Contrasting effects of different acid rain types on soil nitrogen pools

Area of Science:

• Environmental Science
• Soil Science
• Ecology

Background:

• Acid rain is a major global change driver affecting soil nitrogen (N) pools.
• Atmospheric deposition has shifted from sulfuric acid rain (SAR) to mixed acid rain (MAR) and nitric acid rain (NAR).
• The impact of different acid rain types on soil N pools is not well understood, especially at a large scale.

Purpose of the Study:

• To systematically assess the effects of different acid rain types (SAR, MAR, NAR) on soil N pools.
• To analyze these effects across diverse ecosystems using a large-scale meta-analysis.
• To identify the primary drivers influencing soil N pool responses to acid rain.

Main Methods:

• Conducted a meta-analysis of 1195 paired observations from 74 studies.
• Systematically assessed the effects of SAR, MAR, and NAR on various soil N pools (total N, ammonia N, nitrate N, microbial biomass N, alkaline hydrolyzable N).
• Analyzed ecosystem-dependent responses and the influence of pH, climate, and soil texture.

Main Results:

• SAR increased soil total N and ammonia N, while decreasing nitrate N and microbial biomass N.
• NAR decreased soil total N, ammonia N, and microbial biomass N, but increased alkaline hydrolyzable N.
• MAR showed intermediate effects; responses were stronger in forests and grasslands, and primarily driven by pH.

Conclusions:

• Acid rain composition fundamentally alters soil N dynamics, with pH being the primary controlling factor.
• Nitric acid rain (NAR) poses a particular vulnerability to affected ecosystems.
• Results necessitate incorporating differential acid rain effects into Earth system models for accurate N cycling projections.