Nitrogen addition enhances phytolith-occluded carbon accumulation across particulate and mineral-associated organic matter fractions in a meadow steppe

Anthropogenic nitrogen (N) deposition is increasing worldwide and is modifying soil carbon (C) dynamics in grassland ecosystems. Phytolith-occluded C (PhytOC) represents a relatively stable biogenic C pool, is likewise affected by rising N input. As particulate organic matter (POM) and mineral-associated organic matter (MAOM) differ in C stability, clarifying the distribution of PhytOC between POM and MAOM is essential for assessing its retention potential and relative importance within soil C pools under N enrichment. Using a long-term N addition experiment (0–50 g N m −2 yr −1 ) in a meadow steppe, we quantified PhytOC and its ratio to soil organic C (SOC) in POM and MAOM fractions in 2019 and 2020. Both aboveground PhytOC production flux and PhytOC in bulk soil were significantly enhanced by N addition. In POM fractions, PhytOC increased proportionally with particulate organic C (POC), resulting in a stable PhytOC/POC ratio. In MAOM fractions, PhytOC increased even as the mineral-associated C (MAOC) declined, resulting in a higher relative contribution of PhytOC. Soil chemical properties were the dominant drivers of PhytOC in the POM fractions, whereas microbial biomass played a stronger role in the MAOM fractions. These findings show that N addition altered the distribution of PhytOC among surface soil fractions in grasslands, and indicate that PhytOC becomes a more important C source supporting the MAOC under N enrichment. We emphasize that incorporating the dynamics of PhytOC into soil C sequestration assessments is essential for accurately evaluating response of grassland C pools to N deposition.