Rapid Assays to Predict Nitrogen Mineralization Capacity of Agricultural Soils Used in Pastoral, Arable and Vegetable Production.

A realistic estimate of N mineralized from soil organic matter is essential to optimize fertilizer N use and minimize impacts of excessive N on the environment. A large-scale study was conducted to identify laboratory assays that may enable soil N mineralization potential of New Zealand soils to be estimated reliably and rapidly. To ensure that the study delivered robust conclusions, samples were collected from a wide range of soils in both the North and South Islands. The 130 sites sampled (0-15 cm sampling depth) represented different soil Orders (sedimentary and allophanic soils), management histories (dairy, sheep/beef, intensive and mixed cropping), and textural classes.  Based on a literature search and previous experience, candidate assays were selected. These included biological assays: N mineralized in a 7-day anaerobic incubation at either 40 or 25^oC; CO₂-C evolved in 24-h following re-wetting of air-dry soil (“CO₂ burst test”); and N mineralized in a two-week aerobic incubation.  Dissolved organic matter was determined using “mild” extractants: cold and hot water and 0.01 M sodium bicarbonate. Particulate organic matter was included as it is known to be labile and can be rapidly quantified (e.g., using mid-infrared spectroscopy). These assays were evaluated against mineralization measured in 14-week aerobic incubation at 25^oC, with soil moisture maintained at -10 kPa. The assays that correlated closely with mineralization potential included anaerobically mineralizable N and CO₂burst test values. Particularly strong correlations were obtained for hot water extractable N, suggesting that this easily-measured organic N fraction can be used to predict N supply potential across the range of soil types and land uses found in New Zealand.