Sources and Sinks of N in Ecosystem Solutions Along the Water Path Through a Tropical Montane Forest in Ecuador Assessed With δ15N Values of Total Dissolved Nitrogen

The globally increasing reactive N richness affects even remote ecosystems such as the tropical montane forests in Ecuador. We tested whether the δ¹⁵N values of total dissolved N (TDN), measured directly in solution with a TOC-IRMS, can be used to help elucidate N sources and sinks along the water path and thus might be suitable for ecosystem monitoring. From 2013 to 2016, the δ¹⁵N values of TDN in bulk deposition showed the most pronounced temporal variation of all ecosystem solutions (δ¹⁵N values: 1.9–5.9‰). In throughfall (TF), TDN was on average ¹⁵N-depleted (−1.8 ± s.d. 0.4‰) relative to rainfall (3.4 ± 0.9‰), resulting from net retention of isotopically heavy N, mainly as NH₄⁺. Simultaneously, N-isotopically light NO₃⁻-N and dissolved organic nitrogen (DON) with a δ¹⁵N value between NO₃⁻-N and NH₄⁺-N were leached from the canopy (leaves: −3.5 ± 0.5‰). The increasing δ¹⁵N values in the order, TF < stemflow (SF, 0.1 ± 0.6‰)< litter leachate (LL, 1.3 ± 0.7‰) concurred with an increasing DON contribution to TDN reflecting the δ¹⁵N value of the organic layer (1.9 ± 0.9‰). The lower δ¹⁵N value of the mineral soil solution at the 0.15 m soil depth (SS15, −1.5 ± 0.3‰) than in LL can be explained by the retention of DON and NH₄⁺ and the addition of NO₃⁻ from mineralization and nitrification. The increasing δ¹⁵N values in the order, SS15 < SS30 (−0.6 ± 0.2‰) < streamflow (ST, 0.5 ± 0.6‰) suggested gaseous N losses because of increasing denitrification. There was no seasonality of the δ¹⁵N values. Our results demonstrate that the δ¹⁵N values of TDN in ecosystem solutions help identify N sources and sinks in forest ecosystems.