Elevated UV-B radiation modifies the extractability of carbohydrates from leaf litter of Quercus robur

We examined the influence of elevated UV-B radiation on the extractability of carbohydrates from leaf litter of Quercus robur. Saplings were exposed to a 30% elevation above the ambient level of erythemally weighted UV-B (280-315 nm) radiation for eight months at an outdoor facility. UV-B radiation was applied under arrays of fluorescent lamps filtered with cellulose diacetate, which transmitted both UV-B and UV-A (315-400 nm) radiation. Saplings were also exposed to elevated UV-A radiation under arrays of polyester-filtered lamps and to ambient radiation under arrays of non-energised lamps. Abscised leaves were collected, ground and sequentially treated with seven solvents in order to fractionate extractable carbohydrates based on the way in which they are held in the cell wall. Elevated UV-B radiation reduced the extractability of carbohydrates from cell walls of Q. robur. Sodium phosphate buffer at pH 7 extracted 10% less total carbohydrate from leaf material exposed during growth to elevated UV-B radiation under cellulose diacetate-filtered lamps than from leaf material grown under polyester-filtered and non-energised lamps. The cumulative amount of carbohydrate released by sequential extraction with phosphate buffer, CDTA, urea and sodium carbonate was between 5.1% and 7.8% lower from leaf material grown under cellulose diacetate-filtered lamps relative to that from leaves grown under non-energised lamps. Abscised leaves were also digested with Driselase, an enzyme mixture extracted from a basidiomycete fungus. No effects of elevated UV radiation were recorded on the amount of carbohydrate released by Driselase digestion. Regression analyses, using data from a previous field decomposition study, suggested that reduced availability of carbohydrates enhanced the colonisation of Q. robur litter by basidiomycete fungi, which then accelerated the decomposition rate of the litter in soil. We recommend that future studies into the effects of UV-B radiation on plant litter decomposition measure not only the concentrations of chemical constituents of litter, but also determine the availability of litter carbon sources to soil microbes, using methods similar to those used here.     

Nonadditive effects of litter mixtures on decomposition and correlation with initial litter N and P concentrations in grassland plant species of northern China

We studied the occurrence of nonadditive effects of litter mixtures on the decomposition (the deviation of decomposition rate of litter mixtures from the expected values based on the arithmetic means of individual litter types) of litters from three plant species (i.e., Stipa krylovii Roshev., Artemisia frigida Willd., and Allium bidentatum Fisch. ex Prokh. & Ikonn.-Gal.) endemic to the grassland ecosystems of Inner Mongolia, northern China and the possible role of initial litter N and P on such effects. We mixed litters of the same plant species that differed in N and P concentrations (four gradients for each species) in litterbags and measured mass losses of these paired mixtures after 30 and 80 days under field conditions. We found the occurrence of positive, nonadditive effects of litter mixtures and showed that the magnitude of the nonadditive effects were related to the relative difference in the initial litter N and P concentrations of the paired litters.     

Nitrogen transfer between decomposing leaves of different N status

Nitrogen movement among microsites is thought to be an important control on patterns of ecosystem-level N cycling. In particular, N transfer between decomposing leaves may explain why litter mixtures sometimes decompose differently than would be predicted from the decomposition dynamics of each species separately. We evaluated how N moves between leaves of differing N status in leaf-pair microcosms. We collected litter from six species of trees from French Guiana (three with high N concentration, three with low) and ¹⁵N-labeled the microbial communities growing on each species. We then established microcosms with one labeled and one unlabeled leaf in a fully factorial design (each species with every species, ¹⁵N on each species) and measured ¹⁵N transfer over 28 days. There was substantial transfer of the ¹⁵N label in all cases, averaging between 15% and 30% of the ¹⁵N originally on the labeled leaf. Net N transfer from high-N to low-N leaves resulted from greater gross ¹⁵N transfer from high-N to low-N leaves than in reverse. Gross ¹⁵N transfer was controlled entirely by the N status of the source leaf, rather than by the difference in N-status of the leaves or by the characteristics of the sink leaf. For example, as much ¹⁵N was transferred from a high-N leaf to another high-N leaf as to a low-N leaf. These results support the assumption from N mineralization theories that microbes at a specific site have first access to that N and therefore control how much N is available to move to other microsites in the soil system. The strength of the gradient between microsites may then control the rate at which available N moves, but not how much N is available to move. If N transfer among different litter species is important for synergistic effects on decomposition of litter mixtures it would not be driven by the N gradient as is often hypothesized, but by the characteristics of the source leaf.     

