Microbial activity in herbivore dung affects nodulation and growth of Trifolium pratense

Herbivore dung varies among species in terms of nutrients and microbial composition, but the effect of the microbial activity in the dung on plant growth and symbiotic association is largely unknown. This study aimed to investigate the influence of cow dung quantity and dung microbial activity on plant-microbe interactions (mycorrhiza, nodulation) and the growth of Trifolium pratense seedlings.

A mesocosm experiment was conducted with gradients of unsterilized or sterilized cow dung applied to pots with T. pratense seedlings. Biomass, relative growth rate (RGR), root phosphomonoesterase (PME) activity, nodulation, and root mycorrhizal colonization of the seedling were measured after 7 weeks.

Growth of T. pratense increased with increasing dung supply, while sterilization marginally decreased growth by 15–20% (P = 0.068). Root nodulation increased with increasing dung supply but was significantly lower with sterilized dung. Both root PME activity and mycorrhizal colonization decreased with increasing dung supply but were unaffected by sterilization.

The decreased growth and nodulation due to dung sterilization aligned with our predictions but could not be attributed to reduced dung decomposition and nutrient availability. Instead, the reduced microbial activity from sterilization likely negatively impacted bacterial N₂-fixing activity and, consequently, plant growth. This suggests that herbivore dung’s effect on plant growth and interactions is more complex than previously anticipated, influenced not only by nutrient supply and stoichiometry but also by microbial composition and activity.