Tibetan legumes supply nitrogen that supports grassland carbon uptake
A peer-reviewed field survey and ecosystem model link nitrogen-fixing legumes with carbon uptake in Tibetan Plateau grasslands. Measurements at 323 sites connected nitrogen supply with legume biomass, while comparisons with historical observations found gains in vegetation and plant nitrogen. Removing nitrogen fixation turned a carbon sink into a source in simulations; that switch was a model result rather than an observed regional change.
Science··Morning
Root microbes supply a separate nitrogen source
Legumes in Tibetan Plateau grasslands obtain nitrogen through microbes associated with their roots. Those microbes convert atmospheric nitrogen into a form plants can use. A single peer-reviewed study combined field measurements, historical comparisons and ecosystem modeling to examine how that supply relates to carbon uptake. It surveyed 323 sites during peak growing seasons in 2019 and 2020, including 60 permafrost sites and 263 on seasonally frozen ground.[1]
Nitrogen fixation follows legume biomass
Sampling yielded 1,047 specimens drawn from legumes, non-legume plants and litter. Nitrogen content and stable isotopes helped estimate the share obtained from the atmosphere. Legumes occurred at 233 sites. For each square metre of aboveground biomass, estimated nitrogen fixation ranged annually from zero to 7.43 grams. Peaks occurred at elevations around 3,600–4,000 metres. Greater legume biomass accompanied higher fixation estimates. Areas with more fixation also overlapped with gains in vegetation biomass and nitrogen density relative to historical observations.[1]
Removing fixation changes the modeled carbon balance
The process-based DyN-LPJ ecosystem model tested carbon and nitrogen cycling. Excluding ecosystem-scale symbiotic fixation intensified nitrogen limitation and changed the simulated grasslands from a carbon sink into a source. The switch belongs to the model’s sensitivity experiment; field observations did not document a regional transition. The isotope comparison requires reference assumptions, and scaling up plant measurements introduces further assumptions about the whole ecosystem. Water conditions and vegetation differed among sites; spatial associations cannot establish the effect of warming on every measured change.[1]
