Balanced fertilization management to protect soil inorganic carbon stocks and reduce soil CO2 emissions

Abstract

Decalcification, especially due to acidity induced by nitrogen (N) fertilization, generates an often-underestimated source of atmospheric CO₂ in agroecosystems. Complete depletion of soil inorganic carbon (SIC), i.e. carbonates, intensifies the decomposition of soil organic carbon (SOC) to an extent not yet experimentally demonstrated. Six fertilization management practices including application of urea, urea + superphosphate + potassium chloride, ammonium phosphate, ammonium phosphate + potassium chloride, chicken manure along a control i.e. no fertilization were used to quantify the effects of N fertilization on soil acidification and the percentage of SIC-originated CO₂ in total soil CO₂ emissions. Gas samples were collected during a 56-day incubation experiment to determine total emitted CO₂ and its δ¹³C value. The presence of SIC, kept the total CO₂ emissions after inorganic fertilization at levels comparable to unfertilized soil and a balanced fertilization reduced SIC-originated CO₂ emissions ($\approx$ 15 % after NPK vs. 35 % with N applications) due to better nutrient use efficiency and comparatively less proton generation after nitrification. When inorganic N fertilization led to complete SIC depletion after shifting in soil pH from circumneutral (pH = 7.4) to slightly-moderately acidic pH (pH = 6.5 to about 5.8) ranges, a sudden increase in total CO₂ emissions indicated the loss of the protective effects of SIC, and the extreme decomposition of the indigenous SOC. Complete depletion of SIC activates a negative feedback loop: the more fertilizer is added for more crop production, the more SOC, and soil productivity will be lost. We conclude that balanced fertilization and the use of organic fertilizers not only ensure sustainable productivity, but also significantly reduce CO₂ emissions from agroecosystems by preventing SIC depletion.