Rapid Assays to Predict Nitrogen Mineralization Capacity of Agricultural Soils

Core Ideas Mineralized soil N is an important source of N for crops, but difficult to predict. A reliable, “laboratory‐friendly” test for mineralizable N is not yet available. Chemical and bioassays were assessed to identify a “quick‐test” for mineralizable N. Assays were evaluated against N mineralized in 14‐week incubation (130 soils). Best tests: hot water‐extractable N or CO 2 evolved after re‐wetting air‐dry soil. 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°C; CO 2 –C evolved in 24‐h following re‐wetting of air‐dry soil (“CO 2 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 NaHCO 3 . 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°C. The assays that correlated closely with mineralization potential included anaerobically mineralizable N and CO 2 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.