The weathering of silicates is a major control on
atmospheric CO2 at geologic timescales. It was proposed to enhance
this process to actively remove CO2 from the atmosphere. While there
are some studies that propose and theoretically analyze the application of
rock powder to agricultural land, results from field experiments are still
scarce.
In order to evaluate the efficiency and side effects of Enhanced Weathering
(EW), a mesocosm experiment was set up and agricultural soil from Belgium
was amended with olivine-bearing dunite ground to two different grain sizes,
while distinguishing setups with and without crops.
Based on measurements of Mg, Si, pH, and DIC, the additional weathering
effect of olivine could be confirmed. Calculated weathering rates are up to
3 orders of magnitude lower than found in other studies. The calculated
CO2 consumption by weathering based on the outlet water of the mesocosm
systems was low with 2.3–4.9 tCO2km-2a-1 if compared
with previous theoretical estimates. Suspected causes were the removal or
dilution of Mg as a weathering product by processes like adsorption,
mineralization, plant uptake, evapotranspiration, and/or preferential flow,
not specifically addressed in previous EW experiments for CO2
consumption. The observation that Mg concentrations in the upper soil layers
were about 1 order of magnitude higher than in the outlet water indicates
that a careful tracking of weathering indicators like Mg in the field is
essential for a precise estimate of the CO2 consumption potential of
EW, specifically under global deployment scenarios with a high diversity of
ecosystem settings. Porewater Mg/Si molar ratios suggest that dissolved Si
is reprecipitating, forming a cation-depleted Si layer on the reactive
mineral surface of freshly ground rocks.
The release of potentially harmful trace elements is an acknowledged side
effect of EW. Primarily Ni and Cr are elevated in the soil solution, while
Ni concentrations exceed the limits of drinking water quality. The use of
olivine, rich in Ni and Cr, is not recommended, and alternative rock sources
are suggested for the application.
Introduction
The application of rock powder on agricultural soils has long been used to
improve soil properties to achieve a productivity increase (De Villiers,
1961; Kronberg, 1977; Leonardos et al., 1987; Anda et al., 2015a, b, 2013; Shamshuddin and Anda, 2012), predominantly in the form of
liming. The application of carbonate rock powder to agricultural soils is a
process to adjust soil pH (Cregan et al., 1989) in order to increase crop
production (Haynes and Naidu, 1998). In addition to pH adjustment, the additional
release of cations and anions into the soil–rock system alters the chemical
composition of the soil solution. Alternative amendment materials are
gaining increased attention, one of which is silicate rock powder. Silicate
rocks can provide geogenic nutrients via the chemical weathering of the
additional minerals (Hartmann et al., 2013; van Straaten, 2006). Additionally, it has the potential additional advantage of enhancing atmospheric
CO2 sequestration: on geological timescales, natural silicate
weathering is one of the most important controls on atmospheric C
concentrations (Berner, 2003). The silicate weathering process releases
cations like Mg2+, Ca2+, and others, and CO2 is stored as
alkalinity in the ocean, whereas carbonate weathering yields no net CO2
uptake on longer timescales (Hartmann et al., 2013). Enhanced Weathering
(EW) has therefore been put forward as a method/technique with strong
potential to contribute to climate mitigation. In order to achieve COP21
atmospheric CO2 concentration targets, it becomes more likely that not
only emission reduction is required (Fuss et al., 2014; Rogelj et al., 2018;
Peters, 2016). Focus should also be put on applying effective CO2
sequestration techniques (Sanderson et al., 2016; Beerling et al., 2018;
Minx et al., 2018).
With dwindling resources of rocks with concentrated content of widely
applied macronutrients, which might lead to a shortage of traditional
fertilizers (Cordell et al., 2009; Manning, 2015), geogenic nutrient
replacement by EW will become a valid alternative to supply not only
phosphorus or potassium but also further geogenic nutrients, with
potentially important local impact on food security (van Straaten, 2002;
Cordell et al., 2009). In addition, alternative regional fertilizer concepts
for certain regions need to be developed to enhance productivity, as for
example in Africa (Ciceri and Allanore, 2019).
However, the application of silicate rock products requires knowledge of
soil mineral properties, hydrology, soil solution composition, and element
uptake by plants to enable predictions on its consequences. Specifically
this knowledge is lacking at the broader scale (Beerling et al., 2018;
Beerling, 2017; Kantola et al., 2017; Edwards et al., 2017; Taylor et al.,
2017), despite several experiments in the past (Anda et al., 2015a, 2013;
Shamshuddin and Anda, 2012; Shamshuddin et al., 2011). One of the main gaps
is the evolution of soil solution composition and its migration in the
treated soil, considering a broad variety of possible combinations of soil
type, rock product, and plant species (Hartmann et al., 2013). The timescale at which changes in weathering fluxes can be expected at the scale of
large catchments was shown for the Mississippi River basin, where alkalinity fluxes
increased by more than 50 % over less than a century, which was partly
attributed to liming and land management processes (Raymond and Cole, 2003;
Raymond and Hamilton, 2018). In general, past land use change and management
of catchments can affect the chemical baselines of rivers draining to the
ocean over decades (Hartmann et al., 2011, 2007; Meybeck,
2003; Radach and Pätsch, 2007). The large-scale application of rock
products will likely lead to an alteration of river chemistry, and
consequences for adjacent coastal zones remain to be assessed.
In the future, increasing food and bioenergy demand will probably lead to
more efforts to improve soil conditions for optimized biomass production
(Fuss et al., 2018; IPCC, 2019). The future application of customized rock
products to provide slow-release geogenic nutrient fertilizers, adjust pH,
increase cation exchange capacities (CECs), or adjust soil hydrology is
therefore likely. The replenishment of geogenic macro- and micronutrients is
needed because the natural supply cannot keep up with the permanent removal
from the soil–rock system under intensive harvest scenarios for crops or
timber (de Oliveira Garcia et al., 2018; van Straaten, 2006). The
application of rock products will therefore change the fluxes of elements
within and from the soils, while being mediated by the biological pump.
