It is well recognized in the literature that topography can influence soil nutrient stocks and dynamics in temperate regions, but for tropical forests, this source of variation has sometimes been ...ignored. The nature of such variations may depend upon the soil type, which in turn, is closely linked to local or regional topography. This study characterizes the soil and describes the status of carbon and nitrogen in vegetation, litterfall, litter‐layer and soil upper layers along the main positions of a topographic gradient (plateau, slope and valley), 60 km north of Manaus, on Cuieiras Reserve watershed. Nitrogen concentrations in living leaves, fresh litterfall, litter‐layer and soil upper layers were lower in the valley than in both slope and plateau plots. Carbon concentrations in plant material were not significantly different among the three topographic positions, resulting in higher C : N ratios in valley plots. Local topography (plateau, slope and valley) clearly was an influential factor in the nutrient distribution along the study locations. Lower rates of N cycling processes in the valley are probably related to its sandy soil texture and seasonal flooding.
We used forest inventories, conducted in seventy-two 1-ha permanent plots to assess the variation in tree and palm aboveground live biomass (AGLB), and its relation with soil gradients (texture and ...nutrients) and topography (altitude and slope). Our plots, located at Reserva Florestal Adolpho Ducke, a 10,000
ha reserve in central Amazonia, near the city of Manaus, were systematically spread over 64
km
2. The plots were long (250
m) and narrow (up to 40
m), following elevational contours. Chemical and physical soil analyses were undertaken using topsoil samples, collected in each plot. The plots covered a soil textural gradient ranging from 8% to 98% percent of sand, slopes varying from 0.5° to 27°, and the maximum difference of altitude among plots was 70
m, ranging from 39 to 109
m
a.s.l. The mean total AGLB (palm
+
trees) for stems over 1
cm diameter breast height (dbh
=
1.3
m), was 327.8
Mg/ha (ranging from 210.9 to 426.3) and was similar to other studies in the region. We found a two-fold variation in total AGLB estimates among plots, with soil or topography explaining around 20% of this variation. AGLB was positively related to either a soil textural gradient (represented by the percentage of clay) or altitude, but was insensitive to slope. Although slope had no detectable effect on the mean variation of AGLB per plot, it explained 14% of the AGLB in understory and emergent trees. On slopes, AGLB is concentrated mostly in tree classes of small size (1
≤
dbh
<
10
cm), whereas in flat areas, most of the biomass is concentrated in a few big trees. When considering palms and trees separately, we found that tree biomass tended to increase in clay-rich soils (mostly located in high areas), whereas palm biomass was higher in sandy soils (low slopes and bottomlands). This study shows that forests in central Amazonia grow in a heterogeneous environment in relation to soil and topography, and this heterogeneity seems to be in part responsible for differences in structure and AGLB accumulation. In the forest around Manaus, topography can be used to predict aboveground live biomass as an alternative to soil variables, with similar predictive power. Topographic variables can be easily assessed using satellite imagery and could improve current estimates of carbon stocks over large areas in the Amazon.
Experiments were carried out to test the effects of liming and nutrient additions on plant growth and soil processes such as C and N mineralisation in three contrasting forest types in central ...Amazonia: the stunted facies of heath forest (SHF), the tall facies of heath forest (THF) and the surrounding lowland evergreen rain forest (LERF). Calcium-carbonate additions increased soil respiration in the field plots in the SHF; in laboratory incubations, soil respiration was higher in the SHF when soils were fertilised with N, and in THF and LERF after S additions. The addition of N alone or in different combinations generally induced a net immobilisation of soil N. Net nitrification increased during the incubation in SHF and THF soils fertilised with N+P, and in LERF soils fertilised with either N, or P, or CaCO₃. In a field experiment using ingrowth bags, a higher fine root production was observed in all forest types when bags were fertilised with CaCl₂ or CaCO₃, suggesting that Ca may be a limiting nutrient in these soils. Calcium-carbonate addition in a glasshouse bioassay experiment with rice showed an overall positive effect on the survival and growth of the seedlings. In other treatments where soil pH was not raised, the rice showed acute toxicity symptoms, poor root and shoot growth and high mortality. Similar results were yielded in a field experiment, using naturally established seedlings in the field plots in SHF, THF and LERF. It is concluded that the acute H⁺ ion toxicity is a major growth-limiting factor for non-adapted plants in heath forest soils in central Amazonia.
