Nitrogen transfer in litter mixture enhances decomposition rate,temperature sensitivity,and C quality changes

Background and aims

Litter decomposition is a critical process in terrestrial ecosystems and, since in natural conditions plant litter occurs in mixtures, understanding the interactive effects of mixed litter is of great ecological relevance. In this context, we test the hypothesis that N transfer between high quality litter to N-poor substrates are at the base of synergistic interactions, positively affecting litter decay rate, temperature sensitivity, and changes of organic C quality.

Methods

We carried out a manipulative experiment using four organic substrates, encompassing a wide range of biochemical quality (Hedera helix and Quercus ilex leaf litter, cellulose strips and woody sticks), each decomposing either separately or in matched pair mixtures for 360 days. Organic substrates were characterized for mass loss, C and N content and by ¹³C CPMAS NMR to assess biochemical quality changes.

Results

Litter response to mixing was related to the biochemical quality of the components in the mixture: additive when substrates with similarly high (H. helix and Q. ilex) or low (cellulose and wood) N content were paired, but synergistic when substrates with contrasting N content were associated (either of the two leaf litters with either cellulose or wood). Overall, no antagonist effects were observed in this experiment. Interestingly, decomposition of cellulose and wood showed an higher temperature sensitivity, compared to monospecific substrates, when paired with N rich materials. Significant N transfer was found from N rich litter to N poor substrates and ¹³C CPMAS NMR showed rapid changes of C quality of cellulose and wood sticks only when paired with N rich litter.

Conclusions

Our findings support the hypothesis that mixing litters of different quality, with quality expressed in terms of C/N ratio and N content, increases decomposition rate and temperature sensitivity of the lower quality substrates.     

Carbon flux from decomposing wood and its dependency on temperature,wood N2 fixation rate,moisture and fungal composition in a Norway spruce forest

Globally 40–70 Pg of carbon (C) are stored in coarse woody debris on the forest floor. Climate change may reduce the function of this stock as a C sink in the future due to increasing temperature. However, current knowledge on the drivers of wood decomposition is inadequate for detailed predictions. To define the factors that control wood respiration rate of Norway spruce and to produce a model that adequately describes the decomposition process of this species as a function of time, we used an unprecedentedly diverse analytical approach, which included measurements of respiration, fungal community sequencing, N₂ fixation rate, nifH copy number, ¹⁴C‐dating as well as N%, δ¹³C and C% values of wood. Our results suggest that climate change will accelerate C flux from deadwood in boreal conditions, due to the observed strong temperature dependency of deadwood respiration. At the research site, the annual C flux from deadwood would increase by 27% from the current 117 g C/kg wood with the projected climate warming (RCP4.5). The second most important control on respiration rate was the stage of wood decomposition; at early stages of decomposition low nitrogen content and low wood moisture limited fungal activity while reduced wood resource quality decreased the respiration rate at the final stages of decomposition. Wood decomposition process was best described by a Sigmoidal model, where after 116 years of wood decomposition mass loss of 95% was reached. Our results on deadwood decomposition are important for C budget calculations in ecosystem and climate change models. We observed for the first time that the temperature dependency of N₂ fixation, which has a major role at providing N for wood‐inhabiting fungi, was not constant but varied between wood density classes due to source supply and wood quality. This has significant consequences on projecting N₂ fixation rates for deadwood in changing climate.     

Effects of nutrient enrichment on the decomposition of wood and associated microbial activity in streams

