Litter Lignin Content Research Articles

Dynamics of litter decomposition rate and soil organic carbon sequestration following vegetation succession on the Loess Plateau, China

Litter quality determines the decomposability of litter and the proportion of recalcitrant residues, which in turn can affect the accumulation of soil organic carbon (SOC). However, the impact of litter decomposition on SOC turnover following vegetation succession has not been systematically studied. To clarify the dynamics and mechanisms of litter decomposition and SOC sequestration following vegetation succession, this study conducted a decomposition experiment across four successional stages: grassland (∼30 years), shrubland (∼60 years), pioneer forest (∼110 years), and climax forest (∼160 years). This study also analyzed the litter quality and the SOC turnover rate. The results showed that litter decomposition rates gradually decreased with vegetation succession. Litter lignin content (LC), and nitrogen content (LN) were the major factors controlling the change of litter decomposition rate. Litter with a high LC: LN ratio and lignin: LN ratio exhibited a lower decomposition rate, whereas the opposite was found for litter with a higher LN content. The increase in the SOC content mainly occurred in the early successional stage (<110 years), with a higher litter decomposition rate and SOC turnover rate. At the later stage (≥110 years), the SOC was close to saturation, together with low rates of litter decomposition and SOC turnover. The variation trend of the mineral-associated organic carbon (MAOC) content was the same as that of the SOC content, resulting from continuous litter inputs. Based on the above results, litter inputs can significantly increase the SOC content in the early successional stage, and soil carbon sequestrations gradually reach saturation in the late successional stage. The findings of this study strengthen our understanding of the changes in litter decomposition and its contribution to SOC sequestration following vegetation succession.

Amazonian deforestation and its influence on soil biotic factors and abiotic properties

Deforestation in the Amazonian rainforest is the main cause of biodiversity loss, soil erosion, and climate change. Land covers influence the soil properties, soil biota community composition, and litter properties by providing habitat and nutrients. Our aim here was to present a quantitative analysis of the soil biochemical properties, soil biota assemblage, and litter properties associated with four land covers (primary Amazon rainforest, pasture, Hevea brasiliensis, and deforested sites) from the Brazil’s Legal Amazon, Rio Branco, Acre. The non-metric multidimensional scaling (NMDS) grouped the land covers in four groups considering the dissimilarities on soil biota abundance between each other. The SEM models showed that deforestation increased the soil pH, and decreased SOC, TN, P, microbial N biomass, microbial C biomass, litter lignin content, litter N, C and P contents, and biodiversity indices (Margalef index, and α diversity). The results of our study highlighted the importance of considering land covers in Brazil’s Legal Amazon, based on a sustainable way to improve soil quality and soil biodiversity, since even the studied monocropping system (pasture) showed decreases in habitat provision (litter deposition) and nutrient availability. Thus, the use of forest systems (H. brasiliensis) or the preservation of primary Amazon rainforest may positively influence the soil chemical properties, soil biota, and litter properties.

Warming and grazing enhance litter decomposition and nutrient release independent of litter quality in an alpine meadow

Abstract Warming and grazing, and litter quality jointly determine litter decomposition and nutrient releases in grazing ecosystems. However, their effects have previously been studied in isolation. We conducted a two factorial experiment with asymmetric warming using infrared heaters and moderate grazing in an alpine meadow. Litter samples were collected from all plots in each treatment, among which some subsamples were placed in their original plots and other samples were translocated to other treatment plots to test the relative effects of each treatment on litter decomposition and nutrient releases. We found that warming rather than grazing alone significantly increased total losses of litter mass, total organic carbon, total nitrogen (TN) and total phosphorus (TP) per unit area due to increases in both mass loss rates and litter biomass. However, grazing with warming did not affect their total mass losses because increased mass loss was offset by decreased litter biomass compared with the control. Seasonal mean soil temperature better predicted litter decomposition than litter lignin content or carbon to nitrogen ratio. There were interactions between warming and grazing, but there were no interactions between them and litter quality on litter decomposition. The temperature sensitivity of TN loss was higher than that of TP loss per unit area. Our results suggest that increased temperature has a greater effect on litter decomposition and nutrient release than change in litter quality, and that more N release from litter could result in greater P deficiency in the alpine meadow.

