Forest soil CO2 efflux models improved by incorporating topographic controls on carbon content and sorption capacity of soils

Abstract

Improved models are needed to predict the fate of carbon in forest soils under changing environmental conditions. Within a temperate sugar maple forest, soil CO 2 efflux averaged 3.58 lmol m -2 s -1 but ranged from 0.02 to 25.35 lmol m -2 s -1 . Soil CO 2 efflux models based on temperature and moisture explained approximately the same amount of variance on gentle and steep hillslopes (r 2 = 0.506, p \ 0.05 and r 2 = 0.470, p \ 0.05 respectively). When soil carbon content and sorption capacity were added to the models, the amount of explanation increased slightly on a gentle hillslope (r 2 = 0.567, p \ 0.05) and substantially on a steep hillslope (r 2 = 0.803, p \ 0.05). Within the organic-rich surface of the mineral soil, carbon content was positively related and sorption capacity was negatively related to soil CO 2 efflux rates. There were general patterns of smaller carbon pools and lower sorption capacity in the upland positions than in the lowland and wetland positions, likely a result of hydrological transport of particulate and dissolved substances downslope, leading to higher soil CO 2 efflux in the upland positions. However, the magnitude of the soil CO 2 efflux was mitigated by the higher sorption capacity of the organic-rich surface layer of the mineral soils, which was negatively correlated to soil CO 2 efflux. More accurate estimates of forest soil CO 2 efflux must take into account topographic influences on the carbon pool, the environmental factors that affect rates of carbon transformation, as well as the physicochemical factors that determine the fraction of the carbon pool that can be transformed.

Catchment‐Scale Shifts in the Magnitude and Partitioning of Carbon Export in Response to Changing Hydrologic Connectivity in a Northern Hardwood Forest

“…Among these determinants, soil temperature and moisture are more sensitive to changes in meteorological conditions at daily to annual time scales (Stielstra et al, ). Since no differences in soil temperature among positions were detected in c38 (Lecki & Creed, ), spatial differences in atmospheric C export could be explained by topography‐driven differences in soil moisture retention (Webster, Creed, Beall, & Bourbonnière, ). Previous studies have identified the role of topography on soil microbial respiration (Lecki & Creed, ).…”

“…Since no differences in soil temperature among positions were detected in c38 (Lecki & Creed, ), spatial differences in atmospheric C export could be explained by topography‐driven differences in soil moisture retention (Webster, Creed, Beall, & Bourbonnière, ). Previous studies have identified the role of topography on soil microbial respiration (Lecki & Creed, ). Even though soil temperature and moisture were identified as the major drivers of atmospheric C export, both soil C distribution and temperature were identified to be more relevant predictors of soil microbial respiration in steeper slopes compared to gentle slopes.…”

“…The same effect of increasing soil moisture supressing atmospheric C export is observed during the snow‐free period in the ecotone and wetland (Figure b, top). This process reflects rises in the water table, which saturates surface soils in lowlands (ecotone and wetland), leading to reducing conditions that limit aerobic respiration (Lecki & Creed, ; Smith et al, ). While the ecotone presents a more diverse microbial community (Du et al, ), with different microbial populations thriving during drier and wetter conditions (Goldman et al, ), increasing soil moisture could induce oxygen limitation and supress overall atmospheric C export from this area (Figure c).…”

“…Daily soil CO 2 efflux (μmol m −2 d −1 ) was calculated for each of seven features on both the shallow (T15) and steep (T35) slope transects using the empirical model developed by Lecki and Creed (). Chamber‐based measurements of CO 2 fluxes collected during the same time period from the same study area were used to create this model.…”

“…CO 2 emissions through respiration include autotrophic and heterotrophic sources and have been estimated at 98 ± 12 Pg C yr À1 globally (Bond-Lamberty & Thomson, 2010). Heterotrophic sources are controlled by microbes influenced by environmental conditions, primarily soil temperature and moisture, C availability, and C quality (Lecki & Creed, 2016;Webster et al, 2014). Climatic shifts toward warmer conditions enhance rates of heterotrophic respiration (Kirschbaum, 1995), but simultaneous changes in soil moisture can limit microbial respiration by reducing the diffusion of dissolved organic C (DOC; low soil moisture) or oxygen (high soil moisture) to aerobic microbes (Lohse et al, 2009).…”