Biological versus geochemical control and environmental change drivers of the base metal budgets of a tropical montane forest in Ecuador during 15 years

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 year⁻¹ Ca, ND-1770 mg m⁻² year⁻¹ K, 287–597 mg m⁻² year⁻¹ Mg and 403–540 mg m⁻² year⁻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.