Seasonal dynamics of soil microbial carbon sequestration

Rising air temperatures caused by increasing CO2 concentrations in the atmosphere call for efficient counteractive strategies. One strategy is the transfer of carbon (C) into soil where it can be stabilized. The majority of C in soils is remains of dead microbial biomass. A first step to transfer C into stabile soil pool is thus the efficient uptake of C into soil microorganisms. Carbon-use-efficiency describes how much of the C that microbes take up is used for microbial growth and how much is respired. After cell death microbial compounds can get attached to clay minerals in the soil and become stabilized. Up to now, there is little direct evidence for a connection of carbon-use-efficiency, sorption on minerals, and persistence of C in soils. Furthermore, little is known about how temperature, nutrient availability, substrate chemistry, and adaptations of the active microbial community influence carbon-use-efficiency. Since these factors vary seasonally in temperate systems, the interactions of controls could result in seasonal variations of carbon-use-efficiency. Well timed amendments to soils at times of high carbon-use-efficiency, might lead to accumulation of stabile soil C and could counteract climate change. In this project we will, for the first time, measure carbon-use-efficiency in a temperate forest and an agricultural field, both with or without litter incorporation in fall, 18 times over the course of two years. We will use this data of carbon-use-efficiency, nutrient availability, substrate chemistry, temperature and microbial community composition to statistically determine the importance of direct and indirect controls on carbon-use-efficiency. We will further set up short term laboratory experiments addressing microbial adjustments to temperature, nutrient availability, and substrate chemistry. To investigate the connection of carbon-use-efficiency and stabilization of C on clay minerals we will set up another laboratory incubation experiment. In this experiment we will add labelled substances to soils taken at times of high and low carbon-use-efficiency and we will trace the uptake of the label by the soil microbes and the transfer into the different soil C pools over time. To assess the long term persistence of C in dependence of timing and nature of substrate amendments we will use a mathematical soil C model. To our knowledge this is the first project to investigate the seasonality carbon-use-efficiency and the importance and interactions of environmental factors and the microbial community. The combination of field and laboratory experiments as well as modeling approaches will clarify the role of carbon-use-efficiency in the formation of stabile soil C. Our findings will eventually help to develop temporally accurate management practices to counteract climate change.

Microorganisms are the driving force for carbon (C) fluxes into and out of soil ecosystems. They take up C, previously fixed by plants through photosynthesis, to grow or to produce energy through respiration. Respired C is lost from the soil as CO2, while C incorporated in soil microbial biomass remains in the soil and can be stabilized in the soils. The ratio of C that stays in the microbial biomass and total taken up C is defined as carbon use efficiency (CUE). CUE is thus considered an important parameter, that is sought to be optimized to increase soil C sequestration to e.g. counteract rising CO2 concentrations in the atmosphere. In this project we investigated how these microbial parameters change throughout the year, what the main controls over CUE, microbial growth and respiration are, and how these parameters are related to soil C sequestration, i.e. increase in stable C in soils. To do this we determined microbial growth, respiration, and CUE 18 times over the course of two years in an agricultural soil and a forest soil. Microbial respiration, microbial growth, and microbial biomass displayed high seasonal changes. Microbial respiration followed soil temperature and was high in summer and low in winter. Microbial growth also followed temperature but also depended on the availability of C. In contrast, microbial CUE did not clearly follow seasonal temperature fluctuations and exhibited peaks during winter. We further observed surprising microbial biomass increases during winter. Because of our surprising findings of microbial dynamics in winter we conducted an additional laboratory experiment. We looked into microbial respiration and growth, as well as microbial glucose uptake (glucose is directly released from plant roots) in response to soil cooling in the same soils as before. In both soils, respiration and cell division, were strongly reduced when soils were cooled from 11 C to 1 C, while glucose uptake was unchanged. We found adaptations of microbial cell walls to retain fluidity at lower temperatures. The discrepancy between C uptake and cell division, further hints at the production of storage compounds in the microbial cells. Our findings suggest that soil microorganisms might increase internal storage instead of dividing, which could be the mechanism behind the increase in microbial biomass in winter. The decline of microbial biomass in spring, might have been caused by microbial death and thus an increase in microbial necromass that might have been stabilized on the soil matrix. While the concrete mechanisms still have to be elucidated further, our findings indicate that winter with its bloom and following decline in microbial biomass could constitute the main season for microbial C sequestration in temperate soil systems.