1. We determined the effects of nutrient enrichment on wood decomposition rates and microbial activity during a 3‐year study in two headwater streams at Coweeta Hydrologic Laboratory, NC, U.S.A. After a 1‐year pretreatment period, one of the streams was continuously enriched with inorganic nutrients (nitrogen and phosphorus) for 2 years while the other stream served as a reference. We determined the effects of enrichment on both wood veneers and sticks, which have similar carbon quality but differ in physical characteristics (e.g. surface area to volume ratios, presence of bark) that potentially affect microbial colonisation and activity. 2. Oak wood veneers (0.5 mm thick) were placed in streams monthly and allowed to decompose for approximately 90 days. Nutrient addition stimulated ash‐free dry mass loss and increased mean nitrogen content, fungal biomass and microbial respiration on veneers in the treatment stream compared with the reference. The magnitude of the response to enrichment was great, with mass loss 6.1 times, and per cent N, fungal biomass and microbial respiration approximately four times greater in the treatment versus reference stream. 3. Decomposition rate and nitrogen content of maple sticks (ca. 1–2 cm diameter) also increased; however, the effect was less pronounced than for veneers. Wood response overall was greater than that determined for leaves in a comparable study, supporting the hypothesis that response to enrichment may be greater for lower quality organic matter (high C : N) than for higher quality (low C : N) substrates. 4. Our results show that moderate nutrient enrichment can profoundly affect decomposition rate and microbial activity on wood in streams. Thus, the timing and availability of wood that provides retention, structure, attachment sites and food in stream ecosystems may be affected by nutrient concentrations raised by human activities.     

Pine Forest Floor Carbon Accumulation in Response to N and PK Additions: Bomb <Superscript>14</Superscript>C Modelling and Respiration Studies

The addition of nitrogen via deposition alters the carbon balance of temperate forest ecosystems by affecting both production and decomposition rates. The effects of 20 years of nitrogen (N) and phosphorus and potassium (PK) additions were studied in a 40-year-old pine stand in northern Sweden. Carbon fluxes of the forest floor were reconstructed using a combination of data on soil ¹⁴C, tree growth, and litter decomposition. N-only additions caused an increase in needle litterfall, whereas both N and PK additions reduced long-term decomposition rates. Soil respiration measurements showed a 40% reduction in soil respiration for treated compared to control plots. The average age of forest floor carbon was 17 years. Predictions of future soil carbon storage indicate an increase of around 100% in the next 100 years for the N plots and 200% for the NPK plots. As much as 70% of the increase in soil carbon was attributed to the decreased decomposition rate, whereas only 20% was attributable to increased litter production. A reduction in decomposition was observed at a rate of N addition of 30 kg C ha^–1 y^–1, which is not an uncommon rate of N deposition in central Europe. A model based on the continuous-quality decomposition theory was applied to interpret decomposer and substrate parameters. The most likely explanations for the decreased decomposition rate were a fertilizer-induced increase in decomposer efficiency (production-to-assimilation ratio), a more rapid rate of decrease in litter quality, and a decrease in decomposer basic growth rate.     

模拟氮沉降对华西雨屏区天然常绿阔叶林凋落叶分解过程中基质质量的影响

为理解氮沉降对华西雨屏区天然常绿阔叶林凋落物分解过程的影响,采用立地控制实验和凋落物分解袋法,研究了低氮沉降(L,50 kg N hm~(-2)a~(-1))、中氮沉降(M,150 kg N hm~(-2)a~(-1))和高氮沉降(H,300 kg N hm~(-2)a~(-1))对华西雨屏区天然常绿阔叶林凋落叶分解过程中基质质量的影响。结果表明:N沉降抑制了凋落叶的分解,并随着N沉降量的增加,抑制作用增强。N沉降遏制了凋落叶的C、N释放和纤维素降解,促进了P释放。N沉降提高了凋落叶的C/P比,中氮和高氮处理提高了凋落叶C/N比。N沉降显著增加了凋落叶N、木质素和纤维素的含量,分解1年后,各N沉降处理的木质素/N和纤维素/N均显著高于对照。N沉降提高了质量残留率与C/N、木质素/N和纤维素/N的相关性,降低了与C/P的相关性。可见,模拟N沉降显著影响了华西雨屏区天然常绿阔叶林凋落叶分解过程中的基质质量,进而影响了凋落叶的分解过程。     

Chemical Changes During 6 Years of Decomposition of 11 Litters in Some Canadian Forest Sites. Part 2. 13C Abundance, Solid-State 13C NMR Spectroscopy and the Meaning of “Lignin”

