Response of Soil Microbial Communities and Nutrient Stoichiometry to Stand Age in Chinese Fir Plantations: Insights at the Aggregate Scale

Afforestation is often considered to be an effective solution to alleviate soil degradation; however, unreasonable successive monoculture of Chinese fir results in further soil nutrient losses in impoverished soil systems. To determine the nutritional deficiency type of soil, it is critical to clarify the interactions between soil microbial and soil nutrients since the nutritional need of the microbial biomass is restricted by specific nutrients. Thus, the evaluation of such interactions can further reflect the situation of nutrient levels in soil. Nutrient stoichiometry and microbial communities within soil aggregates (> 2 mm, 2–1 mm, 1–0.25 mm, and < 0.25 mm) were determined in soils collected from Chinese fir plantations along the chronosequence (control, 9-, 17-, and 26-yr) in 2019, Guangxi, China. All soil microbial taxonomic groups (total microbial biomass, general bacteria, fungi, Gram-positive bacteria, Gram-negative bacteria, and actinomycete) showed the highest concentrations in micro-aggregates (< 0.25 mm), in line with the trends of soil carbon to nitrogen (C:N) ratios, carbon to phosphorus (C:P) ratios, and nitrogen to phosphorus (N:P) ratios. Compared with control (CK), soil C:N, C:P ratios, and all microbial taxonomic groups were significantly higher in the 17-yr of plantations, which elevated by 91–108%, 23–53%, and 31–204%, respectively. In contrast, N:P ratios significantly decreased by 30–65% in 9–26 yr plantations compared with CK. These results indicate that afforestation with Chinese fir contributed to the accumulation of soil nutrients and microbial biomass. In this area, soil phosphorus (P) was the main nutrient limiting soil quality in Chinese fir plantation. To prevent significant soil nutrient losses and the decline of soil microbial biomass, it is crucial to monitor the dynamic changes in microbial activities and P losses within aggregates, especially in micro-aggregates.