Dynamics of leaf litter decomposition in the timberline zone of western Himalaya

Litter decomposition plays a vital role in carbon and nutrient budget of forest ecosystems, but our understanding of this process for high altitude ecosystems, especially the timberline zone, is not very clear. We studied leaf litter decomposition of Betula utilis, which is the main timberline species of Western Himalaya. We placed 144 litterbags (each containing 10 g of oven dried litter) at three ‘spatially well separated sites’ in Oct 2014, to study the direction and magnitude of change in key variables associated with leaf litter decomposition and to test for generality of decomposition process across these spatially distant sites. The results revealed that initial nutrient concentration and lignin content of litter was significantly different for the sites. Further, magnitude of mass loss and decay rate was found to be greater for up to 34 weeks (19.03 ± 4.55% and 0.35 ± 0.06 yr−1 respectively), which reached 47.01 ± 1.98% and 0.17 ± 0.01 yr−1 respectively, after 4 years. N, P and K concentrations showed sharp decease for up to 34 weeks, after which N consistently increased with incubation time, whereas P and K concentrations were relatively stable. The lignin and cellulose contents decreased consistently with incubation time (from 12.29 ± 1.21% to 5.21 ± 0.50% and from 16.87 ± 1.01% to 8.25 ± 0.50% respectively). There were significant increases in C:N and lignin:N ratios for up to 34 weeks, followed by a continuous decrease thereafter. The change in various variables associated with litter decomposition was not consistent among the sites and the magnitude of change was greater during initial stages due to leaching of labile components. The increase in N and decreases in C:N and lignin:N ratios with incubation time might be due to microbial immobilization of N and are important for nutrient conservation in timberline ecosystems, as such changes are related to faster decomposition of recalcitrant fraction of litter, N mineralization and building of soil nutrient pool.

Estimating relative cellulolytic and ligninolytic enzyme activities as functions of lignin and cellulose content in decomposing plant litter

Extracellular enzymes catalyze plant litter decomposition, including enzymes that degrade holocellulose (E2) and lignin (E3). To estimate relative enzyme activities associated with observed patterns of hollocellulose (C2) and lignin (C3) decay, we set observed decay rates equal to reverse Michaelis-Menten equations. Results were consistent with empirical studies, showing a negative relationship of E2/(E2+E3) to litter lignin content, C3/(C2+C3), above a minimum threshold at which lignin begins to decay. This threshold was previously reported to be 40% lignin content, but our results demonstrated substantial variability with litter type and environment. To our knowledge, this is the first mechanistic explanation of microbial allocation of cellulolytic and ligninolytic enzymes as a function of the lignin concentration of the lignocellulose complex but raises further questions about factors controlling the threshold for lignin decay, such as nitrogen availability.

Effects of elevated CO2, increased nitrogen deposition, and plant diversity on aboveground litter and root decomposition

AbstractGlobal change‐induced litter decomposition strongly affects the carbon (C) and nitrogen (N) dynamics in grassland ecosystems. However, few studies show the interactive effects of global change factors on litter and root decomposition. We conducted a four‐year grassland field experiment to examine the quality and decomposition of litter and root in a three‐factorial experiment with elevated CO2, increased N deposition, and plant species richness. We found that elevated CO2 decreased the litter N content and root lignin content. N addition increased the root N content and decreased the litter lignin content. Increasing plant richness decreased the N and lignin contents in litter and root. In contrast to the quality changes, elevated CO2 had no effect on decomposition of litter and root. N addition increased the C loss of the litter by 4.8%, but did not affect C and N loss in root. Increasing plant richness affected the C and N loss in litter and root. ANCOVAs showed that tissue quality and root biomass affected the C and N loss in litter and root, and soil C and N affected the N loss of litter and root. However, changes in tissue quality, biomass, and soil as covariates did not significantly change the effects of CO2, N, and plant richness on decomposition. The structural equation model showed that elevated CO2 indirectly decreased litter N loss and increased root N loss, while N addition indirectly increased the C and N loss in litter and root, via their effects on tissue quality. Increasing plant richness increased litter C and N loss, but indirectly decreased root C and N loss. N deposition can accelerate litter and root decomposition, thus modifying the limitation of elevated CO2 on soil N availability. Biodiversity loss greatly alters litter and root decomposition, potentially driving any changes in C and N cycling. Our study clearly demonstrates a relative certainty of a predicted increase in the C loss and N release in litter and root decomposition with increased N deposition, whereas the effects of elevated CO2 and plant diversity changes on decomposition strongly differ between litter and root in grassland ecosystems.

