Controlled-release urea reduces nitrogen fertilizer-induced proton release: a column experiment

Nitrogen (N) fertilization is a major driver of soil acidification, yet quantitative evidence on how controlled-release urea (CRU) alters H + production (H pro ) pathways remains limited. We conducted a 90-day soil column experiment using a calcareous fluvo-aquic soil collected from a 14-year long-term field site. Five treatments were established with four replicates: no N (CK), full- and high-rate CRU (CRF1 and CRF2), and full- and high-rate conventional urea (BBF1 and BBF2). Leachates were collected on days 7, 30, 60 and 90 and analyzed for inorganic N, major cations and anions. The H pro was quantified using an input-output balance approach by separating the N-cycle component (associated with net <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im1"> <mml:mrow> <mml:mi>N</mml:mi> <mml:msubsup> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> export) and the carbonate-related C-cycle component (associated with net <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im2"> <mml:mrow> <mml:mi>H</mml:mi> <mml:mi>C</mml:mi> <mml:msubsup> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> export). Compared with conventional urea at the same N rate, CRU reduced cumulative <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im3"> <mml:mrow> <mml:mi>N</mml:mi> <mml:msubsup> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> leaching by 54.17% (CRF1 vs. BBF1) and 44.93% (CRF2 vs. BBF2), accompanied by lower base-cation losses. Total H pro over 90 days decreased by 42.47% (CRF1 vs. BBF1) and 33.66% (CRF2 vs. BBF2). These results indicate that CRU mitigates fertilizer-induced acidification potential mainly by moderating nitrification-driven <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="im4"> <mml:mrow> <mml:mi>N</mml:mi> <mml:msubsup> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:mrow> </mml:math> losses and associated charge-balanced cation leaching, thereby lowering the net H + load exported from the soil profile.