Large mammalian herbivores and the paradox of soil carbon in grazing ecosystems: Role of microbial decomposers and their enzymes

Shamik Roy, Sumanta Bagchi

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14 Citations (Scopus)
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Abstract

Grazing is the dominant land use across the world, and large mammalian herbivores exert strong influence over biogeochemical cycles. Grazing ecosystems feature C-rich soils, even though herbivores consume a major fraction of plant production to reduce detrital input to soil. Yet, counter-intuitively, moderate grazing can promote net soil-C storage in many ecosystems compared to grazer-exclusion. We address this enigmatic influence of grazers on soil-C and test their indirect effect on proximate drivers of decomposition: microbial extracellular enzyme activity. We used a replicated long-term grazer-exclusion experiment to measure responses in above- and belowground plant biomass, soil-C stock, microbial biomass, labile/recalcitrant C pools and three enzymes relevant to the C-cycle: peroxidase—which initiates decomposition of recalcitrant matter, alongside beta-glucosidase and cellobiohydrolase—which act further downstream on more labile fractions. Consistent with other ecosystems, upto 12 years of herbivore exclusion did not increase soil-C in the fenced plots despite higher plant biomass and higher potential detrital C-inputs. Grazer-exclusion did not alter microbial biomass; peroxidase increased threefold and beta-glucosidase was doubled; cellobiohydrolase was unaffected. Grazer-exclusion also led to twofold increase in recalcitrant-C and in microbial respiration, but it did not influence labile-C. Structural equation models supported the hypothesis that grazing favours soil-C via its indirect effect on peroxidase, but they did not support that the effects can run in the opposite direction where soil-C affects enzymes. Grazer-mediated shifts in how microbes deploy enzymes emerge as a plausible mechanism that affects soil-C. These linkages may be important to maintain soil-C sequestration in drylands which support large mammalian herbivores.
Original languageEnglish
Pages (from-to)976-988
Number of pages13
JournalEcosystems
Volume25
Issue number4
Early online date10 Sep 2021
DOIs
Publication statusPublished - Jun 2022
Externally publishedYes

Keywords

  • Carbon sequestration
  • Decomposition
  • Grassland
  • Herbivory
  • Lignin
  • Soil organic matter
  • Structural equation modelling (SEM)

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