Researchers from the Institute of Soil Science (ISS) and the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences (NIGLAS) and their collaborators have revealed that soil inorganic carbon (SIC) is not temporally stable due to soil acidification, which is different from the traditional viewpoint.  

The study was published in National Science Review.  

Unlike soil organic carbon (SOC), SIC is frequently disregarded, as its cycling is much slow.

"Environmental changes, including the use of chemical fertilizers, global warming and atmospheric acid deposition, can cause significant global soil acidification, especially in cropland, which may accelerate SIC turnover," said Dr. ZHANG Ganlin from NIGLAS.  

How much SIC has been lost during last decades at a continental scale like China due to data limitations. To quantify these changes, the researchers conducted a national resampling campaign in the last decade and collected legacy soil data as much as possible.  

The national soil data in the 1980s were taken as control plots. By comparing the SIC data in the 1980s and 2010s, they found that approximately half of the pairs exhibited a clear declining trend. 

"It is clear evidence of SIC loss," said Dr. ZHANG. "This result is consistent with the findings of similar studies conducted at the regional scale and can provide an important observational benchmark."  

They also conducted modeling to map the spatial distribution of SIC and estimated the total changes in SIC stock over the last 30 years. The machine learning technique was used to combine soil data at different sites with the environmental covariates that control SIC turnover.  

"It is evidenced that soil acidification caused by enhanced nitrogen precipitation, together with nitrogen fertilizer application, is accounted for SIC reduction. A mass of SIC was found in the subsoils regardless of the ecosystems," said Dr. ZHANG. 

Most of the lost SIC is believed to be converted to CO₂ and released to the atmosphere. In general, the SIC losses across China and in cropland can offset approximately 18%-24% of the terrestrial biomass organic carbon sink and 57% of the SOC sink in cropland, respectively.  

The study reveals that the consumption of SIC may offset a large portion of the global efforts aimed at ecosystem carbon sequestration, which emphasizes the importance of better understanding the indirect coupling mechanisms of nitrogen and carbon cycling as well as effective countermeasures to minimize SIC loss.