Rhizosphere soil C:P stoichiometry differentially regulates microbial necromass carbon responses to nitrogen and phosphorus addition in subtropical forests

Description

Subtropical forest soils are typically nitrogen (N)-rich but phosphorus (P)-deficient, creating a strong stoichiometric imbalance that may reshape microbial pathways responsible for soil organic carbon (SOC) stabilization, particularly in the microbially active rhizosphere. Microbial necromass carbon (MNC) is a stable component of SOC, but the mechanisms by which microbial traits (e.g., functional diversity, life-history strategies, and network interactions) regulate MNC accumulation under N and P addition remain unclear. This study conducted N and P addition experiments in a subtropical forest, and collected rhizosphere soil from four representative broadleaf tree species, to assess the microbial mechanisms and pathways driving MNC variation in response to soil ecological stoichiometry. The results showed that N addition significantly increased FNC and MNC, whereas P addition significantly reduced both; BNC increased under both treatments. The rhizosphere soil C:P ratio was strongly associated with MNC variation, and it increased markedly under N addition while it decreased under P addition. Furthermore, variation in FNC was closely associated with microbial r-strategists, r/K ratios, and network degree, whereas variation in BNC was linked to microbial functional diversity and network degree. These findings demonstrate that rhizosphere soil C:P stoichiometry integrates nutrient availability, microbial community traits, and network interactions to regulate MNC accumulation. By revealing how N and P addition shift this balance in the rhizosphere soil, our study provides mechanistic insights with broader implications for understanding microbial contributions to long-term SOC stabilization in forest ecosystems.

Steps to reproduce

In July 2015, a typical secondary evergreen broadleaf forest was selected for an N and P addition experiment comprising four treatments: control (CK: no addition), N addition (N: 100 kg N ha-1 year-1), P addition (P: 50 kg P ha-1 year-1), and combined N+P addition (N+P: 100 kg N ha-1 year-1+50 kg P ha-1 year-1). Experimental plots were located at ~550 m elevation on acidic mountain yellow-red soils, with slopes <20° and southern exposure. N was applied as ammonium nitrate/urea (NH4NO3/CO(NH2)2) and P as sodium dihydrogen phosphate (NaH2PO4). Nutrient additions were conducted following a standardized protocol, with half of the annual amount applied in solid form in March and September. The experiment followed a completely randomized design with three replicates per treatment. Each plot measured 20 × 20 m, and the adjacent plots were spaced more than 20 m apart. In July 2021, all plots were surveyed. Four dominant native tree species—Schima superba, Machilus nanmu, Castanopsis carlesii, and Castanopsis eyrei—were identified in each plot. For each species within each plot, rhizosphere soil and root exudates were collected from 3–5 representative trees. Samples from the same species within a plot were thoroughly homogenized to obtain one composite sample, which was retained for subsequent analyses. Rhizosphere soils were collected using the shaking method. After removing surface litter, roots were excavated, and loosely attached soil was shaken off. Soil adhering within 4 mm of the root surface was considered rhizosphere soil. Multiple individuals per species were sampled and pooled to form representative composite samples.

Institutions

• Jiangxi Agricultural University

  Jiangxi, Nanchang

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