Direct and indirect controls on organic matter decomposition in four coastal wetland communities along a landscape salinity gradient

AbstractCoastal wetlands store more carbon than most ecosystems globally. As sea level rises, changes in flooding and salinity will potentially impact ecological functions, such as organic matter decomposition, that influence carbon storage. However, little is known about the mechanisms that control organic matter loss in coastal wetlands at the landscape scale. As sea level rises, how will the shift from fresh to salt‐tolerant plant communities impact organic matter decomposition? Do long‐term, plant‐mediated, effects of sea‐level rise differ from direct effects of elevated salinity and flooding?We identified internal and external factors that regulated indirect and direct pathways of sea‐level rise impacts, respectively, along a landscape‐scale salinity gradient that incorporated changes in wetland type (fresh, oligohaline, mesohaline and polyhaline marshes). We found that indirect and direct impacts of sea‐level rise had opposing effects on organic matter decomposition.Salinity had an indirect effect on litter decomposition that was mediated through litter quality. Despite significant variation in environmental conditions along the landscape gradient, the best predictors of above‐ and below‐ground litter decomposition were internal drivers, initial litter nitrogen content and initial litter lignin content respectively. Litter decay constants were greatest in the oligohaline marsh and declined with increasing salinity, and the fraction of litter remaining (asymptote) was greatest in the mesohaline marsh. In contrast, direct effects of salinity and flooding were positive. External drivers, salinity and flooding, stimulated cellulytic activity, which was highest in the polyhaline marsh.Synthesis. Our results indicate that as sea level rises, initial direct effects of salinity will stimulate decay of labile carbon, but over time as plant communities shift from fresh to polyhaline marsh, litter decay will decline, yielding greater potential for long‐term carbon storage. These findings highlight the importance of quantifying carbon loss at multiple temporal scales, not only in coastal wetlands but also in other ecosystems where plant‐mediated responses to climate change will have significant impacts on carbon cycling.

Effects of Increased Summer Precipitation and Nitrogen Addition on Root Decomposition in a Temperate Desert.

BackgroundClimate change scenarios that include precipitation shifts and nitrogen (N) deposition are impacting carbon (C) budgets in arid ecosystems. Roots constitute an important part of the C cycle, but it is still unclear which factors control root mass loss and nutrient release in arid lands.Methodology/Principal FindingsLitterbags were used to investigate the decomposition rate and nutrient dynamics in root litter with water and N-addition treatments in the Gurbantunggut Desert in China. Water and N addition had no significant effect on root mass loss and the N and phosphorus content of litter residue. The loss of root litter and nutrient releases were strongly controlled by the initial lignin content and the lignin:N ratio, as evidenced by the negative correlations between decomposition rate and litter lignin content and the lignin:N ratio. Fine roots of Seriphidium santolinum (with higher initial lignin content) had a slower decomposition rate in comparison to coarse roots.Conclusion/SignificanceResults from this study indicate that small and temporary changes in rainfall and N deposition do not affect root decomposition patterns in the Gurbantunggut Desert. Root decomposition rates were significantly different between species, and also between fine and coarse roots, and were determined by carbon components, especially lignin content, suggesting that root litter quality may be the primary driver of belowground carbon turnover.

Control of climate and litter quality on leaf litter decomposition in different climatic zones.

