Harnessing soil communities for N retention

Background/Question/Methods

Soil biota play an important role in carbon (C) and nitrogen (N) cycling, which are processes that underpin ecosystem services of food production and climate mitigation. During ecosystem succession, the abundance of fungi and fungal-feeding fauna generally increase, whereas bacteria and bacterial-feeding fauna decrease. It is well known that land use intensification can reverse this trend by shifting soil food webs back from being fungal-based to being bacterial-based. Because fungal-based soil food webs are linked to more efficient N cycling, these shifts in the relative abundance of fungi and bacteria likely have consequences for ecosystem retention of N, which contributes to eutrophication and climate warming when lost from soil. However, it is not clear what the relative importance is of land use, climate, and plant communities for the composition of soil communities and processes of N cycling. Here, I use the results of three experiments - ranging from a landscape-scale field survey linking climate, soil properties, and plant traits to soil microbial communities, to a field-scale N addition study, and, finally, a mesocosm-scale mechanistic N retention experiment – to show how land use, climate, and plant traits can select for fungal- or bacterial-based soil food webs, with consequences for N cycling.

Results/Conclusions

In a field survey across 180 grasslands in the UK, we found that climate and land use strongly affected community weighted mean (CWM) plant traits, and that, in turn, CWM plant traits explained variation in microbial community composition. Specifically, conservative plant traits such as low specific leaf area (SLA) were linked to a greater biomass of fungi relative to bacteria. In a linked experiment, using intact soil columns from a subset of these sites, the retention of added labelled N was greater in extensively managed grassland with fungal-dominated microbial communities because of a greater immobilisation into microbial biomass, leading to lower N leaching losses from these soils. Finally, preliminary results from a mechanistic glasshouse experiment with constructed plant communities show that plant traits both directly, through plant N uptake, and indirectly, through microbial immobilisation, impact on ecosystem N retention. These combined results provide insight into the mechanisms of ecosystem N retention on different temporal and spatial scales. Moreover, in my talk, I will highlight that there is scope for harnessing the benefits of fungal-based soil food webs through agricultural management for optimising N retention.