SPATIAL HETEROGENEITY OF SUBSTRATES: EFFECTS ON HYDROLYSIS, IMMOBILIZATION AND NITRIFICATION OF UREA-N

Abstract

Hydrolysis, immobilization and nitrification of urea-N was measured in samples of Ap horizons of a Black Chernozemic and a Luvisolic soil incubated in the laboratory. Urea was either placed as a nest or mixed throughout the soil. Samples were removed over time to determine the two-dimensional redistribution of urea, .**GRAPHIC**. .**GRAPHIC**. and .**GRAPHIC**. Localizing urea in a nest reduced both its rate of hydrolysis and subsequent nitrification, and increased recovery of added N in the presence of straw equivalant to 4 t ha-1. In contrast, urea mixed into the soil was nearly completely hydrolyzed and oxidized in 8 d or completely immobilized in the presence of straw. Kinetic studies showed urea hydrolysis was inhibited in a Luvisolic soil sample with increasing substrate concentration beyond 5 mM. The apparent Michaelis-Menten constant (Ka) was 19 mM; the inhibition constant (Ki) was 7 mM; and apparent maximum velocity (Va) was 34.5 .mu.g N g-1 h-1. Urease activity in the Black Chernozemic soil was described by normal Michaelis-Menten kinetics with a Km value of 3.4 mM and Vmax equal to 18.2 .mu.g N g-1 h-1. When urea was localized in a nest, most of the nitrite oxidizers originally present in the soil were killed during the first 24 d of incubation. Factors such as solubilized organics, in addition to .**GRAPHIC**. and .**GRAPHIC**. were considered to be involved. It was concluded that nest placement of urea influenced N transformations in two ways. First, the low surface area:mass ratio reduced exposure of urea to the soil, thereby slowing processes such as immobilization by organisms on decomposing straw. Second, at the microsite level, high concentrations of urea can inhibit urease; and NH3, once generated in such concentrated localized areas, can itself inhibit nitrification through direct toxic effects or possibly through dissolution of inhibitory organics.