Redox fluctuation structures microbial communities in a wet tropical soil

Frequent high-amplitude redox fluctuation may be a strong selective force on the phylogenetic and physiological composition of soil bacterial communities and may promote metabolic plasticity or redox tolerance mechanisms. To determine effects of fluctuating oxygen regimens, we incubated tropical soils under four treatments: aerobic, anaerobic, 12-h oxic/anoxic fluctuation, and 4-day oxic/anoxic fluctuation. Changes in soil bacterial community structure and diversity were monitored with terminal restriction fragment length polymorphism (T-RFLP) fingerprints. These profiles were correlated with gross N cycling rates, and a Web-based phylogenetic assignment tool was used to infer putative community composition from multiple fragment patterns. T-RFLP ordinations indicated that bacterial communities from 4-day oxic/anoxic incubations were most similar to field communities, whereas those incubated under consistently aerobic or anaerobic regimens developed distinctly different molecular profiles. Terminal fragments found in field soils persisted either in 4-day fluctuation/aerobic conditions or in anaerobic/12-h treatments but rarely in both. Only 3 of 179 total fragments were ubiquitous in all soils. Soil bacterial communities inferred from in silico phylogenetic assignment appeared to be dominated by Actinobacteria (especially Micrococcus and Streptomycetes), "Bacilli," "Clostridia," and Burkholderia and lost significant diversity under consistently or frequently anoxic incubations. Community patterns correlated well with redox-sensitive processes such as nitrification, dissimilatory nitrate reduction to ammonium (DNRA), and denitrification but did not predict patterns of more general functions such as N mineralization and consumption. The results suggest that this soil's indigenous bacteria are highly adapted to fluctuating redox regimens and generally possess physiological tolerance mechanisms which allow them to withstand unfavorable redox periods.     

Experimental litterfall manipulation drives large and rapid changes in soil carbon cycling in a wet tropical forest

Global changes such as variations in plant net primary production are likely to drive shifts in leaf litterfall inputs to forest soils, but the effects of such changes on soil carbon (C) cycling and storage remain largely unknown, especially in C‐rich tropical forest ecosystems. We initiated a leaf litterfall manipulation experiment in a tropical rain forest in Costa Rica to test the sensitivity of surface soil C pools and fluxes to different litter inputs. After only 2 years of treatment, doubling litterfall inputs increased surface soil C concentrations by 31%, removing litter from the forest floor drove a 26% reduction over the same time period, and these changes in soil C concentrations were associated with variations in dissolved organic matter fluxes, fine root biomass, microbial biomass, soil moisture, and nutrient fluxes. However, the litter manipulations had only small effects on soil organic C (SOC) chemistry, suggesting that changes in C cycling, nutrient cycling, and microbial processes in response to litter manipulation reflect shifts in the quantity rather than quality of SOC. The manipulation also affected soil CO ₂ fluxes; the relative decline in CO ₂ production was greater in the litter removal plots (?22%) than the increase in the litter addition plots (+15%). Our analysis showed that variations in CO ₂ fluxes were strongly correlated with microbial biomass pools, soil C and nitrogen (N) pools, soil inorganic P fluxes, dissolved organic C fluxes, and fine root biomass. Together, our data suggest that shifts in leaf litter inputs in response to localized human disturbances and global environmental change could have rapid and important consequences for belowground C storage and fluxes in tropical rain forests, and highlight differences between tropical and temperate ecosystems, where belowground C cycling responses to changes in litterfall are generally slower and more subtle.     

Multiple nutrients limit litterfall and decomposition in a tropical forest

To explore the importance of 12 elements in litter production and decomposition, we fertilized 36 1600 m²-plots with combinations of N, P, K, or micronutrients (i.e. B, Ca, Cu, Fe, Mg, Mn, Mo, S, Zn) for 6 years in a lowland Panamanian forest. The 90% of litter falling as leaves and twigs failed to increase with fertilization, but reproductive litter (fruits and flowers) increased by 43% with N. K enhanced cellulose decomposition; one or more micronutrients enhanced leaf-litter decomposition; P enhanced both. Our results suggest tropical forests are a non-Liebig world of multiple nutrient limitations, with at least four elements shaping rates of litterfall and decomposition. Multiple metallomic enzymes and cofactors likely create gradients in the break down of leaf litter. Selection favours individuals that make more propagules, and even in an N-rich forest, N is a non-substitutable resource for reproduction.     

