Hydrologic properties of biochars produced at different temperatures
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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Present adrress: Center for Environmental Policy, Bard College, 30 Campus Road, Annandale-on-Hudson, NY 12504, USA