One of the key issues is the dissolution rate of applied rock material.
While the kinetics are relatively well understood at the laboratory scale
for singular minerals (Rosso and Rimstidt, 2000; Wogelius and Walther,
1992), the dissolution rate of a rock powder mixture as soil amendment, with
fresh surfaces, which have not been in contact with an aquatic phase before,
is nearly unknown. Several points of the rock powder application on soils
have to be considered. First, the upper parts of soils are not permanently
saturated with water, which may lead to mineral dissolution–precipitation
reactions. Second, it can be expected that mineral surfaces initially need
to equilibrate with the new system and varying water content and that
dissolution rates of minerals will be different from those in
long-term equilibrium within the natural soil system. Third, trace elements
from the applied rock material will eventually be released and migrate
downwards, reprecipitated if oversaturation with a specific mineral phase
occurs, or adsorbed to soil minerals or organic matter.
To understand these processes in an agricultural setting with typical crops,
dunite could serve as model rock material, often containing more than 90 %
of olivine, a mineral often used as a model mineral to theoretically study
effects of EW (Schuiling and Krijgsman, 2006; Hartmann et al., 2013;
Köhler et al., 2010; Taylor et al., 2015; Renforth et al., 2015;
Montserrat et al., 2017). Using near-monomineralic rocks decreases the
complexity of observable effects. Discussed alternatives like basalt have a
much greater complexity (considerable quantities of plagioclase and
pyroxene, and to a lesser extent olivine and other trace minerals). In
addition, basalt has the potential to provide the nutrient phosphorus, which
is typically low in dunite. The release of phosphorus could potentially
influence plant–weathering interactions in the soil, complicating the
analysis of the weathering process. In our present study, we applied
dunite to agricultural soils to quantify the impact on inorganic carbon and
dissolved silica fluxes in the presence and absence of crop plants.
We studied the release of the major elements Mg and Si predominantly derived
from Mg olivine, as indicators for the inorganic CO2 sequestration
potential, and we assessed whether the release of elements into the soil
solution occurs stoichiometrically, or whether a secondary layer covering
the fresh surfaces of minerals will develop, potentially enriched in Si and
depleted in Mg (Daval et al., 2013a; Hellmann et al., 2012; Pokrovsky and
Schott, 2000), which could influence weathering and subsequently
sequestration rates. In addition, the release of trace metals was used to
understand how these behave in a near-natural environment to evaluate the
impacts on the environment.
MethodsMesocosm setup
In October 2013, a fully replicated setup (five replicates per treatment
combination) of mesocosms was built up and left running for 730 d. The
experimental setup was not specifically tailored to this study of weathering
fluxes as we piggybacked on an experiment to evaluate elemental cycling into
plants. Here we report on data of the first year. Rain barrel mesocosms
with a diameter of 46 cm were filled with a natural loamy sandy soil from
Belgium (detailed characterization including grain size distribution in
Supplement S1). Controlled factors were the application of olivine-rich
dunite (henceforth referred to as olivine amendment) in the top layer of the
soils (22 kgm-2, a high mass to induce observable effects, and a
similar value as the maximum mass applied in an experiment by ten Berge et
al., 2012) using two different olivine grain size fractions (roughly coarse
sand and fine sand to silt), two crop plants (wheat and barley), and two
irrigation regimes (daily and weekly precipitation), while the total amount
of rain was equal (about 800 mma-1). Controls were established by
using the same setup without olivine application (blanks) and without
plants. Waters were sampled at 1.5, 12.5, and 24.5 cm depth and at the
bottom of the mesocosms (Fig. 1).
(a) Schematic mesocosm configuration; (b) status of the experiment
in April 2014 (6 months in).
Material
The experiment material was produced from dunite rock, containing
approximately 90 % olivine, of which 92 % was forsterite (Mg end-member
olivine). The rest is comprised of lizardite (Mg-rich serpentine), Cr-bearing chlorite (including chromite or chrome-spinel inclusions), and
traces of chabazite (zeolite group) and Mg hornblende (amphibole),
determined by energy-dispersive X-ray spectrometry (Zeiss LEO 1455 VP
coupled with an EDX detector by Oxford Instruments). It originates from the
Almklovdalen peridotite complex (Åheim mineral deposit mined by the North
Cape Minerals Company, Norway). More insights into the geochemistry of the
material can be found in Hövelmann et al. (2012), and Beyer (2006)
describes the geological setting. The bulk chemical composition (Panalytical
Magix Pro wavelength dispersive X-ray fluorescence (XRF) analysis) of the
sample is given in Table 1. The particle size distribution of the two
grain size classes used was analyzed by Sympatec Helos KFMagic laser
granulometry (Table 2). The sample was analyzed for specific surface area,
measured using N2 and Kr adsorption during BET analyses (Brunauer et
al., 1938) with a Quantachrome autosorb iQ (Table 2). Only the Kr-based
measurements were used in calculations since the use of Kr ensures more
precise results, especially at lower surface areas (Naderi, 2015).
Geochemical composition of source dunite, derived from XRF runs
(n=3).
a Sieve
mesh at which 20 % is retained; thus 80 % is smaller than the given
diameter. b Class with the largest class weight. c This class is
divided into five smaller classes but was summed to show the share below
1 µm.
