Biochar alters soil microbial communities and potential functions 3–4 years after amendment in a double rice cropping system

Highlights

• Straw biochar increased the abundances of paddy soil bacteria and fungi 3–4 years after amendment.

• Biochar amendment decreased soil bacteria/fungi ratios.

• Biochar improved soil bacterial composition to favor TOC accumulation.

• Biochar improved soil fungal composition to favor plant growth and TOC degradation.

• Biochar reduced fungal abundances of plant pathogen in paddy soil.

Abstract

The effects of biochar application on soil microbial communities and functional characteristics and their correlations with soil fertility properties were explored in a double rice cropping system three to four years after a single biochar amendment. Three treatments including a control, a low (24 t ha⁻¹), and a high (48 t ha⁻¹) application rate of straw-derived biochar were constructed. Biochar amendment significantly increased the abundance of bacteria and fungi by up to 102 % and 178 %, respectively, which might be probably caused by the increases in soil total organic carbon (TOC), total nitrogen, and rice biomass as compared with the control. However, the abundance of archaea was only slightly elevated after biochar amendments. Bacteria/fungi ratios were significantly decreased by up to 61.4 % in the biochar treatments, probably because fungi were the dominant decomposers of increased recalcitrant carbon from biochar and rice biomass. Biochar stimulated the relative abundance of Acidobacteria, which favours soil organic carbon accumulation. Biochar increased the relative abundances of Mortierella and Westerdykella, which are more beneficial to plant growth and TOC degradation. Furthermore, potential phytopathogens of Athelia and Penicillium were decreased with biochar amendment. The results demonstrate that biochar application should be sustained as an effective measure for improving the microbial characteristics of paddy field by ameliorating its soil properties.

Introduction

Biochar, a recalcitrant, carbon-rich material produced via biomass pyrolysis under oxygen-limited conditions, can store carbon in soils on a millennium scale and is porous and alkaline (Lehmann and Joseph, 2015). It has extensive potential application in improving soil nutrient cycling (Haider et al., 2017; Lehmann and Joseph, 2015). Biochar contains available nutrients and can retain soil nutrients by its functional categories, pore structure, and high cation exchange capacity, as well as change microbial communities and increase microbial activity and abundance (Lehmann and Joseph, 2015; Ye et al., 2017). In a 140 d field experiment, Yang et al. (2019) observed that biochar addition enhanced soil organic carbon, ammonium (NH₄⁺-N), dissolved organic carbon (DOC), total nitrogen (TN), and rice yields in paddy fields. Other studies have reported increased soil mineral N content (NH₄⁺-N and nitrate (NO₃⁻-N)), and plant biomass in one year field experiments (Nelissen et al., 2015). Haider et al. (2017) found that the pH value and cation exchange capacity for biochar treatments decreased with biochar aging in the soil in a four-year field experiment. Similarly, Wang et al. (2018a) reported that biochar amendment increased soil DOC, NH₄⁺-N, NO₃⁻-N, microbial biomass N, and C in the first year after biochar addition, but there was no significant effect after three years. Biochar application has also been shown to increase soil total organic C (TOC), TN, and total phosphorus (TP) content in a four year field experiment, owing to the accumulation of native organic matter in the paddy soil by stabilizing rhizodeposits (Weng et al., 2017). Consequently, the impacts of biochar application on soil fertility change with biochar ageing in soils. Because the labile components in aged biochar are exhausted, the long-term biochar effects on soil fertility can be investigated by studying field-aged biochar.

Microorganisms play a critical role in maintaining and improving soil fertility through biogeochemical processes (Sahu et al., 2017). Biochar can affect the soil microbial community by changing soil physiochemical properties, using biochar as a source of mineral nutrients or energy, and changing the soil-plant-microbe feedback loop (Ameloot et al., 2013). For example, the addition of anthropogenic or wildfire-produced biochar enhanced the microbial abundance and activity in a loamy sand soil in an incubation experiment, which might be due to the increases in microbial habitat and available carbon (Kolb et al., 2009). As reported by Chen et al. (2013), the addition of biochar to a paddy field for 1.5 years increased the bacterial abundance by 28 %–64 %, but decreased the fungal abundance by 35 %–46 %, thus strongly affecting bacterial and fungal community structures; this might be due to the stimulation of bacterial growth and inhibition of fungal growth by neutral or slightly alkaline soil conditions. Furthermore, biochar increased the native soil organic carbon in paddy soils, which might be due to the decreases in microbial mineralization rates of native soil organic carbon caused by stabilized rhizodeposits from microstructures (Weng et al., 2017). Based on the 16S rRNA gene sequencing, Nan et al. (2020) found that the low rate application (8 t ha⁻¹ yr⁻¹) of biochar annually enhanced the complexity and stability of the soil bacterial community structure by increasing soil TOC content.

The impact of biochar amendment on the functional characteristics of microbial community have been studied. Zhou et al. (2019) showed that biochar addition increased the amino acid, carbohydrate, and energy metabolism, and changed the bacterial community by increasing the pH value and TN content in a 45 d incubation experiment, which accelerated the decomposition and transformation of pig manure and rice straw. Dai et al. (2018) found that the easily mineralizable C in biochar increased soil saprotroph abundance in a 150 d incubation experiment, leading to a relative decrease in soil fungal pathogens. The bacterial and fungal communities and functional characteristics affected by biochar remain poorly understood in durations longer than two years after a single application.

Paddy fields account for approximately 20 % (165 million ha) of the global irrigated croplands and provide the main source of food for more than 50 % of the population on earth (FAO, 2016). Paddy soils have been experiencing degradation due to long-term application of synthetic fertilizers during recent decades, thus becoming a serious threat to food security (Godfray, 2015). Biochar was tentatively applied into paddy fields and was effective in increasing soil fertility and improving soil health in field experiments (Nan et al., 2020; Wang et al., 2018a). Biochar properties (e.g. aromatic moieties, labile fractions, and physical structures) and its effects on soil properties vary with time, which may further induce microbial community change over time (Jones et al., 2011; Wang et al., 2019). For example, Jones et al. (2011) revealed that the rate of microbial respiration was increased by the easily degradable materials in fresh biochar but was decreased by 3 y aged biochar at 15, 20, and 25 °C in incubation experiments, which might be due to the increase in the soil aggregate stability (Chen et al., 2018b). Chen et al. (2013) found decreased fungal abundance because of the increase in soil pH 1.5 years after a single biochar amendment. In contrast, Yao et al. (2017) observed that aged biochar enhanced soil fungal abundance in the third year after a single biochar addition, which might be caused by the increased soil total C, TN, NO₃⁻-N, available potassium, and water content. However, most recent studies were obtained using a few samples limited only to the first two years after a single biochar application. Therefore, the impacts of biochar aging on the bacterial, archaeal, and fungal communities and on their potential functions are still not fully known (Ladygina and Rineau, 2013; Lehmann and Joseph, 2015).

In the current study, it was hypothesized that aged biochar could alter the abundance, community composition, and potential functions of soil bacteria, archaea, and fungi by changing soil fertility properties after a single application in paddy fields. To test this, the impacts of biochar application on the soil fertility properties, abundance, community composition, and functional categories of bacteria and fungi three to four years after a single application (2014–2016) in a typical double rice paddy were investigated.