C

1
Carbon and Nutrient Stocks in Agroforestry
Systems and Secondary Forest in the Central Amazon
ND04_FEER_04
Karen A. McCaffery1, Marco A. Rondon 1, Jorge
Gallardo-Ordinola, Steven A. Welch2, Ted R.
Feldpausch1, Erick C.M. Fernandes1, Susan J.
Riha2 and Elisa V.Wandelli
Department of Crop and Soil Sciences 1,
Department of Earth and Atmospheric Sciences2,
Cornell University, Ithaca, NY 14853 Instituto
Nacional de Pesquisas da Amazônia, Rua Nelson
Batista Sales 114, Cj Petro-Aleixo, Manaus, AM.
Centro de Pesquisa Agroflorestal da Amazônia da
Embrapa (EMBRAPA-CPAA), AM-10, km 29, Manaus, AM,
69011-970 Correspondence ecf3_at_cornell.edu
1. INTRODUCTION Since 1970, an estimated 25-40 m
ha of forest have been cleared in the Brazilian
Amazon and approximately 70 of this land
converted to pasture. Due to poor management,
most of the pastures degrade and are abandoned
within 5-10 years. More than half of these become
woody, secondary forest and the remaining
continue as unproductive grassland (Fearnside
1995). There are at present 20-35 million
hectares of abandoned, degraded pastures in the
Amazon characterized by low economic
productivity, low rates of carbon accumulation
and greatly diminished biodiversity. If
degraded pasture lands can be rehabilitated and
the productivity significantly increased, policy
makers would have an alternative strategy to
minimize the future clearing of primary forests.
We are evaluating two strategies to improve the
productivity of abandoned pasture land (1)
adding modest levels of key, limiting nutrients
(P and Ca) to secondary vegetation, and (2)
harnessing the acid-soil tolerance of native tree
and fruit species and adapted crop and pasture
species in complex, multi canopy Agroforestry
systems (AFS). In contrast to naturally
regenerating secondary forests, only a few
studies (De Camargo et al. 1999, Buschbacher et
al. 1988) have measured carbon and nutrient
stocks in post-pasture secondary forest and
managed agroecosystems in the Amazon.
2. OBJECTIVES 1. To determine carbon and
nutrient stocks in aerial biomass, litter, roots
and soil in three 9-year-old agroforestry
systems established on abandoned pastures and in
same-age secondary forests. 2. To evaluate
photosynthetic potential of agroforestry systems
and secondary forests by measuring LAI responses
to seasonal changes and management. 3. To study
fertilizer effects on biomass accumulation in a
chronosequence of secondary forests following
pasture.
Brazil Nut
Mahogany
Peach palm
Columbrina
Schizolobium
Açaí
Banana
Gliricidia

• 4. RESULTS
• Aerial biomass Regression models were developed
  for 13 agroforestry species for estimating aerial
  biomass (Table 1).The palm agroforestry system
  (AS1) contained the highest biomass (70.6 t/ha)
  and largest C stock (32.4 t/ha) of the three
  managed systems (Table 2). In AS2 Brasil nut
  accounted for 50 of C. In AS1, Peach palm
  accounted for 46 of C. Secondary forest (9 yrs)
  contained higher biomass than the AFS.
• Light capture More fertilizer, less grazing,
  and less pruning produce more leaf area (Figure
  1) ASP-H (3.6), SF (3.3), AS1 (3.2), ASP-L
  (2.9), AS2 (2.8) and pasture (2.1). The weak dry
  season in Manaus (September RF gt100 mm) lowered
  LAI by 0.3 in most areas.
• Root biomass The palm-system had the highest
  root biomass (6.2 t/ha) compared to SF (1.8 t/ha)
  and ASP (0.9 t/ha) based on single repetitions
  (Figure 2).
• Soil carbon Little variation between land-uses
  has been observed thus far in SOC (repetitions
  pending analysis) although the pasture system
  showed a slight decline over time compared to
  other land-uses (Figure 3). Nearly 30 of soil C
  is concentrated in the top 15 cm.
• Total C and P stocks Soil is the dominant C
  pool in all sites accounting for 90 of total C
  in the SF on abandoned pasture compared to 70-77
  in other systems studied (Figure 4). The largest
  P pool was the aerial biomass (40.5 kg/ha)
  (Figure 5). While SFs had higher C accumulation
  than the AFS, AS1 and AS2 had higher P stocks.
• Secondary forest management Current data from
  seven secondary forests indicate a reduction in
  soil available P over time (Figure 6). Biomass
  accumulation in the first 15 years of regrowth
  was rapid (120.8 t/ha).

