Nitrogen fertilizers control CO2 emission from calcareous soils: implications for land management and global warming

Soil acidification has increasingly become a critical issue for sustainable production due to the excessive nitrogen (N) fertilization in agricultural systems. Application of N fertilizers and the consequent nitrification yield protons (H⁺), which strongly and irreversibly accelerate dissolution of soil inorganic carbon (SIC) e.g., CaCO₃, leading to CO₂ release in the atmosphere. Here, ¹⁴C-labeled CaCO₃ was added to calcareous soil (0.75% CaCO₃) to investigate the effects of chicken manure, urea, NH₄NO_3, KNO₃ and (NH₄)₂SO₄ on soil acidification and to estimate the SIC contribution to CO₂ emission. 250 mL gas-tight jars were filled with a cropland soil (pH = 7.2), homogenously mixed with 1.3% Ca¹⁴CO₃ powder (¹⁴C activity = 11.3 kBq pot^-1). Following fertilization in rates of 0.1, 0.15, 0.25 g N kg^-1 soil, NaOH was applied to trap the emitted CO₂ and to determine ¹⁴C activity. CaCO₃ addition increased soil pH values by 0.17-0.43 units. Addition of ammonium-based fertilizers ((NH₄)₂SO₄, NH₄NO₃) strongly decreased pH up to 0.3 units. All fertilizers increased CO₂ emission (5.1%-180%) compared to the unfertilized soil after 44 days of incubation except KNO₃. SIC-originated CO₂ due to fertilization was ranged from 2.9 to 160 mg C kg^-1 (1.1% to 48% of total emitted CO₂). Manure and urea had lowest impacts on SIC-driven CO₂ during the first 5 days (2.9-34 mg C kg^-1) irrespective of the application rate. Thereafter, the effects of fertilizers on SIC-originated CO₂ increased in the order: urea < manure < KNO₃ < NH₄NO₃ < (NH₄)₂SO₄. As nitrification of (NH₄)₂SO₄ yields in 4 mol H⁺, which neutralizes 2 mol carbonates, it initially caused the highest SIC-originated CO₂ until 9 days. Urea and NH₄NO₃ release by nitrification 2 mol H⁺ per mole of fertilizer, but urea initially hydrolyses to NH₄OH, which increases soil pH. So, urea addition had the minimum SIC loss as CO₂ in the first 5 days, but starting from 16^th day, CO₂ emission sharply increased and reached to highest values among the fertilizers. Manure increased SIC-originated CO₂ emission from 23^rd day of incubation. Gradual and incomplete mineralization of organic N of chicken manure duration 44 days explains the smallest released CO₂ from CaCO₃ and slowest acidification in the first 16 days. Furthermore, Ca²⁺ and Mg²⁺ in manure may be precipitated as carbonates, which decrease the SIC share in the emitted CO₂. Generally, the higher the applied fertilizer amounts, the larger was the proportion of CO₂ released from SIC. Both the fertilizer chemistry and the application rate played significant roles in dissolution of carbonates. Summarizing, the correct selection of the type and amount of fertilizers based on soil properties and plant demand is necessary to decrease SIC-originated CO₂ emission to mitigate global warming, and also save various ecosystem services such as organic matter stability and increase C sequestration.