When Wet Gets Wetter: Decoupling of Moisture, Redox Biogeochemistry, and Greenhouse Gas Fluxes in a Humid Tropical Forest Soil

Upland humid tropical forest soils are often characterized by fluctuating redox dynamics that vary temporally and spatially across the landscape. An increase in the frequency and intensity of rainfall events with climate change is likely to affect soil redox reactions that control the production and emissions of greenhouse gases. We used a 24-day rainfall manipulation experiment to evaluate temporal and spatial trends of surface soil (0–20 cm) redox-active chemical species and greenhouse gas fluxes in the Luquillo Experimental Forest, Puerto Rico. Treatments consisted of a high rainfall simulation (60 mm day^?1), a fluctuating rainfall regime, and a control. Water addition generated high temporal and spatial variation in soil moisture (0.3–0.6 m³ m^?3), but had no significant effect on soil oxygen (O₂) concentrations. Extractable nitrate (NO₃ ^?) concentrations decreased with daily water additions and reduced iron (Fe(II)) concentrations increased towards the end of the experiment. Overall, redox indicators displayed a weak, non-deterministic, nonlinear relationship with soil moisture. High concentrations of Fe(II) and manganese (Mn) were present even where moisture was relatively low, and net Mn reduction occurred in all plots including controls. Mean CO₂ fluxes were best explained by soil C concentrations and a composite redox indicator, and not water addition. Several plots were CH₄ sources irrespective of water addition, whereas other plots oscillated between weak CH₄ sources and sinks. Fluxes of N₂O were highest in control plots and were consistently low in water-addition plots. Together, these data suggest (1) a relative decoupling between soil moisture and redox processes at our spatial and temporal scales of measurement, (2) the co-occurrence of aerobic and anaerobic biogeochemical processes in well-drained surface soils, and (3) an absence of threshold effects from sustained precipitation on redox reactions over the scale of weeks. Our data suggest a need to re-evaluate representations of moisture in biogeochemical models.     

Long‐term nitrogen addition modifies microbial composition and functions for slow carbon cycling and increased sequestration in tropical forest soil

Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N‐limited temperate forests. In N‐rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old‐growth tropical forest soil responded to 13 years of N addition at four rates: 0 (Control), 50 (Low‐N), 100 (Medium‐N), and 150 (High‐N) kg N ha^?1 year^?1. Soil organic carbon (SOC) content increased under High‐N, corresponding to a 33% decrease in CO₂ efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N₂O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes (narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High‐N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N₂O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems.     

Suppression of methanogenesis by dissimilatory Fe(III)-reducing bacteria in tropical rain forest soils: implications for ecosystem methane flux

Tropical forests are an important source of atmospheric methane (CH₄), and recent work suggests that CH₄ fluxes from humid tropical environments are driven by variations in CH₄ production, rather than by bacterial CH₄ oxidation. Competition for acetate between methanogenic archaea and Fe(III)‐reducing bacteria is one of the principal controls on CH₄ flux in many Fe‐rich anoxic environments. Upland humid tropical forests are also abundant in Fe and are characterized by high organic matter inputs, steep soil oxygen (O₂) gradients, and fluctuating redox conditions, yielding concomitant methanogenesis and bacterial Fe(III) reduction. However, whether Fe(III)‐reducing bacteria coexist with methanogens or competitively suppress methanogenic acetate use in wet tropical soils is uncertain. To address this question, we conducted a process‐based laboratory experiment to determine if competition for acetate between methanogens and Fe(III)‐reducing bacteria influenced CH₄ production and C isotope composition in humid tropical forest soils. We collected soils from a poor to moderately drained upland rain forest and incubated them with combinations of ¹³C‐bicarbonate, ¹³C‐methyl labeled acetate (¹³CH₃COO^?), poorly crystalline Fe(III), or fluoroacetate. CH₄ production showed a greater proportional increase than Fe²⁺ production after competition for acetate was alleviated, suggesting that Fe(III)‐reducing bacteria were suppressing methanogenesis. Methanogenesis increased by approximately 67 times while Fe²⁺ production only doubled after the addition of ¹³CH₃COO^?. Large increases in both CH₄ and Fe²⁺ production also indicate that the two process were acetate limited, suggesting that acetate may be a key substrate for anoxic carbon (C) metabolism in humid tropical forest soils. C isotope analysis suggests that competition for acetate was not the only factor driving CH₄ production, as ¹³C partitioning did not vary significantly between ¹³CH₃COO^? and ¹³CH₃COO^?+Fe(III) treatments. This suggests that dissimilatory Fe(III)‐reduction suppressed both hydrogenotrophic and aceticlastic methanogenesis. These findings have implications for understanding the CH₄ biogeochemistry of highly weathered wet tropical soils, where CH₄ efflux is driven largely by CH₄ production.     

