A systematic examination of nitrogen cycling in disturbed forest ecosystems demonstrates that eight processes, operating at three stages in the nitrogen cycle, could delay or prevent solution losses of nitrate from disturbed forests. An experimental and comparative study of nitrate losses from trenched plots in 19 forest sites throughout the United States suggests that four of these processes (nitrogen uptake by regrowing vegetation, nitrogen immobilization, lags in nitrification, and a lack of water for nitrate transport) are the most important in practice. The net effect of all of these processes except uptake by regrowing vegetation is insufficient to prevent or delay losses from relatively fertile sites, and hence such sites have the potential for very high nitrate losses following disturbance.
Chronic defoliation by the fall cankerworm, Alsophila pometaria (Harris), accompanied substantial increases in the stream export of nitrate nitrogen (NO-N) from three mixed hardwood forests in the southern Appalachians. These integrated results clearly demonstrate a measurable effect of insect consumers on ecosystem processes, and provide support for the regulatory importance of insects on a landscape scale.
A microtechnique based on the most-probable-number (MPN) method has been developed for the enumeration of the ammonium-oxidizing population in soil samples. An MPN table for a research design ([8 by 12] i.e., 12 dilutions, 8 replicates per dilution) is presented. A correlation of 0.68 was found between MPNs determined by the microtechnique and the standard tube technique. Higher MPNs were obtained with the microtechnique with increased accuracy in endpoint determinations being a possible cause. Considerable savings oftime, space, equipment, and reagents are observed using this method. The microtechnique described may be adapted to other microbial populations using various types of media and endpoint determinations.
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