Asynchronous responses of soil microbial community and understory plant community to simulated nitrogen deposition in a subtropical forest

Atmospheric nitrogen (N) deposition greatly affects ecosystem processes and properties. However, few studies have simultaneously examined the responses of both the above- and belowground communities to N deposition. Here, we investigated the effects of 8 years of simulated N deposition on soil microbial communities and plant diversity in a subtropical forest. The quantities of experimental N added (g of N m⁻² year⁻¹) and treatment codes were 0 (N0, control), 6 (N1), 12 (N2), and 24 (N3). Phospholipid fatty acids (PLFAs) analysis was used to characterize the soil microbial community while plant diversity and coverage were determined in the permanent field plots. Microbial abundance was reduced by the N3 treatment, and plant species richness and coverage were reduced by both N2 and N3 treatments. Declines in plant species richness were associated with decreased abundance of arbuscular mycorrhizal fungi, increased bacterial stress index, and reduced soil pH. The plasticity of soil microbial community would be more related to the different responses among treatments when compared with plant community. These results indicate that long-term N deposition has greater effects on the understory plant community than on the soil microbial community and different conservation strategies should be considered.     

蚂蚁筑巢对西双版纳热带森林土壤微生物生物量碳及熵的影响

蚂蚁筑巢能够改变热带森林土壤理化环境,从而对土壤微生物生物量碳及熵的时空动态产生重要影响.本研究以西双版纳高檐蒲桃热带森林群落为对象,采用氯仿熏蒸法对蚂蚁巢地和非巢地土壤微生物生物量碳及熵时空动态进行测定.结果表明: 1)蚁巢地平均微生物生物量碳及熵(1.95 g·kg^-1,6.8%)显著高于非巢穴(1.76 g·kg^-1,5.1%);蚁巢地和非蚁巢地土壤微生物生物量碳呈单峰型时间变化趋势,而土壤微生物熵呈“V”型变化格局.2)蚁巢地和非巢地土壤微生物生物量碳及熵均具有明显的垂直变化:微生物生物量碳随土层加深显著降低,微生物熵则沿土层加深显著升高,但蚁巢微生物生物量碳及熵的垂直变化较非巢穴显著. 3)蚂蚁筑巢引起了巢内水分和温度的显著改变,进而影响土壤微生物生物量碳及熵的时空动态.土壤水分分别解释微生物生物量碳及熵的66%～83%和54%～69%,而土壤温度分别解释土壤微生物生物量碳及熵的71%～86%和67%～76%. 4)蚂蚁筑巢引起土壤理化性质变化对土壤微生物生物量碳和熵产生重要影响.蚁巢土壤微生物生物量碳与土壤有机碳、温度、全氮、含水率呈极显著正相关,与容重、硝态氮,水解氮呈显著正相关,与土壤pH呈极显著负相关;除土壤微生物熵与pH呈显著正相关外,与其他土壤理化指标均呈显著负相关.土壤总有机碳、全氮和温度对微生物生物量碳的贡献最大,而土壤总有机碳和全氮对微生物熵的负作用最小.因此,蚂蚁筑巢能够显著改变微生境(如土壤水分与温度)及土壤理化性质(如总有机碳及全氮),进而调控热带森林土壤微生物生物量碳及熵的时空动态.     

北亚热带次生栎林与火炬松人工林土壤微生物生物量碳的季节动态

在南京林业大学下蜀森林生态定位研究站次生栎林和火炬松人工林内,采用随机区组设计,连续两年测定了林下土壤微生物生物量碳、土壤温度、土壤湿度和林地凋落物输入量.结果表明:两种林分的土壤微生物生物量碳呈明显的季节性波动,均在植物生长旺季维持在较低水平,而在植物休眠季节维持在较高水平;在0～10 cm土层内,火炬松林和次生栎林土壤微生物生物量碳变幅分别在267.8～459.8 mg·kg-1和278.6～467.8 mg·kg-1;土壤微生物生物量碳与土壤温度之间具有显著的负相关关系,而与森林地上凋落物输入量、土壤湿度无显著相关关系.说明该区域土壤微生物生物量碳的季节波动可能与土壤中有效碳及其它土壤养分的可利用状况、植物根系对营养的竞争以及林木的生长节律有关.     

