Decades‐long effects of high CO₂ concentration on soil nitrogen dynamics at a natural CO₂ spring

Abstract

The effects of high atmospheric CO2 concentration ([CO2]) on ecosystem processes have been explored using temporal facilities such as open‐top‐chambers and free‐air CO2 enrichment. However, the effects of high [CO2] on soil properties takes decades and may not be captured by short‐term experiments. Natural CO2 springs provide a unique opportunity to study the long‐term effects of high [CO2]. In this study, we investigated soil properties at a natural CO2 spring. We found that the amounts of total carbon (C) and nitrogen (N) stored in the soil at the high [CO2] site exceeded those in the reference site by 60 and 30%, respectively. The effects of high [CO2] were large in the upper slope position where the canopy openness was high and plants grew faster, but no effects were detected in the lowest position where the canopy openness was lower (half of that at the upper slope position). In contrast, effects of high [CO2] on soil N dynamics, such as N mineralization and nitrification rates, did not exhibit a slope gradient. This suggests that effects of high [CO2] differed among soil stoichiometric characteristics and N dynamics. These complicated effects of high [CO2] imply that the future effects of high [CO2] on ecosystems could vary widely in conjunction with environmental conditions such as light availability and/or topographic conditions.

FACE facts hold for multiple generations; Evidence from natural CO2 springs

“…Trees in contrast showed similarly enhanced photosynthetic rate and reduced stomatal conductance but a significant decrease in nitrogen content of the leaves at spring sites. These differences in leaf nitrogen content response between functional groups could be due to several factors not quantified here, including differences in nitrogen allocation, differential biotic interactions such as the association of mycorrhiza to trees versus herbs, or abiotic factors such as differential light availability or soil accessibility (Osada, Onoda, & Hikosaka, ; Ueda, Onoda, Kamiyama, & Hikosaka, ).…”

“…Interpretation of plant responses at CO 2 springs would clearly be improved by further characterization of soil properties across the sites including nitrogen source (ammonium and nitrate availability), pH (characterized in just under half of sites globally) and soil CO 2 concentration (Pfanz et al, ; Ueda et al, ). For example, there is limited information available on soil nitrogen at natural CO 2 springs, but where quantified, total nitrogen pools have generally been found to be larger in spring than control soils (Newton, Bell, & Clark, ; Ross, Tate, Newton, Wilde, & Clark, ; Ueda et al, ).…”

“…Interpretation of plant responses at CO 2 springs would clearly be improved by further characterization of soil properties across the sites including nitrogen source (ammonium and nitrate availability), pH (characterized in just under half of sites globally) and soil CO 2 concentration (Pfanz et al, ; Ueda et al, ). For example, there is limited information available on soil nitrogen at natural CO 2 springs, but where quantified, total nitrogen pools have generally been found to be larger in spring than control soils (Newton, Bell, & Clark, ; Ross, Tate, Newton, Wilde, & Clark, ; Ueda et al, ). Of total soil nitrogen content, smaller inorganic nitrogen pools in CO 2 spring sites may be indicative of increased uptake by plants under elevated [CO 2 ] (Ueda et al, ), though nitrogen content of leaf litter returning to the soil generally shows decreased or unchanged nitrogen content at natural CO 2 springs (Coûteaux, Kurz, Bottner, & Raschi, , Cotrufo, Raschi, Lanini, & Ineson, , Gahrooee, , Ross, Tate, Newton, & Clark, ) suggesting changes in plant nitrogen allocation that may impact plant‐soil nitrogen cycling (see Gamage et al, ).…”

“…For example, there is limited information available on soil nitrogen at natural CO 2 springs, but where quantified, total nitrogen pools have generally been found to be larger in spring than control soils (Newton, Bell, & Clark, ; Ross, Tate, Newton, Wilde, & Clark, ; Ueda et al, ). Of total soil nitrogen content, smaller inorganic nitrogen pools in CO 2 spring sites may be indicative of increased uptake by plants under elevated [CO 2 ] (Ueda et al, ), though nitrogen content of leaf litter returning to the soil generally shows decreased or unchanged nitrogen content at natural CO 2 springs (Coûteaux, Kurz, Bottner, & Raschi, , Cotrufo, Raschi, Lanini, & Ineson, , Gahrooee, , Ross, Tate, Newton, & Clark, ) suggesting changes in plant nitrogen allocation that may impact plant‐soil nitrogen cycling (see Gamage et al, ). Where investigated, and likely as a result of anaerobic and acidic soil conditions characteristic of natural CO 2 springs, ammonium is the predominant form of inorganic nitrogen (Onoda et al, ; Osada et al, ; Ueda et al, ), which may facilitate the positive response of spring plant photosynthetic rate to elevated [CO 2 ], since plants primarily utilizing ammonium as an inorganic nitrogen source will be less impacted by inhibition of nitrate assimilation by elevated [CO 2 ] than plants utilizing nitrate (Bloom, ; Rubio‐Asensio & Bloom, ).…”