Nitrogen release, tree uptake, and ecosystem retention in a mid-rotation loblolly pine plantation following fertilization with 15N-enriched enhanced efficiency fertilizers
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Nitrogen is the most frequently limiting nutrient in southern pine plantations. Previous studies found that only 10 to 25% of applied urea fertilizer N is taken up by trees. Enhanced efficiency fertilizers could increase tree uptake efficiency by controlling the release of N and/or stabilize N. Three enhanced efficiency fertilizers were selected as a representation of fertilizers that could be used in forestry: 1) NBPT treated urea (NBPT urea), 2) polymer coated urea (PC urea), and 3) monoammonium phosphate coated NBPT treated urea (MC NBPT urea). Urea, MC NBPT urea, and NBPT urea fertilizer treatments showed an extractable NH4+ spike 14 days after fertilization while the polymer coated urea showed a spike in NH4+ 49 days after fertilization. Total ecosystem recovery of fertilizer in each treatment was; MC NBPT urea, 51.29 g N; NBPT urea, 48.87 g N; urea, 45.09 g N; and PC urea, 31.30 g N which represents 78.7%, 74.7%, 72.1%, and 47.6% respectively of the total N applied. For the MC NBPT urea, NBPT urea, and PC urea treatments the largest sinks for N were the forest floor and mineral soil. The largest sink for fertilizer applied N in the urea treatment was in the tree. The 2011 foliage cohort was the largest sinks for fertilizer N recovered by the tree. N volatilization was around 20% for all fertilizer treatments except polymer coated fertilizer, which was 1.1%. Urea preformed the same as the soluble enhanced efficiency fertilizers and better than the PC urea fertilizer. The results emphasize the importance of climatic conditions on fertilizer release and effectiveness.
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