Spatial distribution of ammonium and calcium in optimally fertilized pine plantation soils
Commercial timber production is increasingly reliant on long-term fertilization to maximize stand productivity, yet we do not understand the extent to which this practice homogenizes soil properties. The effects of 16 yr of optimal fertilization and optimal fertilization with irrigation (fertigation) on forest floor depth, pH, total organic carbon (TOC) and total nitrogen (TN) content, and concentrations of potentially plant available NH+4 ,Ca2+,Mg2, and Al3+ in the fine root zone of monoculture loblolly pine (Pinus taeda L.) stands in North Carolina were examined. Generally, optimal fertilization significantly increased forest floor depth and the concentrations of potentially plant available NH+4, Ca2+, and Mg2. No significant effects on TOC or TN were observed. However, differences were observed between optimally fertilized and fertigated treatments. Specifically, fertilization alone tended to retain more Ca2+ and Mg2+ in the root zone and to increase pH more than fertigation. Semivariogram analysis indicated optimal fertilization generally lead to a significant increase in the effective spatial autocorrelation ranges of NH+4 and Ca2+ at the end of the growing season, from approximately 25 m in controls to more than 100 m. However, optimally fertilized and fertigated plots differed in terms of their NH+4 and Ca2+ spatial autocorrelation ranges, with the ranges in fertigated plots being 1.5-to 2-fold greater. A comparison of kriged maps of NH+4 and Ca2+ availability in fall 2000 and spring 2001 suggested that the spatial distribution of Ca2+, which has not been added to these soils for 6 yr, was stable, whereas the heterogeneity of NH+4 distribution increased post fertilization.