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 degrees 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 (similar to 80%) of organic N, with the remainder being NH4-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 (NH4-N, amino acid-N, amino sugar N) when treated with I M HCl for 6 h at 80 degrees 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 NH4-N released by hot 2 M KCl. The best predictor of PNDM was N mineralized in a 28-d aerobic incubation at 20 degrees C (79% of variability in PNDM explained).