Changes in microbial metabolic C- and N- limitations in the rhizosphere and bulk soils along afforestation chronosequence in desertified ecosystems

The resource acquisition strategy of soil microorganisms can be reflected by soil extracellular enzyme activity (EEA). However, there are few reports on the application of extracellular enzyme stoichiometry (EES) method to study the difference in microbial metabolic nutrient limitation between rhizosphere and bulk soil. Here, we choose the rhizosphere and bulk soils of Pinus sylvestris var. mongolica (P. sylvestris) plantations with five stand ages in the Mu Us sandy land, and analyzed the variation and differences of microbial metabolic limitation between rhizosphere and bulk soils with stand age by EES method. The results showed that the microbial metabolic C-limitation in the rhizosphere and bulk soil gradually increased with stand age. Almost all the vector angles were less than 45°, which indicated that the soil microbial metabolism was relatively limited by N rather than P. Furthermore, the microbial C- and N-limitation in rhizosphere soils were generally stronger than bulk soils. Soil physical properties (59.73%) explained most of the variations in soil EES based on the variation-partitioning analysis, followed by total nutrients (43.00%). The partial least squares path model suggested that the main driving factor for the variation of soil microbial metabolic C-limitation in the rhizosphere and bulk soils was physical properties, while the microbial N-limitation was for total nutrients. In general, the study emphasized the application of EES methods to assess the dynamic equilibrium between soil microbial resource acquisition and nutrient availability in desert ecosystems. These insights provide guidance for formulating afforestation strategies, such as nutrient management of sandy plantations.