Neutral soil pH conditions favor the inhibition of phenol on hydrolase activities and soil organic carbon mineralization

Abstract

The enzymic latch theory established the relationships between hydrolases, oxidases and soil organic carbon (SOC) stock in peatlands, desert or agricultural soils. The vital role of phenolics and soil pH still lack experimental evidences and has yet to be addressed simultaneously. The objective of this study was to validate the role of phenolics in SOC mineralization and whether and how pH regulates the role of phenolics through microbial activity. We conducted a 28-day laboratory experiment to tested the effects of three levels of phenol, 0 mg kg⁻¹ (control), 20 mg kg⁻¹ (LPh) and 100 mg kg⁻¹ (HPh), under four pH values (i.e., pH 5.6, 6.4, 7.2 and 7.9) maintaining 20 % soil moisture, on soil phenol oxidase (PO), hydrolases (α-glucosidase (AG), β-glucosidase (BG), β-xylosidase (XYL), cellobiohydrolase (CBH), and total hydrolases (SUM-H)), microbial indices (microbial biomass carbon (MBC) and dehydrogenases (DHA)), and dissolved organic carbon (DOC). The results showed that soil pH and phenol interacted with CBH, XYL and DHA activities. Both hydrolase and PO activities increased with pH and were highest at pH 7.9 (AG: 38.62 mg kg⁻¹ h⁻¹, BG: 258.88 mg kg⁻¹ h⁻¹, SUM-H: 426.93 mg kg⁻¹ h⁻¹, PO: 1.35 mg kg⁻¹ h⁻¹). CBH activity was reduced by up to 17.74 % at pH 6.4, 20.54 % at pH 7.2 and 21.98 % at pH 7.9. LPh and HPh reduced XYL activity throughout the incubation period up to 24.93 % and 19.88 % at pH 6.4 and 23.43 % and 32.38 % at pH 7.2, respectively. DOC increased with hydrolases activities (AG, BG and SUM-H) and microbial indices (DHA and MBC). Phenolic accumulation limited soil hydrolase and microbial activities and slowed down SOC mineralization, especially at nearly neutral soil pH. SOC stability increased with the transformation of soil labile C to MBC at LPh while reduced with the consumption of SOC by microorganisms at HPh. Overall, the inhibition of phenol on hydrolase activities and SOC mineralization was enhanced under neutral soil pH conditions, helping to better understand the SOC accumulation in agroecosystems.