Do biodegradable microplastics cause soil inorganic carbon loss in calcareous soils?

The presence of biodegradable microplastics (MPs) has the potential to affect soil pH, and possibly accelerate or inhibit the loss of soil inorganic carbon (SIC) in calcareous soils. However, most researchers have focused on the release of biotic carbon dioxide (CO2) from soils following MP amendments, and few studies have investigated SIC-derived CO2. In this experiment, three typical biodegradable MPs were applied to three calcareous soils amended with 1 % 13C-labeled (99 % atom) carbonate, and the release of CO2 originating from SIC was quantified. The total CO2 emissions, soil pH, and microbial functional genes involved in soil nitrification and denitrification were also detected. Throughout the experiment, the contribution of 13C-labeled carbonate to total CO2 emissions ranged from 0.42 % to 3.31 %. The impact of biodegradable MPs on SIC-derived CO2 varied with incubation period. At the early stage (≤20 days), the amendment of three biodegradable MPs increased the abiotic CO2 in some cases, and the CO2 emissions from 13C-labeled SIC were positively correlated with the total CO2 originating from the decomposition of SOC and MPs. At the late stage (20–70 days), the presence of biodegradable MPs inhibited the release of CO2 from 13C-labeled carbonate in most treatments. Moreover, there were negative relationships of SIC-derived CO2 with soil pH and the amoA gene of ammonia-oxidizing archaea (AOA), but positive correlations of SIC-derived CO2 with amoA of ammonia oxidizing bacteria (AOB) and nirK and nirS genes encoding nitrate reductase in denitrification. Our results indicate that long-term exposure to biodegradable MPs probably regulates the release of H+ in the nitrification process by controlling AOB, and then controlling the dynamics of SIC in calcareous soils.