Effect of biochar addition on soil microbial community in a wheat crop

Highlights

• The impact of biochar on soil microbial biomass, activity and diversity was evaluated.

• Two loads of biochar incorporation in soil and two exposure times were considered.

• Microbial activity increased 3 months after treatment.

• After 3 months functional diversity changed, but not bacterial genetic diversity.

• No effect of biochar was found 14 months after treatment.

Abstract

Biochar is known to enhance soil fertility and C sequestration, but relatively little information is currently available about its effect on soil microbial community, a component of terrestrial ecosystems that plays a key role in nutrient cycling. This study tested the effects of soil amendment with two loads of wood-derived biochar (30 and 60 t ha⁻¹) in a wheat crop in Tuscany (Italy). Soil samples were collected 3 and 14 months after treatments over two successive growing seasons, and analysed for pH, total organic C (C_org), extractable C (C_ext), microbial biomass-C (C_mic), 25 specific microbial activities, mean substrate-induced respiration (mSIR) for 25 substrates, functional microbial diversity and bacterial genetic diversity. No significant effect of biochar treatment was observed on C_org, C_ext, C_mic, microbial quotient (C_mic % C_org) or genetic diversity. An increase in mSIR, some specific microbial activities and soil pH, and a significant change in functional diversity were observed 3 months after treatment. In contrast, no effect of biochar was detected 14 months after treatment for the parameters considered, except for a small but significant increase in pH. Our data suggest that biochar addition stimulated soil microbial activity without causing any apparent disturbance, but this positive effect was very short-lived.

Introduction

Biochar is produced by low-temperature (400–500 °C) pyrolysis of biomass (such as manure, organic waste, bioenergy crops, crop residues) in oxygen-free or low-oxygen environment. After pyrolysis of the biomass, about 50% of the original biomass C remains in the biochar [1]. Used as a soil amendment, biochar can enhance nutrient availability to plants and improve physical and biological properties of the soil [2], [3], [4], [5], [6]. Moreover, because of its molecular structure dominated by aromatic C blocks, biochar is much more resistant to microbial decomposition than uncharred organic matter [7] and can persist in the soil from 1000 to 10,000 years [8], thus increasing soil C storage [9]. In addition, biochar might improve soil fertility on the long term, mainly by indirect effects, such as increase of cation exchange capacity, surface area and water retention in soil pores, which decreases nutrient leaching, and, to a minor extent, as a consequence of direct nutrient release [2], [10]. The use of biochar has been advocated not only in addressing the widespread problem of loss of soil organic matter and consequent reduction of fertility, but also as a manageable option to dispose of organic waste and favour C sequestration in soil [11].

Beneficial effects of biochar application on crop yield have been documented [12], [13], [14], but specific effects on the soil microbial community are still poorly explored. Yet, soil microorganisms play a central role in nutrient cycling and provide thus an important ecosystem service [15]. Biochar, or at least some labile biochar compounds, has been found to be a potential source of organic matter and inorganic nutrients for microorganisms [16] and can act as a refuge protecting microbes from predators [8]. In fact, biochar pores may be below 5 μm in diameter [17], thereby being accessible to bacteria and fungal hyphae but not their larger predators such as mites and collembolans or most nematodes and protozoans [18], [19]. In addition, biochar can bind toxic compounds such as heavy metals, polycyclic aromatic hydrocarbons and organic pesticides, thus reducing their bioavailability [20], [21], [22], [23], [24]. However, the current information on the effect of biochar on microbial community is not univocal. Organic pyrolytic products, such as phenolics and polyphenolics, may be present in biochar and negatively affect soil microorganisms [25]. Negative, null or positive effects of biochar on soil microbial community have been reported depending on the biochar employed and type of soil [11], [16], [26], [27], [28], [29]. Moreover, only few of these studies have been performed in the field [26], [29], where usually a greater number of factors interacts compared to laboratory conditions. The topic clearly demands further investigation to be performed in the field considering specific biochar, soil and plant cover types. A further aspect that needs consideration is how long the impact of biochar addition, if evidence is provided of positive or negative effects, lasts in time. With the aim to address the above questions, the present study was performed in an agricultural soil in the Mediterranean area, with a focus on microbial quotient (microbial C % organic C), microbial activities, and microbial functional and genetic diversity. These parameters are known to vary in response to several environmental factors including plant cover, land use, agricultural management, pH, salinity, heavy-metal contamination and fire [30], [31], [32], [33], [34], [35], [36], [37], [38], and only rarely have been considered simultaneously in field studies on biochar [29]. Moreover, no data are currently available on the topic for Mediterranean crop soils. This research is a part of a larger study also including biochar effects on crop yield [14] and on greenhouse gas fluxes from soil [39].