Abstract
Abstract. Biochar is purported to provide agricultural benefits when added to the soil, through changes in saturated 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 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 micropore specific 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.
Highlights
The ability of biochar to chemically and physically alter soil environments for specific agronomic benefits is the subject of increased investigation, as evidenced by the recent rise in published biochar studies (Web of Science, 2021) and United States trademark and patent applications listing the word “biochar” (US Patent and Trademark Office, 2021)
Biochars were sieved to 2 mm and characterized using procedures recommended by the International Biochar Initiative (2015): pH and electrical conductivity (EC) were measured at a 1 : 20 biochar to 18.2 M cm water (Barnstead Nanopore, Thermo Fisher) dilution (w : v) after solutions were shaken for 90 min; total carbon, nitrogen, hydrogen, and oxygen were measured using a dry-combustion elemental analyzer (Costech ECS4010); and moisture, volatile, and ash content were measured as a percent of total dry weight through sequential shifts in furnace temperature (ASTM D 176284, 2011)
That SSAμp measured by CO2 adsorption frequently results in higher values than surface area measured by N2, as CO2 can access micropores unavailable to N2 (Maziarka et al, 2021; Zeng et al, 2013)
Summary
The ability of biochar to chemically and physically alter soil environments for specific agronomic benefits is the subject of increased investigation, as evidenced by the recent rise in published biochar studies (Web of Science, 2021) and United States trademark and patent applications listing the word “biochar” (US Patent and Trademark Office, 2021). Biochar properties typically include a low bulk density, high porosity, high surface area, reactive surface functional groups, and recalcitrant carbon (Downie et al, 2009). These attributes make it a promising material for amendment to agricultural soils, as biochar may help improve soil water-holding capacity, hydraulic conductivity, and nutrient retention. Several authors have concluded that, due to differences in biochar production parameters and those of the soil environment, material- and site-specific investigation is required before conclusions can be drawn about the potential of biochar to provide agricultural benefits (Hassan et al, 2020; Jeffery et al, 2011; Zhang et al, 2016)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