Substantial labile carbon stocks and microbial activity in deeply weathered soils below a tropical wet forest

Contrary to large areas in Amazonia of tropical moist forests with a pronounced dry season, tropical wet forests in Costa Rica do not depend on deep roots to maintain an evergreen forest canopy through the year. At our Costa Rican tropical wet forest sites, we found a large carbon stock in the subsoil of deeply weathered Oxisols, even though only 0.04–0.2% of the measured root biomass (>2 mm diameter) to 3 m depth was below 2 m. In addition, we demonstrate that 20% or more of this deep soil carbon (depending on soil type) can be mobilized after forest clearing for pasture establishment. Microbial activity between 0.3 and 3 m depth contributed about 50% to the microbial activity in these soils, confirming the importance of the subsoil in C cycling. Depending on soil type, forest clearing for pasture establishment led from no change to a slight addition of carbon in the topsoil (0–0.3 m depth). However, this effect was countered by a substantial loss of C stocks in the subsoil (1–3 m depth). Our results show that large stocks of relatively labile carbon are not limited to areas with a prolonged dry season, but can also be found in deeply weathered soils below tropical wet forests. Forest clearing in such areas may produce unexpectedly high C losses from the subsoil.     

Organic matter inputs shift soil enzyme activity and allocation patterns in a wet tropical forest

Soil extracellular enzymes mediate organic matter turnover and nutrient cycling yet remain little studied in one of Earth’s most rapidly changing, productive biomes: tropical forests. Using a long-term leaf litter and throughfall manipulation, we explored relationships between organic matter (OM) inputs, soil chemical properties and enzyme activities in a lowland tropical forest. We assayed six hydrolytic soil enzymes responsible for liberating carbon (C), nitrogen (N) and phosphorus (P), calculated enzyme activities and ratios in control plots versus treatments, and related these to soil biogeochemical variables. While leaf litter addition and removal tended to increase and decrease enzyme activities per gram soil, respectively, shifts in enzyme allocation patterns implied changes in relative nutrient constraints with altered OM inputs. Enzyme activity ratios in control plots suggested strong belowground P constraints; this was exacerbated when litter inputs were curtailed. Conversely, with double litter inputs, increased enzymatic investment in N acquisition indicated elevated N demand. Across all treatments, total soil C correlated more strongly with enzyme activities than soluble C fluxes, and enzyme ratios were sensitive to resource stoichiometry (soil C:N) and N availability (net N mineralization). Despite high annual precipitation in this site (MAP ~5 m), soil moisture positively correlated with five of six enzymes. Our results suggest resource availability regulates tropical soil enzyme activities, soil moisture plays an additional role even in very wet forests, and relative investment in C, N and P degrading enzymes in tropical soils will often be distinct from higher latitude ecosystems yet is sensitive to OM inputs.     

亚热带森林土壤微生物生物量及群落功能特征的城乡梯度变化

土壤微生物在土壤养分循环以及生物地球化学循环中起着重要作用,土壤微生物对环境变化响应灵敏.为研究城乡梯度环境变化对亚热带森林土壤微生物的影响,本研究选取合肥市大蜀山国家森林公园(城市林)、紫蓬山国家森林公园(远郊林)、六安市万佛山(乡村自然林)为样地,分析比较其微生物生物量碳(MBC)、微生物生物量氮(MBN)及微生物...     

Soil respiration and microbial population in a tropical deciduous forest soil of Orissa, India

An investigation was carried out to estimate soil respiration rate and its relationship with microbial population in natural tropical forest soil, deforested soil and deforested-and-cultivated soil of Orissa, India. Soil respiration measurements and microbial isolation were performed following standard procedures. Monthly variation of soil respiration was observed to be governed by soil moisture. Considering respiration as a function of microbial population a regression analysis was made. The microfungal population showed positive relationship with the rate of soil respiration. The study revealed that conversion of natural forest led to a reduction of soil microbes and rate of soil respiration. Considering the importance of the microbial component in soil, we conclude that the conversion of natural forests to different land uses leads to the loss of biological stability of the soil.     

寒温带兴安落叶松林凋落物层对土壤呼吸的影响

为了进一步探讨土壤凋落物层对土壤呼吸的影响,用Li-6400对大兴安岭北部3种林型(白桦-落叶松林、樟子松-落叶松林和落叶松纯林)自然状态的土壤呼吸(R_S)、去凋落物后的土壤呼吸(R_D)以及凋落物呼吸(R_L)进行测定,结果表明:凋落物层的去除会使土壤呼吸速率降低,3种林型观测期内平均R_S分别为7.32μmol m^-2 s^-1、8.55μmol m^-2 s^-1和6.66μmol m^-2 s^-1,平均R_D分别为6.46μmol m^-2 s^-1、7.98μmol m^-2 s^-1和5.74μmol m^-2 s^-1。但去除凋落物后的土壤总呼吸速率较自然状态下分别升高了13.85%、16.21%和13.73%;凋落物的去除并不影响...     

