Effects of biochar soil amendments on soil properties and plant recruitment in coastal climate change adaptation projects

Biochar is a carbon-rich material produced through the pyrolysis of organic waste from the agricultural and forestry industries. Because black carbon is chemically stable, its application to agricultural fields is being touted as a method to mitigate greenhouse gas emissions as it has been shown to improve the fertility of the soil and crop yields as well as increase carbon sequestration. This data publication archives data from a set of paired restoration projects that incorporated biochar soil amendments to test the ability of biochar to provide positive ecosystem or carbon sequestration benefits.

Methods
We used two field experiments to help determine whether the incorporation of biochar in coastal wetland restoration projects utilizing sediment addition promoted plant recruitment, sequestered carbon, or altered soil conditions. In the first experiment, replicated field plots (13 m2) were constructed within a large-scale (50 ha) sediment addition project, where the effect of 10% (v/v) Eucalyptus biochar soil amendments on plant growth and soil conditions were examined for three different sediment types. In the second experiment, we conducted a replicated field experiment at three tidal marshes where we simulated thin layer sediment placement at the scale of smaller plots (0.7 m x 0.7 m), examining the effect of two different biochar types (softwood biochar and biochar mixed with compost). These field experiments allow us to examine any commonalities in the effects of biochar incorporation into projects aimed to enhance salt marshes' condition and function through sediment additions.

Landscape-scale Restoration Experiment

To study the effect of biochar on enhancing restoration success in coastal marshes, we sampled several large-scale plots established in August 2018 as part of a 50-ha coastal restoration project in Central Monterey Bay, CA. The three areas of interest each had three large plots (11m x 3.5m) that were designed with three nested plots (3.5m x 3.5m) that aimed to compare two different sediment types and depth mixtures used in sediment addition projects at Elkhorn Slough. These sediment types included granite fines, locally available sediment of crushed granite available at low or no cost, and floodplain sediments imported from a river channel widening project conducted to increase flow capacity and mitigate flooding along the nearby Pajaro River. The Pajaro River in the nearby town of Watsonville is a flood-prone area that experienced devastating flooding affecting largely low-income farm worker communities in 1995 and 2023 (https://www.pajarowatershed.org/bench-excavation/). Soil compositions included: granite fines to a depth of 0.6 m (three 3.5 m x 3.5 m plots with biochar and three without), granite fines to a depth of 0.6 m, but capped with 15 cm of floodplain sediment (three 3.5 m x 3.5 m plots with biochar and three without), a 50/50 mix of granite fines and floodplain sediments to a depth of 0.6 m (again three 3.5 m x 3.5 m plots with biochar and three without), and reference plots with 100% floodplain sediments (nine 3.5 m x 3.5 m plots with biochar and nine without).

In the summer of 2022, two 50 cc samples were collected from each plot using a soil ring ca. 3.5cm diameter and 5 cm deep to measure soil characteristics, including KCl-extractable ammonium and nitrate, pH, bulk density, redox potential, salinity, and loss on ignition (LOI). This yielded a total number of six samples per treatment for the three different soil mixtures with granite fines, and 18 samples per treatment for the floodplain sediments, for a total of 72 soil samples total.

Soil nutrients were measured as KCl extractable ammonium and nitrate. Sediment samples (10 g wet mass) were extracted with a 40 mL 2.0 M KCl solution. The slurries were vortexed and centrifuged, and the supernatants vacuum filtrated into Nalgene propylene vials and stored frozen. The samples were brought to room temperature and were analyzed for NH3+4-N and NO3-N using a Lachat QC8500 Flow Injection Colorimeter, at the Center for Clean Water Technology, Stony Brook University. Soil pH was measured on 5 cc samples of 1:1 dry sediment to water mixtures (weight/weight) which were vortexed three times at one-minute intervals before measures. The pH measures were taken directly using a Sensorex pH probe on an Orion Star A211 benchtop pH meter. Soil redox was measured on room temperature soil samples using an Oakton double-junction gel-filled ORP probe on a SPER Scientific Benchtop mV meter, where the probe was surrounded by soil. Soil salinity was measured on a 10:1 dilution of water soil: dry soil, utilizing 2.0 g of wet mass soil. Samples were vortexed for one minute and then centrifuged at 2500 rpm for 10 minutes. Supernatant salinity was measured using a YSI Professional multiprobe meter and back-calculated for a 1:1 mixture of soil and water. Loss on ignition (LOI), a surrogate for soil organic content, was measured as the weight lost upon ignition for four hours at 550°C of oven-dried samples. Sediments were not dried before measurements of pH and salinity. Rather, estimates of oven-dry weight were made for wet sediments using the field moist/oven-dry ratio. Sediments were processed for particle size distribution after pretreatment with hydrogen peroxide and dispersal with sodium hexametaphosphate and analyzed using a Beckman Coulter LS 13 300 particle size analyzer.