Gliricidia
Euterpe
Brachiaria Desmodium pasture
Palm-based Agroforestry System (AS1)
Fruit and Timber Agroforestry System (AS2)
Silvopastoral System (ASP)
3. METHODS Site Three repetitions of four AFS
were planted (3/92) at the EMBRAPA/CPAA Research
station, km 54 on BR-174 on a Xanthic Hapludox in
an area of abandoned degraded pastures
Palm-based (AS1) Fruit/Timber-based (AS2)
Silvopastoral-High Inputs (ASP-H)
Silvopastoral-Low Inputs (ASP-L) were established
after a slash and burn in 11/91 and three
secondary forest (SF) control plots demarcated
(Fernandes et al., 2001). Carbon Stocks
Evaluation Aerial Biomass in Agroforestry
Systems Seven to 23 trees of varied size of each
species were destructively harvested. Trees were
felled, divided into trunk, branches and leaves
and total FW determined. Duplicate composite
samples of each component were weighed, oven
dried and subsamples analyzed for C and
nutrients. Linear regression models were
developed and used to estimate the above-ground
biomass contribution of each species to the AFS.
Aerial Biomass in Secondary Forest All
standing live stems gt 1 cm DBH were measured in
20 of the area of each plot. Six
species-specific allometric equations
representing the dominant plants and one
mixed-species equation (Nelson et al. 1999) were
used to estimate aerial DW. Leaf Area
Monitoring Hemispheric canopy images were
captured bi-monthly in all study areas. Leaf
area indices were analyzed using a digital canopy
analyzer. Root Biomass Fine (lt2 mm) and coarse
(gt2 mm) root biomass are being measured in AS1,
ASP and SF to capture principal root interactions
between species. Fine roots were sampled with an
auger to 50 cm depth. Coarse roots were excavated
from trenches in 10 cm layers to 150 cm depth.
Soil pits were 3 x 3 m (AS1) and 0.5 x 3 m (ASP,
SF) and all 1.5 m deep. Roots were weighed fresh,
subsampled, washed, dried, re-weighed and
analyzed for C and nutrients. Soil Carbon
Composite soil samples were collected with an
auger at five depths to 1 m at points
representing the principal species interactions
in the AFSs and SF. Samples were dried,
homogenized, subdivided and separated into three
aggregate size classes (gt2 mm 0.25-2 mm and
lt0.25 mm). Charcoal contamination is being
manually removed from the two larger size classes
and chemically (Kuhlbusch, 1995) from the
smallest for analysis of total carbon (dry
combustion). Dissolved Organic Carbon (DOC) Soil
water samples are being collected at 0.2, 1 and 2
m depths using Teflon suction caps and free
drainage lysimeters. Soil water and stem flow
will be analyzed by TOC. Secondary Forest
Management Inputs of P (50 kg/ha), Ca (0.8 t/ha)
and gypsum (1 t/ha) were applied to a
chronosequence of 1-14 year old secondary
forests. Biomass, succession, LAI, soil water
uptake (TDR) and nutrient movement are being
evaluated.
5. DISCUSSION Carbon and Nutrient Stocks The
regression models developed may be used as tools
in designing AFS to maximize C accumulation.
Species such as Brazil Nut, Peach palm, Cupuaçu
and Columbrina have good potential to increase C
stocks and the added value of providing products
of economic value. Although SF had higher biomass
and C than the AFS, the latter are export systems
providing fruit, green manure and timber. Stocks
of P were slightly higher in the palm and fruit
tree systems than SF suggesting that despite
exports, the tree species may be mobilizing more
P from soil pools than SF. Root Biomass
Estimates The proportion of root biomass is low
compared to aerial biomass although this may be
an artifact of incomplete samples (Salamão et al.
1998). We are continuing root excavation in
additional soil pits. Soil Plant Carbon C
stocks are greatest in the soil (to 1 m) and
provide a potentially more stable way to store C.
Our results are similar to what has been reported
for the southern Amazon (Fujisaka et al, 1998).
Converting forests into pastures resulted in a
net flux of C to the atmosphere of 6.6 t/ha/yr.
The establishment of AFS on degraded pastures
enables net C recovery, and after 8 years of
growth, total biomass is around 25 of that of
primary forest. Light capture is most
influenced by management activities. In July,
after a palm heart harvest in AS1 and pruning in
AS2, leaf area was less than SF, ungrazed ASP-H
and ASP-L, but greater than grazed pasture. No
areas studied attained or maintained LAI similar
to estimates for Manaus area primary forest
(5.7-6.1 as per Roberts et al., 1996). Biomass
accumulation on Abandoned Pastures We estimate a
mean annual increment of 10 t/ha, which is
similar to other Amazonian findings (Uhl et al.
1988). If this rate were sustained, equivalent
primary forest biomass of 300 t/ha (Fujisaka et
al. 1998) would be achieved in only 30 years,
which contradicts present estimated forest
regeneration times of 100 years (Fearnside,
1996). The concomitant decrease in available soil
P as plant biomass increases over time, however,
suggests that the growth rates of our secondary
forests on abandoned pastures may start to slow
down significantly over the next five years. We
are monitoring the growth effect of nutrient
applications to the secondary forests on
abandoned pastures.
Table 2. Aerial biomass and carbon estimates for
three, 9-year-old AFS and a secondary forest on
degraded pasture land near Manaus, Brazil.
Table 1. Regression models developed to estimate
aerial biomass of tree components in three 9 year
old Agroforestry Systems.
6. REFERENCES CITED see Handout