DOI: 10.5061/dryad.r2280gbnm
title: Supplementing enhanced weathering with organic amendments accelerates the net climate benefit in rangeland soils

description: Carbon dioxide removal (CDR) technology that combines negative greenhouse
 gas emission pathways alongside substantial emissions reductions is needed
 to limit climate change. Rock dust amendments to soils is an emerging CDR
 technology that has the potential to decrease atmospheric CO2 via enhanced
 rock weathering (ERW), although there is considerable uncertainty
 regarding the efficacy of ERW with few multi-year field studies conducted
 to date. We explored combining finely-ground metabasaltic rock dust with
 other soil CDR technologies (compost, biochar) to stimulate carbon (C)
 sequestration and support plant growth. A combined amendment of
  ground rock, compost and biochar observed the highest increases
 in soil C stocks (15.3 ± 6.1 Mg C ha-1) over three years. All treatments
 with compost applied alone or in combination increased soil C stocks by
 2.0 ± 2.9 to 15.3 ± 6.1 Mg C ha-1.Ground rock only reduced rates of soil C
 decreases relative to the control, although still soil C stocks still
 decreased over time. . Continuous greenhouse gas fluxes showed that ground
 rock amendments lowered nitrous oxide (N2O) emissions up to 11.0 ± 0.6 kg
 CO2e ha-1 yr-1 and increased methane (CH4) consumption by -9.5 ± 3.5 to
 -18.4 ± 4.4 kg CO2e ha-1 yr-1 (N2O and CH4 reduction after three years:
 -0.06 ± 0.01 Mg CO2e ha-1). Combined compost, ground rock, and biochar
 yielded the highest estimated net ecosystem benefit of -86.0 ± 7.2 Mg CO2e
 ha-1 after three years. Net benefits (sum of relative changes in soil C
 stocks, estimated enhanced weathering rates, and annual greenhouse gas
 emissions) across all amendment treatments included slowing or reversing
 background losses in soil C stocks (-16.3 ± 5.0 to -50.5 ± 5.3 Mg CO2e
 ha-1). Benefits were dominated by increases in soil organic C stocks, both
 directly from organic matter amendments and likely increases in
 belowground plant C allocation. To determine an initial theoretical
 potential, we scaled to 8% of California rangelands and found
 supplementing enhanced weathering with compost and biochar could increase
 the net CO2e benefit by 43 Mt CO2e y-1 relative to enhanced weathering
 alone. The use of mixed inorganic and organic amendments significantly
 increased estimated weathering rates and provided additional soil organic
 C sequestration.
description: <strong>Site description</strong>
 The field site was located at the University of California Sierra
 Foothill Research and Extension Center (SFREC) in Browns Valley,
 California. Soils are derived from Mesozoic and Franciscan volcanic rock
 and classified as xeric Inceptisols and Alfisols in the Auburn-Sobrante
 complex (Soil Survey Staff, 2020). Soil depth is approximately 30 cm
 overlaying unconsolidated rock. The area is classified as oak woodland and
 rangeland and has been exclusively grazed by cattle since at least 1960.
 Recent stocking rates were approximately 8 acres per cow. The field sites
 were not otherwise seeded, irrigated, fertilized, or tilled. The study
 region has a Mediterranean climate with average annual precipitation of
 700 mm with high interannual variability and a mean annual temperature of
 16.6 ºC (California Irrigation Management Information System, 2023), but
 experienced an extended drought throughout the experiment (Liu et al.,
 2022). The growing season occurs during the cool, wet weather conditions,
 typically from October to May while summers are hot and dry. The site was
 dominated by naturalized stands of annual grasses (Ryals et al.,
 2016). <strong>Experimental
 design</strong> A control treatment and five
 combinations of ground rock (GR), compost (CP), and biochar (BC) soil
 amendments were applied in a randomized block design to account for
 background spatial variability of the site conditions. This included six
 15 x 60 m plots randomly located in each of three blocks, with each block
 containing a control plot and plots amended with GR or CP only, ground
 rock plus compost (GR+CP); ground rock plus compost plus biochar
 (GR+CP+BC) was added in year 2 (Table 1). To measure continuous soil
 greenhouse gas emissions, a fourth block was established with nine, with
 three 10 x 10 m plot replicates of control, GR only, and CP only
 treatments (Figure S1). Treatments were applied
 individually and were applied to the soil surface with soil incorporation
 or tilling. Ground rock for enhanced silicate weathering was finely ground
 metabasaltic rock sourced as waste material from a regional mining
 operation (Specialty Granules, Ione, CA). A summary of relevant soil
 amendment properties, chemistry, total C and N content, and mineralogy are
 provided in Tables S1-S3. Compost amendments were derived from a mixture
 of organic yard debris (garden trimmings and wood chips) and cow, chicken,
 and horse manures and were commercially produced at the West Marin Compost
 Facility (Nicasio, California) by maturing in watered piles with weekly
 aeration for approximately three months (Vergara & Silver, 2019).
