Nitrogen fertilizers control CO2 emission from calcareous soils:&#160;implications for land management and global warming
Authors: Tao Jingjing , Fan Lichao , Zhou Jianbin , Kuzyakov Yakov , Zamanian Kazem 
Published: March 2022

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