It has been proposed that the C/N ratio, or quality, of litter or mulch mixtures affects N release. Although total N release from these mixtures and the effects on soil N are relatively well ...understood, a mechanistic understanding of the interactions between litter species with respect to their N release is still lacking. This study examines decomposition and N dynamics in mixtures of high-quality leguminous mulch, gliricidia
Gliricidia sepium (Jacq.) Kunth. ex Walp. with a C/N ratio of 13, and low-quality cupuaçu
Theobroma grandiflorum (Wild. ex Spring) Schumann litter with a C/N ratio of 42, which occur in combination in agroforestry systems. Ratios of 100:0, 80:20, 50:50, 20:80, 0:100 of fresh
15N-enriched gliricidia leaves and senescent cupuaçu leaves, totaling the same dry weight of 6.64
t
ha
−1, were applied to an Oxisol and sampled at 6, 14, 38, and 96 days after application. After more than 40% of the N in the gliricidia leaves had been released and the microbial biomass N reached its peak, a significant increase in available soil N occurred at day 14, which was more pronounced with greater amounts of gliricidia in the leaf mixture. However, relative to the N applied in the leaf mixture, there was no significant difference in available soil N with greater proportions of gliricidia. Total N release from the mixtures corresponded to the total N applied by gliricidia. Until day 38, cupuaçu C mineralization was significantly faster in the presence of the highest proportion of gliricidia compared to lower proportions. This faster C mineralization of more than 0.5% per day, however, did not increase total C loss or N release from cupuaçu leaves after 96 days. The use of
15N tracers identified an N transfer from gliricidia leaves and the soil to cupuaçu leaves and consequently, a lower N release from gliricidia to the soil in the presence of cupuaçu leaves. Though we expected that available N in the soil would also decrease with greater amounts of cupuaçu litter in the mixture, our results indicated an additive effect of the two species on N release and soil mineral N, with gross interactions between them canceling net interactive effects. Therefore, N release of leaf mixtures behaved as predicted from a calculated sum of individual release patterns, in spite of a transfer of N from the high- to the low-quality leaves.
Sustainable management for existing Amazonian forests requires an extensive knowledge about the limits of ecosystem nutrient cycles. Therefore, symbiotic nitrogen (N₂) fixation of legumes was ...investigated in a periodically flooded forest of the central Amazon floodplain (Várzea) over two hydrological cycles (20 months) using the ¹⁵N natural abundance method. No seasonal variation in ¹⁵N abundance (δ ¹⁵N values) in trees which would suggest differences in N₂ fixation rates between the terrestrial and the aquatic phase was found. Estimations of the percentage of N derived from atmosphere (%Ndfa) for the nodulated legumes with Neptunia oleracea on the one side and Teramnus volubilis on the other resulted in mean %Ndfa values between 9 and 66%, respectively. More than half of the nodulated legume species had %Ndfa values above 45%. These relatively high N gains are important for the nodulated legumes during the whole hydrological cycle. With a %Ndfa of 4-5% for the entire Várzea forest, N₂ fixation is important for the ecosystem and therefore, has to be taken into consideration for new sustainable land-use strategies in this area.
Summary
The future flora of Amazonia will include significant areas of secondary forest as degraded pastures are abandoned and secondary succession proceeds. The rate at which secondary forests ...regain carbon (C) stocks and re‐establish biogeochemical cycles that resemble those of primary forests will influence the biogeochemistry of the region. Most studies have focused on the effects of deforestation on biogeochemical cycles. In this study, we present data on the recuperation of carbon stocks and carbon fluxes within a secondary forest of the eastern Amazon, and we compare these measurements to those for primary forest, degraded pasture, and productive pasture. Along a transect from a 23‐y‐old degraded pasture, through a 7‐y‐old secondary forest, through a 16‐year‐old secondary forest, and to a primary forest, the δ13C values of soil organic matter (SOM) in the top 10 cm of soil were – 21.0, – 26.5, – 27.4, and – 27.9‰, respectively, indicating that the isotopic signature of SOM from C3 forest plants was rapidly re‐established. The degraded pasture also had significant inputs of C from C3 plants. Radiocarbon data indicated that most of the C in the top 10 cm of soil had been fixed by plants during the last 30 years. Differences in soil C inventory among land use types were small compared to uncertainties in their measurement. Root inputs were nearly identical in primary and secondary forests, and litterfall in the secondary forest was 88% of the litterfall rate of the primary forest. In contrast, the secondary forest had only 17% of the above ground biomass. Because of rapid cycling rates of soil C and rapid recovery of C fluxes to and from the soil, the below ground C cycle in this secondary forest was nearly identical with those of the unaltered primary forest.
