Moist tropical forests in Amazonia and elsewhere are subjected to increasingly severe drought episodes through the El Niño–Southern Oscillation (ENSO) and possibly through deforestation‐driven reductions in rainfall. The effects of this trend on tropical forest canopy dynamics, emissions of greenhouse gases, and other ecological functions are potentially large but poorly understood. We established a throughfall exclusion experiment in an east‐central Amazon forest (Tapajós National Forest, Brazil) to help understand these effects. After 1‐year intercalibration period of two 1‐ha forest plots, we installed plastic panels and wooden gutters in the understory of one of the plots, thereby excluding ∼890 mm of throughfall during the exclusion period of 2000 (late January to early August) and ∼680 mm thus far in the exclusion period of 2001 (early January to late May). Average daily throughfall reaching the soil during the exclusion period in 2000 was 4.9 and 8.3 mm in the treatment and control plots and was 4.8 and 8.1 mm in 2001, respectively. During the first exclusion period, surface soil water content (0–2 m) declined by ∼100 mm, while deep soil water (2–11 m) was unaffected. During the second exclusion period, which began shortly after the dry season when soil water content was low, surface and deep soil water content declined by ∼140 and 160 mm, respectively. Although this depletion of soil water provoked no detectable increase in leaf drought stress (i.e., no reduction in predawn leaf water potential), photosynthetic capacity declined for some species, the canopy thinned (greater canopy openness and lower leaf area index) during the second exclusion period, stem radial growth of trees <15 m tall declined, and fine litterfall declined in the treatment plot, as did tree fruiting. Aboveground net primary productivity (NPP) (stemwood increment and fine litter production) declined by one fourth, from 15.1 to 11.4 Mg ha−1 yr−1, in the treatment plot and decreased slightly, from 11.9 to 11.5 Mg ha−1 yr−1, in the control plot. Stem respiration varied seasonally and was correlated with stem radial growth but showed no treatment response. The fastest response to the throughfall exclusion, and the surface soil moisture deficits that it provoked, was found in the soil itself. The treatment reduced N2O emissions and increased CH4 consumption relative to the control plot, presumably in response to the improved soil aeration that is associated with soil drying. Our hypothesis that NO emissions would increase following exclusion was not supported. The conductivity and alkalinity of water percolating through the litter layer and through the mineral soil to a depth of 200 cm was higher in the treatment plot, perhaps because of the lower volume of water that was moving through these soil layers in this plot. Decomposition of the litter showed no difference between plots. In sum, the small soil water reductions provoked during the first 2 years of partial throughfall exclusion were sufficient to lower aboveground NPP, including th...

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Using a new approach involving one-time measurements of radiocarbon (C-14) in fine (<2 min diameter) root tissues we have directly measured the mean age of fine-root carbon. We find that the carbon making up the standing stock of fine roots in deciduous and coniferous forests of the eastern United States has a mean age of 3-18 years for live fine roots, 10-18 years for dead fine roots, and 3-18 years for mixed live+dead fine roots. These C-14-derived mean ages represent the time C was stored in the plant before being allocated for root growth, plus the average lifespan (for live roots), plus the average time for the root to decompose (for dead roots and mixtures). Comparison of the C-14 content of roots known to have grown within I year with the C-14 of atmospheric CO2 for the same period shows that root tissues are derived from recently fixed carbon, and the storage time prior to allocation is <2 years and likely <1 year. Fine-root mean ages tend to increase with depth in the soil. Live roots in the organic horizons are made of C fixed 3-8 years ago compared with 11-18 years in the mineral B horizons. The mean age of C in roots increases with root diameter and also is related to branching order. Our results differ dramatically from previous estimates of fine-root mean ages made using mass balance approaches and root-viewing cameras, which generally report life spans (mean ages for live roots) of a few months to 1-2 years. Each method for estimating fine-root dynamics, including this new radiocarbon method, has biases. Root-viewing approaches tend to emphasize more rapidly cycling roots, while radiocarbon ages tend to reflect those components that persist longest in the soil. Our C-14-derived estimates of long mean ages can be reconciled with faster estimates only if fine-root populations have varying rates of root mortality and decomposition. Our results indicate that it standard definition of fine roots, as those with diameters of <2 mm, is inadequate to determine the most dynamic portion of the root population. Recognition of the variability in fine-root dynamics is necessary to obtain better estimates of belowground C inputs

Abstract. Over the past three decades, tropical forest clearing and burning have greatly altered the Amazonian landscape by increasing the cover of pastures and secondary forests. The alteration of biogeochemical processes on these lands is of particular interest on highly weathered Oxisols that cover large areas in the region because of concerns regarding possible nutrient limitation in agricultural land uses and during forest regrowth. The objectives of this study were to quantify (1) the reaccumulation of nutrients in biomass of secondary land uses, (2) changes in soil nutrient contents, (3) internal nutrient cycles, and (4) input-output budgets for the landscape mosaic.Nutrient stocks and fluxes were quantified from 1996 to 1998 in mature forest, 19-yrold secondary forest, degraded pastureland, and managed pastureland in the Brazilian state of Pará. Mature forests contain 130 Mg C/ha in aboveground biomass while secondary forest, degraded pasture, and managed pasture contain 34, 4, and 3 Mg C/ha, respectively. Reaccumulation of N, P, K, Ca, and Mg in aboveground biomass of secondary forest was 20%, 21%, 42%, 50%, and 27% of that present in mature forest, while degraded pasture contained 2%, 4%, 15%, 11%, and 6%. Managed pasture had similar accumulations as degraded pasture except for Ca (3%).Changes in soil stocks of C, N, and P were not detected among land uses, except in fertilized managed pastures, where total soil P (0-10 cm) was elevated. Conversely, Mehlich-III-extractable P of all secondary lands were very low (Ͻ1 g/g) and were 1 kg/ha less than contents (0-10 cm) in mature forest. NaOH-extractable P was present in 100-fold higher concentrations and may gradually contribute to meeting plant demands over decadal time scales. Soil cation contents (0-20 cm) were elevated in secondary lands with increases of ϳ85, 500, and 75 kg/ha for K, Ca, and Mg, respectively. These increases could account for a substantial portion of cation contents originally in the aboveground biomass of mature forest.The recycling of nutrients through ϳ9.0 Mg·ha Ϫ1 ·yr Ϫ1 of litterfall in secondary forest of 132, 2.8, 32, 106, and 23 kg·ha Ϫ1 ·yr Ϫ1 for N, P, K, Ca, and Mg, respectively, is similar to mature forest. Nutrient returns in both pasturelands were smaller for all elements except K, which was similar to the forested sites. In these pasture ecosystems, grass turnover has replaced litterfall return as the predominate mechanism of nutrient recycling.Soil solution fluxes of total N were higher in mature forest (12 kg·ha Ϫ1 ·yr Ϫ1 at 25 cm depth) compared to secondary lands (Ͻ4 kg·ha Ϫ1 ·yr Ϫ1), indicating that cycling of available forms of N has diminished. Conversely, fluxes of cationic elements appear elevated in secondary lands and are charge balanced in solution by HCO 3 Ϫ derived from biological activity in the soil surface. Despite detectable increases in soil cation fluxes, rainwater inputs and stream water outputs of these elements across the watershed were not significantly different.The aggregate picture for this la...

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