Impacts of multiple applications of fertilizer on stream chemistry in the Ouachita MountainsThis article is part of a larger document. View the larger document here.
We have previously reported changes in stream chemistry following a late winter application of urea and diammonium phosphate to a loblolly pine (Pinus taeda L.) plantation located in a 176-ha subwatershed in the Ouachita Mountains. This stand was again fertilized with 437 kg/ha of urea in March of 2001. Water chemistry prior to, during, and after fertilization was monitored downstream of the stand at the outlet of the subwatershed. Current Best Management Practices prohibit fertilizer entry in streamside management zones (SMZs) by either direct application or aerial drift. Fertilizer traps were located in a SMZ and within unprotected stream channels to document improper entry of fertilizer into the SMZ and quantify rates of applications in the unprotected stream channels. Nitrogen (N) concentrations at the subwatershed outlet increased immediately during application, and a number of the traps within the SMZ collected significant amounts of fertilizer. Application of urea upstream from the SMZ had only minor immediate impacts on stream chemistry. N concentrations increased dramatically during the first storm following fertilization. This increase indicated that the urea, which fell in unprotected stream channels or surrounding upland areas, was washed downstream to the main channel. In May, almost 3 months after application, NO3--N concentrations peaked at 15.4 mg/l during a small storm event. Concentrations of NO3--N remained elevated for at least 2 years after application. Concentrations of NO3--N were also greater than those observed following the first application of fertilizer, suggesting that repeated application of fertilizer could have a cumulative impact on N levels in water draining from intensively managed forests.