Iron oxidation stimulates organic matter decomposition in humid tropical forest soils

Humid tropical forests have the fastest rates of organic matter decomposition globally, which often coincide with fluctuating oxygen (O₂) availability in surface soils. Microbial iron (Fe) reduction generates reduced iron [Fe(II)] under anaerobic conditions, which oxidizes to Fe(III) under subsequent aerobic conditions. We demonstrate that Fe (II) oxidation stimulates organic matter decomposition via two mechanisms: (i) organic matter oxidation, likely driven by reactive oxygen species; and (ii) increased dissolved organic carbon (DOC) availability, likely driven by acidification. Phenol oxidative activity increased linearly with Fe(II) concentrations (P < 0.0001, pseudo R² = 0.79) in soils sampled within and among five tropical forest sites. A similar pattern occurred in the absence of soil, suggesting an abiotic driver of this reaction. No phenol oxidative activity occurred in soils under anaerobic conditions, implying the importance of oxidants such as O₂ or hydrogen peroxide (H₂O₂) in addition to Fe(II). Reactions between Fe(II) and H₂O₂ generate hydroxyl radical, a strong nonselective oxidant of organic compounds. We found increasing consumption of H₂O₂ as soil Fe(II) concentrations increased, suggesting that reactive oxygen species produced by Fe(II) oxidation explained variation in phenol oxidative activity among samples. Amending soils with Fe(II) at field concentrations stimulated short‐term C mineralization by up to 270%, likely via a second mechanism. Oxidation of Fe(II) drove a decrease in pH and a monotonic increase in DOC; a decline of two pH units doubled DOC, likely stimulating microbial respiration. We obtained similar results by manipulating soil acidity independently of Fe(II), implying that Fe(II) oxidation affected C substrate availability via pH fluctuations, in addition to producing reactive oxygen species. Iron oxidation coupled to organic matter decomposition contributes to rapid rates of C cycling across humid tropical forests in spite of periodic O₂ limitation, and may help explain the rapid turnover of complex C molecules in these soils.     

Canopy soil greenhouse gas dynamics in response to indirect fertilization across an elevation gradient of tropical montane forests

Canopy soils can significantly contribute to aboveground labile biomass, especially in tropical montane forests. Whether they also contribute to the exchange of greenhouse gases is unknown. To examine the importance of canopy soils to tropical forest‐soil greenhouse gas exchange, we quantified gas fluxes from canopy soil cores along an elevation gradient with 4 yr of nutrient addition to the forest floor. Canopy soil contributed 5–12 percent of combined (canopy + forest floor) soil CO₂ emissions but CH₄ and N₂O fluxes were low. At 2000 m, phosphorus decreased CO₂ emissions (>40%) and nitrogen slightly increased CH₄ uptake and N₂O emissions. Our results show that canopy soils may contribute significantly to combined soil greenhouse gas fluxes in montane regions with high accumulations of canopy soil. We also show that changes in fluxes could occur with chronic nutrient deposition.     

