Controls Over Leaf Litter and Soil Nitrogen Fixation in Two Lowland Tropical Rain Forests

Global comparisons suggest that rates of N fixation in tropical rain forests may be among the highest on earth. However, data supporting this contention are rare, and the factors that regulate N fixation within the biome remain largely unknown. We conducted a full-factorial (N × P) fertilization experiment in two lowland tropical rain forests in Costa Rica to explore the effects of nutrient availability on rates of free-living N fixation in leaf litter and soil. P fertilization significantly increased N fixation rates in both leaf litter and soil, and the effect was dependent on sampling date. Fertilization with N did not affect rates of N fixation at any time. In addition, variation in N fixation rates measured in unfertilized plots at four sampling time points suggested seasonal variability in N fixation: leaf litter N fixation ranged from 0.36 kg/ha/yr in the dry season to 5.48 kg/ha/yr in the wet season. Soil N fixation showed similar patterns ranging from a dry season low of 0.26 kg/ha/yr to a wet season high of 2.71 kg/ha/yr. While the observed temporal variability suggests potential climatic control over free-living N fixation in these forests, data suggest that neither soil nor leaf litter moisture alone regulate N fixation rates. Instead, we hypothesize that a combination of ample C availability, low leaf litter N:P ratios, and high rainfall coincide during the latter portions of the rainy season and drive the highest free-living N fixation rates of the year.     

Variable Responses of Lowland Tropical Forest Nutrient Status to Fertilization and Litter Manipulation

Predicting future impacts of anthropogenic change on tropical forests requires a clear understanding of nutrient constraints on productivity. We compared experimental fertilization and litter manipulation treatments in an old-growth lowland tropical forest to distinguish between the effects of inorganic nutrient amendments and changes in nutrient cycling via litterfall. We measured the changes in soil and litter nutrient pools, litterfall, and fine root biomass in plots fertilized with nitrogen (N), phosphorus (P), or potassium (K), and in litter addition and litter removal treatments during 7 years. Soil inorganic N and litter N increased in double-litter plots but not in N-fertilized plots. Conversely, litter P and soil pools of P and K increased in fertilized plots but not in the double-litter plots. Soil and litter pools of N and K decreased in the no-litter plots. Changes in litterfall with added nutrients or litter were only marginally significant, but fine root biomass decreased with both the litter and the K addition. Differences between the two experiments are mostly attributable to the coupled cycling of carbon and nutrients in litter. Increased nutrient inputs in litter may improve plant uptake of some nutrients compared to fertilization with similar amounts. The litter layer also appears to play a key role in nutrient retention. We discuss our findings in the context of possible impacts of anthropogenic change on tropical forests.     

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.     

Ecoenzymatic stoichiometry of microbial nutrient acquisition in tropical soils

The relative activities of soil enzymes involved in mineralizing organic carbon (C), nitrogen (N), and phosphorus (P) reveal stoichiometric and energetic constraints on microbial biomass growth. Although tropical forests and grasslands are a major component of the global C cycle, the effects of soil nutrient availability on microbial activity and C dynamics in these ecosystems are poorly understood. To explore potential microbial nutrient limitation in relation to enzyme allocation in low latitude ecosystems, we performed a meta-analysis of acid/alkaline phosphatase (AP), β-1,4-glucosidase (BG), and β-1,4-N-acetyl-glucosaminidase (NAG) activities in tropical soils. We found that BG:AP and NAG:AP ratios in tropical soils are significantly lower than those of temperate ecosystems overall. The lowest BG:AP and NAG:AP ratios were associated with old or acid soils, consistent with greater biological phosphorus demand relative to P availability. Additionally, correlations between enzyme activities and mean annual temperature and precipitation suggest some climatic regulation of microbial enzyme allocation in tropical soils. We used the results of our analysis in conjunction with previously published data on soil and biomass C:N:P stoichiometry to parameterize a biogeochemical equilibrium model that relates microbial growth efficiency to extracellular enzyme activity. The model predicts low microbial growth efficiencies in P-limited soils, indicating that P availability may influence C cycling in the highly weathered soils that underlie many tropical ecosystems. Therefore, we suggest that P availability be included in models that simulate microbial enzyme allocation, biomass growth, and C mineralization.     

