Get documents about "Methane Fluxes
Document Sample
ND04_FEER_05
Fluxes of methane from soils in the Central Amazon: The role of agroforestry systems
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, 69011-970;
Correspondence:mrondon@argo.com.br
1. INTRODUCTION
2. OBJECTIVES Figure 3. Methane fluxes in agroforestry systems in the Figure 4. Methane fluxes in managed pastures
Forests soils have been found to be net sinks for atmospheric methane during most of the annual cycle in several Central Amazon
tropical regions (Davidson et al, 1995) and are considered to play an important role in global methane budgets (Dorr * To assess net fluxes of methane from soils under three agroforestry systems (AS1,AS2 90
90
60
et al, 1993). When primary forest is converted to agriculture or pastures, the methane sink is usually reduced and and ASP), primary rainforest and secondary forest (SF). 60
30
ug CH4m -2h-1
ug CH4m -2h-1
30
0
eventually soils could become net sources (Keller et al, 1997). * To determine the effect of fertilization (P, Ca and Gypsum) on fluxes of methane from -30
0
-30
pastures. -60
-90 Nov
-60
-90
Dec
In the last three decades at least 30 x 106
ha of Amazon forest has been converted to pasture. Poor management * To make preliminary estimates of methane fluxes from termite mounds under -120
Aug/00 Sep Oct
Jan/01 -120
Oct/00 Nov Dec Jan/01
AS1 ASP AS2 Control Fertilized
leading to a drastic decline in pasture productivity and invasions by persistent, herbaceous and woody weeds have agroforestry systems and secondary vegetation.
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 5. DISCUSSION
associated with the continued conversion of Amazonian forest into pasture could have effects at regional and global
scales. 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
It is well known that termites emit methane to the atmosphere. In degraded lands in the Central Amazon, termite (high gas diffusivity) showed higher sink fluxes compared to primary "terra firme" forest on a clayey
mounds are abundant and they appear to negatively impact primary productivity by inhibiting re-colonization by soil (75% clay). Native forest (campina and terra firme) are clearly the strongest methane sink (figure
SF
vegetation (see Fernandes et al, this session). They may also contribute to net methane fluxes. 6), with an estimated annual oxidation rate of 4,5 to 6,9 g CH4.ha -1y-1 (Only the drier months have been
ASP evaluated so far). These estimates coincide with reported values for tropical rainforest (Keller et al,
Our LBA project considers two strategies for regenerating degraded pasture land aimed at breaking the vicious 1997).
AS 1 Figure 5. Methane emission from termite mounds under Figure 6. Estimated methane oxidation during the dry
circle of deforestation: Restoring productivity through modest inputs of P and Ca; and the establishment of various land uses in the Central Amazon season in the Central Amazon
AS 2
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 40 9
kgCH4 ha -1y-1
Aerial View of the plateau with experimental sites. The 3 blocks are surrounded by secondary forest Systems studied
soil faunal activity and diversity (Fernandes et al, 1995) and would likely also result in positive impacts on methane
mgCH4 m-2 h-1
6
included: Palm -based (AS1), Fruit based (AS2), tree-grass-legume pastures (ASP), Secondary forest (SF). Plots of 20
balances. However, data on methane fluxes between soils and the atmosphere from AFS in the tropics is notably Primary forest, control and fertilized pastures are located 1 km apart. 3
scarce. 0 0
AS1
AS2
Forest
SF
ASP
Pasture
Campina
-20
AS1 AS2 Forest SF Pasture
4. PRELIMINARY RESULTS
Precipitation: Figure 1 shows weekly rainfall distribution for the experimental area. Conversion of forest to pastures resulted in drastic net reductions in methane oxidation by soils, almost
(data from McCaffery, 2001). eliminating net sinks during the sampled period. This is in agreement with findings for other rainforests
Methane fluxes in primary and secondary forests: Figure 2 shows that in agreement (Keller and Reiners, 1994, Goureau et al, 1993). Secondary forest also showed only negligible net
with results from other tropical areas (Scharfe et al, 1990), forest soils in the Central methane sinks, comparable to those of degraded pastures. This could be attributed to "leaks"coming
Amazon are net sinks of methane during the dry season. Sandy "Campina"soils from abundant termite mounds that are found in this vegetation. (see Fernandes et al, this session)
Millions of hectares of the Amazon rainforest
are converted each year to pasture. Oxidation of
Because of poor management, pastures only Degraded pastures are soon abandoned and constitute the largest sink within the land uses studied, although we only have late dry Figure 6 shows how AS1 and AS2 Agroforestry systems allowed a recovery of methane sinks to around
maintain their productivity for a few years before secondary vegetation start to colonize the land. season data evaluated for this ecosystem. Early wet season data are currently being
methane by the soils is drastically reduced.
becoming degraded.
