Spatial Distribution of Ammonium and Calcium in Optimally Fertilized Pine Plantation Soils

mately 7 to 10% of the area available for loblolly pine production (Schultz, 1997), optimum nutrition may beCommercial timber production is increasingly reliant on long-term come strategically important to meet global demand for fertilization to maximize stand productivity, yet we do not understand loblolly derived forest products (Albaugh et al., 1998). the extent to which this practice homogenizes soil properties. The Natural forest soils are rarely homogenous, and variaeffects of 16 yr of optimal fertilization and optimal fertilization with irrigation (fertigation) on forest floor depth, pH, total organic carbon tions in microtopography, organic matter content, parti(TOC) and total nitrogen (TN) content, and concentrations of potencle-size distribution, and hydrologic properties across tially plant available NH4 , Ca2 , Mg2 , and Al3 in the fine root zone the soil create a patchy mosaic of different nutrient-rich of monoculture loblolly pine (Pinus taeda L.) stands in North Carolina and nutrient-poor areas. Soil nutrient distributions have were examined. Generally, optimal fertilization significantly increased been found to vary significantly both between and within forest floor depth and the concentrations of potentially plant available forest soils (Zinke, 1962; Trangmar et al., 1987; RobertNH4 , Ca2 , and Mg2 . No significant effects on TOC or TN were son et al., 1988; Gallardo, 2003). This patchy distribution observed. However, differences were observed between optimally of nutrients is believed to be the main cause of growth fertilized and fertigated treatments. Specifically, fertilization alone variation between individual trees in most monoculture tended to retain more Ca2 and Mg2 in the root zone and to increase stands (Schultz, 1997). Variations in nutrient distribupH more than fertigation. Semivariogram analysis indicated optimal tion within a stand are therefore of particular importance fertilization generally lead to a significant increase in the effective in commercial forestry as they may affect the uniformity spatial autocorrelation ranges of NH4 and Ca2 at the end of the growing season, from approximately 25 m in controls to more than of stand development and hence final productivity. A 100 m. However, optimally fertilized and fertigated plots differed in more homogeneous distribution of water and nutrients terms of their NH4 and Ca2 spatial autocorrelation ranges, with the may allow for increased production of loblolly pine and ranges in fertigated plots being 1.5to 2-fold greater. A comparison other intensively managed species (Bergh et al., 1999). of kriged maps of NH4 and Ca2 availability in fall 2000 and spring Although intensive management strategies such as op2001 suggested that the spatial distribution of Ca2 , which has not been timal nutrition have been shown to successfully boost added to these soils for 6 yr, was stable, whereas the heterogeneity of productivity, there is little knowledge of the impact of such NH4 distribution increased post fertilization. management options on nutrient distributions. Such knowledge will likely become more important as industry moves to maximize production through the introduction L pine is grown commercially throughout the of more intensive agricultural-type management systems Southeastern USA, and is considered the most im(Bradshaw and Strauss, 2000) and genetically modified portant softwood species of the region (Schultz, 1997). clonal tree stock (McKeand et al., 1997). On the sandy soils of the North Carolina Sandhills and In this study, we evaluated the effect of long-term Piedmont regions, N and water availability are the bigoptimal nutrition on the spatial distribution of selected gest constraints to loblolly pine growth (Schultz, 1997). soil chemical properties in soils of experimental loblolly Recently, it has been demonstrated that application of pine plantations in southern North Carolina. The prian intensive management system and optimal nutrition mary objective of this study was to determine the extent can significantly increase the productivity of loblolly to which optimal fertilization and optimal fertilization pine monocultures on these N-poor and xeric soils (Alwith irrigation (fertigation) altered the spatial distribubaugh et al., 1998). Optimal nutrition involves adding tion of growth-limiting nutrients at the plot scale. Our a balanced mix of limiting nutrients annually at a rate approach was to fit semivariogram models to soil data determined through foliar analysis to be optimal for and determine the effective range of spatial dependence. immediate plant growth needs (Albaugh et al., 1998; Spatial dependence of soil properties defines how soil Bergh et al., 1999). As sandy soils represent approxisamples vary from one another based on the relative distance between sampled areas (Rossi et al., 1991). The smaller the effective range, the more heterogeneous the I. Edwards and R. Turco, Dep. of Agronomy, Purdue Univ., Lilly spatial distribution. Soil samples were taken at the end Hall of Life Sciences, 915 West State Street, West Lafayette, IN of the growth season in October 2000, and again in 47907-2054; A. Gillespie and J. Chen, Dep. of Forestry and Natural Resources, Pfendler Hall, 715 West State St., West Lafayette IN 47907April 2001. 2061; K. Johnsen, USDA Forest Service Southern Research Station, Forestry Sciences Lab., P.O. Box 12254, 3041 East Cornwallis Rd., MATERIALS AND METHODS Research Triangle Park, NC 27709. Received 18 Mar. 2004. *Corresponding author ([email protected]). Site Description Published in Soil Sci. Soc. Am. J. 69:1813–1821 (2005). This study was conducted at the Southeastern Tree ReForest, Range & Wildland Soils search and Education Site (SETRES)—a loblolly pine plantadoi:10.2136/sssaj2004.0112 © Soil Science Society of America Abbreviations: SETRES, Southeastern Tree Research and Education Site; TN, total nitrogen; TOC, total organic carbon. 677 S. Segoe Rd., Madison, WI 53711 USA 1813 Published online September 29, 2005