Analysis
The sampled pore and outlet waters were filtered through 0.45 µm
nitrocellulose Chromafil syringe filters (A-45/25) into sample bottles and
stored cool (4 ∘C) until analysis. Soil material was obtained by
extracting sediment cores (20 cm long and 28 mm in diameter) using a hammer
auger with a removable plastic lining (Eijkelkamp 04.15.SA foil sampler,
Giesbeek, the Netherlands). In each container, one core was taken at the
center. Immediately after sampling, each core was sub-sectioned into 10
slices of 2 cm, packed in vacuum plastic bags and stored cool (4 ∘C) on return to the laboratory. Sediment samples were dried for 72 h at
40 ∘C and homogenized by manual grinding. Three slices were
analyzed: slice 2 (-2 to -4cm; in dunite-amended layer), slice 6 (-10 to
-12cm; just below dunite-amended layer), and slice 10 (-18 to -20cm; in
untreated soil below dunite-amended layer).
Silica and magnesium
Dissolved silica (Si) and magnesium (Mg) were measured with an inductively
coupled plasma atomic emission spectrophotometer (ICP-AES, Thermo
Scientific, ICAP 6000 series).
pH
Water from the bottom outlet was drained into a bucket and pH was measured
using a WTW pH meter, calibrated with three NIST buffer standards (pH 4, 7,
and 10).
Trace metals
In the soil solution as well as in the soil material, concentrations of
aluminum (Al), barium (Ba) chromium (Cr), cobalt (Co), iron (Fe), manganese
(Mn), nickel (Ni), strontium (Sr), and zinc (Zn) were analyzed using
inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima
2100, PerkinElmer).
For analysis of the total content of substances within the soil material, a
digestive procedure was followed according to Heinrichs and Herrmann (2013). In
brief, soil was dried at 40 ∘C and milled with a planetary ball
mill. A total of 150 mg of soil was weighed into PTFE crucibles and a mixture of 4 mL
nitric acid (65 % Suprapur grade), 2 mL hydrofluoric acid (40 % Suprapur grade), and 2 mL
perchloric acid (70 % Suprapur grade) was added. The crucibles were sealed and
placed for 10 h in a closed digestion aperture (PicoTrace GmbH) at 170 ∘C to
ensure complete dissolution. Subsequently, the acids were vaporized in a
closed system and the residues were dissolved with 2 mL nitric acid (65 %
Suprapur grade), 0.6 mL hydrochloric acid (37 % Suprapur grade), and 20 mL high-purity water
at 90 ∘C for 1 h. The solutions were standardized to 50 mL with
high-purity water and underwent atomic emission spectroscopy (ICP-AES) as
described above.
Dissolved inorganic carbon (DIC)
DIC was measured with a Picarro G2131-i cavity ring-down spectrometer
coupled to a preparation device (AutoMate FX, Inc.) for discrete sample
measurement. Enough sample volume was not available
for DIC analyses at all sampling times, as priority was given to other major compounds based on
the original purpose of the experiment. Samples were preserved with
HgCl2 and stored dark and cool until analysis.
Calculation of weathering and CO2 sequestration rate
The average flux of Mg from dunite-amended soils at the outlet can be
calculated by
fluxMg2+=[Mg2+]treated-[Mg2+]untreated×q,
with q as water volume discharged at the outlet per sampled time interval.
The sequestration rate can subsequently be calculated by
CO2 sequestration=fluxMg2+×molweightMgfraction of Mg in olivine×RCO2×ω,
with a fraction of Mg in olivine of about 1.8 (inferred from XRF analysis,
Table 1). RCO2 is the theoretical maximum uptake of CO2 in metric tons
per metric ton of rock (1.25), which is corrected by ω (=0.85), to
account for carbonate system equilibration in the ocean (after Renforth,
2012; Renforth et al., 2013, and references therein). The global CO2
sequestration potential was then calculated by multiplying with the
available arable land in an optimistic and a pessimistic scenario (Moosdorf
et al., 2014).
The weathering rate can be estimated by
weathering rate Rmol olivinem2s=fluxMg2+1.8applied massolivine×specific surface area×t.
The numerator converts the molar flux of Mg to molar flux of olivine
(1.8 molMg per 1 mol olivine). Time factor t is used to convert the flux
measured in 340 d to seconds.
Calculation of the amorphous Si layer
The Mg-depleted and Si-enriched layer that forms during the dissolution
process (Daval et al., 2011) was roughly estimated using the release of Mg
in conjunction with the Mg/Si ratio and the available surface area of the
forsterite.
The mass of SiO2 that precipitated per square meter and year as
amorphous Si can be estimated by
mSiO2amorph.gSiO2m2a=RMgMg/Sitheoretical-RSi×MSiO2×t,
with the dissolution (weathering) rates RMg and RSi calculated from
experimental data, the theoretical Mg/Si ratio (1.8), MSiO2 as the
molar mass of SiO2, and time factor t to convert seconds to years.
The depletion layer thickness can then be calculated as
growth rate of SiO2 layer nma=mSiO2amorph.ρSiO2amorph.×1-φSiO2amorph.×109,
with the density ρSiO2amorph as 2.23×106gm-3 (Iler,
1979) and with the porosity ϕSiO2amorph as 0.3 (20 %–40 %; Maher et
al., 2016).
ResultsHydrology
Two rain regimes, with daily and with weekly rainfall, delivering the same
total annual precipitation volume, were used. Since there were no
significant and/or systematic differences between results of both rain
treatments (Fig. S2-1 in the Supplement), all discussed data integrate values from both
rain treatments. After the experiment start, it took between 7 and 23 d
until water reached the bottom of the mesocosms. The amount of irrigation
water and the water collected at the outlet of each barrel were used to
roughly estimate the loss of water through evaporation and transpiration,
not accounting for water storage in biomass and changes in soil water
storage capacity. Sample volume which could be extracted varied. Between
days 200 and 300, growth of plants and elevated ambient temperatures caused
strong evapotranspiration, which reduced the outflowing water volume to a
minimum. At these times, no or only a little sample volume could be
obtained. Data clearly show elevated evapotranspiration in the mesocosm
seeded with crops (Fig. 2).
