Can biochar conserve water in Oregon agricultural soils?

Soil & Tillage Research 198 (2020) 104525

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Can biochar conserve water in Oregon agricultural soils? a

a,1

b

a

Claire L. Phillips , Sarah E. Light , Hero T. Gollany , Stephanie Chiu , Thomas Wanzek Kylie Meyera, Kristin M. Trippea,* a b

T a,2

,

USDA-ARS Forage Seed and Cereal Research Unit, 3450 SW Campus Way, Corvallis, OR 97331, United States USDA-ARS Columbia Plateau Conservation Research Center, 48037 Tubbs Ranch Road Adams, OR, 97810, United States

A R T I C LE I N FO

A B S T R A C T

Keywords: Gasification Drought Soil hydraulics Porosity Plant available water Hydrus-1D

Few studies have evaluated the ability of biochar to improve soil water content under field conditions. Our objective was to assess the potential drought benefits of biochar across a range of soil textures by measuring indicators of hydraulic capacity and water-retention curves for field-amended soils. Two biochars, conifer wood and wheat straw, were incorporated by rotary tillage at amendment rates of 9−36 Mg ha−1 in four locations. All four soils showed a significant increase (p≤0.04) in saturated water content of 0.05–1.7 % volume per Mg ha−1 biochar added, but no increase in water content at field capacity. There were significant (p < 0.01) decreases in volumetric water content at wilting point by 0.09 to 0.8 % per Mg ha−1 of biochar added in all soil textures. This translated to small, significant increases in plant-available water capacity of the same magnitude. Although the conifer wood biochar was less hydrophobic and had 14 % more internal pore space than the wheat straw biochar, both biochars had similar impacts on volumetric soil water content (θ). Simulations of soil drying and recharge using moisture retention curves and the Hydrus-1D model indicated that biochar amendment would generally decrease θ in the loam soil, and would have minimal impact on θ ( ± 1 %) in the more coarse textured soils. Monitoring of in situ θ in two of these soils showed similar results. Biochar sped up soil drying in the loam following irrigation, but had no impact on θ in the loamy sand. These results suggest limited utility for biochar as a drought adaptation tool in the first year following incorporation into these medium- to coarse-textured soils.

1. Introduction

Soil management practices that improve the plant-available water capacity (θAW) of soils may provide drought adaptation by increasing rooting-zone water that could otherwise be lost to runoff or evaporation (Falkenmark and Rockström, 2006; Orth and Destouni, 2018; Rockström, 1999). Increasing plant-available water capacity has been shown to enhance carry-over of soil water received from winter precipitation into the growing season (López et al., 1996) as well as improving soil water content (θ) during deficit periods (Bescansa et al., 2006; Jones et al., 1994). A growing body of evidence demonstrates the potential for biochar to increase plant-available water capacity (Masiello et al., 2015; Omondi et al., 2016). For instance, several studies conducted in soil columns or pots have shown increased plant-available water capacity in sandy soils (Abel et al., 2013; Basso et al., 2013; Hansen et al., 2016; Liu et al., 2017; Suliman et al., 2017; Zhang et al., 2016). Similar techniques have been used to show decreased hydraulic conductivity in sandy

Loss of winter snowpack and increasing summer temperatures are creating transformative drought conditions in the Pacific Northwest (PNW) region of the United States (Abatzoglou et al., 2014; Mote et al., 2013, 2016). Management practices that can improve soil water capture and storage may serve as drought adaption tools for both irrigated and dryland farms. Biochar is a soil amendment with high porosity that has been shown to increase soil water-holding capacity, particularly in coarse-textured soils (Masiello et al., 2015). However, much of the research on hydraulic impacts of biochar have been conducted in the laboratory, with few studies evaluating agriculturally-relevant field incorporation (Hardie et al., 2014; Major et al., 2012; Obia et al., 2016). The PNW is characterized by a Mediterranean climate, with most precipitation occurring during the fall to spring and comparatively dry conditions during the summer growing season.

Abbreviations: CW, Conifer wood biochar; θ, volumetric soil water content; θFC, volumetric water content at field capacity; θsat, volumetric water content at saturation; θWP, volumetric water content at wilting point; WS, wheat straw biochar ⁎ Corresponding author. E-mail address: [email protected] (K.M. Trippe). 1 Present Address: University of California Cooperative Extension, 142A Garden Highway, Yuba City, CA 95991. 2 Present Address: Dept. of Crop and Soil Science, Oregon State University, Agriculture and Life Sciences Building, Corvallis, OR 97331. https://doi.org/10.1016/j.still.2019.104525 Received 2 April 2019; Received in revised form 23 October 2019; Accepted 28 November 2019 0167-1987/ Published by Elsevier B.V.

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what practitioners can expect when using biochar. Another central component to assessing biochar utility is to evaluate biochars that have good “system-fit” (Sohi et al., 2015), meaning they are from regionally-available feedstocks, and are produced with charring methods that are available, economical, and minimize life-cycle negative environmental impacts. In the PNW, low-value woody biomass is a primary source of biochar feedstock (Campbell et al., 2018; PageDumroese et al., 2016), with straw residues also representing a major potential feedstock source (Banowetz et al., 2008). In this study, commercially produced gasified conifer wood (CW) and wheat straw (WS) biochars were used to represent types that could be produced at a large scale. Gasified biochars, which are produced at high temperatures in the presence of low oxygen concentrations, are anticipated to be especially well suited for moisture retention. High production temperatures increase internal biochar porosity and water-holding capacity (Zhang and You, 2013), and the high abundance of oxygenated functional groups on gasified biochar have been shown to increase hydrophilic characteristics and water retention (Suliman et al., 2017). Our objective was to assess the potential for biochar as a droughtadaptation tool in the PNW using three approaches: 1) comparing indicators of hydraulic capacity six months after biochar addition to field plots, 2) using hydraulic properties and the Hydrus-1D model to simulate soil water content during drying periods and precipitation events, and 3) monitoring water content (θ) with buried sensors over the growing season at two irrigated field sites. This work builds on the small number of field studies investigating the hydraulic impacts of biochar, and encompasses multiple soil textures, two biochars, and three amendment rates. Based on the body of evidence for biochars specifically and organic matter more generally, we hypothesized that biochar would increase available water capacity in sandy soil, but have small or no increases in loam soils. The results of this study will help to clarify the potential use of biochar to conserve water in soil types relevant to Oregon agriculture.