Biological Nitrogen Fixation in Two Tropical Forests: Ecosystem-Level Patterns and Effects of Nitrogen Fertilization

Humid tropical forests are often characterized by large nitrogen (N) pools, and are known to have large potential N losses. Although rarely measured, tropical forests likely maintain considerable biological N fixation (BNF) to balance N losses. We estimated inputs of N via BNF by free-living microbes for two tropical forests in Puerto Rico, and assessed the response to increased N availability using an on-going N fertilization experiment. Nitrogenase activity was measured across forest strata, including the soil, forest floor, mosses, canopy epiphylls, and lichens using acetylene (C₂H₂) reduction assays. BNF varied significantly among ecosystem compartments in both forests. Mosses had the highest rates of nitrogenase activity per gram of sample, with 11 ± 6 nmol C₂H₂ reduced/g dry weight/h (mean ± SE) in a lower elevation forest, and 6 ± 1 nmol C₂H₂/g/h in an upper elevation forest. We calculated potential N fluxes via BNF to each forest compartment using surveys of standing stocks. Soils and mosses provided the largest potential inputs of N via BNF to these ecosystems. Summing all components, total background BNF inputs were 120 ± 29 μg N/m²/h in the lower elevation forest, and 95 ± 15 μg N/m²/h in the upper elevation forest, with added N significantly suppressing BNF in soils and forest floor. Moisture content was significantly positively correlated with BNF rates for soils and the forest floor. We conclude that BNF is an active biological process across forest strata for these tropical forests, and is likely to be sensitive to increases in N deposition in tropical regions.     

Changes in soil heterotrophic respiration,carbon availability,and microbial function in seven forests along a climate gradient

Soil microbial communities play an essential role in soil carbon (C) emission and C sequestration in forest ecosystems. However, little information is available regarding the relationship between soil C dynamics and microbial substrate utilization at large scales. Along the North–South Transect of Eastern China (NSTEC), seven forests representative of boreal, temperate and tropical biomes were examined. Soil heterotrophic respiration (R_h), soil dissolved organic C (DOC), microbial biomass C (MBC), and microbial community-level physiological profiles (CLPPs) were investigated using biochemical measurements, static chamber-gas chromatography analysis, and Biolog-Eco microplates, respectively. We found that soil R_h rates were significantly higher in subtropical and boreal forests than in temperate forests. Conversely, the concentrations of soil DOC and MBC, as well as microbial metabolic activity and functional diversity, were consistently higher in temperate forests than in subtropical forests. There were considerably different substrate utilization profiles among the boreal, temperate, and subtropical forests. Soil microorganisms from the temperate and boreal forests mainly metabolized high-energy substrates, while those from the subtropical forests used all substrates equally. In addition, soil R_h rates were significantly and negatively related to soil labile C concentrations, total metabolic activity, and the intensity of individual substrate utilization, indicating that soil microbes assimilated more soil substrates, thereby reducing CO₂ emissions. Overall, our study suggests that climate factors, as well as substrate availability, dominate the activities and functions of soil microbes at large scales.     

Oxygen effects on methane production and oxidation in humid tropical forest soils