Climate and initial litter quality are the major factors influencing decomposition rates on large scales. We established a comprehensive database of terrestrial leaf litter decomposition, including 785 datasets, to examine the relationship between climate and litter quality and evaluate the factors controlling decomposition on a global scale, the arid and semi-arid (AS) zone, the humid middle and humid low (HL) latitude zones. Initial litter nitrogen (N) and phosphorus (P) concentration only increased with mean annual temperature (MAT) in the AS zone and decreased with mean annual precipitation (MAP) in the HL zone. Compared with nutrient content, MAT imposed less effect on initial litter lignin content than MAP. MAT were the most important decomposition driving factors on a global scale as well as in different climatic zones. MAP only significantly affected decomposition constants in AS zone. Although litter quality parameters also showed significant influence on decomposition, their importance was less than the climatic factors. Besides, different litter quality parameters exerted significant influence on decomposition in different climatic zones. Our results emphasized that climate consistently exerted important effects on decomposition constants across different climatic zones.

The decomposition of fine and coarse roots: their global patterns and controlling factors.

Fine root decomposition represents a large carbon (C) cost to plants, and serves as a potential soil C source, as well as a substantial proportion of net primary productivity. Coarse roots differ markedly from fine roots in morphology, nutrient concentrations, functions, and decomposition mechanisms. Still poorly understood is whether a consistent global pattern exists between the decomposition of fine (<2 mm root diameter) and coarse (≥2 mm) roots. A comprehensive terrestrial root decomposition dataset, including 530 observations from 71 sampling sites, was thus used to compare global patterns of decomposition of fine and coarse roots. Fine roots decomposed significantly faster than coarse roots in middle latitude areas, but their decomposition in low latitude regions was not significantly different from that of coarse roots. Coarse root decomposition showed more dependence on climate, especially mean annual temperature (MAT), than did fine roots. Initial litter lignin content was the most important predictor of fine root decomposition, while lignin to nitrogen ratios, MAT, and mean annual precipitation were the most important predictors of coarse root decomposition. Our study emphasizes the necessity of separating fine roots and coarse roots when predicting the response of belowground C release to future climate changes.

Tree communities rapidly alter soil microbial resistance and resilience to drought

Summary The ability of soil microbial communities to withstand and recover from disturbance or stress is important for the functional stability of forest ecosystems. However, the relationship between the community responses of soil microbes and variation in tree mixtures vs functional composition remains poorly understood. We investigated soil biochemical properties and soil microbial resistance and resilience to drought in three 4‐year‐old tree monocultures (Acer saccharum Marsh, Larix laricina (Duroi) K. Koch and Pinus strobus L.) and two tree species combinations (L. laricina/A. saccharum and L. laricina/P. strobus) planted in a high‐density tree field experiment located in southern Quebec, Canada. The experimentally imposed drought stress consisted of maintaining soil material for 30 days at 25% of water‐holding capacity (WHC). Microbial biomass was assessed immediately after the water stress (resistance) and 15 and 30 days following drought (resilience). Results showed that tree communities influenced soil chemistry, soil respirometry properties and microbial resistance and resilience. We measured significant non‐additive (i.e. both synergistic and antagonistic) effects of mixing tree species in some of the soil biochemical properties measured, mostly in the L. laricina/A. saccharum mixture. However, we did not find non‐additive effects of tree mixtures on microbial resistance and resilience. A structural equation modelling analysis revealed that resistance and resilience were mostly modulated by direct effects of community‐weighted means (CWM) of leaf litter lignin content and mineralizable N, and by indirect links from tree density and CWM of leaf litter N content via mineralizable N. This study suggests that tree species identity surpassed species mixtures as a key driver of soil microbial resistance and resilience. We showed a trade‐off between microbial resistance and resilience in soil food webs, which is consistent with ecological theory. Our results indicate that differences in functional traits between tree species may rapidly be reflected in divergent soil biochemical properties and that these differences can in turn drive soil microbial resistance and resilience to drought.

Influences of evergreen gymnosperm and deciduous angiosperm tree species on the functioning of temperate and boreal forests.

