Nitrogen mineralization in selected soils from the maize producing areas of South Africa

Abstract

The aim of this study was to relate nitrogen mineralization from soil to selected soil properties more easily obtained in the laboratory. Net nitrogen mineralization was determined to varying depths (0–200, 200–400 and 400–600 mm) in 21 soil profiles from the main maize-producing areas in South Africa. Samples were selected to obtain a wide range of clay and organic matter contents. Soil samples were packed in specially designed leaching tubes and treated with a nutrient solution free of nitrogen, before incubating them at 30°C under optimum conditions. Nitrogen mineralization was measured from nitrogen released after several consecutive incubations. The mineral nitrogen was leached with 0.01 M KCl from the soil in a leaching tube and the soil was then treated with a nitrogen-free nutrient solution before continuing with the incubation. Determinations of mineral N were carried out after incubation for 2, 4, 10, 16 and 22 weeks. These data were used to calculate the cumulative net nitrogen mineralized after 22 weeks (N₂₂) and also the nitrogen mineralization potential (N₀). Both N22 and N₀ were correlated with organic carbon, total nitrogen and clay content. Simple regressions showed the greatest correlation to be with total nitrogen (N₂₂: r ² = 75% and N₀: r ² = 71%) for all the sample depths and with clay (N₂₂: r ² = 80% and N₀ r ² = 81%) when only topsoils were considered. Multiple regression analysis showed organic carbon to be the least significant parameter but both N₂₂ and N₀were positively correlated with total nitrogen and clay content (N₂₂: r ² = 79% and N₀: r ² = 76%) for all depths; for topsoils (N₂₂: r ² = 81% and N₀: r ² = 82%); for intermediate soils (N₂₂: r ² = 92% and N₀: r ² = 76%) and for subsoils (N₂₂: r ² = 76% and N₀: r ² = 88%). Nitrogen mineralized from soil under optimum moisture and temperature conditions can therefore be estimated from total nitrogen and clay content.

Keywords:

• Clay content
• organic carbon
• total nitrogen