Carbon, Nitrogen, and Phosphorus Stoichiometric Characteristics and Enzyme Activity Dynamics in Soils of Navel Orange Orchards with Different Ages

Soil C, N, and P, along with their stoichiometric ratios, are important indicators for evaluating soil fertility. However, the dynamics of C:N:P stoichiometry in soils under citrus cultivation of varying durations remain unclear. This study investigated soil C:N:P stoichiometry and extracellular enzyme activities (βG, NAG, and AP) across different soil depths (0–10, 10–20, and 20–40 cm) in navel orange orchards with different ages (3-, 5-, 10-, and 15-year-old). Soils cultivated for 10 years exhibited significantly higher DOC, available N, and P contents compared to other durations. The TC:TN and TC:TP ratios at 0–10 cm were significantly 0.17–4.78 and 3.94–16.1 higher than those at 10–40 cm, respectively (p < 0.05). Notably, the TC:TP and TN:TP ratios observed in the 5-, 10-, and 15-year-old orchards were lower than in the 3-year-old orchard, suggesting an alleviation of P limitation in older orchards. The MBC:MBN ratio remained largely stable with orchard age at 0–20 cm (p > 0.05), whereas younger orchards (3- and 5-year-old) exhibited significantly 18.2–168 and 2.17–17.4 higher MBC:MBP and MBN:MBP ratios than older ones (10- and 15-year-old) at 10–40 cm (p < 0.05). At 0–10 cm, the 15-year-old orchard exhibited higher βG:NAG and βG:AP ratios than the younger orchards. Soil total C:P and microbial C:P ratios explained 29.7% and 20.1% of the variation in enzyme activity composition among abiotic and biotic factors, respectively (p < 0.01). These findings highlight the importance of monitoring P availability in mature orchards to maintain sustainable soil fertility. Future research should focus on long-term field experiments to validate optimal P fertilization thresholds and investigate the specific microbial community shifts driving these stoichiometric changes across different soil depths.