Litter-soil Interface Research Articles

Thermocouple Probe Orientation Affects Prescribed Fire Behavior Estimation.

Understanding the relationship between fire intensity and fuel mass is essential information for scientists and forest managers seeking to manage forests using prescribed fires. Peak burning temperature, duration of heating, and area under the temperature profile are fire behavior metrics obtained from thermocouple-datalogger assemblies used to characterize prescribed burns. Despite their recurrent usage in prescribed burn studies, there is no simple protocol established to guide the orientation of thermocouple installation. Our results from dormant and growing season burns in coastal longleaf pine ( Mill.) forests in South Carolina suggest that thermocouples located horizontally at the litter-soil interface record significantly higher estimates of peak burning temperature, duration of heating, and area under the temperature profile than thermocouples extending 28 cm vertically above the litter-soil interface ( < 0.01). Surprisingly, vertical and horizontal estimates of these measures did not show strong correlation with one another ( ≤ 0.14). The horizontal duration of heating values were greater in growing season burns than in dormant season burns ( < 0.01), but the vertical values did not indicate this difference ( = 0.52). Field measures of fuel mass and depth before and after fire showed promise as significant predictive variables ( ≤ 0.05) for the fire behavior metrics. However, all correlation coefficients were less than or equal to = 0.41. Given these findings, we encourage scientists, researchers, and managers to carefully consider thermocouple orientation when investigating fire behavior metrics, as orientation may affect estimates of fire intensity and the distinction of fire treatment effects, particularly in forests with litter-dominated surface fuels.

Stoichiometric linkages between plant litter, trophic interactions and nitrogen mineralization across the litter–soil interface

The common notion for describing N mineralization in models is that it results from decomposer organisms trying to meet their stoichiometric demands based on their own elemental composition and that of the resource. However, in addition to influencing C and nutrient availability, plant litter also influences the composition of both the litter and mineral soil community – importantly not in the same manner – resulting in altered trophic interactions. Since decomposer groups and their consumers vary in their elemental composition and demands, a change in composition and abundance of soil functional groups may result in a change in the stoichiometry of the whole soil food web, thus altering their stoichiometric relations with the available resource with potential functional consequences. We use experimental data and quantitative food web modeling to investigate the impact of the changes in the litter and soil food webs brought about by the differing stoichiometry of plant litter on (a) N mineralization, (b) the contribution of different functional groups to mineralization, and (c) the stoichiometric flexibility of the system, assessed as the ability to mineralize materials with different stoichiometry. Our simulations suggested that the effects of litter stoichiometry on trophic interactions, their impacts on N mineralization and the relative contribution of functional groups may not behave as a continuum across the litter and soil interface. Further, changes in food webs associated with variation in plant stoichiometric traits can influence the relative importance of functional groups, which given their particular stoichiometric demands may affect ecosystem-level N cycling. Our results suggested that litter materials of intermediate N contents, or litter mixtures encompassing materials with different nutrient contents and thus resulting in mixtures of intermediate stoichiometry, may promote food webs that are better suited to deal with changing substrate stoichiometry.

The influence of litter composition across the litter–soil interface on mass loss, nitrogen dynamics and the decomposer community

Many studies have investigated the influence of plant litter species composition on decomposition, but results have been context-dependent. Litter and soil are considered to constitute a decomposition continuum, but whether litter and soil ecosystems respond to litter identity and mixing in the same manner is unsure. In a field experiment utilizing 5 litter species and their mixture, we investigated whether the effects of litter identity and mixing on mass loss, nutrient dynamics, and decomposer communities are consistent across the litter–soil interface. In monoculture, mass loss and nitrogen (N) dynamics in the litter layer corresponded to the underlying soil N availability, demonstrating the continuum of resources from litter to soil. Litter microbial biomass and mesofauna abundance tended to be greater on litter with a faster decay rate and greater N release. However, soil decomposer abundance and diversity were not greater with higher soil N, causing litter and soil communities to respond differently to litter identity. Non-additive mass loss and N dynamics were observed after 6 months, and were correlated with non-additive litter microbial community composition and litter mesofauna communities, but all other aspects of the litter community and all measures of the soil community were additive. Decomposer communities and N dynamics did not respond similarly to the litter mixture across the litter–soil interface. This study is one of the few to comprehensively examine how a litter mixture influences decomposition dynamics and communities across the soil–litter interface, including multiple taxa and trophic levels. Our results demonstrate that processes associated with decomposition are decoupled for litter and soil, particularly in that litter showed non-additivity in mass loss, N release and decomposer community, but soil responses were largely additive.

