A comparison of direct and indirect 15N isotope techniques for estimating crop N uptake from organic residues

Abstract

Experiments were carried out to compare the direct approach for estimating crop N uptake from ¹⁵N labelled organic inputs, to two indirect approaches, ¹⁵N isotope dilution and A value. In the first experiment soils received 25, 50, 75, or 100 mg N kg soil⁻¹ in the form of Casuarina equisitifolia residues in addition to ammonium sulphate fertiliser, to give a total of 100 mg N kg soil⁻¹ added. This was a cross labelling design, thus two matching sets of treatments, were set up, identical in all but the position of the ¹⁵N label. Maize (Zea mays L.) plants were grown in the soils amended with residues for 11 weeks and N derived from residues (Ndfr) estimated using the A-value or the direct approach. The A-value approach appeared to significantly overestimate %Ndfr compared to the direct method. In the second experiment contrasting residues were added to soil, fababean (Vicia faba L. var. minor), alfalfa (Medicago sativa L.), soyabean fixing, (Glycine max (L.) Merrill), soyabean non-fixing, barley (Hordeum vulgare L.) and maize. This was also cross-labelling design, labelled and unlabelled residues were used. Maize plants were grown in these soils for 11 weeks and %Ndfr in the maize plants estimated using ¹⁵ N isotope dilution and the direct approach. The ¹⁵ N isotope dilution approach also overestimated %Ndfr compared to the direct method in this experiment. Pool substitution appeared to be responsible for the discrepancy between the direct and indirect techniques. It was concluded that ¹⁵N isotope dilution and A-value approaches as used in these experiments (i.e where residues and ¹⁵N label are added simultaneously) were not appropriate techniques for estimating N derived from organic residues in soils.