Terrestrial ecosystem development, involving changes in ecosystem structure and function over centuries to millennia, is widely believed to be controlled by five state factors: time (soil age), climate, topography, parent material, and organisms (vegetation). Hypotheses suggest soil age is a major ecosystem driver locally, influencing carbon stocks, C:N:P ratios, and soil pH. However, soil age isn't the sole driver; ecosystems with the same soil age can differ. A comprehensive assessment comparing the relative importance of soil age to other state factors (parent material, climate, vegetation, topography) across global biomes is lacking. Most studies focus on spatial (climate, vegetation) or temporal (soil age) gradients separately. Understanding the interplay of these factors is crucial for forecasting and managing ecosystems facing climate and land-use changes. Previous chronosequence studies quantified how local ecosystem development affects ecosystem properties over time. However, the role of environmental context in driving absolute values, trajectories, and rates of development across diverse climates, biomes, and parent materials is poorly understood. A regional study suggested drier environments show weaker trends in ecosystem development than mesic ecosystems. This research aims to quantify the contribution of soil age relative to other state factors in controlling ecosystem properties across biomes and investigate changes in multiple ecosystem properties during development across ecosystem types.

Several studies have utilized long-term soil chronosequences to examine the effects of local ecosystem development on above- and belowground properties, biogeochemical cycling, and community structure over centuries to millennia. However, information on the role of environmental context in driving the absolute values, trajectories, and rates of development for multiple ecosystem properties across contrasting climates, biomes, and parent material types is limited. A recent regional study highlighted that drier environments exhibit different trends in ecosystem development compared to more mesic ecosystems. This lack of a unified understanding of the role of environmental context in driving terrestrial ecosystem development across diverse global biomes motivated this study.

This study combined two approaches: a global field survey and a meta-analysis of existing data. The field survey collected data from 16 chronosequences across six continents, focusing on topsoil (0-10 cm). This depth was chosen due to its biological activity and common usage in similar studies. The chronosequences covered a wide range of soil ages (hundreds to millions of years), vegetation types (grasslands, shrublands, forests, croplands), origins (volcanic, sedimentary, dunes, glacier), and climates. Thirty-two topsoil, plant, and microbial properties were measured (Supplementary Table 3). The meta-analysis included data from 48 additional chronosequences from the literature, expanding the range of conditions considered. Variation Partitioning Modeling was used to determine the unique and shared contributions of soil age, parent material, climate, vegetation type, and topography to the variation in ecosystem properties. Partial Spearman correlations controlled for soil age were used to investigate associations between environmental factors and ecosystem properties. Finally, Spearman rank correlations were used to analyze changes in ecosystem properties within chronosequences, identifying consistent patterns of change over time.

Soil age was a significant but relatively weak ecosystem driver across biomes. Parent material, climate, vegetation type, and topography collectively explained 24 times more variation in ecosystem properties than soil age alone. Soil age explained a small percentage (2.1% unique and 3.5% shared) of the total variation. Parent material, climate, and vegetation explained significantly more variation than soil age. Parent material strongly influenced properties like bacterial biomass, climate affected soil pH, and vegetation type influenced texture. Despite its relatively weak overall influence, soil age explained significant unique variation in soil N:P and C:P ratios, soil pH, TBR (total base cation reserve), plant productivity, and C stocks. The meta-analysis of published data supported these findings. Further analyses revealed that environmental context, rather than soil age, determined the structure and function of terrestrial ecosystems. Drier and sandy ecosystems had lower values for multiple properties than wetter ecosystems. Sandy soils showed slower development of microbial biomass, C stocks, and nutrient availability compared to other substrates. Irrespective of soil age, drier ecosystems had more alkaline soils and less plant productivity. Consistent patterns in ecosystem property changes with soil age were observed. Increases in soil N:P and C:P ratios, microbial biomass, C stocks, and available P with increasing soil age were common. Declines in soil total P and pH were also observed. However, other properties showed inconsistent patterns, highlighting the influence of local conditions.

This study challenges the hypothesis that soil age is a primary driver of ecosystem structure and function across biomes. While significant at a local scale, its influence is overshadowed by parent material, climate, and vegetation at broader scales. The finding that environmental context is a major determinant of ecosystem development highlights the importance of considering these factors when predicting ecosystem responses to global change. The study confirms the well-established paradigm that ecosystems of similar age can be at different stages of development. The reduced influence of time in arid and cold environments might be due to dust inputs and erosion. Changes in environmental context, such as those associated with deforestation and shifts to drier conditions, could have significant negative impacts on ecosystem services, including carbon storage, soil fertility, and plant production. While soil age's overall contribution is relatively small, it still accounts for unique variation in certain properties, suggesting its inclusion in global ecosystem models to enhance predictive accuracy.

This research demonstrates that while soil age is a significant ecosystem driver at the local scale, its influence on ecosystem structure and function across biomes is less substantial than that of environmental context factors such as parent material, climate, and vegetation type. This finding underscores the critical role of environmental context in determining the trajectories of ecosystem development. Future research should focus on disentangling the complex interactions between soil age and these environmental factors, and on refining ecosystem models to incorporate these findings for improved prediction of ecosystem responses to ongoing global change.

The study acknowledged some limitations. Certain geographic locations (e.g., continental Africa) were underrepresented in the datasets, potentially affecting the generalizability of the findings. The reliance on current climate data as a proxy for past climates is a simplification, as past climatic conditions may have influenced ecosystem development trajectories. Further, the study primarily focused on topsoil, limiting the insights into deeper soil layers.