Combined methods for the determination of lignin and cellulose in leaf litter

Precise and specific methods for the simultaneous quantitative determination of lignin and cellulose are discussed in this paper, enabling the monitoring of even slight changes in the content of lignocellulose in dead plant material during decomposition processes. Results from different leaf litter samples as well as a comparison of the lignocellulose content of freshly fallen leaf litter, leached and microbially inoculated leaf litter, and feces of saprophagous soil animals fed on this food source are presented. The suitability of the described methods for investigating decomposition processes is discussed.     

“Afterlife” effects of mycorrhization on the decomposition of plant residues

The symbiosis with arbuscular mycorrhizal fungi is known to affect growth and tissue quality of plants. Therefore, mycorrhization may also have “afterlife” effects on decomposition dynamics. We tested this hypothesis with plant material of mycorrhized and non-mycorrhized plants of seven grassland species. We found that mycorrhization increased the decomposition rate and interpret this result as a consequence of the enhanced nutritive status of the plant tissue with positive effects on decomposer activity. The turn-over of organic matter and nutrients in ecosystems may therefore be indirectly influenced by the symbiosis with mycorrhizal fungi.     

Impacts of anthropogenic N additions on nitrogen mineralization from plant litter in exotic annual grasslands

Urban regions of southern California receive up to 45 kg N ha^-1 y^-1 from nitrogen (N) deposition. A field decomposition study was done using ¹⁵N-labelled litter of the widespread exotic annual grass Bromus diandrus to determine whether elevated soil N is strictly from N deposition or whether N mineralization rates from litter are also increased under N deposition. Tissue N and lignin concentrations, which are inversely related in field sites with high and low N deposition, determine the rate at which N moves from plant litter to soil and becomes available to plants. The effect of soil N on N movement from litter to soil was tested by placing litter on high and low N soil in a factorial experiment with two levels of litter N and two levels of soil N. The litter quality changes associated with N deposition resulted in faster rates of N cycling from litter to soil. Concentrations of litter-derived N in total N, NH₄⁺, NO₃⁻, microbial N and organic N were all higher from high N/low lignin litter than from low N/high lignin litter. Litter contributed more N to soil NH₄⁺ and microbial N in high N than low N soil. At the end of the study, N mineralized from high N litter on high N soil accounted for 46% of soil NH₄⁺ and 11% of soil NO₃⁻, compared to 35% of soil NH₄⁺ and 6% of soil NO₃⁻ from low N litter on low N soil. The study showed that in high N deposition areas, elevated inorganic soil N concentrations at the end of the summer N deposition season are a result of N mineralized from plant litter as well as from N deposition.     

Reciprocal transfer of carbon and nitrogen by decomposer fungi at the soil-litter interface

We have investigated whether decomposer fungi translocate litter-derived C into the underlying soil while simultaneously translocating soil-derived inorganic N up into the litter layer. We also located and quantified where the translocated C is deposited within the soil aggregate structure. When ¹³C-labeled wheat straw was decomposed on the surface of soil amended with ¹⁵N-labeled inorganic N, we found that C and N were reciprocally transferred by fungi, with a significant quantity (121-151 μg C g⁻¹ whole soil) of litter-derived C being deposited into newly formed macroaggregates (>250 μm sized aggregates). Fungal inhibition reduced fungal biomass and the bidirectional C and N flux by approximately 50%. The amount of litter-derived C found in macroaggregates was positively correlated with litter-associated fungal biomass. This fungal-mediated litter-to-soil C transfer, which to our knowledge has not been demonstrated before for saprophytic fungi, may represent an important mechanism by which litter C enters the soil and becomes stabilized as soil organic matter within the macroaggregate structure.     

