Quantifying the whole farm systems impact of nitrogen best practice on dairy farms

Soil N mineralisation is the process by which organic N is converted into plant-available forms, while soil N immobilisation is the transformation of inorganic soil N into organic matter and microbial biomass, thereafter becoming bio-unavailable to plants. These cyclical processes are known as “mineralisation-immobilisation turnover” and are governed by many factors, including soil N status, organic matter, clay content, soil biota, as well as soil moisture and temperature. Mechanistic models can be used to explore the contribution of mineralised N to pasture growth through simulation of plant, soil and environment interactions driven by management. Here, our objectives were (1) compare the performance of three agro-ecosystems models (APSIM, DayCent and DairyMod) in simulating soil N using the same experimental data collected in three diverse environments, and (2), to determine if tactical application of N in different seasons could be used to leverage seasonal trends in N mineralisation to influence pasture growth. We found that despite considerable variation in model sophistication, no model consistently outperformed the other models with respect to simulation of soil N, shoot biomass or soil water. Differences in the accuracy of simulated soil NH₄ and NO₃ were greater between sites than between models; overall, all models performed well in simulating cumulative N₂O. Further scenario analyses showed that while tactical N application had immediate effects on NO₃, NH₄, N mineralisation and pasture growth, no long-term relationship between mineralisation and pasture growth could be discerned. These results suggest that while superfluous N application generally stimulates immobilisation and a pulse of N₂O emissions, subsequent effects through N mineralisation on pasture growth are variable. We suggest that further controlled environment soil incubation research may help separate successive and overlapping cycles of mineralisation and immobilisation that make it difficult to diagnose long-term implications for (and associations with) pasture growth.