soils (Barnes et al., 2014; Lim et al., 2016), demonstrating the potential for biochar to reduce water losses to deep drainage. However, results have been mixed in clay soils (Aller et al., 2017; Castellini et al., 2015; Sun and Lu, 2014) and medium-textured soils (Aller et al., 2017). In a meta-analysis, Omondi et al. (2016) reported significant increases in plant-available water content only in coarse textured soil, but not in fine and medium-textured soil. In a clay soil, Castellini et al. (2015) showed that biochar amendment augmented the largest pores that readily drain, and do not contribute to plant-available water. The ability to draw conclusions about biochar’s utility for droughtadaptation requires more information on textural interactions, as well as on biochar performance in the field. Omondi et al. (2016) showed significantly smaller improvements in available-water content in field studies than in laboratory or greenhouse studies, which was likely related to the fact that higher amendment rates were tested in laboratory studies. Results from long-term field studies have also been contradictory. In a sandy-loam soil in Tasmania, Hardie et al. (2014) found no improvement in soil water content or plant available water capacity 30 months after biochar amendment. However, in a sandy-loam soil in Brazil, de Melo Carvalho et al. (2014) observed improvements two and three years after biochar amendment. Obia et al. (2016) also found improvements in θ for a sandy-loam soil in Zambia, but mixed results in a loamy sand and sand soil. This apparent inconsistency in biochar impacts in field studies, particularly in medium-and fine-textured soils, is not surprising given the decades-long debate over the extent to which organic matter generally improves plant-available water capacity (Hudson, 1994; Huntington, 2007; Stevenson, 1974). Although biochar is quite different chemically than uncharred organic residues, it retains many of the physical characteristics of organic feedstocks (Chia et al., 2015), and both biochar and organic matter are widely touted to increase soil water content (Hudson, 1994; Woolf et al., 2010). It has been only recently, by analyzing very large datasets including thousands of samples and a large range in soil organic matter content, that global relationships between organic matter content, textural components, and plant-available water can be identified (Libohova et al., 2018; Minasny and McBratney, 2018; Rawls et al., 2003). These large datasets have indicated that: organic matter improves available water capacity more in sandy soils than in loams and clays; correlations are weak when organic matter content is less than 8 % (Libohova et al., 2018); and the increase in available water capacity is small (Minasny and McBratney, 2018), about 1.5 % for a 1 % increase in soil organic matter content (Libohova et al., 2018). Assuming that biochar affects soil hydraulics through at least some of the mechanisms of uncharred organic matter, this suggests biochar will have smaller benefits in fine-textured soils compared to sandy soils, that variable results can be expected, and that large incorporation rates may be needed to see a soil moisture benefit. This also suggests that a large data set across soil types and with realistic incorporation methods is necessary for establishing guidelines on

2. Materials and methods 2.1. Soil and biochar properties Biochar field plots were established at four agricultural experimental stations in Oregon representing different climatic regions and soil textures (Table 1). Soil texture was determined with the hydrometer method (Bouyoucos, 1951) and C content was determined by combustion-IR of oven-dry ground samples using a LECO CNS elemental analyzer (LECO Corp, MI, USA). At each site two biochars were incorporated: a CW biochar produced from lumber mill waste by BioLogical, Inc. (Philomath, OR) and a WS biochar produced by AgEnergy (Spokane, WA). Both biochars were produced by gasification, with highest heating temperatures of approximately 1250 °C for the CW and 800 °C for the WS.

Table 1 Field site locations, climate, and soil characteristics (mean ± SD, N = 3). Site Name

Central Oregon Agricultural Research Center (COARC)

Columbia Plateau Conservation Research Center (CPCRC)

North Willamette Research and Extension Center (NWREC)

Klamath Basin Research and Extension Center (KBREC)

Location MAT (°C) MAP (cm) Water management Soil taxonomy Soil series Textural class Sand (%) Silt (%) Clay (%) Carbon (% mass)

44.679104, -121.149958 9 25 Irrigated Aridic Argixeroll Madras Loam 40.2 ± 1.9 41.7 ± 2.2 18.1 ± 0.6 1.24 ± 0.40

45.717775, -118.628545 10.5 36 Dryland Typic Haploxeroll Walla Walla Silt Loam 29.4 ± 0.4 63.3 ± 0.4 7.3 ± 0 1.20 ± 0.04

45.2808, -122.7512 11 114 Dryland Pachic Ultic Argixeroll Willamette Sandy Loam 54.9 ± 1.3 36.6 ± 1.2 8.5 ± 0.4 1.34 ± 0.04

42.163209, -121.757131 9 36 Irrigated Typic Durixerept Poe Loamy Sand 82.7 ± 0.5 11.5 ± 0.7 5.8 ± 0.5 0.51 ± 0.02

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with very large pore diameters, the 95th percentile of SEM measurements (rather than the 100th percentile) was used to represent the upper limit of internal biochar pore diameters. The volume associated with pores larger than this limit were assumed to be between-particles, and accounted for 31 % and 42 % of the total packed sample volume for the CW and WS biochars, respectively. Biochar internal pore volume was calculated by subtracting the volume associated with these larger pores from the total packed sample volume.

