Rhizodeposition of N by pea and barley and its effect on soil N dynamics

Rhizodeposition of N during plant growth influences the microbial activity in the rhizosphere and constitutes a source of labile organic N, but has not been quantified to the same degree as the rhizodeposition of C. The rhizodeposition of N, defined as root-derived N present in the soil after removal of visible roots and root fragments, was determined during field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) growth in a sandy soil at a low concentration of mineral N using a continuous split-root N-15-labelling technique. The N rhizodeposition constituted 15 and 48% of the below-ground N in pea when determined 7 and 14 (maturity) wk after planting (WAP), respectively. In barley 32 and 71% of the below-ground N were present in rhizodeposits at the two samplings. At maturity the rhizodeposition of N amounted to 19 mg N plant(-1) (7% of total plant N) for pea and 17 mg N plant(-1) (20% of total plant N) for barley. Incubation of soils, after removal of roots, showed that the N rhizodeposits were labile; 79% of the pea and 48% of the barley root-derived N present in soil at 7 WAP were mineralizable. The root-derived N present in soil at maturity was less labile, since only 30 and 23% of the N rhizodeposition from pea and barley, respectively, were mineralized upon incubation. The mineralization of N in soils was studied during 3 months after harvest of plants at maturity with roots in situ and was found to be greater after pea than after barley. This was due to differences in the net mineralization of N from roots and rhizodeposits, which was greater after pea than after barley. Rhizodeposits and roots contributed 35% of the N mineralized after pea and 12% of the N mineralized after barley. After removing roots and root fragments from the soil the total net mineralization of N was similar in soil previously grown to pea and barley.