Carbon and nutrient limitation of soil microorganisms and microbial grazers in a tropical montane rain forest

We investigated the role of carbon, nitrogen and phosphorus as limiting factors of microorganisms and microbial grazers (testate amoebae) in a montane tropical rain forest in southern Ecuador. Carbon (as glucose), nitrogen (as NH₄NO₃) and phosphorus (as NaH₂PO₄) were added separately and in combination bimonthly to experimental plots for 20 months. By adding glucose and nutrients we expected to increase the growth of microorganisms as the major food resource of testate amoebae. The response of microorganisms to experimental treatments was determined by analysing microbial biomass (SIR), fungal biomass and microbial community composition as measured by phospholipid fatty acids (PLFAs). We hypothesized that the response of testate amoebae is closely linked to that of microorganisms. Carbon addition strongly increased ergosterol concentration and, less pronounced, the amount of linoleic acid as fungal biomarker, suggesting that saprotrophic fungi are limited by carbon. Microbial biomass and ergosterol concentrations reached a maximum in the combined treatment with C, N and P indicating that both N and P also were in short supply. In contrast to saprotrophic fungi and microorganisms in total, testate amoebae suffered from the addition of C and reached maximum density by the addition of N. The results indicate that saprotrophic fungi in tropical montane rain forests are mainly limited by carbon whereas gram positive and negative bacteria benefit from increased availability of P. Testate amoebae suffered from increased dominance of saprotrophic fungi in glucose treatments but benefited from increased supply of N. The results show that testate amoebae of tropical montane rain forests are controlled by bottom–up forces relying on specific food resources rather than the amount of bacterial biomass with saprotrophic fungi functioning as major antagonists. Compared to temperate systems microbial food webs in tropical forests therefore may be much more complex than previously assumed with trophic links being rather specific and antagonistic interactions overriding trophic interactions.     

Cultivation impacts soil microbial dynamics in dry tropical forest ecosystem in India

We studied for two years the seasonal changes in plant available nitrate and ammonium nitrogen (N), nitrification, N-mineralization, microbial biomass carbon (MBC), nitrogen (MBN) and phosphorus (MBP) in two forest and three cropland sites, derived from a tropical forest ecosystem of India. Results indicated that seasonal values of nitrate N, ammonium N and phosphate P ranged from 7.33–12.99, 5.1–10.22 and 4.0–7.8 μg g^?1 in forest and 4.13–9.26, 9.35–14.46 and 2.8–5.8 μg g^?1 in cropland ecosystems, respectively, with maximum values in summer and minimum in rainy seasons. Nitrification and N-mineralization values varied from 6–28 and 4–26 μg g^?1 mo^?1 in forest and 3–14 μg g^?1 mo^?1 and 4–17 μg g^?1 mo^?1 in cropland, with maximum values in rainy season and minimum in summer season.MBC, MBN MBP ranged from 393–753, 34–80 and 16–36 μg g^?1 in forests and 186–414, 21–41 and 11–22 μg g^?1 in croplands, being maximum in summer and minimum in rainy seasons. There was gradual increase in the values of inorganic N, nitrification, N-mineralization and MBC, MBN and MBP along the age of cropland. Analysis of variance indicated significant difference in the concentration of inorganic N, nitrification and N-mineralization and MBC, MBN and MBP due to sites and seasons.Cultivation caused decline in the mean annual organic C, N and P by 42%, 29% and 13%. The values of nitrate N were decreased by 23–38%, while ammonium N was increased by 39–74%. Nitrification and N-mineralization values were reduced by 39–63% and 40–60%, respectively. Microbial C, N and P were reduced by 44–54%, 41–50% and 28–44%, respectively. Nonetheless, the contribution of soil microbial biomass reflected in total N was enhanced from 4.76% in forest to 5.03% in cropland ecosystem. Enhancement of plant available ammonium-N and microbial contribution in total N are an indicator of natural conserving mechanism to check the nitrogen loss from the nutrient poor agro-ecosystem.     