Experimental removal and addition of leaf litter inputs reduces nitrate production and loss in a lowland tropical forest

Environmental perturbations such as changes in land use, climate, and atmospheric carbon dioxide concentrations may alter organic matter inputs to surface soils. While the carbon (C) cycle response to such perturbations has received considerable attention, potential responses of the soil nitrogen (N) cycle to changing organic matter inputs have been less well characterized. Changing litter inputs to surface to soils may alter the soil N cycle directly, by controlling N substrate availability, or indirectly, via interactions with soil C biogeochemistry. We investigated soil N-cycling responses to a leaf litter manipulation in a lowland tropical forest using isotopic and molecular techniques. Both removing and doubling leaf litter inputs decreased the size of the soil nitrate pool, gross nitrification rates, and the relative abundance of ammonia-oxidizing microorganisms. Gross nitrification rates were correlated with the relative abundance of ammonia-oxidizing archaea, and shifts in the N-cycling microbial community composition correlated with concurrent changes in edaphic properties, notably pH and C:N ratios. These results highlight the importance of understanding coupled biogeochemical cycles in global change scenarios and suggest that environmental perturbations that alter organic matter inputs in tropical forests could reduce inorganic N losses to surface waters and the atmosphere by limiting nitrate production.     

Relationships among precipitation regime, nutrient availability, and carbon turnover in tropical rain forests

The effect of high precipitation regime in tropical forests is poorly known despite indications of its potentially negative effects on nutrient availability and carbon (C) cycling. Our goal was to determine if there was an effect of high rainfall on nitrogen (N) and phosphorous (P) availability and indexes of C cycling in lowland tropical rain forests exposed to a broad range of mean annual precipitation (MAP). We predicted that C turnover time would increase with MAP while the availability of N and P would decrease. We studied seven Neotropical lowland forests covering a MAP range between 2,700 and 9,500 mm. We used radiocarbon (?¹⁴C) from the atmosphere and respired from soil organic matter to estimate residence time of C in plants and soils. We also used C, N, and P concentrations and the stable isotope ratio of N (δ¹⁵N) in live and dead plant tissues and in soils as proxies for nutrient availability. Negative δ¹⁵N values indicated that the wettest forests had N cycles that did not exhibit isotope-fractionating losses and were potentially N-limited. Element ratios (N:P and C:P) in senescent leaves, litter, and live roots showed that P resorption increased considerably with MAP, which points towards increasing P-limitation under high MAP regimes. Soil C content increased with MAP but C turnover time only showed a weak relationship with MAP, probably due to variations in soil parent material and age along the MAP gradient. In contrast, comparing C turnover directly to nutrient availability showed strong relationships between C turnover time, N availability (δ¹⁵N), and P availability (N:P) in senescent leaves and litter. Thus, an effect of MAP on carbon cycling appeared to be indirectly mediated by nutrient availability. Our results suggest that soil nutrient availability plays a central role in the dynamic of C cycling in tropical rain forests.     

模拟干旱和磷添加对热带低地雨林叶凋落物分解的影响

凋落物是植物在其生长发育过程中新陈代谢的产物,是土壤有机质输入的重要途径,凋落物分解是生态系统养分循环的关键过程之一。在全球气候变化背景下,热带地区干旱事件发生的频率和强度均在增加,同时,普遍认为热带地区受磷(P)限制,所以探讨干旱胁迫和土壤磷可用性对热带地区叶凋落物分解的影响及两者是否存在交互效应十分必要,有助于了解干旱对该区叶凋落物分解的影响机制以及是否受土壤磷调控。依据植物多度、碳固持类型、叶质地,以海南三亚甘什岭热带低地雨林的4个树种叶凋落物(铁凌 Hopea exalata、白茶树 Koilodepas bainanense、黑叶谷木 Memecylon nigrescens、山油柑 Acronychia pedunculata)为实验材料,依托2019年在该区建成的热带低地雨林模拟穿透雨减少、磷(P)添加双因素交互控制实验平台,包括干旱(D -50%穿透雨)、P添加(P +50Kg P hm^-2a^-1)、模拟干旱×P添加(DP -50%穿透雨×+50Kg P hm^-2a^-1)、对照(CK)4个处理,且4种处理随机分布于3个区组,即设置了3个重复。使用常规的凋落物分解袋法探究实验处理对4个树种叶凋落物的分解系数、碳(C)、氮(N)元素动态变化的影响。结果表明:不同树种的叶凋落物因基质质量不同分解存在差异。模拟干旱处理对叶凋落物C、N损失产生抑制作用,但是对不同树种叶凋落物的抑制作用不同,原因是干旱处理通过抑制土壤分解者活动、减弱凋落物的物理破碎作用,间接抑制凋落物分解,并且由于高质量(含N量高)凋落物受微生物分解者影响较大,所以该凋落物分解受干旱抑制程度较大;P添加处理对叶凋落物C损失存在促进作用、N损失存在抑制作用,原因是土壤中P含量的升高,提高了微生物分解高C物质的能力,以及当土壤中P含量较高时,间接抑制微生物通过分解凋落物获取养分或者促进微生物优先完成自身生长代谢需要而不是合成分解凋落物所需要的酶,导致叶凋落物N损失下降;模拟干旱与P添加处理存在显著交互效应,P添加处理缓解或反转了干旱胁迫对叶凋落物分解的抑制作用。以上结果表明,不同基质质量的凋落物分解存在差异,对干旱胁迫的响应不同;在叶凋落物分解过程中,P添加促进C损失、抑制N损失;此外,在热带低地雨林,土壤中P可用性变化可调节干旱对凋落物分解的影响。     

