Soil and leaf litter at the millimeter scale. This photograph depicts the heterogeneity of the soil environment and micro-scale carbon dynamics. Photography by Katerina Georgiou

There are as many microbes in a square meter of soil as there are stars in our galaxy. Scientists that simulate how soil microbes respond to changes in organic matter inputs from plants may have overlooked how these microbes interact as a community, according to new research from EESA scientists whose work appears in the journal Nature Communications.

The study was led by Katerina Georgiou, a Department of Energy SCGSR Fellow within Berkeley Lab’s Climate & Ecosystem Sciences Division. Results indicate that accounting for this microbial community regulation within Earth system models could help improve long-term predictions of how carbon within soils will respond to changes in precipitation, temperature, or land use in the long run. These factors affect the amount of plant carbon inputs entering the soil through changes in plant productivity, rooting depth, and species distribution.

Microbes in soil mediate soil carbon decomposition. Scientists curious to know whether this affects soil carbon accumulation in response to increased plant inputs have recently begun using microbial models of soil carbon decomposition to better predict soil carbon cycling.

“Microorganisms in the soil play a critical role in soil carbon decomposition,” says Georgiou. “We found that using microbial models that don’t consider microbial density-dependence – or how microbes behave as a community – could underestimate the impact of increasing plant inputs on soil carbon over decades. This has potentially large consequences for predicting long-term regional and global carbon feedbacks.”

Most existing microbial models are based on theory of how individual microbes behave, disregarding the regulatory effects that occur within microbial communities competing for resources. For their study, the researchers sought ways to represent this microbial community regulation within microbial models.

Response of soil organic carbon (SOC) to doubling and removal of C inputs in long-term litter manipulations. a) Percent change in SOC at Detritus Input and Removal Treatment (DIRT) experiments after a sustained 2× step increase in inputs. b) Percent change in SOC at DIRT and Long-Term Bare Fallow (LTBF) experiments after sustained 0× inputs. Points indicate means and bars the s.e.m.

Georgiou and her colleagues synthesized observations from 24 existing litter manipulation experiments conducted over periods of five or more years at sites spanning grassland and temperate forest ecosystems.

They compared data obtained from these studies to a range of existing soil carbon models. The team noted that prominent microbial models of soil carbon decomposition fail to impose limitations on microbial population size, resulting in unrealistic oscillatory behavior. With microbial biomass allowed to grow unchecked, the authors show that whether carbon inputs double or increase ten-fold, the steady-state microbial population will expand in proportion. This ends up driving soil carbon back to its pre-disturbance steady state, rendering it insensitive to long-term changes in carbon inputs from plants – a response not observed in the existing long-term data.

Ultimately, the team proposed modifying microbial models to reflect density-dependent microbial turnover, which arises from community interactions, to improve predictions of soil carbon accumulation in response to long-term litter manipulations. The proposed modification improves microbial models for inclusion in earth system models, with potentially large implications for global carbon feedbacks.