 Biochar amendments were purchased commercially (Rogue Biochar, Oregon
 Biochar Solutions, White City, OR) and made with a feedstock of Douglas
 fir and pine woody biomass sourced from local logging and mill residues.
 Compost and biochar amendments were applied once during the study
 following typical rancher practice in the region, and ground rock was
 applied annually in 2019, 2020, and 2021 (Table 1). We note that there
 were no field-scale data available for ground rock amendments to
 rangelands, so we adopted suggested best practices for cropland
 applications (Andrews & Taylor, 2019; Larkin et al.,
 2022)<sup>,</sup>. All amendments were applied before the
 first rainfall in the fall prior to onset of the growing
 season.  <strong>Soil and aboveground biomass
 analysis</strong> To quantify changes in total
 soil C and N content, samples were collected prior to amendment
 application and annually at the end of each of three growing seasons. Soil
 was collected from five random locations within each plot using hand
 augers (6 cm diameter). Soil was collected from 0-10 cm, 10-20 cm, and
 20-30 cm depths. Samples were air-dried, sieved to &lt; 2 mm, visible
 roots removed, and a subsample ground to a fine powder. Samples were then
 analyzed in duplicate for total C and N on a elemental analyzer (CE
 Elantech, Inc., Lakewood, New Jersey). To quantify changes in soil
 inorganic C, air-dried soil samples were analyzed on an soliTOC cube
 (Elementar, Ronkonkoma, NY) using temperature ramping method DIN 19539
 which can separately calculate total organic C (TOC), residual organic C
 (ROC), and total inorganic C (TIC). Subsamples were
 analyzed for soil pH, gravimetric soil moisture, and extracted with
 potassium chloride (KCl) to quantify inorganic N pools within 24 h of
 sample collection. Soil pH was determined by vortexing a 1:1 soil to water
 solution (5 g soil to 5 ml deionized water) for 1 minute then measuring
 the solution pH after 10 minutes (McLean, 1982). Soil moisture was
 determined gravimetrically by weighing fresh soil, oven drying for 24
 hours at 105 °C, reweighing the dried soil, and calculating the difference
 as percent soil moisture. Nitrate
 (NO<sub>3</sub><sup>-</sup>) plus nitrite
 (NO<sub>2</sub><sup>-</sup>) and ammonium
 (NH<sub>4</sub><sup>+</sup>) were measured after
 extraction of 15 g of field-fresh soil in 75 ml of 2 M KCl
 solution<sup>36</sup>. Soil KCl extracts were stored at -20
 <sup>º</sup>C until colorimetrically analyzed using a discrete
 analyzer (Model AQ300, Seal Instruments, Mequon, WI). Bulk density was
 sampled at 0-10, 10-20, and 20-30 cm in each plot prior to amendment
 application and after three growing seasons using a 6.35 cm diameter bulk
 density corer (Table S9). Subsamples were dried at 105
 <sup>º</sup>C to a constant weight. Bulk density was
 calculated as the rock-free dry volume of the total soil core.
 Aboveground vegetation was sampled at peak standing crop for
 biomass production at the end of each growing season (n = 9 per plot). In
 annual grasslands, peak standing crop is generally considered an index of
 net primary productivity (Chiariello, 1989). Live aboveground plant tissue
 was clipped from randomly located 20 cm diameter rings and collected into
 pre-dried and weighed paper bags. Samples were dried at 65 ºC and
 subsequently weighed for biomass.  Aboveground biomass C content was
 assumed to be 50% of the oven dried mass.
 <em>Soil pore water analyses</em>
 In Fall 2021, soil lysimeters (Soil Water Sampler, Soil Moisture
 Corp., Santa Barbara, CA) were installed at 30 cm depth to sample soil
 pore water following rain events. Lysimeter sampling was attempted
 approximately 24 h after each rain event, but sample collection was not
 always possible due to low solute volumes. Sample collection occurred from
 November 2021 following the first rain event through January 2022 until
 collection was impossible due to decreases in soil moisture content.
 Following sample collection, dissolved organic carbon (DOC) and dissolved
 inorganic carbon (DIC) were analyzed with a combustion oxidation
 nondispersive infrared absorption method on a total dissolved C analyzer
 (varioTOC, Elementar, Hanau, Germany), with DIC calculated as the
 difference between acidified and non-acidified samples. Sample
 acidification to pH &lt; 2 was performed by adding a small amount of
 pure hydrochloric acid (Tisserand et al., 2024). To quantify another
 metric of silicate weathering, soil lysimeter subsamples were also
 analyzed for calcium (Ca), magnesium (Mg), and sodium (Na) in triplicate
 via inductively coupled plasma optical emission spectroscopy (ICP-OES;
 Model Optima 5300 DV, Perkin Elmer, Waltham, MA).
