Abstract
Nitrogen (N) availability is projected to increase in a warming climate. But whether the more available N is immobilized by microbes (thus stimulates soil carbon (C) decomposition), or is absorbed by plants (thus intensifies C uptake) remains unknown in the alpine meadow ecosystem. Infrared heaters were used to simulate climate warming with a paired experimental design. Soil ammonification, nitrification, and net mineralization were obtained by in situ incubation in a permafrost region of the Qinghai‐Tibet Plateau (QTP). Available N significantly increased due to the stimulation of net nitrification and mineralization in 0–30 cm soil layer. Microbes immobilized N in the end of growing season in both warming and control plots. The magnitude of immobilized N was lower in the warming plots. The root N concentration significantly reduced, but root N pool intensified due to the significant increase in root biomass in the warming treatment. Our results suggest that a warming‐induced increase in biomass is the major N sink and will continue to stimulate plant growth until plant N saturation, which could sustain the positive warming effect on ecosystem productivity.
Highlights
Nitrogen (N) is considered to be the most important abiotic factor limiting net primary production in many terrestrial ecosystems like temperate and boreal forests, temperate grassland (Vitousek & Howarth, 1991), and the Arctic and alpine tundra (Chapin & Shaver, 1996; Jonasson, Michelsen, Schmidt, & Nielsen, 1999; Jonasson, Michelsen, Schmidt, Nielsen, & Callaghan, 1996; Zhang & Cao, 1999)
Progressive N limitation may occur due to the coupling of carbon (C) and N in primary production if the warming-induced increase in ecosystem C sequestration capacity is not accompanied by an increase in available N
A large amount of inorganic N is immobilized by microbes during the growing season (Hobbie & Chapin, 1998; Schmidt, Jonasson, & Michelsen, 1999), which results in N limitation for plant growth in tundra and the Arctic ecosystems (Clemmensen, Sorensen, Michelsen, Jonasson, & Ström, 2008; Schmidt et al, 1999; Sistla, Asao, & Schimel, 2012) yet increase in microbial decomposition of soil organic matter (SOM) (Jonasson, Michelsen, & Schmidt, 1999)
Summary
Nitrogen (N) is considered to be the most important abiotic factor limiting net primary production in many terrestrial ecosystems like temperate and boreal forests, temperate grassland (Vitousek & Howarth, 1991), and the Arctic and alpine tundra (Chapin & Shaver, 1996; Jonasson, Michelsen, Schmidt, & Nielsen, 1999; Jonasson, Michelsen, Schmidt, Nielsen, & Callaghan, 1996; Zhang & Cao, 1999). The elevated temperature is likely to precipitate the N cycling and SOM decomposition in the Arctic tundra Permafrost soil in Qinghai-Tibet Plateau (QTP) stores as much as 160 Pg C in the 0–25 m depth (Mu et al, 2015), equivalent to nearly 12% of global soil C storage in the top 1 m It has a large potential for C emission under warming climate as permafrost C is very sensitive to global warming (Schuur et al, 2008). We conducted a field experiment to examine the response of net mineralization rate, different N and C pools, and the coupled C and N responses to climate warming in an alpine meadow in the permafrost region of QTP
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