Identification of the Park Grass Experiment soil metaproteome

The Park Grass Experiment, is an international reference soil with an impressive repository of temperate grassland metadata, however, it still lacks documentation of its soil metaproteome. The identification of these proteins is crucial to our understanding of soil ecology and their role in major biogeochemical processes. However, protein extraction can be fraught with technical difficulties including co-extraction of humic material and lack of a compatible databases to identify proteins. To address these issues, we used two compatible soil protein extraction techniques on Park Grass soil, one that removed humic material, namely a modified freeze-dry, heat,thaw, phenol, chloroform (HTPC) method and another which co-extracted humic material, namely an established surfactant method. Proteins were identified by matching mass spectra against a tailored Park Grass metagenome database. We identified a broad range of proteins from Park Grass soil, mainly in protein metabolism , membrane transport, carbohydrate metabolism, respiration and ribosome associated categories, enabling reconstitution of specific processes active in grassland soil. The soil microbiome was dominated by Proteobacteria, Actinobacteria, Acidobacteria and Firmicutes at phyla level and Bradyrhizobium, Rhizobium, Acidobacteria, Streptomyces and Pseudolabrys at genus level. Further functional enrichment analysis enabled us to identify many proteins in regulatory and signalling networks of key biogeochemical cycles such as the nitrogen cycle. The combined extraction methods connected previous Park Grass metadata with the metaproteome, biogeochemistry and soil ecology. This could provide a base on which future targeted studies of important soil processes and their regulation can be built.

Stomatal conductance limited the CO2 response of grassland in the last century

Background The anthropogenic increase of atmospheric CO2 concentration (ca) is impacting carbon (C), water, and nitrogen (N) cycles in grassland and other terrestrial biomes. Plant canopy stomatal conductance is a key player in these coupled cycles: it is a physiological control of vegetation water use efficiency (the ratio of C gain by photosynthesis to water loss by transpiration), and it responds to photosynthetic activity, which is influenced by vegetation N status. It is unknown if the ca-increase and climate change over the last century have already affected canopy stomatal conductance and its links with C and N processes in grassland. Results Here, we assessed two independent proxies of (growing season-integrating canopy-scale) stomatal conductance changes over the last century: trends of δ18O in cellulose (δ18Ocellulose) in archived herbage from a wide range of grassland communities on the Park Grass Experiment at Rothamsted (U.K.) and changes of the ratio of yields to the CO2 concentration gradient between the atmosphere and the leaf internal gas space (ca – ci). The two proxies correlated closely (R2 = 0.70), in agreement with the hypothesis. In addition, the sensitivity of δ18Ocellulose changes to estimated stomatal conductance changes agreed broadly with published sensitivities across a range of contemporary field and controlled environment studies, further supporting the utility of δ18Ocellulose changes for historical reconstruction of stomatal conductance changes at Park Grass. Trends of δ18Ocellulose differed strongly between plots and indicated much greater reductions of stomatal conductance in grass-rich than dicot-rich communities. Reductions of stomatal conductance were connected with reductions of yield trends, nitrogen acquisition, and nitrogen nutrition index. Although all plots were nitrogen-limited or phosphorus- and nitrogen-co-limited to different degrees, long-term reductions of stomatal conductance were largely independent of fertilizer regimes and soil pH, except for nitrogen fertilizer supply which promoted the abundance of grasses. Conclusions Our data indicate that some types of temperate grassland may have attained saturation of C sink activity more than one century ago. Increasing N fertilizer supply may not be an effective climate change mitigation strategy in many grasslands, as it promotes the expansion of grasses at the disadvantage of the more CO2 responsive forbs and N-fixing legumes.