Soil organic matter priming: The pH effects.

Area of Science:

• Soil Science
• Biogeochemistry
• Microbial Ecology

Background:

• Priming effects (PEs) on soil organic matter (SOM) decomposition are crucial for global carbon (C) cycling.
• Soil pH is a major determinant of PEs, affecting microbial communities, SOM stability, and biogeochemical processes.

Purpose of the Study:

• To critically review the prerequisites and mechanisms of PEs influenced by soil pH.
• To assess the consequences of global change on pH-dependent PEs.

Main Methods:

• Literature review focusing on pH-mediated priming effects.
• Analysis of microbial community composition, enzyme activities, and SOM stabilization mechanisms across different pH levels.
• Evaluation of short-term and long-term effects of soil acidification on PEs.

Main Results:

• Highest PEs observed in soils with pH 5.5–7.5, particularly around pH 6.5, where positive PEs could be 20 times higher than baseline SOM decomposition.
• Low molecular weight organic compounds primarily trigger PEs in slightly acidic soils.
• Negative PEs occur at pH < 4.5 or > 7, indicating suboptimal microbial conditions or specific SOM stabilization.
• Short-term acidification (e.g., in rhizosphere) impacts PEs via mineral-SOM complexation, SOM oxidation, enzymatic depolymerization, and nutrient availability.
• Nitrogen fertilization and land use intensification decrease soil pH, thereby increasing PEs.

Conclusions:

• Soil pH is a critical, yet often overlooked, factor controlling SOM decomposition and PEs.
• PEs are modulated by both short-term microbial metabolic shifts and long-term microbial community adaptations to pH changes.
• Understanding pH-PE relationships is vital for predicting carbon cycling responses to global change.