Abstract
Microbial decomposers face large stoichiometric imbalances when feeding on nutrient-poor plant residues. To meet the challenges of nutrient limitation, microorganisms might: (i) allocate less carbon (C) to growth vs. respiration or excretion (i.e., flexible C-use efficiency, CUE), (ii) produce extracellular enzymes to target compounds that supply the most limiting element, (iii) modify their cellular composition according to the external nutrient availability, and (iv) preferentially retain nutrients at senescence. These four resource use modes can have different consequences on the litter C and nitrogen (N) dynamics–modes that selectively remove C from the system can reduce C storage in soil, whereas modes that delay C mineralization and increase internal N recycling could promote storage of C and N. Since we do not know which modes are dominant in litter decomposers, we cannot predict the fate of C and N released from plant residues, in particular under conditions of microbial nutrient limitation. To address this question, we developed a process-based model of litter decomposition in which these four resource use modes were implemented. We then parameterized the model using ∼80 litter decomposition datasets spanning a broad range of litter qualities. The calibrated model variants were able to capture most of the variability in litter C, N, and lignin fractions during decomposition regardless of which modes were included. This suggests that different modes can lead to similar litter decomposition trajectories (thanks to the multiple alternative resource acquisition pathways), and that identification of dominant modes is not possible using “standard” litter decomposition data (an equifinality problem). Our results thus point to the need of exploring microbial adaptations to nutrient limitation with empirical estimates of microbial traits and to develop models flexible enough to consider a range of hypothesized microbial responses.
Highlights
The products of litter decomposition represent the first step toward long-term soil organic carbon (C) stabilization (Berg and McClaugherty, 2003; Cotrufo et al, 2013), but decomposing nutrient poor and/or chemically recalcitrant litter poses challenges for microorganisms
The first challenge is imposed by stoichiometric imbalances between litter and decomposers, which decomposers can confront by four different modes of resource use: (i) flexible C-use efficiency (CUE), (ii) synthesis of extra-cellular enzymes to target the most limiting element, (iii) adjustment of microbial cellular composition, and (iv) by retaining limiting resources during senescence
To facilitate the interpretation of this figure, oxidizable compounds were not considered (i.e., CO = 0 and g(l) = 1), microbial biomass was assumed not to be limiting decomposition (Kr = 0), microbial biomass C was set to a constant value, and the C:N ratio of the substrate was changed by varying the proportion of hydrolysable compounds and proteins (N-rich)
Summary
The products of litter decomposition represent the first step toward long-term soil organic carbon (C) stabilization (Berg and McClaugherty, 2003; Cotrufo et al, 2013), but decomposing nutrient poor and/or chemically recalcitrant litter poses challenges for microorganisms. The adaptations of decomposers to face these challenges affect rates of litter decomposition and organic matter stabilization. The first challenge is imposed by stoichiometric imbalances between litter and decomposers, which decomposers can confront by four different modes of resource use: (i) flexible C-use efficiency (CUE), (ii) synthesis of extra-cellular enzymes to target the most limiting element (selective enzymes), (iii) adjustment of microbial cellular composition (plastic microbial C:N), and (iv) by retaining limiting resources during senescence (nutrient retention). Depending on the net effect of these adaptations, litter with low nutrient contents may promote or reduce C stabilization
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