There is still a poor understanding of how changes in the organic composition of litter contribute to slowing or even cessation of decomposition. Using ¹³C nuclear magnetic resonance (NMR) spectroscopy of samples from the Canadian Intersite Decomposition Experiment (CIDET), we asked whether increasing lignin per se could account for the well-known increase in acid-unhydrolyzable residue (AUR), and secondly, using three litters from four sites with different mean annual temperatures, whether changes in organic composition would follow similar trajectories with C mass loss. At 6 years, there was 16–39% C remaining for 10 foliar litters and wood blocks at a site with rapid initial decomposition, and higher amounts remaining for three species at three colder sites. ¹³C NMR spectra obtained with rapid cross-polarization (CP) mainly showed increasing similarity among the foliar litters, although wood showed little change in composition. Foliage generally showed loss of O- and di-O-alkyl C, mainly from carbohydrate, and increase in alkyl, aromatic, phenolic and carboxyl C. However, O-alkyl C loss was limited, especially for litters with slow initial decomposition, and many litters showed relatively small changes in intensity distribution. Quantitative ¹³C (“BD”) spectra showed similar trends, but even smaller changes in C composition, and 6-year CP difference spectra showed that C was lost across the whole range of structures. Changes in δ¹³C were small and variable, but could be correlated to some extent with loss of carbohydrates versus tannins. Lignin was not selectively preserved, and the increase of resistant structures derived from lignin, tannins, and cutin collectively accounts for increasing AUR. Compositional changes of NMR C fractions across sites with different temperatures were small and inconsistent, likely due to the influence of other site factors; however, changes in their contents did largely follow consistent trajectories with %C remaining.     

Decomposition rates of bryophytes in managed boreal forests: influence of bryophyte species and forest harvesting

The slow decomposition rate of boreal forest floor bryophytes contributes both to maintaining high soil C reserves as well as affecting conditions for tree growth by maintaining excessively high soil water content, cooling the soil and slowing nutrient cycles. In this study, mass loss of three bryophyte species (Pleurozium schreberi, Sphagnum capillifolium, S. fuscum) was measured in unharvested, partial cut and low-retention cut forest blocks. Mesh decomposition bags containing the three species and wood sticks were placed at two depths in colonies of either P. schreberi or S. capillifolium (environment) in the three harvest treatments and retrieved after two growing seasons. Mass loss was primarily related to substrate type (P. schreberi > S. capillifolium > wood sticks > S. fuscum) and secondarily to depth. Harvest treatment and environment (P. schreberi or S. capillifolium) only weakly affected sphagna mass loss. The weak effect of harvest treatment suggests that conditions created by low retention cuts do not to stimulate decomposition in this system and are not important enough to stimulate carbon loss, or to counteract paludification. On the other hand, the strong effect of bryophyte type indicates that conditions affecting bryophyte colonization and succession are of great importance in driving carbon and nutrient cycles.     

Quality comparison of umbilical cord blood cryopreserved with conventional versus automated systems

Umbilical cord blood (CB) banks usually freeze and store CB for clinical transplantation using conventional controlled-rate freezer or the automated BioArchive system. The aim of this study is to compare the quality of CB cryopreserved with conventional and automated methods and to make clear the cause of the quality difference between the two methods. The experiment used 80 CB units: 40 were conventionally cryopreserved and the remainder were cryopreserved with a BioArchive. After thawing, the following measures of CB quality were compared: recovery rates of cell count, cell viability of total nucleated cells (TNCs), mononuclear cells (MNCs), and CD34+ cells, as well as colony-forming unit-granulocyte/macrophage (CFU-GM) content. Additionally, processing and storage records were reviewed to quantify the number of exposures of CB units at room temperature (transient warming event, TWE), which was analyzed in relation to CB quality. MNC and CD34+ cell viability were as follows: MNC, 78.2% ± 6.8% (conventional), 81.7% ± 7.2% (automated); CD34+ cell, 90.6% ± 6.9% (conventional), 94.7% ± 3.5% (automated). The absolute CFU-GM content per CB unit was 7.1 × 10⁵ ± 5.9 × 10⁵ with conventional cryopreservation and 12.3 × 10⁵ ± 12.0 × 10⁵ with automated cryopreservation. CBs cryopreserved with BioArchive showed significantly higher MNC and CD34+ cell viability, and CFU-GM content than those conventionally cryopreserved. The CB quality comparison depending on the amount of TWEs showed no significant quality difference between groups that were more exposed to TWEs and groups that were less exposed. CBs cryopreserved with BioArchive were of higher quality than conventionally cryopreserved CBs, and the cause of quality difference might be due to the difference of freezing conditions rather than the TWE effect.     

