01 March 2019

Biological soil fertility: potentially mineralisable nitrogen

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Biological soil fertility: potentially mineralisable nitrogen

The identification and management of soil fertility are crucial issues in agriculture. How can we determine the ability of the soil to provide nutrients for plants, and especially nitrogen which is essential to growth?

If in conventional agriculture it is easy to cover a lack of nitrogen supply from the soil by applying  mineral fertilisers, the aim in organic farming and other low input systems is to meet the needs of the crops by maximising the supply from the soil. In these systems inputs are organic: farm manure (slurry, raw or composted manure, ...), crop residues, plant cover crops (catch crops, intercrop cover crops, intercropping, decomposition of stable organic matter - "humus", ...).

The conversion of organic nitrogen into mineral nitrogen is a microbial process involving aerobic bacteria (nitrobacter, nitrosomonas...), known as nitrification. There are several methods used to measure the potentially mineralisable nitrogen in soils, including those based on laboratory incubation. This is widely used on soils, and also to study the release of nitrogen by organic matter supplied to the soil. At the CRA-W, it has been used to describe and manage soil fertility in organic farming (BIO2020), and also for other projects (Soilveg, Bioecosys). However, these laboratory measurements only provide a theoretical indication of nitrogen soil fertility.

The originality of the approach is that it compares these laboratory measurements with field monitoring of the mineral nitrogen found in the soil profile horizons, in bare soil parcels, parcels with crops (cereals, vegetables), and a transposition to the field. This is based on a close correlation between nitrification rate and soil temperature. This approach makes it possible to convert the number of incubation days in the laboratory into an equivalent number of days under field conditions, and thus to predict the dynamics of nitrogen release in the field over the agricultural season. For this purpose, our work is based on daily medians of soil temperature calculated over a 20-year period in reference meteorological stations, which are representative of the various bioclimatic regions of Wallonia.

Processing of all cumulative data will make it possible to fine-tune this transposition of laboratory measurements to the field, in our crop systems and our pedoclimatic conditions.

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