Trace Gas Emissions

Tropical forests are important sources and sinks of greenhouse gases, particularly C02, N20, and methane (CH4). The high NPP typical of moist and humid tropical forests is a significant component of the global C cycle (Melillo et al., 1993; Clark et al., 2003). Tropical forests also have the highest rates of soil respiration, which releases C02 back to the atmosphere (Raich and Schlesinger, 1992). Drought, fire, clearing, and disease—all factors that rapidly increase rates of tree mortality—can alter mi-crobial activity or community structure, resulting in pulses of C02 and other greenhouse gases (Schimel and Gulledge, 1998). Methane is produced under anaerobic conditions and tropical forests have generally been considered a weak sink (Keller and Reiners, 1994; Steudler et al., 1996). However, upland tropical forest soils have recently been identified as a significant source of CH4 at local (Keller et al., 1986; Silver et al., 1999; Teh et al., 2005) and regional (Frankenberg et al., 2005) scales.

Humid tropical forests are the largest natural source of N20 globally (Lashof and Ahuja, 1990). Nitrous oxide is produced via nitrification and denitrification. Factors that stimulate nitrification—such as the death of N-rich tissues and anthropogenic N deposition—can increase N20 emissions (Hall and Matson, 1999; Silver et al., 2005a); similarly, if soils become reduced, denitrification rates to N20 and dinitrogen (N2) may increase (Firestone et al., 1980; Silver et al., 2001). In the humid tropics, denitrification is likely to be the dominant source of N20 emissions from soils. The potential effects of climate change on the ratio of N20 : N2 are poorly understood. Theory suggests that denitrification to N2 is favored under low-N0^ and low-redox conditions, so it is possible that if increased rainfall and temperature lead to more strongly reducing conditions less N0^ will be available for denitrification and N2 production will be favored. It is extremely difficult to accurately estimate N2 fluxes from ecosystems, although this is an active area of research.

There have been few controlled experiments looking at the effects of climate changes on greenhouse gas production in tropical forests. Throughfall exclusion experiments—mimicking enhanced drought conditions—decreased N20 emissions from seasonal forests in tropical Brazil (Cattanio et al., 2002; Nepstad et al., 2002;

Table 11.2. Summary of potential effects of climate change on nutrient cycling in tropical forests.

Increased temperature

Altered rainfall

Other effects

Research directions

Soil nutrient availability

Increase in mineralization rates for both N and P

+Increased P availability due to shift in redox conditions, increased N losses via nitrification, denitrification, and leaching

Increased soil organic matter due as a result of elevated C02 could increase nutrient retention capacity and organic coatings could decrease P sorption capacity

What is the impact of increases in nutrient deposition due to urbanization and intensification of agriculture in addition to climate shifts and elevated CO2 in these systems?

Litter nutrient inputs and decomposition

Data show both + and — correlations between rainfall and litter nutrient concentrations Shifts in seasonality could decouple decomposition of litter from nutrient demand

Some evidence for increased C: nutrient ratios under elevated C02 potentially leading to decreased decomposition rates and/or increased nutrient immobilization during decomposition

What are the effects of climate change on below-ground nutrient inputs and decomposition?

Nutrient uptake

Root uptake kinetics are positively related to soil temperature.

Mycorrhizal infection appears to be positively correlated to soil temperature

Stronger drought/ dry-season dynamics may reduce fine root and microbial biomass in surface soils, thus reducing nutrient uptake

Shifts in relative and absolute below-ground plant biomass under elevated C02 appear to be species-specific, thus hard to predict in a diverse tropical forest

Are changes in nutrient uptake dynamics translated into shifts in net productivity?

(continued)

Table 11.2 (cont.)

Increased temperature

Altered rainfall

(+/-)

0ther effects

Research directions

Trace gas emissions

N20 emissions are sensitive to soil moisture conditions and have been found to positively correlate to rainfall. Under wet conditions upland tropical forests can switch from a slight sink to a net source of

Will trace gas dynamics in tropical forests under wetter and warmer conditions lead to positive feedbacks for the global climate?

Davidson et al., 2004a). Similar results were found during an induced drought in a tropical megacosm experiment (van Haren et al., 2005). Observational data along a tropical montane rainfall gradient showed increased N20 emissions with increasing rainfall (Keller et al., 1986) and decreased soil 02 availability (Silver et al., 1999).

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