| |
Pine straw producers tend to apply luxury consumption fertilization rates to increase yield, creating a risk of groundwater and surface water contamination. Controlled-release fertilizers reduce leaching risks, but have not been used in forestry due to high cost. The purpose of our study was to investigate the effects of applying controlled-release polymer-coated urea (ESN) compared to conventional urea in raked and non-raked slash pine plantations on the ammonium (NH4-N) and nitrate-nitrite (NOx-N) nitrogen concentration in the soil solution. Five June fertilization treatments (ESN at 0, 28, 56, or 140 kg N ha-1, urea at 56 kg N ha-1) and two February raking treatments (raked and non-raked) were tested in a randomized complete block design with three replications. These fertilization rates were applied annually in 2014, 2015, and 2016 and annual pine straw harvests were done in 2014-2017. Soil solution was collected by suction lysimeters at 30-cm depth for one year following each fertilization. All fertilization treatments increased NH4-N and NOx-N concentrations in soil solution compared to the non-fertilized control and to pre-fertilization levels. The period of elevated NH4-N concentrations occurred between one and four weeks after the second and third fertilization, and the period of elevated NOx-N concentrations from one to 13 or 26 weeks after each fertilization, with peaks at four or eight weeks. Secondary minor NOx-N concentration spikes were recorded almost a year after the first and second fertilization. NH4-N and NOx-N concentrations increased with increasing N application rate. Conventional urea resulted in greater NH4-N concentration increase than ESN applied at the same N rate, especially in the non-raked plots. Effects of pine straw harvesting were not consistent, but generally NH4-N concentrations were higher in non-raked plots, while NOx-N concentrations were higher in non-raked plots after ESN application, but in raked plots after urea application. |
