Crop diversity gradients alter molecular composition of mineral-associated organic matter and its susceptibility to phosphate-induced desorption

Mineral-associated organic matter (MAOM) contributes to soil carbon (C) persistence but also includes a fraction susceptible to mobilization, partly driven by its molecular composition. Agricultural practices that alter organic matter inputs, such as increasing crop diversity or perenniality, may influence MAOM chemistry and thus its exchangeability. We examined how a crop diversity gradient in a temperate agricultural system changes MAOM molecular composition and its susceptibility to inorganic phosphorus (Pi)-induced desorption. Soils were collected two years after establishment of wheat monoculture (W), wheat-bean intercrop (WB), and wheat-bean intercropped with two perennial cover crops (WBCC). We quantified MAOM C concentrations, characterized MAOM composition using pyrolysis-GC/MS, and assessed Pi-induced mobilization through short-term incubations. Crop diversity did not affect MAOM C concentrations or compound richness after the 2-yr field trial. However, MAOM composition progressively diverged across the crop diversity gradient. The perennial-inclusive system (WBCC) had higher relative abundances of lipids and lower proportions of N-bearing and phenolic compounds compared to monoculture wheat. In desorption incubations, WBCC soils exhibited C desorption following Pi addition, whereas wheat monoculture soils showed net sorption. Pi-induced C mobilization correlated positively with unclassified compounds and negatively with phenols. Though MAOM under higher crop diversity exhibited greater susceptibility to Pi-induced desorption compared to wheat monoculture, this did not translate to greater C losses via mineralization. Our findings demonstrate that MAOM molecular composition is a sensitive measure of soil responses to crop diversification and nutrient inputs, highlighting the importance of MAOM chemistry in regulating the balance between MAOM persistence and exchangeability.