Cellulose and lignin degradation in forest soils: Response to moisture,temperature, and acidity

The concentration of lignin in plant tissue is a major factor controlling organic matter degradation rates in forest ecosystems. Microbial biomass and lignin and cellulose decomposition were measured for six weeks in forest soil microcosms in order to determine the influence of pH, moisture, and temperature on organic matter decomposition. Microbial biomass was determined by chloroform fumigation; lignin and cellulose decomposition were measured radiometrically. The experiment was designed as a Latin square with soils of pH of 4.5, 5.5, and 6.5 adjusted to 20, 40, or 60% moisture content, and incubated at temperatures of 4, 12, or 24°C. Microbial biomass and lignin and cellulose decomposition were not significantly affected by soil acidity. Microbial biomass was greater at higher soil moisture contents. Lignin and cellulose decomposition significantly increased at higher soil temperatures and moisture contents. Soil moisture was more important in affecting microbial biomass than either soil temperature or soil pH.     

Interactive effects of wildfire and permafrost on microbial communities and soil processes in an Alaskan black spruce forest

Boreal forests contain significant quantities of soil carbon that may be oxidized to CO₂ given future increases in climate warming and wildfire behavior. At the ecosystem scale, decomposition and heterotrophic respiration are strongly controlled by temperature and moisture, but we questioned whether changes in microbial biomass, activity, or community structure induced by fire might also affect these processes. We particularly wanted to understand whether postfire reductions in microbial biomass could affect rates of decomposition. Additionally, we compared the short‐term effects of wildfire to the long‐term effects of climate warming and permafrost decline. We compared soil microbial communities between control and recently burned soils that were located in areas with and without permafrost near Delta Junction, AK. In addition to soil physical variables, we quantified changes in microbial biomass, fungal biomass, fungal community composition, and C cycling processes (phenol oxidase enzyme activity, lignin decomposition, and microbial respiration). Five years following fire, organic surface horizons had lower microbial biomass, fungal biomass, and dissolved organic carbon (DOC) concentrations compared with control soils. Reductions in soil fungi were associated with reductions in phenol oxidase activity and lignin decomposition. Effects of wildfire on microbial biomass and activity in the mineral soil were minor. Microbial community composition was affected by wildfire, but the effect was greater in nonpermafrost soils. Although the presence of permafrost increased soil moisture contents, effects on microbial biomass and activity were limited to mineral soils that showed lower fungal biomass but higher activity compared with soils without permafrost. Fungal abundance and moisture were strong predictors of phenol oxidase enzyme activity in soil. Phenol oxidase enzyme activity, in turn, was linearly related to both ¹³C lignin decomposition and microbial respiration in incubation studies. Taken together, these results indicate that reductions in fungal biomass in postfire soils and lower soil moisture in nonpermafrost soils reduced the potential of soil heterotrophs to decompose soil carbon. Although in the field increased rates of microbial respiration can be observed in postfire soils due to warmer soil conditions, reductions in fungal biomass and activity may limit rates of decomposition.     

Lignin characteristics in soil profiles in different plant communities in a subtropical mixed forest

Aims Lignin is generally considered as an important indicator of soil organic carbon (SOC) storage and dynamics. To evaluate the effects of plant communities and soil depth on soil lignin is critical to better understand forest carbon cycling.Methods We compared lignin content and chemical signature in three soil depths of four major plant communities in a subtropical forest, which located in the north part of Wuling Mountains, China. Lignin was measured using CuO oxidation method.Important findings Both lignin content and its biochemical signature in plant litter varied among communities. However, these differences were mostly no longer exist in the upper soil layers. Lignin chemistry in soils inherited some of the biochemical signature of lignin in litter, but in a diminished magnitude. These results suggest that different plant communities had similar decomposition process with varying rates, caused diminished differences in lignin content and its biochemical signature. Lignin content decreased with soil depth, but the biochemical signature of lignin was not significantly different among soil layers for all communities, which suggests that vertical movement of lignin within the soil profile is very likely a key process causing this similar biochemical signature. These results emphasized the important roles of lignin inputs and soil eluviation in shaping lignin characteristics and distribution in forest soils, which pinpoint the urgent need to consider hydrological processes in studying forest soil carbon cycling.     