It has been recognized for a long time that the overstorey composition of a forest partly determines its biological and physical-chemical functioning. Here, we review evidence of the influence of evergreen gymnosperm (EG) tree species and deciduous angiosperm (DA) tree species on the water balance, physical-chemical soil properties and biogeochemical cycling of carbon and nutrients. We used scientific publications based on experimental designs where all species grew on the same parent material and initial soil, and were similar in stage of stand development, former land use and current management. We present the current state of the art, define knowledge gaps, and briefly discuss how selection of tree species can be used to mitigate pollution or enhance accumulation of stable organic carbon in the soil. The presence of EGs generally induces a lower rate of precipitation input into the soil than DAs, resulting in drier soil conditions and lower water discharge. Soil temperature is generally not different, or slightly lower, under an EG canopy compared to a DA canopy. Chemical properties, such as soil pH, can also be significantly modified by taxonomic groups of tree species. Biomass production is usually similar or lower in DA stands than in stands of EGs. Aboveground production of dead organic matter appears to be of the same order of magnitude between tree species groups growing on the same site. Some DAs induce more rapid decomposition of litter than EGs because of the chemical properties of their tissues, higher soil moisture and favourable conditions for earthworms. Forest floors consequently tend to be thicker in EG forests compared to DA forests. Many factors, such as litter lignin content, influence litter decomposition and it is difficult to identify specific litter-quality parameters that distinguish litter decomposition rates of EGs from DAs. Although it has been suggested that DAs can result in higher accumulation of soil carbon stocks, evidence from field studies does not show any obvious trend. Further research is required to clarify if accumulation of carbon in soils (i.e. forest floor + mineral soil) is different between the two types of trees. Production of belowground dead organic matter appears to be of similar magnitude in DA and EG forests, and root decomposition rate lower under EGs than DAs. However there are some discrepancies and still are insufficient data about belowground pools and processes that require further research. Relatively larger amounts of nutrients enter the soil-plant biogeochemical cycle under the influence of EGs than DAs, but recycling of nutrients appears to be slightly enhanced by DAs. Understanding the mechanisms underlying forest ecosystem functioning is essential to predicting the consequences of the expected tree species migration under global change. This knowledge can also be used as a mitigation tool regarding carbon sequestration or management of surface waters because the type of tree species affects forest growth, carbon, water and nutrient cycling.

Functional leaf traits and biodiversity effects on litter decomposition in a stream

Rapid loss of biodiversity worldwide has raised concerns about the consequences to ecosystem functioning, including processes such as litter decomposition. Consequent experiments with litter mixtures to assess effects of changing tree composition and diversity on decomposition have given mixed results, but the causes are not clear. Reasoning that such conflicting accounts reported in the literature may be reconciled by considering differences in functional litter traits, we conducted a field experiment in a stream with leaf litter from nine deciduous tree species mixed in a total of 40 combinations. Fine-mesh and coarse-mesh litter bags were used to distinguish between potential effects mediated by microbial decomposers and detritivores. We hypothesized that diversity effects would emerge in species mixtures containing functionally dissimilar leaves but be absent in mixtures of functionally similar litter as assessed by determining proximate lignin, nitrogen, and phosphorus contents of the litter. Mean decomposition rates of litter mixtures did not lend support to our hypothesis for either microbial decomposition (as inferred from mass loss in fine-mesh litter bags) or decomposition caused by both microbes and detritivores (mass loss from coarse-mesh bags). Decomposition rates were largely controlled by litter lignin content, whereas P and especially N were not important. In line with our hypothesis, the most recalcitrant (slowly decomposing) and most labile (rapidly decomposing) species decomposed slower and faster, respectively, in litter mixtures comprising different decay categories than in homogenous mixtures composed of a single decay category or in single-species litter bags. However, across decay categories, evidence was weak in support of the idea that functional litter dissimilarity matters, in spite of plausible mechanisms that could cause such effects.

