The Predominance of Nongrowing Season Emissions to the Annual Methane Budget of a Semiarid Alpine Meadow on the Northeastern Qinghai-Tibetan Plateau

The terrestrial methane budget varies between different vegetation types and soil conditions and is highly uncertain for alpine grasslands. This work used eddy covariance techniques to continuously measure CH4 flux (NEEm) over a semiarid alpine meadow on the northeastern Qinghai-Tibetan Plateau from January 2017 to August 2019. The diel NEEm averaged 0.14 +/- 0.98 nmol CH4 m(-2) s(-1) (mean +/- S.D.), with a rough pattern of daytime release and nocturnal uptake. The 8-day NEEm exhibited a similar sinusoid variation, with a peak of 6.8 mg CH4 m(-2) d(-1) at the end of April and a minimum of -1.5 mg CH4 m(-2) d(-1) at the end of August. The maximum release probably coincided with the thawing of frozen soil in the root zone, and the peak uptake may be related to high soil temperature. Monthly CH4 uptake was highest from June to September and consumed 51.7 mg CH4 m(-2) from the atmosphere. CH4 production in the other months totaled 647.6 mg CH4 m(-2). The semiarid alpine meadow thus acted as a weak net CH4 source, releasing ca 0.6 g CH4 m(-2) year(-1) to the atmosphere. The boosted regression trees analysis shows that the sensible heat flux (H) is positively related to half-hour NEEm and accounted for 34% of its variability. The piecewise structural equation models reveal that the magnitude of the effects from soil temperature and vapor pressure deficit on 8-day and monthly NEEm were almost equal, but acted in opposite directions. Vegetation growth and soil moisture exerted little direct influence on NEEm variability at half-hour, 8-day, or monthly scales. Our results show that CH4 emissions of the nongrowing season dominate the annual methane budget for this alpine meadow area. Methane consumption during the growing season was significantly constrained by low soil temperature and high soil water content. These findings imply that semiarid alpine meadows may consume more methane during the growing season if soil temperatures increase and soil moisture levels decrease as projected by future warming scenarios, thus constituting a climate change negative feedback.