华北落叶松与阔叶树种混合凋落叶分解过程中养分释放和酶活性变化

森林凋落物是森林土壤的重要组成部分,凋落物分解在调控森林生态系统养分循环中发挥了关键作用。采用凋落物分解袋法,研究河北塞罕坝地区华北落叶松与白桦,华北落叶松与蒙古栎,华北落叶松、白桦和蒙古栎混合凋落叶及纯华北落叶松凋落叶分解过程中分解速率、养分释放和酶活性的变化。结果表明: 经过近2年的分解,混合凋落叶分解速率均显著高于纯华北落叶松凋落物叶;在所有处理中,华北落叶松与白桦混合凋落叶分解速率最高。在凋落叶分解过程中,不同处理养分含量变化一致,凋落叶N、P含量呈上升趋势,C、K含量和C/N呈下降趋势;相对纯华北落叶松凋落叶,各混合凋落叶分解可以促进凋落叶C、K的释放,但对N、P的释放有一定的抑制作用。在凋落叶分解过程中,不同处理凋落叶过氧化氢酶、脲酶、酸性磷酸酶活性呈上升趋势,蔗糖酶活性呈下降趋势;凋落叶分解速率与凋落叶过氧化氢酶、脲酶、酸性磷酸酶活性呈正相关,与蔗糖酶活性呈负相关。总体来看,华北落叶松和白桦、蒙古栎凋落叶混合可以促进华北落叶松凋落叶的分解,且凋落叶中酶活性动态变化与凋落叶的分解密切相关。     

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.     

Effects of Single Chinese Fir and Mixed Leaf Litters on Soil Chemical, Microbial Properties and Soil Enzyme Activities

The quality of leaf litter can control decomposition processes and affect the nutrient availability for plant uptake. In this study, we investigated the effect of single leaf litter (Chinese fir – Cunninghamia lamcealata (Lamb.) Hook) and mixed leaf litters (C. lamcealata, Liquidamba formosana Hance and Alnus cremastogyne Burk) on soil chemical properties, soil microbial properties and soil enzyme activities during 2 years decomposition. The results showed that soil microbial biomass C, the ratio of soil microbial biomass C to total soil organic C (soil microbial quotient, Cmic/Corg) and soil enzymes (urease, invertase, dehydrogenase) activities increased significantly in mixed leaf litters treatments whereas soil chemical properties remained unchanged. However, soil microbial metabolic quotient (qCO₂) values and soil polyphenol oxidase activity were higher in the single Chinese fir leaf litter treatment that had a higher C:N (carbon:nitrogen) ratio (79.53) compared with the mixed leaf litter (C:N ratios of 76.32, 56.90, 61.20, respectively). Our results demonstrated that the mixed leaf litter can improve forest soil quality, and that soil microbial properties and soil enzyme activities are more sensitive in response to litter quality change than soil chemical properties.     