Leaf traits capture the effects of land use changes and climate on litter decomposability of grasslands across Europe

Land use and climate changes induce shifts in plant functional diversity and community structure, thereby modifying ecosystem processes. This is particularly true for litter decomposition, an essential process in the biogeochemical cycles of carbon and nutrients. In this study, we asked whether changes in functional traits of living leaves in response to changes in land use and climate were related to rates of litter potential decomposition, hereafter denoted litter decomposability, across a range of 10 contrasting sites. To disentangle the different control factors on litter decomposition, we conducted a microcosm experiment to determine the decomposability under standard conditions of litters collected in herbaceous communities from Europe and Israel. We tested how environmental factors (disturbance and climate) affected functional traits of living leaves and how these traits then modified litter quality and subsequent litter decomposability. Litter decomposability appeared proximately linked to initial litter quality, with particularly clear negative correlations with lignin-dependent indices (litter lignin concentr tion, lignin:nitrogen ratio, and fiber component). Litter quality was directly related to community-weighted mean traits. Lignin-dependent indices of litter quality were positively correlated with community-weighted mean leaf dry matter content (LDMC), and negatively correlated with community-weighted mean leaf nitrogen concentration (LNC). Consequently, litter decomposability was correlated negatively with community-weighted mean LDMC, and positively with community-weighted mean LNC. Environmental factors (disturbance and climate) influenced community-weighted mean traits. Plant communities experiencing less frequent or less intense disturbance exhibited higher community-weighted mean LDMC, and therefore higher litter lignin content and slower litter decomposability. LDMC therefore appears as a powerful marker of both changes in land use and of the pace of nutrient cycling across 10 contrasting sites.

Decomposition of Scots pine fine woody debris in boreal conditions: Implications for estimating carbon pools and fluxes

Litter quality and environmental effects on Scots pine ( Pinus sylvestris L.) fine woody debris (FWD) decomposition were examined in three forestry-drained peatlands representing different site types along a climatic gradient from the north boreal (Northern Finland) to south (Southern Finland) and hemiboreal (Central Estonia) conditions. Decomposition (percent mass loss) of FWD with diameter ≤10 mm (twigs) and FWD with diameter >10 mm (branches) was measured using the litter bag method over 1–4-year periods. Overall, decomposition rates increased from north to south, the rate constants ( k values) varying from 0.128 to 0.188 year −1 and from 0.066 to 0.127 year −1 for twigs and branches, respectively. On average, twigs had lost 34%, 19% and 19%, and branches 25%, 17% and 11% of their initial mass after 2 years of decomposition at the hemiboreal, south boreal and north boreal sites, respectively. After 4 years at the south boreal site the values were 48% for twigs and 42% for branches. Based on earlier studies, we suggest that the decomposition rates that we determined may be used for estimating Scots pine FWD decomposition in the boreal zone, also in upland forests. Explanatory models accounted for 50.4% and 71.2% of the total variation in FWD decomposition rates when the first two and all years were considered, respectively. The variables most related to FWD decomposition included the initial ash, water extractives and Klason lignin content of litter, and cumulative site precipitation minus potential evapotranspiration. Simulations of inputs and decomposition of Scots pine FWD and needle litter in south boreal conditions over a 60-year period showed that 72 g m −2 of organic matter from FWD vs. 365 g m −2 from needles accumulated in the forest floor. The annual inputs varied from 5.7 to 15.6 g m −2 and from 92 to 152 g m −2 for FWD and needles, respectively. Each thinning caused an increase in FWD inputs, up to 510 g m −2, while the needle inputs did not change dramatically. Because the annual FWD inputs were lowered following the thinnings, the overall effect of thinnings on C accumulation from FWD was slightly negative. The contribution of FWD to soil C accumulation, relative to needle litter, seems to be rather minor in boreal Scots pine forests.

Does Atmospheric NO 3 − Deposition Alter the Abundance and Activity of Ligninolytic Fungi in Forest Soils?

