Using the CROPGRO-Perennial Forage Model to simulate the effect of asymbiotic nitrogen fixation on the growth of brachiariagrass

CONTEXT

There is an increase in global demand to reduce fossil fuel use and to create sustainable intensification of animal production systems. Alternative sources of mineral nitrogen fertilizers have been shown to increase herbage accumulation. The environment in which plants thrive is dynamic and complex due to the nature of the soil-plant-atmosphere interactions. Modeling can evaluate and explain these aspects.

OBJECTIVE

The objective of this study was to adapt the CROPGRO-Perennial Forage Model (PFM) for its ability to simulate asymbiotic N-fixation and growth of brachiariagrass (Brachiaria hybrid ‘BRS RB331’ Ipyporã) under four N sources: 0 N (0 N), 80 kg ha⁻¹ yr⁻¹ of mineral nitrogen fertilizer (80 N), 0 N + Azospirillum brasilense (Az), and 80 kg N ha⁻¹ yr⁻¹ + A. brasilense (80 N + Az).

METHODS

The study was conducted from November 2014 to March 2017 in Sinop, Brazil. The parameterization of the CROPGRO-PFM started with published values for ‘Marandu’ palisadegrass but required minor modifications for Brachiaria hybrid ‘BRS RB331’ Ipyporã. A limited number of parameter values were re-calibrated to include root senescence, dormancy sensitivity to daylength, and dry matter partitioning to leaf and stem. To mimic asymbiotic N fixation by A. brasilense, the biological nitrogen fixation (BNF) module from CROPGRO-PFM-Alfalfa was used, along with a modified additional cost for BNF and modified specific activity.

RESULTS

Important model modifications were to increase partitioning to leaf while decreasing partitioning to stem, along with modifying the daylength (winter) effect on partitioning to shoot. After modifying these model parameters, the simulation of herbage accumulation was improved, with a RMSE of 459 kg DM ha⁻¹, and d-statistic of 0.70. The model was successfully adapted to simulate BNF by A. brasilense which resulted in equivalent of 61.8 kg N fixed ha⁻¹ yr⁻¹.

CONCLUSIONS

Model adaptation for BNF by A. brasilense can effectively mimic the physiology of asymbiotic biological nitrogen fixation in the CROPGRO-PFM for simulating herbage accumulation of Ipyporã brachiariagrass.

IMPLICATIONS AND SIGNIFICANCE

Replacing mineral fertilizers with a sustainable N source can assist with sustainable food production, mitigation of greenhouse gas emissions and environmental pollution. However, the simulations indicate that asymbiotic BNF is costly (nearly 10 times more than BNF of soybean nodules) and it can be overcome/saturated by higher fertilization levels that provide N at a lower energy cost, although there is the fossil fuel cost of mineral N fertilization but not for BNF.