Physiochemical properties of the biochars were characterized using the following methods. Oven-dried and ground biochar samples C and H content were determined in triplicate by combustion-IR (Elementar Macro Cube CHNS analyzer, Elementar, Germany). Biochar pH was measured in a 1:20 biochar-water solution (Camps-Arbestain et al., 2015)). Hydrophobicity was assessed by comparing the ability of the biochars to imbibe water in comparison to ethanol, which is able to more completely wet polar surfaces, as described by Gray et al. (2014). Cores measuring 5 cm diameter ×2.5 cm high were sealed on the bottom by affixing 200 μm nylon mesh, and then were filled with 9.2 g WS or 9.5 g CW biochar. Mesh was also attached to the top of the packed cores with a rubber band to prevent biochar from floating. Four replicate cores of each biochar type were placed in individual jars filled and allowed to saturate from the bottom for 21 days, with the jar lids sealed to prevent evaporation. The cores were periodically removed and weighed to determine whether they were taking up additional water. After the final wet weights were determined, the cores were dried at 105 °C for 24 h to determine dry mass. The wetting procedure was repeated using 100 % ethanol. Biochar particle size was determined by a combination of laser light scattering for size fractions < 2 mm, and dry sieving for larger fractions. For the sieve analysis, a 20 g oven-dry sample was placed in a stack of sieves with 8, 4, and 2 mm mesh size, which was then capped with a solid lid and bottom pan and placed on a shaker table at 200 rpm for an hour. The mass fraction in each sieve was determined. Fractions that passed through a 2 mm sieve were analyzed using a Mastersizer 2000 (Malvern Panalytical, Malvern, UK) by Micromeritics Analytical Lab Services (Norcross, GA, USA). A single sample was analyzed using dry dispersion, with an assumed refractive index of 2.421. Laser light scattering estimates particle size distribution on a volume basis. To combine these data with those from sieve analyses, which were on a mass basis, we made the assumption that all particle size classes had the same particle density (Table 2), and therefore had the same distribution on both a volume and mass basis. Bulk density, particle density, surface area, total porosity and pore size distribution were determined by Hg porosimetry on a single 160–250 mg sample (Micromeritics Analytical Lab Services, Norcross, GA, USA). Because Hg porosimetry does not distinguish internal biochar pores and between-particle pores in packed biochar samples, we also measured pore throat sizes by SEM images to estimate the upper limit of internal biochar pore diameters. Transverse images of biochar particles were obtained at 500–5000 times magnification using a VEGA3 SEM (TESCAN USA Inc, Warrendale, PA). Pore diameters were measured manually using Fiji image analysis software (Schindelin et al., 2012), providing a total sample of 131 and 333 pores for the CW and WS biochars, respectively. Because there were a small number of pores

2.2. Experimental design In order to characterize multiple soil types, biochar types, and amendment rates under field conditions, we established small plots (1 m × 0.5 m) at each field site, which were sufficiently large to allow the biochar to be mechanically incorporated, but small enough that the quantity of biochar was practical to obtain. Plots were established in a partially-randomized block design that was replicated at each site in September-November 2016. At each site, rows (blocks) were amended with either CW or WS biochar, which allowed the row to be tilled in a continuous path without cross-contamination of biochar types. A 1-m wide untilled buffer was left between the rows. Within rows, plots were established in randomized order with amendment rates of 0, 9, 18, and 36 Mg ha−1, and 0.5 m wide buffers between plots to prevent carryover of biochar. Each biochar type and rate combination was replicated three times, for a total of 24 plots per site. Biochar was spread evenly over each plot, lightly raked into the soil surface to weight it down, and then incorporated to 12 cm depth using a small rototiller (Honda FG110) in a single pass. Visual inspection indicated that the biochar was evenly mixed through the tilled layer with no stratification. The soil was allowed to settle over the winter, and in March-April 2017 volumetric soil cores were collected from plot centers for moisture retention analysis. Field cropping history varied by site, but all sites had minimal vegetation at the time soil cores were collected. The silt loam site (CPCRC) was planted into winter wheat by drill in the fall following biochar incorporation, and cores were subsequently sampled between the rows in the spring. The sandy loam site (NWREC) was left fallow for the duration of the study. The loam site (COARC) and loamy sand site (KBREC) were planted to cover crops (mustard and spring wheat, respectively) following spring sampling but while in situ water content was monitored. 2.3. Moisture retention measurements and indicators of hydraulic capacity A volumetric soil core (5 cm diameter ×3 cm height) was removed from each plot for analysis on ceramic pressure plates at matric potentials (ψ) of -0.05, -0.1, -0.5 MPa (Soil Moisture Equipment, Santa Barbara, Inc). Cores were lined on the bottom with a piece of cotton fabric held by a rubber band, placed on ceramic plates, saturated by soaking on the ceramic plates for 16−24 hours, and sealed in pressurized chambers with compressed air for 10 days (at -0.05 MPa) up to 4 weeks (at -0.3 MPa). After water had stopped dripping from the chamber outlet for two days, which indicated that all cores had reached equilibrium, cores were removed and weights determined. Following all the pressure measurements, the cores were oven dried to calculate water contents at each pressure, as well as to determine soil bulk density and total porosity. Wilting point water content at -1.5 MPa (θWP) was then determined with the WP4 Water PotentiaMeter (METER USA, Pullman, WA). Subsamples from the oven dry soil cores were wetted to a known moisture content estimated to be slightly above wilting point. The actual water potential of the samples was then determined using the WP4 instrument. Because the relationship between water content and the natural logarithm of water potential (pF, or ln cm H2O) is linear in this very dry range, the water content at wilting point was then calculated by linearly interpolating between the measured value and an oven-dry

Table 2 Biochar production conditions and physical properties (mean ± SD). Property

Conifer Wood (CW)

Wheat Straw (WS)

Highest heating temperature (°C) C (%) N=3 H (%) N=3 pHH2O N=3 Bulk (tap) density (g cm−3) Particle (skeletal) density (g cm−3) Pore volume of packed sample (cm3 g−1) Estimated internal pore volume (cm3 g−1) 95th Percentile of pore diameters in SEMs (μm) Saturated H2O content (cm3 g−1) N=4 Saturated H2O/ethanol content (ratio) N = 4

ca. 1250 82.6 ± 0.6 1.2 ± 0.1 8.3 ± 0.2 0.17 1.10 5.11

ca. 800 60.1 ± 1.8 1.3 ± 0.3 10.9 ± 0.1 0.17 1.66 5.29

3.52

3.09

17.2 (N = 131)

20.1 (N = 333)