Nutrient fluxes via litterfall and leaf litter decomposition vary across a gradient of soil nutrient supply in a lowland tropical rain forest

The extent to which plant communities are determined by resource availability is a central theme in ecosystem science, but patterns of small-scale variation in resource availability are poorly known. Studies of carbon (C) and nutrient cycling provide insights into factors limiting tree growth and forest productivity. To investigate rates of tropical forest litter production and decomposition in relation to nutrient availability and topography in the absence of confounding large-scale variation in climate and altitude we quantified nutrient fluxes via litterfall and leaf litter decomposition within three distinct floristic associations of tropical rain forest growing along a soil fertility gradient at the Sepilok Forest Reserve (SFR), Sabah, Malaysia. The quantity and nutrient content of small litter decreased along a gradient of soil nutrient availability from alluvial forest (most fertile) through sandstone forest to heath forest (least fertile). Temporal variation in litterfall was greatest in the sandstone forest, where the amount of litter was correlated negatively with rainfall in the previous month. Mass loss and N and P release were fastest from alluvial forest litter, and slowest from heath forest litter. All litter types decomposed most rapidly in the alluvial forest. Stand-level N and P use efficiencies (ratios of litter dry mass to nutrient content) were greatest for the heath forest followed by the sandstone ridge, sandstone valley and alluvial forests, respectively. We conclude that nutrient supply limits productivity most in the heath forest and least in the alluvial forest. Nutrient supply limited productivity in sandstone forest, especially on ridge and hill top sites where nutrient limitation may be exacerbated by reduced rates of litter decomposition during dry periods. The fluxes of N and P varied significantly between the different floristic communities at SFR and these differences may contribute to small-scale variation in species composition.     

Root biomass distribution in a moist tropical montane forest

Effects of N sources (ammonium, nitrate and ammonitrate) and VA mycorrhizae (Glomus intraradices) on rhizosphere soil characteristics (pH, exchangeable acidity, exchangeable cations, inorganic N concentrations) growth and nutrient acquisition of coffee seedlings (Coffea arabica L. cv guatemala) were investigated in a pot study with an acid soil (Red Bluff Loam) sterilized by autoclaving. Ammonium addition decreased rhizosphere pH while nitrate and ammonitrate additions both increased rhizosphere pH. Mycorrhizae induced a higher pH, a lower exchangeable acidity and higher values of exchangeable cations in the rhizosphere. Ammonium addition resulted in a lower mycorrhizal infection than the two other N sources. Mycorrhizal plants grew better and accumulated more N, Ca and Mg than non-mycorrhizal plants.     

Dissolved organic carbon affects soil microbial activity and nitrogen dynamics in a Mexican tropical deciduous forest

Seasonal variation of dissolved organic C (DOC) and its effects on microbial activity and N dynamics were studied during two consecutive years in soils with different organic C concentrations (hilltop and hillslope) in a tropical deciduous forest of Mexico. We found that DOC concentrations were higher at the hilltop than at the hillslope soils, and in both soils generally decreased from the dry to the rainy season during the two study years. Microbial biomass and potential C mineralization rates, as well as dissolved organic N (DON) and NH₄⁺ concentrations and net N immobilization were higher in soils with higher DOC than in soils with lower DOC. In contrast, net N immobilization and NH₄⁺ concentration were depleted in the soil with lowest DOC, whereas NO₃⁻ concentrations and net nitrification increased. Negative correlations between net nitrification and DOC concentration suggested that NH₄⁺ was transformed to NO₃⁻ by nitrifiers when the C availability was depleted. Taken together, our results suggest that available C appears to control soil microbial activity and N dynamics, and that microbial N immobilization is facilitated by active heterotrophic microorganisms stimulated by high C availability. Soil autotrophic nitrification is magnified by decreases in C availability for heterotrophic microbial activity. This study provides an experimental data set that supports the conceptual model to show and highlight that microbial dynamics and N transformations could be functionally coupled with DOC availability in the tropical deciduous forest soils. Responsible Editor: Chris Neill     

Hurricane-induced nitrous oxide fluxes from a wet tropical forest

Hurricane activity is predicted to increase over the mid-Atlantic as global temperatures rise. Nitrous oxide (N₂O), a greenhouse gas with a substantial source from tropical soils, may increase after hurricanes yet this effect has been insufficiently documented. On September 21, 1998, Hurricane Georges crossed Puerto Rico causing extensive defoliation. We used a before–after design to assess the effect of Georges on N₂O emissions, and factors likely influencing N₂O fluxes including soil inorganic nitrogen pools and soil water content in a humid tropical forest at El Verde, Puerto Rico. Emissions of N₂O up to 7 months post-Georges ranged from 5.92 to 4.26 ng cm⁻² h⁻¹ and averaged five times greater than fluxes previously measured at the site. N₂O emissions 27 months after the hurricane remained over two times greater than previously measured fluxes. Soil ammonium pools decreased after Georges and remained low. The first year after the hurricane, nitrate pools increased, but not significantly when compared against a single measurement made before the hurricane. Soil moisture and temperature did not differ significantly in the two sampling periods. These results suggest that hurricanes increase N₂O fluxes in these forests by altering soil N transformations and the relative availabilities of inorganic nitrogen.