The relationship between rooting depth and above‐ground hydraulic traits can potentially define drought resistance strategies that are important in determining species distribution and coexistence in seasonal tropical forests, and understanding this is important for predicting the effects of future climate change in these ecosystems. We assessed the rooting depth of 12 dominant tree species (representing c. 42% of the forest basal area) in a seasonal Amazon forest using the stable isotope ratios (δ18O and δ2H) of water collected from tree xylem and soils from a range of depths. We took advantage of a major ENSO‐related drought in 2015/2016 that caused substantial evaporative isotope enrichment in the soil and revealed water use strategies of each species under extreme conditions. We measured the minimum dry season leaf water potential both in a normal year (2014; Ψnon‐ENSO) and in an extreme drought year (2015; ΨENSO). Furthermore, we measured xylem hydraulic traits that indicate water potential thresholds trees tolerate without risking hydraulic failure (P50 and P88). We demonstrate that coexisting trees are largely segregated along a single hydrological niche axis defined by root depth differences, access to light and tolerance of low water potential. These differences in rooting depth were strongly related to tree size; diameter at breast height (DBH) explained 72% of the variation in the δ18Oxylem. Additionally, δ18Oxylem explained 49% of the variation in P50 and 70% of P88, with shallow‐rooted species more tolerant of low water potentials, while δ18O of xylem water explained 47% and 77% of the variation of minimum Ψnon‐ENSO and ΨENSO. We propose a new formulation to estimate an effective functional rooting depth, i.e. the likely soil depth from which roots can sustain water uptake for physiological functions, using DBH as predictor of root depth at this site. Based on these estimates, we conclude that rooting depth varies systematically across the most abundant families, genera and species at the Tapajós forest, and that understorey species in particular are limited to shallow rooting depths. Our results support the theory of hydrological niche segregation and its underlying trade‐off related to drought resistance, which also affect the dominance structure of trees in this seasonal eastern Amazon forest. Synthesis. Our results support the theory of hydrological niche segregation and demonstrate its underlying trade‐off related to drought resistance (access to deep water vs. tolerance of very low water potentials). We found that the single hydrological axis defining water use traits was strongly related to tree size, and infer that periodic extreme droughts influence community composition and the dominance structure of trees in this seasonal eastern Amazon forest.

Selective logging is an extensive land use in the Brazilian Amazon region. The soil-atmosphere fluxes of nitrous oxide (N 2 O), nitric oxide (NO), methane (CH 4 ), and carbon dioxide (CO 2 ) are studied on two soil types (clay Oxisol and sandy loam Ultisol) over two years in both undisturbed forest and forest recently logged using reduced impact forest management in the Tapajos National Forest, near Santarem, Para, Brazil. In undisturbed forest, annual soil-atmosphere fluxes of N 2 O (mean ± standard error) were 7.9 ± 0.7 and 7.0 ± 0.6 ng N cm −2 h −1 for the Oxisol and 1.7 ± 0.1 and 1.6 ± 0.3 ng N cm −2 h −1 for the Ultisol for 2000 and 2001, respectively. The annual fluxes of NO from undisturbed forest soil in 2001 were 9.0 ± 2.8 ng N cm −2 h −1 for the Oxisol and 8.8 ± 5.0 ng N cm −2 h −1 for the Ultisol. Consumption of CH 4 from the atmosphere dominated over production on undisturbed forest soils. Fluxes averaged −0.3 ± 0.2 and −0.1 ± 0.9 mg CH 4 m −2 day −1 on the Oxisol and −1.0 ± 0.2 and −0.9 ± 0.3 mg CH 4 m −2 day −1 on the Ultisol for years 2000 and 2001. For CO 2 in 2001, the annual fluxes averaged 3.6 ± 0.4 mol m −2 s −1 on the Oxisol and 4.9 ± 1.1 mol m −2 s −1 on the Ultisol. We measured fluxes over one year each from two recently logged forests on the Oxisol in 2000 and on the Ultisol in 2001. Sampling in logged areas was stratified from greatest to least ground disturbance covering log decks, skid trails, tree-fall gaps, and forest matrix. Areas of strong soil compaction, especially the skid trails and logging decks, were prone to significantly greater emissions of N 2 O, NO, and especially CH 4 . In the case of CH 4 , estimated annual emissions from decks reached extremely high rates of 531 ± 419 and 98 ± 41 mg CH 4 m −2 day −1 , for Oxisol and Ultisol sites, respectively, comparable to wetland emissions in the region. We calculated excess fluxes from logged areas by subtraction of a background forest matrix or undisturbed forest flux and adjusted these fluxes for the proportional area of ground disturbance. Our calculations suggest that selective logging increases emissions of N 2 O and NO from 30% to 350% depending upon conditions. While undisturbed forest was a CH 4 sink, logged forest tended to emit methane at moderate rates. Soil-atmosphere CO 2 fluxes were only slightly affected by logging. The regional effects of logging cannot be simply extrapolated based upon one site. We studied sites where reduced impact harvest management was used while in Earth Interactions • Volume 9 (2005) • Paper No. 23 • Page 2 typical conventional logging ground damage is twice as great. Even so, our results indicate that for N 2 O, NO, and CH 4 , logging disturbance may be as important for regional budgets of these gases as other extensive land-use changes in the Amazon such as the conversion of forest to cattle pasture.

