Dark microbial CO2 fixation in temperate forest soils increases with CO2 concentration.

Marie Spohn,Sven Marhan,Carsten W Mueller,Carmen Höschen,Karolin Müller

doi:10.1111/gcb.14937

Abstract

Dark, that is, nonphototrophic, microbial CO2 fixation occurs in a large range of soils. However, it is still not known whether dark microbial CO2 fixation substantially contributes to the C balance of soils and what factors control this process. Therefore, the objective of this study was to quantitate dark microbial CO2 fixation in temperate forest soils, to determine the relationship between the soil CO2 concentration and dark microbial CO2 fixation, and to estimate the relative contribution of different microbial groups to dark CO2 fixation. For this purpose, we conducted a 13 C-CO2 labeling experiment. We found that the rates of dark microbial CO2 fixation were positively correlated with the CO2 concentration in all soils. Dark microbial CO2 fixation amounted to up to 320µgCkg-1 soilday-1 in the Ah horizon. The fixation rates were 2.8-8.9 times higher in the Ah horizon than in the Bw1 horizon. Although the rates of dark microbial fixation were small compared to the respiration rate (1.2%-3.9% of the respiration rate), our findings suggest that organic matter formed by microorganisms from CO2 contributes to the soil organic matter pool, especially given that microbial detritus is more stable in soil than plant detritus. Phospholipid fatty acid analyses indicated that CO2 was mostly fixed by gram-positive bacteria, and not by fungi. In conclusion, our study shows that the dark microbial CO2 fixation rate in temperate forest soils increases in periods of high CO2 concentrations, that dark microbial CO2 fixation is mostly accomplished by gram-positive bacteria, and that dark microbial CO2 fixation contributes to the formation of soil organic matter.

Highlights

Soils have mostly been studied as a source of CO2 during the last decades, and organic carbon (C) in soil is assumed to have been fixed by photosynthesis (Bond-Lamberty, Bailey, Chen, Gough, & Vargas, 2018; Carey et al, 2016; Schlesinger & Andrews, 2000)
The soil was divided, and one part of each soil sample was immediately used for the determination of microbial biomass carbon (MBC), one part was freeze dried and ground using a ball mill (Retsch GmbH), one part was frozen at −14°C for subsequent DNA and phospholipid fatty acids (PLFAs) extraction, and one part was chemically fixed for NanoSIMS measurements
We found that the rate of dark CO2 fixation was positively correlated with the CO2 concentration in soils (Figure 2), which has never been described before, to our knowledge

Summary

| INTRODUCTION

Reported that dark microbial CO2 fixation was higher in alkaline than in acid soils, and the authors explained this by the higher HCO−3 concentration in these soils Aside from these two studies, nothing is known about the relationship between the CO2 concentration and dark microbial CO2 fixation, to our knowledge. We hypothesized that (a) dark microbial CO2 fixation in temperate forest soils contributes substantially to C cycling and the C balance of temperate forest soils; (b) the microbial CO2 fixation rate increases with the CO2 concentration in soil; and (c) CO2 is mostly fixed by prokaryotes and not by fungi in these temperate forest soils. We estimated and visualized the contribution of different microbial groups to dark microbial CO2 fixation based on phospholipid fatty acids (PLFAs) and nanoscale secondary ion mass spectrometry (NanoSIMS)

| MATERIALS AND METHODS

Findings

| DISCUSSION