To test whether plant species influence greenhouse gas production in diverse ecosystems, we measured wet season soil CO
2
and N
2
O fluxes close to ∼300 large (>35 cm in diameter at breast height ...(DBH)) trees of 15 species at three clay‐rich forest sites in central Amazonia. We found that soil CO
2
fluxes were 38% higher near large trees than at control sites >10 m away from any tree (
P
< 0.0001). After adjusting for large tree presence, a multiple linear regression of soil temperature, bulk density, and liana DBH explained 19% of remaining CO
2
flux variability. Soil N
2
O fluxes adjacent to
Caryocar villosum
,
Lecythis lurida
,
Schefflera morototoni
, and
Manilkara huberi
were 84%−196% greater than
Erisma uncinatum
and
Vochysia maxima
, both Vochysiaceae. Tree species identity was the most important explanatory factor for N
2
O fluxes, accounting for more than twice the N
2
O flux variability as all other factors combined. Two observations suggest a mechanism for this finding: (1) sugar addition increased N
2
O fluxes near
C. villosum
twice as much (
P
< 0.05) as near Vochysiaceae and (2) species mean N
2
O fluxes were strongly negatively correlated with tree growth rate (
P
= 0.002). These observations imply that through enhanced belowground carbon allocation liana and tree species can stimulate soil CO
2
and N
2
O fluxes (by enhancing denitrification when carbon limits microbial metabolism). Alternatively, low N
2
O fluxes potentially result from strong competition of tree species with microbes for nutrients. Species‐specific patterns in CO
2
and N
2
O fluxes demonstrate that plant species can influence soil biogeochemical processes in a diverse tropical forest.
The controls on uptake and release of CO
2 by tropical rainforests, and the responses to a changing climate, are major uncertainties in global climate change models. Eddy-covariance measurements ...potentially provide detailed data on CO
2 exchange and responses to the environment in these forests, but accurate estimates of the net ecosystem exchange of CO
2 (NEE) and ecosystem respiration (
R
eco) require careful analysis of data representativity, treatment of data gaps, and correction for systematic errors. This study uses the comprehensive data from our study site in an old-growth tropical rainforest near Santarem, Brazil, to examine the biases in NEE and
R
eco potentially associated with the two most important sources of systematic error in Eddy-covariance data: lost nighttime flux and missing canopy storage measurements. We present multiple estimates for the net carbon balance and
R
eco at our site, including the conventional “
u* filter”, a detailed bottom-up budget for respiration, estimates by similarity with
222Rn, and an independent estimate of respiration by extrapolation of daytime Eddy flux data to zero light. Eddy-covariance measurements between 2002 and 2006 showed a mean net ecosystem carbon loss of 0.25
±
0.04
μmol
m
−2
s
−1, with a mean respiration rate of 8.60
±
0.11
μmol
m
−2
s
−1 at our site. We found that lost nocturnal flux can potentially introduce significant bias into these results. We develop robust approaches to correct for these biases, showing that, where appropriate, a site-specific
u* threshold can be used to avoid systematic bias in estimates of carbon exchange. Because of the presence of gaps in the data and the day–night asymmetry between storage and turbulence, inclusion of canopy storage is essential to accurate assessments of NEE. We found that short-term measurements of storage may be adequate to accurately model storage for use in obtaining ecosystem carbon balance, at sites where storage is not routinely measured. The analytical framework utilized in this study can be applied to other Eddy-covariance sites to help correct and validate measurements of the carbon cycle and its components.
The stable isotope content of samples of precipitation and of the river water throughout the Piracicaba basin in Brazil was measured over a two-year period. The isotope values of precipitation follow ...a consistent pattern of relatively depleted values of both deuterium and oxygen 18 during the rainy summers and enriched ones during the dry winters, with all values aligned slightly above the Global Meteoric Water Line. The isotopic composition of the river water throughout the basin shows a remarkable spatial coherence and much smaller scatter of data than those of the precipitation. The isotope composition of river water is close to that of the precipitation in the rainy season, however, with a consistent lower d-excess value by 1‰-2‰. This is attributed to evaporative water loss in the basin, in part an expression of the recycling of water due to the anthropogenic activity in the region. The more divergent values are recorded during high-water stages in the rivers. In many cases, the floods during the beginning of the rainy season are characterized by an enrichment of the heavy isotopes and lower d-excess values when compared to the precipitation, with the opposite situation later in the rainy season. This is interpreted as resulting from the watershed/riverflow interaction pattern, and it thus suggests that the isotope composition can monitor the hydrologic situation in the basin and its changes.
Higher levels of taxonomic and evolutionary diversity are expected to maximize ecosystem function, yet their relative importance in driving variation in ecosystem function at large scales in diverse ...forests is unknown. Using 90 inventory plots across intact, lowland, terra firme, Amazonian forests and a new phylogeny including 526 angiosperm genera, we investigated the association between taxonomic and evolutionary metrics of diversity and two key measures of ecosystem function: aboveground wood productivity and biomass storage. While taxonomic and phylogenetic diversity were not important predictors of variation in biomass, both emerged as independent predictors of wood productivity. Amazon forests that contain greater evolutionary diversity and a higher proportion of rare species have higher productivity. While climatic and edaphic variables are together the strongest predictors of productivity, our results show that the evolutionary diversity of tree species in diverse forest stands also influences productivity. As our models accounted for wood density and tree size, they also suggest that additional, unstudied, evolutionarily correlated traits have significant effects on ecosystem function in tropical forests. Overall, our pan-Amazonian analysis shows that greater phylogenetic diversity translates into higher levels of ecosystem function: tropical forest communities with more distantly related taxa have greater wood productivity.
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