Plant cover was assessed using drone images collected yearly in the summer (2018-2023) to track restoration progress. Plots were delineated digitally in ArcGIS (version 10.3, ESRI, Redlands, CA, USA), and a grid of 50 points was placed over each plot; with a visual count made of the number of points intersecting vegetation. Carbon dioxide and methane emissions were also quantified during the summer of 2022 using an LGR (Los Gatos Research)-ABA ultraportable greenhouse gas analyzer, at a sampling density of three samples from each plot (n=3 per plot; n=12-36 per treatment; 108 measures total). The analyzer was connected by nylon tubing to a 0.6 m x 0.3 m transparent polycarbonate chamber (42 L) seated to PVC collars installed in the marsh soil (30 cm diameter). Gas measurements were conducted for three min per incubation. Greenhouse gas fluxes were calculated using chamber volume and footprint. The Ideal Gas Law (PV = nRT) was used to calculate changes in gas concentrations over time using measured air temperatures and atmospheric pressure. 

Plot-scale Thin-layer Sediment Placement Experiment

In 2018, we conducted a replicated experiment with thin layer sediment placement in small plots (0.7 x 0.7 m) at tidal marshes located at eight different national estuarine research reserves (in New Hampshire, Massachusetts, Rhode Island, Chesapeake Bay, Maryland, Chesapeake Bay, Virginia, North Carolina, the San Francisco Estuary, California, and Elkhorn Slough, also in California). The goal of this experiment was to evaluate the effects of sediment placement on vegetation recovery in low and high marshes and compare this to control and reference plots, across a range of environmental conditions and in locations with different common plant species. Here, we report on the results of biochar incorporation, which was conducted at three locations: Elkhorn Slough, in California (36.811°N, -121.749°W), Sage Lot Pond marsh, on Cape Cod, Massachusetts (41.555°N, -70.511°W), and Prudence Island, Rhode Island (41.650°N, -71.342°W). At Elkhorn Slough and Prudence Island, the biochar soil amendment was a mixture of biochar and compost (Blacklite mix#6, Pacific Biochar, Santa Rosa CA), while at Sage Lot Pond, the biochar soil amendment was pure biochar (Blacklite Pure): both biochars were produced from a feedstock of softwood forest residue, and the soil amendments were incorporated into the sediment additions at 10% v/v. Sites had similar salinity regimes (five-year averages of 28-30% at all three sites) but varied somewhat in tidal range and mean high water, and tidal range.

We report on vegetation cover and height tracked over time (fall 2017, 2018, 2019, and 2020, as well as spring 2019) in plots with 14 cm of sediment added, 14 cm of sediment added with 10% biochar amendment, control plots, which were similar plots with no sediment or biochar added, and reference plots, which generally were higher elevation with more complete plant cover. Plant cover was assessed using the point-intercept method for a grid of 25 points. Sediments were sampled in the spring of 2019 (20 cc), and processed for KCl extractable ammonium, pH, redox, salinity, and loss on ignition, using the methods described above. In the summer of 2019, carbon dioxide emissions were measured at Waquoit Bay and Elkhorn Slough using a Licor LI-8100A automated soil CO2 flux system with a chamber and dome. Collars were placed in areas lacking in vegetation.