the model predictions of annual N‐leaching are relatively insensitive to all but two of the plant parameters. However, the model predictions of annual N‐harvested and dry mass production are sensitive to numerous plant parameters.

Simulating carbon dynamics along the Boreal Forest Transect Case Study (BFTCS) in central Canada: 1. Model testing

CENTURY 4.0, a simulation model of carbon and nitrogen dynamics of terrestrial ecosystems based on the relationships between climate, soil texture, plant productivity, decomposition and human management, was tested against observed data along the boreal forest transect case study (BFTCS) in central Canada. The results show that the simulated average aboveground biomass and net N mineralization were consistent with observed data. The modeled estimates for soil carbon were consistent with those from regional‐scale empirical regression models. High correlation (R2 = 0.92) with data was obtained for the simulation of soil carbon dynamics of the boreal forest, but the model overestimated soil carbon (O–20 cm) by 2–8% for fine‐textured soil and underestimated soil carbon by 5–18% for sandy soil. The effects of climatic variation on temporal changes in biomass and soil carbon storage over the past century were found to be very different for southern and northern sites but relatively insensitive to site‐specific soil texture. The main discrepancies between observed data and CENTURY 4.0 results are associated with the effects of soil texture and an inadequate representation of fire disturbance on C dynamics of boreal forests. Further improvements, particularly in the representation of disturbance regimes and in the simulation of slow pool C dynamics, are suggested to enhance its predictive capability.

Modeling of water and nitrogen dynamics on an undisturbed soil and a restored soil after open-cast mining

The water movement, mass transport and nitrogen transformations were evaluated on a restored and on an undisturbed soil. During the opencast coal-mining at Acklington in north-east England the soil-layers were removed and stored in large heaps. After the completion of the coal exploitation the soil was restored layer by layer. An identical and undisturbed soil in the vicinity of the opencast mine served as a benchmark. The same experimental treatments were given to both soils. The water and nitrogen dynamics were evaluated by an one-dimensional model based on the Galerkin finite element method. The nitrogen submodel incorporates mineralization, immobilization, nitrification, denitrification, ammonium exchange, uptake and mass transport. The simulations of water content and nitrogen in the soils were compared with the biweekly water content and irregular inorganic nitrogen measurements. Good agreement was found between measured and simulated water distribution during simulation period. At 10 cm depth, the simulation was not as good especially during the autumn period. The soils received the same amount of inorganic nitrogen fertilizer and agronomic treatment. The dynamics of the simulated and observed ammonia and nitrate concentrations can be largely explained as a result of differences in soil structure.