Development of a lucerne model in APSIM next generation: 3 Biomass accumulation and partitioning for different fall dormancy ratings

Modelling lucerne growth requires response functions that represent seasonal partitioning of biomass into above-ground and below-ground organs. An additional challenge is to parameterize perennial organ responses across contrasting fall dormancy (FD) genotypes. Current models use empirical approaches to simulate biomass accumulation and partitioning. This research integrated knowledge of lucerne biomass accumulation and partitioning into the Agricultural Production Systems sIMulator (APSIM) next generation (APSIM NextGen) model framework. Biomass supply was calculated from light interception and total radiation use efficiency (RUEtotal), and then allocated based on the relative demand of each organ. Leaf biomass demand was parameterized as a function of specific leaf area (SLA). Stem biomass demand was parameterized as a positive power function of shoot biomass. Root biomass (taproots and crowns) showed a strong seasonal pattern. The observed decrease of root biomass in periods of an increasing photoperiod (mid-winter to mid-summer) was assumed as remobilization to shoots and carbon loss from maintenance respiration. Periods of decreasing photoperiod showed increased biomass of root caused by greater carbon partitioning to this organ. To capture this, a model optimization approach was used to fit required parameters. Fitted parameters included a remobilization coefficient (percentage of storage biomass per day) of 0.01common to all FD cultivars tested (FD5, FD2 and FD10). The regrowth coefficient (remobilization duration) remained constant at 0.01 post-defoliation until 250 °Cd for FD5, 200 °Cd for FD2 and 300 °Cd FD 10, and then declined to 0 after another 50 °Cd. The model was parameterized to have maximal root demand in a decreasing photoperiod to capture carbon partitioning. The model had good prediction of shoot biomass (NSE=0.70) and fair prediction (NSE=0.60) of root biomass for 42 day defoliation treatments. It was less accurate for predictions of shoot biomass under a frequent (28 day) defoliation regime. This highlights the importance to include the response to limitations caused by depleted root N reserves in future model versions. The APSIM NextGen lucerne model provided a mechanistic framework to model perennial organ biomass dynamics with structural and storage components, root maintenance respiration, remobilization in spring, partitioning in autumn and the regrowth effect. This framework accounted for differences in fall dormancy of genotypes and provided a methodology that can be integrated into models of other perennial crops.