Burning, watering, litter quality and time effects on N, P, and K uptake by pitch pine (Pinus rigida) seedlings in a greenhouse study

Young pitch pine seedlings are frequently subjected to prescribed burns in the New Jersey pine barrens as part of the management to eliminate fuel load in these forests, thereby reducing the risk of wildfire. The burns are relatively cool and release nutrients without killing the vegetation. Not all nutrients mineralized are immediately available to the plants, however. To determine factors that may contribute to nutrient release and to link this to availability of nutrients to plants following a fire in the pine barrens, a greenhouse study was conducted. The experimental design was a 3 × 2 × 2 × 3 factorial with three types of litter, high and low watering regime, burned and unburned litter, and three sample dates. Nitrogen, phosphorus, and potassium (through a rubidium analog) uptake rates were measured using natural isotope or radioisotope root bioassays. Though in the short time the experiment ran, neither an overall burned litter nor a litter quality effect was seen, high watering made P less available and K more available. N and K were the most available immediately following application of treatments, whereas P was not available until after 6 weeks following treatment application. Net mineralization of nitrogen, phosphorus, and potassium occurred in burned litter, whereas net immobilization of N and P occurred in highly watered, unburned litter. Since watering and timing were most influential in making nutrients available to seedlings following application of treatments in this greenhouse study, these factors may also be important in making nutrients available to mature trees in the New Jersey pine barrens.