(a) Water flow from outlet; values refer to daily fluxes from the
preceding interval. (b) Average daily temperature in the greenhouse and
evapotranspiration, calculated from the difference of precipitation input
and barrel outflow, relative to precipitation.
Release patterns of weathering tracers
The fine-olivine fraction shows about a 9-fold higher specific surface
area in both krypton- and nitrogen-based measurements than the coarse
fraction (Table 2). Observed results are therefore differentiated by fine- and
coarse-rock treatment. Elevated concentrations of the major studied
parameters DIC, Mg, and Si are only observed at 1.5 and 12.5 cm depth, with
the largest increase in the top sampling point if compared to the base level
setup without olivine. Changes in DIC and Mg concentrations are most
pronounced in the mesocosms supplied with fine-olivine amendment, with
values markedly above base concentrations in the setup with coarser olivine
(Figs. 3, 5). A pronounced increase in pH at the beginning of the
experiment (Fig. 4), with values near 9 for the fine-grain-size setup, can
be observed. If values are compared against untreated mesocosms, the pH of
soil solutions increases by up to 1.0 and 0.3 pH units in mesocosms treated
with fine and coarse olivine, respectively (Fig. S14-1 in the Supplement). Over the
course of the experiment, the observed pH approaches values around 8.
Depending on the setup, the pH in the fine setup is about 0.5 pH units
higher than in the others after 1 year. Si concentrations develop
dissimilarly, with most pronounced increases in the coarse setup, whereas the
fine setup releases less than half of the Si into the soil solution in the
surface level, i.e., at 1.5 cm depth (Fig. 6). The effect is less obvious in
the second sampled depth at 12.5 cm, and no changes are visible below.
Interestingly the Si concentrations in the top sampling for treatments with
the fine material are also lower than if no olivine was supplied. With the
exception of visible differences in Si concentrations, with lower values in
the setups with plants, no clear difference pattern can be identified if
crop plants are present (Figs. 3–6 and S3-1 to S6-1).
The general pattern is a large variation in concentrations, suggesting that
the variability between mesocosms is high and that five replicas per setup
are probably not enough to derive a differentiated signal as presented for
the major element concentrations. Despite the large variability, it is clear
that the weathering signal from the amended olivine travels slowly downwards
in the soil pore space. Within the first year, it was not moving much beyond
the 12.5 cm level, as elevated Mg concentrations, which provide the clearest
signal for olivine dissolution, were not clearly detectable at the third
level (24.5 cm), with two exceptions in the case of the fine-grain setup.
Development of average DIC concentrations over 1 year at 1.5 cm
depth, differentiated by olivine and crop treatment. For more information on
the subsequent layers and error bars, please refer to Fig. S3-1 in the Supplement.
Development of pH values (averaged proton concentrations, converted
to pH) over the experiment period at 1.5 cm depth, differentiated by olivine
and crop treatment. For more data and error bars, please refer to Fig. S4-1.
Development of Mg concentrations over the experiment period,
differentiated by olivine and crop treatment. Data points are averages but
error indicators were omitted to provide a better overview. For a more
differentiated view and standard deviations, please refer to Fig. S5-1.
Development of Si concentrations over the experiment period in the
surface layer, differentiated by olivine and crop treatment. Data points are
averages, but error indicators were omitted to provide a better overview. For
a more differentiated view and standard deviations, please refer to Fig. S6-1.
Generally, Mg/Si is clearly above 2 at the 1.5 and 12.5 cm levels below
surface in mesocosms amended with fine olivine. The ratio is roughly in the
range of 1–10 in the lower sampled depths of fine treatments and in all
depths of the setups without coarse and no olivine (Fig. 7). Shortly after
the start of the experiment Mg/Si ratios (Fig. 7) are high (Mg/Si>50) in the soil water at the surface of the fine-grain
treatment, due to a strong increase in Mg and the comparably low increase in
Si. The effect is weaker for the coarse-grain treatment (Mg/Si<30, but still above 2). There is no distinct difference in Mg/Si in the
three setups (fine, coarse, no olivine) in the deepest soil sampling point
and the outlet (with the exception of two outlier points in the deepest sampled
layer for the fine olivine with crop setup).
Development of Mg/Si ratios over the experiment period in the
surface layer, differentiated by olivine and crop treatment. The dashed grey
line indicates the stoichiometric Mg/Si ratio of 1.8 based on the rock
chemistry. Data points are averages but error indicators were omitted to
provide a better overview. For a more differentiated view and standard
deviations, please refer to Fig. S7-1.
CO2 sequestration rates
Ideally, the CO2 consumption by weathering can be calculated based on
DIC or alkalinity. As too few samples were available for DIC analysis, the
additional CO2 consumption by olivine amendment was calculated based on
the release of Mg2+, considering the average geochemical composition of
the material and the background values from applied soils and irrigation
water. Based on the stoichiometric composition, the ideal dissolution of 1 molMg olivine yields 2 molMg and consumes 4 molCO2:
Mg2SiO4+4H2O+4CO2→2Mg2++4HCO3-+H4SiO4.
