Availability of residual N-15 in a coniferous forest soil: a greenhouse bioassay and comparison with chemical extractions

The assessment of soil N availability by chemical extraction methods often needs to be checked by methods which directly measure plant N uptake such a s a greenhouse bioassay. In this paper, the recovery of residual N-15, from humus material samples with N-15 labelled for 24-h, seven-month, and 31-mo nth, in western redcedar (Thuja plicata Donn ex D. Don) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings was investigated in a 342-day greenhouse incubation study and was compared to chemical extraction studies on the same samples. Apparently higher N availability in the 24-h treatmen t resulted in greater shoot mass in that treatment than in the other two tr eatments. However, root and whole plant mass were not significantly differe nt among treatments and there were no differences between the species in an y of the above measurements. Plants in the 24-h treatment also proportional ly took up more residual N-15 from the humus material than those in the oth er two treatments and thus significantly greater availability ratios were o btained in the former than in the latter two treatments. At the end of the 342-day incubation, a significant amount of N-15 had been immobilised by th e soil in the 24-h treatment compared to net N-15 mineralisation in the oth er two treatments. The high soil mineral N and N-15 contents in the 24-h tr eatment at the end of the 342-day incubation compared to the low soil miner al N and N-15 contents in the seven-month treatment at the beginning of gre enhouse incubation means immobilisation of fertiliser N in the greenhouse i ncubation was dramatically reduced compared to field situations. Nitrificat ion was negligible before day 182 but was detected at the end of the incuba tion. Correlation analysis showed that N-15 released during a two-week anae robic incubation or in a 42-day aerobic incubation, N-15 extracted by 0.01 M KMnO4 or 2 M KCl, N-15 released by autoclaving or fumigation (-extraction ), and even N-15 abundance in the fulvic acid fraction of the organic matte r all seemed good indicators of soil residual N availability. (C) 1999 Else vier Science B.V. All rights reserved.