50% of forest values, but the pasture based ASP reached only nearly 25% of forest sink strength.
processed.
Initial data suggest that the application of modest levels of P and Ca to recover degraded pastures could
SF soils exhibit small net methane fluxes during the early dry season, but later become enhance methane sinks in the region.
3. METHODS 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 Fluxes of methane by termites. Though the proportion of the area covered by termite mounds is less
Site: Three repetitions of four AFS were planted (3/92) at the EMBRAPA/CPAA Research station, km 54 on BR-174
triggered a change in fluxes from sinks to small sources. than 2% of total area (Queiroz, 2001), methane fluxes in SF and in Pasture sites are at least two orders
north of Manaus, on a Xanthic Hapludox in an area of abandoned degraded pastures: Palm-based (AS1); Figure 1. Weekly rainfall distribution Figure 2. Methane fluxes in Primary and Secondary
Fruit/Timber-based (AS2); Silvopastoral-High Inputs (ASP); were established after a slash and burn in 11/91 and forest in the Central Amazon of magnitude higher than recorded sinks by soils in the same plots. This suggests that termites could
120
three secondary forest (SF) control plots demarcated. Primary forest on the same soil type (Forest) and "Campina", a 30
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
ug CH4m-2h-1
0
dwarf forest on a Quartzipsament were also studied. In the same area, 12 year old pastures of Brachiaria sp. were 80
-30
limited and therefore any extrapolation is cautioned. Studies will continue during 200.
mm
fertilized on 10/00 with P, Ca and gypsum (Welch, 2000). Twelve termite mounds from each land use were sampled -60
in early January 2001.
40
-90 Data does not yet cover the full annual cycle thus net annual estimates are premature.
Aug/00 Sep Oct Nov Dec Jan/01
-120
Gas sampling: Four closed vented chambers (25cm diameter) were used to sample gas in each of AS1, AS2, ASP, 0
SF Forest Campina
SF, Forest and Campina, as well as in fertilized and control pastures. Weekly to bi-weekly measurements were
Aug/00 Sep Oct Nov Dec JAn01
Aknowledgements:
initiated on August 2000 and to date, they cover part of the dry season and the onset of the rainy season. Gas samples Thanks to Antonio Nobre, Eleusa Barros, and INPA’s Agronomy laboratory personnel for allowing us
were analyzed within 24 hours after collection using a Perkin Elmer model GC400 gas chromatograph. (FID to use their GC and for providing a friendly working environment. Thanks to all students and staff of the
Methane fluxes in Agroforestry systems: As shown in figure 3, during the dry season LBA ND0-4 project for continued support and productive discussions. Photographs by E. Fernandes, K.
detector, 3m Porapak Q column, 75C, carrier gas, He).
all the AFS sites oxidized atmospheric methane. As the soil becomes more humid with McCaffery and M. Renjifo.
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.
Presence of termite mounds is
abundant on degraded pasture land. Methane fluxes in fertilized pastures: Figure 4 illustrates that fertilization appears to
Mounds inhibit vegetation
regrowth. Emission of methane by slightly enhance methane sinks at the end of the rainy season. Fertilizer was applied in
termites could surpass oxidation by September and therefore a time lag seems to exist before an effect is evident. Pastures,
the termite impacted soils.
including ASP, showed consistently lower methane sinks or higher sources. This is in Restoring pasture productivity with modest inputs and Agroforestry systems offer an alternative to regenerate
Heavily degraded pastures are covered by dispersed agreement with studies done in the region (Goreau et al, 1988). selection of proper species, can reduce the necessity to degraded land and to partially recover soil methane sinks.
earthworm casts. Surface sealing occurs limiting the clear additional primary forest.
diffusion of methane from the atmosphere to the
soils. 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 6. REFERENCES CITED see Handout