5.27 ± 0.20

4.42 ± 0.06

0.96 ± 0.003

0.90 ± 0.01

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condition, at which ψ = −1000 MPa or pF 7.01 (Campbell, 2008). Because our wettest pressure plate measurement was at -0.05 MPa, water content at field capacity (θFC, g H2O cm−3 soil) was estimated by fitting the data from each core (θ at 0, -0.05, -0.1, -0.5, and -1.5 MPa) to the van Genuchten (1980) model using the program SWRC Fit (Seki, 2007) to interpolate between saturated water content (θsat) and the -0.05 MPa measurement. Field capacity is commonly approximated at a matric potential of -0.033 MPa (pF 2.5, Kirkham, 2014) or less commonly as -0.01 MPa (pF 2.0, Hillel, 2004). Here we present results assuming field capacity at -0.033 MPa, but also provide results for θFC at -0.01 MPa in the supplemental material (Fig. S1 and Table S1.).Additionally, a larger core (18 cm diameter ×5 cm height) from one plot of each treatment was collected for moisture retention analysis using the HYPROP instrument (METER Group AG, Munich, Germany). While the pressure plate and WP4 methods are capable of characterizing a large number of samples at a small number of pressure points, the HYPROP provides high resolution data for a single replicate per treatment, with measurements concentrated at the wet end of the moisture release curve. The HYPROP continuously weighs a core as it evaporates, providing water content based on mass loss and water potential from two mini-tensiometers inserted to 1.25 and 3.75 cm depth. Unsaturated hydraulic conductivity (K) is calculated from the potential gradient between the two tensiometers following the simplified evaporation method (Pertassek et al., 2015) The spatially-replicated small cores were used to investigate biochar impacts on indicators of hydraulic capacity (θsat, θFC, θWP, and available water, θAW computed as θFC−θWP). The data from the small cores were combined with the temporally-intensive HYPROP core to estimate a single set of van Genuchten model parameters for each treatment. Using the HYPROP-FIT software, the θ (ψ) relationship was fit to either the original van Genuchten-Mualem model (van Genuchten, 1980) or a bimodal variant (Durner, 1994), based on visual inspection and Aikake’s Information Criterion. The original van Genuchten-Mualem model was used for all the treatments in the NWREC soil, and the unamended COARC soiland the bimodal variant was used for the amended COARC treatments, and all treatments in the CPCRC, and KBREC soils.

ponding. The lower boundary condition allowed for free drainage. Soil profiles were initialized at a matric potential of -0.01 MPa (-100 cm H2O) across all layers, and then saturated by imposing 5 cm day−1 of precipitation for ten days. This was followed by 21 days of drying under a potential evaporative demand of 0.5 cm day−1, which approximates the long term daily average reference evapotranspiration in May at the two irrigated sites (AgriMet, 2019). On days 32, 39, and 53 precipitation events of 1 cm, 2 cm, and 3 cm, respectively, were simulated. The potential evaporative demand was held at 0.5 cm day−1 between precipitation events. 2.5. In situ volumetric water content At the loam and loamy sand sites, which are both irrigated, in situ soil water content was monitored from late May-October 2017 using TDR sensors buried at 10 cm depth, with tines parallel to the soil surface (CS-655, Campbell Scientific, Logan, Utah, USA). Because of the large number of sensors required, only two of three replicate plots of each treatment were monitored at each site. At both sites, plots were planted in cover crops that were not harvested during the monitoring period. At the loam site (COARC), the mustard cover crop senesced shortly after sensor installation. Prior to sensor installation, soil-specific calibrations relating permittivity and volumetric water content were established both with and without biochar. Kameyama et al. (2014) showed high-temperature biochars increase apparent permittivity, resulting in moisture contents that are erroneously high if calibration coefficients from unamended soil are used. To calibrate sensors, air-dried soil from each site was mixed with biochar and packed into 19 L buckets to bulk densities determined from field plots. Separate calibrations were determined for unamended soil, and soil amended with either CW or WS biochar at 1 % and 2 % by dry mass (equivalent to the 18 and 36 Mg ha−1 amendment rates, respectively). Two TDR sensors were inserted vertically into the center of each bucket with at least 8 cm of soil surrounding all sides. Known amounts of water were sprinkled on the soil surface to achieve a range of moisture contents, and after each watering event the buckets were covered with foil to prevent evaporation, and allowed to equilibrate for 48−72 h until sensor permittivity reached a steady-state value. Because water was not evenly distributed across the bucket depth, a small soil sample was collected from the edge of each bucket to confirm water content at the sensor depth of 0−10 cm. Binomial calibration curves were fit to four measurements between 5 % and 35 % volumetric water content, using the average permittivity of two TDR sensors. Consistent with Kameyama et al. (2012), we found apparent permittivity at a given water content increased with biochar amendment rate (data not shown). We therefore used an average of the calibration coefficients from the unamended and 18 Mg ha−1 treatments for the intermediate 9 Mg ha−1 amendment rate.

2.4. Hydrus-1D simulations To explore how differences in soil water retention translate to differences in θ over time, we simulated soil responses to precipitation and evaporation for a subset of the treatments using the van Genuchten parameters determined as described above. The primary question was: when biochar increases θsat but does not increase θFC, how much precipitation is needed to see a transient moisture benefit? Secondly, for how long does the benefit last? Field capacity is typically defined as water content three days following a saturating rain event, but the actual water content can vary with antecedent moisture conditions, and does not necessarily match the laboratory approximation of water content at -0.03 MPa (Hillel, 2004). Therefore, we expected to find that biochar-amended soils could have a soil moisture benefit shorter or longer than three days. Simulations were performed only on the unamended and 36 Mg ha−1 WS treatments to answer these questions. Hydrus-1D numerically solves the Richards equation for variablysaturated water flow (Šimůnek et al., 2013). For our simulations, each soil was described as a 100 cm profile discretized into 200 even layers. The hydraulic parameters for the top 12 cm, which corresponded with the biochar incorporation depth, were determined from soil cores as described above. For 13−100 cm depth, parameters for the van Genuchten-Mualem model were prescribed using typical values for each soil textural type as determined from pedotransfer functions implemented in the ROSETTA model (Schaap et al., 2001), which is included in Hydrus-1D. The upper boundary condition for the profile was an atmospheric layer, where water was added as rainfall or removed by specifying an evaporative demand. When precipitation exceeded infiltration rate, water was allowed to runoff so that there was no surface

2.6. Data analysis Statistical analyses tested whether indicators of hydraulic capacity (θsat, θFC, θWP, and θAW) varied with biochar amendment rate and biochar type. Because some variables had large variances and others had unequal variances across sites, analyses were conducted separately for each site. Analyses used a linear mixed effects model with biochar type, amendment rate, and biochar type × amendment rate as fixed factors, and block as a random effect. Analyses were implemented with the lmer function from the lme4 package in the R statistical language (R Development Core Team, 2014). 3. Results 3.1. Biochar properties Physical characterization demonstrated differences between the CW 4

