Per-Erik Jansson

Simulated nitrogen dynamics and losses in a layered agricultural soil

Abstract A soil nitrogen model emphasizing mineral nitrogen dynamics and losses is presented. The model has a one-dimensional layered structure and considers plant uptake, mineralization, immobilization, leaching and denitrification processes. Fertilization and manure additions are included as management inputs. A physically based soil-water and heat model provides daily values of temperature, unfrozen water content, water flow and drainage, at different depths in the profile. Input data requirements include standard meteorological variables, basic soil physical and biological properties and crop management characteristics. Soil nitrogen dynamics were simulated for a 3-year period in N-fertilized and unfertilized barley. Model predictions were compared with measurements of nitrate leaching and mineral N content of the soil. Simulation of mineral N levels and leaching generally agreed with field data. Prediction of mineral N dynamics in both N-fertilized and unfertilized barley was better for surface layers than for deeper layers in the profile. Discrepancies between simulated and measured mineral N content in the topsoil were mainly related to mineralization and plant uptake. In deeper soil layers, differences between measured and predicted values were primarily related to the water flow and the drainage pathway. Simulated amounts of nitrate leached were close to measured values. Discrepancies in the temporal distribution of nitrate leached were mainly attributable to the simulated water flow. The model predicted that nitrate leaching to drainage tiles occurred mainly from the upper layers. Prediction of leaching depended as much on the simulation of drainage flow pathways and the vertical distribution of nitrate in the profile as on the simulation of water flow rates.

Soil moisture redistribution and infiltration in frozen sandy soils

Infiltration into frozen soil is a result of the whole climate dynamics of the preceding winter with all its importance for the freezing of the soil. Therefore a predictive infiltration model needs to include a proper description of the main processes of soil water and heat transfer during season-long periods. Such a model may assume two water-conducting flow domains. A lysimeter experiment was set up with the aim of studying these processes in two different sandy soils. Frequent measurements of total and liquid soil water content, soil temperature, and groundwater level were made during two winters with contrasting meteorological conditions. The main problems in the simulation of the two winters were (1) frost-induced upward water redistribution, (2) rate of infiltration in the initially air-filled pores, and (3) heat transfer caused by snowmelt refreezing in the frozen soil. An extensive calibration of the model suggested that some key empirical parameters were not constant for the two soils and the two seasons. Complementary methods for determining the hydraulic conductivity of frozen unsaturated field soils are necessary to further improve the model.

Modelling water flow, nitrogen uptake and production for wheat

Soil water and temperature conditions were simulated for three years at three sites in the Netherlands, using a model named SOIL. Observed water table depths from one site with a sandy loam soil indicated bypass flow in macropores. Nitrogen turnover was simulated using the output of SOIL as input to a nitrogen model. To improve the nitrogen model, a crop-growth submodel was introduced, and simulations were compared with measured data for two seasons and three fertilizer treatments at the three sites. Mineral-N in the soil after application of fertilizer was substantially higher in the simulation than indicated by measurements in 4 out of 18 simulations. Regression analyses showed that simulated mineral-N content in the uppermost metre explained 64% of the observed variation. The corresponding values for nitrogen content (Nta) and biomass (Wta) of aboveground tissues were 86 and 93%, respectively. With a few exceptions annual values ofWta andNta were simulated with an accuracy of approximately 20%. A sensitivity test showed that growth parameters and especially the light use efficiency parameter strongly influenced biomass production for fertilized treatments whereas the control of nitrogen uptake from soil was most important for non-fertilized treatments.

PREFERENTIAL WATER FLOW IN A FROZEN SOIL — A TWO-DOMAIN MODEL APPROACH

Earlier modelling studies have shown the difficulty of accurately simulating snowmelt infiltration into frozen soil using the hydraulic model approach. Comparison of model outputs and field measurements have inferred the occurrence of rapid flow even during periods when the soil is still partly frozen. A one-dimensional, physically based soil water and heat model (SOIL) has been complemented with a new two-domain approach option to simulate preferential flow through frozen layers. The ice is assumed to be first formed at the largest water filled pore upon freezing. Infiltrating water may be conducted rapidly through previously air-filled pores which are not occupied by ice. A minor fraction of water is slowly transferred within the liquid water domain, which is absorbed by the solid particles. A model validation with field measurements at a location in the middle-east of Sweden indicated that the two-domain approach was suitable for improving the prediction of drainage during snowmelting. In particular, the correlation between simulated and observed onset of drainage in spring was improved. The validation also showed that the effect of the high flow domain was highly sensitive to the degree of saturation in the topsoil during freezing, as well as to the hydraulic properties at the lower frost boundary regulating the upward water flow to the frozen soil and ice formation.

Model for evaporation, moisture and temperature of bare soil: calibration and sensitivity analysis

Abstract A modelling approach for predicting soil surface temperature and soil evaporation is presented. The procedure is based on the equations for heat flow at the soil surface and includes vapour diffusion and a semi-empirical correction function for the surface vapour pressure. The effects of changes in three important model parameters were studied by means of multiple model simulations. The first parameter determines the steepness of the water potential gradient close to the surface. The second parameter is the water vapour enhancement factor and the third one limits the lowest possible hydraulic conductivity during drying. Measurements of soil water content and soil temperature in a bare sandy loam were used to evaluate the models behaviour. Vapour pressure at the soil surface was found to be substantially lower than saturated vapour pressure at topsoil moisture potentials as high as −100 hPa. The difficulty to distinguish between vapour and liquid water flow at low moisture contents was demonstrated. Results from the temperature tests indicated enhanced vapour diffusion and a probable value of the diffusion tortuosity coefficient close to 1.0, whereas a value close to 0.7 was more likely according to the soil water contents and calculated evaporation.

