Stimulation of soil respiration by elevated CO2 is enhanced under nitrogen limitation in a decade-long grassland study

Qun Gao; Gangsheng Wang; Kai Xue; Yunfeng Yang; Jianping Xie; Hao Yu; Shijie Bai; Feifei Liu; Zhili He; Daliang Ning; Sarah E. Hobbie; Peter B. Reich; Jizhong Zhou

doi:10.1073/pnas.2002780117

New Research In

Edited by David M. Karl, University of Hawaii at Manoa, Honolulu, HI, and approved October 27, 2020 (received for review February 17, 2020)

Significance

The magnitude of CO₂ efflux from soils (resulting from autotrophic and heterotrophic respiration) is one of the largest uncertainties in projecting future carbon–climate feedbacks. Despite research over several decades, the magnitude, direction, and duration of such feedbacks and their underlying microbial mechanisms are poorly understood, especially in the context of potentially interacting global environmental changes. In a decade-long experiment examining the interactive effects of CO₂ and N enrichment, N limitation strengthened the stimulatory effects of elevated CO₂ on soil respiration, primarily via N mining during the decomposition of more recalcitrant organic compounds. This study also provides a strategy for integrating genomics information into ecosystem and Earth system models to improve carbon-cycle predictions.

Abstract

Whether and how CO₂ and nitrogen (N) availability interact to influence carbon (C) cycling processes such as soil respiration remains a question of considerable uncertainty in projecting future C–climate feedbacks, which are strongly influenced by multiple global change drivers, including elevated atmospheric CO₂ concentrations (eCO₂) and increased N deposition. However, because decades of research on the responses of ecosystems to eCO₂ and N enrichment have been done largely independently, their interactive effects on soil respiratory CO₂ efflux remain unresolved. Here, we show that in a multifactor free-air CO₂ enrichment experiment, BioCON (Biodiversity, CO₂, and N deposition) in Minnesota, the positive response of soil respiration to eCO₂ gradually strengthened at ambient (low) N supply but not enriched (high) N supply for the 12-y experimental period from 1998 to 2009. In contrast to earlier years, eCO₂ stimulated soil respiration twice as much at low than at high N supply from 2006 to 2009. In parallel, microbial C degradation genes were significantly boosted by eCO₂ at low but not high N supply. Incorporating those functional genes into a coupled C–N ecosystem model reduced model parameter uncertainty and improved the projections of the effects of different CO₂ and N levels on soil respiration. If our observed results generalize to other ecosystems, they imply widely positive effects of eCO₂ on soil respiration even in infertile systems.

Footnotes

↵¹Q.G. and G.W. contributed equally to this work.
↵²To whom correspondence may be addressed. Email: yangyf{at}tsinghua.edu.cn or jzhou{at}ou.edu.

Author contributions: Y.Y., S.E.H., P.B.R., and J.Z. designed research; Q.G., K.X., Y.Y., J.X., H.Y., S.B., F.L., Z.H., S.E.H., P.B.R., and J.Z. performed research; G.W. contributed new reagents/analytic tools; Q.G., G.W., and D.N. analyzed data; and Q.G., G.W., Y.Y., S.E.H., P.B.R., and J.Z. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2002780117/-/DCSupplemental.