We investigated the effects of oxygen (O₂) concentration on methane (CH₄) production and oxidation in two humid tropical forests that differ in long‐term, time‐averaged soil O₂ concentrations. We identified sources and sinks of CH₄ through the analysis of soil gas concentrations, surface emissions, and carbon isotope measurements. Isotope mass balance models were used to calculate the fraction of CH₄ oxidized in situ. Complementary laboratory experiments were conducted to determine the effects of O₂ concentration on gross and net rates of methanogenesis. Field and laboratory experiments indicated that high levels of CH₄ production occurred in soils that contained between 9±1.1% and 19±0.2% O₂. For example, we observed CH₄ concentrations in excess of 3% in soils with 9±1.1% O₂. CH₄ emissions from the lower O₂ sites were high (22–101 nmol CH₄ m^?2 s^?1), and were equal in magnitude to CH₄ emissions from natural wetlands. During peak periods of CH₄ efflux, carbon dioxide (CO₂) emissions became enriched in ¹³C because of high methanogenic activity. Gross CH₄ production was probably greater than flux measurements indicated, as isotope mass balance calculations suggested that 48–78% of the CH₄ produced was oxidized prior to atmospheric egress. O₂ availability influenced CH₄ oxidation more strongly than methanogenesis. Gross CH₄ production was relatively insensitive to O₂ concentrations in laboratory experiments. In contrast, methanotrophic bacteria oxidized a greater fraction of total CH₄ production with increasing O₂ concentration, shifting the δ¹³C composition of CH₄ to values that were more positive. Isotopic measurements suggested that CO₂ was an important source of carbon for methanogenesis in humid forests. The δ¹³C value of methanogenesis was between ?84‰ and ?98‰, which is well within the range of CH₄ produced from CO₂ reduction, and considerably more depleted in ¹³C than CH₄ formed from acetate.     

Tree stem bases are sources of CH4 and N2O in a tropical forest on upland soil during the dry to wet season transition

Tropical forests on upland soils are assumed to be a methane (CH₄) sink and a weak source of nitrous oxide (N₂O), but studies of wetland forests have demonstrated that tree stems can be a substantial source of CH₄, and recent evidence from temperate woodlands suggests that tree stems can also emit N₂O. Here, we measured CH₄ and N₂O fluxes from the soil and from tree stems in a semi‐evergreen tropical forest on upland soil. To examine the influence of seasonality, soil abiotic conditions and substrate availability (litter inputs) on trace greenhouse gas (GHG) fluxes, we conducted our study during the transition from the dry to the wet season in a long‐term litter manipulation experiment in Panama, Central America. Trace GHG fluxes were measured from individual stem bases of two common tree species and from soils beneath the same trees. Soil CH₄ fluxes varied from uptake in the dry season to minor emissions in the wet season. Soil N₂O fluxes were negligible during the dry season but increased markedly after the start of the wet season. By contrast, tree stem bases emitted CH₄ and N₂O throughout the study. Although we observed no clear effect of litter manipulation on trace GHG fluxes, tree species and litter treatments interacted to influence CH₄ fluxes from stems and N₂O fluxes from stems and soil, indicating complex relationships between tree species traits and decomposition processes that can influence trace GHG dynamics. Collectively, our results show that tropical trees can act as conduits for trace GHGs that most likely originate from deeper soil horizons, even when they are growing on upland soils. Coupled with the finding that the soils may be a weaker sink for CH₄ than previously thought, our research highlights the need to reappraise trace gas budgets in tropical forests.     

Land use and season affect fluxes of CO2, CH4, CO,N2O,H2 and isotopic source signatures in Panama: evidence from nocturnal boundary layer profiles

Conversion of tropical rainforests to pastures and plantations is associated with changes in soil properties and biogeochemical cycling, with implications for carbon cycling and trace gas fluxes. The stable isotopic composition of ecosystem respiration (δ¹³C_R and δ¹⁸O_R) is used in inversion models to quantify regional patterns of CO₂ sources and sinks, but models are limited by sparse measurements in tropical regions. We measured soil respiration rates, concentrations of CO₂, CH₄, CO, N₂O and H₂ and the isotopic composition of CO₂, CH₄ and H₂ at four heights in the nocturnal boundary layer (NBL) above three common land‐use types in central Panama, during dry and rainy seasons. Soil respiration rates were lowest in Plantation (average 3.4 μmol m^?2 s^?1), highest in Pasture (8.3 μmol m^?2 s^?1) and intermediate in Rainforest (5.2 μmol m^?2 s^?1). δ¹³C_R closely reflected land use and increased during the dry season where C₃ vegetation was present. δ¹⁸O_R did not differ by land use but was lower during the rainy than the dry season. CO₂ was correlated with other species in approximately half of the NBL profiles, allowing us to estimate trace gas fluxes that were generally within the range of literature values. The Rainforest soil was a sink for CH₄ but emissions were observed in Pasture and Plantation, especially during the wet season. N₂O emissions were higher in Pasture and Plantation than Rainforest, contrary to expectations. Soil H₂ uptake was highest in Rainforest and was not observable in Pasture and Plantation during the wet season. We observed soil CO uptake during the dry season and emissions during the wet season across land‐use types. This study demonstrated that strong impacts of land‐use change on soil–atmosphere trace gas exchange can be detected in the NBL, and provides useful observational constraints for top‐down and bottom‐up biogeochemistry models.     