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3.2. Soil porosity Incorporating biochar into soil substantially increased the total pore volume of soil per unit soil mass (Fig. 2), with significant relationships between amendment rate and soil pore volume (p < 0.01 for each site, regression parameters given in Table S1). While the biochars differed in particle size and internal porosity, they had similar impacts on soil pore volume in the loam and silt-loam soil, as indicated by non-significant interaction terms. In the coarser soils interactions were significant, and the larger-sized, more porous CW biochar increased soil pore volume more than the WS biochar (p = 0.066 in the sandy-loam and p < 0.01 in the loamy sand, Table S1). The relative portion of the additional pore volume that was associated with internal biochar pores, in contrast to pores between soil and biochar particles, was estimated. The volume of internal pores was estimated by multiplying the specific internal pore volume of the biochars (Table 2) by their amendment rates. The internal pore volume, shown as light colors in Fig. 2, contributed only 11–45 % of the total increase in soil pore volume, which suggests that the majority of the increase was from between-particle pore volume. While the absolute internal pore volume increased with amendment rate, the fractional contribution from internal pores had considerable variability, and no trend was apparent as amendment rate increased. When pore volume was expressed per unit soil volume, rather than per unit soil mass, a significant increase (p < 0.01 for each site) in pore volume with biochar amendment was still observed, but the magnitude of the effect was much smaller (Fig. S1 and Table S1). The difference in these results was related to the fact that biochar amendment substantially increased soil total volume, as well as pore volume. Thus, when pore volume increases were normalized by total volume increases, the apparent impacts of biochar were smaller. The bulking effect of biochar was also related to the low particle densities of the biochars (Table 2), which were approximately half that of quartz mineral (2.65 g cm−3), demonstrating that biochar particles occupy about twice the volume of soil particles.

Fig. 1. Biochar (A) cumulative pore volume determined by Hg porosimetry, and (B) particle size distribution determined by a combination of laser light scattering and sieving. Pore size designations follow Soil Science Society of America (SSSA) definitions.

3.3. Indicators of hydraulic capacity As biochar amendment rate increased, θsat increased significantly (p ≤ 0.04) but θFC did not increase across any of the sites and soil textures (Fig. 3, regression parameters given in Table S1). Furthermore, θWP decreased with amendment rate, by a small but significant (p < 0.01) amount. Expressed as percent volume, θsat increased by 0.05–1.7, and θWP decreased by 0.09 to 0.8 per Mg ha−1 of biochar. The decrease in θWP and lack of change in θFC translated to a significant (p ≤ 0.01) but small increase in θAW with increasing amendment rate. The two biochar types had similar effects on θsat, θFC, θWP, and θAW as indicated by non-significant interactions between amendment rate and biochar type (Fig. 3). An exception was for θWP in the loamy sand soil, where CW addition resulted in a larger decrease in θWP than the WS biochar.

and WS biochars that may influence their ability to uptake water (Table 2). Although the two biochars had a similar bulk density and a similar total pore volume as estimated by Hg porosimetry, our estimate of internal pore volume, which comes from subtracting the volume associated with pores larger than those detected in SEM images, was 14 % higher for the CW than the WS biochar. CW biochar water uptake was also significantly (p < 0.001) higher by 19 % than the WS biochar, when saturated over 21 days. The reduced water uptake of the WS biochar was consistent with greater hydrophobic characteristics than the CW. The ratio of saturated water content to ethanol content, an indicator of hydrophobicity (Gray et al., 2014), averaged 0.90 (S.D. = 0.013) in the WS biochar, and was significantly (p < 0.001) lower than the average ratio of 0.96 (S.D. = 0.003) for the CW biochar. Pore size distributions of the biochars (Fig. 1A) show no internal porosity in meso- or macropores for either biochar, and all the pore volume in either micropore (5–30 μm) or smaller (< 5 μm) size classes. The micropore size class is generally considered the smallest class of pores containing plant-available water (Soil Science Society of America, 2008). About 49 % and 43 % of the internal pore volume in WS and CW biochars, respectively, was comprised of pores smaller than this size class, and thus too small to be considered in the plant-available water budget. Particle size analysis (Fig. 1B) indicated that the CW biochar had larger particles than the WS biochar. Almost 50 % of the CW biochar was in particle size of 4 mm or larger, whereas only 3 % of the WS biochar was larger than 4 mm.

3.4. Hydrus-1D simulations using van Genuchten parameters Simulations were used to evaluate biochar impacts on soil water over time, and assess whether an increase in θsat without an increase in θFC confers any increase in volumetric water content during the growing season. Because the CW and WS biochars had similar impacts on θsat and θFC (Fig. 3), this analysis employed only the WS treatments as an illustrative example. The spatially-replicated data described above were combined with a single HYPROP core for each treatment to estimate van Genuchten model parameters (Table S2). The small cores generally had higher measured θsat than the HYPROP cores, and therefore fitted θ(ψ) relationships tended towards the mean of all the cores at saturation. The addition of the HYPROP core gave somewhat different 5

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Fig. 2. Impacts of biochar incorporation on soil pore volume, and contributions of biochar internal and between-particle pores. Error bars are SD of total soil pore volume (n = 4). Dashed horizontal lines indicate porosity of unamended plots.

patterns for fitted θFC compared to the observations in Fig. 3. These fitted relationships indicated that biochar caused rapid loss of saturated water and decreased θFC by as much as 10 % for the loam soil and 7 % for the silt loam soil at the highest WS amendment rate. Saturated water content was lost more slowly in the coarser-textured soils. Modeled θFC decreased by 2.8 % in the sandy loam, and θFC increased by up to 3.5 % in the loamy sand at the highest amendment rate. The more rapid loss of saturated water content in fine-textured soils was also indicated by predicted K(θ) relationships (Fig. 5), which show biochar generally increased K at high moisture levels in the loam and silt loam soils, and had no impact or decreased K in the sandy loam and loamy sand soils. Hydrus-1D simulations employing these relationships suggested that biochar had modest impacts on θ over the course of multiple drying and rewetting cycles in three of the soil types, but biochar led to much drier conditions in the loam soil (Fig. 6). An application of 36 Mg ha−1 WS biochar increased θ in the loam soil following an initial saturating event, but this water was rapidly lost. The larger reservoir of water

created by biochar amendment allowed evaporation to meet the imposed potential evaporation rate of 0.5 cm day−1 for 9.1 days, in comparison to only 2.3 days in the unamended soil. However, the water was also susceptible to drainage, and as drying commenced the water content of the amended soil dropped below that of the unamended soil at 7.4 days, reached wilting point (θWP = 9.2 %) at 13.8 days, and remained lower than in the unamended soil for the duration of simulation. By contrast, the unamended soil did not reach wilting point at any time, and remained above 30 % volumetric water content for the duration of the simulation. Subsequent rain events of 2 and 3 cm were sufficient to fill the pore volume of the unamended soil back to saturation, but filled only 33–43% of the pore volume in the biocharamended soil. In the silt loam soil, the highest amendment rate of WS biochar also sped soil drying and decreased θ, but by a much smaller increment than in the loam soil. Biochar decreased θ most substantially during the initial drainage phase following saturation. This is consistent with the K 6

