Biochar increases arbuscular mycorrhizal plant growth enhancement and ameliorates salinity stress

Highlights

• AM fungi and biochar had a moderately additive effect on plant yield.

• AM fungi and/or biochar treatment improved plant nutrition, especially P, K and Mn.

• Biochar alleviated salinity stress in plants, likely via salt sorption.

Abstract

We examined combined effects of biochar, arbuscular mycorrhizal (AM) fungi and salinity on plant growth and physiology to test whether and how biochar influences AM fungi mediated growth and nutrition enhancements, and whether and how biochar provides amelioration in salt stressed soils. We carried out a full three-factorial greenhouse experiment with Lactuca sativa; and a second study with a wider range of biochar and salt additions to examine physicochemical effects on soil parameters. Biochar together with AM fungal inoculation resulted in an additional plant yield increase compared to each alone under non-saline conditions. In parallel with increased plant growth, we found increased uptake of P and Mn with AM fungi and biochar addition, but to a lesser extent than biochar-induced growth promotion. Both factors, but especially biochar alleviated salinity-caused growth depressions, and improved Na/K ratio in salinity stressed plants. Reduced Na uptake of plants and reduced conductivity in biochar-ameliorated soils suggest that a likely mechanism involves ion adsorption to biochar surfaces. Our results suggest that plants depend on symbiotic microorganisms to fully exploit biochar benefits in soils, suggesting avenues for joint management in agriculture. Biochar may be advantageous in saline soils, but long-term studies are required before recommendations should be given.

Introduction

Biochar is a collective term for carbon rich soil amendments of pyrolyzed organic material (Lehmann and Joseph, 2015). Biochars decompose very slowly, and their addition to soils is therefore thought to be an effective way of storing carbon to curtail the increasing concentration of CO₂ in the atmosphere (Sohi, 2013). Despite numerous observations in both field and controlled experiments showing that plant yield increases due to biochar additions (Chan and Xu, 2009, Graber et al., 2010, Jeffery et al., 2011, Lehmann et al., 2011, Van Zwieten et al., 2010), the exact mechanistic background of biochar effects is not known. Biochars affect soil properties differently depending on feedstock and pyrolysis conditions, strongly differing in pH, nutrient contents and ion exchange capacities (Lehmann and Joseph, 2015). Plant responses to biochars may also be mediated via feedback through biochar effects on soil microbes (Thies et al., 2015). Very little is known about the interactions of biochar with root symbionts such as arbuscular mycorrhizal (AM) fungi. AM fungi provide their host plants with mineral nutrients and receive photosynthetically derived carbohydrates in return (Smith and Read, 2008). The few studies of the effects of biochar on mycorrhiza have mainly considered root colonization and show variable results; some researchers report root colonization levels to be strongly enhanced by biochar (Ishii and Kadoya, 1994, Blackwell et al., 2010), whereas others present evidence that colonization is diminished (Birk et al., 2010, Warnock et al., 2010). Biochars, also those produced from wood materials, typically have a large surface area and cation exchange capacity (CEC), and therefore, its addition to soils also increases the CEC of the soil (Joseph et al., 2009, Blackwell et al., 2010). They can adsorb ions (Liang et al., 2006) and prevent leaching of nutrients (Yao et al., 2011). However, those nutrients may not necessarily be available to plants, as most roots are unable to access the fine porous structure of the biochar due to their size, as the finest part of the roots of most plant species are of several hundred μm in diameter (Fitter, 2002). AM fungal hyphae are much finer in diameter and can re-capture some of the adsorbed nutrients and return them to their host plants (Hammer et al., 2014). By this, a combined management of AM fungi and biochar may lead to more closed nutrient cycles and more efficient fertilizer usage. However, this has not yet been shown empirically. Our first aim in this study is thus to examine whether possible biochar-related enhancement of plant performance and mineral nutrition can be further increased by the presence of AM fungi.

Our second aim is to test whether biochar ameliorates salinity stress in plants, and whether it also may interact with AM fungi under these conditions. It is estimated that more than 7% of the Earth’s land is occupied by saline soils, especially in semiarid and arid regions (Tester and Davenport, 2003), leading to major constraints on agricultural production (Dodd and Pérez-Alfocea, 2012). AM fungi are known to ameliorate salinity stress in plants (Evelin et al., 2009, Porcel et al., 2012) by increasing phosphorus or general nutritional status of the plant (Giri et al., 2007), improving water supply (Ruiz-Lozano and Azcón, 2000), and by aiding nutrient discrimination as K⁺ over Na⁺ ions (Hajiboland et al., 2009, Hammer et al., 2011). Little is known about the interaction of biochars with soil salinity. It was advised to use biochar with caution in soils as certain kinds of biochars themselves, like those derived from poultry feedstocks, contain a high amount of salts (Novak et al., 2009, Lehmann et al., 2011, Revell et al., 2012). Biochar is known to sequester inorganic and organic contaminants due to its ionic exchange, electrostatic interactions or specific ligand binding (Ippolito et al., 2012, Xu et al., 2012). A study by Thomas et al. (2013) found that their biochar (hardwood sawdust) adsorbed significant amounts of added salt in a field experiment. Thus, it might be that biochar immobilizes salt ions in moderately saline soils, or that it may create non-saline microsites for enhanced nutrient uptake.

We tested in a fully factorial setup the effects of biochar, AM fungi and salt addition on growth and performance of Lactuca sativa in a greenhouse experiment. We hypothesized that biochar and AM fungi will interact additively on plant growth and nutrition, i.e. that the combination of both treatments will result in a higher plant growth than each single factor. We further hypothesized that biochar, as AM fungi, will ameliorate salinity stress in lettuce plants.