Salvage harvest effects on advance tree regeneration, soil nitrogen, and fuels following mountain pine beetle outbreak in lodgepole pine

The extent and severity of recent native bark beetle (Dendroctonae) outbreaks in western North America have created a pressing need for forest managers to understand potential consequences of post-disturbance management. For example, post-outbreak timber harvest (i.e., salvage harvest) could alter future forest development, productivity and susceptibility to subsequent disturbance. To assess the potential for such consequences, we measured first-year effects of post-outbreak timber harvest on tree regeneration, soil nitrogen (N) availability and fuels by using a paired and replicated before–after-control-impact (BACI) experimental design with eight pairs of 0.25-ha plots in beetle-killed lodgepole pine (Pinus contorta var. latifolia) in Greater Yellowstone (Wyoming, USA). Post-outbreak timber harvest reduced total (live +dead) lodgepole pine basal area by 90%. Total sapling density (advance regeneration) declined by about 50% following harvest, with tall (30–140cm) saplings declining most, but mean post-harvest sapling density still exceeded 1600 stems ha−1. Relative species density was unaffected and remained dominated by lodgepole pine. Soil temperature at the litter–soil interface was warmer during summer in harvested stands, and soil NO3- concentration increased with harvest relative to untreated plots. Soil NH4+ concentration and resin bag N accumulation increased through time in all beetle-killed plots and were not affected by harvest. Following harvest, dead woody surface fuels in all size categories doubled, and canopy fuel load and canopy bulk density both were reduced; dead fuel depth, duff depth, and canopy base height did not differ between untreated and harvested plots. Harvest did reduce canopy fuels, but the natural progression of needle shedding after beetle-kill accounted for 25–40% of this total canopy fuel reduction. Salvage harvest seems unlikely to alter post-outbreak successional trajectories in these lodgepole pine forests. However, the altered fuel complex (immediate increase in dead woody surface fuels and expected long-term reduction in large-diameter fuels) in harvested plots could cause subsequent fire behavior and effects to differ between harvested and untreated stands.

Plant litter and soil type drive abundance, activity and community structure of alkB harbouring microbes in different soil compartments.

Alkanes are major constituents of plant-derived waxy materials. In this study, we investigated the abundance, community structure and activity of bacteria harbouring the alkane monooxygenase gene alkB, which catalyses a major step in the pathway of aerobic alkane degradation in the litter layer, the litter-soil interface and in bulk soil at three time points during the degradation of maize and pea plant litter (2, 8 and 30 weeks) to improve our understanding about drivers for microbial performance in different soil compartments. Soil cores of different soil textures (sandy and silty) were taken from an agricultural field and incubated at constant laboratory conditions. The abundance of alkB genes and transcripts (by qPCR) as well as the community structure (by terminal restriction fragment polymorphism fingerprinting) were measured in combination with the concentrations and composition of alkanes. The results obtained indicate a clear response pattern of all investigated biotic and abiotic parameters depending on the applied litter material, the type of soil used, the time point of sampling and the soil compartment studied. As expected the distribution of alkanes of different chain length formed a steep gradient from the litter layer to the bulk soil. Mainly in the two upper soil compartments community structure and abundance patterns of alkB were driven by the applied litter type and its degradation. Surprisingly, the differences between the compartments in one soil were more pronounced than the differences between similar compartments in the two soils studied. This indicates the necessity for analysing processes in different soil compartments to improve our mechanistic understanding of the dynamics of distinct functional groups of microbes.

