Conference Talk: EW with ultramafic rock: Field experiments with alfalfa in Greece

Here is the conference presentation by our head of research, Dr. Ingrid Smet, at the 3rd International Conference on Negative CO2 Emissions, 2024 in Oxford.

Introduction

Rock weathering involves the dissolution of minerals, a chemical process that creates cations and captures atmospheric CO2 by converting it into bicarbonate (HCO3-). Enhanced weathering (EW) aims to accelerate this by finely crushing appropriate rocks and putting them in optimal weathering conditions. Soils contain up to 1000 times more CO2 than air, making agricultural areas in warm climates with plenty of water a primary focus for EW. Greece’s geology boasts several rock types suitable for EW, including at least two active quarries in olivine-rich ultramafic rocks. The plain of Thessaly is one of three large agricultural regions in Greece with arable land where mainly annual crops are cultivated and which is heavily irrigated. Here we present initial results of an EW field experiment with alfalfa cultivation in central Greece. 

Methods

In June 2022 a plot (2mx2m) experiment with irrigated alfalfa was set up in a soil representative for the agricultural area of Thessaly (clay loam; pH 7.7; 2.7% CaCO3). An ultramafic rock dust (<125 µm; 72% olivine; 44 wt% MgO) was added at high application rates to maximize chances of observing an EW signal. The field was divided into 5 blocks and 3 treatments were randomly assigned within each block. The 3 treatments consist of the control without any rock dust, and applications of 50 ton/ha and 100 ton/ha. Upon spreading, rock dust was incorporated into the soil to a depth of about 20 cm and 5 macrorhizons were installed in each plot for soil pore water sampling by applying a vacuum. A yield estimation is performed of the central 1mx1m of each plot 2-4 times/year. Plants and soils are collected each spring and autumn and soil water is sampled every 2-6 weeks depending on the season. Plants, soil and water samples are analyzed at the Institute2 accredited laboratory for a set of parameters representing those relevant to plant nutrition and soil quality and those expected to be influenced by dissolution of the rock dust (cations, HCO3-, pH, Ni, Cr, …). 

Results 

   The alfalfa yield is not negatively affected by the rock dust application. No statistically significant difference was observed between the control and the two treatments except for one harvest where the yield of the 100 ton/ha treatment was significantly higher than the other 2 treatments.
  Alfalfa plant nutrition shows a similar trend, its basic nutrients (N, P, K, Ca, Mg, etc.) do not differ significantly between the 3 treatments. The exception is Ni which consistently shows increased contents the more rock dust was applied (Figure 1). And whereas Cr in plants is first significantly increased at higher applications, in the latest plant analysis it shows a distinct negative correlation with rock dust (Figure 1). 

   Soil samples were analyzed for 19 different parameters, but only after 1 year some of them showed statistically significant differences between the treatments. Four of these are linked to EW (Figure 2): soil pH, CaCO3 and Ni are elevated in the rock dust treatments compared to the control, but Cr shows a decrease with added rock dust as also observed in the plants. 

   Soil water parameters connected to the dissolution of the rock dust occasionally show statistically significant differences between the treatments. Throughout the first year, the control tends to have the lowest pH with increasingly higher values as more rock dust is added. At the same time, the HCO3- content was also generally lower in the control than in the treatments (Figure 3). Ni and Cr only start to show significant differences between treatments after the first year. In late 2023, a tendency of increased heavy metal contents with more rock dust is especially clear for Cr (Figure 3). 

Conclusions

   In this experiment with a clay-rich alkaline soil, EW signals are observed in plants, soil and soil water up to 2 years after application. Changes to the inorganic carbon system are reflected in increased pH and CaCO3 in the soil and elevated pH and HCO3- of the soil pore water. The high amounts of ultramafic rock dust however also led to an increase of Ni, mainly in the plants and soil, and a delayed increase of Cr in the soil water more than one year after application. Although EW usually involves lower amounts of rock dust, this underscores the need for an appropriate monitoring period for Ni and Cr to exclude accidental heavy metal contamination of soils and soil water. The quantitative and qualitative yield characteristics of alfalfa were not negatively affected by the high doses of rock dust, highlighting alfalfa as a potential crop for CDR through EW.
  Further research is needed to identify more sensitive parameters to detect EW at reduced application rates of rock dust and to quantify with sufficient precision the amount of CO2 that is removed in each EW project.

Thanks to the conference organizers CO2RE co2re.org for sharing the video with us and giving us the permission to share it.