Variations in Belowground Carbon Storage and Soil CO₂ Flux Rates along a Wet Tropical Climate Gradient¹

Abstract

We used a humid tropical elevation gradient to examine the relationships among climate, edaphic conditions, belowground carbon storage, and soil respiration rates.We also compared open and closed canopy sites to increase the range of microclimate conditions sampled along the gradient, and determine the effects of canopy openings on C and P storage, and C dynamics. Total soil C, the light C fraction, and all of the component fractions of the P pool were significantly related to soil moisture, and all but total soil C were also significantly related to temperature. Both labile and recalcitrant soil P fractions were negatively correlated with the light C fraction, while the dilute HCI-extractable P pool, generally thought of as intermediate in availability, was positively correlated with light C, suggesting that P may play an important role in C cycling within these systems. Total fine root biomass was greatest at 1000 m elevation and lowest at 150 m, and was strongly and positively correlated with soil moisture content. Soil respiration rates were significantly and negatively correlated with fine root biomass and the light C fraction. In forested sites, soil respiration rates were strongly and negatively correlated with total belowground C pools (soils -I-roots + forest floor). Belowground C pools did not follow the expected increasing trend with decreases in temperature along the gradient. Our results indicated that in humid tropical forests, the relationships among soil C and nutrient pools, soil respiration rates, and climate are complex. We suggest that frequent and prolonged anaerobic events could be important features of these environments that may explain the observed trends.

Topographic variation in heterotrophic and autotrophic soil respiration in a tropical seasonal forest in Thailand

“…We have no data on the soil porosity of the monitoring plots, and the available data on the soil physical properties (Table 1) do not explain the reason for the trend in water content. However, the responses of soil respiration to soil water condition, as expressed by the exponential quadratic functions, seem to be reasonable for the upper slope because under a soil water condition of nearsaturation, soil respiration is often depressed by the increase in the water-filled pore space (Doff sotta et al 2004;Kang et al 2003) or by the decline in microbial activity due to the oxygen deficit (McGroddy and Silver 2004;Skopp et al 1990). Similar results were found in the other areas of Thailand; depression of soil respiration occurred under high soil moisture content over 0.21 (Adachi et al 2009).…”

Nitrogen additions mobilize soil base cations in two tropical forests

“…Nonetheless, soil N in both forests is relatively high and plant productivity has not responded to added N in either site (Cusack et al 2011). Upper elevation soils also have relatively lower soil redox potential (Silver et al 1999), and higher bulk soil phosphorous (P) (McGroddy and Silver 2000) compared with the lower elevation sites.…”

“…Nutrient cycling in both forests is generally not at steady-state because of the frequent occurrence of severe tropical storms and hurricanes (Heartsill-Scalley et al 2010;Scatena et al 1993;Silver et al 1996). Despite significant similarities, background soil N levels, climate and soil type vary between the two sites (McGroddy and Silver 2000). Both forests were included to assess heterogeneity in base cation responses to N addition in different tropical forests.…”

Spatial patterns in oxygen and redox sensitive biogeochemistry in tropical forest soils

“…Total Fe in these extracts was quantified on an inductively coupled plasma atomic emission spectrometer (Perkin-Elmer, USA). Phosphorus (P) was quantified following Tiessen and Moir (1993) with two exceptions: (1) the resin P step was omitted because the concentrations are typically very low for these forest soils (McGroddy and Silver 2000), and (2) fresh soils were extracted to prevent any binding of P via Fe oxidation during the air drying process. Labile P was quantified with a NaHCO 3 extraction followed by a sequential extraction with 0.1 M NaOH to quantify the P associated Al or Fe (Al/Fe P).…”

“…However, reduction of Fe(III) can liberate P from Fe-P bonds increasing P availability (Baldwin and Mitchell 2000, Peretyazhko and Sposito 2005, Chacon et al 2006. Soil P pools vary across environmental gradients, and circumstantial evidence provides linkages with redox (Schimel et al 1985, McGroddy and Silver 2000, Litaor et al 2005, Mage and Porder 2013. For example, P availability generally increases when soils become saturated (Ponnamperuma 1972) and temporal changes in labile P associated with the onset of the rainy season in seasonal tropical rain forests has been linked to changes in redox (Vandecar et al 2011).…”