 <em>Automated soil and greenhouse gas flux
 measurements </em> Surface fluxes of
 N<sub>2</sub>O, CH<sub>4</sub>, and CO<sub>2
 </sub>were measured continuously during three complete growing
 seasons (October to June) from 2019 to 2022 using an automated chamber
 system. The system consisted of nine opaque, automated gas flux chambers
 (eosAC, Eosense, Nova Scotia, Canada) plumbed to a multiplexer (eosMX,
 Eosense, Nova Scotia, Canada). Each chamber was randomly deployed in the
 10 x 10 m replicate, and plants were included in the soil collars and only
 trimmed to ensure complete chamber closure. The multiplexer allowed for
 dynamically signaled chamber deployment and routed gases to a cavity
 ring-down spectrometer (Model G2508, Picarro Inc., Santa Clara, CA, USA).
 Chambers were measured sequentially approximately every two hours, with  a
 10 min sampling period with a 1.5 min flushing period before and after
 each measurement. Chamber volumes were used to calculate the minimum
 detectable flux of 0.004 nmol N<sub>2</sub>O
 m<sup>−2</sup> s<sup>−1</sup>, 0.02 nmol
 CO<sub>2</sub> m<sup>−2</sup>
 s<sup>−1</sup>, and 0.004 nmol CH<sub>4</sub>
 m<sup>−2</sup> s<sup>−1 </sup>(Nickerson,
 2016). Flux calculations and analyses were first
 performed using Eosense eosAnalyze-AC v. 3.7.7 software, then data quality
 assessment and control were subsequently performed in R (RStudio,
 v.1.1.4633). Fluxes were removed from the final dataset if they were
 associated with erroneous spectrometer cavity temperature or pressure
 readings or if any gas concentrations were negative, corresponding to
 instrument malfunction. Fluxes were also removed if the chamber deployment
 period was less than 9 min or greater than 11 min, indicative of chamber
 malfunction. Calculated linear and exponential fluxes were compared using
 estimate uncertainty to estimate ratios, and in cases where both the
 linear and exponential models produced high uncertainty, the individual
 flux was eliminated from the dataset. Data filtering removed 8.7% of flux
 measurement periods, generating a final dataset of 47,762 simultaneous
 flux measurements of CO<sub>2</sub>,
 N<sub>2</sub>O, and CH<sub>4</sub>. Daily mean
 flux values across years and treatments are presented in Figure S1. To
 convert flux measurements to CO<sub>2</sub>e, we used the IPCC
 AR5 100-year GWP values of 28 CO<sub>2</sub>e for
 CH<sub>4</sub> and 298 CO<sub>2</sub>e for
 N<sub>2</sub>O<sup> </sup>(Myhre et al.,
 2013). <em>Estimation of weathering rates and net
 ecosystem C benefits </em> Bicarbonate production
 via enhanced weathering of basaltic minerals is largely dependent on
 Ca<sup>2+</sup> and Mg<sup>2+</sup>
 concentrations. As a conservative estimate to account for potential
 CO<sub>2</sub> production via carbonate
 (CaCO<sub>3</sub>, MgCO<sub>3</sub>)
 precipitation, we assume 1 mol of CO<sub>2</sub> is converted
 to bicarbonate (HCO<sub>3</sub><sup>-</sup>) per
 Ca<sup>2+</sup> and Mg<sup>2+ </sup>produced via
 weathering (Beerling et al. 2018). The weathered metabasaltic material in
 this study was applied annually for three years at a rate of 37.8 Mg
 ha<sup>-1</sup> yr<sup>-1 </sup>and contained
 4.25-6.46% Ca and 1.02-4.14% Mg by mass (Table S2). During the study
 period, annual rainfall in this location varied from 266.7 to 548.2 mm
 yr<sup>-1</sup> (1 mm is equivalent to 10000 L
 ha<sup>-1</sup>), with 266.7 mm yr<sup>-1</sup> of
 rainfall during the lysimeter observation period(California Irrigation
 Management Information System, 2023). To calculate net rates of enhanced
 weathering, all rainfall was assumed to become soil water with similar DIC
 concentrations to our lysimeter observations. As this approach likely
 overestimates solute movement through the soil values should be treated
 with appropriate caution. We used the 2021-2022 value as an estimate for
 weathering rates during the other two growing seasons; since 2021-2022 was
 the lowest rainfall year of the study these should be considered
 conservative estimates. We estimated the net ecosystem
 C benefit (reduction of CO<sub>2</sub> or
 CO<sub>2</sub>e added or increased removal of
 CO<sub>2</sub> or CO<sub>2</sub>e from the
 atmosphere) of each treatment after three growing seasons. This was done
 by combining annual changes in aboveground biomass, net changes in soil C
 stocks from the initial baseline, estimates of annual enhanced weathering
 rates, and measured or estimated changes in annual
 N<sub>2</sub>O and CH<sub>4</sub> fluxes in
 CO<sub>2</sub>e from the individual compost and ground rock
 treatment flux measurements. Changes in soil C stocks within blocks were
 calculated as the difference in soil C stocks at each depth (0-10 cm,
 10-20 cm, and 20-30 cm) from prior to treatment application in Fall 2019
 and the end of third growing season in Spring 2022 and converted to
 CO<sub>2</sub>e values. As a first approximation of the
 changes in N<sub>2</sub>O and CH<sub>4</sub>
 fluxes for combined treatments, we calculated the summed differences from
 both compost and/or ground rock only treatments relative to the control.