Although field studies have demonstrated an ecosystem-specific effect of experimental atmospheric nitrogen (N) deposition on litter decomposition, a mechanistic understanding of how ligninolytic microbial communities respond to atmospheric deposition is lacking. Because high levels of inorganic N suppress lignin decomposition by some basidiomycetes, it is plausible that the abundance and activity of these key microorganisms underlies differential ecosystem responses of decomposition to atmospheric N deposition. We hypothesize that: (a) atmospheric N deposition will cause an ecosystem-specific reduction in basidiomycete activity and abundance with greatest decreases in ecosystems with lignin-rich forest litter and (b) the abundance of lignin degrading basidiomycetes will be positively correlated with ligninolytic enzyme activity. To test these hypotheses, we measured the effects of experimental N deposition on the potential activity of phenol oxidase enzymes, and the abundance of basidiomycete genes encoding laccase, a primary phenol oxidase enzyme, in three hardwood forests spanning a range of leaf litter lignin content. The black oak-white oak (BOWO) contains high lignin litter, the sugar maple-basswood (SMBW) has low lignin litter, and the sugar maple-red oak (SMRO) is intermediate. An ecosystem by N deposition interaction significantly influenced phenol oxidase activity in the surface soil (P = 0.05), where phenol oxidase activity decreased with increasing experimental N deposition in the BOWO ecosystem. No consistent response to N deposition was evident for surface soil phenol oxidase activity within either the SMRO or SMBW ecosystem. This interaction did not influence laccase gene abundance. Instead, basidiomycete laccase gene abundance was reduced by experimental N deposition (main effect) in surface soil. There was only a weak correlation between basidiomycete laccase gene abundance and potential phenol oxidase enzyme activity, suggesting that the abundance of organisms possessing laccase genes may not control phenol oxidase activity in soil. Our results suggest that the regulation of laccase gene expression may mediate the decomposition response to atmospheric N deposition.

Response of Oxidative Enzyme Activities to Nitrogen Deposition Affects Soil Concentrations of Dissolved Organic Carbon

Recent evidence suggests that atmospheric nitrate (NO 3 − ) deposition can alter soil carbon (C) storage by directly affecting the activity of lignin-degrading soil fungi. In a laboratory experiment, we studied the direct influence of increasing soil NO 3 − concentration on microbial C cycling in three different ecosystems: black oak–white oak (BOWO), sugar maple–red oak (SMRO), and sugar maple–basswood (SMBW). These ecosystems span a broad range of litter biochemistry and recalcitrance; the BOWO ecosystem contains the highest litter lignin content, SMRO had intermediate lignin content, and SMBW leaf litter has the lowest lignin content. We hypothesized that increasing soil solution NO 3 − would reduce lignolytic activity in the BOWO ecosystem, due to a high abundance of white-rot fungi and lignin-rich leaf litter. Due to the low lignin content of litter in the SMBW, we further reasoned that the NO 3 − repression of lignolytic activity would be less dramatic due to a lower relative abundance of white-rot basidiomycetes; the response in the SMRO ecosystem should be intermediate. We increased soil solution NO 3 − concentrations in a 73-day laboratory incubation and measured microbial respiration and soil solution dissolved organic carbon (DOC) and phenolics concentrations. At the end of the incubation, we measured the activity of β-glucosidase, N-acetyl-glucosaminidase, phenol oxidase, and peroxidase, which are extracellular enzymes involved with cellulose and lignin degradation. We quantified the fungal biomass, and we also used fungal ribosomal intergenic spacer analysis (RISA) to gain insight into fungal community composition. In the BOWO ecosystem, increasing NO 3 − significantly decreased oxidative enzyme activities (−30% to −54%) and increased DOC (+32% upper limit) and phenolic (+77% upper limit) concentrations. In the SMRO ecosystem, we observed a significant decrease in phenol oxidase activity (−73% lower limit) and an increase in soluble phenolic concentrations (+57% upper limit) in response to increasing NO 3 − in soil solution, but there was no significant change in DOC concentration. In contrast to these patterns, increasing soil solution NO 3 − in the SMBW soil resulted in significantly greater phenol oxidase activity (+700% upper limit) and a trend toward lower DOC production (−52% lower limit). Nitrate concentration had no effect on microbial respiration or β-glucosidase or N-acetyl-glucosaminidase activities. Fungal abundance and basidiomycete diversity tended to be highest in the BOWO soil and lowest in the SMBW, but neither displayed a consistent response to NO 3 − additions. Taken together, our results demonstrate that oxidative enzyme production by microbial communities responds directly to NO 3 − deposition, controlling extracellular enzyme activity and DOC flux. The regulation of oxidative enzymes by different microbial communities in response to NO 3 − deposition highlights the fact that the composition and function of soil microbial communities directly control ecosystem-level responses to environmental change.

