Once a carbon sink, grasslands have evolved to become a net positive source of greenhouse gas over the last few hundreds years.

In a new study, published Tuesday in the journal Nature Communications, researchers traced the influence of grasslands on the planet's climate. Their analysis revealed the ways human activities have transformed Earth's largest terrestrial biome.

Like all biomes and ecosystems, grasslands emit and absorb carbon dioxide. Grassland soils also release nitrous oxide, while grazers that call grasslands home emit methane.

For the new study, researchers examined the fluxes in these three gases on managed and natural grasslands over the last several centuries.

"We built and applied a new spatially explicit global grassland model that includes mechanisms of soil organic matter and plant productivity changes driven by historical shifts in livestock and the reduction of wild grazers in each region," lead study author Jinfeng Chang said in a news release.

Chang led the researchers while working at the International Institute for Applied Systems in Austria, but now works at Zhejiang University in China

"This model is one of the first to simulate the regional details of land use change and degradation from livestock overload," Chang said. "We also looked at the effect of fires and soil carbon losses by water erosion; CH4 emissions from animals; N2O emissions from animal excrement, manure, and mineral fertilizer applications; and atmospheric nitrogen deposition."

The data showed that since 1750, as humans have introduced more and more livestock to the planet's grasslands, the biome's CH4 and N20 emissions have increased by a factor of 2.5.

Sparsely grazed and natural grasslands have enhanced their carbon absorption and storage services over the last century, but intensively managed grasslands have become a net source of greenhouse gas emissions.

In fact, greenhouse gas emissions levels on heavily grazed grasslands rival those of croplands.

"Our results show that the different human activities that have affected grasslands have shifted the balance of greenhouse gas removals and emissions more towards warming in intensively exploited pastures, and more towards cooling in natural and semi-natural systems," said co-author Thomas Gasser, researcher at IIASA. "Coincidently, until recently the two types of grasslands have almost been canceling each other out."

Unfortunately, as pasture lands have expanded and livestock numbers have risen dramatically, global grasslands have become a net source of greenhouse gas emissions.

"Global grasslands will accelerate climate warming if better policies are not put in place to favor soil carbon increases, stop deforestation for ranching, and develop climate-smart livestock production systems," Gasser said.

Authors of the new study suggest land managers and policy makers should take a closer look at the ecosystem services, including greenhouse gas absorption and sequestration, that sparsely grazed or wild grasslands can offer.

"In the context of low-warming climate targets, the mitigating or amplifying role of grasslands will depend on a number of aspects," said study co-author Philippe Ciais.

"This includes future changes in grass-fed livestock numbers; the stability of accumulated soil carbon in grasslands; and whether carbon storage can be further increased over time or if it will saturate, as observed in long-term experiments," said Ciais, a researcher at the Laboratory for Sciences of Climate and Environment.

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