Effects of manipulated herbivore inputs on nutrient flux and decomposition in a tropical rainforest in Puerto Rico

Forest canopy herbivores are known to increase rates of nutrient fluxes to the forest floor in a number of temperate and boreal forests, but few studies have measured effects of herbivore-enhanced nutrient fluxes in tropical forests. We simulated herbivore-induced fluxes in a tropical rainforest in Puerto Rico by augmenting greenfall (fresh foliage fragments), frassfall (insect feces), and throughfall (precipitation enriched with foliar leachates) in replicated experimental plots on the forest floor. Background rates of greenfall and frassfall were measured monthly using litterfall collectors and augmented by adding 10× greenfall or 10× frassfall to designated plots. Throughfall fluxes of NH(4), NO(3) and PO(4) (but not water) were doubled in treatment plots, based on published rates of fluxes of these nutrients in throughfall. Control plots received only background flux rates for these compounds but the same minimum amount of distilled water. We evaluated treatment effects as changes in flux rates for NO(3), NH(4) and PO(4), measured as decomposition rate of leaf litter in litterbags and as adsorption in ion-exchange resin bags at the litter-soil interface. Frass addition significantly increased NO(3) and NH(4) fluxes, and frass and throughfall additions significantly reduced decay rate, compared to controls. Reduced decay rate suggests that nitrogen flux was sufficient to inhibit microbial decomposition activity. Our treatments represented fluxes expected from low-moderate herbivore outbreaks and demonstrated that herbivores, at these outbreak levels, increase ecosystem-level N and P fluxes by >30% in this tropical rainforest.

Survival, Dispersal, and Potential Soil-Mediated Suppression of Phytophthora ramorum in a California Redwood-Tanoak Forest

Because the role of soil inoculum of Phytophthora ramorum in the sudden oak death disease cycle is not well understood, this work addresses survival, chlamydospore production, pathogen suppression, and splash dispersal of the pathogen in infested forest soils. Colonized rhododendron and bay laurel leaf disks were placed in mesh sachets before transfer to the field in January 2005 and 2006. Sachets were placed under tanoak, bay laurel, and redwood at three vertical locations: leaf litter surface, litter-soil interface, and below the soil surface. Sachets were retrieved after 4, 8, 20, and 49 weeks. Pathogen survival was higher in rhododendron leaf tissue than in bay tissue, with >80% survival observed in rhododendron tissue after 49 weeks in the field. Chlamydospore production was determined by clearing infected tissue in KOH. Moist redwood-associated soils suppressed chlamydospore production. Rain events splashed inoculum as high as 30 cm from the soil surface, inciting aerial infection of bay laurel and tanoak. Leaf litter may provide an incomplete barrier to splash dispersal. This 2-year study illustrates annual P. ramorum survival in soil and the suppressive nature of redwood-associated soils to chlamydospore production. Infested soil may serve as primary inoculum for foliar infections by splash dispersal during rain events.

Drought‐induced nitrous oxide flux dynamics in an enclosed tropical forest

AbstractEl Niño–La Niña cycles strongly influence dry and wet seasons in the tropics and consequently nitrous oxide (N2O) emissions from tropical rainforest soils. We monitored whole‐system and soil chamber N2O fluxes during 5‐month‐long droughts in the Biosphere 2 tropical forest to determine how rainfall changes N2O production. A consistent pattern of N2O flux changes during drought and subsequent wetting emerged from our experiments. Soil surface drying during the first days of drought, presumably increased gas transport out of the soil, which increased N2O fluxes. Subsequent drying caused an exponential decrease in whole‐system (4.0±0.1% day−1) and soil chamber N2O flux (8.9±0.8% day−1; south chamber; and 13.7±1.1% day−1; north chamber), which was highly correlated with soil moisture content. Soil air N2O concentration ([N2O]) and flux measurements revealed that surface N2O production persisted during drought. The first rainfall after drought triggered a N2O pulse, which amounted to 25% of drought‐associated reduction in N2O flux and 1.3±0.4% of annual N2O emissions. Physical displacement of soil air by water and soil chemistry changes during drought could not account for the observed N2O pulse. We contend that osmotic stress on the microbial biomass must have supplied the N source for pulse N2O, which was produced at the litter–soil interface. After the pulse, N2O fluxes were consistently 90% of predrought values. Nitrate change rate, nutrient, [N2O], and flux analyses suggested that nitrifiers dominated N2O production during the pulse and denitrifiers during wet conditions. N2O flux measurements in Biosphere 2, especially during the N2O pulse, demonstrate that large‐scale integration methods, such as flux towers, are essential for improving ecosystem N2O flux estimates.

