Work Package 2

Shifts in plant and microbial N uptake strategies with secondary succession
WP2 aims to understand how the form of nitrogen taken up by plants and microbes changes over time following forest fire. We investigate the pathways through which fixed N in feather mosses is made available to plants - the role of disturbance and mycorrhizae - we also aim to quantify the amount of fixed N that is retained in an organic form in the moss carpet.

Plant species composition and N uptake strategy

Early successional plant species appear to be uniquely well adapted to the increase in inorganic N resources after fire. The grass Deschampsia flexuosa exhibits a strong preference for NO3 - (Persson et al. 2003), as does birch (Betula spp.) (Kurth and DeLuca, unpublished). Nitrate is extremely soluble and therefore highly mobile in the soil environment with diffusion rates nearly fifty fold greater than for NH4  (Jones et al. 2002). This makes NO3 an important source of N for developing vegetation in post-fire boreal forest systems when roots are still small prior to full mycorrhizal establishment. In mid-to late secondary forests succession, it is likely that organic N and NH4+ become the primary form of N for plant uptake. While numerous studies have demonstrated organic N uptake in ericaceous shrubs commonly associated with mid to late successional boreal forests (i.e. Empetrum hemaphroditum, Vaccinium vitus-idea), and Norway spruce (Picea abies), there has been limited effort to contrast microbial amino acid and peptide uptake with plant amino acid uptake and inorganic N uptake in the context of disturbance and succession.

Following the fate of fixed nitrogen

Efforts to quantify plant-microbial competition for N resources in soil are incomplete as a result of difficulties in separating roots from soil, a lack of spatial information of 15N distribution, poor consideration of isotopic pool dilution and the respiratory loss of 13C from dual-labelled substrates (Jones et al. 2009) and shoot 15N and 13C accumulation represent a poor proxy for root uptake. In this work package we will use nanoscale secondary ion mass spectroscopy (NanoSIMS) which allows for high resolution spatial imaging of stable isotopes in intact plant-microbial-soil systems (Clode et al. 2009). This approach will allow us to visualize and  trace the fate of microbially fixed 15N in feathermosses, and trace organic N from the feathermoss-humus layer into higher plants. To properly understand plant-microbial competition for organic N we will differentiate between roots, mycorrhiza and other organisms in the bryosphere.