氮、磷养分有效性对森林凋落物分解的影响研究进展

通过对相关研究文献的综述结果表明,氮(N)和磷(P)是构成蛋白质和遗传物质的两种重要组成元素,限制森林生产力和其他生态系统过程,对凋落物分解产生深刻影响。大量的凋落物分解试验发现在土壤N有效性较低的温带和北方森林,凋落物分解速率常与底物初始N浓度、木质素/N比等有很好的相关关系,也受外源N输入的影响;而在土壤高度风化的热带亚热带森林生态系统中,P可能是比N更为重要的分解限制因子。然而控制试验表明,N、P添加对凋落物分解速率的影响并不一致,既有促进效应也有抑制效应。为了深入揭示N、P养分有效性对凋落物分解的调控机制,"底物的C、N化学计量学"假说、"微生物的N开采"假说以及养分平衡的理论都常被用于解释凋落物分解速率的变化。由于微生物分解者具有较为稳定的C、N、P等养分需求比例,在不同的养分供应的周围环境中会体现出不同的活性,某种最缺乏的养分可能就是分解的最重要限制因子。未来的凋落物分解研究,应延长实验时间、加强室内和野外不同条件下的N、P等养分添加控制试验,探讨驱动分解进程的微生物群落结构和酶活性的变化。     

The response of microbial biomass and hydrolytic enzymes to a decade of nitrogen, phosphorus, and potassium addition in a lowland tropical rain forest

Nutrient availability is widely considered to constrain primary productivity in lowland tropical forests, yet there is little comparable information for the soil microbial biomass. We assessed microbial nutrient limitation by quantifying soil microbial biomass and hydrolytic enzyme activities in a long-term nutrient addition experiment in lowland tropical rain forest in central Panama. Multiple measurements were made over an annual cycle in plots that had received a decade of nitrogen, phosphorus, potassium, and micronutrient addition. Phosphorus addition increased soil microbial carbon (13 %), nitrogen (21 %), and phosphorus (49 %), decreased phosphatase activity by ~65 % and N-acetyl β-glucosaminidase activity by 24 %, but did not affect β-glucosidase activity. In contrast, addition of nitrogen, potassium, or micronutrients did not significantly affect microbial biomass or the activity of any enzyme. Microbial nutrients and hydrolytic enzyme activities all declined markedly in the dry season, with the change in microbial biomass equivalent to or greater than the annual nutrient flux in fine litter fall. Although multiple nutrients limit tree productivity at this site, we conclude that phosphorus limits microbial biomass in this strongly-weathered lowland tropical forest soil. This finding indicates that efforts to include enzymes in biogeochemical models must account for the disproportionate microbial investment in phosphorus acquisition in strongly-weathered soils.     

Asynchronous fluctuation of soil microbial biomass and plant litterfall in a tropical wet forest

Carbon availability often controls soil microbial growth and there is evidence that at regional scales soil microbial biomass is positively correlated with aboveground forest litter input. We examined the influence of plant litterfall on annual variation of soil microbial biomass in control and litter-excluded plots in a tropical wet forest of Puerto Rico. We also measured soil moisture, soil temperature, and plant litterfall in these treatment plots. Aboveground plant litter input had no effect on soil microbial biomass or on its pattern of fluctuation. Monthly changes in soil microbial biomass were not synchronized with aboveground litter inputs, but rather preceeded litterfall by one month. Soil microbial biomass did not correlate with soil temperature, moisture, or rainfall. Our results suggest that changes in soil microbial biomass are not directly regulated by soil temperature, moisture, or aboveground litter input at local scales within a tropical wet forest, and there were asynchronous fluctuation between soil microbial biomass and plant litterfall. Potential mechanisms for this asynchronous fluctuation include soil microbial biomass regulation by competition for soil nutrients between microorganisms and plants, and regulation by below-ground carbon inputs associated with the annual solar and drying-rewetting cycles in tropical wet forests.     

云南药山自然保护区黄背栎林和巧家五针松林生态化学计量特征

以云南药山自然保护区黄背栎林和巧家五针松林的4块样地为研究对象,旨在揭示这2种森林生态系统营养元素含量状况和土壤养分的供给能力。通过分析4块样地"叶片-凋落物-土壤"C、N、P含量、生态化学计量特征及其相关性,结果表明:(1)不同样地同一组分的C、N含量差异显著,P含量差异不显著,同一样地各组分间C、N含量差异显著,均为叶片凋落物土壤,P含量则为叶片土壤凋落物;(2)不同样地叶片C∶P、N∶P比值和凋落物与土壤C∶N比值差异不显著,其余指标差异均显著。同一样地叶片、凋落物、土壤的C∶N、C∶P、N∶P比值差异显著,均为凋落物叶片土壤;(3)黄背栎林叶片-土壤C含量、C∶P比值和凋落物-叶片N∶P比值呈极显著或显著相关,巧家五针松林凋落物-叶片C、N含量和叶片-土壤P含量、C∶N、N∶P比值呈极显著或显著相关;(4)土壤N元素缺乏是限制植物生长的主要因素,P元素主要源于土壤矿物风化释放,而非生物小循环。     

