Mediation of soil aggregate stability and carbon sequestration by meadow steppe soil microorganisms under long-term nitrogen and phosphorus nutrient additions

Abstract

Soil aggregates are key for soil organic carbon (SOC) storage and microbial habitats, but how long-term nitrogen (N) and phosphorus (P) additions affect aggregate stability and C retention via microbes is unclear. We studied this in a meadow steppe under control, N, P, and NP, measuring SOC, mean weight diameter (MWD), and microbial communities in bulk soil and aggregate sizes across seasons. N and NP significantly increased MWD across seasons (by 14%−42%) but did not consistently elevate SOC. Both treatments reduced microbial biomass (30%−50%) across all aggregate sizes by lowering soil pH and available potassium. Microbial taxa influencing MWD differed by aggregate size: arbuscular mycorrhizal fungi (AMF) biomass in bulk soil, mega-aggregates, and macro-aggregates primarily affected MWD, while bacterial biomass became more significant in micro-aggregates. Microbial biomass showed a consistent negative correlation with MWD across all fractions. The increase in MWD was primarily driven by soil acidification, as N and NP treatments reduced pH from 7.2 to 5.9, which stimulated abiotic cementation and simultaneously inhibited AMF biomass. The positive effect of acidification on MWD outweighed the negative effect of reduced AMF. In micro-aggregates, the pH decline enhanced bacterial regulation of MWD by suppressing bacterial decomposition of organic binding agents. Changes in SOC content were mainly regulated by total N content, with only a weak indirect influence from microbial biomass.

Graphical abstract

Keywords

nitrogen and phosphorus additions / soil aggregate size / soil organic carbon / soil microorganisms / mean weight diameter

Highlight

	● N and NP addition enhanced aggregate stability but reduced microbial biomass.
	● AMF biomass stabilized bulk and mega-aggregates; bacteria regulated micro-aggregates.
	● SOC was primarily driven by pH and total N, with little direct microbial effect.
	● MWD is co-regulated by microbial biomass in bulk soil and aggregates, as well as by pH.
	● The decrease in pH is the main driving factor for the increase in MWD.

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