The ability to sequester atmospheric CO2 is material specific and
depends here on the Mg2+ that can be released during hydrolysis from
the Mg-rich olivine. It is defined as the carbon dioxide removal (RCO2) in metric tons of CO2 per metric ton of Mg olivine (estimated to be 1.1 for
ultramafic (i.e., Mg rich) rocks; Moosdorf et al., 2014). This assumption considers that impurities (like Fe abundance), in contrast to the ideal
Mg olivine and equilibration effects, reduce the theoretical maximum
RCO2 of 1.25 for forsterite. Based on the average of Mg concentrations in
the outlet water over the first year (340 d), the experiment leads to a
total annual CO2 sequestration of
2.3–4.9 tCO2km-2a-1, depending on the applied grain size
(Table 3).
To evaluate the potential order of magnitude of Mg uptake or dilution,
possibly introduced by the experimental setup, Mg concentrations in the
outlet water might be compared to those in the surface layer pore water. The
ratio of surface layer Mg concentration to outlet Mg concentration is 12.2
in the coarse setup and 13.8 in the fine setup (Table 3).
Mg and water flux averages (±SD) throughout the period of
the experiment, excluding background contributions from soil and irrigation
water, for crop and no crop treatment. The potential of Mg removal and its
effect on CO2 consumption is provided, assuming that all water would
percolate the pore space of the upper soil and equilibrate towards the
measured Mg concentrations. Details on the calculation of CO2
consumption are found in Sect. 2.4.
a Calculated directly from original Mg concentrations per sample time step (not averages, hence the slight difference to the ratio of average Mg concentrations from the first two columns). Data were taken only from day 79 onwards because fluctuations were too inconsistent in the first weeks of the experiment. b Calculated net flux from mesocosm based on observed Mg concentrations in the outflow. cCO2 consumption “observed” at outlet multiplied with reduction ratio to account for Mg removal or dilution.
Trace metalsSoil
Analyses of the soil elemental composition show that some trace element
concentrations are elevated, where olivine was applied (Fig. 8). Markedly,
this is the case for Co, Cr, Ni, Mn, Al, and Fe. There are no statistically
significant differences between the applied grain sizes and crop types.
Averaged trace element concentrations in the solid soil material,
differentiated by olivine treatment (grouping) and depth (blue: 2–4 cm; red:
10–12 cm; black: 18–20 cm below surface). Error bars indicate 1 standard
deviation. Data are separated by plant type in Supplement S8 (topsoil
only).
Soil solution
Ni and Cr concentrations in the soil solution are elevated in the surface
layer where fine olivine grain sizes were applied (Fig. 9) during the first
100 d. The coarse-grain setup does not show any visible Cr concentration
difference compared to the control; Ni concentrations in barrels amended
with olivine are higher than in the mesocosms without olivine on average
(Fig. 9). The existence of plants does not cause a distinct pattern change
in any setup compared to no-plant treatments. Base values of Cr without
olivine treatment are already above 50 nmolL-1. The dissolution of
dunite predominantly leads to elevated levels of Ni and Cr concentrations in
the soil solution over the control (Fig. S11-1.).
Development of Cr (a) and Ni (b) concentrations in soil pore water
over the first 5 months in the layer 1.5 cm below the surface, differentiated by
olivine and crop treatment. For more data and error bars, please refer to
Figs. S9-1 and S10-1.
For the other trace elements, no distinct pattern between treatments with
and without olivine was identified, apart from a general variability found
in the solutions.
DiscussionTracing the weathering effect
Based on the released Mg and BET surface area measurements of the ground
rock material, weathering rates can be estimated (see Sect. 2.4). Derived
rates of 10-13.12 and 10-13.75molOlm-2s-1 for
coarse and fine material, respectively, based on the outlet water, are about
1 order of magnitude lower than values published for an olivine-amended
soil column experiment (Renforth et al., 2015) and about 3 orders
of magnitude lower than theoretical optimum dissolution rates given in
Strefler et al. (2018). These differences are not unexpected, considering
that Mg is not acting conservatively, and our experimental setup simulates
natural processes like extended periods of drying out, with potential
secondary mineral formation, and subsequent slowed-down or ceased chemical
weathering processes, as well as cation exchange, and dilution by
preferential flow.
The large difference in available surface area for reaction between fine and
coarse material (Table 2) has implications for the release rate of elements
as it is proportional to the available surface area. Averages of Mg, with
the clearest dissolution signal, show a strong difference between Mg/Si
release ratios of the fine- and coarse-material setups, suggesting that the
dissolution of the dunite and predominantly olivine is clearly not
stoichiometric. This was also observed in results from laboratory
experiments (Pokrovsky and Schott, 2000). We assume that the potential
formation of cation-depleted silica layers around minerals might affect the
dissolution rates. Further effects are related to the distribution of water
in the pore space, which is steered by grain size distribution effects, via
differences of the water contact time with grain surfaces. Also, climatic
conditions lead to drying-up during the warm period in the greenhouse caused
by evapotranspiration under presence of plants, resulting in low soil water
content and varying elemental concentrations in the remaining water. If the
Mg/Si ratio is 2, the ideal stoichiometric molar ratio of element release
from forsteritic olivine is reached. In the case of the applied material,
the Mg/Si ratio is about 1.8 (inferring from XRF results). The experimental
data show that Si and Mg concentrations in the upper layer are often
decreased if plants were present (Figs. 5, 6). But even with this
effect, Mg/Si ratios are still far from the equilibrium release ratio of
1.8. This effect is widely recognized as incongruent dissolution (Casey et
al., 1993; Ruiz-Agudo et al., 2012). Considering that there are large
amounts of Mg released, Si determines the ratio assuming that removal by
plants is minor. This suggests that there is an active retention of Si,
potentially leading to a cation-depleted amorphous silica layer growing
around olivine minerals (Daval et al., 2013b, 2011; Hellmann
et al., 2012). This effect has been described in detail for forsterite by
Maher et al. (2016). High Mg/Si ratios indicate that in the beginning of the
experiment the dissolution rate is controlled by the exchange of protons for
Mg, while declining ratios over the course of the experiment indicate an
approach to steady-state conditions (Maher et al., 2016). Since this effect
may eventually determine the CO2 sequestration rate, an estimation of
the thickness of those layers would lead to a better understanding of
weathering kinetics for silicate application schemes in general, as this
process may affect other silicate minerals too. From the mesocosm
experiments, it is only feasible to calculate a rough 1st-order estimate
given the rather basic setup of the mesocosms. Calculated amorphous layer
growth rates (details Sect. 2.5) for the mesocosms without plants range
from 0.02 nma-1 (fine setup) to 0.08 nma-1 (coarse setup). These
values are above or near observations of surface layers around “aged”
minerals, e.g., from Hellmann et al. (2012): 14.7 kyr old feldspar with a
surface layer of 50 nm (≈0.0034nma-1) and a layer of
150–200 nm on a younger (assuming 10 kyr) serpentine (≈0.02nma-1). However, if assuming that the fresh surfaces of the forsterite
are weathering faster in the beginning and a decrease in reaction rate can
be caused by the formation of the amorphous layer due to processes related
to the diffusion of released elements through the layer (Nugent et al.,
1998; Daval et al., 2013b), this might explain why the calculated growth
rates are comparable to aged material, as with time weathering rates
decrease.
