Strontium isotopes trace biological activity in the Critical Zone along a climate and vegetation gradient

Abstract

Weathering and ecosystem nutrition are intimately linked through the supply of fresh mineral nutrients from regolith and bedrock (the "geogenic nutrient pathway"). However, the prominence of this link is dependent on the efficiency of nutrient recycling from plant litter (the "organic nutrient cycle"). Isotope ratios of strontium (Sr), an element that behaves similarly to Ca in ecosystems, confer two types of information: radiogenic Sr isotopes inform as to the sources of Sr and the degree of weathering, while stable Sr isotopes constrain partitioning between compartments of the Critical Zone (bedrock, water, secondary solids, and plants). To date, however, neither the reactions nor the mass balance between compartments that fractionate Sr isotopes, nor the fractionation factors involved, are well understood. Here, we present geochemical budgets of Sr (using radio genic and stable Sr isotopes, and Ca/Sr ratios) at four sites along a substantial climate and primary production gradient in the coastal mountains of Chile. We found that Sr release through weathering is isotopically congruent, and released Sr is not strongly isotopically fractionated either during secondary mineral formation or transfer into the exchangeable pool. Despite this, the Sr-88/Sr-86 ratio of bio-available Sr, which should reflect the ratio of dissolved Sr, is higher than that of rock and regolith. We propose that this offset is caused by plants: while Sr-88/Sr-86 in plant organs at the four study sites systematically increased from roots towards their leaves, whole-plant Sr isotope compositions indicate preferential uptake of light Sr into plants (with a fractionation of up to -0.3 parts per thousand relative to the bio-available pool). Despite this strong biological fractionation, Sr-88/Sr-86 ratios in bio-available Sr do not covary with biomass production across our study sites, because with greater plant growth Sr is recycled more times after release by weathering - an isotope-neutral process. Rather, the loss of Sr from the ecosystem in solid organic material sets the isotope ratio of dissolved or bio-available Sr. Organic solids thus appear to constitute a significant export path of elements released during weathering, with the removal of solid plant debris reducing the recycling factor of Sr, and possibly that of other mineral nutrients too.