The Five-Year Fate of a 15N Tracer in a Mixed Deciduous Forest: Retention, Redistribution, and Differences by Mycorrhizal Association

Abstract:

The impact of nitrogen deposition on forest ecosystems depends in large part on its fate: uptake by trees can stimulate growth, while gaseous or leaching losses contribute to air and water pollution and represent the loss of a limiting nutrient. Past tracer studies have shown that soils dominate the short-term fate of added ¹⁵N, but its longer-term term fate remains uncertain. This study examined how much ¹⁵N tracer moved plant or soil pools or was lost over 5-6 years. In 2007, a ¹⁵N tracer (0.21 kg/ha as 99% enriched ¹⁵N-KNO₃) was added to 0.25 ha mixed hardwood forest in central NY. All of the tracer was recovered in the days after its addition, but recovery fell to 78% by the end of this year (25% surface litter, 48% 0-10 cm soil, 5% roots). One year later, recovery in these pools fell (to 51%), with losses from surface litter (-11%) and the 0-10 cm soils (-15%), including losses from the “heavy” soil fraction. Additional tracer moved to other plant pools (+5%) and to deeper soil (+13%; up to 30 cm), for a total recovery of 69% of the added tracer. Between years 1 and 5-6, only total tracer recovery decreased by only 1.4%. Recovery decreased in foliage (-0.2%), all roots (-3.5%), and surface litter (-9.8%), while increasing in woody biomass (+0.9%), 0-10 cm soil (+8.9%), and deep soil (+2.3%; up to 50 cm). Tracer recovery in live and dead plant N pools (11%) did not change, as 3% moved from roots into aboveground plant tissues and 3% moved from live plant pools into leaf litter; these results imply no net transfer of ¹⁵N from soil to plants during this period. Over all 5-6 years, only 1.6% of the tracer moved into bark or wood, a small but important sink because of its high C:N ratio; however, roughly one-third of this total was in wood formed prior to the start of the tracer addition. Tree species differed in their recovery of ¹⁵N: the six species with ectomycorrhizal associations showed more enrichment than the four species with arbuscular mycorrhizae. It is not yet apparent whether ectomycorrhizal species were more effective at scavenging ¹⁵N at the time of tracer addition, or if they were better at acquiring ¹⁵N from its initial fate in litter and soil. Only continued long-term resampling can distinguish between these alternatives, and to what extent ¹⁵N initially retained in soil eventually moves into plants over decadal timescales.