Variations in microbial C-limitation and N-limitation dependent on stand age within soil aggregates of coastal saline Robinia plantations

The application of ecoenzymatic stoichiometric theory (EEST) models is a simple and effective method to estimate microbial nutrient limitation in terrestrial ecosystems. Ecological restoration of afforestation is a topic of broad interest, yet our understanding of microbial metabolic status in coastal restored soils, particularly at the aggregate scale, remains limited. Here, we investigated the activities of C-, N-, and P-acquiring enzymes within soil aggregates from different plantations of Robinia pseudoacacia (established in the spring of 1988, 2002, 2012, and 2018 using one-year-old seedlings in coastal undisturbed grasslands). Then, we applied the EEST models to examine how afforestation affects microbial nutrient limitation. We found that (1) soil C and N contents and their acquiring enzyme activities were significantly increased along the afforestation chronosequence, and were higher in macro-aggregates (> 0.25 mm) than in micro-aggregates (< 0.25 mm). (2) However, microbial metabolism in coastal soils was colimited by C and N, with a significant coupling relationship between C-limitation and N-limitation in macro-aggregates. Notably, afforestation significantly promoted the proportion of soil macro-aggregates, and alleviating their N limitation but intensifying C limitation, and the effect was more pronounced when the stand age exceeded 22 years. (3) Most plant traits significantly correlated with microbial nutrient limitation in coastal soils of R. pseudoacacia plantations. Therefore, plant trait modulation through ecological measures, such as improving the species composition and age structure of R. pseudoacacia plantations, might serve as an effective approach for managing microbial nutrient limitation and improving soil health in coastal saline-alkali ecosystems. • Afforestation improved soil aggregate structure, enhanced soil C and N fertility. • Microbial metabolism was co-limited by C and N across different aggregates in coastal soils. • Plant traits, soil salinity and bulk density jointly regulated microbial nutrient limitation.