Dramatic loss of inorganic carbon by nitrogen-induced soil acidification in Chinese croplands.

Intensive crop production systems worldwide, particularly in China, rely heavily on nitrogen (N) fertilization, but left more than 50% of it in the environment. Nitrogen (over)fertilization and atmospheric N deposition induce soil acidification, which is neutralized by soil inorganic carbon (SIC; carbonates) and carbon dioxide (CO2 ) is released to the atmosphere. For the first time, the loss of SIC stocks in response to N-induced soil acidification was estimated from Chinese croplands from 1980 to 2020 and forecasts were made up to 2100. The SIC stocks in croplands in 1980 were 2.16 Pg C (16.3 Mg C ha(-1) ) in the upper 40 cm, 7% (0.15 Pg C; 1.1 Mg C ha(-1) ) of which were lost from 1980 to 2020. During these 40 years, 7 million ha of cropland has become carbonate free. Another 37% of the SIC stocks may be lost up to 2100 in China, leaving 30 million ha of cropland (37.8%) without carbonates if N fertilization follows the business as usual (BAU) scenario. Compared to the BAU scenario, the reduction of N input by 15%-30% after 2020 (scenarios S1 and S2) will decrease carbonates dissolution by 18%-41%. If N input remains constant as noted in 2020 (S3) or decreases by 1% annually (S4), a reduction of up to 52%-67% in carbonates dissolution is expected compared to the BAU. The presence of CaCO3 in the soil is important for various processes including acidity buffering, aggregates formation and stabilization, organic matter stabilization, microbial and enzyme activities, nutrients cycling and availability, and water permeability and plants productivity. Therefore, optimizing N fertilization and improving N use efficiency are important for decreasing SIC losses from acidification. N application should be strictly calculated based on crop demand, and any overfertilization should be avoided to prevent environmental problems and soil fertility decline associated with CaCO3 losses.