Factors affecting ammonia volatilization from land-applied broiler litter
Cassity-Duffey, Kate Banks
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Broiler litter is commonly surface-applied to pasture in the southeastern U.S. as a method of waste management and to provide an inexpensive source of nutrients, such as nitrogen (N). Predicting plant-available N derived from litter can be difficult due to losses through ammonia (NH3) volatilization. Field studies were conducted to determine overall NH3 loss as affected by environmental variables and litter characteristics. Loss as a percentage of the applied total N (TN) ranged from 0.9 to 10.5% in eleven 28-d studies. Average vapor pressure (VP) and initial ammonium-N plus uric acid-N explained 79% of the variability in cumulative NH3 loss in nine studies. To better understand the effects of soil water content (WC) and VP on NH3 loss, two 15-d laboratory studies were performed with two soil WCs (0.03 vs 0.13 g H2O g-1) at 32 and 92% relative humidity (RH). Results showed that litter can absorb or lose water at a relatively fast rate depending on RH. At 92% RH, NH3 loss at a soil WC of 0.13 g g-1 was much larger than that at 0.03 g g-1 (21% vs 11% of TN), whereas at 32% RH, NH3 loss was only slightly greater at 0.13 than at 0.03 g g-1 (5.2 vs 3.2 % of TN). Additionally, litter pH buffering capacity (pHBC) was investigated in 37 samples of broiler litter. Values of pHBC ranged from 187 to 537 mmol H (pH unit kg dry litter)-1. Litter pHBC was calibrated to near infrared spectroscopy with R2=0.90. Combining laboratory and field data, a mechanistic model of litter WC and NH3 volatilization was developed. Simulated litter WC fluctuations on a diurnal basis ranged from 0.06 to 1.3 g H2O g litter-1, impacting both mineralization and the pool of ammoniacal-N subject to NH3 volatilization. The calibrated model simulated NH3-N loss for 28-d field studies with R2=0.81. Our data suggest that high initial uric acid-N, ammonium-N, and high VP/RH increase NH3 volatilization, the latter through rewetting of the litter, which leads to increased N mineralization and NH3 loss.