As the formation of a secondary layer through Si reprecipitation should
preferentially include Si previously released by the amended material, this
process alone cannot explain why Si concentrations in the fine setup are
about 2-fold below values of the control setup without olivine and without
plants (Fig. 6). We speculate that the increased release of Si from the
finest grains leads to short-term oversaturation of Si (about 1.9 mmolL-1 at 25 ∘C; Stumm and Morgan, 1996), extending beyond the
typical area of the secondary silica layer formation around the grains,
which may facilitate the formation of secondary minerals such as smectites
(Prudêncio et al., 2002) or a mixture of different hydrous silicates of
iron and magnesium, known as iddingsite (Smith, 1987).
CO2 sequestration by olivine amendment
The pH increase, as an indicator of rock dissolution of silicates, is most
pronounced in the upper layer and during the first 6 months of the
experiment. This indicates an enhanced reaction with the added rock powder,
which contains a large fraction of very fine material, providing an
increased reactive surface area. The effect indicates the generation of
alkalinity by chemical weathering consuming CO2 and can partly be seen
in the DIC concentrations for the fine-grain experiments (Fig. 3). As DIC
was handled with the lowest priority during the sampling campaign (regarding
low sampling volumes), only a few measurements are available, which makes it
hard to truly differentiate the treatments based on DIC.
Due to limits in the acquired data, it is only possible to give a rough
estimate of the CO2 drawdown effect by weathering. The elevated
elemental concentrations from the surface sample point do not progress
evenly downwards, which means that several processes influence Mg
concentrations during percolation. A Mg concentration difference between
surface soil pore water and outflow of about an order of magnitude could
primarily stem from element removal by plants, mineral precipitation,
de-mobilization by cation absorption through clay minerals, or dilution
through preferential flow along soil macropores and the rims of the mesocosm
barrel. Significant and systematic Mg removal by plants might be ruled out,
as differences between Mg concentrations of the surface pore waters in
planted and unplanted mesocosms are mostly not significant (Fig. S5). As
Mg in the surface layer and the outflow increases in nearly all treatment
types when evapotranspiration values rise due to increasing temperatures
from around day 100 on, a process also observed in another mesocosm
experiments (Thaysen et al., 2014), the loss of Mg by secondary
mineralization can be assumed but not quantified. However, the precipitation
of Mg carbonates at ambient conditions is kinetically hindered (Case et al.,
2011; Giammar et al., 2005), and therefore the Mg removal effect is
potentially small for carbonates in the mesocosm environment, but further
investigation for amorphous phases or precursors for secondary mineral
formation would be necessary.
The soil is characterized by an average CEC of 8.6 meq (100 g dry
soil)-1. While the value is comparably low (Batjes, 1997), it can be
expected that Mg is adsorbed onto surfaces of clay minerals and organic
matter at the given elevated pH values.
It is furthermore possible that some parts of the irrigation water
bypass the bulk material as preferential flow, possibly along the
barrel's rims or potentially through the soil, facilitated by plant roots or
evoked by macropores, which lead to the preferential transportation of water
downwards, effectively decreasing rock–water interaction times (Nielsen et
al., 1986) and therefore dissolution rates in the affected material (Maher,
2011). The process of preferential flow is well established for natural
soils (Beven and Germann, 2013), and we assume it asserts some influence on
the outflow elemental concentrations in this mesocosm experiment. Future EW
experiments using mesocosms or field experiments may therefore include
tracking of the effect of preferential flow paths, as it seems from this
experiment that hydrology maybe a factor introducing large uncertainty, also
considering the seasonality and periods of drying affecting the
concentrations and the flux from the system (cf. Fig. 2 with Fig. S12-1).