 It is important to recognize that these values may not capture the
 synergistic effects of combined amendments and are used here to capture
 likely order-of-magnitude scale impacts. Emissions associated with
 amendment production, transport, and application were not included, thus
 reported values should be considered net C benefit estimates from within
 ecosystem boundaries. <em>Calculations and
 statistical analyses</em> All statistical
 analyses were performed using JMP Pro 15 (SAS Institute Inc., Cary, NC).
 Differences in treatment year total C and inorganic soil C stocks,
 aboveground biomass, soil characteristics (soil moisture, soil pH, mineral
 N), lysimeter cation concentrations, and annual
 CO<sub>2</sub>, CH<sub>4</sub>, and
 N<sub>2</sub>O fluxes were analyzed with one-way ANOVAs
 followed by post-hoc Tukey tests. Figures were created using ggplot2 in R
 (Wickham, 2009). Values reported in the text are means ± standard errors
 unless otherwise noted.
description: # Supplementing enhanced weathering with organic amendments accelerates
 the net climate benefit in rangeland soils ## Description of the data and
 file structure Treatments:  Treatments included control (CT), ground rock
 (GR), compost (CP), ground rock plus compost (GR + CP), and ground rock
 plus compost plus biochar (GR + CP + BC) Sampling periods Greenhouse gas
 sampling years: Year 1 = 2019-2020, Year 2 = 2020-2021, Year 3 =
 2021-2022, Soil carbon (organic TC and inorganic TIC), years: 2019, 2020,
 2021, or 2022 Dissolved inorganic carbon (DIC), Dissolved organic carbon
 (DOC) and dissolved cations (calcium = Ca, Magnesium = Mg, and sodium
 (Na)), years: 2021-2022 Blocks Block replicate of each treatment (B1, B2,
 or B3) Depths Soil sampled depth in cm (0-10, 10-20, 20-30, 30-40, or
 40-50) **Statistical values in all datasets:** Mean = Mean, sd = standard
 deviation, se = standard error, ci = confidence interval **The following
 subsets used to create figures are included in this dataset:**  * Annual
 mean and daily mean Greenhouse gas data:                      SFREC_Annual
 Mean_CH4_fluxes.csv                      SFREC_Annual Mean_CO2_fluxes.csv
                      SFREC_Annual Mean_N2O_fluxes.csv                    
  SFREC_Daily Mean_CH4_fluxes.csv                      SFREC_Daily
 Mean_CO2_fluxes.csv                      SFREC_Daily Mean_N2O_fluxes.csv -
 Greenhouse gas data is presented in meters squared per second for carbon
 dioxide: umol CO2 m^-2 s^-1, methane: nmol CH4 m^-2 s^-1m, and nitrous
 oxide: nmol N2O m^-2 s^-1 * Date = sampling date in Month.day.year format
 - - SFREC_summaryTC.csv: Mean Soil C % and C stocks :Mean Soil C in %
 carbon (TC) and by soil mass in kg per hectare (TC.kg.ha), respective
 statistical values, and mean change from beginning of experiment by
 treatment in kg per hectare (Change_treatment_kgTC_ha) -
 SFREC_summaryTIC.csv: Mean Inorganic Soil C % and inorganic C stocks :
 Mean Inorganic Soil C in in kg per hectare (TIC.kg.ha), respective
 statistical values, and mean change from beginning of experiment by
 treatment in kg per hectare (Change_treatment_kgTIC_ha) -
 SFREC_DOC_DIC.csv: Lysimeter dissolved organic (DOC) and inorganic carbon
 (DIC): Mean dissolved Inorganic Soil C in in kg per hectare (TIC.kg.ha)
 for all individual sample dates and treatments - Soil pH, nitrate,
 ammonium, dissolved cations, and biomass data is included in the
 supplemental information - Missing data: NA ## Code/Software All
 statistical analyses were performed in JMP Pro 16. Figures were processed
 in R Studio (version: 2022.07.1 Build 554; R version 4.0.5 (2021-03-31))
 with the attached R Script: FINAL_SFREC_Dataprocessing_plots.R