Leaf litter inputs reinforce islands of nitrogen fertility in a lowland tropical forest

The role of lowland tropical forest tree communities in shaping soil nutrient cycling has been challenging to elucidate in the face of high species diversity. Previously, we showed that differences in tree species composition and canopy foliar nitrogen (N) concentrations correlated with differences in soil N availability in a mature Costa Rican rainforest. Here, we investigate potential mechanisms explaining this correlation. We used imaging spectroscopy to identify study plots containing 10–20 canopy trees with either high or low mean canopy N relative to the landscape mean. Plots were restricted to an uplifted terrace with relatively uniform parent material and climate. In order to assess whether canopy and soil N could be linked by litterfall inputs, we tracked litter production in the plots and measured rates of litter decay and the carbon and N content of leaf litter and leaf litter leachate. We also compared the abundance of putative N fixing trees and rates of free-living N fixation as well as soil pH, texture, cation exchange capacity, and topographic curvature to assess whether biological N fixation and/or soil properties could account for differences in soil N that were, in turn, imprinted on the canopy. We found no evidence of differences in legume communities, free-living N fixation, or abiotic properties. However, soils beneath high canopy N assemblages received ~ 60% more N via leaf litterfall due to variability in litter N content between plot types. The correlation of N in canopy leaves, leaf litter, and soil suggests that, under similar abiotic conditions, litterfall-mediated feedbacks can help maintain soil N differences among tropical tree assemblages in this diverse tropical forest.

     

Relationships among phosphorus, molybdenum and free-living nitrogen fixation in tropical rain forests: results from observational and experimental analyses

Biological nitrogen (N) fixation is the primary source of “new” N to unmanaged ecosystems, and recent analyses suggest that terrestrial N inputs via free-living N fixation may be more important than previously assumed. This may be particularly true in some tropical rain forests, where free-living fixation could outpace symbiotic N fixation to represent the dominant source of new N inputs. However, our understanding of the controls over free-living N fixation in tropical rain forests remains poor, which directly constrains our ability to predict how N cycling will respond to changing environmental conditions. Although both phosphorus (P) and molybdenum (Mo) availability have been shown to limit free-living N fixation rates in the tropics, few studies have simultaneously explored P versus Mo limitation or the potential importance of P × Mo interactions. Here, an archived set of foliar, litter, and soil samples from a Costa Rican tropical rain forest provided an opportunity to simultaneously assess the relative strength of P versus Mo relationships with free-living N fixation rates. We also conducted a short-term, full-factorial (P × Mo) litter incubation experiment to directly assess nutrient limitation, allowing us to explore P and Mo controls over free-living N fixation rates using both observational and experimental approaches. We previously showed that N fixation rates were positively correlated with P concentrations in all substrates and, using the archived samples, we now show that Mo concentrations correlated with N fixation only in canopy leaves (where total Mo concentrations were extremely low). Likewise, fertilization with P alone (and not Mo) stimulated leaf litter N fixation rates. Thus, our results suggest that P availability dominantly controls free-living N fixation at this site, and when taken with data from other studies, our results suggest that attempts to identify “the nutrient” that limits N fixation in “the tropics” may be misguided. Rather, nutrient controls over free-living N fixation appear to be more nuanced—and the true nature of nutrient limitation to N fixation likely varies over a variety of scales across the vast tropical rain forest biome.     

Soil organic matter,nutrient distribution,fungal and microbial biomass and enzyme activities in a forest beech stand on the Apennines of southern Italy

The aim of this study was to analyse the amount and qualitative characteristics of organic matter (OM) in the litter horizon (considering leaf litter at different decomposition stages) and underlying soil to a 30-cm depth in a beech stand on the Apennines in southern Italy. Distribution of major nutrients as well as fungal and microbial biomass were also evaluated, in addition to beech leaf nutrient content monitor from full expansion to abscission in order to estimate annual nutrient input to soil from litterfall and nutrient retranslocation before abscission. OM was significantly higher in leaf litter. C/N ratio and the Na, Mn, Fe levels also decreased along the decomposition continuum, whereas N and S contents slowly decreased with soil depth. Generally, leaf nutrient content was also significantly lower in dead leaves, indicating efficient retranslocation to persistent organs. Fungal biomass was the highest in leaf layers, with no significant changes between spring and autumn samplings. Enzyme activities did not differ significantly along the decomposition continuum but marked decreases were found in the upper soil layer; these remained relatively constant, with the exception of laccase, at deeper soil depths. No seasonal effect on enzyme activities and OM content was found.     

