pH-dependence of organic matter solubility: Base type effects on dissolved organic C, N, P, and S in soils with contrasting mineralogy

• We measured dissolved organic matter (DOM) in soils treated with Ca(OH)2 or KOH.
• Dissolved organic C, N, P, and S all increased as soil pH was raised.
• KOH induced larger increases in DOM than Ca(OH)2, especially at higher rates.
• KOH (analog for NH4OH from urea hydrolysis) may desorb stable C bound to clays.
• DOM released by both bases was rapidly biodegraded (~ 45% in 7 days at 20 °C).

Dissolved organic matter (DOM) is an important substrate for soil biological processes. Raising pH is known to cause release of DOM but less is known of the impact of different bases on the quantity and quality of DOM in soils. Three acid (pH 5.3–5.7) soils of contrasting mineralogy were used to examine how the quantity and quality of DOM changes when soil pH is raised using lime (Ca(OH)2) or KOH (chemical analog of NH4OH, a product of urea hydrolysis). Dissolved organic carbon (DOC) increased as the base addition rate increased, with the response to KOH being up to 9 times larger than to Ca(OH)2. The proportion of carbohydrate-C (hexose- and pentose-C) in DOM did not show consistent effects of base rate or type and was similar for the three soils (35% of DOC, on average). Dissolved phenol-C increased as base addition rate increased. Only monomeric phenols were solubilized in Ca(OH)2-treated soils whereas polyphenols were solubilized in increasing amounts as the KOH rate was raised. Although the quantity of DOM differed considerably between treatments, its bioavailability (45% on average; measured using a 7-day bioassay at 20 °C) was similar across treatments and soil types, with good correspondence between carbohydrate-C and bioavailable C. Large base type effects on organically bound nutrients (dissolved organic N, P, and S) were also observed (KOH ≫ Ca(OH)2). A distinguishing feature of DOM released by KOH was the presence of large amounts of complexed Al and Fe. We concluded that KOH (and, by extension, NH4OH) may render stable organic matter accessible to microbes by liberating it from the protective influence of soil minerals. Our results emphasize the importance of choosing an appropriate base when examining pH effects on DOM and associated biological processes.