Topography Mediates the Response of Soil CO₂ Efflux to Precipitation Over Days, Seasons, and Years

Abstract

Spatiotemporal heterogeneity in soil CO₂ efflux (F_S) underlies one of our greatest gaps in understanding global carbon (C) cycles. Though scientists recognize this heterogeneity, F_S sampling schemes often average across spatial heterogeneity or fail to capture fine temporal heterogeneity, and many ecosystem models assume flat terrain. Here, we test the idea that simple, remotely sensible terrain variables improve regression models of spatiotemporal variation in F_S. We used automatic chambers that, for the first time, capture F_S in complex temperate forest terrain at fine temporal resolution with 177,477 hourly F_S measurements at 8 locations from ridgetop to valley along planar and swale hillslopes, across three years ranging from dry to record wet precipitation. In two of these years, we measured F_S weekly at 50 additional locations distributed across the 8-ha catchment. Growing season Fs estimates were 1.25 times greater when sampling hourly versus weekly. At ridgetops, growing season F_S increased by an average of 463 gC m⁻² 180 day⁻¹ (75.9%) from dry to wet years, while valleys decreased by 208 gC m⁻² 180 day⁻¹ (− 20.1%). This bidirectional response to interannual moisture was identified in distinct Random Forest models of Fs for convergent (water accumulating) or non-convergent (water shedding) hillslope positions. We hypothesize that different F_S constraints drive these opposing responses—water availably to biota limits F_S from ridgetops while slow oxygen diffusion limits F_S from wet valleys. Accounting for hillslope position and shape reduces variance of F_S estimates in complex terrain, which could improve F_S sampling, C budgets, and modeling.