Substantial labile carbon stocks and microbial activity in deeply weathered soils below a tropical wet forest

Abstract

Contrary to large areas in Amazonia of tropical moist forests with a pronounced dry season, tropical wet forests in Costa Rica do not depend on deep roots to maintain an evergreen forest canopy through the year. At our Costa Rican tropical wet forest sites, we found a large carbon stock in the subsoil of deeply weathered Oxisols, even though only 0.04–0.2% of the measured root biomass (>2 mm diameter) to 3 m depth was below 2 m. In addition, we demonstrate that 20% or more of this deep soil carbon (depending on soil type) can be mobilized after forest clearing for pasture establishment. Microbial activity between 0.3 and 3 m depth contributed about 50% to the microbial activity in these soils, confirming the importance of the subsoil in C cycling. Depending on soil type, forest clearing for pasture establishment led from no change to a slight addition of carbon in the topsoil (0–0.3 m depth). However, this effect was countered by a substantial loss of C stocks in the subsoil (1–3 m depth). Our results show that large stocks of relatively labile carbon are not limited to areas with a prolonged dry season, but can also be found in deeply weathered soils below tropical wet forests. Forest clearing in such areas may produce unexpectedly high C losses from the subsoil.

Reviews and syntheses: Field data to benchmark the carbon cycle models for tropical forests

“…There is accumulating evidence that the huge C stocks in the deep soils underlying many of these forests are not inert (e.g., Trumbore et al, 1995; Veldkamp et al, 2003). At the Costa Rican LS site (Table 6), the SOC at 2-3 m depth was found to be strongly temperature-responsive (Schwendenmann and Veldkamp, 2006), indicating a vulnerability of this large tropicalforest C stock to future warming.…”

“…While the range of these estimates is the only available guidance for Table 7) Twig litterfall > 0.9-2.5 Mg ha −1 yr −1 Lower bound 2.7-8.7 d 1.5 e Always an underestimate; excludes precollection losses (see Table 7 b Ratio between the 8-year maximum and 8-year minimum of stocks of live fine roots (< 2 mm, 0-50 cm depth) in old Oxisols; LS site (Espeleta and Clark, 2007). c Ratio of soil organic carbon to 3 or 4 m depth in old Oxisols vs. in younger Oxisols; LS site (Table 6; Veldkamp et al, 2003). d Range of ratios of maximum to minimum values from 18 0.5 ha plots in each of 12 successive years; LS site .…”

“…In tropical forests, the field sampling for these spatially variable organs has been confined to harvesting the root systems of selected individual trees (e.g., Niiyama et al, 2010) or to sampling coarse roots in pits or trenches away from trees, thus missing their tap roots and other large roots (e.g., Castellanos et al, 1991;Veldkamp et al, 2003). A recent survey of the available harvest data (Waring and Powers, 2017) found that root : shoot ratios for individual trees from old-growth tropical forests averaged ca.…”

“…Deep SOC (1-4 m depth) at this forest site was also found to mobilize with forest-topasture conversion (e.g., 30 Mg C ha −1 lost from this subsurface soil layer in ca. 30 years; Veldkamp et al, 2003). Changes in tropical-forest SOC, particularly in the deeper soil layers, could strongly impact the total forest C stocks and net C balance of this biome.…”

Response of tree biomass and wood litter to disturbance in a Central Amazon forest

“…lb), and is also the pool that is most vulnerable to rapid changes in stocks in response to disturbance. Soil also contains a large amount of carbon, but compared to wood carbon, soil is much less susceptible to changes in carbon storage over yearly to decadal time scales Telles et al 2003) although studies at some sites show significant changes (Veldkamp et al 2003). Third, years with high tree mortality rates (Nelson et al 1994;Condit et al 1995;Williamson et al 2000) can cause massive shifts of wood carbon from live to dead pools, with consequent changes in ecosystem structure and dynamics.…”