The below‐ground perspective of forest plants: soil provides mainly organic nitrogen for plants and mycorrhizal fungi

Abstract

Summary• Nitrogen (N) availability has a major impact on a wide range of biogeochemical processes in terrestrial ecosystems. Changes in N availability modify the capacity of plants to sequester carbon (C), but despite the crucial importance for our understanding of terrestrial ecosystems, the relative contribution of different N forms to plant N nutrition in the field is not known. Until now, reliably assessing the highly dynamic pool of plant-available N in soil microsites was virtually impossible, because of the lack of adequate sampling techniques.• For the first time we have applied a novel microdialysis technique for disturbance-free monitoring of diffusive fluxes of inorganic and organic N in 15 contrasting boreal forest soils in situ.• We found that amino acids accounted for 80% of the soil N supply, while ammonium and nitrate contributed only 10% each. In contrast to common soil extractions, microdialysis revealed that the majority of amino acids are available for plant and mycorrhizal uptake.• Our results suggest that the N supply of boreal forest soils is dominated by organic N as a major component of plant-available N and thus as a regulator of growth and C sequestration.

Short-term carbon input increases microbial nitrogen demand, but not microbial nitrogen mining, in a set of boreal forest soils

“…Low N availability typically limits net primary production in this biome (LeBauer and Treseder 2008) and organic N forms likely dominate plant N uptake (Inselsbacher and Näsholm 2012). Given the mechanistic focus of our study, we tested our hypotheses in a laboratory incubation experiment, by manipulating C and N availability in six types of boreal forest soils that covered a range of SOM contents and decomposition states.…”

Wheat roots efflux a diverse array of organic N compounds and are highly proficient at their recapture

“…First, the pool and/or diffusive fluxes of small organic N compounds in the soil solution can be as large as those of inorganic N (Inselsbacher and Näsholm 2012;Warren 2014a). Second, it has been known for at least 100 years that plants can grow with, or take up from hydroponic solution, different forms of organic N (e.g.…”

Interplay between N-form and N-dose influences ecosystem effects of N addition to boreal forest

“…Organic N in the form of amino acids naturally dominates the soluble soil N pool directly available to plants and ectomycorrhizal (EcM) fungi in coniferous nutrient poor boreal forests (Jones and Kielland 2002;Kielland et al 2007;McFarland et al 2010;Inselsbacher and Näsholm 2012). Numerous shortterm uptake studies have confirmed a high capacity of boreal forest plants Nordin et al 2001;Persson et al 2003) as well as temperate trees (Scott and Rothstein 2011) to take up different amino acids.…”

“…The indications lately that organic N in the form of amino acids may be the dominant soluble N source directly available to uptake by plants in boreal forest (Inselsbacher and Näsholm 2012) may open up new solutions for increasing boreal forest productivity and carbon sequestration. We report the results of the first large-scale experiment investigating and comparing forest ecosystem effects of amino acid and AN addition in the field.…”