Hot Water‐Extractable Nitrogen as an Indicator of Soil Nitrogen Availability

There is keen interest among soil scientists in identifying chemical assays that may be used as predictors of soil N mineralization potential. Our objective was to determine if hot water‐extractable N (16‐h extraction at 80°C) is a useful predictor of mineralizable N and plant N availability. In a group of 30 New Zealand soils, representing different management histories and parent materials, hot water extracted between 2.6 and 8.7% of total N. The extracted N consisted mainly (∼80%) of organic N, with the remainder being NH 4 –N, generated by hydrolysis of heat‐labile organic N. The C/N ratio of the extracted organic matter was relatively low (mean 8:1 vs. 11:1 for total organic matter), indicating that it included N‐rich substrates (i.e., substrates likely to have high mineralization potential). However, about three‐quarters of the extracted organic N was relatively recalcitrant, i.e., it did not hydrolyze to ninhydrin‐reactive N (NH 4 –N, amino acid‐N, amino sugar N) when treated with 1 M HCl for 6 h at 80°C. The contribution of mineralized N to plant N uptake was measured using a greenhouse‐grown oat ( Avena sativa L.) crop, which received no added N. Hot water‐extractable N accounted for 50% of the variation in plant N derived from mineralization (PNDM), compared with 16% for total soil N, 32% for anaerobically mineralizable N (AMN), and 24% for NH 4 –N released by hot 2 M KCl. The best predictor of PNDM was N mineralized in a 28‐d aerobic incubation at 20°C (79% of variability in PNDM explained).