Plant attributes interact with fungal pathogens and nitrogen addition to drive soil enzymatic activities and their temporal variation

Nitrogen enrichment can alter soil communities and their functioning directly, via changes in nutrient availability and stoichiometry, or indirectly, by changing plant communities or the abundance of consumers. However, most studies have only focused on one of these potential drivers and we know little about the relative importance of the different mechanisms (changes in nutrient availability, in plant diversity or functional composition or in consumer abundance) by which nitrogen enrichment affects soil functioning. In addition, soil functions could vary dramatically between seasons; however, they are typically measured only once during the peak growing season. We therefore know little about the drivers of intra-annual stability in soil functioning.In this study, we measured activities of beta-glucosidase and acid phosphatase, two extracellular enzymes that indicate soil functioning. We did so in a large grassland experiment which tested the effects, and relative importance, of nitrogen enrichment, plant functional composition and diversity, and foliar pathogen presence (controlled by fungicide) on soil functioning. We measured the activity of the two enzymes across seasons and years to assess the stability and temporal dynamics of soil functioning.Overall beta-glucosidase activity was slightly increased by nitrogen enrichment over time but did not respond to the other experimental treatments. Conversely, plant functional diversity and interactions between plant attributes and fungicide application were important drivers of mean acid phosphatase activity. The temporal stability of both soil enzymes was differently affected by two facets of plant diversity: species richness increased temporal stability and functional diversity decreased it; however, these effects were dampened when nitrogen and fungicide were added.The fungicide effects on soil enzyme activities suggest that foliar pathogens can also affect below-ground processes and the interacting effect of fungicide and plant diversity suggests that these plant enemies can modulate the relationship between plant diversity and ecosystem functioning. The contrasting effects of our treatments on the mean versus stability of soil enzyme activities clearly show the need to consider temporal dynamics in below-ground processes, to better understand the responses of soil microbes to environmental changes such as nutrient enrichment.

Automated summary

Nitrogen enrichment can affect soil communities and their functioning through changes in nutrient availability and stoichiometry, as well as by altering plant communities and consumer abundance. However, the relative importance of these mechanisms, and the drivers of intra-annual stability in soil functioning, are not well understood. This study examined the effects of nitrogen enrichment, plant functional composition and diversity, and foliar pathogen presence on soil functioning using two enzymes as indicators. The results showed that nitrogen enrichment had a slight impact on one enzyme, while plant diversity and fungicide application were important drivers of the other enzyme. The study highlights the need to consider temporal dynamics and the interactions between plants and pathogens when studying soil functioning under environmental changes.