杨树刺槐混交林枯落物分解速率的研究

试验研究杨树纯林、刺槐纯林以及杨树刺槐混交林枯落物分解速率结果表明 ,枯落物分解速率随土壤水分的增大而增加 ,土壤含水量达最大持水量时其分解速率达最大值。起始分解速率以 70 %田间持水量为最大 ,其达最大分解速率的时间为 4d ,其他土壤含水量起始分解存在滞后现象 ,达最大分解速率的时间为 7d左右。不同枯落物分解速率为刺槐纯林 >杨树刺槐混交林 >杨树纯林 ,其原因是由刺槐纯林枯落物C N值较高 ,而杨树纯林枯落物C N值较小缘故所致。刺槐与杨树混交可降低枯落物C N值 ,加快枯落物分解速率     

Medium-term effects of corn biochar addition on soil biota activities and functions in a temperate soil cropped to corn

Biochar addition to soil has been generally associated with crop yield increases observed in some soils, and increased nutrient availability is one of the mechanisms proposed. Any impact of biochar on soil organisms can potentially translate to changes in nutrient availability and crop productivity, possibly explaining some of the beneficial and detrimental yield effects reported in literature. Therefore, the main aim of this study was to assess the medium-term impact of biochar addition on microbial and faunal activities in a temperate soil cropped to corn and the consequences for their main functions, litter decomposition and mineralization. Biochar was added to a corn field at rates of 0, 3, 12, 30 tons ha⁻¹ three years prior to this study, in comparison to an annual application of 1 t ha⁻¹.Biochar application increased microbial abundance, which nearly doubled at the highest addition rate, while mesofauna activity, and litter decomposition facilitated by mesofauna were not increased significantly but were positively influenced by biochar addition when these responses were modeled, and in the last case directly and positively associated to the higher microbial abundance. In addition, in short-term laboratory experiments after the addition of litter, biochar presence increased NO₂ + NO₃ mineralization, and decreased that of SO₄ and Cl. However, those nutrient effects were not shown to be of concern at the field scale, where only some significant increases in SOC, pH, Cl and PO₄ were observed.Therefore, no negative impacts in the soil biota activities and functions assessed were observed for the tested alkaline biochar after three years of the application, although this trend needs to be verified for other soil and biochar types.     

施加内生真菌拟茎点霉（Phomopsis sp.）对茅苍术凋落物降解及土壤降解酶活性的影响

通过向盆栽土壤中分别添加茅苍术(Atractylodes lancea(thunb)DC.)凋落物(CK)、内生真菌拟茎点霉(Phomopsissp.编号B3)和灭菌凋落物(MB)、内生真菌和凋落物(WB)、内生真菌和凋落物及菌体发酵液(WBF)、凋落物和灭菌发酵液(WMF)后,定期检测凋落物纤维素、木质素降解率,测定土壤纤维素酶和木质素酶活性,并利用巢式PCR方法跟踪土壤中活体内生真菌B3动态变化,研究了土壤施加内生真菌对掉落的茅苍术残体的分解及土壤降解酶活性影响。结果表明,植物内生真菌离开宿主进入土壤仍具有生理活性,能适应非宿主环境,存活30 d之久。在此期间,内生真菌在富含凋落物的土壤中明显加快纤维素、木质素的降解,在10 d、30 d、60 d取样测定,处理组中WBF和MB的土壤纤维素酶活性显著高于其他处理组,处理组MB和WB的土壤木质素酶活性显著高于其他处理组。     

Nutrient immobilization and release patterns during plant decomposition in a dry tropical bamboo savanna,India

