Chapter 4.1: Isotope labelling methods for evaluating contributions of organic sources to plant nutrition

Production in the field and glasshouse of plant material labelled with 15N - for investigation of organic sources of crop nutrition - is described in detail, including that from trees. Also discussed are approaches for labelling animal manure with 15N, and for dual labelling of plant material with isotopes of carbon and nitrogen. The indirect technique for measuring nitrogen derived from organic sources is discussed with special reference to pool substitution; where 15N label and residues are applied simultaneously, the isotope dilution and A-value techniques are inappropriate for determination of plant N uptake from organic sources. Crop residue and green manure studies using the direct method are relatively simple. Labelled green manure can be easily obtained by fertilizing with 15N and the above or below ground material then added to unlabelled soil. The percentage N in the next crop derived from added residue (Ndfr) is calculated using the equations given