1. 6. REFERENCES CITED see Handout
Fluxes of methane from soils in the Central Amazon: The role of agroforestry systems
ND04_FEER_05
Marco A. Rondón*, Erick C.M. Fernandes*, Elisa Wandelli‡ and Rubenildo Lima Da Silva‡
*Department of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853; ‡Centro de Pesquisa Agroflorestal da Amazônia (EMBRAPA-CPAA), AM-10, km 29, Manaus, AM, ;
1. INTRODUCTION
Forests soils have been found to be net sinks for atmospheric methane during most of the annual cycle in several tropical regions (Davidson et al, 1995) and are considered to play an important role in global methane budgets (Dorr et al, 1993). When primary forest is converted to agriculture or pastures, the methane sink is usually reduced and eventually soils could become net sources (Keller et al, 1997).
In the last three decades at least 30 x 106 ha of Amazon forest has been converted to pasture. Poor management leading to a drastic decline in pasture productivity and invasions by persistent, herbaceous and woody weeds have led to most being abandoned within a few years of use. This provokes clearing of additional primary forest for new pasture establishment. Considering the vast area involved, net changes in fluxes of methane and other GHG associated with the continued conversion of Amazonian forest into pasture could have effects at regional and global scales.
It is well known that termites emit methane to the atmosphere. In degraded lands in the Central Amazon, termite mounds are abundant and they appear to negatively impact primary productivity by inhibiting re-colonization by vegetation (see Fernandes et al, this session). They may also contribute to net methane fluxes.
Our LBA project considers two strategies for regenerating degraded pasture land aimed at breaking the vicious circle of deforestation: Restoring productivity through modest inputs of P and Ca; and the establishment of agroforestry systems (AFS) [see McCaffery et al, this session]. Regenerated pastures are expected to enhance methane sinks, while AFS growing on formerly degraded lands have been proven to improve soil quality, increase soil faunal activity and diversity (Fernandes et al, 1995) and would likely also result in positive impacts on methane balances. However, data on methane fluxes between soils and the atmosphere from AFS in the tropics is notably scarce.
2. OBJECTIVES
* To assess net fluxes of methane from soils under three agroforestry systems (AS1,AS2 and ASP), primary rainforest and secondary forest (SF).
* To determine the effect of fertilization (P, Ca and Gypsum) on fluxes of methane from pastures.
* To make preliminary estimates of methane fluxes from termite mounds under agroforestry systems and secondary vegetation.
5. DISCUSSION
Due to its influence on gas diffusion, soil texture is an important factor associated with fluxes of methane under native vegetation (Dorr et al, 1993). Campina forest located in a soil with 94% sand (high gas diffusivity) showed higher sink fluxes compared to primary "terra firme" forest on a clayey soil (75% clay). Native forest (campina and terra firme) are clearly the strongest methane sink (figure 6), with an estimated annual oxidation rate of 4,5 to 6,9 g CH4.ha -1y-1 (Only the drier months have been evaluated so far). These estimates coincide with reported values for tropical rainforest (Keller et al, 1997).
Conversion of forest to pastures resulted in drastic net reductions in methane oxidation by soils, almost eliminating net sinks during the sampled period. This is in agreement with findings for other rainforests (Keller and Reiners, 1994, Goureau et al, 1993). Secondary forest also showed only negligible net methane sinks, comparable to those of degraded pastures. This could be attributed to "leaks"coming from abundant termite mounds that are found in this vegetation. (see Fernandes et al, this session) Figure 6 shows how AS1 and AS2 Agroforestry systems allowed a recovery of methane sinks to around 50% of forest values, but the pasture based ASP reached only nearly 25% of forest sink strength.
Initial data suggest that the application of modest levels of P and Ca to recover degraded pastures could enhance methane sinks in the region.
Fluxes of methane by termites. Though the proportion of the area covered by termite mounds is less than 2% of total area (Queiroz, 2001), methane fluxes in SF and in Pasture sites are at least two orders of magnitude higher than recorded sinks by soils in the same plots. This suggests that termites could control methane budgets in such areas. Our estimates resulted in higher emission rate from soil termite mounds than reported data by Martius et al (1993) for above-ground termite mounds. Our data set is still limited and therefore any extrapolation is cautioned. Studies will continue during 200.
Data does not yet cover the full annual cycle thus net annual estimates are premature.