Microbial communities acclimate to recurring changes in soil redox potential status

Rapidly fluctuating environmental conditions can significantly stress organisms, particularly when fluctuations cross thresholds of normal physiological tolerance. Redox potential fluctuations are common in humid tropical soils, and microbial community acclimation or avoidance strategies for survival will in turn shape microbial community diversity and biogeochemistry. To assess the extent to which indigenous bacterial and archaeal communities are adapted to changing in redox potential, soils were incubated under static anoxic, static oxic or fluctuating redox potential conditions, and the standing (DNA‐based) and active (RNA‐based) communities and biogeochemistry were determined. Fluctuating redox potential conditions permitted simultaneous CO₂ respiration, methanogenesis, N₂O production and iron reduction. Exposure to static anaerobic conditions significantly changed community composition, while 4‐day redox potential fluctuations did not. Using RNA : DNA ratios as a measure of activity, 285 taxa were more active under fluctuating than static conditions, compared with three taxa that were more active under static compared with fluctuating conditions. These data suggest an indigenous microbial community adapted to fluctuating redox potential.     

微生物法降解木质素的研究进展

生物质是代替石化资源生产能源和化学品的关键资源,木质素作为植物细胞壁的主要成分已经在很多行业中得到了广泛的应用。然而,由于木质素结构复杂且难以降解,成为生物质资源利用的最大障碍,因此,去除或者降解木质素是利用细胞壁中其他成分的关键步骤。许多行业使用有害化学物质降解木质素,严重危害了生态环境,自然界中木质素经常被包括真菌和细菌在内的微生物降解,因此,研究微生物降解木质素的机制为解决这一问题提供了可能性。本文讨论了木质素的化学组成成分,重点讨论了自然界降解木质素的微生物种类及其降解机制,包括各种真菌和细菌的木质素降解活性,描述了由各种微生物特别是白腐真菌、褐腐真菌和细菌产生的木质素降解酶,并展望了今后木质素生物降解的研究和应用的可能方向。     

Redox Fluctuations Frame Microbial Community Impacts on N-cycling Rates in a Humid Tropical Forest Soil

Fluctuating soil redox regimes may facilitate the co-occurrence of microbial nitrogen transformations with significantly different sensitivities to soil oxygen availability. In an upland humid tropical forest, we explored the impact of fluctuating redox regimes on gross nitrogen cycling rates and microbial community composition. Our results suggest that the rapidly fluctuating redox conditions that characterize these upland soils allow anoxic and oxic N processing to co-occur. Gross nitrogen mineralization was insensitive to soil redox fluctuations. In contrast, nitrifiers in this soil were directly affected by low redox periods, yet retained some activity even after 3–6 weeks of anoxia. Dissimilatory nitrate reduction to ammonium (DNRA) was less sensitive to oxygen exposure than expected, indicating that the organisms mediating this reductive process were also tolerant of unfavorable (oxic) conditions. Denitrification was a stronger sink for NO₃⁻ in consistently anoxic soils than in variable redox soils. Microbial biomass and community composition were maintained with redox fluctuation, but biomass decreased and composition changed under static oxic and anoxic soil regimes. Bacterial community structure was significantly correlated with rates of nitrification, denitrification and DNRA, suggesting that redox-control of soil microbial community structure was an important determinant of soil N-cycling rates. Specific nitrogen cycling functional groups in this environment (such as nitrifiers, DNRA organisms, and denitrifiers) appear to have adapted to nutrient resources that are spatially and temporally variable. In soils where oxygen is frequently depleted and re-supplied, characteristics of microbial tolerance and resilience can frame N cycling patterns.     

Microbial community structure in soils with decomposing residues from plants with genetic modifications to lignin biosynthesis

Lignin is a major determinant of the decomposition of plant materials in soils. Advances in transgenic technology have led to the possibility of modifying lignin to improve the pulping properties of plant materials for papermaking. Previous studies have shown that lignin modifications also affect the rate of plant material decay in soil. The aim of this work was to investigate short-term changes in soil microbial community structures when tobacco residues with reduced activity of enzymes in the monolignol pathway decompose. The residues from lignin-modified plants all decomposed faster than unmodified plant materials. The relative proportions of some of the structural groups of microbial phospholipid fatty acids were affected by genetic modifications, especially the proportion of double unsaturated chain fatty acids, indicative of fungi.     

