Roles of soil organic acids and base cations in Mollisol acidification during the peak growth stage of crops

Abstract

Changes in low-molecular-weight organic acids (LMWOAs), plant uptake or leaching of base cations (BCs) during peak crop growth affect soil acidity, particularly owing to excessive fertilizer use and intensive farming. The effect of LMWOAs and BCs on the acidity of Mollisol farmland is poorly understood. A long-term experiment with different fertilizer treatments under continuous cropping or corn–soybean rotation modules, was performed to examine the variation of LMWOAs and BCs during peak growth stages of corn and soybean and their effects on soil acidity. The LMWOAs concentrations ranged from 33.6 to 45.9 mg kg^–1 at the jointing stage of corn and 71.2–114 mg kg^–1 at the flowering stage of soybean. The predominant LMWOAs for corn are malic, acetic, and propanedioic acids, and malic, propanedioic, and succinic acids for soybean. Acetic acid was the main acidity determinant for corn (r = 0.714, p < 0.001), whereas that for soybean was malic acid (r = 0.704, p < 0.001). Acidification of the Mollisol farmland by LMWOAs and BCs during the peak crop growth stage is crop-specific and management-dependent. The higher total amount of LMWOAs may be one of the reasons for soil acidification in soybean. Continuous corn significantly increases the total LMWOAs concentration but reduces the content of BCs, exchangeable BCs (Ca²⁺, Mg²⁺, and Na⁺), whereas continuous soybean reduces no effect on either LMWOAs or BCs. Continuous corn cultivation, compared to rotational corn, reduces soil pH from 5.76 to 5.63, due to an increase in the total LMWOAs concentration, as well as a reduction in BCs and exchangeable BCs (Ca²⁺, Mg²⁺, and Na⁺) levels. There is no significant difference in soil pH between continuous and rotational soybean treatments but soil pH below 5.5 in both treatments. Rotation with chemical fertilizer for corn and dairy manure alone for soybean increases soil pH significantly owing to higher levels of BCs and exchangeable BCs (Ca²⁺, Mg²⁺, and K⁺). Manure application is more effective than straw return in mitigating soil acidity. Understanding the decomposition dynamics of LMWOAs during the crop cycle on soil acidity is required to lay a robust scientific basis for mitigating soil acidification in Mollisol farmlands.