Accurate measurement of the gain and loss of N by plants and soil under field conditions is difficult. Experiments where all aspects of the N pathway have been monitored usually do not account for the gain or loss of N. The most difficult aspect to measure is the loss due to denitrification. Essentially, N not accounted for elsewhere is assumed to have been lost by denitrification. Many techniques are being researched to help overcome this problem with our experimental methods and are reported here.
Papers in this book result from symposiums held by the Soil Science Society of America (SSSA) at their annual meetings. Experts were solicited to discuss selected topics and we want to thank them for their chapters which make up this book.
The SSSA is pleased to publish Field Measurement of Dinitrogen Fixation and Denitrification and make these contributions available to all researchers interested in the subject.
E.C.A. Runge, president, 1985
Soil Science Society of America
Many approaches have been taken to accurately measure or estimate N gain by plants and soils via biological dinitrogen (N2) fixation or N loss from soils via denitrification. The number and variety of the approaches attest to the difficulty of quantifying gaseous N exchanges between the atmosphere and plant-soil systems.
With a few exceptions, most attempts to quantify biological N2 fixation or denitrification by direct means under field conditions are of recent origin (since the mid-1970s). Although the methods used often have provided data that appear to be reasonable estimates of N gain or loss, none of the methods are entirely satisfactory. Moreover, lack of an absolute reference standard is a major obstacle in evaluating their reliability.
Six of the articles published here focus on two techniques that are being used by an increasing number of scientists to investigate biological N2 fixation or denitrification. Three chapters discuss the isotopedilution technique for estimating biological N2 fixation, and three discuss the acetylene blockage technique for estimating denitrification. Both techniques employ the use of assumptions for data interpretation that may not be readily apparent. For those interested in using data obtained from these methods, but not directly involved in these research areas, discussion of the advantages and limitations of the two methods highlighted here would seem of value.
Each set of three articles is preceded by an overview of the main approaches that have been taken to measure or estimate N gain by biological N2 fixation or N loss via denitrification. The overviews are not intended to be comprehensive reviews but to serve as guides to methods other than isotope dilution or acetylene blockage techniques for estimation of biological N2 fixation or denitrification, respectively.
No attempt was made by the editors to eliminate contradictory statements among the articles. On the contrary, the format was chosen to expose differences of opinion for or against the measurement techniques under discussion. A major emphasis is on exposing the advantages and limitations of each method. To this end, we anticipate that readers will find this organizational approach to be of value.
Roland D. Hauck, editor
Richard W. Weaver, editor