A novel methodology to track nitrogen transfer in a grass-legume mixture using enriched 15N2

Abstract

Background and Aims – Legumes are a potentially important N source in pasture systems, but quantifying the transfer of biologically fixed N from the legume to the grass component is difficult. A greenhouse H-pot system was developed to directly estimate biological N₂ fixation (BNF) and belowground N transfer using ¹⁵N₂. The system was tested with annual ryegrass (Lolium multiflorum L.) and crimson clover (Trifolium incarnatum L.). Methods – Legume and grass root systems growing in either individual or H pots were exposed to ¹⁵N₂. Control H pots were separated by mesh to prevent contact between roots and mycorrhizae from each side of the pot. To reduce volume demand and avoid cross-contamination in the greenhouse, the gas was supplied through underground tubes in the root zone. Results – Ryegrass and clover were enriched in ¹⁵N when the respective root system was supplied with ¹⁵N₂. Ryegrass was also enriched when clover roots were supplied with the gas and there was free root and mycorrhizal contact between both sides of the H pot, but not when this contact was precluded. Plants grown singly did not enrich when the gas was not supplied to their root systems. Conclusions – The H-pot construction allows the evaluation of belowground transmission, an important mechanism of N transfer. The method of gradually supplying ¹⁵N₂ directly to the root system may be a valuable labeling technique for monitoring the transfer of nitrogen. The lack of enrichment when plants were not directly supplied indicates negligible atmospheric enrichment. Ryegrass enrichment, when supplied with the gas, suggests BNF by other mechanisms.