Biochar alters hydraulic conductivity and inhibits nutrient leaching in two agricultural soils

June 13, 2021

Danielle L. Gelardi (1), Irfan Ainuddin (2), Devin A. Rippner (3), Majdi Abou Najm (1), Sanjai J. Parikh (1)
SOIL, May 2021. DOI: 10.5194/soil-2021-45


Biochar is purported to provide agricultural benefits when added to the soil, through changes in soil water hydraulic conductivity (Ksat), and increased nutrient retention through chemical or physical means. Despite increased interest and investigation, there remains uncertainty regarding the ability of biochar to deliver these agronomic benefits due to differences in biochar feedstock, production method, production temperature and soil texture. In this project, a suite of experiments was carried out using biochars of diverse feedstocks and production temperatures, in order to determine the biochar parameters which may optimize agricultural benefits. Sorption experiments were performed with seven distinct biochars to determine sorption efficiencies for ammonium and nitrate. Only one biochar effectively retained nitrate, while all biochars bound ammonium. The three biochars with the highest binding capacities (produced from almond shell at 500 and 800°C (AS500 and AS800) and softwood at 500°C (SW500)) were chosen for column experiments. Biochars were amended to a sandy loam and a silt loam at 0 and 2% (w/w) and saturated hydraulic conductivity (Ksat) was measured. Biochars reduced Ksat in both soils by 64–80%, with the exception of AS800, which increased Ksat by 98% in the silt loam. Breakthrough curves for nitrate and ammonium, as well as leachate nutrient concentration, were also measured in the sandy loam columns. All biochars significantly decreased the quantity of ammonium in the leachate, by 22 to 78%, and slowed its movement through the soil profile. Ammonium retention was linked to high cation exchange capacity and a high oxygen to carbon ratio, indicating that the primary control of ammonium retention in biochar-amended soils is the chemical affinity between biochar surfaces and ammonium. Biochars had little to no effect on the timing of nitrate release, and only SW500 decreased total quantity, by 27 to 36%. The ability of biochar to retain nitrate may be linked to high surface area, suggesting a physical entrapment rather than a chemical binding. Together, this work sheds new light on the combined chemical and physical means by which biochar may alter soils to impact nutrient leaching and hydraulic conductivity for agricultural production.

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Author Affiliation

(1) Land, Air and Water Resources, University of California, Davis, 1 Shields Ave, Davis CA 95616, USA.
(2) California State University Chico, 400 West First Street, Chico, CA, 95929, USA.
(3) United States Department of Agriculture, Horticulture Crops Research Unit, Davis, Prosser, WA.