Organic nitrogen modulates not only cadmium toxicity but also microbial activity in plants

Highlights: • Impact of additional nitrogen (nitrate, urea or allantoin) on Cd toxicity was studied. • Organic N elevated growth, chlorophylls and photosynthetic activity under Cd excess. • Organic N affected mineral nutrients, amino acids and proteins under Cd excess. • Allantoin and partially urea affected accumulation of citrate and tartrate. • Allantoin was decomposed within 24 h into the urea and elevated microbial activity. It is known that organic nitrogen may modify uptake and toxicity of metals but direct metabolic and microbial comparison of various organic N sources is not available. We therefore studied comparative impact of additional N sources (nitrate, urea or allantoin as 1 mM of N for each compound in addition to 15 mM of inorganic N in the Hoagland solution) on Cd toxicity and microbial activity in common crop cucumber. Organic N significantly elevated the growth, chlorophyll content and photosynthetic activity under Cd excess in comparison with inorganic N though the impact on Cd uptake was negligible. Both organic N compounds also affected accumulation of mineral nutrients, total N, amino acids, and protein content in Cd-stressed plants. Among organic acids, mainly allantoin and partially urea affected accumulation of citrate and tartrate. The most notably, we detected that allantoin was decomposed even within 24 h by microbes into the urea, but it significantly elevated rhizosphere microbial activity. All these data indicate that allantoin is metabolized by plants/microbes into the urea and that it affects microbes mainly in the rhizosphere, which could contribute to amelioration of Cd toxicity.