Long-term soil nutrient dynamics and lateral nutrient movement in fertilized and unfertilized red pine plantations

In this study, we use a repeated-measures analysis to test the hypothesis that soil fertility under potassium-limited red pine (Pinus resinosa Ait.) stands at the Charles Lathrop Pack Demonstration Forest in Warrensburg, New York is increasing toward a steady state that was artificially induced in fertilized stands by K-fertilization over 50 years ago. We measured soil K by horizon and added new data to a 53-year database. We examine one mechanism that explains the higher rate of K accumulation in unfertilized stands compared with fertilized - lateral movement of fertilizer K from treated plots to untreated - using the rubidium/potassium reverse tracer method. Over the past five decades, soil K concentrations under both fertilized and unfertilized red pine have increased significantly. The trends under fertilized and unfertilized plots demonstrate the gradual convergence of soil K under unfertilized plots toward concentrations in fertilized plots. Five decades after fertilization, treated soils still contain greater concentrations of exchangeable K and lower bulk densities than unfertilized plots. Analysis of Rb/K ratios in the forest floor of fertilized and unfertilized plots confirms the hypothesis that lateral transport of surface broadcast fertilizer, applied over 50 years ago, extends approximately 11-16 m from the edges of fertilized plots. The four unfertilized plots closest to fertilized plots have been significantly affected by inputs of fertilizer K, while the remaining five plots are relatively unimpacted. Approximately 36% of the K in fertilized plots, and 23% of the K in unfertilized plots affected by fertilizer migration were derived directly from the fertilizer applied 5 decades ago, demonstrating the highly conservative nature of mineral nutrient cycling in aggrading forests.