Consistent effects of inorganic and organic N addition on microbial necromass carbon in subtropical forest soils: A mesocosm study

Atmospheric nitrogen (N) deposition can substantially influence the accumulation of microbial residues in forest soils. However, the effects of deposited N forms on soil microbial necromass carbon (MNC) and the underlying microbial mechanisms remain unclear. In this study, an N addition experiment was conducted in a subtropical plantation to investigate the effects of inorganic N (NH4Cl) and organic N (urea and glycine) on soil microbial biomass, hydrolytic enzyme activities, and MNC at 0–10 cm and 10–20 cm depths. The results showed that, despite unchanged soil organic carbon (SOC) content and mineralization, additions of NH4Cl, urea, and glycine reduced soil MNC by 20.7%, 17.4%, and 20.1%, respectively. This decline in soil MNC was mainly attributable to a reduction in fungal necromass carbon, whereas bacterial necromass carbon remained unchanged following N addition. Regardless of N form, N addition increased microbial biomass, intensified microbial carbon and phosphorus limitations, and shifted microbial communities from r- to K-strategists. Furthermore, both inorganic and organic N addition decreased the MNC:SOC ratio, indicating a reduced contribution of microbial residues to the SOC pool. Shifts in microbial community composition toward K-strategists, together with aggravated phosphorus limitation, accounted for the decline in soil MNC under N addition. Overall, these findings suggest that, irrespective of N form, atmospheric N deposition weakens the contribution of MNC to SOC by regulating microbial life-history strategies and nutrient limitations in subtropical plantations.大气氮（N）沉降可显著影响森林土壤中微生物残体的积累。然而，不同形态沉积氮对土壤微生物残体碳（MNC）的影响及其潜在微生物学机制仍不明确。本研究在亚热带人工林中开展氮添加试验，探究无机氮（NH4Cl）与有机氮（尿素、甘氨酸）对0–10 cm和10–20 cm土层的土壤微生物生物量、水解酶活性及MNC的影响。结果显示，尽管土壤有机碳（SOC）含量与矿化作用未发生改变，氯化铵、尿素和甘氨酸添加分别使土壤MNC降低了20.7%、17.4%和20.1%。土壤MNC的下降主要源于真菌残体碳的减少，而氮添加后细菌残体碳未出现显著变化。无论氮形态如何，氮添加均提升了微生物生物量，加剧了微生物碳与磷限制，并使微生物群落从r-策略菌向K-策略菌转变。此外，无机氮与有机氮添加均降低了MNC:SOC比值，表明微生物残体对SOC库的贡献有所减弱。微生物群落组成向K-策略菌转变，加之磷限制加剧，共同解释了氮添加下土壤MNC的下降。总体而言，本研究结果表明，无论氮形态如何，大气氮沉降可通过调控亚热带人工林的微生物生活史策略与养分限制，削弱MNC对SOC的贡献。