Rapid carbon accumulation in soil rapidly forming in the Southern Alps of New Zealand

Mr Eron Raines1

1Wollongong Isotope Geochronology Laboratory, School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, 2Australian Nuclear Science and Technology Organisation, Lucas Heights, Australia, 3School of Natural Sciences, University of California , Merced, United States of America, 4School of Biological Sciences, Victoria University of Wellington, Wellington, New Zealand, 5School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand

Biota contribute 3-7 orders of magnitude more potential energy to landscapes than climate or tectonics alone. This potential energy is quantified as the system’s net primary productivity (NPP), i.e., the net gain of photosynthetically sourced carbon. The effects of biological energy on landscape evolution is likely highly non-negligible, yet, has proven difficult to properly quantify in the past. Current methods for quantifying NPP vary in accuracy and can involve careful and costly study over the course of many years. The associated costs are often prohibitive for geomorphic studies. Therefore, NPP is not a commonly included measurement made in such studies. While relating biological to geomorphic processes in rapidly forming soils could help increase the predictive ability of current geomorphic models, a more suitable method for quantifying NPP is required to make this possible.
Here, we present a novel method combining uranium and carbon isotopes that can be used for quantifying soil NPP. The study was carried out on a rapidly forming, New Zealand soil. The uranium isotope composition of the soil was used to derive a soil age of 178 years. Given the soil’s age, the soil production rate is 1.7 mm yr-1 which is one of the most rapid every quantified. Geomorphic models fail to predict such rapid soil production by a factor of ~2. Carbon-14 (14C) was also isolated from the same soil and quantified by AMS. The 14C measurements allow for the soil organic carbon (SOC) mean residence time (MRT) to be calculated. Utilizing a commonly employed biogeochemical model, the MRT allows for the calculation of the concentration of SOC as a function of time. In the rapidly forming soil, we measured a SOC content of 536 g-C m-1. Employing MRT and SOC to calculate the expected age of soil yielded a predicted soil age of 408 years. The discrepancy in MRT predicted age and the observed soil age indicates that the biogeochemical model fails to predict the rate of carbon accretion in the rapidly forming soil by a factor of ~2.
The work presented here is the first biogeochemical characterization of a soil forming more rapidly than current geomorphic models can accurately determine. Both the observed soil NPP and the soil formation rate exceed current model predictions. It is possible that a causal relationship exists, however, further co-characterization of biological energy input rates and soil formation rates is needed to test this hypothesis


Eron Raines is a PhD candidate in geology at Victoria University of Wellington and in isotope geochemistry at the University of Wollongong.

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Nov 08 - 19 2021