Simulating root growth
By Anders Pedersen, Kristian Thorup-Kristensen, Hanne L Kristensen and Jørgen Berntsen
Dynamic plant models have been used to predict crop production for decades. Much research has been done to improve modelling of soil organic matter turnover, water movement in the soil and plant development above ground. In contrast, there is still a lack in our ability to model root growth, proliferation, and water and nutrient uptake. To draw on the high level of development of other submodules, it is of great importance that submodules for simulation of root growth are improved.
In the BIOMOD Project field trials with winter wheat, spring barley and sugar beets were preformed at Research Centre Aarslev and Research Centre Foulum. These two sites represent two different soil types in Denmark (sandy loam and loamy sand). Root penetration was observed using minirhizotrones. In autumn 2000 and 2001 winter wheat were sown and in spring 2001 and 2002 spring barley and sugar beet were sown.
Depth of root penetration
The measured penetration depths for all three crops are shown in figure 1. In figure 1a, results from Aarslev shows that the crops had nearly the same development in root penetration in the two years. However, winter wheat shows a variation between years with a rooting depth of 150 cm in 2002 against 188 cm in 2001. For sugar beet one measurement is lacking preventing comparison of root depth at harvest between the years.
In figure 1b, results from Foulum shows smaller differences in root development for each crop species for the two years. On average for the two years root depths for cereals are nearly the same for both sites, but the roots of sugar beets grew significantly deeper at the sandy loam than on the loamy sand.
In table 1 the estimated rooting depth penetration rates are shown for the three crops. The growth rates of the cereals are close to rates previously estimated on cereal crops grown as cover crops (Thorup-Kristensen, 2001).
Effect of clay content
The Foulum soil has a clay content of about 8 pct (0-50 cm) and the Aarslev soil about 14 pct (0-50 cm). Usually, root penetration is slower in soils with low clay content. However, for the two soil types examined here, there seems to be limited difference in root growth within the range from 8 to 14 pct clay. Therefore, the factor modifying root penetration was chosen to be constant in the range from 8 to 14 pct and decreasing only for lower levels of clay content (figure 2). This factor is then used in the existing root module of the FASSET model (Berntsen et al., submitted).
This work has just started the important task of improving the description of root growth by crop models. However, only two soil types and three crop species were included, and for a further improvement of root models more soil types, crop species and climate regions should investigated.
Berntsen, J., Petersen, B.M., Olesen, J.E., Eriksen, J., Søegaard, K. Simulation of residual effects and nitrate leaching after incorporation of different ley types (submitted)
Thorup-Kristensen, K. (2001) Are differences in root growth of nitrogen catch crops important for their ability to reduce soil nitrate-N content, and how can this be measured? Plant and Soil, 230: 185-195.