Understanding the interdependent cycles of soil carbon, nitrogen and phosphorus during soil saturation events

With climate change, much of the world will experience devastating shifts in weather patterns like increased flooding, intensifying periods of soil saturation. Soil carbon (C), nitrogen (N) and phosphorus (P) cycles are sensitive to changes in soil saturation, where exchange between the mineral-bound and the soluble bioavailable pools can occur with increases in moisture content. With soil saturation, C, N, and P may be mobilized either through greater diffusion or reduced conditions that cause desorption of mineral-bound C, N and P into their respective soluble pools. De-sorption, resorption and diffusion dynamics of C, N, and P may or may not reflect the stoichiometry of the mineral bound pool. Changes in bioavailable soluble C, N and P that could occur with soil saturation and drying may cause unknown consequences for microbial biomass C:N:P. With increases in soil moisture, simultaneous changes in both substrate stoichiometry and microbial growth may occur that impact microbial biomass stoichiometry.  Such changes in microbial stoichiometry and microbial retention of C, N, and P may affect the post-flood fate of soluble C, N, and P. Understanding how releases in mineral bound C, N and P alter the bioavailable C:N:P and how this in turn impacts microbial activity and accumulation of these substrates can inform predictions of retention or losses of C, N and P following soil saturation events.

To determine if mineral-bound, soluble and microbial biomass stoichiometry is maintained or altered during and after soil saturation events, we used a laboratory incubation approach with manipulated soil saturation and duration. Soil incubations were maintained at three water-holding capacity (WHC) levels: 20% (control), 50%, (moderate) and 100% (severe). We maintained the moderate and severe water-logging treatments for  0.5 h, 24 h, 1 week, followed by air-drying to 20% WHC to examine the influence of flood duration. To understand the exchanges of C, N and P between different pools during flooding, we compared changes in soluble and mineral bound soil C, N and P and impacts on microbial C, N, and P exo-cellular enzymes, and microbial biomass C:N:P. Preliminary results indicate that greater soil moisture content increases soluble P and that the 24 hour flood period captures shifts in the mineral bound P pool that do not remain for the longer flood period (1 week). Enzyme activity similarly reflects an increase in microbial activity in the soil held at 50% and 100% moisture content for 24 hours. We also discuss how soil moisture levels and flood duration impact soluble and mineral bound C relative to P, and how microbial biomass C:N:P tracks these fractions. By exploring the combined response of mineral-bound and soluble C, N, and P to variation in soil saturation, we can better understand how different flood scenarios will impact soil C, N and P retention.