Grasslands comprised of grass-legume mixtures could become a substitute for nitrogen fertiliser through biological nitrogen fixation (BNF) which in turn can reduce nitrous oxide emissions directly from soils without negative impacts on productivity. Models can test how legumes can be used to meet environmental and production goals, but many models used to simulate greenhouse gas (GHG) emissions from grasslands have either a poor representation of grass-legume mixtures and BNF, or poor validation of these features. Our objective is to examine how such systems are currently represented in two process-based biogeochemical models, APSIM and DayCent, when compared against an experimental dataset with different grass-legume mixtures at three nitrogen (N) fertiliser rates. Here, we propose a novel approach for coupling DayCent, a single species model to APSIM, a multi-species model, to increase the capability of DayCent when representing a range of grass-legume fractions. While dependent on specific assumptions, both models can capture the key aspects of the grass-legume growth, including biomass production and BNF and to correctly simulate the interactions between changing legume and grass fractions, particularly mixtures with a high clover fraction. Our work suggests that single species models should not be used for grass-legume mixtures beyond about 30% legume content, unless using a similar approach to that adopted here.
This work was conducted by the Models4Pastures consortium project under the auspices of FACCE-JPI. Funding was provided by: the New Zealand Government to support the objectives of the Livestock Research Group of the Global Research Alliance on Agricultural Greenhouse Gases; AgResearch’s Strategic Science Investment Fund as a contribution to the Forages for Reduced Nitrate Leaching (FRNL) research programme; the input of UK partners was funded by DEFRA and also contributes to the RCUK-funded projects: N-Circle (BB/N013484/1), UGRASS (NE/M016900/1) and GREENHOUSE (NE/K002589/1). R.M. Rees and C.F.E. Topp also received funding from the Scottish Government Strategic Research Programme. Lutz Merbold and Kathrin Fuchs acknowledge funding received for the Swiss contribution to Models4Pastures (FACCE-JPI project, SNSF funded contract: 40FA40_154245/1) and for the Doc.Mobility fellowship (SNSF funded project: P1EZP2_172121). Lorenzo Brilli, Camilla Dibari and Marco Bindi acknowledge funding received from the Italian Ministry of Agricultural Food and Forestry Policies (MiPAAF).
- nitrogen uptake
- species composition
- model validation