Lignin decomposition is sustained under fluctuating redox conditions in humid tropical forest soils

Lignin mineralization represents a critical flux in the terrestrial carbon (C) cycle, yet little is known about mechanisms and environmental factors controlling lignin breakdown in mineral soils. Hypoxia is thought to suppress lignin decomposition, yet potential effects of oxygen (O₂) variability in surface soils have not been explored. Here, we tested the impact of redox fluctuations on lignin breakdown in humid tropical forest soils during ten‐week laboratory incubations. We used synthetic lignins labeled with ¹³C in either of two positions (aromatic methoxyl or propyl side chain C_β) to provide highly sensitive and specific measures of lignin mineralization seldom employed in soils. Four‐day redox fluctuations increased the percent contribution of methoxyl C to soil respiration relative to static aerobic conditions, and cumulative methoxyl‐C mineralization was statistically equivalent under static aerobic and fluctuating redox conditions despite lower soil respiration in the latter treatment. Contributions of the less labile lignin C_β to soil respiration were equivalent in the static aerobic and fluctuating redox treatments during periods of O₂ exposure, and tended to decline during periods of O₂ limitation, resulting in lower cumulative C_β mineralization in the fluctuating treatment relative to the static aerobic treatment. However, cumulative mineralization of both the C_β‐ and methoxyl‐labeled lignins nearly doubled in the fluctuating treatment relative to the static aerobic treatment when total lignin mineralization was normalized to total O₂ exposure. Oxygen fluctuations are thought to be suboptimal for canonical lignin‐degrading microorganisms. However, O₂ fluctuations drove substantial Fe reduction and oxidation, and reactive oxygen species generated during abiotic Fe oxidation might explain the elevated contribution of lignin to C mineralization. Iron redox cycling provides a potential mechanism for lignin depletion in soil organic matter. Couplings between soil moisture, redox fluctuations, and lignin breakdown provide a potential link between climate variability and the biochemical composition of soil organic matter.     

Microbial community utilization of recalcitrant and simple carbon compounds: impact of oak-woodland plant communities

Little is known about how the structure of microbial communities impacts carbon cycling or how soil microbial community composition mediates plant effects on C-decomposition processes. We examined the degradation of four ¹³C-labeled compounds (starch, xylose, vanillin, and pine litter), quantified rates of associated enzyme activities, and identified microbial groups utilizing the ¹³C-labeled substrates in soils under oaks and in adjacent open grasslands. By quantifying increases in non-¹³C-labeled carbon in microbial biomarkers, we were also able to identify functional groups responsible for the metabolism of indigenous soil organic matter. Although microbial community composition differed between oak and grassland soils, the microbial groups responsible for starch, xylose, and vanillin degradation, as defined by ¹³C-PLFA, did not differ significantly between oak and grassland soils. Microbial groups responsible for pine litter and SOM-C degradation did differ between the two soils. Enhanced degradation of SOM resulting from substrate addition (priming) was greater in grassland soils, particularly in response to pine litter addition; under these conditions, fungal and Gram + biomarkers showed more incorporation of SOM-C than did Gram – biomarkers. In contrast, the oak soil microbial community primarily incorporated C from the added substrates. More ¹³C (from both simple and recalcitrant sources) was incorporated into the Gram – biomarkers than Gram + biomarkers despite the fact that the Gram + group generally comprised a greater portion of the bacterial biomass than did markers for the Gram – group. These experiments begin to identify components of the soil microbial community responsible for decomposition of different types of C-substrates. The results demonstrate that the presence of distinctly different plant communities did not alter the microbial community profile responsible for decomposition of relatively labile C-substrates but did alter the profiles of microbial communities responsible for decomposition of the more recalcitrant substrates, pine litter and indigenous soil organic matter.     

Lignin decomposition along an Alpine elevation gradient in relation to physicochemical and soil microbial parameters

Lignin is an aromatic plant compound that decomposes more slowly than other organic matter compounds; however, it was recently shown that lignin could decompose as fast as litter bulk carbon in minerals soils. In alpine Histosols, where organic matter dynamics is largely unaffected by mineral constituents, lignin may be an important part of soil organic matter (SOM). These soils are expected to experience alterations in temperature and/or physicochemical parameters as a result of global climate change. The effect of these changes on lignin dynamics remains to be examined and the importance of lignin as SOM compound in these soils evaluated. Here, we investigated the decomposition of individual lignin phenols of maize litter incubated for 2 years in‐situ in Histosols on an Alpine elevation gradient (900, 1300, and 1900 m above sea level); to this end, we used the cupric oxide oxidation method and determined the phenols’ ¹³C signature. Maize lignin decomposed faster than bulk maize carbon in the first year (86 vs. 78% decomposed); however, after the second year, lignin and bulk C decomposition did not differ significantly. Lignin mass loss did not correlate with soil temperature after the first year, and even correlated negatively at the end of the second year. Lignin mass loss also correlated negatively with the remaining maize N at the end of the second year, and we interpreted this result as a possible negative influence of nitrogen on lignin degradation, although other factors (notably the depletion of easily degradable carbon sources) may also have played a role at this stage of decomposition. Microbial community composition did not correlate with lignin mass loss, but it did so with the lignin degradation indicators (Ac/Al)s and S/V after 2 years of decomposition. Progressing substrate decomposition toward the final stages thus appears to be linked with microbial community differentiation.     

