Strong priming of soil organic matter induced by frequent input of labile carbon

Highlights

• Kinetic approaches link microbial functional traits to rare fungal taxa.

• Frequent carbon (C) inputs doubles CO₂ losses by priming compared to seldom addition.

• Frequent C inputs increase microbial turnover and decreased C use efficiency.

• Frequent C inputs shift fungal community to K-strategy fungal species.

Abstract

Labile carbon (C) inputs to soil (e.g., litter and root exudation) can prime soil organic matter (SOM) decomposition, and strongly influence SOM dynamics. The direction and intensity of priming, as well as the net C balance in soil, depend on the amount and frequency of labile C inputs. Most recent priming studies are based on single C additions, which are not truly representative of common litter inputs or root exudation in terrestrial ecosystems. Here, we evaluated the effects of ¹⁴C-labeled glucose addition to soil in the same final amounts (360 μg C g⁻¹) split into two temporal patterns: seldom (20% of microbial biomass every two months) and frequent addition (4% of microbial biomass every 10 days) on the dynamics of CO₂ production and SOM priming over a 200-day incubation. For the first time, we combined enzyme kinetics with substrate-induced growth respiration and fungal diversity to monitor microbially mediated SOM mineralization in response to the labile C input frequency. Frequent glucose addition decreased ¹⁴C incorporation into microbial biomass and almost doubled cumulative priming compared to seldom addition, resulting in a net loss of SOM for seldom and frequent C additions of −94 and −367 μg C g⁻¹ respectively. Larger priming loss of SOM with frequent C inputs was accompanied by increased activities of β-glucosidase, chitinase, and acid phosphatase, and by a shift in fungal community towards increased abundance of K-strategist fungal species (mainly Mortierellales sp. and Trichoderma sp.) capable of SOM mineralization. In conclusion, frequent labile C inputs (e.g., rhizodeposits in rhizosphere or litterfall in disturephere) to soil will stimulate a shift in fungal community structure and functions, resulting in intensive priming of SOM decomposition and CO₂ losses.

Introduction

The stability of carbon (C) stored in soil is dependent on an equilibrium between C inputs through assimilated CO₂ and C returned to the atmosphere through microbial and root respiration (Todd-Brown et al., 2013; Wieder et al., 2013). Plant growth and C inputs (e.g., plant litter, root exudates) to soils are predicted to increase under climate warming and elevated CO₂ (Bai et al., 2013; Yin et al., 2013), which may increase soil C storage (Zang et al., 2019). However, labile C inputs can promote microbial growth and affect the stability of soil C. This is known as the ‘priming effect’ (Kuzyakov, 2010; Blagodayskaya et al., 2014) and it is large enough to influence soil organic matter (SOM) dynamics and stocks (Wieder et al., 2013). Under natural conditions, the priming effects are dependent on the frequency and amount of labile C inputs, such as ongoing inputs via rhizodeposits or litter decomposition (Kuzyakov, 2010; Qiao et al., 2014). Most studies to date have only considered single substrate pulse additions, thus, limiting the potential to represent realistic priming effect mechanisms under real soil C conditions (Ostle et al., 2009).

Single C additions may not be representative of natural C inputs in terrestrial ecosystems, which continuously receive C via root exudates, slowly decomposing dead roots, leaves and shoot residues (Kuzyakov, 2010). To date, nevertheless, only a few studies have investigated the effects of labile C addition frequency on the priming effect, and these have resulted in conflicting conclusions (Mau et al., 2015; Wang et al., 2019). For example, Qiao et al. (2014) found that single labile C addition in forest soils induced a stronger priming effect than repeated inputs. However, another study found the opposite result which was explained by the stimulation of bacterial diversity with repeated C inputs (Mau et al., 2015). Specifically, if the soil is very densely rooted (such as in the upper few cm of grassland soil), the rhizodeposition input is more or less continuous and individual hotspots are joined to form large zones of high activity (i.e. grass rhizosphere). Therefore, a comparison of single and repeated labile C additions in grasslands is needed to reveal microbial mechanisms of priming-induced SOM decomposition.

Despite the great diversity of the soil microbial community, recent priming effect studies have mainly focused on bacteria-driven SOM decomposition (Six et al., 2006; Fontaine et al., 2011; Mau et al., 2015; Morrissey et al., 2017). However, fungi have extensive roles in SOM turnover due to their greater ability to depolymerize complex compounds (i.e. lignin) through enzyme production (Nannipieri et al., 2003; Blagodatskaya and Kuzyakov, 2008; Schneider et al., 2012; Zhou et al., 2020). Based on their C source utilization patterns, many known functions of fungi are also mediated by specific guilds. For example, saprotrophs are regarded as decomposers related to recalcitrant SOM, whilst mycorrhizal fungi are the symbionts which acquires C from host plants rather than from SOM (Johnsen et al., 2001; Eldridge and Delgado-Baquerizo, 2018). Changes in fungal community structure have been detected during priming induced by cellulose (Fontaine et al., 2011), plant residues (Bernard et al., 2007; Nottingham et al., 2009), and biochar (Yu et al., 2018; Song et al., 2020). Despite the evident driving role of fungi in the priming effect (Fontaine et al., 2011), most studies considering the fungal community have focused on a single substrate input. Thus, the question, how does frequent labile C inputs affect priming intensity, fungal community structure and functional diversity, has not been addressed. We hypothesized that multiple glucose additions would stimulate a more diverse fungal community with a relative increase in SOM-utilizing guilds, causing greater positive priming (Fontaine et al., 2003).

The net balance of plant C inputs and primed SOM determines the direction and magnitude of SOM stocks. Increased C inputs to soils may reduce SOM storage due to the priming effect (Shahbaz et al., 2017; Zang et al., 2019), resulting in a positive feedback to climate change (van Groenigen et al., 2014). Besides that which is respired as CO₂, a fraction of the added labile C will be stabilized within the soil C pool (Hoosbeek et al., 2007; Liang et al., 2017; Cui et al., 2020), either by incorporation into living microbial cells or microbial residues (i.e. necromass), thus counterbalancing the priming effect (Liu et al., 2017; Finley et al., 2018; Liang et al., 2018; Cui et al., 2020). Quantifying the balance between retained C and primed SOM loss is therefore critical when investigating the influences of the priming effect on SOM dynamics. To date, most priming studies have emphasized SOM losses and have not reported the net SOM balance between primed C lost and labile C retained (Kuzyakov, 2010; Liu et al., 2017). Therefore, despite the overall importance of priming, the net effects of labile C inputs on SOM dynamics and stabilization remain unclear, especially when considering the frequency of labile C inputs.

Here, we performed large but seldom (once every two months; 20% microbial biomass C) and frequent but small (five times every two months; 4% microbial biomass C) glucose additions over 200-days incubation, to clarify whether labile C input frequency affects SOM priming and soil C balance. Each round (i.e. every two months) the same total amount of glucose was added to the soil. We hypothesized that: (1) although frequent very small C additions would not increase microbial biomass, it could activate a diverse fungal community similar to the stimulated bacterial taxa as reported by Mau et al. (2015); and (2) the net C balance between primed C losses and retention of added glucose-C would be positive and accelerate SOM decomposition, irrespective of addition frequency.