Significance of microbial asynchronous anabolism to soil carbon dynamics driven by litter inputs

Abstract

Soil organic carbon (SOC) plays an important role in the global carbon cycle. However, it remains largely unknown how plant litter inputs impact magnitude, composition and source configuration of the SOC stocks over long term through microbial catabolism and anabolism, mostly due to uncoupled research on litter decomposition and SOC formation. This limits our ability to predict soil system responses to changes in land-use and climate. Here, we examine how microbes act as a valve controlling carbon sequestrated from plant litters versus released to the atmosphere in natural ecosystems amended with plant litters varying in quantity and quality. We find that litter quality - not quantity - regulates long-term SOC dynamics under different plausible scenarios. Long-term changes in bulk SOC stock occur only when the quality of carbon inputs causes asynchronous change in a microbial physiological trait, defined as "microbial biosynthesis acceleration" (MBA). This is the first theoretical demonstration that the response of the SOC stocks to litter inputs is critically determined by the microbial physiology. Our work suggests that total SOC at an equilibrium state may be an intrinsic property of a given ecosystem, which ultimately is controlled by the asynchronous MBA between microbial functional groups.

Figure 1. Schematic of the model framework that was used to simultaneously simulate the interactions among dynamics of litter C input (i.e., quantity and quality), dynamics of biomass and necromass of microbial functional groups (i.e., fungi and bacteria), and dynamics of soil non-living C pools (i.e., labile and recalcitrant C).

Figure 2. Modelled responses of SOC dynamics to different plant litter input scenarios.

The simulations were conducted with the assumption that 1) fungal biomass C equals to bacterial biomass C and 2) fungi and bacteria are equally sensitive to the litter input. B – Bacteria; F – Fungi; M – Microbial; Rec – Recalcitrant; P – Plant; LF – doubled quantity of litter input; LQ – lower quality of litter input; HQ – higher quality of litter input.

Figure 3. A stylized illustration integrating modelled results under various scenarios.

It depicts the model simulation results with the assumption that the ratio between fungal and bacterial biomass C is 1.0 at the initial equilibrium state. The filled square (biomass C pool) and circle (non-living C pool) shapes are drawn to scale with the length or diameter derived from the simulated pool sizes shown in Figure 2 and Appendix; however, different scale number was used for the biomass (square shape) and non-living (circle shape) C pools. The ratios (for example, 1.0:1.7) indicate the ratios between plant-derived and microbial-derived non-living C. S₀ denotes the control simulation with no changes in quality and quantity of litter C input; S_t denotes the simulation with change in quantity of litter C input; S_l denotes the simulation with change in quality of litter C input. The red and blue line color denotes the simulation results with positive and negative microbial biosynthesis acceleration (MBA) to litter C input, respectively, and fungi and bacteria have same magnitude (indicated as same filled color) or different magnitude (indicated as different color with darker color for greater magnitude) of such MBA.