Seminar: Radiocarbon and mineral insights on topsoil C stabilization

The Crop Production Ecology Seminar series, the "VPE seminars", is a free and online platform for scientific debate about agricultural production and sustainability between academics, stakeholders, and the general public.

Shane Stoner

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Shane Stoner's abstract:

Increased carbon inputs from management intensification are rapidly respired: Radiocarbon insights into C sequestration in a long-term pasture trial

Managed grasslands are widely distributed and have shown potential to store carbon (C) as a method for partially mitigating anthropogenic climate change. However, it remains difficult to predict potential C storage under a given soil or management practice, or the amount of time for which C will remain sequestered. To study C storage dynamics due to long-term (1952-2009) phosphorus (P) fertilizer and irrigation treatments in New Zealand grasslands, we measured radiocarbon (14C) in archived soil along with observed changes in C stocks to constrain a compartmental soil model and estimate transit time distributions.

Productivity increases due to P application and irrigation in these trials resulted in very similar C accumulation rates between 1959 and 2009. Model results indicated that decomposition rates of fast cycling C (0.1 to 0.2 yr-1) increased to nearly offset increases in inputs. Thus, transit times decreased with increased inputs, but there was no significant difference in the amount of C stored annually. With increasing P fertilization, decomposition rates of slow pool C also increased (0.005 to 0.008 yr-1).

Long-term data show sustained, significant (i.e. greater than 4 per mille) increases in C stocks regardless of treatment or inputs. Increased inputs associated with fertilizer and irrigation thus did not result in increased C storage, as transit times increased and greater inputs were more rapidly decomposed. As the majority of fresh inputs remain in the soil for less than 10 years, these long-term increases reflect the dynamics of the slow pool. Overall lower C stocks were found in plots subjected to frequent irrigation, though this was attributed to rapid losses associated with the onset of water addition. Rates of C gain and decay highlight trade-offs between productivity, nutrient availability, and soil C sequestration as a climate change mitigation strategy.

Sophie von Fromm

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Sophie von Fromm's abstract:

Mineral and climate controls on timescales of soil carbon stabilization in sub-Saharan Africa

Given the importance of soils for the global carbon cycle, it is essential to understand not only how much carbon soils can store, but also how long this carbon remains belowground.

Previous studies have shown that the interaction of climate and soil mineralogy has a strong influence on carbon storage and age; i.e. carbon stabilization. However, these findings are primarily based on studies conducted on local gradients from temperate regions, leaving large knowledge gaps for tropical and subtropical soils.

Here, we present a systematic continental-scale soil radiocarbon and mineral dataset showing that processes controlling carbon stabilization vary strongly across and within climate zones, following patterns of soil development. We find that intermediately weathered soils under seasonal climates, where poorly-crystalline and reactive clay minerals dominate, stabilize more carbon over centuries to millennia than highly weathered soils from humid regions where less reactive minerals dominate.

Soils under arid climate store carbon for periods similar to those under seasonal climate, but reflect climatic constraints that limit microbial decomposition rather than efficient mineral carbon stabilization. Based on climate projections for sub-Saharan Africa, we argue that responses of the soil carbon cycle will vary strongly with pedo-climatic conditions.