Land-use intensification reshapes microbial phosphorus cycling, organic matter composition, and phosphorus fractions in Amazonian soils.

Soil phosphorus (P) is a limiting factor for vegetation growth in the Amazon rainforest, where plants depend on microorganisms for organic matter cycling and nutrient uptake. While forest-to-agriculture conversion fundamentally reshapes plant-microbe-soil interactions and P cycling, these dynamics are further modulated by the intensity of land management. This study examined the 30-year effects of converting a primary forest into two contrasting systems: a low-intensity agroforest and a high-intensity citrus monoculture. We investigated how microbial and low molecular weight organic compounds (LMWCs) composition interacted with soil physicochemical attributes, acid phosphatase activity, and P fractions (labile, moderately labile, non-labile, and residual). Agroforest soils retained physicochemical and enzymatic attributes similar to the primary forest, while soils of the citrus plantation showed increased P in all fractions due to mineral fertilization and reduced soil organic matter content, mainly in deeper layers. Microbial and LMWC composition patterns reflected land-use, with agroforest representing an intermediate state between primary forest and citrus monoculture. Pseudomonadota and nutrient-rich LMWC were more abundant in the agroforest, whereas Ascomycota and nutrient-poor LMWC predominated the citrus plantation. Genes related to "P acquisition" were more abundant in forest and agroforest soils, while genes related to "P-compound synthesis" were more abundant in the citrus plantation. Labile P was negatively correlated with genes related to microbial metabolism, suggesting that reduced P availability may induce a boost in microbial activity for internal P-cycling. These findings demonstrate that forest-to-agriculture conversion strongly affects microbial functions, with responses aligning with land-use intensity and LMWC resource availability. Nonetheless, microbes adapt by shifting strategies: prioritizing mineralization and solubilization or favoring biosynthesis depending on P availability.