TRANSFORMATIONS OF NITROGEN ADDED AS AMMONIUM AND MANURE TO SOIL WITH A HIGH AMMONIUM-FIXING CAPACITY UNDER LABORATORY CONDITIONS

Abstract

Ammonium sulfate, dairy cattle liquid manure and the “liquid" and “solid" fractions from the manure were separately added to 300-g samples of a loam soil at the rate of 250 μg N/g of soil and incubated in the laboratory at 22 C and 60% water-holding capacity. The Brunisolic soil sample used contained 20% clay which was largely vermiculite. Subsamples were removed at intervals for analysis of nitrate, exchangeable and fixed ammonium, Warburg studies of respiration rate and the distribution of organic N. One series of duplicate samples was leached with water three times during the course of the experiment which was over a year. About half of the nitrogen of the manure was in the NH₄⁺ form and the exchangeable N from this as well as that from (NH₄)₂SO₄ was rapidly nitrified (in about 3 wk) in both the leached and unleached series. About 40% of the NH₄⁺-N of the manure and fertilizer was fixed by the clay and this was nitrified more slowly. With the fertilizer-NH₄⁺, the fixed-N was reduced after about a year to a value slightly above that of the control soil at the beginning of the experiment. With the manure and its liquid fraction a smaller proportion of the "added" fixed NH₄⁺ was nitrified. There was no increase in fixed-NH₄⁺, nitrate or exchangeable NH₄⁺ throughout the experiment with the solid fraction, although with the control soil more than 100 μg/g of nitrate was formed. There was a slight increase in amino compounds, amino acids and amino sugars, from 31 to 245 days. The amounts of these compounds were highest with the solid -fraction-treated soil and lowest with the control. The oxygen uptake was highest with solid -amended soil at all times; after about 180 days the soils treated with the other materials had the same oxygen uptake as the control soil. Leaching of the soil removed the nitrate formed but did not appear to have any other effect. Three leachings, the last after 245 days, removed 113 μg mineral-N/g soil from the control and only 144 μg/g from the manure-treated soil although 142 μg NH₄⁺-N/g soil had been added to the latter. Clay fixation and immobilization had apparently "tied up" most of the added mineral-N.