What happens to allochthonous material that falls into streams? A synthesis of new and published information from Coweeta

1. ,One of two things can happen to allochthonous material once it enters a stream: it can be broken down or it can be transported downstream. The efficiency with which allochthonous material is used is the result of these two opposing factors: breakdown and transport. 2. ,The present synthesis of new and published studies at Coweeta Hydrologic Laboratory compares biological use versus transport for four categories of particulate organic material: (1) large wood (logs); (2) small wood (sticks); (3) leaves; and (4) fine particulate organic matter (FPOM). 3. ,Over 8_years, logs showed no breakdown or movement. 4. ,The breakdown rate of sticks (≤₃_cm diameter) ranged from 0.00017 to 0.00103_day^?1, while their rate of transport, although varying considerably with discharge, ranged from 0 to 0.1_m_day^?1. 5. ,Based on 40 published measurements, the average rate of leaf breakdown was 0.0098_day^?1. The leaf transport rate depended on stream size and discharge. 6. ,The average respiration rate of FPOM was 1.4_mg_O₂_g_AFDM^?1_day^?1 over a temperature range of 6–22_°C, which implies a decomposition rate of 0.00104_day^?1. Transport distances of both corn pollen and glass beads, surrogates of natural FPOM, were short (<_10_m) except during high discharge. 7. , Estimates of transport rate were substantially larger than the breakdown rates for sticks, leaves and FPOM. Thus, an organic particle on the stream bottom is more likely to be transported than broken down by biological processes, although estimates of turnover length suggest that sticks and leaves do not travel far. However, once these larger particles are converted to refractory FPOM, either by physical or biological processes, they may be transported long distances before being metabolized.     

Transformation and stabilization of pyrogenic organic matter in a temperate forest field experiment

Pyrogenic organic matter (PyOM) decomposes on centennial timescale in soils, but the processes regulating its decay are poorly understood. We conducted one of the first studies of PyOM and wood decomposition in a temperate forest using isotopically labeled organic substrate, and quantified microbial incorporation and physico‐chemical transformations of PyOM in situ. Stable‐isotope (¹³C and ¹⁵N) enriched PyOM and its precursor wood were added to the soil at 2 cm depth at ambient (N0) and increased (N+) levels of nitrogen fertilization. The carbon (C) and nitrogen (N) of added PyOM or wood were tracked through soil to 15 cm depth, in physically separated soil density fractions and in benzene polycarboxylic acids (BPCA) molecular markers. After 10 months in situ, more PyOM‐derived C (>99% of initial ¹³C‐PyOM) and N (90% of initial ¹⁵N‐PyOM) was recovered than wood derived C (48% of ¹³C‐wood) and N (89% under N0 and 48% under N+). PyOM‐C and wood‐C migrated at the rate of 126 mm yr^?1 with 3–4% of PyOM‐C and 4–8% of wood‐C recovered below the application depth. Most PyOM C was recovered in the free light fraction (fLF) (74%), with 20% in aggregate‐occluded and 6% in mineral associated fractions – fractions that typically have much slower turnover times. In contrast, wood C was recovered mainly in occluded (33%) or dense fraction (27%). PyOM addition induced loss of native C from soil (priming effect), particularly in fLF (13%). The total BPCA‐C content did not change but after 10 months the degree of aromatic condensation of PyOM decreased, as determined by relative contribution of benzene hexa‐carboxylic acid (B6CA) to the total BPCA C. Soil microbial biomass assimilated 6–10% of C from the wood, while PyOM contributions was negligible (0.14–0.18%). The addition of N had no effect on the dynamics of PyOM while limited effect on wood.     

