Comparison of Available Soil Nitrogen Assays in Control and Burned Forested Sites

The existence of several different methods for measuring net N mineralization and nitrification rates and indexing N availability has raised questions about the comparability of these methods. We compared in situ covered cores, in situ buried bags, aerobic laboratory incubations, and tension lysimetry on control and treated plots of a prescribed burn experiment in the southern Appalachians. Environmental influences were examined with soil moisture and temperature measurements. All methods detected significant differences in net N mineralization rates between treated and control plots; P = 0.04, 0.007,0.001, and 0.07 for covered cores, buried bags, lab incubations, and lysimeters, respectively. Mean rates of N mineralization during the growing season were similar for the three soil incubation methods. The laboratory incubation of soil from treated plots produced significantly greater potential nitrification rates than the in situ methods. The four methods were not well correlated; this indicates the need for caution when comparing N transformation data derived from different methods. We conclude that the in situ covered core method is superior because it best incorporates site-specific soil temperature and moisture changes into N transformation measurements. N often limits forest growth and productivity (Keeney, 1980). The importance of N in site productivity and its responsiveness to disturbance has prompted the design of several methods to assess potential soil N availability. Measurement of N mineralization potentials in controlled environment incubations began in the 1940s and has been widely used ever since (Harmsen and Lindenbergh, 1949). These methods allow determination of potential N availability; however, they are carried out in ideal environmental conditions and may overestimate site processes. Eno (1960) introduced the in situ buried bag incubation technique. While this method incorporates in situ soil temperature variations into estimates of net N mineralization and nitrification, soil moisture remains unchanged during the incubation period. Adams and Attiwill (1986) developed the in situ covered core method for soil N mineralization measurements. Their system uses perforated PVC tubes covered with petri dishes, which allows soil moisture as well as temperature to equilibrate with the surrounding soil environment. Soil solution collection using tension porous cup lysimeters and ion exchange resin bags have also been used to estimate N availability (Hansen and Harris, 1975; Binkley and Matson, 1983; Montagnini et al., 1986). Binkley and Hart (1989) extensively reviewed the literature to identify the strengths and weaknesses of most methods for determining N availability. There have also been numerous studies comparing two or three methods (Binkley and Matson, 1983; Montagnini et al., 1986; Adams and Attiwill, 1986; Raison et al., 1987; Binkley et al., 1986; Carlyle and Malcolm, 1986; Fyles et al., USDA Forest Service, Coweeta Hydrplogic Lab., 999 Coweeta Lab Road, Otto, NC 28763, Received 28 July 1994. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 59:1750-1754 (1995). 1990; Hart and Firestone, 1989; Hill and Shackleton, 1989). However, these comparisons were limited by the number of methods tested or by the sites on which they were tested. This study compared four methods of determining N availability: measurement of net N mineralization (NHI-N plus NOs"-N production) and net nitrification (NOs-N production) using in situ covered cores, in situ buried bags, and aerobic laboratory incubations and soil solution concentrations of NH^-N and NC>3"-N collected using tension lysimetry. All measurements took place concurrently during the growing season on treated and control plots in a fell and burn forest regeneration experiment. These plots provided a wide range of N transformation rates and environmental conditions. The objective was to compare the methods to: (i) test the hypothesis that all methods discriminate between treatment effects and that soil incubation data are interchangeable, and (ii) examine the relationship of soil incubation methods with lysimeter N, extractable soil N, and soil temperature and moisture. MATERIALS AND METHODS