Summary Decomposition and changes in nutrient content of six litter types (leaves, sheaths, roots, twigs, and wood of bamboo, and grass shoots) were studied in nylon net bags for 2 years. The annual weight loss was (% of initial) bamboo leaves 56.5, bamboo sheaths 79.5, bamboo roots 65.8, bamboo twigs 49.6, bamboo wood 31.2, and grass shoots 74.9. Elemental mobility followed the order K>Na>C>P>Ca>N in all components except wood. Generally, an initial increase was followed by a consistent decrease in the contents of N (leaves), P (leaves, roots, wood) and Ca (leaves, roots, grass), and Na (wood). Most of the nutrients were immobilized in the rainy season. C and K contents showed a constant decrease throughout the decomposition period. Materials with a greater C:N ratio (>50) tended to accumulate more nutrients and retain them for longer, except for the bamboo twigs. The critical C:N ratio (at which a net release of N occured) for the leaf material was 25. Litter components with more initial N (sheaths) showed greater weight loss than those with less N (leaves, twigs, and wood). Overall, N and P were lost at the slowest rates while C and K were lost at faster rates. Initial lignin, lignin: N, C:N and C concentrations had a better predictive value for annual weight loss and nutrient release in bivariate relationships. A combination of the initial lignin value and the C: N ratio explained 93% of the variation in annual weight loss. A significant relationship was also observed between the annual weight loss rate and the nutrient mineralization/release rate.     

Microbial activities during the early stage of laboratory decomposition of tropical leaf litters: the effect of interactions between litter quality and exogenous inorganic nitrogen

The comparative decomposition of tropical leaf litters (e.g. Andropogon gayanus, Casuarina equisetifolia, Faidherbia albida) of different qualities was investigated under laboratory conditions during a 60-day incubation period conducted with a typical oxisol. Total CO₂-C, soil inorganic N, microbial biomass (fumigation-extraction), -glucosidase and dehydrogenase activities were determined over the incubation to assess how they responded to the addition of inorganic N (+N). Cumulative CO₂-C evolved from the litter-amended soils was higher than that recorded for the unamended control soil. For the unfertilized treatment (0 N), correlation coefficients calculated between initial chemical data and CO₂ flux during the first day of incubation were r =0.963 for water soluble-C and 0.869 for soluble carbohydrates (P <0.05). At the end of the incubation, the amounts of CO₂-C in the F. albida- and A. gayanus-amended soils were higher than that in the C. equisetifolia-amended treatment. Cumulative net N immobilization increased during the first 30 days of incubation, the amounts being similar for A. gayanus- and C. equisetifolia-amended soil and higher than that recorded in the F. albida-amended treatment. Soil microbial biomass and enzyme activities increased in the litter-amended soils during the first 15 days of incubation and decreased (except for the dehydrogenase activity) thereafter. The addition of inorganic N modified the patterns of CO₂-C respiration and net N immobilization. The magnitude of these modifications varied according to the litter quality. The use of an accurate indicator based on several litter components to predict the amplitude of organic material decomposition is discussed.     

杉木凋落物及其生物质炭对土壤原有有机碳矿化的影响

利用¹³C同位素技术和培养实验,研究不同添加量(0、10、20、30、40、50 g·kg^-1)杉木凋落物和生物质炭(Biochar,BC,350℃热解)对土壤原有有机碳(原SOC)矿化及外源碳自身分解的影响。实验进行28 d,培养温度为25℃,水分保持为60%土壤持水量(Water holding capacity,WHC)。结果表明:凋落物及BC添加显著提高了土壤总CO₂累积排放量(P<0.05),且凋落物的影响更为明显;来源于外源碳及原SOC的CO₂累积排放量均随添加量的增加而增加。培养结束时,凋落物碳(LR-C)分解率为5.71%～13.68%,生物质炭碳(BC-C)分解率仅0.34%～0.50%,凋落物和BC处理下原SOC分解率分别为对照土壤的6.42倍～13.58倍与2.06倍～3.94倍。回归分析发现,2种外源碳处理下原SOC分解率与添加量均呈极显著的线性关系(P<0.01);LR-C分解率亦随添加量的增加而增加;但BC-C分解率则与添加量呈显著的开口向上的抛物线关系(P&...