Intensive agriculture uses increasingly large amounts of nitrogen fertilizers to increase yields because nitrogen is one of the limiting factor of plant growth. Plants are in direct competition with denitrifying bacteria for nitrate, but biological denitrification inhibition (BDI) is a strategy developed by some plants in which procyanidins are produced that inhibit denitrification. This phenomenon increases the available nitrate in the soil. Previous results showed that the addition of procyanidins to lettuce fields allowed effective BDI and increased soil nitrate and plant growth. However, agriculture is performed in different soils, and the action of procyanidins could depend on the type of soil. In this study, we tested the effect of procyanidin amendment on lettuce growth in two types of soils: loamy sand and sandy clay loam. Our results show that in both soils, the addition of procyanidins causes inhibition of denitrification, an increase in available nitrate, counter-selection of denitrification communities and a gain in plant mass without modification of the soil structure. This study highlights a sustainable agricultural method effective in a variety of soils.

Intensive agriculture uses a lot of nitrogen fertilizers to increase crop productivity. These crops are in competition with soil-denitrifying microorganisms that assimilate nitrogen in the form of nitrate and transform it into N2O, a greenhouse gas, or N2. However, certain plant species exude secondary metabolites, called procyanidins, which inhibit denitrifiers and increase the nitrate pool in the soil available for plant nutrition. This phenomenon is called biological denitrification inhibition. Previously, we showed that the addition of exogenous procyanidins to a lettuce crop induces denitrifier inhibition and increases nitrate content in the soil, affecting lettuce morphological traits. Here, the effects of procyanidin amendments in the field on a more long-term and nitrogen-consuming crop species such as celery were tested. The effects of procyanidin amendment on celery growth with those of conventional ammonium nitrate amendments were, therefore, compared. Denitrification activity, nitrate concentration, the abundance of denitrifying bacteria in the soil, and traits related to celery growth were measured. It was shown that the addition of procyanidins inhibits denitrifiers and increases the soil nitrate level, inducing an improvement in celery morphological traits. In addition, procyanidin amendment induces the lowest nitrogen concentration in tissues and reduces N2O emissions.