Impacts of forest management type and season on soil carbon fluxes in Eastern Mau Forest,Kenya

The value of ecosystems functions performed by forests in the climate change era has prompted increasing attention towards assessment of carbon stocks and fluxes in tropical forests. The aim of this study was to understand how forest management approaches and environmental controls impacted on soil CO₂ efflux in a tropical Eastern Mau forest which is one of the blocks of the greater Mau complex in Kenya. Nested experimental design approach was employed where 32 plots were nested into four blocks (disturbed natural, undisturbed natural, plantation and glades). In 10 m² plots, data were collected on soil CO₂ efflux, soil temperature and soil moisture using soda lime methods, direct measurement and proxy techniques, respectively. There was significant forest management type effect (F_3,127 = 3.01, p = 0.033) and seasonality effect (t test = 3.31, df = 1, p < 0.05) on mean soil CO₂ efflux. The recorded mean soil CO₂ efflux levels were as follows: plantation forest (9.219 ± 3.067 g C M^?2 day^?1), undisturbed natural forest (8.665 ± 4.818 g C M^?2 day^?1), glades (8.592 ± 3.253 g C M^?2 day^?1) and disturbed natural forest (7.198 ± 3.457 g C M^?2 day^?1). The study concludes that managing a forest in plantation form is primarily responsible for forest soil CO₂ efflux levels due to aspects such as increased microbial activity and root respiration. However, further studies are required to understand the role and impact of soil CO₂ efflux on the greater forest carbon budget.     

土壤湿度对中国南部热带森林土壤甲烷吸收的影响

森林土壤是一个重要的大气甲烷的汇。然而,相较于寒带和温带,在热带尤其是东南亚地区,森林土壤甲烷通量的观测较少,这限制了目前对热带森林土壤甲烷通量与环境因子之间关系的认识,也给热带森林土壤甲烷汇的估算带来了一定的不确定性。在中国海南省吊罗山国家森林公园的热带森林土壤,采用激光光谱法测量了2016年9月至2018年9月逐月的土壤甲烷通量,并分析了其与周围环境因子的关系。结果表明：研究区土壤是甲烷的汇,山顶样地的年平均吸收量为0.95 kg CH₄-C hm^-2 a^-1,山脚样地的年平均吸收量为1.93 kg CH₄-C hm^-2 a^-1。干季(11月—次年4月)的甲烷吸收通量明显高于湿季(5—10月),占到全年甲烷吸收的68%。山顶样地年平均土壤湿度为19.2%,年内的波动较小(2.8%)。而山脚样地的年平均湿度相对较低,为12.7%,且年内波动大(5.4%)。土壤湿度是控制甲烷吸收最主要的环境因子,可以解释月际甲烷吸收变化的76%,甲烷吸收通量与土壤温度的相...     

