Publisher Summary This chapter reviews important papers of research in the field of mineralization of organic nitrogen in soil, beginning with the years preceding World War II. Although mineralization of nitrogen must consequently be recognized as a very old problem—even the overestimation of the importance of humus for plant nutrition by Albrecht Thaer at the beginning of the 19th century was a result of the conviction of mineralization of organic compounds in soil—it still occupies a prominent position in studies about plant nutrition. The general character of the nitrogen balance in grassland has been investigated long ago, but the spectacular ability of perennial grass to absorb even very high amounts of applied nitrogen and to retain nearly all mineralized and added nitrogen in organically bound form is still considered surprising. Consequently, much attention is paid to this characteristic of grassland and the whole problem of nitrogen mineralization in grassland remains unsolved. Investigations on nitrogen mineralization in uncultivated virgin bogs are very scarce. Cyplenkin and Schilin showed NO3-N formation in the waterlogged peaty soil of the tundra only locally on some hill crests and southbound slopes. Ammonification is much more common in the tundra soil, but is still very slow compared to well-drained neutral soils. Michniewicz determined the rate of nitrogen mineralization and the number of nitrifying organisms in the upper layers of the forest floor and of peat bogs.

Denitrification in Some California Soils

Mineralization of Organic Nitrogen in Soil

1. The factors affecting denitrification in soil have been studied by determining loss of nitrogen from soil under various conditions by total-N analysis.2. It was found that the rate of denitrification of nitrate in soil was dependent upon various factors such as the pH, temperature and water content of the soil and that, under conditions conducive to denitrification, 80–86% of nitrate-N added to Rothamsted soils was lost by denitrification in 5 days.3. The rate of denitrification was greatly affected by the pH of the soil. It was very slow at low pH (below 4·8), increased with rise in soil pH and was very rapid at pH 8·0–8·6.4. The rate of denitrification increased rapidly with rise in temperature from 2° to 25° C. The optimum temperature for denitrification was about 60° C.5. The degree of water saturation of the soil had a profound influence on the rate of denitrification. Below a certain moisture level practically no denitrification occurred; above this level denitrification increased rapidly with increase in moisture content. The critical moisture level was about 60% of the water-holding capacity of the soil.

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Denitrification in soil. II. Factors affecting denitrification

Until recently it has been difficult to determine quantitatively the extent of nitrogen fixation and denitrification in soil, because by conventional methods, only the sum of these two processes their equilibrium state is found. In order to observe these reactions independently, the heavy isotope of nitrogen, N 15, was used. I t is now generally agreed that denitrification implies only the production of nitrogen or gaseous oxides of nitrogen, through the reduction of nitrate, the nitrate acting as a hydrogen acceptor. This concept is the one which will be followed.

Investigations on the denitrifying process in soil

Publisher Summary During the past years, many attempts have been made to draw up nitrogen balance sheets for both cropped and uncropped soils. In general, these attempts have met with only mediocre success because quantitative data is usually not available for some of the items that enter into the calculations. On the expenditure side of the balance sheet, accurate values for the nitrogen removed in harvested crops and in animals are commonly available. After the balance sheet has been drawn up, income and outgo seldom balance, even though account has been made of all known soil nitrogen gains and losses. Usually all of the nitrogen that went into the system has not been recovered. Only on rare occasions does income exceed outgo, if all of the nitrogen added by legumes is excluded. This failure of nitrogen balance sheets to balance constitutes the enigma of discussion. In a broad sense, all items that enter into the nitrogen balance sheet are involved; in other words, there are several enigmas. Such a discussion of nitrogen balance sheets should help to explain why recoveries of nitrogen in the crop are often only 50 percent of that added as fertilizer. The chapter tabulates the nitrogen balance sheet for the United States.

The Enigma of Soil Nitrogen Balance Sheets

1. Methods of investigating denitrification in soil are critically discussed with special reference to methods based on total-N analysis.2. A modified Kjeldahl method of determining nitrogen which includes nitrate and nitrite and is applicable to waterlogged soil is described and the use of this method in studies on denitrification in soil is illustrated and discussed.3. It is shown that rapid denitrification of nitrate in soil can be induced by incubating the soil under waterlogged conditions with organic materials such as glucose and that denitrification can be followed by total-N analyses if the organic material used to induce denitrification is not added in such excess that it also promotes significant fixation of atmospheric nitrogen.4. The percentage of added nitrate-N lost by denitrification on incubation of waterlogged soils with different amounts of nitrate and sufficient glucose for denitrification was found to be the same whatever the level of application of nitrate.5. Denitrification of nitrate in waterlogged soil containing glucose was found to be accompanied by a rapid but temporary accumulation of large quantities of nitrite and by the formation of smaller amounts of ammonia. Hydroxylamine could not be detected during denitrification, but it was found that this compound was rapidly decomposed in the soils examined by a process which appeared to be purely chemical.6. It is shown that denitrification of nitrate in soil is a microbiological process and that the viability of the micro-organisms responsible for denitrification is not affected by air-drying and storage of the soil.

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Denitrification in Some California Soils

Citations

Mineralization of Organic Nitrogen in Soil

Denitrification in soil. II. Factors affecting denitrification

Investigations on the denitrifying process in soil

The Enigma of Soil Nitrogen Balance Sheets

Denitrification in soil. I. Methods of investigation

Related Papers (5)

Investigations on the denitrifying process in soil

The Effect of Partial Pressure of Oxygen on Some Soil Nitrogen Transformations1

Investigations on Denitrification in Soil

Denitrification in soil. I. Methods of investigation

Denitrification in soil. II. Factors affecting denitrification