Biochemical changes assessed by 13C-CPMAS NMR spectroscopy control fungal growth on water extracts of decaying plant litter

The mechanistic bases of saprotrophic fungal dynamics in soil are not fully clarified. By assessing hyphal density and radial expansion of Aspergillus niger on extracts 45 plant litter types (15 species at 3 decomposition stages), encompassing a broad range of organic quality, we investigated how changes in litter biochemistry affected fungal growth. Plant litter were characterized by classic proximate chemical analyses (total C and N, labile C, cellulose and lignin content, C/N and lignin/N ratios) and, at molecular level, by solid-state ¹³C-CPMAS NMR. The growth of A. niger decreased during the decomposition process over all organic matter types, consistently with the well-known disappearance of this species during the early successional stages. The litter suitability as a substrate to A. niger progressively decreased during decomposition, both considering proximate parameters and C types corresponding to spectral regions, with the latter being also invariably predictive of fungal growth over the 45 substrates. A. niger growth was positively associated with the content of labile C, and with di-O-alkyl C and O-alkyl C spectral regions, but negatively with lignin content and with methoxyl C region. Our results suggest that organic matter quality may control saprotrophic fungal dynamics, at least for the tested species.     

模拟氮沉降对华西雨屏区苦竹林凋落物基质质量的影响

凋凋落物基质质量是影响凋落物分解速率的决定性因子之一,本研究旨在探究模拟氮沉降对苦竹林凋落物基质质量的影响。2007年11月至2010年12月每月一次连续对华西雨屏区苦竹人工林进行了模拟氮沉降试验,施氮水平分别为：低氮（5 g N?m–2?a–1）,中氮（15 g N?m–2?a–1）和高氮（30 g N?m–2?a–1）。在施氮2 a后,于2010年1月开始收集各样方的凋落物样品,连续收集12个月,分析测定凋落物基质质量。结果表明：施氮显著增加了凋落叶中N、P元素含量,中氮处理显著增加了凋落枝中N元素含量,中氮和高氮处理均显著增加了凋落枝中P元素含量;施氮对凋落物中C元素含量影响很微弱,显著降低了凋落叶中的C/N,中氮处理显著降低了凋落枝中的C/N,对木质素和纤维素含量均未造成显著影响。由于模拟氮沉降增加了苦竹凋落物的N、P含量,降低了其C/N,因此氮沉降可能会促进苦竹凋落物的初期分解速率。     

Cardiac Myosin-binding protein C modulates the tuning of the molecular motor in the heart

Cardiac myosin binding protein C (cMyBP-C) is an important regulator of cardiac contractility. Its precise effect on myosin cross-bridges (CBs) remains unclear. Using a cMyBP-C^−/− mouse model, we determined how cMyBP-C modulates the cyclic interaction of CBs with actin. From papillary muscle mechanics, CB characteristics were provided using A. F. Huxley's equations. The probability of myosin being weakly bound to actin was higher in cMyBP-C^−/− than in cMyBP-C^+/+. However, the number of CBs in strongly bound, high-force generated state and the force generated per CB were lower in cMyBP-C^−/−. Overall CB cycling and the velocity of CB tilting were accelerated in cMyBP-C^−/−. Taking advantage of the presence of cMyBP-C in cMyBP-C^+/+ myosin solution but not in cMyBP-C^−/−, we also analyzed the effects of cMyBP-C on the myosin-based sliding velocity of actin filaments. At baseline, sliding velocity and the relative isometric CB force, as determined by the amount of α-actinin required to arrest thin filament motility, were lower in cMyBP-C^−/− than in cMyBP-C^+/+. cAMP-dependent protein kinase-mediated cMyBP-C phosphorylation further increased the force produced by CBs. We conclude that cMyBP-C prevents inefficient, weak binding of the myosin CB to actin and has a critical effect on the power-stroke step of the myosin molecular motor.     

13C NMR study of pine needle decomposition

The quality of substrates in plantation forest litter, and their chemistry, can influence decomposition and N cycling. We studied the decomposition of Pinus radiata D. Don needles suspended on branches in windrows, for 3 yr after clear-cutting, using improved solid-state ¹³C NMR and chemical analysis. The NMR spectra suggested that the concentration of condensed tannins was 12–22%, and showed they were chemically altered during the period 4–12 months after clear-cutting. The spectra showed no evidence for further chemical modification of the tannins during the second or third years. Data for P. radiata needle decomposition in New Zealand indicated rapid loss of mass in the first 3 months, and condensed tannins did not appear to prevent mineralization of C or N. The tannin and lignin concentrations increased with decomposition of the needles, which was consistent with the early mineralization of readily available C compounds.     