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Fig. 3. Biochar impacts on soil water content (% volume) at saturation (θsat), field capacity (θFC), and wilting point (θWP), and plant-available water content (θAW). Error bars are standard deviations of three plots. Only significant correlations are shown. Two lines are shown where the interaction between amendment rate and biochar type was significant.

sensor (Fig. S6), possibly due to installation conditions, and therefore was considered an outlier and omitted from further analysis. COARC sensors were installed in late May and captured one irrigation event prior to crop maturation, as well as the first fall rain event in October (Fig. 7). The COARC sensors were quite consistent within treatments during the dry period from mid-July to October (Fig. S6), and show clearly a trend of lower residual water content with increasing biochar content. Irrigation brought all the plots to a similar saturated soil water content, but the biochar-amended plots lost water more rapidly and reached a lower final water content. Fifteen days after irrigation, biochar amendment rate correlated significantly with soil water content (water content declined by 0.002 cm3 cm-3 per every Mg ha-1 biochar added, p= 0.02), with no significant difference between biochar types. The relationship between θ and biochar amendment rate was still significant at the end of the summer (on day 280 p = 0.04), until the first fall rain equalized the treatments. At the loamy sand site (KBREC), sensors captured six irrigation

(θ) relationships (Fig. 5), which show the greatest differences in hydraulic conductivity between the treatments under high moisture, and a gradual convergence at θ = 20 %. At the time of the first rain event three weeks after saturation, θ was only 1 % lower in the biochar treatment than the unamended control, an offset that remained for the duration of the simulation. In the more coarse-textured sandy loam and loamy sand soils, biochar had similarly small, although positive impacts, increasing θ by ≤1 % from days 10-60.

3.5. In situ soil water content Soil moisture sensors at the two irrigated sites were used to monitor the net impacts of changes in hydraulic properties on in situ soil moisture. For clarity, only treatment averages are shown here (Fig. 7), while individual sensors are shown in the supplemental material (Figs. S6-S7). At the loam site (COARC), one sensor from the CW 36 Mg ha−1 treatment measured water contents 2.5–3 times greater than any other 7

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approaches be used to assess biochar impacts on soil water retention, rather than comparing water content at an arbitrary water potential. We utilized the Hydrus-1D simulations to estimate biochar impacts on water content over time, and focused on the question of whether increased θsat could provide some improvement in soil water during the growing season. We posited that if soils are frequently irrigated to nearsaturation, irrigators may benefit from a few more days of higher soil moisture following each event. Additionally, the concept of applying field capacity as the upper limit of water that plants are able to use has received some criticism (Kirkham, 2014). For instance, da Silva et al. (1994) proposed “least-limiting water range” as an alternative expression, which uses 10 % air-filled porosity as the upper limit of water that plants can utilize for growth and other functions, a level that is considerably wetter than “field capacity”. The Hydrus-1D simulations showed that in the loam soil (COARC), where 36 Mg ha−1 of WS biochar increased simulated θsat by 18 %, soil water content remained higher in the biochar treatment for about one week following a saturating rain event. Excluding the period of time when air-filled porosity was < 10 %, and thus too wet for plant use following da Silva et al. (1994) definition, the period of benefit decreased to 2.1 days. Furthermore, biochar caused much more rapid and complete soil drying, so that subsequent rain events of up to 3 cm were insufficient to refill the moisture deficit. These simulations clearly indicated that for the loam soil, an increase in θsat without an increase in θFC was not beneficial for drought-adaptation. However, the loam soil provided the most dramatic case. In the loamy sand (KBREC), where biochar increased fitted θsat by a large increment of 10 % (Fig. 4), the majority of the additional water was lost within about 9 days following saturation, but a small residual benefit of about 1–3 % water content remained for the duration of the simulation, and increased slightly following each precipitation event (Fig. 6). However, this increase can be explained by the modestly higher water retention (Fig. 4) and lower K (Fig. 5) that were estimated by fitting the van Genuchten-Mulaem model to the pooled data. These factors, rather than the large increase in θsat, likely explains the small increase in unsaturated θ. Similarly, in the sandy loam soil (NWREC), biochar increased θ by about 1 % for much of the simulation (Fig. 6). However, the difference between the treatments was greatest when they were most dry, which suggests no benefit from increased θsat. The silt loam soil presented another case, where biochar increased K near saturation, and thus the higher θsat was immediately lost within the first day (Fig. 6). Overall, these results show the potential for positive or negative impacts of biochar, and indicate that no consistent case can be made across soil textures for whether higher θsat would benefit soil water content during the growing season. The inference that can be drawn from these Hydrus-1D simulations is limited, due in part to the small number of cores used to generate parameter estimates. Nevertheless, they demonstrated patterns that were remarkably consistent with in situ measurements of θ at the loam and loamy-sand sites. The Hydrus-1D simulations predicted that biochar would decrease θ in the loam soil by enhancing soil drying, and would have little impact on θ in the sandy-loam. These simulations were consistent with in situ soil moisture measurements from the buried sensors. The in situ measurements show how the hydraulic capacitive indicators (Fig. 3) provided a useful, but incomplete picture of biochar impacts on growing season soil moisture. While the hydraulic capacitive indicators had a consistent response to biochar amendment across all the soil textures, growing season θ was negatively impacted by biochar in one soil texture, and minimally impacted in another. Our results are consistent with other field studies that have shown no improvement in soil moisture or available water capacity with biochar amendment. Hardie et al. (2014) showed no impact on θFC or θAW of incorporating 47 Mg ha−1 acacia green waste biochar to a sandy loam soil, 30 months after incorporation. Furthermore, Major et al. (2012) found no effect of a 20 Mg ha−1 biochar application on soil

