Abstract
Abstract 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.
Highlights
Biological nitrogen fixation (BNF) is the source of large annual additions of nitrogen (N) to the terrestrial biosphere
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
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
Summary
Biological nitrogen fixation (BNF) is the source of large annual additions of nitrogen (N) to the terrestrial biosphere. Despite the importance of manufactured fertilisers in crop production, inputs of N from BNF to the terrestrial environment have been estimated to be more than double that from fertiliser inputs (Fowler et al, 2015;Galloway et al, 2004). It is predicted that by the end of the current century, BNF in agricultural systems will increase from 33 to. 65 Tg N y−1 as a consequence of demand for increased production and climate change (Fowler et al, 2015). This is partly credited to an attempt to increase the utilisation of BNF in agricultural systems due to a perceived benefit in environmental outcomes. As N2O concentrations have increased (Bates et al, 2008), and indirect
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