The amount of Mg released can theoretically be estimated by multiplying its
concentrations with the calculated average outflow water volume at the
mesocosm bottom (Fig. S12-1). Yet, due to the observed
difference between surface layer and outflow Mg concentrations, it must be
assumed that only a small proportion of the initial reaction with the
applied grains eventually leads to CO2 sequestration. While this Mg
removal may be specific to the experimental setup, there will be comparable
effects in natural environments. To determine the potential order of
magnitude of this effect, Mg concentrations in surface layer pore water and
outlet water were compared, and the ratio is near 13 in both setups (Table 3),
indicating that the CO2 uptake potential, measured in dissolved Mg, is
decreased by more than 90 % compared to the surface layer. The estimated
total annual CO2 sequestration at a
maximum of 4.9 tCO2km-2a-1 is 2 orders of magnitude lower
than what was observed in a soil column of 10 cm diameter and without plants
(Renforth et al., 2015). Applied at the global scale, i.e., on all
potentially available arable land (minimum and maximum taken from Moosdorf et al.,
2014), this yields a comparably low CO2 sequestration potential of
maximum 0.07 GtCO2a-1, or less than 0.2 % of the global fossil
CO2 emissions of 2017 (based on Le Quéré et al., 2018). When
the calculations are based on a Mg flux neglecting the obvious removal or
up-concentration of Mg due to evapotranspiration, values are about one order
of magnitude higher (Table 3), which makes them comparable to lower-end
sequestration rates reported from a smaller pot experiment in ten Berge et
al. (2012), which was mainly watered at the bottom to avoid percolation,
thus leading to a different environment for elemental cycling.
Also, the globally upscaled estimates do not take geographic variability
into account. Since weathering rates are elevated in humid (sub)tropical
regions, the global potential based on temperate conditions, and given a
comparably low amount of rainfall, is clearly underestimated. Data on
pCO2 in the mesocosm soils (Fig. S13-1) corroborate that
weathering effects must be less pronounced compared to humid, tropical areas
as values in the experiment were 850–1300 µatm in the surface layer
(depth=5cm), whereas they are up to 30 times higher in areas with
high evapotranspiration (Brook et al., 1983). Soil water content also seems
to be an important control on the soil–rock pCO2 (Romero-Mujalli et
al., 2018), and therefore seasonality controlling soil water content is
likely to be a relevant factor influencing the dissolution kinetics, via the
control on soil pCO2, being an important agent in the dissolution
process of minerals. Overall, the assessment of global potentials by
mesocosm experiments requires setups simulating humid (sub)tropical
conditions, which would promote larger fluxes through the soil column.
The large stretch of results shows (a) that cation removal by different
chemical and physical processes in soils is an important parameter to
include in flux estimates at larger scales, which has not been considered in
detail so far, (b) that it is fundamental for such an experiment setup to
monitor the water and elemental fluxes through the entire mesocosm, and (c) that the role of seasonal dynamics and amount of the weathering agents like
CO2 in the soil should be considered.
Trace elements and processes
The release of trace metals which can be potentially harmful to the
environment was mentioned as one of the side effects of terrestrial EW
(Hartmann et al., 2013). The effect is especially pronounced when rocks like
the dunite in this experiment are applied, since they contain larger amounts
of Cr and Ni (>2000 ppm each, Table 1). Nickel (Ni) in olivine is
expected to be released as it substitutes for Mg. Chromium (Cr), on the other
hand, here present in chromite and chromochlorite from the dunite source
rock, is not expected to be released strongly at the observed pH levels in
the system. Soil analysis confirms an increase in these trace metals, due to
the added material. The data also show that the lower untreated layers are
little affected by the dunite treatment (Fig. 8). Focusing on Ni and Cr,
which are the predominant trace metals in the applied material, soil
solution concentrations fluctuate strongly (Fig. 9) due to warmer periods,
and subsequent drying-out, causing enrichment of dissolved elements in the
solution.
Ni is partly mobile at the given pH values. Observed Ni concentrations
exceed drinking water quality thresholds in the surface layer, e.g., formulated by the WHO (2017), with 0.02 mgL-1 (=∧339.2nmolL-1), yet are within the recommended limits for
agricultural irrigation water (0.2 mgL-1; Ayers and Westcot, 1985).
This demonstrates that a close water monitoring is necessary to understand
implications of a widespread deployment of EW with source materials
containing such elevated concentrations of mobile trace metals. Avoiding
these particular rock type materials might therefore be the best
alternative.
When comparing the theoretical Mg/Ni ratio in the olivine with measured data
in the soil solution of the surface layer (Table 4), it can be seen that less
Ni is in the solution than theoretically possible (a factor of 10–20
difference, depending on the grain size). Under the given physicochemical
conditions it is possible that Fe and Al hydroxides lead to the partial
sorption of Ni (Young, 2013; Rieuwerts, 2007).
Preferential release of Mg over Ni into pore water solution during
the dissolution of dunite. Molar ratios of Mg/Ni in the dunite are based on
XRF analysis. Surface layer solution values for fine and coarse setups are
averaged over the experimental period.
RockSurface layer solution Fine setupCoarse setup(molkg-1)(µmolkg-1)(µmolkg-1)Mg11.24175638Ni0.051.030.26Mg/Ni molar ratio22440532454
At the same time, Cr is apparently less mobile (Fig. S11-1) in
the mesocosm experiment, considering the sample approach, which matches
the general behavior of Cr mobility at pH values measured in the soils (at
pH values of 7–9; Kabata-Pendias, 1993). However, elevated Cr values have
been shown for a column experiment, which is to some extent comparable to
the mesocosm experiment here. In contrast to our experiment, Cr values
increased more strongly by up to 9 ngg-1 (≈173nmolkg-1),
including background Cr (Renforth et al., 2015). Interestingly, Cr seems to
be actively removed from the solution at a later stage of the experiment,
shown by Cr concentrations in the untreated mesocosms being higher than in
the treated mesocosms (Fig. 9a), probably due to the higher pH compared to
the control, supporting the idea of pH management to immobilize Cr.
If trace elements within the applied dunite remain immobile, they accumulate
in the soils and can potentially be released when the pH drops or
redox conditions change (McClain and Maher, 2016, and references therein).
Grain size effects are visible, shown by elevated concentrations of Ni in
the mesocosms amended with fine olivine. Other trace elements are
represented only in smaller amounts in the composition of the source rock,
thus not releasing relevant amounts into the pore water.
