Nutrient cycling in a tropical montane rainforest under a supply-limited weathering regime traced by elemental mass balances and Mg stable isotopes

Forest ecosystem nutrition and rock weathering are intimately linked through the supply of mineral nutrients and recycling (re-utilisation of nutrients released from plant litter during mineralisation). Here, we report on novel applications of Mg stable isotopes in combination with mass balance calculations to quantify nutrient supply (from bedrock, atmospheric wet and dry deposition) and recycling in a tropical ecosystem, characterised by a supply-limited weathering regime in the Central Highlands of Sri Lanka. We measured Mg stable isotopes, radiogenic Sr isotope ratios, and element concentrations in the different compartments of the Earth's Critical Zone (bedrock, saprolite, soil, soil pore water, stream water and vegetation) as tracers for element mass fluxes and pathways. We find that in the clay-rich saprolite Mg isotopes are fractionated towards isotopically heavy Mg during secondary mineral formation. Soil pore water and stream water are the complementary isotopically light reservoirs. The Mg isotope composition of above-ground vegetation is on average identical to soil pore water. Based on these observations we suggest that a tight near-surface mineral nutrient loop between plants and soil sustains the tropical forest ecosystem. This loop is disconnected from the rock weathering nutrient source in the saprolite beneath soil. External atmospheric wet and dry inputs (mainly Mg and Ca) strongly contribute to the plant-available nutrient budget that the nutrient depleted regolith does not supply. These findings, based on Mg stable isotopes, agree with independently-obtained information on bio-element (P, K, Ca, Mg, Si) recycling. This nutrient supply setting contrasts with that reported for the Southern Sierra Nevada (USA), where a kinetically limited weathering regime prevails. Our study illustrates the application of new flux estimators based on elemental and metal stable isotope measurements across a range of climate and weathering regimes.