Effect of wood density and water permeability on wood decomposition rates of 32 Bornean rainforest trees

Aims A better understanding of wood litter decomposition is essential for predicting responses of forest ecosystems to global climate change. Recent studies suggest that chemical properties of wood litters, rather than physical ones such as wood density, are more important for interspecific differences in wood decomposition rates. However, empirical data are still limited, especially for tropical trees. In addition, decomposition rate of wood litter often varies with time, which makes interspecific comparison difficult. We studied the wood decomposition of 32 rainforest trees to elucidate (i) the degree of interspecific variation in wood decomposition rate of a given size and configuration and (ii) if initial wood density and water permeability are consistent predictors of the overall decomposition rate and its pattern over time.Methods A common garden decomposition experiment was conducted in a tropical rainforest in Malaysian Borneo for 32 native tree species. Small wood sticks were set on the forest floor and the weight loss was monitored monthly for 2.7 years.Important findings We found large variation in the wood decomposition rate (a 49-fold range), suggesting that we need to consider this variation when calculating community-level carbon dynamics of tropical rain forests. The physical traits of wood, i.e. wood density and water permeability, were related to wood decomposition rate and its pattern over time. Decomposition half-time related positively and negatively to initial wood density and water permeability, respectively. The time-dependent-rate model fitted better for 18 species (56% of the study species) that had higher water permeabilities than the others, suggesting that micelle porosity in wood relates to temporal changes in decomposition rate.     

湿地枯落物分解及其对全球变化的响应

综述了当前湿地枯落物分解及其对全球变化响应的研究动态。湿地枯落物分解研究已随研究方法的改进而不断深化;当前湿地枯落物分解过程研究主要集中在有机质组分和元素含量变化特征的探讨上;湿地枯落物分解同时受生物因素（即枯落物性质以及参与分解的异养微生物和土壤动物的种类、数量和活性等）和非生物因素（即枯落物分解过程的外部环境条件,包括气候条件、水分条件、酸碱度与盐分条件以及湿地沉积的行为与特征等）的制约;模型已成为湿地枯落物分解研究的重要手段,对其研究也在不断深化。还讨论了湿地枯落物分解对于全球变化的响应,指出全球变暖、大气CO2浓度上升、干湿沉降及其化学组成改变可能对枯落物分解产生的直接、间接和综合影响。最后,指出了当前该领域研究尚存在的问题以及今后亟需加强的几个研究方面。     

Effect of nitrogen addition on decomposition of Calamagrostis angustifolia litters from freshwater marshes of Northeast China

Wetland ecosystems store a large amount of organic carbon (C) in soils, due to the slow decomposition rates of plant litter and soil organic matter. Increased nitrogen (N) availability induced by human activities and global warming may accelerate litter decomposition and affect soil organic C dynamics in wetlands. In the present study, we investigated the effect of N addition on decomposition of Calamagrostis angustifolia litters from freshwater marshes in the Sanjiang Plain of Northeast China under field and laboratory conditions. First, we assessed the changes in initial litter chemical composition and subsequent decomposition following three years of N addition at the rate of 24 g N m⁻² year⁻¹ under field conditions. Our results showed that N addition increased litter N concentration and decreased C/N ratio, and thus stimulated litter decomposition. Secondly, we examined the effect of increased N availability (0, 25, 50 and 100 mg N g⁻¹ litter) on litter decomposition under laboratory conditions. Increased exterior N availability also enhanced microbial respiration and increased litter mass loss under both waterlogging and non-waterlogging conditions. In addition, waterlogging conditions inhibited microbial respiration and suppressed litter mass loss. These findings demonstrated that N addition increased litter decomposition rates through improved litter quality and enhanced microbial activity in freshwater marshes of Northeast China. This implies that increased N availability accelerates litter decomposition rates, and thus may cause substantial losses of soil C and diminish and even reverse the C sink function of wetlands in the Sanjiang Plain of Northeast China.     