Seasonal Dynamics of Microbial Community Composition and Function in Oak Canopy and Open Grassland Soils

Soil microbial communities are closely associated with aboveground plant communities, with multiple potential drivers of this relationship. Plants can affect available soil carbon, temperature, and water content, which each have the potential to affect microbial community composition and function. These same variables change seasonally, and thus plant control on microbial community composition may be modulated or overshadowed by annual climatic patterns. We examined microbial community composition, C cycling processes, and environmental data in California annual grassland soils from beneath oak canopies and in open grassland areas to distinguish factors controlling microbial community composition and function seasonally and in association with the two plant overstory communities. Every 3 months for up to 2 years, we monitored microbial community composition using phospholipid fatty acid (PLFA) analysis, microbial biomass, respiration rates, microbial enzyme activities, and the activity of microbial groups using isotope labeling of PLFA biomarkers (¹³C-PLFA). Distinct microbial communities were associated with oak canopy soils and open grassland soils and microbial communities displayed seasonal patterns from year to year. The effects of plant species and seasonal climate on microbial community composition were similar in magnitude. In this Mediterranean ecosystem, plant control of microbial community composition was primarily due to effects on soil water content, whereas the changes in microbial community composition seasonally appeared to be due, in large part, to soil temperature. Available soil carbon was not a significant control on microbial community composition. Microbial community composition (PLFA) and ¹³C-PLFA ordination values were strongly related to intra-annual variability in soil enzyme activities and soil respiration, but microbial biomass was not. In this Mediterranean climate, soil microclimate appeared to be the master variable controlling microbial community composition and function.     

Carbon and nitrogen limitations of soil microbial biomass in desert ecosystems

Microbial biomass nitrogen was measured in unamended (dry) and wetted soils in ten shrubland and grassland communities of the Chihuahuan desert, southern New Mexico, by the fumigation-extraction method. Microbial biomass-N in dry soils was undetectable. Average microbial biomass-N in wetted soils among all plant communities was 15.3 μg g^-1 soil. Highest values were found in the communities with the lowest topographic positions, and the minimum values were detected in the spaces between shrubs. Microbial biomass was positively and significantly correlated to soil organic carbon and extractable nitrogen (NH₄ ⁺ + NO₃ ^-). In a stepwise multiple regression, organic carbon and extractable nitrogen accounted for 40.9 and 5.6%, respectively, of the variance in microbial biomass-N among all the samples. Among communities, the soil microbial biomass was affected by the ratio of carbon to extractable nitrogen. Our results suggest a succession in the control of microbial biomass from nitrogen to carbon when the ratio of carbon to nitrogen decreases during desertification.     

林下植物根系对森林凋落物分解过程中微生物及酶活性的影响

为深入理解进入凋落物层生长的林下植物根系对森林凋落物分解的影响,本研究通过分解袋模拟试验探讨不同生物量多花黑麦草根系对中亚热带常绿阔叶林优势树种四川山矾凋落叶分解中微生物及酶活性的影响.结果表明: 在分解的240 d进程中,无根(N)、少根(L)、多根(M)3种处理下凋落叶表面细菌和真菌群落多样性指数均表现为多根＞少根＞无根处理,并且不同根生物量处理对真菌群落组成和数量的影响较细菌更为显著.随着多花黑麦草生长季结束,生长进入分解袋中的活根生物量逐渐减少,根系对真菌群落组成的影响减小.同一分解阶段,凋落叶表面酸性磷酸酶、β-葡萄糖苷酶、多酚氧化酶、过氧化物酶活性在有根条件下均高于无根条件.表明根系的生长能够改变微生物群落组成与数量,并提高微生物胞外酶活性,从而对分解产生促进作用.     

