Low molecular weight organic acids mobilize soil organic phosphorus for enzymatic hydrolysis in a temperate montane peatland

Abstract

The stability of carbon (C) stocks in peatlands is intricately linked to phosphorus (P) bioavailability. Given that organic P compounds (P_o) can make up to 89% of total soil P in these ecosystems, it is vital to understand their role in regulating plant productivity and organic matter decomposition. Despite this significance, the mechanisms controlling P bioavailability remain poorly understood. Plants and soil microorganisms primarily regulate the release of soil P via low-molecular-weight organic acids (LMWOAs) and modulate the hydrolysis of P_o through phosphatase enzymes, particularly phosphomonoesterase, phytase, and phosphodiesterase. This study investigated the role of LMWOAs, derived from root exudates of dominant vascular plants and Sphagnum leachates in a temperate montane peatland, in facilitating the release of P. We also quantified the ability of these plants to hydrolyze P_o from various LMWOA-extracted fractions by adding phosphomonoesterase, phytase, and phosphodiesterase. The results show that peatland plants predominantly exuded muconic, azelaic, 3-hydroxybutyric, and malonic acids. The concentration of enzymatically hydrolyzed P_o in the water-extracted fraction was 8.1 ± 3.4 mg kg⁻¹. Notably, azelaic and malonic acids were effective in releasing over 58% of soil P (330–798 mg kg⁻¹), with more than 88% of this P being in organic form. In the azelaic and malonic acid-extracted fractions, the concentration of enzymatically hydrolyzed P_o concentration was 123.7 ± 32.1 mg kg⁻¹, accounting for 23% of the LMWOA-extracted P_o. Phytase, the most important phosphatase enzyme, accounts for 66% (47–88%) of the enzymatically hydrolyzed P_o (81.9 ± 20.9 mg kg⁻¹). Our study demonstrates that LMWOA-mediated release of P_o is an essential prerequisite for enzymatic hydrolysis of P_o in organic peat soils. However, only a small portion of LMWOA-extracted P_o can be hydrolyzed by phosphatase enzymes. The different composition and efficacy of LMWOAs from species of different plant functional types highlight the necessity to consider changes in vegetation composition, as this could significantly impact P dynamics in peatlands and, consequently, the stability of their C stocks.