The relationships between potential laboratory indices for plant-available nitrogen (N) and the plant N uptake in a pot experiment with ryegrass were assessed for 13 mineral soils and 2 peat soils. The methods included aerobic soil incubation, soil incubation in a bioreactor, hot potassium chloride (KCl)-extractable mineral N, 0.01 M calcium chloride (CaCl2)-extractable N, and N loss at heating. The indices for total plant-available N accounted for 63-93% of the variance in N uptake in a statistical analysis with all soils (n = 15) and 27-89% for the mineral soils (n = 13). Most indices were not a direct quantitative measure of the plant N uptake. The N mineralization indices accounted for 57-86% of the variance in N mineralization for all soils and 5-50% for the mineral soils. Hot KCl-extractable mineral N and 0.01 M CaCl2-extractable N were the most promising rapid indices for plant-available N.
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