A Mechanistic Model for Predicting Ammonia Loss and Nitrogen Availability from Surface-Applied Broiler Litter.

Ammonia (NH₃) volatilization from surface-applied broiler litter can lead to large losses of plant available nitrogen (N) to the atmosphere, reducing the efficacy of broiler litter as fertilizer.  Ammonia volatilization is a function of both initial litter characteristics and weather conditions after application.  A mechanistic submodel was first developed to simulate the influence of temperature and relative humidity on litter water content (WC), which affects N mineralization. In laboratory and field studies, the model accurately (r²=0.82) simulated the effect of diurnal changes of RH on litter WC, which fluctuated from 0.05 to 1.5 g H₂O g litter^-1.  The litter WC submodel was incorporated into a mechanistic NH₃ volatilization model to simulate the influence of environmental conditions on organic N mineralization, ammoniacal-N equilibrium, adsorption, and gas partitioning. The full model was able to simulate NH₃-N losses for eleven 28-d field studies conducted in Georgia with r²=0.81. In addition, the model estimated that a range of 15 to 33 % of the total N applied was plant available in the eleven field studies; this indicates the need for in-season estimates of N availability. Results from this study suggest that modeling the effect of relative humidity on litter WC will improve estimations of litter mineralization and volatilization as well as increase our understanding of the mechanisms of NH₃ loss from surface-applied broiler litter.