M. Scott Smith

Phases of denitrification following oxygen depletion in soil

The short-term response of soil denitrification to reduced aeration was studied using the acetylene inhibition method for the assay of denitrification. Two distinct phases of denitrification rate were observed. An initial constant rate, termed phase I, was not decreased by chloramphenicol, was increased slightly or not at all by organic carbon amendment, and lasted for 1–3 h. Phase I was attributed to the activity of pre-existing denitrifying enzymes in the soil microflora. Following phase I the denitrification rate increased; chloramphenicol inhibited this increase. In soils without organic-C amendment a second linear phase, termed phase II, was attained after 4–8 h of anaerobic incubation. The linearity of this phase was attributed to the full derepression of denitrifying enzyme synthesis by the indigenous population and to the lack of significant growth of denitrifiers. Phase I rate was dependent on the initial or in situ aeration state of the soil sample; phase II was not. Therefore, phase I may be more directly related to field denitrification rates. Denitrification rate changes following water saturation of soils in aerobic atmospheres were also examined. Rates were greatly increased by wetting but only after a lag of several hours. Our interpretation is that following wetting of natural soils, anaerobic or partially anaerobic conditions are established by respiration and reduced O2 diffusion rate; this first eliminates O2 inhibition then derepresses the synthesis of denitrifying enzymes. Although denitrifying enzymes are apparently present even in relatively dry soils, their activity is low until O2 inhibition is eliminated. From this evidence we reason that most N is lost from soils during brief periods beginning a few hours after irrigation or a rainfall.

Legume Winter Cover Crops

Throughout virtually all of the history of agriculture, the nitrogen harvested from cropped soils has been replenished, if it has been replenished at all, by leguminous nitrogen fixation. Although animal wastes, nonsymbiotic fixation, and atmospheric deposition can be significant sources of N, a large fraction of the first can be traced to legume sources and the latter two are generally insufficient to maintain productivity of cropland. In the Mediterranean Civilizations, documented recognition of the value of green manures can be found as early as the writings of Xenophon, who lived from 434 to 355 B.C. (according to Wedderbuan and Collingwood, 1976). Semple (1928), in a review of ancient agricultural practices, indicated that several writers have specifically discussed the use of legumes for soil improvement. Theophrastus (373–287 B.C.) wrote of bean crops being used as green manure by farmers of Macedonia and Thessaly. Cato (234–149 B.C.) and Columella (about 45 A.D.) compared the value of various legumes in soil improvement. Lupine was a favored legume for this purpose. According to Pieters (1927), Chinese writers recognized more than 2,000 years ago that legumes increased production of the crops that followed. As is often the case now, development of these practices by farmers may considerably predate their consideration by academics.

THE ROLE OF MICROORGANISMS IN THE SOIL NITROGEN CYCLE

In attempting to review microbial transformations of soil N in one brief chapter, ambition is perhaps as important a qualification as good sense and knowledge. This subject currently occupies the attention of a large percentage of all those active in soil microbiology and microbial ecology. Monographs, symposia, and stacks of research articles related to N transformations appear at a rate considerably beyond any one individual’s capacity to absorb them. At least some of what is reported here will probably be made obsolete by current developments in the more active research areas: certain aspects of N2-fixation, denitrification, and whole system analysis. Multiple volumes have been devoted to individual segments of the cycle. We begin by abandoning all pretense of comprehensive coverage. We will describe some of the significant microbial transformations and fluxes of soil N, which relate to the main themes of this book. Our review will concentrate on specific processes, for which mechanisms and organisms will be considered. Although some consideration of overall ecosystem N dynamics will be presented here, the reader interested in an integrative approach to N cycling is referred to several excellent reviews in a recent book (Clark and Rosswall 1979). We could not hope to provide a more eloquent or entertaining overview of N cycle studies than that by F. E. Clark in the volume just cited.