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

Abstract

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.

Keywords: dissolved organic nitrogen; endocytosis; mineralization; nitrogen cycle; oligopeptide; phagomixotrophy.

Figure 1

Rate of uptake of ¹⁵N supplied as nitrate, l-alanine, l-tetraalanine or ¹⁵N-labelled microbial culture to sterile wheat (Triticum aestivum) plant roots and roots of plants growing in soil. Values are mean ± SEM; n = 3 for sterile plants and n = 4 for plants grown in soil. Values for both plants in sterile culture and plants grown in soil assume uptake over the entire root system.

Figure 2

Concentrations of ¹⁵N and ¹³C recovered in tissues of wheat (Triticum aestivum) plants grown in sterile culture after supply of ¹⁵N¹³C-dual-labelled l-alanine or l-tetraalanine. Values are data for individual plants. The solid line is the line of best fit using data for plants supplied with both alanine and tetraalanine (r² = 0.994; slope = 1.85). The dashed line represents the relationship between ¹⁵N and ¹³C in the compounds supplied to roots. Closed circles, alanine; open circles, tetraalanine.