Forest Floor Composition and Ammonia Loss from Urea in a Loblolly Pine Plantation

Surface application of urea to pine forests may lead to ammonia (NH3) volatilization, which can reduce forest production. The objective of this study was to characterize urea reactions with the forest soil and to determine how various forest floor components interact with weather to affect NH3 loss. Three field studies were performed in a midrotation loblolly pine (Pinus taeda L.) plantation with up to three treatments in each study, where NH3 volatilization chambers were fertilized with 200 kg N ha(-1) that contained either (i) intact forest floor, (ii) forest floor removed by hand raking, or (iii) forest floor burned. In a complementary laboratory study, NH3 loss was evaluated from cores comprised of: (i) mineral soil only, (ii) mineral soil + humus layer, (iii) mineral soil + humus + old needles, and (iv) mineral soil + humus + old needles + new needles. Loss of NH3 in the field was generally greatest when urea was applied to a mineral soil with no forest floor, resulting in a loss of 53 and 59% of the applied N in two 29-d studies. Loss of NH3 was intermediate from a complete forest floor (11 and 35% loss), and rates of loss appeared to be most affected by relative humidity. Losses were least from a recently burned forest floor (6 and 7% loss), due partly to slower urea hydrolysis. Loss of NH3 in the laboratory study was also greater from the bare soil than from the complete forest floor. Retention of NH4+ in the soil layers was related to the soil pH buffering capacity below pH 7. Because the humus layer was shown to have a large pH buffering capacity to retain NH4+, these results suggest forest management that enhances humus formation should increase NH4+ sorption and decrease NH3 volatilization.