Isotopic labelling contribution to nitrogen transfer evaluation between organic and mineral compartments in soil-plant systems

This report aims at studying some pathways of the nitrogen cycle by means of the stable isotope ¹⁵N. ¹⁵N-tracer techniques are explained and comments made on terms commonly used. Emission spectrometry is proposed as a substitute for mass spectrometry for ¹⁵N analysis. This technique is simpler than mass spectrometry and is described briefly. Nitrogen in the soil is continuously involved in two opposite transformation processes, namely mineralization and immobilization. The turning under of ¹⁵N-enriched crop residues with differing C/N values (green manure and cereal straw) makes it possible to follow the mineralization process of both soil and crop residue nitrogen. C/N values over 20 favours the immobilization process. Mineral N is incorporated into various organic fractions which are separated by acid hydrolysis; the tracer is recovered mainly in the amino acid forms. When evaluating the simultaneous transfers between mineral and organic nitrogen forms it was found that the amounts actually at work (gross processes) are much higher than those observed directly (net processes). A Method is suggested for measuring the amounts involved in the gross processes. It is based on the variation of the tracer amounts in the mineral form. This method allows also the determination of nitrogen leaving the soil-plant system (losses). Losses may be due to ammonia volatilization, denitrification and lixiviation. Volatilization losses were small in our experiments. Denitrification losses were high in some cases; the evaluation of these losses through an 'in vitro' method is described but the actual denitrification is difficult to be valuated in the field. Lixiviation losses as determined by lysimeter experiments were very small in normally cultivated soils but high in bare soils. The true percent plant recovery of fertilizer N was measured in several pot, lysimeter and field experiments. The average recovery value ranged between 50 and 70% though some individual values were either much lower or much higher. A nearly 100% plant recovery value is possible in thoroughly controlled conditions. Such a goal is not advisable in the long run as the amount of soil organic matter would decrease even if crop residues are ploughed in or turned under. Determination of the A value is usually utilized as a measure of the soil nitrogen available to the crop. This value bears little interest and ought to be used with care. The biological processes involved in the nitrogen cycle are complex as they are often antagonistic. ¹⁵N is a useful tool and is sometimes required for quantitative evaluations. (author)