Plant footprint decreases the functional diversity of molecules in topsoil organic matter after millions of years of ecosystem development

Abstract Aim Theory suggests that the diversity of molecules in soil organic matter (SOM functional diversity) provides key insights on multiple ecosystem services. We aimed to investigate how and why SOM functional diversity and composition change as topsoils develop, and its implications for key soil functions (e.g., from nutrient pool to water regulation). Location We reported data on 16 soil chronosequences globally distributed in nine countries from six continents. Time Period 2016–2017. Major Taxa Studied Soil microbes (bacteria and fungi) and vascular plants. Methods SOM functional diversity and composition without mineral interference were measured using diffuse reflectance mid‐infrared Fourier transform spectroscopy (DRIFT). We aimed to characterize the main environmental factors related to SOM functional diversity and composition. Also, we calculated the links among SOM functional diversity and key soil functions. Results We found that SOM functional diversity declines after millions of years of soil formation (pedogenesis). We further showed that increases in plant cover and productivity led to a higher ratio of reduced (e.g., alkanes) over oxidized carbon forms (i.e., C: O‐functional groups ratio), which was positively correlated to SOM functional diversity as soils age. Our findings indicated that the plant footprint (i.e., the accumulation of plant‐derived material promoting the C: O‐functional group ratio) would explain the reduction of SOM functional diversity as ecosystems develop. Moreover, the dissimilarity in SOM composition consistently increased with soil age, with the soil development stage emerging as the main predictor of SOM dissimilarity across contrasting biomes. Main Conclusions Our global survey contextualized the natural history of SOM functional diversity and composition during long‐term soil development. Together, we showed how plant footprint drives the losses of SOM functional diversity with increasing age, which might provide a novel mechanism to explain typically reported losses in ecosystem functions during ecosystem retrogression.