Fine root dynamics have the potential to contribute significantly to ecosystem-scale biogeochemical cycling, including the production and emission of greenhouse gases. This is particularly true in tropical forests which are often characterized as having large fine root biomass and rapid rates of root production and decomposition. We examined patterns in fine root dynamics on two soil types in a lowland moist Amazonian forest, and determined the effect of root decay on rates of C and N trace gas fluxes. Root production averaged 229 ( AE 35) and 153 ( AE 27) g m À2 yr À1 for years 1 and 2 of the study, respectively, and did not vary significantly with soil texture. Root decay was sensitive to soil texture with faster rates in the clay soil (k 5À0.96 year À1 ) than in the sandy loam soil (k 5À0.61 year À1 ), leading to greater standing stocks of dead roots in the sandy loam. Rates of nitrous oxide (N 2 O) emissions were significantly greater in the clay soil (13 AE 1 ng N cm À2 h À1 ) than in the sandy loam (1.4 AE 0.2 ng N cm À2 h À1 ). Root mortality and decay following trenching doubled rates of N 2 O emissions in the clay and tripled them in sandy loam over a 1-year period. Trenching also increased nitric oxide fluxes, which were greater in the sandy loam than in the clay. We used trenching (clay only) and a mass balance approach to estimate the root contribution to soil respiration. In clay soil root respiration was 264-380 g C m À2 yr À1 , accounting for 24% to 35% of the total soil CO 2 efflux. Estimates were similar using both approaches. In sandy loam, root respiration rates were slightly higher and more variable (521 AE 206 g C m 2 yr À1 ) and contributed 35% of the total soil respiration. Our results show that soil heterotrophs strongly dominate soil respiration in this forest, regardless of soil texture. Our results also suggest that fine root mortality and decomposition associated with disturbance and land-use change can contribute significantly to increased rates of nitrogen trace gas emissions.

[1] We sampled air in the canopy layer of undisturbed upland forests during wet and dry seasons at three sites in the Brazilian Amazon region and found that both methane (CH 4 ) and carbon dioxide (CO 2 ) mixing ratios increased at night. Such increases were consistent across sites and seasons. A canopy layer budget model based on measured soil-atmosphere fluxes of CO 2 was constructed to estimate ecosystem CH 4 emission. We estimate that net CH 4 emission in upland forests ranged from 2 to 21 mg CH 4 m À2 d À1 . While the origin of this CH 4 source is unknown, these ground based measurements are consistent with recent findings based on satellite observations that indicate a large, unidentified source of CH 4 in tropical forest regions.

[1] We conducted an experiment on sand and clay tropical forest soils to test the short-term effect of root mortality on the soil-atmosphere flux of nitrous oxide, nitric oxide, methane, and carbon dioxide. We induced root mortality by isolating blocks of land to 1 m using trenching and root exclusion screening. Gas fluxes were measured weekly for ten weeks following the trenching treatment. For nitrous oxide there was a highly significant increase in soil-atmosphere flux over the ten weeks following treatment for trenched plots compared to control plots. N 2 O flux averaged 37.5 and 18.5 ng N cm À2 h À1 from clay trenched and control plots and 4.7 and 1.5 ng N cm À2 h À1 from sand trenched and control plots. In contrast, there was no effect for soil-atmosphere flux of nitric oxide, carbon dioxide, or methane.