Can plants absorb and utilize phosphate buffer extractable soil organic nitrogen without its prior mineralization?

Several researchers have hypothesized that organic nitrogen extracted using neutral phosphate buffer, termed phosphate buffer extractable organic nitrogen (PEON), is absorbed and utilized by some plant species as a nitrogen source without prior mineralization. Earlier reports have suggested that leaf lettuce (Lactuca sativa L.) depends mainly on inorganic nitrogen, whereas qing-geng-cai (Brassica chinensis L.) can absorb PEON. We tested leaf lettuce and qing-geng-cai growth under aseptic conditions using three nitrogen sources under different light conditions to investigate this hypothesis. Nitrate, amino acids or purified PEON were supplied as the sole nitrogen source. Five different levels of light intensity were used to detect the contribution of carbon to plant growth supplied from PEON. Leaf lettuce growth was promoted only by nitrate, whereas growth of qing-geng-cai was promoted by nitrate and amino acids. The positive effect of amino acids on qing-geng-cai was pronounced at higher light intensity. However, PEON imparted no significant effect on either of the plants under any of the light conditions. Our results do not support the hypothesis that PEON is used as a nitrogen source by plants.