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Whether nitrogen (N) availability will limit plant growth and removal of atmospheric CO2 by the terrestrial biosphere this century is controversial. Studies have suggested that N could progressively limit plant growth, as trees and soils accumulate N in slowly cycling biomass pools in response to increases in carbon sequestration. However, a question remains over whether longer‐term (decadal to century) feedbacks between climate, CO2 and plant N uptake could emerge to reduce ecosystem‐level N limitations. The symbioses between plants and microbes can help plants to acquire N from the soil or from the atmosphere via biological N2 fixation—the pathway through which N can be rapidly brought into ecosystems and thereby partially or completely alleviate N limitation on plant productivity. Here we present measurements of plant N isotope composition (δ15N) in a peat core that dates to 15,000 cal. year BP to ascertain ecosystem‐level N cycling responses to rising atmospheric CO2 concentrations. We find that pre‐industrial increases in global atmospheric CO2 concentrations corresponded with a decrease in the δ15N of both Sphagnum moss and Ericaceae when constrained for climatic factors. A modern experiment demonstrates that the δ15N of Sphagnum decreases with increasing N2‐fixation rates. These findings suggest that plant‐microbe symbioses that facilitate N acquisition are, over the long term, enhanced under rising atmospheric CO2 concentrations, highlighting an ecosystem‐level feedback mechanism whereby N constraints on terrestrial carbon storage can be overcome. [ABSTRACT FROM AUTHOR]

• The Dongfanghong peatland originated in 1011 cal. CE post Millennium Eruption, expanding laterally through paludification. • The carbon accumulation averaged 118 g C/m2/yr, ranging from 38 to 374 g C/m2/yr. • Climate mainly governed multi-centennial carbon accumulation rates. • Hydrological conditions regulated both lateral expansion and carbon accumulation. Peatlands are terrestrial ecosystems with high carbon sequestration efficiency, providing critical feedback on climate change. It is crucial to understand how peatland development and carbon accumulation respond to various factors to reveal the future direction of carbon reservoirs under changing climatic conditions. To assess the impact of climate and hydrological conditions on the development and carbon accumulation of peatlands, 15 peat core samples were analyzed to examine the processes of lateral expansion in the Dongfanghong (DFH) peatland, a minerotrophic fen located in the Changbai Mountains of northeast China. Accelerator Mass Spectrometry 14C, total organic carbon, dry bulk density, testate amoebae, δ13C of α-cellulose in Carex spp. and plant macrofossil analyses were conducted on a high-resolution peat core. The proxies were used to calculate carbon accumulation rate (CAR) and reconstruct water table depth as well as surface moisture and vegetation development of the DFH peatland. Our findings indicated that the DFH peatland was initiated at 1011 cal. CE and experienced paludification during the last millennium. The expansion of the DFH peatland was primarily determined by hydrological conditions rather than climatic factors such as total solar irradiance and regional temperature. The rapid expansion between 1500 and 1700 cal. CE was likely resulted from relatively low precipitation and a decrease in water table/surface moisture. CAR generally correlated with total solar irradiance, regional temperature, and precipitation, suggesting that climate may be crucial for CAR in DFH peatlands over multi-centennial timescales. However, the increased CAR under relatively dry conditions suggested that the CAR were regulated by hydrological conditions over a centennial timescale. This study highlighted the important roles of climate and hydrological conditions in the lateral expansion and CAR of the peatland in minerotrophic fen and that there may be a threshold for hydrology during the process of peatland development and carbon accumulation. [ABSTRACT FROM AUTHOR]

• Hillslope topography is essential for local hydrology and peatland formation. • Microtopographic patterning enhances the positive water balance of the peatland. • Feedback between plant growth and pool development reinforces the stability of the peatland. Some sloping peatlands in northern regions often develop surface microtopographic patterns to maintain their water balance and ecosystem functioning. However, we do not know whether and how spatial patterning would influence the water balance and peat formation of permafrost-affected peatlands in relatively dry regions. Here we used data from the field observations and Unmanned Aerial Vehicle (UAV) survey of a slope peatland at an elevation of around 4800 m in the hinterland of the Qinghai-Tibetan Plateau (QTP) to document and understand the topographic controls of water balance and vegetation growth. Our terrain analysis result shows that the peatland—located on the middle section of a hillslope—has a gentle slope of 5.6° ± 2.5°, while the non-peatland upper section has a steep slope of 12° ± 4.5°. The great upstream catchment area and the presence of shallow impermeable permafrost likely create a saturated condition for peat formation. Our UAV results show obvious spatial patterning of abundant pools and ridges across this peatland, and pool sizes and ridge abundance increase with increasing slopes, suggesting that slope-controlled water flow gradient is the main driver of ridge formation and that ridges is to slow down the runoff. UAV-derived greenness values show a positive relationship with the total pool extent locally (R2 = 0.60) and decrease with increasing distance from the individual pools, suggesting sensitive responses of vegetation growth to surface moisture. Thus, enhanced vegetation growth and likely resultant great peat accumulation immediately around pools potentially further differentiate surface microtopography, strengthening the pool stability. We conclude that the local slope gradient, surface patterning (pools and ridges) and permafrost interact together to regulate water flow and maintain water balance, which in turn regulate the vegetation growth, peat accumulation and peatland stability. Our study implies that the delicate water balance maintained partly by microtopography is sensitive to climate change—especially potential extreme hydroclimate events—and natural and human-induced disturbances that may modify the surface patterning and weaken the peatland's stability, affecting the carbon sequestration ability of this type of peatlands. [ABSTRACT FROM AUTHOR]