image: The study combined a global synthesis of 466 forest topsoil observations (a), regional field sampling and carbonate rock weathering measurements in southwestern China (b), and controlled laboratory experiments (c). Laboratory treatments compared limestone powder with a quartz control, with separate additions of water, glucose or glycine. Glucose and glycine labelled with carbon-13 allowed the researchers to track carbon entering microbial necromass.
Credit: ©Science China Press
Carbonate rocks such as limestone and dolostone help soils hold on to carbon as they weather. A new study in National Science Review finds that carbonate rock weathering promotes the buildup of microbial necromass, the remains of dead microbes, and helps protect it from decomposition. The study, led by researchers at the Institute of Subtropical Agriculture, Chinese Academy of Sciences, combined global soil datasets, field surveys in southwestern China and controlled laboratory experiments.
Soil organic carbon is critical for climate regulation, nutrient cycling, water retention and biodiversity maintenance. Microbes make up a large portion of soil organic carbon: they feed on plant-derived carbon, grow and multiply, then leave behind their necromass when they die. How much of microbial necromass accumulates depends on how efficiently microbes use carbon to grow and how well their necromass is protected from decomposition.
Both processes are strongly shaped by soil conditions, which are in turn largely influenced by underlying bedrock. Carbonate rocks underlie about 15% of Earth's ice-free land and dissolve at least ten times as fast as many silicate rocks. Their weathering releases calcium and nutrients and alters soil acidity, affecting microbial activity and the binding of organic matter to minerals. Yet research on carbonate rock weathering and carbon cycling has largely focused on carbon dioxide uptake during rock dissolution. Its effects on soil organic carbon are less well understood.
The team analyzed 466 observations of forest topsoils worldwide, using amino sugars as biomarkers of microbial necromass. Soils in karst landscapes, shaped by the dissolution of carbonate rocks, had 43.4% higher amino sugar concentrations and 57.6% higher organic carbon concentrations than non-karst soils. These differences remained significant after accounting for climate and vegetation covariates.
Field surveys across six regions of southwestern China found 19.3% more amino sugars per unit of organic carbon in karst than in non-karst soils. Within the karst sites, carbonate rock weathering rate was the strongest measured predictor of microbial necromass accumulation. Faster weathering was also associated with more microbial genes for breaking down complex plant compounds, including lignin. Together with evidence of more extensive lignin degradation, these findings suggest that carbonate rock weathering promotes microbial processing of plant-derived carbon.
To simulate the effects of carbonate rock weathering, the team further conducted a controlled laboratory incubation experiment by adding powdered limestone to soils. Limestone addition raised soil pH and calcium availability and increased enzyme activity linked to the breakdown of complex organic matter. By tracing carbon from two simple compounds, glucose and glycine, the researchers found that limestone addition increased the efficiency of carbon accumulation in microbial necromass by 22% to 71%, depending on the compound.
The team also showed that karst soils had 57.8% higher concentrations of amino sugars bound to minerals than non-karst soils. Their abundance was closely linked to certain forms of calcium. Calcium can help organic matter bind to mineral surfaces, making it harder for microbes to access and decompose. This mineral protection offers an explanation for the greater retention of microbial necromass in karst than in non-karst soils.
The findings reveal that carbonate rock weathering supports both the formation and protection of microbial-derived carbon, a role beyond its well-documented function in inorganic carbon uptake. Including rock type and weathering in soil carbon models could improve predictions of where organic carbon accumulates and how long it persists. The findings also provide a basis for testing whether enhanced carbonate rock weathering could help soils retain more organic carbon.
Peilei Hu, from the Institute of Subtropical Agriculture, is the first author of the paper. Wei Zhang and Kelin Wang are the corresponding authors. This work was supported by the National Natural Science Foundation of China and the Guangxi Bagui Young Scholars Program.