events and two precipitation events during the fall (Fig. 7). At this site there was no clear impact of biochar amendment. Neither peak water contents during wetting events nor minimum water contents during drying periods correlated with amendment rate. Treatment replicates had considerable variability, giving no consistent patterns overall (Fig. S7). 4. Discussion Biochar amendment was hypothesized to improve θAW in the coarsest-textured loamy sand soil, and have diminished benefits in finer-textured soils. Our results instead suggest a small increase in θAW, amounting to 0.1 to 0.6 % volumetrically per Mg ha−1 of biochar added, across all the soil types. Importantly, however, this increase was driven by a decrease in the amount of water at wilting point in contrast to an increase in the amount of water at field capacity. This complicates interpretation of the potential drought-benefit of biochar, because the decreased θWP suggests that as plants approach a physiological stress condition there is less water available, which is not favorable for plant growth. Others have shown no impact of biochar on θWP in a sandy soil (Suliman et al., 2017) or mixed results across several biochar-soil combinations (Obia et al., 2016; Sun and Lu, 2014). Still others found that biochar increased θWP in sandy soils (Abel et al., 2013; Lei and Zhang, 2013; Liu et al., 2017) and a sandy loam (Hardie et al., 2014). However, it is important to consider that in this study, biochar had no impact on water content by weight at wilting point (Table S1). It is also clear that biochar significantly increased soil porosity (Fig. 2). Thus, the decrease in θWP may have been driven primarily by an increase in total pore volume, rather than by biochar actually increasing the residual water available for plant extraction. In other words, biochar increased the denominator of the water volume/soil volume ratio to a greater extent than it increased the numerator. Other hydraulic capacitive indicators suggested little moisture benefit from biochar (Fig. 3). While biochar significantly increased θsat, the fact that it did not impact θFC suggests that the additional pore volume created by biochar amendment was in size classes susceptible to rapid drainage, and thus not part of what is typically considered the plant-available water budget. The addition of the HYPROP cores to these data (Fig. 4) further supported the conclusion that biochar resulted in more water loss at high moisture contents in the loam and silt loam soils. The related concepts of field capacity and available-water content are somewhat problematic, because they specify an arbitrary pressure head as the cutoff point for water that is susceptible to gravitational drainage, when in reality drainage slows gradually over a range of water potential values, and this range is furthermore known to depend on soil texture, structure, and OM content (Kirkham, 2014). Nevertheless, the field capacity concept persists as a convenient way to compare hydraulic capacity of different soils. To evaluate what impact a higher water potential value for field capacity would have on our findings, we also computed θFC at -0.01 MPa. At this higher field capacity, only one soil-biochar combination had a significant positive relationship between θFC and amendment rate (Fig S1). The sandy loam soil (NWREC) amended with CW biochar had an increase in θFC of 0.1 % for every Mg ha−1 biochar applied (Table S1), which amounts to an increase of 4.0 % at the highest amendment rate. The fact that the other seven soil-biochar combinations still showed no impact of biochar amendment on θFC at the higher water potential indicates that our findings are not a result of selecting too low a value to represent field capacity. 4.1. Simulations show biochar has little benefit for growing season soil water The limitations of the field capacity concept require that other 8

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Fig. 4. Modeled van Genuchten θ(ψ) relationships fitted to pressure plate, WP4, and HYPROP data, for unamended soil and the highest WS biochar application rate.

Fig. 5. Modeled van Genuchten K(θ) relationships (lines) and HYPROP-derived observations (points), for unamended soil and highest WS biochar application rate. 9

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Fig. 6. Simulated volumetric water content (θ) at 5 cm depth with Hydrus-1D. Day 0 coincides with the end of an initial saturating precipitation event. Consecutively larger precipitation events totaling 1 cm, 2 cm, and 3 cm were simulated after 3, 4, and 6 weeks, respectively.

being in micropore and smaller size classes (e.g. Baltrėnas et al., 2015; Zhang and You, 2013; although see Hyväluoma et al., 2017 for contradictory evidence). In this study, between-particle porosity was estimated to account for at least half of the pore space created by biochar amendment (Fig. 2, although see S1 for a lower contribution when pore volume is normalized by the increase in total volume). The hydraulic capacitive indicators further suggested that between-particle pore space had dominant effects on water retention. Despite the fact that the CW and WS biochars had extensive microporosity, the additional pore volume did not translate to increased water content at wilting point. The importance of between-particle porosity may also help to explain why the WS and CW biochar had similar impacts on hydraulic capacitive indicators, despite differing in hydrophobicity, saturated water content, and internal pore volume (Table 2). Suliman et al. (2017) showed that oxygenating biochar to enhance its hydrophilic

water retention or drainage of a clay soil. However, because of the important influence of texture, the variability in biochar properties, including hydrophobicity (Abel et al., 2013; Gray et al., 2014), surface charge (Suliman et al., 2017), and extent of surface weathering (Aller et al., 2017), and the potentially large impact of incorporation techniques, inconsistent findings across studies are to be expected. 4.2. Biochar enhanced large pore size classes Our results are also consistent with others who have shown that biochar amendment increases total soil porosity, and in particular increases the volume of pore sizes that hold water between saturation and field capacity (Abel et al., 2013; Castellini et al., 2015; Liu et al., 2017; Masiello et al., 2015; Obia et al., 2016). This contrasts with the small size of pores inside biochar particles, which are typically reported as

Fig. 7. Volumetric soil water content (θ) at 10 cm depth at two irrigated locations. Each line is the average of two plots. 10