Black carbon (BC) is an important fraction of many soils worldwide and plays an important role in global C biogeochemistry. However, few studies have examined how it influences the mineralization of ...added organic matter (AOM) and its incorporation into soil physical fractions and whether BC decomposition is increased by AOM. BC-rich Anthrosols and BC-poor adjacent soils from the Central Amazon (Brazil) were incubated for 532
days either with or without addition of
13C-isotopically different plant residue. Total C mineralization from the BC-rich Anthrosols with AOM was 25.5% (
P
<
0.05) lower than with mineralization from the BC-poor adjacent soils. The AOM contributed to a significantly (
P
<
0.05) higher proportion to the total C mineralized in the BC-rich Anthrosols (91–92%) than the BC-poor adjacent soils (69–80%). The AOM was incorporated more rapidly in BC-rich than BC-poor soils from the separated free light fraction through the intra-aggregate light fraction into the stable organo-mineral fraction and up to 340% more AOM was found in the organo-mineral fraction. This more rapid stabilization was observed despite a significantly (
P
<
0.05) lower metabolic quotient for BC-rich Anthrosols. The microbial biomass (MB) was up to 125% greater (
P
<
0.05) in BC-rich Anthrosols than BC-poor adjacent soils. To account for increased MB adsorption onto BC during fumigation extraction, a correction factor was developed via addition of a
13C-enriched microbial culture. The recovery was found to be 21–41% lower (
P
<
0.05) for BC-rich than BC-poor soils due to re-adsorption of MB onto BC. Mineralization of native soil C was enhanced to a significantly greater degree in BC-poor adjacent soils compared to BC-rich Anthrosols as a result of AOM. No positive priming by way of cometabolism due to AOM could be found for aged BC in the soils.
We monitored seventy-two 1 ha permanent plots spread over 64 km² of terra firme forest at Reserva Ducke (Manaus, Amazonas, Brazil) over 2-yr intervals to assess the effects of a soil and topographic ...gradient on the rate of change in the aboveground tree live biomass (AGLB). AGLB increased significantly over the 2-yr intervals, exhibiting a mean rate of change of 1.65 Mg/ha/yr (bootstrapped 95% CI: 1.15, 2.79). The rate of change varied according to tree size class; understory and sub-canopy trees exhibited higher rates of change. Over the whole period, the rate of change was not related to soil or topographic features of the plots, but there was evidence that the relationships varied depending on the year of measurement. In the plots monitored between 2001 and 2003 we found a significant relationship between AGLB change and the soil textural gradient, but this relationship was not evident in plots monitored between 2002 and 2004. This suggests that both the temporal variation in the soil-biomass change relationship and the size structure of the forest need to be included in models of biomass change in Amazonia. We also noted that the rate of biomass change is sensitive to the equation used to estimate AGLB. Allometric models that incorporate wood-density data provide higher per plot AGLB estimates, but lower rates of change, suggesting that variations in floristic composition have important implications for carbon cycling in diverse tropical forests. Abstract in Portuguese is available at http://www.blackwell-synergy.com/loi/btp.
Subsistence agriculture, cattle ranching, and periodical land abandonment are common land‐use practices in Amazonia. Because changes in land use affect biogeochemical cycles, secondary forests ...growing after land abandonment develop at varying speeds and spatial patterns, due in part to varying nutrient dynamics. Leaf and soil nutrient concentrations can provide useful information on nutrient cycling processes and strategies of nutrient use by trees that are suitable for introduction to abandoned areas. To understand nutrient dynamics in secondary forests from different regeneration stages, as well as the importance of pioneer species in the regeneration process, we measured the concentration of macronutrients in leaves of three pioneer tree species (Vismia cayennensis, Cecropia sciadophylla, and Bellucia dichotoma) in central Amazon secondary forests. We also measured macronutrients in the topsoil under the trees. We found that type of prior land use, time since abandonment, and number of fire events were significantly correlated with the concentrations of leaf and soil macronutrients, explaining between 10 and 38% of the variation in macronutrient concentrations. The observed patterns suggest that management practices affect the processes involved in N cycling and availability. Of the three pioneer species, C. sciadophylla showed the highest nutrient resorption efficiency and the highest leaf nutritional quality. We suggest that these two features determine a higher potential of C. sciadophylla for natural regeneration and restoration of abandoned lands.