Aknowledgements:
Thanks to Antonio Nobre, Eleusa Barros, and INPA’s Agronomy laboratory personnel for allowing us to use their GC and for providing a friendly working environment. Thanks to all students and staff of the LBA ND0-4 project for continued support and productive discussions. Photographs by E. Fernandes, K. McCaffery and M. Renjifo.
Aerial View of the plateau with experimental sites. The 3 blocks are surrounded by secondary forest Systems studied included: Palm -based (AS1), Fruit based (AS2), tree-grass-legume pastures (ASP), Secondary forest (SF). Plots of Primary forest, control and fertilized pastures are located 1 km apart.
A S 1
AS 2
ASP SF
4. PRELIMINARY RESULTS
Precipitation: Figure 1 shows weekly rainfall distribution for the experimental area. (data from McCaffery, 2001).
Methane fluxes in primary and secondary forests: Figure 2 shows that in agreement with results from other tropical areas (Scharfe et al, 1990), forest soils in the Central Amazon are net sinks of methane during the dry season. Sandy "Campina"soils constitute the largest sink within the land uses studied, although we only have late dry season data evaluated for this ecosystem. Early wet season data are currently being processed.
SF soils exhibit small net methane fluxes during the early dry season, but later become a small sink again until the onset of rains. It is feasible that observed fluxes in SF are related to high termite populations and activity. Intense rainfalls in late December triggered a change in fluxes from sinks to small sources.
Methane fluxes in Agroforestry systems: As shown in figure 3, during the dry season all the AFS sites oxidized atmospheric methane. As the soil becomes more humid with the arrival of the rainy season, soils reduced their sink strength and became a net source of atmospheric methane. Both AS1 and AS2 were successful in enhancing methane sink compared to ASP which consistently showed reduced sink capacity. Temporal variability in flux estimates was high especially in pastures and was more homogeneous in AS2.
Methane fluxes in fertilized pastures: Figure 4 illustrates that fertilization appears to slightly enhance methane sinks at the end of the rainy season. Fertilizer was applied in September and therefore a time lag seems to exist before an effect is evident. Pastures, including ASP, showed consistently lower methane sinks or higher sources. This is in agreement with studies done in the region (Goreau et al, 1988).
Methane emissions from termite mounds: Figure 5 shows average fluxes from 12 mounds sampled in each land use. All mounds sampled in SF were active (emitting methane) while in other sites only 60% to 70% were active.
Millions of hectares of the Amazon rainforest
are converted each year to pasture. Oxidation of methane by the soils is drastically reduced.
Because of poor management, pastures only maintain their productivity for a few years before becoming degraded.
Degraded pastures are soon abandoned and
secondary vegetation start to colonize the land.
3. METHODS
Site: Three repetitions of four AFS were planted (3/92) at the EMBRAPA/CPAA Research station, km 54 on BR-174 north of Manaus, on a Xanthic Hapludox in an area of abandoned degraded pastures: Palm-based (AS1); Fruit/Timber-based (AS2); Silvopastoral-High Inputs (ASP); were established after a slash and burn in 11/91 and three secondary forest (SF) control plots demarcated. Primary forest on the same soil type (Forest) and "Campina", a dwarf forest on a Quartzipsament were also studied. In the same area, 12 year old pastures of Brachiaria sp. were fertilized on 10/00 with P, Ca and gypsum (Welch, 2000). Twelve termite mounds from each land use were sampled in early January 2001.
Gas sampling: Four closed vented chambers (25cm diameter) were used to sample gas in each of AS1, AS2, ASP, SF, Forest and Campina, as well as in fertilized and control pastures. Weekly to bi-weekly measurements were initiated on August 2000 and to date, they cover part of the dry season and the onset of the rainy season. Gas samples were analyzed within 24 hours after collection using a Perkin Elmer model GC400 gas chromatograph. (FID detector, 3m Porapak Q column, 75C, carrier gas, He).
Presence of termite mounds is abundant on degraded pasture land. Mounds inhibit vegetation regrowth. Emission of methane by termites could surpass oxidation by the termite impacted soils.
Heavily degraded pastures are covered by dispersed
earthworm casts. Surface sealing occurs limiting the
diffusion of methane from the atmosphere to the soils.
Restoring pasture productivity with modest inputs and selection of proper species, can reduce the necessity to clear additional primary forest.
Agroforestry systems offer an alternative to regenerate
degraded land and to partially recover soil methane sinks.
6. REFERENCES CITED see Handout