Carbon Nitrogen and Water Cycle

In the carbon, nitrogen and water cycle component, IMAGE 2.4 includes models for describing the global carbon cycle (Terrestrial Carbon Model) and the global nitrogen and phosphate cycle.

Terrestrial Carbon Model	Global Nitrogen and Phosphate Cycle
Model documentation, input and output	Introduction
Model description	Point sources
Land-cover conversions	Nonpoint sources, denitrification and leaching
	Nonpoint sources, transport and retention in groundwater and surface water

Global Nitrogen and Phosphate Cycle, nonpoint sources, denitrification and leaching

General

We developed a model for soils under rainfed crops that combines the effect of temperature, crop type, soil properties and hydrological conditions on annual mean nitrate leaching and denitrification rates, relying on simplifications of empirical models from the literature (e.g. Kolenbrander (1981), Kragt et al. (1990) and Simmelsgaard et al. (2000)).

Potential loss

The potential loss from the plant-soil system, N_pot is calculated as the difference between N_sur and the NH3 volatilization N_vol:

N_pot = N_sur - N_vol

see Managed Land
see Land-use Emissions

Denitrification

Denitrification in soil is calculated as an empirical fraction f_den of N_pot:

N_den = f_den N_pot

The denitrification fraction f_den is composed empirically of four factors, each ranging individually from 0 to 1:

f_den = MIN[(f_climate + f_text + f_drain + f_soc),1]

where f_climate (-) represents the effect of climate on denitrification rates, combining the effects of temperature and residence time of water and nitrate in the root zone; f_text, f_drain f_soc (-) are factors representing effects of soil texture, soil drainage and soil organic carbon content, respectively.

Leaching

Nitrate leaching (N_lea) – the nitrate in percolating water from the root zone to the subsoil where it is no longer available to plant roots – is calculated as follows:

N_lea = N_pot - N_den = ( 1 - f_den ) N_pot

The concentration of nitrate in the percolating water can be calculated for each grid cell from the excess of precipitation over evapotranspiration and Nlea. It should be noted that Nlea includes surface runoff and water drained by drainage systems, which are explicit in our simplified approach (Van Drecht et al., 2003).

Climate

In the IMAGE 2.4 approach climates with a large annual precipitation excess have high leaching rates and short residence times of nitrate in the soil solution. In dry climates the annual water flow through the soil is small and the residence time of nitrate is longer than in humid climates. N transformation rates are higher in tropical climates than in temperate ones. In arid climates the precipitation excess is small and soil water percolation rates are low. As a consequence, the residence time of nitrate in the soil solution is long and nitrate leaching limited. This implies that gaseous N loss processes dominate in dry climates as discussed elsewhere (Seitzinger et al., 2006). In temperate and humid climates, precipitation excess generally exceeds the available soil moisture capacity and, as a consequence, soilwater percolation and nitrate leaching rates are high.

Soil conditions

The factors f_text, f_drain and f_soc account for soil properties that influence denitrification through the soil’s water and oxygen status. Fine-textured soils have more capillary pores and hold water more tightly than sandy soils do. As a result, anaerobic conditions favouring denitrification may be more easily reached and maintained for longer periods in fine-textured soils than in coarse-textured ones. Soil drainage conditions also influence soil aeration, and denitrification rates are generally higher in poorly drained soils than in well-drained soils. Finally, the soil oxygen status is influenced by root respiration and microbial activity. Oxygen consumption by micro-organisms is driven by temperature, supply of carbon and water availability. Since temperature and soil water are already represented in fclimate, we use soil organic carbon content as a proxy for the carbon supply. We use 0.5 by 0.5 degree resolution information on derived soil properties taken from the WISE database (Batjes, 1997; Batjes, 2002).

Irrigation

Since data on irrigation water use, its efficiency and leaching requirements are not available on the scale of our calculations, we ignored irrigation in all crops except wetland rice which may lead to underestimation of nitrate leaching rates in about 10% of the global arable area of ~1400 Mha.

Wetland rice

In wetland rice systems the efficiency of fertilizer use is generally low due to high NH³ volatilization and denitrification rates (Bouwman et al., 2002). The effects of soil texture, soil organic carbon, drainage and precipitation surplus for these systems are ignored, because they are assumed to be predominantly anaerobic. Since rice is primarily produced in subtropical and tropical climates, we assumed denitrification to always be 75% of the surface N balance surplus, so that fden = 0.75. This value was selected on the basis of measurements, indicating that about 30% of the total N input or 75% of the surface balance N surplus is lost through denitrification (Van Drecht et al., 2003).