Evaluation of a New Approach to the 15Nitrogen Isotope Dilution Technique, for Estimating, in the Field, Crop Nitrogen Uptake from Organic Residues

Crop residue return is a nutrient conservation practice that positively impacts crop production and improves the soil quality. One of the challenges scientists faced was to find a reliable technique to properly measure the N release from these residues and follow their fate in the plant-soil system.

The direct isotopic labeling method, which consists of labeling crop residues with 15N, has the disadvantage of being very expensive and often complex.

The indirect technique based on the principle of isotopic dilution and which consists in using the 15N tracer and the unlabelled residues simultaneously, has the advantage of being simpler and less expensive than the direct technique. However, it has limitations due to the temporary organisation of an amount of nitrogen of the 15N tracer in the presence of the organic residues and which causes a dilution of the isotopic labeling which is not related to the mineralisation of the residues applied, Which leads to an over estimation of the nitrogen-residues use by the plant. In order to minimise errors related to N pools substitution, a modified approach to this technique was suggested. It consists of labeling the soil with 15N and allowing it to stabilise for several months before applying the unlabeled residues. By this soil pre-labeling, the mineral nitrogen native to the soil and the nitrogen organised by the microorganisms have substantially a similar enrichment in 15N. A field experiment was conducted to test the new approach for estimating crop N uptake from suflower residues. The soil was prelabeled with 15N by applying 15N-fertiliser to sunflower crop (Helianthus annuus L. var. Viki). 14N plots which received unlabelled fertiliser were also set up. At harvest, 15N-labelled residues were added to the unlabelled soils (direct technique) and unlabelled residues were added to the 15N-labelled soils (indirect technique). Control plots without residues were also established. All plots were sown with wheat (Triticum aestivum L. var merchouch) – sunflower (Helianthus annuus L. var. Viki) cropping sequence. The objective of this study is to test in the field the new approach to the dilution technique and to compare it with the direct technique. If the estimates of crop N uptake from organic residues, obtained by the two techniques are similar, this will strongly argue for the validity of the new approach tested. In the cropping sequence, the first and second crop derived respectively 6,92% and 3,62% from crop residues estimated by the direct method, 7,9% and 4.24% estimated by the indirect method. The results showed no significant difference between the two techniques, which suggests that the soil pre-labeling new technique compares well with the direct technique. It also has the advantage of being less expensive and easier to use for a wide range of organic residues, even the most complex ones.