Decrease in heathland soil labile organic carbon under future atmospheric and climatic conditions

Abstract

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Resistance of soil protein depolymerization rates to eight years of elevated CO2, warming, and summer drought in a temperate heathland

“…Elevated CO 2 has been found to increase plant C/N ratios (Larsen et al 2011;Arndal et al 2013), root growth (Arndal et al 2013(Arndal et al , 2014, and the belowground plant N stock (Arndal et al 2013), suggesting enhanced plant N demand and uptake. Nitrogen stocks in the light soil density fraction were significantly reduced (Thaysen et al 2017), whereas extractable soil N pools remained constant (Larsen et al 2011), and gross N mineralization rates were constant or even enhanced (Larsen et al 2011;Björsne et al 2014). Based on these findings, we hypothesize that elevated CO 2 had stimulated gross protein depolymerization rates that provided additional N for plant uptake without reducing N availability for soil microorganisms, resulting in constant or enhanced gross N mineralization rates.…”

“…In combination, elevated CO 2 , warming, and drought led to a deceleration of soil N turnover after the first two years of the experiment, with stronger single treatment than combined treatment effects (Larsen et al 2011). A more recent study after eight years, however, suggests significant N losses from the light soil fraction after longer exposure to the combined treatments (Thaysen et al 2017).…”

“…Summer drought was realized using rain exclusion curtains that were automatically unfolded during summer precipitation events, typically in May or June, and removed 8-11% of the annual precipitation. On average, this resulted in a reduction of soil water content of 3.2 ± 0.5% during the drought period, and of 1.9 ± 0.3% in total (comparison of ambient and drought plots 2006-2013; see Thaysen et al 2017 for details). In 2013, summer drought was applied from April 29th to May 27th.…”

“…Warming has been shown to increase root growth and the belowground plant N stock in Calluna vulgaris (L.), one of the two dominant plants at the CLIMAITE site (Arndal et al 2013), as well as N losses from the light and heavy soil fraction (Thaysen et al 2017), and microbial C and N concentrations (Haugwitz et al 2014), at constant or enhanced gross N mineralization rates (Larsen et al 2011;Björsne et al 2014). We hypothesize that also warming had stimulated gross protein depolymerization rates, promoting the allocation of soil N to microbial and plant biomass at constant or enhanced gross N mineralization rates.…”

Drought Stress and Root-Associated Bacterial Communities

“…By contrast, Gram-negative bacteria contain characteristics of copiotrophs, as they prefer labile carbon compounds and organic nitrogen ( Treseder et al, 2011 ), particularly in the form of plant root exudates ( Balasooriya et al, 2014 ); and indeed it has been observed that Gram-negative bacteria incorporated almost ten times as much plant-derived carbon as Gram-positive bacteria under well-watered conditions ( Fuchslueger et al, 2014 ). Under drought, labile organic carbon is increasingly scarce within soils ( Thaysen et al, 2017 ), and in turn the rate of transfer of plant-derived carbon to microbes has been observed to go down ( Ruehr et al, 2009 ), possibly as microbial communities switch to degrading more recalcitrant carbon sources within plant organic matter ( Bradford et al, 2008 ). Karst et al (2016) posited that plants close protein channels to prevent sugar transport to the rhizosphere as part of osmotic adjustment under drought.…”