Substrate, climate, and land use controls over soil N dynamics and N-oxide emissions in Borneo

Nitrogen (N) enrichment of tropical ecosystems is likely to increase with rapid industrial and agricultural development, but the ecological consequences of N additions in these systems are not well understood. We measured soil N- oxide emissions and N transformations in primary rain forest ecosystems at four elevations and across two substrate types on Mt. Kinabalu, Borneo, before and after short-term experimental N additions. We also measured N pools and fluxes across a land use gradient of primary forest, burned secondary forest, and fertilized agriculture. Background soil N₂O and NO emissions in primary forest decreased with elevation, and soils derived from sedimentary substrates had larger pools of inorganic N, rates of nitrification, and N-oxide fluxes than ultrabasic soils when there were significant differences between substrate types. N-oxide emissions after N additions and background rates of nitrification were low in all soils derived from ultrabasic substrates compared to sedimentary substrates, even at lowland sites supporting, diverse Dipterocarp forests growing on morphologically similar Oxisols. Rates of potential nitrification were good predictors of N-oxide emissions after N additions. N₂O and NO fluxes were largest at low elevations and on sedimentary-derived soils compared to ultrabasic-derived soils, even at the smallest addition of N, 15kgNha^–1. Because current methods of soil classification do not explicitly characterize a number of soil chemical properties important to nutrient cycling, the use of soil maps to extrapolate biogeochemical processes to the region or globe may be limited in its accuracy and usefulness. In agricultural systems, management practices were more important than substrate type in controlling N-oxide emissions and soil N cycling. N-oxide fluxes from agricultural fields were more than an order of magnitude greater than from primary forests on the same substrate type and at the same elevation. As primary forests are cleared for intensive agriculture, soil N₂O and NO emissions are likely to far exceed those from the most N-saturated tropical forest ecosystems. This study highlights the inter-dependence of climate, substrate age, N deposition, and land-use practices determining N cycling and N-oxide emissions in humid tropical regions.     

Effects of nitrogen additions on above- and belowground carbon dynamics in two tropical forests

Anthropogenic nitrogen (N) deposition is increasing rapidly in tropical regions, adding N to ecosystems that often have high background N availability. Tropical forests play an important role in the global carbon (C) cycle, yet the effects of N deposition on C cycling in these ecosystems are poorly understood. We used a field N-fertilization experiment in lower and upper elevation tropical rain forests in Puerto Rico to explore the responses of above- and belowground C pools to N addition. As expected, tree stem growth and litterfall productivity did not respond to N fertilization in either of these N-rich forests, indicating a lack of N limitation to net primary productivity (NPP). In contrast, soil C concentrations increased significantly with N fertilization in both forests, leading to larger C stocks in fertilized plots. However, different soil C pools responded to N fertilization differently. Labile (low density) soil C fractions and live fine roots declined with fertilization, while mineral-associated soil C increased in both forests. Decreased soil CO₂ fluxes in fertilized plots were correlated with smaller labile soil C pools in the lower elevation forest (R² = 0.65, p < 0.05), and with lower live fine root biomass in the upper elevation forest (R² = 0.90, p < 0.05). Our results indicate that soil C storage is sensitive to N deposition in tropical forests, even where plant productivity is not N-limited. The mineral-associated soil C pool has the potential to respond relatively quickly to N additions, and can drive increases in bulk soil C stocks in tropical forests.     

Microbial and Environmental Controls of Methane Fluxes Along a Soil Moisture Gradient in a Pacific Coastal Temperate Rainforest

Most studies of greenhouse gas fluxes from forest soils in the coastal rainforest have considered carbon dioxide (CO₂), whereas methane (CH₄) has not received the same attention. Soil hydrology is a key driver of CH₄ dynamics in ecosystems, but the impact on the function and distribution of the underlying microbial communities involved in CH₄ cycling and the resultant net CH₄ exchange is not well understood at this scale. We studied the growing season variations of in situ CH₄ fluxes, microbial gene abundances of methanotrophs (CH₄ oxidizers) and methanogens (CH₄ producers), soil hydrology, and nutrient availability in three typical forest types across a soil moisture gradient. CH₄ displayed a spatial variability changing from a net uptake in the upland soils (3.9–46 µmol CH₄ m^?2 h^?1) to a net emission in the wetter soils (0–90 μmol CH₄ m^?2 h^?1). Seasonal variations of CH₄ fluxes were related to soil hydrology in both upland and wet soils. Thus, in the upland soils, uptake rates increased with the decreasing soil moisture, whereas CH₄ emission was inversely related to the water table depth in the wet soils. Spatial variability of CH₄ exchange was related to the abundance of genes involved in CH₄ oxidation and production, but there was no indication of a temporal link between microbial groups and CH₄ exchange. Our data show that the abundances of genes involved in CH₄ oxidation and production are strongly influenced by soil moisture and each other and grouped by the upland–wetland classification but not forest type.     