Litter Fall and Nutrient Turnover in Two Eucalypt Woodlands

The components and chemical composition of litter fall and the litter layer of two woodlands growing close together near Narrabeen Lagoon, New South Wales, were examined over a 5-year period. Total litter fall peaked in the summer but some tree and shrub species in both communities showed peaks of litter fall in summer and autumn. Litter fall averaged 540 g m-2 year-1 for a hillside Angophora costata-Eucalyptus gummifera-dominated woodland on a gradational sand, and 745 g m-2 year-1 for a E. botryoides-E. robusta-A. floribunda-dominated community on podzolized sand at the slope foot. The accumulated litter masses averaged 2100 and 1900 g m-2, respectively: calculated litter half-lives were 2.7 years and 1.8 years. The higher rates of litter fall and turnover of organic matter and nutrients in the E. botryoides-E. robusta-A. floribunda community, despite its poorer soil nutrient status, are postulated to result from a more reliable water supply and differing microbial activity at the litter-soil interface, leading to more rapid recycling of nutrients.

Influence of Topographic Heterogeneity on Deciduous Litter Decomposition

Woodlot floor topography, consisting of rises (high sites), hollows (low sites) and more extensive level areas (level sites) about 1 m in diameter with a 0.5 m vertical maximum, affected structural and functional components of the litter system. Topographic effects were measured on litter accumulation, substrate temperature and moisture, bacterial populations, litter decomposition, and organic matter incorporation in soil. Average litter accumulation (April to October) was 416.2 g m-2, 1209.9 g m-2 and 2438.3 g m-2 on high, level and low sites, respectively. Greater litter weight and depth in low sites reduced moisture loss from underlying litter and soil by up to 63%. At low sites, temperature variability of the litter-soil interface was reduced 13% of the mean by surface litter and by the heat capacity of a greater moisture content. Average bacterial populations were approximately 3.5 times larger in low sites. Litter decomposition in mesh bags averaged an exponential weight loss of -0.015 over 16 months. Interaction of low topographic site and litter composition enhanced decay. Finally, the soil of low sites was characterized by greater incorporation of organic fragments and humus formation.

Microarthropod Response Following Cable Logging and Clear‐Cutting in the Southern Appalachians

Litter and soil microarthropod populations were monitored following cable logging and clear—cutting of a forested watershed (WS 7) at the Coweeta Hydrologic Laboratory in the southern Appalachians of North Carolina. Annual mean densities of microarthropods in litter bags were reduced over 50% on the clear—cut watershed when compared with an adjacent forested watershed (WS 2), and averaged 8.4 individuals/g of litter on WS 7 vs. 20.4/g on WS 2 (P &lt; .01). Density estimates obtained from 5 cm deep sections of litter and soil indicated a 25% reduction in densities on WS 7, with a 17—mo average of 98 900 microarthropods/m2 on WS 7 vs. 133 500 microarthropods/m2 on WS 2 (P &gt; .001). In contrast, densities of microarthropods increased over 100% in deeper soil horizons (5—55 cm), averaging 89 800 microarthropods/m2 on WS 7 vs. 43 700 microarthropods/m2 on WS 2 (P &lt; .001). Clear—cutting by methods that do not destroy the integrity of the litter—soil environment engenders two kinds of perturbations: (1) removal of the canopy, and (2) addition of organic matter inputs, primarily woody litter debris, to the forest floor. Without a protective canopy, temperatures at the litter—soil interface on WS 7 were increased; daily maximum summer temperatures averaged over 40°C. Thus, lethal or nearly lethal temperatures reduced microarthropod densities in the top 5 cm of litter and soil on WS 7; declines were attributed primarily to oribatid mite mortality. Increased densities below 5 cm were attributed primarily to increased prostigmatid mite abundance. The relationships between microarthropod densities, standing crops of organic matter and litter and microbial production were examined. The amount and vertical distribution and organic matter appears to be the best predictor of microarthropod densities and vertical distributions in most forest ecosystems.