Arboriculture & Urban Forestry 35(5): September 2009 259 primarily from increases in foliar total [N]. To a degree, increases in foliar total [N] status enhance photosynthetic capacity and the ability to acquire atmospheric carbon. However, 75% or more of applied fertilizer N could not be accounted for in aboveground biomass, regardless of the rate of application. It must be stressed that whole-plant fertilizer N recovery rates, which include roots, were unquestionably greater than the reported aboveground tis- sue recovery rates. However, substantial increases in fertilizer N recovery based on the observed concentrations of N and the percentages of NDFF in root tissues would require dramatic, and somewhat improbable, changes in the root-to-shoot ratio (Harris 1992; Ågren and Franklin 2003). Consequently, regardless of the rate of application substantial amounts of fertilizer N appears to accumulate in nontarget sinks (e.g., soil microbial biomass), or is potentially lost from the system via leaching or gaseous emissions. CONCLUSION Figure 4. Percent fertilizer recovery in aboveground tissues of common hackberry at (A) Arlington, WI, and (B) Lisle, IL in 2002. Symbols represent the mean (n = 5) ± the adjusted standard error. Scales on the y-axis are different. (Kraimer et al. 2001), and substantially less than 50%–60% fertilizer N recovery rates in fruit-bearing orange trees [Cit- rus sinensis (L.) Osbeck.] (Feigenbaum et al. 1987) and Ken- tucky bluegrass (Poa pratensis L.) (Horgan et al. 2002). Re- covery rates at Lisle, IL for both treatment levels were lower than all cited studies, including those involving conifers. It is important to recognize that the reported fertilizer recov- ery rates do not include fertilizer N accumulation in the root system. A number of studies have indicated significant accumu- lations of N in root tissues, particularly at high rates of appli- cations (Warren 1993; Mattos et al. 2003). The percent NDFF in root tissues at both locations were only slightly lower than values observed in the foliage and current season stem wood, indicating fertilizer N recovery at the whole plant level is sig- nificantly higher than what is reported for aboveground tissues. Despite the observed differences in aboveground biomass between the respective locations, the treatment-induced trends in the percent NDFF and fertilizer N recovery were remarkably consistent on both a mass and percent basis. This study demon- strated fertilizer N uptake at application rates recommended un- der the ANSI A-300 standards were proportional, but not equal to, the amount of fertilizer N applied. Fertilization was also dem- onstrated to enhance the overall aboveground N status, resulting In all tissue types, there were proportional increases in the per- cent nitrogen derived from fertilizer. On both a mass and percent- age basis, the vast majority of fertilizer N in aboveground tis- sues was preferentially partitioned to the foliage. However, only the highest rate of application appeared to produce a significant increase in the aboveground N status, and it was confined to a single harvest period. Support for the recommended rates of ap- plication (ANSI 2004) based on increases in fertilizer N uptake at the higher rate of application must be tempered. Substantial amounts of fertilizer N applied to each tree either accumulated in nontarget sinks (e.g., soil matrix, microbial biomass), or was lost from the planting site. It is the specific nature of what hap- pens to the unrecovered fertilizer N that is of concern. Fertilizer N immobilized within the soil microbial biomass represents a slow-release, long-term nutrient pool. However, there is mount- ing evidence that chronic, high-dose additions of N adversely af- fects soil microbial populations and their ability to immobilize readily available N, alters soil chemistry, and negatively affects tree growth (Wallenstein et al. 2006). Consequently, the low recovery rates observed in this study suggest N fertilization in accordance with the ANSI standards may become increasingly worrisome if applications are made on an annual basis. If the majority of the unaccounted for fertilizer N is lost from the site of application through leaching and/or gaseous emissions the concern for environmental degradation is immediate. The con- sistency of fertilizer recovery rates suggests both rates of appli- cation should be carefully considered in landscapes with a shal- low depth to groundwater, adjacent to surface bodies of water, or possess soil qualities prone to leaching (i.e. coarse texture). The ANSI A-300 standards for the fertilization of landscape trees (2004) explicitly state the application of fertilizers is a pre- scription that must be based on an understanding of the targeted plants’ biology, impact to the environment, and desired man- agement objective(s). The isotopic enrichment of the fertilizer used in this study provided unique and much-needed insights into our understanding of the biology and fate of fertilizer N ap- plied at rates consistent with tree care standards. In addition to defining the extent of recovery for a fast release fertilizer, the study provided direction for continued fertilizer research. For instance, the dynamic nature of N partitioning and storage be- tween above and below ground tissues requires examinations be extended well into the dormant period. Similarly, it is unclear ©2009 International Society of Arboriculture
September 2009
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