Flexible Carbon-Use Efficiency across Litter Types and during Decomposition Partly Compensates Nutrient Imbalances—Results from Analytical Stoichiometric Models

Abstract

Mathematical models involving explicit representations of microbial processes have been developed to infer microbial community properties from laboratory and field measurements. While this approach has been used to estimate the kinetic constants related to microbial activity, it has not been fully exploited for inference of stoichiometric traits, such as carbon-use efficiency (CUE). Here, a hierarchy of analytically-solvable mass-balance models of litter carbon (C) and nitrogen (N) dynamics is developed, to infer decomposer CUE from measured C and N contents during litter decomposition. The models are solved in the phase space—expressing litter remaining N as a function of remaining C—rather than in time, thus focusing on the stoichiometric relations during decomposition rather than the kinetics of degradation. This approach leads to explicit formulas that depend on CUE and other microbial properties, which can then be treated as model parameters and retrieved via nonlinear regression. CUE is either assumed time-invariant or as a function of the fraction of remaining litter C as a substitute for time. In all models, CUE tends to increase with increasing litter N availability across a range of litter types. When temporal trends in CUE are considered, CUE increases during decomposition of N-poor litter cohorts, in which decomposers are initially N-limited, but decreases in N-rich litter possibly due to C-limitation. These patterns of flexible CUE that partly compensate stoichiometric imbalances are robust to moderate shifts in decomposer C:N ratio and hold across wide climatic gradients.

Constraining Carbon and Nutrient Flows in Soil With Ecological Stoichiometry

“…Analytical models of SOM decomposition use different strategies to constrain the cycling of nutrients through microbial biomass. Many models combine the stoichiometric imbalance between organic matter, microorganisms, and their exoenzymes with flexible microbial CUE (Manzoni and Porporato, 2009;Moorhead et al, 2012Moorhead et al, , 2013Manzoni, 2017) and overflow respiration (Schimel and Weintraub, 2003). Others represent stoichiometric biases in SOM decomposition by shifting microbial community composition or microbial community homeostasis (Sinsabaugh and Shah, 2012;Sinsabaugh et al, 2013Sinsabaugh et al, , 2015Waring et al, 2013;Warton et al, 2015;Hartman et al, 2017).…”

Evidence for preferential protein depolymerization in wetland soils in response to external nitrogen availability provided by a novel FTIR routine

“…7). High α values between 1.38 and 1.82 in an environment which provides very little external N are rational and not exceptionally high in relation to α values of 1.3 assumed in some stoichiometric models (Manzoni, 2017). Yet, the increased protein depolymerization activity is accompanied by very low nNUEs (Table 2).…”

“…In order to place the microbial N content of different samples into a relationship with one another and to take variations in C loss into account, we assumed microbial homeostasis and a microbial C/N ratio of 5 (Mouginot et al, 2014). The thereby determinable total amount of newly formed microbial C was set in relation to the litter C loss, which leads to an estimate of the CUE (Manzoni, 2017). The CUE was determined as…”

“…In order to characterize the variation between C depolymerization and N depolymerization, we use the coefficient of preferential protein depolymerization (α). This parameter is the ratio depolymerized N/depolymerized C and is adopted from recent stoichiometric models which introduce this term as the coefficient of preferential N uptake (Manzoni et al, 2010;Manzoni, 2017). Our study for the first time provides empirical data for α. α ranged from 0.74 to 1.82 did not depend on the initial litter N content (R 2 = 0.004, p > 0.05) but appeared to be a decomposition site property, which correlated negatively with the N content of the soil in which the litter decomposed (R 2 = 0.76, p < 0.001) ( Fig.…”

A Mechanistic Model of Microbially Mediated Soil Biogeochemical Processes: A Reality Check

“…Numerical simulations were carried using the ecosystem model T&C (Fatichi et al, , , ; Fatichi & Pappas, ; Mastrotheodoros et al, ; Manoli et al, ; Paschalis et al, , ; Pappas et al, ) combined with new modules simulating soil biogeochemistry and plant nutrient dynamics (T&C‐BG) described in the following and extensively in the supporting information: Figure S1, Texts S1 and S2, and additional references in the supporting information (Ainsworth & Long, ; Batterman et al, ; Chapin et al, ; Curry & Schmidt, ; Daly et al, ; Daly & Porporato, ; Ebrahimi & Or, ; Friend et al, ; Farquhar et al, ; Hassink & Whitmore, ; Hanson et al, ; Jackson et al, ; Jungk, ; Kögel‐Knabner, ; Moorhead & Sinsabaugh, ; Moyano et al, ; Manzoni et al, , , ; Manzoni & Porporato, ; Manzoni, ; Poorter, ; Poorter & Villar, ; Phillips et al, ; Roumet et al, ; Sparks & Carski, ; Stewart et al, ; Smith & Read, ; Smith & Smith, ; Sinsabaugh et al, , ; Thomas & Stoddart, ; Thomas & Martin, ; Yang et al, ; Zhang et al, ). The original T&C is a mechanistic model simulating energy, water, and CO 2 exchanges at the land surface at an hourly time step.…”