Impact of biochar amendments on the quality of a typical Midwestern agricultural soil

Abstract

Biochar, a co-product of thermochemical conversion of lignocellulosic materials into advanced biofuels, may be used as a soil amendment to enhance the sustainability of biomass harvesting. We investigated the impact of biochar amendments (0, 5, 10, and 20 g-biochar kg^− 1 soil) on the quality of a Clarion soil (Mesic Typic Hapludolls), collected (0–15 cm) in Boone County, Iowa. Repacked soil columns were incubated for 500 days at 25 °C and 80% relative humidity. On week 12, 5 g of dried and ground swine manure was incorporated into the upper 3 cm of soil for half of the columns. Once each week, all columns were leached with 200 mL of 0.001 M CaCl₂. Soil bulk density increased with time for all columns and was significantly lower for biochar amended soils relative to the un-amended soils. The biochar amended soils retained more water at gravity drained equilibrium (up to 15%), had greater water retention at − 1 and −5 bars soil water matric potential, (13 and 10% greater, respectively), larger specific surface areas (up to 18%), higher cation exchange capacities (up to 20%), and pH values (up to 1 pH unit) relative to the un-amended controls. No effect of biochar on saturated hydraulic conductivity was detected. The biochar amendments significantly increased total N (up to 7%), organic C (up to 69%), and Mehlich III extractable P, K, Mg and Ca but had no effect on Mehlich III extractable S, Cu, and Zn. The results indicate that biochar amendments have the potential to substantially improve the quality and fertility status of Midwestern agricultural soils.

Introduction

The emerging cellulosic bioenergy industry has been promoted as a means of simultaneously improving energy security, improving weak rural economies, and helping to mitigate the threat of global climate change. Concerns, however, have been raised that the harvesting of crop residues for the production of bioenergy could have adverse impacts on soil and environmental quality (Lal, 2004, Wilhelm et al., 2004, Lal and Pimentel, 2007). The harvesting of crop residue removes substantial amounts of plant nutrients from soil agro-ecosystems. Unless these nutrients are replaced by the addition of synthetic fertilizers, manure, or other soil amendments the productivity of the soil will decline. Even if synthetic fertilizers are added to replace the removed nutrients, the sustained removal of crop residues without compensating organic amendments will cause a decline in levels of soil organic matter, which will lead to degradation of soil structure, a decline in cation exchange capacity, a decline in the capacity of soils to hold nutrients and water, and ultimately a decline in soil productivity (Wilhelm et al., 1986).

The loss of soil organic matter also indicates the loss of soil organic C to the atmosphere as CO₂, and hence the necessity of discounting any C offset credits accrued from biofuels displacing fossil fuels. Furthermore, the removal of above ground residue leaves the soil surface vulnerable to raindrop impact, which increases surface crusting, restricts infiltration of water, and increases surface runoff and erosion (Blanco-Canqui and Lal, 2009). Runoff, erosion and the leaching of nutrients not only degrade soil quality but also adversely impact the quality of water in streams and reservoirs. Thus the emerging cellulosic bioenergy industry will not be sustainable unless new agronomic systems are also deployed that enhance the amount of C that is retained by the soils from which biomass feedstock is harvested.

Application of biochar, a co-product of the pyrolysis platform for transforming lignocelluloses biomass into liquid energy products, to the soils from which biomass was harvested has been proposed as a key component of a potentially sustainable integrated agronomic-biomass–bioenergy production system (Fowles, 2007, Lehmann, 2007, Laird, 2008). During pyrolysis most of the Ca, Mg, K, P, and plant micronutrients, and about half of the N and S in the biomass feedstock are partitioned into the biochar fraction. Thus using the biochar as a soil amendment returns most of those nutrients to the soils from which they came. Biochar also increases the capacity of soils to adsorb plant nutrients (Liang et al., 2006, Cheng et al., 2008) thereby reducing leaching losses of nutrients. Biochar has been shown to decrease soil bulk density, and increase cation exchange capacity, nutrient cycling, and the ability of soils to retain plant available water. Thus the use of biochar as a soil amendment is anticipated to increase both nutrient and water use efficiency and thereby crop productivity (Glaser et al., 2001, Liang et al., 2006). Indeed several reports indicate that soil biochar applications increase crop yields (Iswaran et al., 1980, Kishimoto and Sugiura, 1985, Marjenah, 1994, Yamato et al., 2006).

The C content of biochar varies from < 1 to > 80%, depending on the nature of the feedstock and the thermal–chemical process employed (Antal and Grnli, 2003, Spokas and Reicosky, 2009). In general, the C in biochar is very stable in soil environments (Schmidt et al., 1999, Glaser et al., 2002, Kuzyakov et al., 2009, Lehmann et al., 2009). Radio C dates of naturally occurring wildfire chars in soils are often measured in 1000 s y.b.p. (Skjemstad et al., 1998, Pessenda et al., 2001, Swift, 2001, Preston and Schmidt, 2006). By contrast, the half-life of C in plant and animal residues if returned directly to the soil is measured in weeks or months. Thus the transformation of biomass C into stable forms of biochar coupled with soil application of the biochar is a system that effectively removes CO₂ from the atmosphere through photosynthesis and sequesters the C in soils for millennia. Furthermore, there are several reports indicating that soil biochar applications reduce emissions of N₂O and CH₄ from soils either by preventing the formation of these potent greenhouse gasses or by enhancing their oxidation after the gasses have formed (Yanai et al., 2007, Spokas and Reicosky, 2009).

A key advantage of soil biochar applications is that C offset credits can be easily and accurately quantified based on the amount of biochar C applied to the soil and the stability of the biochar C. Soil biochar applications may also qualify for less easily quantified C offset credits based on reductions in N₂O and CH₄ emissions, increase crop productivity and/or reductions in agricultural inputs due to increased fertilizer and water use efficiency (Laird et al., 2009). Because of C offset credits accrued through soil biochar applications, bioenergy produced through an integrated biomass–bioenergy–biochar platform, may be viewed as C-negative energy and there is a potential for such a system to result in agrading soil quality rather than degrading soil quality.

Much of the previous work on the impact of biochar on soil quality has been conducted in the tropics. The highly weathered Oxisols and Ultisols of the tropics intrinsically have low nutrient retention capacity due to a dominance of Fe- and Al-oxides and 1:1 phyllosilicates in the clay fraction. By contrast, Midwestern Mollisols are typically dominated by 2:1 phyllosilicates clays, have higher levels of soil organic matter, and higher nutrient and water holding capacities. Here we test the hypothesis that soil biochar amendments will enhance the quality of a typical Midwestern Mollisol by quantifying the impact of biochar and manure amendments on various soil quality indicators using a soil column leaching/incubation study. A companion paper (Laird et al., 2010) reports the leaching of nutrients from the same soil columns.