Rapid Assays to Predict Nitrogen Mineralization Capacity of Agricultural Soils

Inability to predict the quantity of nitrogen a soil can supply via mineralization remains a serious obstacle to the improvement of N management. A large-scale study (130 soils) was conducted to identify laboratory assays that may enable N mineralization potential of New Zealand soils to be estimated reliably and rapidly. To ensure that the study delivered robust conclusions, samples (0-15 cm) were collected from a wide range of sedimentary and allophanic soils representing different land uses (pastures; arable cropping). The selected assays included: N mineralized in a 7-d anaerobic incubation at either 40 or 25 degrees C; CO2-C evolved in 24-h following re-wetting of air-dry soil ("CO2 burst test"); and N mineralized in a 2-wk aerobic incubation. Dissolved organic matter was determined using "mild" extractants: cold and hot water and 0.01 mol L-1 NaHCO3. Particulate organic matter was included as it is known to be labile and can be rapidly quantified. These assays were evaluated against N mineralization potential measured in a 14-wk aerobic incubation at 25 degrees C. The assays that correlated closely with mineralization potential included anaerobically mineralizable N and CO2 burst 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 a wide range of soil types and land uses. Inclusion of hot water specific ultraviolet absorbance (260 nm) in a multivariate regression with hot water extractable N and land use produced the best-fit model for explaining variability in N mineralization potential.