Constraints to nitrogen acquisition of terrestrial plants under elevated CO₂

Abstract

A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO 2 (eCO 2 ), and whether or not this constraint will become stronger over time. We explored the ecosystem-scale relationship between responses of plant productivity and N acquisition to eCO 2 in free-air CO 2 enrichment (FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) in all three ecosystem types, this relationship was positive, linear and strong (r 2 = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO 2 caused neutral or modest changes in productivity. As the ecosystems were markedly N limited, plants with minimal productivity responses to eCO 2 likely acquired less N than ambient CO 2 -grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO 2 , and this decrease was independent of the presence or magnitude of eCO 2 -induced productivity enhancement, refuting the long-held hypothesis that this effect results from growth dilution. (iii) Effects of eCO 2 on productivity and N acquisition did not diminish over time, while the typical eCO 2 -induced decrease in plant N concentration did. Our results suggest that, at the decennial timescale covered by FACE studies, N limitation of eCO 2 -induced terrestrial productivity enhancement is associated with negative effects of eCO 2 on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation.

Water availability moderates N2 fixation benefit from elevated [CO2]: A 2‐year free‐air CO2 enrichment study on lentil (Lens culinaris MEDIK.) in a water limited agroecosystem

“…In our second research question, we asked how e[CO 2 ] and seasonal variation in water availability change the use of soil versus aerial N sources, and how N allocation patterns in lentil might change. Our results demonstrate that N 2 fixation increased in e[CO 2 ]‐grown lentil to such an extent that uptake of soil N decreased in absolute terms, which is an independent confirmation of previous findings on Medicago trunculata L. (Guo et al, ), the N 2 fixing tree Robinia pseudoacacia (Feng, Dyckmans, & Flessa, ), or an earlier FACE experiment on Trifolium repens L. (Zanetti et al, ) and in line with a meta‐analysis on the negative effect of e[CO 2 ] on plant N acquisition (Feng et al, ). Decreases of N uptake under e[CO 2 ] are generally associated with the decreases in nitrate uptake (Guo et al, ).…”

Elevated CO2maintains grassland net carbon uptake under a future heat and drought extreme

“…4A). Recent analyses of 12 decadal experiments (44,45) show that such a positive impact of eCO 2 on plant nitrogen acquisition and aboveground net primary production is not decreasing with time and that the associated progressive nitrogen limitation hypothesis (46) is not ubiquitous.…”

Decoupling of soil carbon and nitrogen turnover partly explains increased net ecosystem production in response to nitrogen fertilization

“…the difference between gross primary production and ecosystem respiration. Elevated atmospheric CO 2 concentrations commonly result in at least a temporary stimulation of primary production3031, but also in increased allocation of plant C to root exudates2728. Labile C compounds in these exudates can ‘prime’ the decomposition of soil organic matter (SOM)192232.…”