Litter Quality and Climate Decouple Nitrogen Mineralization and Productivity in Chilean Temperate Rainforests

We measured litter quality, N mineralization, air and soil temperatures, soil moisture, and aboveground net primary production (ANPP) at 18 plots in three forest types typical of full‐stature forests found on western slopes of the Cordillera de Piuchué (CP) on Isla de Chiloé in southern Chile. The primary objectives were to examine the range of net N mineralization across forest types that have different vegetation and litter composition, assess probable controls on mineral N production and leaching, and to determine if the relationship between N mineralization and ANPP was similar to that observed in cool temperate northern hemisphere forests. Average annual litter lignin/N ratios were high in the evergreen montane broadleaf forest plots (50:1) and very high in the montane conifer plots (80:1). Net N mineralization during the summer months was correspondingly low (1.2 and 2.2 kg ha−1, respectively). The high litter lignin content and associated low rate of N mineralization can explain the extremely low concentrations of mineral N in soils and upland streams of this region. In the broadleaf evergreen coastal forest plots, lignin/N ratio was lower (23:1) and N mineralized was considerably greater over the same measurement period than in the montane forest plots (14 kg ha−1). In contrast to findings in many cool temperate northern hemisphere forests, ANNP in the broadleaf forest type was high (average 11.1 Mg ha−1 yr−1) in spite of the low net N mineralization rates, and ANPP and net N mineralized were not correlated. Net N mineralization was best correlated with litter quality, soil temperature, and soil moisture content, and ANPP was best correlated with growing degree days.

Hedychium gardnerianum Invasion into Hawaiian Montane Rainforest: Interactions Among Litter Quality, Decomposition Rate, and Soil Nitrogen Availability

Few studies have examined the invasion of understory species into closed-canopy forests and, despite inter-specific differences in litter quality and quantity between understory and dominant canopy trees, the influence of understory invasions on soil nitrogen (N) cycling remains unknown. This paper examines litter quality and decomposition of kahili ginger (Hedychium gardnerianum), an invasive understory herb, to determine the influence of this species on N cycling in a Hawaiian montane rainforest. To examine the potential feedback between increased soil N availability and litter decomposition, litter from the invasive ginger, a native tree, and native tree fern was collected from unfertilized and fertilized plots and decomposed in a reciprocal transplant design. Hedychium litter decomposed faster than litter from the two native species. Across species, decomposition rates were negatively correlated with litter lignin content. Despite rapid decomposition rates of Hedychium litter, soil nitrogen availability and rates of net mineralization in the soil were similar in invaded and uninvaded plots. Nitrogen cycling at this site may be more strongly influenced by native species, which contribute the most to overall stand biomass. A negative effect of fertilization on the decomposition of Hedychium litter suggests that a negative feedback between litter quality and soil N availability may exist over longer timescales.

Decomposition of Avena sterilis litter under arid conditions

The influence of abiotic conditions on Avena sterilis plant litter decomposition was studied at three sites along a topoclimatic gradient in the Judean Desert, Israel. Decomposition of A. sterilis plant litter followed a two-phase pattern: early and rapid mass loss during the winter season followed by a long period of low mass loss during the remaining seasons. Differences in decomposition rates were found between winter and summer, and between material on the soil surface and buried in the soil. There were significant differences in decomposition rates attributable to topoclimatic location, to which the short rainy-winter season contributed significantly. Lignin concentration was found to increase significantly during the rapid phase of decomposition, however a decrease in this value occurred during the remaining dry seasons. Litter nitrogen content oscillated during the study period such that in the buried litter a bi-phasic pattern similar to the litter decomposition in the above-ground litter was obtained. These results suggest that litter decomposition is not affected by climate but by litter lignin content, since an inverse relationship was found between lignin content and the decomposition rate.