How the Birch effect differs in mechanisms and magnitudes due to soil texture

Earth system models predict frequent extreme weather events with more frequent and intense drought and variable precipitation events at regional and global scales (Jentsch et al., 2007; Stocker et al., 2013), resulting in transient soil moisture conditions (Planton et al., 2008). Prolonged periods of drought decrease microbial respiration and carbon (C) losses (Muhr et al., 2010; Manzoni and Katul, 2014), however, wetting of soils after a drought triggers a spike in microbial respiration known as the “Birch effect” (Birch, 1958). The Birch effect contributes to substantial amounts of C losses from soils to the atmosphere (Kim et al., 2011). Despite almost a century of study on the effects of drying and wetting on soil C (e.g., (Lebedjantzev, 1924; Birch, 1958, 1964; Bottner, 1985), conjecture about major drivers and mechanisms of the Birch effect remains.

One of the main causal mechanisms of the Birch effect may be that reduced enzyme activities under drought conditions lead to accumulation of soluble organic C in the soil (Schaeffer et al., 2017; Singh et al., 2021a, 2021b) that is mineralized upon rewetting (Warren, 2016). Several additional mechanisms, however, could potentially increase organic C availability upon rewetting of drier soils (Fig. 1). These mechanisms include, (i) cell lysis due to desiccation under drought releases cellular metabolites which become available upon rewetting of soils (Griffiths and Birch, 1961; Scheu and Parkinson, 1994; Fierer and Schimel, 2002, 2003), (ii) disruption of macroaggregates by repeated drying-rewetting cycles, exposing aggregate-protected C for microbial uptake (Denef et al., 2001; Homyak et al., 2018; Schimel, 2018), and (iii) changes in the bacterial and fungal communities in response to transient moisture conditions (Scheu and Parkinson, 1994). To date, there is very limited information available to support the concept of microbial cell lysis and release of necromass upon rewetting (Schimel, 2018). Contrarily, accumulation of solutes (metabolites) inside microbial cells under drought conditions to counter the solute potential is reported by several studies (Csonka, 1989; Hasegawa et al., 2000; Wood et al., 2001; Slessarev et al., 2020; Slessarev and Schimel, 2020). However, this mechanism is questioned by other studies (Williams and Xia, 2009; Boot et al., 2013; Kakumanu et al., 2013; Warren, 2014). Additionally, disruption of macroaggregates upon repeated drying and wetting tends to release the protected C in soil for microbial uptake (Denef et al., 2001; Cosentino et al., 2006). In reality, all these mechanisms and more unknown variables could simultaneously contribute to the Birch effect and the relative contribution of these known and unknown mechanisms may change based on the soil biophysical properties including texture, structure, soil organic C (SOC) content, and microbiota (Xiang et al., 2008; Sanaullah et al., 2011). Though many studies reported the occurrence of the Birch effect, the specific mechanism(s) have not been investigated across soil types.

Soil texture exerts a major control on SOC turnover during drying-wetting events (Moyano et al., 2012; Barnard et al., 2020; Li et al., 2020; Singh et al., 2021a). The magnitude of the respiration pulse is generally higher in finer-textured soils than coarse-textured soils (Cable et al., 2008) due to increased water holding capacity (WHC) and pore connectivity of fine-textured soils that prolong the moisture availability to microbes (Franzluebbers et al., 2000; Taylor et al., 2002; Singh et al., 2021a). Furthermore, soils with different textures are differently susceptible to aggregate breakdown and release of C substrates under transient soil moisture conditions as aggregation is directly linked to soil texture. The activities of enzymes in different pore size classes can also influence the depolymerization and transfer of substrate to microbes (Singh et al., 2021a). Soil microbiota could also be altered differently in different soils by repeated drying and rewetting, thereby altering the magnitude of C respiration pulse (Meisner et al., 2015). The relative importance of these multiple mechanisms and their interactions with intrinsic soil properties such as soil texture are still a source of uncertainty for projecting the magnitude of C emissions in response to soil moisture extremes (Falloon et al., 2011; Suseela et al., 2012).

The goals of this study were to quantify the Birch effect in differently textured soils and examine the mechanism(s) contributing to this effect under transient state moisture conditions compared to steady state moisture control. The specific objectives of this study were: (i) to assess the sensitivity of SOC dynamics to transient state moisture conditions compared to steady state moisture control in differently-textured soils, (ii) to understand the changes in microbial community structure, metabolite accumulation, and aggregate C release over time in response to transient and steady state moisture contents in differently textured soils, and (iii) to determine the major mechanisms driving the Birch effect in each soil type. We hypothesized that (i) cumulative CO₂ efflux will be higher in transient than steady state moisture conditions, (ii) metabolite accumulation will be greater under dry-phase of the transient moisture condition, leading to elevated CO₂ emissions upon rewetting, (iii) microbial community structure will undergo considerable changes under transient state but not under steady state moisture conditions and these changes will be favorable for the Birch effect, and (iv) frequent changes in soil moisture will break macroaggregates contributing to the Birch effect by exposing physically protected C to decomposition, and (v) different mechanisms primarily contribute to the Birch effect in different soil textures.