Background: Dissolved organic carbon (DOC) is a major component of the total carbon in headstreams in Amazonia. Long-term measurements of DOC concentration are difficult to obtain in remote areas of ...Amazonia. Aims: To take measurements of electrical conductivity (EC) and DOC concentration in a blackwater stream and to determine whether it is possible to make predictions of the DOC concentration in such streams using continuous measurements of EC alone. Methods: In a forested area, DOC concentration and EC were measured in groundwater along a topographic gradient at weekly intervals over a period of 8 months, and in stream water at half-hourly intervals over 2 years. Results: Strong and positive relationships between EC and DOC concentration were found in stream water in wet (r ² = 0.99) and dry periods (r ² = 0.97). It was thus possible to produce two equations that could be used to derive DOC concentration values from continuous measurements of EC. Both DOC concentration and EC varied according to the discharge, indicating that the largest amounts of DOC are transported during the wet season. The amount of carbon exported annually was estimated to be between 2.5% and 5% of the sink estimated by the eddy covariance method. Conclusions: The relationship found offers an opportunity for a comprehensive assessment of DOC concentration in blackwater catchments based on low-cost and simple EC measurements, rather than on expensive and complex estimates of DOC concentration.
We tested the hypothesis that the growth of fine roots upward into the leaf litter, forming a 'surface root mat', found widely in Amazonian forests, is of adaptive value for plants of poor soils ...because it makes possible more rapid uptake of limiting nutrients. We assessed the effect of invasion by fine roots on the rates of loss of dry mass and nutrient content of leaf litter over 1 year in three plots in each of three contrasting forest types in central Amazonia: the stunted facies of heath forest known as campina (SHF), the taller facies of heath forest known as campinarana (THF), both on spodosols, and the surrounding lowland evergreen rain forest (LERF) on ultisol. Pairs of bags filled with freshly fallen leaves from the trees of Clitoria fairchildiana (Fabaceae) were placed on the litter layer; in order to prevent roots entering the control bag in each pair that bag was lifted from the forest floor and turned over each week, while the treatment bag was left undisturbed. From each plot, four pairs of litter bags were retrieved after 30, 60, 120, 180, 270 and 360 d, and all roots that had grown into the litterbags were carefully removed. The leaves and roots from each bag were oven-dried for nutrient analysis. In no forest type was there a significant difference in the rate of loss of dry matter from the litter between the bags with and without roots. The amounts of roots invading the litter bags increased significantly in the sequence SHF < THF < LERF. In no forest did the presence of roots have a significant effect on the rate of disappearance of N or P from the leaf material, or on the rate of accumulation of Fe and Al. In the SHF there was no significant effect of invasion by roots on the rates of disappearance of Ca, Mg, Mn or Zn, but in the THF, the rates of disappearance of these four elements between 270 and 360 d were significantly greater in the presence of roots. In the LERF the results were similar, but the effects of roots started earlier--the rates of disappearance of Ca and Mg were significantly enhanced between 120 and 360 d. The results from the SHF may be interpreted to suggest that the growth of fine roots (and their fungal associates) upward into leaf litter is unlikely to be explained wholly by their role in the efficient recovery of mineral nutrients.
We analyzed the effects of distance to forest edge and soil texture on finelitter production and on nutrient concentrations in the leaf fall in an experimentally fragmented landscape in Brazilian ...Amazonia. Production of fine litter (leaves, twigs <2 cm in diameter, flowers, and fruits) was measured over a 3-yr period. Litter traps were installed in plots located near (<100 m) and far (>250 m) from forest edges, and in clayey or sandy soils. In total, 28 plots were established, with 10 litter traps per plot. Results reveal a significant effect of distance to forest edge on litter production, but no significant effect of soil type or interaction between soil type and edge distance. On average, annual litter production on edge plots exceeded that on the interior plots by 0.68 Mg/ha ($9.50 \pm 0.23$vs.$8.82 \pm 0.14 Mg\cdot ha^{-1}\cdot yr^{-1}$, mean ± 1 SE, based on a 3-yr period). With regard to nutrient concentrations in the leaf fall, we detected a significant effect of soil type on three of eight nutrients analyzed. Concentrations of N, Mg, and Mn were greater in leaves on clayey than on sandy soils. Distance to forest edge only significantly affected the concentration of Ca, which was greater near than far from edges, perhaps due to strong Ca mobilization by the roots of pioneer trees. Several factors may account for the observed increase in litterfall near forest edges, including the greater prevalence of winds, increased plant desiccation stress, and higher rates of tree recruitment, especially of pioneer trees, near edges. Elevated rates of litterfall are likely to have cascading effects on the ecology of fragmented forests, affecting the invertebrate fauna, increasing seed and seedling mortality, and causing forest fragments to be more vulnerable to destructive surface fires.
