Hydrologic properties of biochars produced at different temperatures

doi:10.1016/j.biombioe.2012.01.033

Biomass and Bioenergy

Volume 41, June 2012, Pages 34-43

https://doi.org/10.1016/j.biombioe.2012.01.033 Get rights and content

Abstract

Adding charcoal to soil (biochar soil amendment) can sequester carbon and improve soil performance, although the extent and exact mechanisms of soil improvement are not clear. Additionally, biochar properties can vary significantly with production conditions. Here we characterize the impact of pyrolysis temperature on two important soil hydrologic properties: field capacity and hydrophobicity. We show that pure biochar exhibits a wide range in both properties depending on feedstock and pyrolysis conditions. We find that both properties can be controlled by choice of pyrolysis temperature; 400 °C–600 °C produced biochars with the most desirable hydrological properties (peak field capacity and minimum hydrophobicity). Further, we show that hydrophobicity is strongly correlated (R² = 0.87; p < 0.001) to the presence of alkyl functionalities in FTIR spectra, suggesting that this property derives from aliphatic domains on the surface of low-temperature biochars. Although we could relate hydrophobicity to biochar chemistry, our chemical characterization techniques were insufficient to describe variation in field capacity of soil–biochar mixtures. Field capacity may be related to large biochar pores, suggesting the need for quantitative techniques to characterize large (greater than 0.1 μm) pores within biochar particles.

Highlights

► Pyrolysis temperature and feedstock drive variation in biochar field capacity and hydrophobicity. ► Hydrologic behavior of pure biochars does not necessarily predict the behavior of biochar-amended soils. ► Optimum hydrologic properties of biochar occur at production temperatures between 400 and 600 °C, depending on feedstock. ► A full constraint of biochar hydrologic behaviors requires better tools to constrain pore size, structure, and connectivity.

Introduction

The pyrolysis of biomass to create charcoal for soil amendment is one of only two rapidly implementable C sequestration techniques [1]. Charcoal made for the purposes of soil amendment from biomass is called biochar [2], and the C sequestration capacity of land-applied biochar is estimated to be equivalent to removing 1.8 Pg CO₂ from the atmosphere per year up to a total of 130 Pg CO₂ over 100 years [3].

Biochar has potential advantages beyond soil carbon sequestration, including improved retention of soil nutrients, improved soil hydrologic properties, and higher crop yields [4], [5], [6], [7], [8]. Potential soil improvements depend on the physical and chemical structure of the biochar, which themselves depend complexly on biochar production conditions [9], [10], [11], [12], [13], [14], [15], and the amount of biochar added to the soil [6], [7], [16], [17]. Here we explore the effects of variation in feedstock properties and pyrolysis temperature on the hydrologic behavior of biochar and soil amended with biochar. We focus on two properties: hydrophobicity and field capacity. Field capacity – measured as the amount of water held in a material that has been saturated and allowed to freely drain – is one standard measure of water availability to plants. We selected feedstocks varying in initial chemical composition and in plant structure (woody versus nonwoody biomass) to test the influence of feedstock structure and chemistry on biochar hydrologic properties. We show that both hydrophobicity and field capacity vary significantly with pyrolysis temperature and feedstock. Further, we show that the hydrophobicity of biochar can be rapidly estimated through either chemical or spectroscopic tests.

Section snippets

Biochar production

We produced biochar from dried magnolia tree (Magnolia grandiflora) leaves, apple wood chips (Malus domestica) from commercial suppliers (Texas Smoke, Brownwood, Texas; Allied Kenco, Houston, Texas), and corn (Zea mays) stover collected from Kellogg Biological Station, NSF Long Term Ecological Research Station (KBS LTER, 42°24′37″N, 85°22′45″W). Magnolia leaves were collected as whole, newly-senesced leaves from a suburban yard in Houston in 2009, TX (29°42′38″N, 95°24′31″W). The magnolia

Controls on biochar and biochar + soil field capacities

Biochar field capacity varied as a function of pyrolysis temperature and as a function of feedstock (Fig. 1). We observed more than an order of magnitude difference among samples, from a low of 0.77 g g⁻¹ biochar for magnolia leaf biochar produced at 300 °C to a high of 11.1 g g⁻¹ biochar for corn stover biochar produced at 500 °C (Table 2; Fig. 1). The largest increase in field capacity occurred between 300 °C and 400 °C for apple wood and corn stover biochars. In magnolia leaf biochars, the

Conclusions

We observed a ten-fold variation in the field capacity of biochar samples generated from three feedstocks (corn stover, magnolia leaves, and apple wood) pyrolyzed at a range of final heat treatment temperatures. However, the trends in the field capacity of pure biochars were not consistent with field capacity measurements for biochar + soil mixtures, where modest gains in field capacity (+25–36%) were observed in sandy soils. Biochar hydrophobicity also varied with feedstock and production

Acknowledgments

We acknowledge support from NSF EAR-0911685, NSF EEC-064742, and DOE SUN grant DE-FG36-08GO88073. We appreciated the help of Krystle Hodge in generating elemental analysis data and the comments of two anonymous reviewers.

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¹: Present adrress: Center for Environmental Policy, Bard College, 30 Campus Road, Annandale-on-Hudson, NY 12504, USA

View full text

Hydrologic properties of biochars produced at different temperatures

Abstract

Highlights

Introduction

Section snippets

Biochar production

Controls on biochar and biochar + soil field capacities

Conclusions

Acknowledgments

Field Crop Res

Mar Chem

Org Geochem

J Hydrol

Org Geochem

Soil Biol Biochem

Carbohydr Res

Org Geochem

Soil Biol Biochem

Soil Biol Biochem

Reducing abrupt climate change risk using the Montreal Protocol and other regulatory actions to complement cuts in CO2 emissions

Proc Natl Acad Sci USA

Bio-char sequestration in terrestrial ecosystems – a review

Mitig Adapt Strateg Global Change

Sustainable biochar to mitigate global climate change

Nat Commun

Effects of biochar from slow pyrolysis of papermill waste on agronomic performance and soil fertility

Plant Soil

Maize yield and nutrition during 4 years after biochar application to a Colombian savanna oxisol

Plant Soil

Agronomic values of greenwaste biochar as a soil amendment

Aust J Soil Res

Using poultry litter biochars as soil amendments

Aust J Soil Res

Reducing abrupt climate change risk using the Montreal Protocol and other regulatory actions to complement cuts in CO₂ emissions