Effects of forest-pasture edge on C,N and P associated with <Emphasis Type="Italic">Caesalpinia eriostachys</Emphasis>, a dominant tree species in a tropical deciduous forest in Mexico

Forest fragmentation in tropical ecosystems can alter nutrient cycling in diverse ways. We have analysed the effects of the forest-pasture edge on nutrient soil dynamics in a tropical deciduous forest (TDF) in Mexico. In two remnant forest fragments, both larger than 10 ha, litterfall, litter and soil samples associated to the tree Caesalpinia eriostachys were collected at five distances from the pasture edge into the inner forest (10 m in the pasture and 0–10, 30–40, 70–80 and 100–110 m towards the forest interior). We measured the concentrations of carbon (C), nitrogen (N) and phosphorus (P) in litterfall, surface litter and soil, and soil microbial C (C_mic) and microbial N (N_mic). Soil nutrient concentrations and C_mic and N_mic were lower in the pasture soils than in the forest soil samples. Total C and N pools, and C_mic and N_mic in the pasture were lower than in the forest. In contrast, net N immobilization and the increase in N_mic from rain to dry season increased from the edge to the inner forest. Soil P concentration was lower in the pasture and at the first distance class in the forest margin (0–10 m) than in the sites located further into the forest, while litter P concentration had the inverse pattern. Litterfall P was also reduced near the edge and increased towards the forest interior. As a consequence, litterfall C:P and N:P ratios decreased from the edge to the inner forest. These results suggest that the forest–pasture edge disrupts P dynamics within the first 10 m in the forest. Thus, plants' use of nutrients and productivity could be altered in the edge of fragmented forests.     

Spatial Variation of Soil CO<Subscript>2</Subscript>, CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O Fluxes Across Topographical Positions in Tropical Forests of the Guiana Shield

The spatial variation of soil greenhouse gas fluxes (GHG; carbon dioxide—CO₂, methane—CH₄ and nitrous oxide—N₂O) remains poorly understood in highly complex ecosystems such as tropical forests. We used 240 individual flux measurements of these three GHGs from different soil types, at three topographical positions and in two extreme hydric conditions in the tropical forests of the Guiana Shield (French Guiana, South America) to (1) test the effect of topographical positions on GHG fluxes and (2) identify the soil characteristics driving flux variation in these nutrient-poor tropical soils. Surprisingly, none of the three GHG flux rates differed with topographical position. CO₂ effluxes covaried with soil pH, soil water content (SWC), available nitrogen and total phosphorus. The CH₄ fluxes were best explained by variation in SWC, with soils acting as a sink under drier conditions and as a source under wetter conditions. Unexpectedly, our study areas were generally sinks for N₂O and N₂O fluxes were partly explained by total phosphorus and available nitrogen concentrations. This first study describing the spatial variation of soil fluxes of the three main GHGs measured simultaneously in forests of the Guiana Shield lays the foundation for specific studies of the processes underlying the observed patterns.     

