Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Abstract:

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.

Abstract:

To assess the susceptibility of the base metal budget of a remote tropical montane forest in Ecuador to environmental change, we determined the extent of biological control of base metal fluxes and explored the impact of atmospheric inputs and precipitation considered as potential drivers of ecosystem change on the base metal fluxes. We quantified all major base metal fluxes in a ca. 9.1 ha forested catchment from 1998 to 2013. Mean (±s.d.) annual flux to the soil via throughfall+ stemflow+litterfall was 13800±1500 mg m-2 Ca, 19000±1510 mg m-2 K, 4690±619 mg m-2 Mg and 846±592 mg m-2 Na of which 22±6%, 45±16%, 39±10% and 84±33%, respectively, were leached to below the organic layer. The mineral soil retained 79-94% of this Ca, K and Mg, while Na was released. Weathering rates estimated with three different approaches ranged from not detected (ND) to 504 mg m-2 yr-1 Ca, ND-1769 mg m-2 yr-1 K, 287-597 mg m-2 yr-1 Mg and 403-540 mg m-2 yr-1 Na. The size of mainly biologically controlled aboveground fluxes of Ca, K and Mg was 1-2 orders of magnitude larger than that of mainly geochemically controlled fluxes (sorption to soil and weathering). The elemental catchment budgets (total deposition-streamflow) were positive for Ca (574±893 mg m-2) and K (1330±773 mg m-2), negative for Na (-370±1300 mg m-2) and neutral for Mg (1.89±304 mg m-2). Our results demonstrate that biological processes controlled element retention for Ca, K and Mg in the biological part of the ecosystem. This was different for Na, which was mainly released by weathering from the study catchment, while the biological part of the ecosystem was Na-poor. The deposition of base metals was the strongest driver of their budgets suggesting that the base metal cycling of the study ecosystem is susceptible to changing deposition.