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 radiogenic 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 88Sr/86Sr 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 88Sr/86Sr 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‰ relative to the bio-available pool). Despite this strong biological fractionation, 88Sr/86Sr 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.
Highlights
To sustain the continuous supply of mineral nutrients to ecosystems, losses of nutritive elements by erosion or as solutes in flowing water need to be balanced by fresh input
Nutrient supply within ecosystems is sustained through a tightly coupled biogeochemical cycle termed the “organic nutrient cycle”. This cycle comprises a set of strategies for efficient nutrient uptake and re-utili zation through microbial release from plant litter and organic matter (e.g. Aerts and Chapin, 1999; Jobbágy and Jackson, 2004; Turner et al, 2012; Uhlig and von Blanckenburg, 2019; Vitousek et al, 1998)
To test the hypothesis that plant uptake exerts the pre dominant control over isotope differences observed between bedrock Sr and dissolved Sr (e.g. Andrews et al, 2016) we explored Sr isotopes during the interactions of plants with the weathering zone along a spectacular climate gradient in the coastal mountains of Chile that features strong gradients in plant growth: the Critical Zone project “EarthShape: Earth Surface Shaping by Biota”
Summary
To sustain the continuous supply of mineral nutrients to ecosystems, losses of nutritive elements by erosion or as solutes in flowing water need to be balanced by fresh input. This balance is established over some 103 yrs. Nutrient supply within ecosystems is sustained through a tightly coupled biogeochemical cycle termed the “organic nutrient cycle”. This cycle comprises a set of strategies for efficient nutrient uptake and re-utili zation through microbial release from plant litter and organic matter These two pathways describe the fundamental workings of the “Critical Zone” – the zone where rock meets life
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