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properties can translate to increased θAW in a sandy soil. Our finding conflicts with this, suggesting instead that hydraulic characteristics of amended soil are comparatively insensitive to the hydraulic characteristics of the biochar themselves. This could be due to their use of comparatively high amendment rate (2 % by mass), the fact that we incorporated biochar in the field, or may even be due to methodological differences for measuring θAW (i.e. pressure plates versus free drainage to quantify θFC). Several explanations have been offered for the increased volume of large pore classes with biochar amendment. They may develop because biochar reduces soil packing density, for instance when biochar particles differ in size from soil particles, or have a non-spherical shape (Lim et al., 2016; Liu et al., 2017). Macro- and meso-porosity may also result from improvements in soil aggregation (Du et al., 2017; Kelly et al., 2017). Our results are consistent with a change in soil packing density given the time-frame of our sampling about six months after biochar incorporation, as aggregation can be a multi-year process. Tillage by itself also decreases the packing density of soil and increases macroporosity (Ahuja et al., 1998). One limitation of this study was the lack of a non-tilled control to demonstrate the impacts of tillage, alone, on hydraulic capacitive indicators. Additionally, tillage creates transient porosity that can be expected to re-consolidate over several years (Ahuja et al., 1998). Our results, therefore, provide only a snapshot of hydraulic impacts six months after incorporation, and may not indicate persistent effects. Over multiple year timespans, the impacts of biochar could diminish as the soil reconsolidates, or alternatively biochar amendment may help to slow and reduce re-consolidation of tilled soils, possibly by facilitating soil aggregation (Kelly et al., 2017; Ma et al., 2016; Sun and Lu, 2014). Longer-term studies will be necessary to evaluate these processes and potential outcomes.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Acknowledgements We thank R.W. Polumsky for field and laboratory support; Tracy Wilson and Hoyt Downing for access to COARC and field assistance; Tom Silberstein for access to KBREC and field assistance; and Marc Anderson for access to NWREC. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.still.2019.104525. References Abatzoglou, J.T., Rupp, D.E., Mote, P.W., 2014. Seasonal climate variability and change in the Pacific Northwest of the United States. J. Clim. 27, 2125–2142. https://doi. org/10.1175/JCLI-D-13-00218.1. Abel, S., Peters, A., Trinks, S., Schonsky, H., Facklam, M., Wessolek, G., 2013. Impact of biochar and hydrochar addition on water retention and water repellency of sandy soil. Geoderma 202–203, 183–191. https://doi.org/10.1016/j.geoderma.2013.03. 003. AgriMet, 2019. Monthly Average Reference Evapotranspiration [WWW Document]. URL https://www.usbr.gov/pn/agrimet/monthlyet.html (Accessed 2.1.19). . Ahuja, L.R., Fiedler, F., Dunn, G.H., Benjamin, J.G., Garrison, A., 1998. Changes in soil water retention curves due to tillage and natural reconsolidation. Soil Sci. Soc. Am. J. 62, 1228. https://doi.org/10.2136/sssaj1998.03615995006200050011x. Aller, D., Rathke, S., Laird, D., Cruse, R., Hatfield, J., 2017. Impacts of fresh and aged biochars on plant available water and water use efficiency. Geoderma 307, 114–121. https://doi.org/10.1016/j.geoderma.2017.08.007. Baltrėnas, P., Baltrėnaitė, E., Spudulis, E., 2015. Biochar from pine and birch morphology and pore structure change by treatment in Biofilter. Water Air Soil Pollut. 226. https://doi.org/10.1007/s11270-015-2295-8. Banowetz, G.M., Boateng, A., Steiner, J.J., Griffith, S.M., Sethi, V., El-Nashaar, H., 2008. Assessment of straw biomass feedstock resources in the Pacific Northwest. Biomass Bioenergy 32, 629–634. https://doi.org/10.1016/j.biombioe.2007.12.014. Barnes, R.T., Gallagher, M.E., Masiello, C.A., Liu, Z., Dugan, B., 2014. Biochar-induced changes in soil hydraulic conductivity and dissolved nutrient fluxes constrained by laboratory experiments. PLoS One 9, e108340. https://doi.org/10.1371/journal. pone.0108340. Basso, A.S., Miguez, F.E., Laird, D.A., Horton, R., Westgate, M., 2013. Assessing potential of biochar for increasing water-holding capacity of sandy soils. Gcb Bioenergy 5, 132–143. https://doi.org/10.1111/gcbb.12026. Bescansa, P., Imaz, M.J., Virto, I., Enrique, A., Hoogmoed, W.B., 2006. Soil water retention as affected by tillage and residue management in semiarid Spain. Soil Tillage Res. 87, 19–27. https://doi.org/10.1016/j.still.2005.02.028. Bouyoucos, G.J., 1951. A recalibration of the hydrometer method for making mechanical analysis of Soils1. Agron. J. 43, 434. https://doi.org/10.2134/agronj1951. 00021962004300090005x. Campbell, G., 2008. Determining the -15 Bar (permanent Wilt) Water Content of Soils With the WP4C. Decagon Devices Appl. Note. Campbell, J.L., Sessions, J., Smith, D., Trippe, K., 2018. Potential carbon storage in biochar made from logging residue: basic principles and Southern Oregon case studies. PLoS One 13, e0203475. https://doi.org/10.1371/journal.pone.0203475. Camps-Arbestain, M., Amonette, J.E., Singh, B., Wang, T., Schmidt, H.P., 2015. A biochar classification system and associated test methods. In: Lehmann, J., Joseph, S. (Eds.), Biochar for Environmental Management: Science, Technology and Implementation. Earthscan, London, Sterling, VA, pp. 28. Castellini, M., Giglio, L., Niedda, M., Palumbo, A.D., Ventrella, D., 2015. Impact of biochar addition on the physical and hydraulic properties of a clay soil. Soil Tillage Res. 154, 1–13. https://doi.org/10.1016/j.still.2015.06.016. Chia, C.H., Downie, A., Munroe, P., 2015. Characteristics of biochar: physical and structural properties in soil. In: Lehmann, J., Joseph, S. (Eds.), Biochar for Environmental Management: Science, Technology and Implementation. Routledge, New York, pp. 89–109. da Silva, A.P., Kay, B.D., Perfect, E., 1994. Characterization of the least limiting water range of soils. Soil Sci. Soc. Am. J. 58, 1775. https://doi.org/10.2136/sssaj1994.

5. Conclusions This study showed no benefit of biochar for drought-adaptation six months following incorporation in four soil textural classes. The hydraulic impacts of the biochar amendments were dominated by large pores that developed between biochar and soil particles, which resulted in an increase in θsat, but no increase in θFC or θWP. While these large pores could provide a brief increase in soil water following a saturating rain or irrigation event, simulations and in situ observations indicated that they readily lose water and provide little moisture benefit over the growing season. In a loam-textured soil, in situ observations showed that biochar amendment resulted in more rapid and complete water loss following an irrigation event, and actually decreased overall θ. Despite differences in the particle size, internal porosity, and hydrophobicity, the two biochars had similar impacts on soil hydraulic properties. Caution should be used in applying biochar as a droughtadaptation tool in medium- to coarse-textured soil.

Funding This work was supported by the was supported by the U.S. Department of Agriculture, Agricultural Research Service in the laboratory of K.T (2072-1410-004) and the U.S. Department of the Interior under award JVA # 16-JV-11261944-089 to C.P. and K.T.

Data statement Data used in this manuscript are available for download from the USGS ScienceBase Catalog as the dataset entitled Moisture retention and hydraulic conductivity for four biochar-amended soils from Oregon, 2018. (https://www.sciencebase.gov/catalog/item/ 5ada1880e4b0e2c2dd293743) 11

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