A measure of the stability of the residual 15N in soilis the availability ratio (AR) defined by Broadbentand Nakashima (1967) as AR = T,(U, + T,)/T,(U, + T,), where T, is the labelled N in the plant,U, is the unlabelled N in the plant, T, is the labelledN in the soil at the beginning of plant growth and U,is the unlabelled N in the soil at the beginning ofplant growth. Alternatively, the isotopic compositionof the inorganic N in the soil can be used in theexpression in the absence of plant growth. Availabilityratios decline as the residual “N becomesprogressively more stable. An availability ratio of 1would indicate that the labelled N and indigenous soilN were equally available.
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