Experimental Site of the ‘Ecology of Arable Land’ Project

Abstract In 1979 a major integrated research project ‘The Ecology of Arable Land’ was initiated, in which recent trends in Swedish crop husbandry are reflected. These trends include less carbon input through crop residues, a higher nitrogen input through fertilizers, less biological fixation of nitrogen, through growing of legumes and simpler crop rotations. Nitrogen is the major limiting nutrient in modern agriculture. Crop uptake efficiency of nitrogen is low and pollution of waters and the atmosphere by nitrogen from fertilizer use is becoming increasingly evident. The project aims at quantitatively determining the fate of fertilizer and biologically-fixed nitrogen in four cropping systems (barely with 0 kg N ha−1 yr−1 barley with 120 kg N ha−1 yr−1, meadow fescue grass ley with 200 kg N ha−1 yr−1, lucerne ley with biological nitrogen fixation), with special attention to the role of soil organisms (microorganisms, fauna and roots) in regulating the biogeochemical nitrogen cycle. The experimental site o...

Water balance and soil moisture dynamics of field plots with barley and grass ley

Abstract A physically based soilwater and heat model was used to estimate the water balance of an arable field in central Sweden for each of three different crop covers (barley with and without N fertilization and grass ley). Annual water balances were calculated for each year from 1981 to 1985. On-site measurements of soil physical properties, meteorological variables and plant development were used as input to the model. Simulated soil forst, snow cover, soilwater contents, soilwater tensions and relative differences in simulated drainage between treatments were in agreement with the corresponding measured values. In the simulation, surface runoff (70 mm year −1 in all treatments) mainly occurred during snowmelt periods and accounted for much of the variation in the total runoff estimate. Annual mean precipitation amounted to 610 mm year −1 , whereas average evapotranspiration was calculated to be 320, 360 and 435 mm year −1 in barley without N fertilization, barley with N fertilization and grass ley, respectively. Soil evaporation accounted for 60, 43 and 23% whereas evaporation of intercepted water accounted for 5, 12 and 19% of the total evapotranspiration, respectively. Drainage estimates amounted to 205, 170 and 110 mm year −1 .

Simulated soil temperature and moisture at a clearcutting in central Sweden

Soil water and temperature measurements were made at a clearcutting in Jadraas, central Sweden, to give appropriate information for nutrient flow calculations and soil biological research. Compared to uncovered plots, slash‐covered plots were 1–2°C colder and had 3–6 volume percent higher water content in the 5 cm thick humus‐layer during the growing season following cutting. Relative to air, soil temperatures became warmer at both treatments during the second season and differences between uncovered and covered plots decreased. Tension dynamics in the mineral soil was most pronounced in uncovered plots, especially during the first dry growing season. The physically based model, SOIL, was used to analyse these observations and to estimate the effects of clearcutting. Simulated snow and frost depths, soil temperatures, water contents and tensions as well as ground water table were compared with measured data during a period of one to four years. Physical parameter values were estimated from independent mea...

Simulated evapotranspiration from the Norunda forest stand during the growing season of a dry year

Abstract A SVAT model was used to simulate evaporation from the Norunda forest stand during the dry growing season of 1994. Daily mean values of meteorological data were used as input and compared with actual data on soil moisture, transpiration and total evaporation. The soil moisture data of TDR were used as a basis for defining combinations of parameters values that resulted in acceptable agreement. Different threshold for reduction of transpiration during the dry spell in July resulted in small changes in soil moisture dynamics. Using also sap-flow measurements the final estimated value of −150xa0hPa as the threshold value below which water uptake is reduced was found in combination with a flexibility coefficient of 0.7 for compensatory water uptake between soil horizons. The agreement with daily rates of evaporation was good, and coefficients of determination of 0.44, 0.78 and 0.89 were obtained when comparing with all days of eddy-flux measurements, days without interception losses with eddy-flux measurements and sap-flow measurements of canopy transpiration, respectively. The total evaporation from May to the end of October was estimated to be 314xa0mm, of which interception losses, canopy transpiration and soil evaporation accounted for 70, 181 and 63xa0mm respectively. However, the eddy-flux measurements indicated that the total sum had been underestimated, and the sap-flow measurements suggested that canopy transpiration had been slightly overestimated. The high correspondence between simulated and measured fluxes was surprising considering the heterogeneity involved and simplistic parameterization of the model. In future studies the resolution of input data to the model will be higher, and spatial variation will be explicity represented when simulating evaporation from this ecosystem.

Diurnal fluctuations of water and heat flows in a bare soil

The complexity of coupled transport of heat and moisture at the soil surface necessitates a combination of field and numerical experiments to evaluate the interactions between liquid and vapor phase flow. The near-surface moisture and temperature conditions of a bare soil were investigated experimentally and by using the SOIL model to assess the importance of water vapor flow. During a 1-month period in early fall, intensive measurements of water content, water tension, and temperature were made in a bare soil plot. Soil thermal conductivity, measured on soil cores extracted for laboratory analysis, was found to agree with estimates based on the Kersten equation. Simulated water content and soil temperature agreed well with observations. Modeled soil vapor flow was significant compared to liquid flow only during the initial dry days when the inclusion of vapor flow improved the predicted diurnal variation in water tension. Model predictions were sensitive to an accurate representation of the soil surface energy balance, including the consideration of steep gradients in tension near the soil surface, and to the enhancement of vapor flow.