Overall these findings from the mesocosm experiment underline the
proposition to focus on alternative sources like basalt (Hartmann et al.,
2013; Taylor et al., 2015; Strefler et al., 2018; Amann and Hartmann, 2019)
to avoid strong environmental impacts from trace element release.
Conclusion
Given the scarcity of data considering the field application of rock
material for EW, or of compilations of research with other purposes, there
are some lessons to be learned from this experiment. The elevated Mg
concentrations indicate the potential of an inorganic CO2 sequestration
effect, and the order of magnitude is possibly large enough for the method
to be considered to be one piece in the puzzle of negative emission
technology (NET) portfolios. However, the calculations are bound to high
uncertainties mainly from water flow and elemental concentration estimates
throughout the mesocosm. It is crucial to understand processes that can
affect the CO2 sequestration potential or impact its assessment, i.e., evapotranspiration, sorption of cations, secondary mineralization, or low
overall residence time due to preferential flow of irrigation or rainwater.
The concentration difference between the upper soil and the outlet water of
about 1 order of magnitude indicates the relevance to include those
processes in future experiments to parameterize standardized flux
calculations, specifically for environmental conditions with promising
CO2 sequestration potential by EW in areas more humid than represented
by the setup of this experiment. This is especially relevant if the EW
CO2 sequestration was to be coupled to a carbon price to regain
application costs (Hartmann and Kempe, 2008). Further beneficial services
by EW like the potential increase in plant biomass via uptake of
growth-limiting elements provided by rocks, or the increase in secondary
minerals positively affecting nutrient retention, were not investigated here
but are an essential part of future studies to assess the full potential of
EW as a method for carbon dioxide removal (Amann and Hartmann, 2019), and
potentially for pH management, avoiding the release of further greenhouse
gases like N2O (Kantola et al., 2017).
One of the main concerns of the rock powder application is the release of
potentially harmful trace elements. It could be shown for the first time in
a dedicated EW experiment in an open system with plants that levels of Ni
in solution are significantly elevated, whereas it was possible to confirm
that Cr levels in solution are low under the given soil conditions.
The experiment also showed that the behavior of silica in the soil is not
well understood if silicate powder of different grain sizes is applied. This
is evidenced by the high Mg/Si ratios and the potential sink of silica in
comparison to the non-silicate-treated mesocosms. Results appear to be
consistent with published observations that the formation of a cation-depleted and Si-enriched grain surface layer is responsible for the missing
silica. The available data do not allow further conclusions here.
Nonetheless, the effect of a growing depleted silica layer on the
dissolution kinetics and of further secondary mineral phases should be
investigated, specifically if a long-term application of EW is envisaged.
Using more complex rock products, like basalt with higher aluminum content,
may produce larger amounts of new phyllosilicates and other products around
the added fresh mineral grains, changing their kinetic behavior in the
long term.
Overall, this shows that mesoscale and field experiments are of the utmost
importance to identify the essential processes, to decrease uncertainties in
process understanding and element releases, and to address the effects of
elevated element fluxes. Only if budgets of EW can be estimated reliably, could
the resulting CO2 consumption be bound to a carbon prize within a
NET deployment strategy.
Data availability
The data used in this experiment are provided as comma-separated files in the
Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/bg-17-103-2020-supplement.
Author contributions
This article was conceived through the joint work of ES, JS, JH, and TA,
who all participated in discussions, planning, and writing, with the lead
of TA. The mesocosm study was conceived and designed by ES, JS, PM,
and IJ. Sampling was primarily conducted by ES and JS. Mg and Si
analyses were performed by ES and JS, trace elements were analyzed by EKF, and
DIC was measured by TA. WdOG contributed to the discussion of trace
elements. JH contributed to the discussion of weathering effects.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This research was executed with the financial support of the Research
Foundation Flanders (FWO), project no. G043313N “Silicate fertilization,
crop production and carbon storage: a new and integrated concept for
sustainable management of agricultural ecosystems”. Jonas Schoelynck is a postdoctoral
fellow of FWO (project no. 12H8616N). Additional support was provided by the
German Research Foundation's priority program DFG SPP 1689 on “Climate
Engineering – Risks, Challenges and Opportunities?” and specifically the
CEMICS2 project to Thorben Amann, Jens Hartmann, and Wagner de Oliveira Garcia. Further support of Thorben Amann and Jens Hartmann
came from the Deutsche Forschungsgemeinschaft (DFG, German Research
Foundation) under Germany's Excellence Strategy – EXC 2037
“Climate, Climatic Change, and Society” – project number 390683824,
contribution to the Center for Earth System Research and Sustainability
(CEN) of Universität Hamburg, and through the previous EXC177 “CLISAP2”,
Universität Hamburg.
We acknowledge Peggy Bartsch, Tom Jäppinen, Marvin Keitzel, and Andreas Weiss for valuable contributions from the wet lab and Sebastian Lindhorst
for providing granulometric analyses (all from the Institute for Geology,
Universität Hamburg). We thank Stephan Jung and Joachim Ludwig (from
the Institute for Mineralogy and Petrography, Universität Hamburg) for
contributing the XRF and XRD analyses. All employees of the Antwerp city
greenhouse are thanked for their practical support.
We thank the editor and reviewers for their insightful comments on our
paper. We would specifically like to acknowledge Søren Jessen for his
thorough reviews.
Financial support
This research has been supported by the Research Foundation Flanders (grant nos. G043313N and 12H8616N) and the German Research Foundation (grant nos. SPP1689, EXC177, and EXC 2037).
Review statement
This paper was edited by Andreas Ibrom and reviewed by Søren Jessen and David Manning.
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