Ammonia Emission from Soybean‐Amended Calcareous Soil with Various Soil Temperature and Moisture Levels

Abstract

AbstractHigh soil temperature, moisture, and crop residue levels were found to combine for maximum volatilization of ammonia from calcareous soil amended with soybean [Glycine max (L.) Merr.] tissue in this study. An unusually large amount of ammonia was liberated to the atmosphere and provided the opportunity to measure accurately isotope separation of 15N and 14N atoms resulting from the diffusion process. Soybean tissue (early reproductive stage) was mixed with a calcareous soil (Aridic Paleustoll) at two different levels (25 and 50 g kg−1). The two soil soybean‐tissue mixtures were wetted to the desired moisture content (20, 60, and 100% field capacity) and incubated 7 d at either 10, 20, or 30 °C. After incubation, ammonia evolution and moisture loss were determined during either a 4‐ or 7‐d dry‐down period in which dry NH3‐free air was passed across the soil surface. We found more NH3 volatilized at 100ɐ FC than at 20 or 60% FC, at 30 than at 10 or 20 °C, and at 50 than at 25 g kg−1 residue rate. Maximum loss of NH3 was about 5% of soybean‐tissue N measured in an experiment in which four wet‐dry cycles were used. Maximum loss was directly related to water evaporation rate. These results indicate that soil temperature, moisture, and crop‐residue rates all influence NH3 losses from soybean‐tissue amended soil. Isotopic ratios of the NH3−N evolved were measured and ammonia content of the first fraction of ammonia volatilized was lower in atom % 15N than the last fraction. These results indicate that, for the conditions of this study, diffusion is an important process since the 14N atoms volatilized more quickly as NH3 than did the 15N atoms.