Nitrogen mineralization, nitrification and nitrogen balance in laboratory incubation of soil from natural grassland in the central Orange Free State

Abstract

Air-dried soil from three layers under grassveld was moistened by the addition of 12 levels of water and incubated for up to two years at five temperatures in screw-capped preserving jars with an air-filled head- space equivalent to seven times the volume of the soil. The jars were opened and the contents mixed to promote soil aeration at increasing intervals. Soil water, ammonium, nitrite and nitrate concentrations were monitored at 11 sampling times. Organic carbon and nitrogen were determined initially and nitrogen at the last sampling. Gross N-mineralization was measured as initial organic minus final organic nitrogen and net N- mineralization as final inorganic minus initial inorganic nitrogen. Nitrification was calculated as (nitrite+nitrate)/(ammonium+nitrite+nitrate) and denitrification as gross minus net mineralization. There was no net mineralization in soils incubated at 7°, 17° or 27°C and containing less than 6% water; its rate increased with water content to optima at 18% water in the 0–5 and 5–20-cm layer and 20% water in the 20–40-cm layer. The temperature coefficient of net mineralization was about two for the range 7–27°C and the kinetics approximated zero order in layers below 5 cm. At 37° and 44°C kinetics resembled first or, at larger water contents, a higher order. A period of negative net mineralization following the initial accumulation of nitrate at the higher temperatures and water contents was attributed to denitrification because it coincided with a negative nitrogen balance. Up to 41 % of the total nitrogen in the 0–5-cm, 29% in the 5–20-cm and 17% in the 20–40-cm layer was mineralized during about 500 days of incubation.

Keywords:

• Cycling in ecosystems
• nitrogen availability
• semi-arid climates
• soil temperature
• soil water