Plant diversity plays a crucial role in shaping ecosystems, impacting everything from productivity to the storage of soil organic carbon (SOC). While numerous studies have examined the effects of plant diversity on SOC, most of these have relied on controlled biodiversity experiments.
These experiments have limitations, such as their short duration and controlled abiotic factors, which might not fully capture the complex relationship between plant diversity and SOC storage. In contrast, real-world gradients of biodiversity, developed over millennia, can provide valuable insights into the interaction between biodiversity and ecosystem functioning.
A comprehensive study was undertaken to explore how plant diversity and soil organic matter are related across a wide range of climate conditions, specifically in grasslands. The hypothesis posited that plant diversity positively influences SOC content through its impact on plant biomass and organic matter input into the soil. Additionally, it was expected that this relationship would be more pronounced in arid grasslands as opposed to more humid ones.
The study involved 84 natural and semi-natural grassland sites across six continents, representing 19 distinct grassland types. At each site, an average of 30 plots were examined, and the sites were part of the Nutrient Network Global Research Cooperative, free from experimental manipulations.
Contrary to the initial hypothesis, the study did not find a significant correlation between the Shannon index of plant diversity and plant biomass across the 84 grasslands. This could be attributed to the counteracting effects of plant diversity, where it increases plant biomass through resource complementarity but also leads to species loss due to shading by larger plants. This finding aligns with a previous study that found no significant relationship between productivity and plant species richness.
The absence of a significant correlation between plant diversity and plant biomass implies that plant diversity is not associated with SOC through the rate of aboveground biomass input into soils.
In contrast to plant biomass, the study found a positive correlation between SOC content and the Shannon index of plant diversity across all 84 sites.
As the Shannon index increased from 0 to 2.5, SOC content increased by a factor of 2.6. This increase in SOC with plant diversity was notably higher than the limited changes observed in biodiversity experiments. This suggests that plant diversity's true impact on SOC is likely underestimated in controlled experiments due to the slow response of SOC content to vegetation changes.
Plant diversity might influence SOC content through the quality of organic matter, particularly the carbon-to-nitrogen ratio. Higher plant diversity leads to higher carbon-to-nitrogen ratios in plant biomass, which, in turn, slows down decomposition rates. The study's findings align with a litter decomposition study that showed that nitrogen addition to plant litter increased early-stage litter decomposition, illustrating the role of carbon-to-nitrogen ratios.
The presence of lignin, which decomposes slowly due to its complex structure, also contributes to higher SOC levels in areas with high plant diversity. This result is consistent with the positive relationship observed between carbon-to-nitrogen ratios and SOC content globally.
Plant diversity not only affects carbon-to-nitrogen ratios but also the diversity of organic compounds in plant litter and soils. A diverse pool of molecules increases the cost of decomposition for soil microorganisms, as it requires a wide range of enzymes. Consequently, a diverse pool of organic compounds from a diverse plant community might decompose more slowly than a less diverse pool, further influencing SOC content. Additionally, high plant diversity can lead to high soil microbial biomass and soil aggregation, both promoting SOC sequestration.
The positive correlation between plant diversity and SOC was found to be climate-dependent. The relationship was significant in sites with higher mean annual temperature (MAT), lower mean annual precipitation (MAP), and a higher aridity index, but not in cooler and moister sites. The strength of the relationship diminished with decreasing MAT, increasing MAP, and higher aridity index.
This climate-dependent relationship is mainly attributed to the significant correlation between the Shannon index and soil carbon-to-nitrogen ratios in warm and arid sites. The higher soil carbon-to-nitrogen ratios lead to lower organic matter decomposition rates and, consequently, higher SOC contents in these regions.
Plant biomass was positively correlated with SOC content across all 84 sites, but this relationship was influenced by climatic conditions. It was more pronounced in sites with higher MAT, while sites with lower MAP or aridity index had weaker correlations between SOC and plant biomass.
Arid and cooler sites appear to be influenced more by decomposition rates than organic carbon input to the soil, leading to a lack of significant correlations between plant biomass and SOC at these sites.
A model emphasizing the relationship between plant diversity and SOC through carbon-to-nitrogen ratios, in conjunction with aridity, provided a better fit for the data compared to a model focusing on plant biomass. This model underscores that plant diversity influences SOC through organic matter quality, with climate acting as a crucial modulator.
In conclusion, the study found a positive relationship between plant diversity and SOC content, with the strongest effect observed in warm and arid grasslands.
Contrary to expectations, the study indicated that plant diversity affects SOC content through organic matter quality, primarily the carbon-to-nitrogen ratio, while plant biomass was related to SOC but not to plant diversity. This research highlights the climate-dependent nature of the relationship between biodiversity and ecosystem processes and underscores the importance of considering climate conditions when studying these interactions, particularly in the context of SOC storage in grassland ecosystems.