黄土高原典型草原优势植物凋落物分解及养分释放对氮添加的响应

利用原位分解袋法研究了黄土高原典型草原优势植物长芒草(Stipa bungeana)和阿尔泰狗娃花(Heteropappus altaicus)凋落物的养分释放过程对氮添加的响应,试验周期为1 a。设置6个氮添加水平,分别为N0(0)、N1(1.15 g N m~(-2)a~(-1))、N2(2.3 g N m~(-2)a~(-1))、N3(4.6 g N m~(-2)a~(-1))、N4(9.2 g N m~(-2)a~(-1))和N5(13.8 g N m~(-2)a~(-1)),氮素类型为尿素((NH_2)_2CO)。结果表明:(1)氮添加处理两年显著改变了长芒草和阿尔泰狗娃花凋落物的初始化学性质。随着氮梯度的增加,凋落物的N(氮)含量逐渐增加,木质素含量先增加后下降,C/N(碳氮比)和木质素/N降低,C(碳)、P(磷)和C/P(碳磷比)没有显著的差异。(2)氮处理对长芒草和阿尔泰狗娃花凋落物的分解速率的影响不显著。长芒草和阿尔泰狗娃花凋落物C含量随分解时间整体为降低过程,N和P含量总体上为增加过程,且整个分解过程中N含量各处理间差异显著。(3)氮处理对长芒草和阿尔泰狗娃花凋落物C和P的分解基本无影响,两种元素都呈现释放过程。氮处理对凋落物的N残留率有显著的影响,在N1—N3(1.15—4.6 g/m~2)处理下的长芒草凋落物N残留率高于其他处理,且呈现富集过程;而阿尔泰狗娃花凋落物中的N呈现富集-释放过程。在土壤养分贫瘠的黄土高原典型草原,适量的氮输入可以促进系统的固氮。     

Organic matter accumulation following fires in a moorland soil chronosequence

Severe fires in 1957 and 1976 removed the vegetation and soil organic matter from the litter layers and organic horizons of soils at two adjacent moorland sites leaving exposed the uppermost mineral horizon of the soil. In the period since, plant recolonization and soil organic matter reaccumulation have occurred to give a chronosequence. Assuming no major changes in the carbon and nitrogen content of the unburned soil since 1957, the rates of accumulation of soil C and N were estimated to be 0.035 kg C m^–2 y^–1 and 0.001 kg N m^–2 y^–1 over the first 19 years, and 0.50 kg C m^–2 y^–1 and 0.023 kg N m^–2 y^–1 over the period from 19 to 38 years after burning. Solid-state ¹³C NMR (cross-polarization, magic angle spinning ¹³C nuclear magnetic resonance spectroscopy) showed that the ratio of alkyl- and methyl-C-to-O-alkyl-C increased with stage of decomposition and in the unburned soil with decreasing particle-size. For the organic matter that had reaccumulated in the 1957-burned soil, the alkyl-C-to-O-alkyl-C ratio of the > 2000 μm and 2000–250 μm particle-size fractions were greater than those of the corresponding size fractions from the unburned soil, indicating that the reaccumulated soil organic matter was subject to decomposition but limited fragmentation or comminution.     

Linking temperature sensitivity of soil organic matter decomposition to its molecular structure,accessibility, and microbial physiology

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme‐kinetic hypothesis suggests that decomposition of low‐quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high‐quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme‐substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low‐density fraction (LF) which represents readily accessible, mineral‐free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30‐days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6–1.8 g cm^?3) and bulk soil was measured by solid‐state ¹³C‐NMR. Decomposition Q₁₀ was significantly correlated with the abundance of aromatic plus alkyl‐C relative to O‐alkyl‐C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two‐ to three‐fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C‐use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme‐kinetic interpretation of widely observed C quality‐temperature relationship for short‐term decomposition. Factors controlling long‐term decomposition Q₁₀ are more complex due to protective effect of mineral matrix and thus remain as a central question.