How do aboveground and belowground consumers impact nitrogen mineralization?

Background/Question/Methods

Terrestrial ecosystems contain both plant-based and detritus-based food chains that are thought to have strong and interactive impacts on nutrient cycling. Previous research has demonstrated that species in the plant-based chain can impact nutrient cycling both directly through inputs into the soil and indirectly by changing the composition or properties of the plant community. Species in the detritus-based chain have similar direct and indirect impacts on nutrient cycling, although the microbial community typically facilitates their indirect effects. Although the impact of species in both chains has been well studied, we have yet to explore potential feedbacks between plant- and detrital-based chains. Our study examines how the number of trophic levels in the plant-based and detritus-based food chains impacts the rate of net nitrogen mineralization in an old-field ecosystem. We employed a fully factorial design by sequentially adding strongly interacting species to either the plant or detritus based chains in experimental mesocsoms. The plant-based food chain contained goldenrod and grasses, a grasshopper herbivore (Melanoplus femurrubrum), and a nursery web spider predator (Pisaurina mira). Similarly, the detrital-based chain contained the soil microbial community, a woodlouse detritivore (Oniscus asellus), and a wolf spider predator (Rabidosa rabida).

Results/Conclusions

Plant-based and detritus-based food chains had unique and interactive effects on nitrogen cycling. Consistent with previous work, nitrogen mineralization was higher in cages containing grasshoppers than in control cages (an increase from 0.023 to 0.028 μg-N/g_{dry weight}×day), and further elevated (0.042 μg-N/g_{dry weight}×day) in cages with grasshoppers and nursery web spiders. In contrast to expectations, nitrogen mineralization was unaffected by the addition of woodlouse or wolf spiders. However, the presence of woodlouse tended to negate the positive effects of grasshoppers and nursery web spiders by reducing soil nitrogen mineralization from the rates reported above to 0.019 and 0.029 μg-N/g_{dry weight}×day, respectively. Monitoring data indicates that the presence of woodlouse decreased the survival of grasshoppers by 20% when nursery web spiders were present. The significant decrease in grasshopper survival suggests a novel interaction between plant-based and detritus-based food chains, and a potential explanation for why woodlouse damped the nitrogen mineralization trophic cascade. Interestingly, woodlouse did not alter the cascading effect of nursery web spiders on plant biomass. Our study demonstrates the potential for interactions between species in the plant-based and detrital-based food chains and suggests that these interactions can have important implications for trophic cascades and ecosystem functioning.