180 Scharenbroch and Lloyd: Particulate Organic Matter and Soil Nitrogen Availability Particulate Organic Matter and Soil Nitrogen Availability in Urban Landscapes Bryant C. Scharenbroch and John E. Lloyd Abstract. Toward developing nitrogen management in amenity tree care, we studied soil organic matter, microbial biomass, and carbon and nitrogen mineralization in an attempt to characterize the plant available nitrogen under a variety of landscape management conditions. Fine particulate organic matter (POM) fractions were significantly correlated with microbial biomass, carbon mineralization, and nitrogen mineralization (R2 landscapes were assigned a site quality index based on landscape age and management practices. Fine POM, microbial biomass N, and N mineralization were significantly and positively correlated with the site quality index (R2 values ranging from 0.42 to 0.89). These urban values of 0.86, 0.90, and 0.84, respectively). We propose that with refinement and further testing, a fine POM measurement can be used to accurately predict soil nitrogen availability in urban landscapes. This research shows that urban landscapes are quite variable in terms of nitrogen availability. As a result of this variability, we recommend that urban landscapes be assessed on a per-site basis for proper nitrogen management. Key Words. Microbial biomass and activity; nitrogen mineralization; particulate organic matter; soil nitrogen avail- ability. Soil nitrogen availability is a chief concern of managers of trees in urban landscapes. Amenity tree fertilization practices are specified by the American National Standards Institute, ANSI A300 for Tree Care Operations—Tree, Shrub, and Other Woody Plant Maintenance—Standard Practices (Fer- tilization) (2001). Standard 7.2.1 states that the reason for fertilization is to “supply nutrients determined to be defi- cient.” According to objective 7.2.6, soil and/or foliar analy- sis should be used to determine the need for fertilizer. Stan- dard 8.1.2 states that in the absence of a soil and/or foliar analysis, a 3:1:2 or 3:1:1 fertilizer ratio should be used, and these ratios should be adjusted based on local knowledge, age, and/or condition of the plant, soil, and environmental conditions. Standard 8.1.4 states that slow-release fertilizers should be applied at rates between 1000 and 2000 kg N ha−1 (2 to 4 lb N 1000 ft−2) per application not to exceed 3,000 kg Nha−1 (6 lb N 1000 ft−2) annually. Plant health care (PHC) and integrated pest management (IPM) programs have promoted site-specific prescription fer- tilization for the maintenance and establishment of urban trees (Smith et al. 2002). The ANSI standards recognize the need for inclusion of extant soil nitrogen in fertilization rec- ommendations, but soil nitrogen analysis is only suggested and not required (TCIA 2001). By providing specific fertil- izer ratios, types, and amounts without consideration of soil nitrogen availability status, Standards 8.1.2 and 8.1.4 endorse nitrogen fertilization without soil assessment. Research that details the soil nitrogen variability among types of urban landscapes is significant because it provides data to support the need of required soil assessment before fertilization. ©2006 International Society of Arboriculture Because a standard soil nitrogen availability index does not currently exist; urban landscape fertilization is routinely per- formed without knowledge of existing soil nitrogen condi- tions (Beverly et al. 1997; Osmond and Platt 2000). Plant health care, integrated pest management, and the ANSI Stan- dards serve as the foundation for prescription tree fertiliza- tion, but their use and value could be increased with a prac- tical and accurate soil nitrogen assessment. To be practical, nitrogen assessments must be affordable, convenient, and easy to interpret. Accurate nitrogen assessments must also be highly correlated with established soil nitrogen availability indices and plant response. The accuracy and practicality of common soil nitrogen assessment methods have been reviewed (Kopinga and van den Burg 1995; Doran and Jones 1996; Scharenbroch and Lloyd 2004). Total soil nitrogen in urban landscapes ranges from 1.10 to 6.07 g kg−1 (51 to 214 lb N 1000 ft−2) (Roberts and Roberts 1984; Beyer et al. 1995; Pouyat et al. 2002). However, much of the total soil nitrogen is found in forms unavailable to plants; thus, measurements of total soil nitro- gen tend to overestimate plant available nitrogen (Mulvaney and Khan 2001). Studies that have measured extractable min- eral nitrogen found ranges of 45 to 1500 gg−1 (0.55 to 5.7 lb N 1000 ft−2) in urban environments (Roberts and Roberts 1984; Goldman et al. 1995; Zhu and Carreiro 1999). Mea- surements of soil mineral nitrogen are inherently variable as a result of environmental factors such as moisture, tempera- ture, and microbial activity (Khan et al. 2001; Mulvaney and Khan 2001). Also, mineral nitrogen measurements include nitrogen available for uptake by plants and soil microorgan-
July 2006
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