Throughfall exclusion and leaf litter addition drive higher rates of soil nitrous oxide emissions from a lowland wet tropical forest

Tropical forests are a significant global source of the greenhouse gas nitrous oxide (N₂O). Predicted environmental changes for this biome highlight the need to understand how simultaneous changes in precipitation and labile carbon (C) availability may affect soil N₂O production. We conducted a small‐scale throughfall and leaf litter manipulation in a lowland tropical forest in southwestern Costa Rica to test how potential changes in both water and litter derived labile C inputs to soils may alter N₂O emissions. Experimentally reducing throughfall in this wet tropical forest significantly increased soil emissions of N₂O, and our data suggest that at least part of this response was driven by an increase in the concentration of dissolved organic carbon [DOC] inputs delivered from litter to soil under the drier conditions. Furthermore, [DOC] was significantly correlated with N₂O emissions across both throughfall and litterfall manipulation plots, despite the fact that native NO₃^? pools in this site were generally small. Our results highlight the importance of understanding not only the potential direct effects of changing precipitation on soil biogeochemistry, but also the indirect effects resulting from interactions between the hydrologic, C and N cycles. Finally, over all sampling events we observed lower mean N₂O emissions (<1 ng N₂O‐N cm^?2 h^?1) than reported for many other lowland tropical forests, perhaps reflecting a more general pattern of increasing relative N constraints to biological activity as one moves from drier to wetter portions of the lowland tropical forest biome.     

Effects of Nutrient Limitation on Aboveground Carbon Dynamics during Tropical Dry Forest Regeneration in Yucatán,Mexico

Tree growth (as diameter increment), litterfall production, and litter biomass were studied in two secondary tropical dry forests of the Yucatán Peninsula under four treatments of nutrient addition. The studys objective was to assess how variations in the nutrient supply affect aboveground net primary production and carbon (C) accumulation on the floor of two forests in different stages of regeneration. The study included an area of young forest (10 years old) with phosphorus (P)-poor soils and an area of old forest (around 60 years old) where soil P was comparatively less limiting. Four replicate plots (12 × 12 m) at each forest were either left intact (controls) or fertilized with nitrogen (N), P, or N plus P during 3 consecutive years. After 3 years of fertilization, relaxation of the constraints on nutrient limitation resulted in increased trunk growth rates at both the young and old forests. This effect was more pronounced with the addition of P or N plus P (trunk growth doubled with respect to controls), whereas N addition increased tree growth by 60% in comparison to trees in plots without nutrient supplements. In both forests, there were no significant differences in litterfall production among treatments during the first 2 years after fertilization. In the 3rd year of nutrient addition, litterfall production was significantly higher in plots fertilized with N plus P compared to control plots at both forest sites; however, changes in litterfall were not accompanied by litter accumulation in the floor of the two forests. The results of this study support the hypothesis that there is nutrient limitation during tropical dry forest regeneration. They further show that it may be maintained in the long term during secondary succession.     

Nitrogen Addition Shapes Soil Phosphorus Availability in Two Reforested Tropical Forests in Southern China

Scant information is available on how soil phosphorus (P) availability responds to atmospheric nitrogen (N) deposition, especially in the tropical zones. This study examined the effect of N addition on soil P availability, and compared this effect between forest sites of contrasting land‐use history. Effects of N addition on soil properties, litterfall production, P release from decomposing litter, and soil P availability were studied in a disturbed (reforested pine forest with previous understory vegetation and litter harvesting) and a rehabilitated (reforested mixed pine/broadleaf forest with no understory vegetation and litter harvesting) tropical forest in southern China. Experimental N‐treatments (above ambient) were the following: Control (no N addition), N50 (50 kg N ha^?1 yr^?1), and N100 (100 kg N ha^?1 yr^?1). Results indicated that N addition significantly decreased soil P availability in the disturbed forest. In the rehabilitated forest, however, soil P availability was significantly increased by N addition. Decreases in soil P availability may be correlated with decreases in rates of P release from decomposing litter in the N‐treated plots, whereas the increase in soil P availability was correlated with an increase in litterfall production. Our results suggest that response of soil P availability to N deposition in the reforested tropical forests in southern China may vary greatly with temporal changes in tree species composition and soil nutrient status, caused by different land‐use practices.