Using Stable Isotopes to Quantify Nitrogen Fates in Container Plants

Nitrogen use efficiency (NUE) is increasingly important in the production of container plants as awareness of the environmental consequences of nitrogen (N) leaching increases and as regulations on nitrogen losses from nurseries tighten. Furthermore, N lost to the environment reduces the economic efficiency of container production. While N leaching is relatively easy to measure, losses due to nitrification have been difficult to quantify. We have demonstrated that fertilizer with unique stable isotope signatures of oxygen and N can be used to accurately quantify N fate following application. To quantify N uptake and loss, fertilizer containing ¹⁵N- and ¹⁷O-labeled ammonium nitrate (NH₄NO₃) and potassium nitrate (KNO₃), respectively, was applied to red maple (Acer rubrum) plants growing in 2-L containers. Mass balance equations were used to quantify N conversions and losses in the system. Approximately 18% to 36% of applied N was leached from the container. Within the leachate, 59% of the nitrate came directly from applied fertilizer and 41% from nitrification. Plant nitrogen in new growth was predominately from fertilizer nitrate (30% to 50%), with fertilizer ammonia (20% to 23%) and unknown N (media, fixation, mineralization) accounting for the remainder.  The nitrification rates suggest a large fraction of the assimilated ammonia may have first been nitrified and taken up as nitrate.   A significant fraction of the applied N was unaccounted for, presumably lost via denitrification, immobilization and/or volatilization. Better quantification of the sources of N loss from container plants will allow us to assess various cultural practices for their effects on N loss and in turn provide valuable information to plant producers.