How significant to plant N nutrition is the direct consumption of soil microbes by roots?

Summary The high degree to which plant roots compete with soil microbes for organic forms of nitrogen ( N ) is becoming increasingly apparent. This has culminated in the finding that plants may consume soil microbes as a source of N , but the functional significance of this process remains unknown. We used 15 N ‐ and 14 C ‐labelled cultures of soil bacteria to measure rates of acquisition of microbes by sterile wheat roots and plants growing in soil. We compared these rates with acquisition of 15 N delivered as nitrate, amino acid monomer ( l ‐alanine) and short peptide ( l ‐tetraalanine), and the rate of decomposition of [ 14 C ] microbes by indigenous soil microbiota. Acquisition of microbe 15 N by both sterile roots and roots growing in soil was one to two orders of magnitude slower than acquisition of all other forms of 15 N . Decomposition of microbes was fast enough to account for all 15 N recovered, but approximately equal recovery of microbe 14 C suggests that microbes entered roots intact. Uptake of soil microbes by wheat ( Triticum aestivum ) roots appears to take place in soil. If wheat is typical, the importance of this process to terrestrial N cycling is probably minor in comparison with fluxes of other forms of soil inorganic and organic N .