Increased nitrate availability in the soil of a mixed mature temperate forest subjected to elevated CO₂ concentration (canopy FACE)

Abstract

In a mature temperate forest in Hofstetten, Switzerland, deciduous tree canopies were subjected to a free-air CO 2 enrichment (FACE) for a period of 8 years. The effect of this treatment on the availability of nitrogen (N) in the soil was assessed along three transects across the experimental area, one under Fagus sylvatica, one under Quercus robur and Q. petraea and one under Carpinus betulus. Nitrate, ammonium and dissolved organic N (DON) were analysed in soil solution obtained with suction cups. Nitrate and ammonium were also captured in buried ion-exchange resin bags. These parameters were related to the local intensity of the FACE treatment as measured from the 13 C depletion of dissolved inorganic carbon in the soil solution. Over the 8 years of experiment, the CO 2 enrichment reduced DON concentrations, did not affect ammonium, but induced higher nitrate concentrations, both in soil solution and resin bags. In the nitrate captured in the resin bags, the natural abundance of the isotope 15 N increased strongly. This indicates that the CO 2 enrichment accelerated net nitrification, probably as an effect of the higher soil moisture resulting from the reduced transpiration of the CO 2 -enriched trees. It is also possible that N mineralization was enhanced by root exudates (priming effect) or that the uptake of inorganic N by these trees decreased slightly as the result of a reduced N demand for fine-root growth. In this mature deciduous forest, we did not observe any progressive N limitation due to elevated atmospheric CO 2 concentrations; on the contrary, we observed an enhanced N availability over the 8 years of our measurements. This may, together with the global warming projected, exacerbate problems related to N saturation and nitrate leaching, although it is uncertain how long the observed trends will last in the future.

The role of soil microbes in the global carbon cycle: tracking the below‐ground microbial processing of plant‐derived carbon for manipulating carbon dynamics in agricultural systems

“…An example of an indirect positive feedback to elevated CO 2 is a consequence of the carbon fertilisation of primary (photosynthetic) production, whereby increased atmospheric CO 2 stimulates photosynthesis and the release of root exudates, which in turn means more labile carbon available for microbial decomposition and respiration . Moreover, increased root deposition of easily available exudates may ‘prime’ the turnover of less available SOM constituents that otherwise would not be subject to decomposition …”

Quantify the loss of major ions induced by CO2enrichment and nitrogen addition in subtropical model forest ecosystems

“…Previous studies concerning the effects of elevated atmospheric CO 2 on soil water chemistry mostly focused on the N availability and carbon (C) leaching. It was found that the elevated atmospheric CO 2 concentration increased [ Schleppi et al ., ], decreased [ Niklaus et al ., ], and had no effect [ Finzi and Schlesinger , ] on N availability in the soil. And the significant effect on C loss induced by the high CO 2 treatment was observed [ Williams et al ., ; Fenner et al ., ; Liu et al ., ].…”

Climate change effects on vegetation in the Pacific Northwest: a review and synthesis of the scientific literature and simulation model projections

“…Recently, however, results from long-term experiments with elevated CO 2 suggest that elevated CO 2 could actually accelerate decomposition of soil organic matter and reduce soil carbon storage (Cheng et al 2012, Drake et al 2011, Kowalchuk 2012, Phillips et al 2012, Schleppi et al 2012, Zak et al 2011. Unfortunately, it is unclear how these results, derived primarily from experiments in temperate deciduous forests, translate to western coniferous forests and rangelands with signifi cant seasonal water limitations that can infl uence soil carbon turnover (Norton et al 2011).…”

“…Evidence for this progressive nitrogen limitation comes from long-term free-air CO 2 enrichment (FACE) studies with fertilization treatments in tallgrass prairie (Reich et al 2006a) Recent results from long-term FACE studies suggest that several feedback processes may work to reduce or reverse the effects of progressive nitrogen limitation on productivity. Schleppi et al (2012) attributed elevated nitrifi cation rates and high levels of soil nitrate availability under trees subjected to elevated CO 2 to higher soil moisture levels produced by reduced evapotranspiration. Recent studies have concluded that observed reductions in soil carbon and increased nitrogen availability could be the result of root "priming," in which trees export carbon to soil biota through the root system, thereby stimulating soil organic matter decomposition by fungi and bacteria, and subsequent release of mineral nutrients bound up in that organic matter (Drake et al 2011, Phillips et al 2012, Schleppi et al 2012, Zak et al 2011).…”