环境微生物介导的木质素代谢及其资源化利用研究进展

木质素是一种丰富的芳烃生物大分子聚合物,其分解代谢与地球元素循环和生物资源利用密切相关。但由于木质素结构的复杂性和无规则性导致其难以降解,使得木质素降解的研究成为全球碳循环和生物质资源利用研究的难点。近年来,来自不同环境的微生物陆续被发现具有木质素降解能力,并解析出参与木质素分解代谢的多种氧化还原酶。然而对木质素详细的代谢过程仍不十分清楚,因此,探究木质素降解酶系、作用机理和代谢网络是研究微生物代谢木质素机理的关键。本文综述环境中参与木质素降解的微生物,重点解析其木质素解聚酶系组成、分泌机制和木质素的代谢途径,并在此基础上阐明近年来木质素生物转化的最新研究进展,以期为今后环境微生物代谢木质素机理及其资源化利用的研究提供参考。     

Subordinate plant species impact on soil microbial communities and ecosystem functioning in grasslands: Findings from a removal experiment

Despite their low relative abundance, subordinate plant species may have larger impacts on ecosystem functioning than expected, but their role in plant communities remains poorly understood. The aim of this study was to test how subordinate plant species influence the functioning of a species-rich semi-natural grasslands. A plant removal experiment was set-up in the mountain grasslands of the Jura Mountains (Switzerland) to test the impact of subordinate plant species on soil microbial communities and ecosystem functioning. The experiment included three treatments: removal of all subordinate species, partial biomass removal of dominant species, and a no biomass removal control. After 2 years of treatments, we determined soil microbial community (bacteria and mycorrhizal fungi) by T-RFLP analysis and measured litter decomposition, soil respiration, soil inorganic nitrogen (DIN) availability and throughout above-ground biomass production as measures of ecosystem function. The removal of subordinate plant species strongly affected bacterial and weakly influenced mycorrhizal fungi communities and decreased rates of plant litter decomposition, soil respiration and DIN availability with larger effects than the partial loss of dominant biomass. The removal of subordinate plant species did not modify plant community structure, but it did reduce total above-ground biomass production compared to the control plots. Collectively, our findings indicate that the loss of subordinate species can have significant consequences for soil microbial communities and ecosystem functions, suggesting that subordinate species are important drivers of ecosystem properties.     

The effects of tree rhizodeposition on soil exoenzyme activity, dissolved organic carbon, and nutrient availability in a subalpine forest ecosystem

Previous studies have found that root carbon inputs to the soil can stimulate the mineralization of existing soil carbon (C) pools. It is still uncertain, however, whether this “primed” C is derived from elevated rates of soil organic matter (SOM) decomposition, greater C release from microbial pools, or both. The goal of this research was to determine how the activities of the microbial exoenzymes that control SOM decomposition are affected by root C inputs. This was done by manipulating rhizodeposition with tree girdling in a coniferous subalpine forest in the Rocky Mountains of Colorado, USA, and following changes in the activities of nine exoenzymes involved in decomposition, as well as soil dissolved organic C, dissolved organic and inorganic nitrogen (N), and microbial biomass C and N. We found that rhizodeposition is high in the spring, when the soils are still snow-covered, and that there are large ephemeral populations of microorganisms dependent upon this C. Microbial N acquisition from peptide degradation increased with increases in microbial biomass when rhizodeposition was highest. However, our data indicate that the breakdown of cellulose, lignin, chitin, and organic phosphorus are not affected by springtime increases in soil microbial biomass associated with increases in rhizodeposition. We conclude that the priming of soil C mineralization by rhizodeposition is due to growth of the microbial biomass and an increase in the breakdown of N-rich proteins, but not due to increases in the degradation of plant litter constituents such as cellulose and lignin.     

Consistent effects of nitrogen amendments on soil microbial communities and processes across biomes

Ecosystems worldwide are receiving increasing amounts of reactive nitrogen (N) via anthropogenic activities with the added N having potentially important impacts on microbially mediated belowground carbon dynamics. However, a comprehensive understanding of how elevated N availability affects soil microbial processes and community dynamics remains incomplete. The mechanisms responsible for the observed responses are poorly resolved and we do not know if soil microbial communities respond in a similar manner across ecosystems. We collected 28 soils from a broad range of ecosystems in North America, amended soils with inorganic N, and incubated the soils under controlled conditions for 1 year. Consistent across nearly all soils, N addition decreased microbial respiration rates, with an average decrease of 11% over the year‐long incubation, and decreased microbial biomass by 35%. High‐throughput pyrosequencing showed that N addition consistently altered bacterial community composition, increasing the relative abundance of Actinobacteria and Firmicutes, and decreasing the relative abundance of Acidobacteria and Verrucomicrobia. Further, N‐amended soils consistently had lower activities in a broad suite of extracellular enzymes and had decreased temperature sensitivity, suggesting a shift to the preferential decomposition of more labile C pools. The observed trends held across strong gradients in climate and soil characteristics, indicating that the soil microbial responses to N addition are likely controlled by similar wide‐spread mechanisms. Our results support the hypothesis that N addition depresses soil microbial activity by shifting the metabolic capabilities of soil bacterial communities, yielding communities that are less capable of decomposing more recalcitrant soil carbon pools and leading to a potential increase in soil carbon sequestration rates.     

