Increased Nitrogen Availability in the Soil Under Mature Picea abies Trees Exposed to Elevated CO2 Concentrations

Abstract

The response of trees to increasing atmospheric CO 2 concentrations is often mediated by the availability of nutrients. However, little is known about the influence of CO 2 enrichment on nutrient availability in forests with mature trees. We studied processes in the soil under five 35-m-tall Norway spruce trees (Picea abies) in NW Switzerland that were exposed to a mean CO 2 concentration of 550 ppm for 5 growing seasons using free air CO 2 enrichment (FACE). We compared them with values from the soil under five control trees. Ceramic suction cups were installed in the soil under each tree to collect soil solution, and ion-exchange resin bags were buried in the soil to absorb ammonium and nitrate. Soil cores taken at the end of the experiment were used to measure the gross production of ammonium and nitrate by the 15 N dilution technique. Although temporally and spatially variable, the nitrate concentration was higher in the soil solution under CO 2 -enriched trees. This effect was reflected in the resin bag extracts, which additionally indicated a trend of increased ammonium availability. Dissolved reduced N concentration (mainly dissolved organic N), however, was lower in the soil solution under CO 2 -enriched trees. K and Mg, and to a lesser extent Na and sulfate concentrations increased in the soil solution. P concentrations mostly remained below the detection limit. In spite of the higher concentrations of nitrate in soil extracts, gross N mineralization and nitrification rates were not affected by FACE. Needles from CO 2 -enriched trees contained slightly more N. No difference was observed for other nutrients. Overall, these results support the hypothesis of a priming effect, i.e., that FACE led to the production of more root exudates, which in turn stimulated soil biological activity, including mineralization, over a time-span of at least several years. However, these tall trees showed no growth response to elevated CO 2 ; hence, they gained no advantage from increased nitrate in the soil solution, presumably owing to other growth constraints including P and Mg availability.

Nitrogen dynamics after two years of elevated CO2 in phosphorus limited Eucalyptus woodland

“…The weak or missing N cycle response under FACE treatment in some ecosystems -grassland, heathland or forest observed here and elsewhere (Wild et al 2018;Schleppi 2019;Rütting 2020), could be due to limitation of another nutrient, such as P (Dijkstra et al 2013;. At EucFACE, evidence derived from Eucalyptus leaf C:N:P stoichiometry revealed strong P re-translocation compared to N in both ambient and CO 2 treated trees (Crous et al 2019), emphasising that P is highly limiting for the trees in this ecosystem.…”

“…Beginning with photosynthesis, CO 2 can stimulate a cascade of potential effects in an ecosystem, leading to an increase in plant growth and in belowground allocation (root growth) (Andresen et al 2016b;Körner 2018). Subsequently, increased rhizodeposition could stimulate organic matter decomposition (rhizosphere priming) and nutrient mineralization, leading to increased nutrient availability, to meet the extra nutrient demand (Dijkstra et al 2013;Kuzyakov 2015;Jilling et al 2018;Moreau et al 2019;Schleppi 2019). A meta-analysis found that gross N mineralization rate was stimulated by CO 2 across N limited ecosystems, but not in ecosystems limited by P , potentially because the cascade response to labile C deposition switches from N mining to P mining (Dijkstra et al 2013).…”

Experimental Design and Interpretation of Terrestrial Ecosystem Studies Using 15N Tracers: Practical and Statistical Considerations

“…Relatively short incubation periods are then applied, in the order of magnitude of one day (e.g. Wessel and Tietema, 1992;Schleppi et al, 2019). Instead, the aim of field studies is to describe the overall fate of the applied tracer.…”

A dynamic rhizosphere interplay between tree roots and soil bacteria under drought stress

“…Interestingly, the importance of both genera to plants has been verified in multiple metagenomic studies ( Loper et al, 2012 ; Mendes et al, 2013 ), however, the interplay between these important root bacteria and forest trees remains a mystery. In the forest soil, root exudation is suspected to enhance rhizobacteria, in turn leading to increased decomposition of soil organic matter, that is, increased C mineralization, a process termed microbial priming ( Schleppi et al, 2019 ). Here, we use this term on a wide perspective, even that P release from soil phosphates, for example, is not strictly considered as priming ( Dijkstra et al, 2013 ).…”