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

Abstract

<p>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<sup>+</sup>), which strongly and irreversibly accelerate dissolution of soil inorganic carbon (SIC) e.g., CaCO<sub>3</sub>, leading to CO<sub>2</sub> release in the atmosphere. Here, <sup>14</sup>C-labeled CaCO<sub>3</sub> was added to calcareous soil (0.75% CaCO<sub>3</sub>) to investigate the effects of chicken manure, urea, NH<sub>4</sub>NO<sub>3, </sub>KNO<sub>3 </sub>and<sub> </sub>(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> on soil acidification and to estimate the SIC contribution to CO<sub>2</sub> emission. 250 mL gas-tight jars were filled with a cropland soil (pH = 7.2), homogenously mixed with 1.3% Ca<sup>14</sup>CO<sub>3</sub> powder (<sup>14</sup>C activity = 11.3 kBq pot<sup>-1</sup>). Following fertilization in rates of 0.1, 0.15, 0.25 g N kg<sup>-1</sup> soil, NaOH was applied to trap the emitted CO<sub>2</sub> and to determine <sup>14</sup>C activity. CaCO<sub>3</sub> addition increased soil pH values by 0.17-0.43 units. Addition of ammonium-based fertilizers ((NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>, NH<sub>4</sub>NO<sub>3</sub>) strongly decreased pH up to 0.3 units. All fertilizers increased CO<sub>2</sub> emission (5.1%-180%) compared to the unfertilized soil after 44 days of incubation except KNO<sub>3</sub>. SIC-originated CO<sub>2</sub> due to fertilization was ranged from 2.9 to 160 mg C kg<sup>-1</sup> (1.1% to 48% of total emitted CO<sub>2</sub>). Manure and urea had lowest impacts on SIC-driven CO<sub>2</sub> during the first 5 days (2.9-34 mg C kg<sup>-1</sup>) irrespective of the application rate. Thereafter, the effects of fertilizers on SIC-originated CO<sub>2</sub> increased in the order: urea < manure < KNO<sub>3</sub> < NH<sub>4</sub>NO<sub>3</sub> < (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>. As nitrification of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> yields in 4 mol H<sup>+</sup>, which neutralizes 2 mol carbonates, it initially caused the highest SIC-originated CO<sub>2</sub><sub> </sub>until 9 days. Urea and NH<sub>4</sub>NO<sub>3</sub> release by nitrification 2 mol H<sup>+ </sup>per mole of fertilizer, but urea initially hydrolyses to NH<sub>4</sub>OH, which increases soil pH. So, urea addition had the minimum SIC loss as CO<sub>2</sub><sub> </sub>in the first 5 days, but starting from 16<sup>th</sup> day, CO<sub>2</sub> emission sharply increased and reached to highest values among the fertilizers. Manure increased SIC-originated CO<sub>2</sub> emission from 23<sup>rd</sup> day of incubation. Gradual and incomplete mineralization of organic N of chicken manure duration 44 days explains the smallest released CO<sub>2</sub> from CaCO<sub>3</sub> and slowest acidification in the first 16 days. Furthermore, Ca<sup>2+</sup> and Mg<sup>2+</sup> in manure may be precipitated as carbonates, which decrease the SIC share in the emitted CO<sub>2</sub>. Generally, the higher the applied fertilizer amounts, the larger was the proportion of CO<sub>2</sub> 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 CO<sub>2</sub> emission to mitigate global warming, and also save various ecosystem services such as organic matter stability and increase C sequestration.</p>