Plant controls on decomposition rates: the benefits of restoring abandoned agricultural lands with native prairie grasses

Plant species can both directly and indirectly affect soil processes in various ways, including through functional traits related to the quantity and chemistry of biomass produced. Understanding how functional traits affect soil processes may be particularly important in restorations that specifically select a target plant community. In this study, I examined how species differing in litter traits alter decomposition, both directly via chemistry and indirectly via influences on soil microclimate. Decomposition dynamics of two old-field grasses were compared with the native prairie grass, Andropogon gerardii, in two Michigan old-fields. Decomposition rates were strongly, negatively related to tissue chemistry, but showed little effect of microclimate differences. Soil bacterial community composition differed between species at one site, while extracellular enzyme activities differed between species at the other site. These findings suggest plant species may be altering microbial community function. Overall, litter chemistry was the dominant factor determining decomposition rates, suggesting that restoring native prairie grasses with recalcitrant litter into grass-dominated old-fields could slow litter decomposition and ultimately lead to changes in soil carbon and nitrogen cycling. Eventually, this could lead to soils that more closely resemble the more organic-rich soils of native prairies and ultimately increase prairie plant community restoration success.     

三种土壤条件下紫茎泽兰根际的酶活性及细菌群落状况

紫茎泽兰被列为我国危害最严重的外侵植物,为探索其侵入机制,以四川省凉山州的3种主要土壤?红壤、黄壤和紫色土为研究对象,比较了根际和非根际土壤(距离根系约20 cm)的酶活性及细菌群落。结果表明,尽管土壤类型不同,根际酶活性(过氧化氢酶、酸性磷酸酶、脲酶和蔗糖酶)及微生物量碳氮显著高于非根际,说明紫茎泽兰的根系生命活动促进根际微生物生长繁殖,数量增加,活性增强,有益于土壤养分供应,促进紫茎泽兰生长,提高生存竞争优势。在不同类型的土壤中,紫茎泽兰根际细菌的分类单元数和主成分方差比非根际降低或无显著变化,说明紫茎泽兰对土壤细菌群落的影响因土壤而异。3种土壤的优势菌株种类差异极大,根际20种优势细菌中仅链霉菌1(Streptomyces1)为共有菌株,非根际仅有绿弯菌(Chloroflexi KD4-96)为共有菌株,说明紫茎泽兰能在细菌群落不同的土壤上生长,具有极强的适应性。但是,3种土壤的优势细菌均为放线菌门、变形菌门和拟杆菌门,合计占细菌总量的60.69%—78.75%;就同一种土壤而言,根际20种优势细菌中有8—11株与非根际相同。因此,土壤类型是决定细菌群落的主要因素,但因紫茎泽兰入侵而发生一定程度的变化。     

Temporal variation overshadows the response of leaf litter microbial communities to simulated global change

Bacteria and fungi drive the decomposition of dead plant biomass (litter), an important step in the terrestrial carbon cycle. Here we investigate the sensitivity of litter microbial communities to simulated global change (drought and nitrogen addition) in a California annual grassland. Using 16S and 28S rDNA amplicon pyrosequencing, we quantify the response of the bacterial and fungal communities to the treatments and compare these results to background, temporal (seasonal and interannual) variability of the communities. We found that the drought and nitrogen treatments both had significant effects on microbial community composition, explaining 2–6% of total compositional variation. However, microbial composition was even more strongly influenced by seasonal and annual variation (explaining 14–39%). The response of microbial composition to drought varied by season, while the effect of the nitrogen addition treatment was constant through time. These compositional responses were similar in magnitude to those seen in microbial enzyme activities and the surrounding plant community, but did not correspond to a consistent effect on leaf litter decomposition rate. Overall, these patterns indicate that, in this ecosystem, temporal variability in the composition of leaf litter microorganisms largely surpasses that expected in a short-term global change experiment. Thus, as for plant communities, future microbial communities will likely be determined by the interplay between rapid, local background variability and slower, global changes.