With climate change altering precipitation patterns, flood characteristics are expected to increase in many parts of the world, with more land flooding annually and remaining flooded for a longer period (Hirabayashi et al. 2013; Jeong et al. 2013). Flood events saturate the soil which drastically changes the environment and may shift small-scale controls on mineral associated carbon (C), nitrogen (N), and phosphorus (P) persistence (Bhattacharyya et al. 2018; Jilling et al. 2018; Lamparter et al. 2009). The soil mineral associated pool is important for C, N, and P retention with turnover times from decades to centuries and is thought to be largely biologically unavailable (Hemingway et al. 2019; Jilling et al. 2018; Miller et al. 2001; Peretyazhko & Sposito 2005). Yet evidence suggests that the mineral associated pool is more dynamic than previously thought, showing rapid changes due to environmental disturbances like increases in moisture or flooding (Huang & Hall 2017). Such flood events can change how (in)organic matter is partitioned between the mineral associated and soluble pools. However, C, N, and P may be impacted differently by flooding depending on how they are protected within the soil and the rate of microbial C, N, and P uptake. Soil C, N, and P cycling are often linked, where a change in one pool influences the behavior of the others, but their potential varying responses to flooding are not well explained as they are typically examined separately (Gross et al. 2020; Mooshammer et al. 2014).
The uncertainty in how flooding affects the simultaneous and often interdependent responses of C, N, and P pools limits how we predict and understand patterns of soil processes and ecological functioning in response to flooding. For instance, flood-induced changes to the mineral associated C, N, and P pools could change element retention, distribution, and stoichiometry in the soil with subsequent impacts on plant and microbial community C and nutrient availability (Mooshammer et al. 2014; Mori et al. 2018; Tian et al. 2017).
Mineral associated C, N, and P can be mobilized by flooding through three key mechanisms: changes in redox potential, disintegration of aggregates, and increases in biological accessibility. With flooding, soil becomes temporarily waterlogged, often lowering the redox potential (Chen et al. 2019; Shaheen et al. 2021), which can cause mineral associated C, N, and P to desorb into their respective soluble pool (Bailey et al. 2019; Lin et al. 2018). At the same time, flooding can break up aggregates, releasing physically protected elements into the water extracted pool (Fierer & Schimel 2003). Once in solution, C, N, and P are more mobile in the soil pore water which can transport previously protected compounds to biologically accessible areas of the soil or out of the soil and into groundwater (Bailey et al. 2019; Lehmann et al. 2020; Marschner & Kalbitz 2003; Patel et al. 2021).
Following an increase in the concentration and mobility of soluble and biologically available substrates under water saturation, microbial uptake of C, N, or P may increase, with the relative C, N, and P assimilation depending on microbial stoichiometric demands (Buchkowski et al. 2015; Mooshammer et al. 2014). However, more prolonged flooding and the resulting increases in anoxia may limit microbial exo-cellular enzyme production and assimilation of newly biologically available C, N, and P as many microbial species are aerobic (Dyckmans et al. 2006; Unger et al. 2009). During short flood periods when there are still aerobic microsites, microbes may be able to both maintain activity and benefit from the increases in substrate availability.
Differences in flood duration may also affect the magnitude of desorption. During short floods or early flood onset, spatial heterogeneity exists where aerobic and anaerobic soil microsites co-occur (Dorau et al. 2022; Schlüter et al. 2022). Increases in flood duration allows more time for water to percolate, causing greater and more uniform waterlogging throughout the soil matrix (Reddy & DeLaune 2008). As such, longer flood periods should create a more reduced and anoxic soil environment, leading to more desorption of mineral associated C, N, and P. Thus, with longer flooding, higher concentrations of soluble compounds may be a result of both greater desorption as well as inhibition of microbial aerobic respiration and uptake of newly available substrates.
When the soil dries following flood retreat and returns to pre-flood conditions soluble C, N, and P can re-enter the mineral associated pool. However, the disturbance from flooding and drying may affect the C, N, and P mineral pool composition due to changes in both the chemistry and stoichiometry of the soluble pool and resorption dynamics (Guggenberger & Kaiser 2003). For instance, as soil matric potential declines, a release of N- and P-rich cytoplasmic material can occur following cell lysis and the microbial production of N-rich osmoregulatory solutes (Fierer & Schimel 2003; Warren 2016). The soluble C, N, and P concentrations post-flood may also be affected by changes in microbial exo-cellular enzyme activity and C, N, and P microbial assimilation during flooding (Gu et al. 2019). These releases and uptake change the C:N:P of compounds available for resorption to the mineral associated pool. Competition for sorption sites of oxidized mineral surfaces, may further shift the C:N:P ratio of the mineral associated pool compared to pre-flood conditions (Bird et al. 2008; Guppy et al. 2005). Both N and P are thought to outcompete C for sorption sites, driving the outcome of resorption (Hatton et al. 2012; Schneider et al. 2010; Sollins et al. 2006).
While flood events are expected to influence the partitioning of organic matter and nutrients between the mineral associated and bioavailable water extracted pool; it is difficult to predict the impacts of flooding on C, N, and P given the inter-relatedness between pools and among the three elements. Thus, it is necessary to examine how flooding and its duration impact C, N, and P both independently and concurrently throughout a flood period and post-flood to understand the cascading effects of flooding on the soil system. To better characterize and explain flood effects on soil C and nutrients, we examined the partitioning of C, N, and P across soil pools and microbial biomass as well as exo-cellular activity responses during a weeklong flood and after drying in a soil incubation. We hypothesized that as flood duration increases, mineral associated C, N, and P and microbial activity will decrease and in turn soluble C, N, and P will increase. However, post-flood microbial activity will increase relative to the flood period, with longer flood periods showing a greater post-flood microbial response due to higher bioavailable, soluble C, N, and P. Lastly, we expect hysteresis with drying because of changes in the concentration and composition of the water extracted and mineral associated pools during the flood, and thus a net change in post-flood mineral associated C, N and P composition compared to pre-flood conditions.