Controls on the Carbon Balance of Tropical Peatlands

The carbon balance of tropical peatlands was investigated using measurements of gaseous fluxes of carbon dioxide (CO₂) and methane (CH₄) at several land-use types, including nondrained forest (NDF), drained forest (DF), drained regenerating forest (DRF) after clear cutting and agricultural land (AL) in Central Kalimantan, Indonesia. Soil greenhouse gas fluxes depended on land-use, water level (WL), microtopography, temperature and vegetation physiology, among which WL was the strongest driver. All sites were CH₄ sources on an annual basis and the emissions were higher in sites providing fresh litter deposition and water logged conditions. Soil CO₂ flux increased exponentially with soil temperature (T _s) even within an amplitude of 4–5°C. In the NDF soil CO₂ flux sharply decreased when WLs rose above −0.2 and 0.1 m for hollows and hummocks, respectively. The sharp decrease suggests that the contribution of surface soil respiration (RS) to total soil CO₂ flux is large. In the DF soil CO₂ flux increased as WL decreased below −0.7 m probably because the fast aerobic decomposition continued in lower peat. Such an increase in CO₂ flux at low WLs was also found at the stand level of the DF. Soil CO₂ flux showed diurnal variation with a peak in the daytime, which would be caused by the circadian rhythm of root respiration. Among the land-use types, annual soil CO₂ flux was the largest in the DRF and the smallest in the AL. Overall, the global warming potential (GWP) of CO₂ emissions in these land-use types was much larger than that of CH₄ fluxes.     

桂北喀斯特峰丛洼地植物群落特征及其与土壤的耦合关系

基于喀斯特峰丛洼地草丛、灌丛、次生林、原生林4个生态系统24个样地(20 m × 20 m)的系统取样调查, 研究了喀斯特峰丛洼地不同生态系统群落的结构组成与生物多样性特征, 选取代表植物群落和土壤性质的35个指标, 对不同生态系统及整个喀斯特脆弱生态系统植物群落与土壤主要养分、土壤矿质养分和土壤微生物间的相互关系进行了主成分分析与典范相关分析。结果表明: 沿草丛、灌丛、次生林、原生林的顺向演替发展, 重要值(importance value, IV)>10.00的科、属、种及物种多样性最大值出现在次生林, 群落结构最佳值出现在顶级群落原生林; 喀斯特峰丛洼地景观异质性高, 各生态系统影响因子不同, 土壤微生物在喀斯特脆弱生态系统处于主导地位, 其次为灌丛; 不同集团因子的典范相关分析表明, 植物多样性指标与土壤氮素、Al₂O₃、Fe₂O₃、土壤微生物生物量碳(C_mic)、真菌和细菌关系密切。因此, 在喀斯特脆弱生态系统恢复与重建过程中, 应针对不同生态系统制定相应的培育管理措施。     

Soil oxygen availability and biogeochemistry along rainfall and topographic gradients in upland wet tropical forest soils

We measured soil oxygen concentrations at 10 and 35 cm depths and indices of biogeochemical cycling in upland forest soils along a rainfall and elevation gradient (3500–5000 mm y^–1; 350–1050 masl) and along topographic gradients (ridge to valley, 150 m) in the Luquillo Experimental Forest, Puerto Rico. Along the rainfall gradient, soil O₂ availability decreased significantly with increasing annual rainfall, and reached very low levels (<3%) in individual chambers for up to 25 consecutive weeks over 82 weeks of study. Along localized topographic gradients, soil O₂ concentrations were variable and decreased significantly from ridges to valleys. In the valleys, up to 35% of the observations at 10–35 cm depth were <3% soil O₂. Cross correlation analyses showed that soil O₂ concentrations were significantly positively correlated along the topographic gradient, and were sensitive to rainfall and hydrologic output. Soil O₂ concentrations in valley soils were correlated with rainfall from the previous day, while ridge sites were correlated with cumulative rainfall inputs over 4 weeks. Soils at the wettest point along the rainfall gradient had very high soil methane concentrations (3–24%) indicating a strong influence of anaerobic processes. We measured net methane emission to the atmosphere at the wettest sites of the rainfall gradient, and in the valleys along topographic gradients. Other measures of biogeochemical function such as soil organic matter content and P availability were sensitive to chronic O₂ depletion along the rainfall gradient, but less sensitive to the variable soil O₂ environment exhibited at lower elevations along topographic gradients.