Landscape-level variation in temperature sensitivity of soil organic carbon decomposition

We examined landscape-level variation in temperature sensitivity of labile SOC across 71 sites at a central North American grassland. The observed range in activation energy of decomposition (E_a), an index of temperature sensitivity, was as great at the landscape scale as has been observed at the continental scale. E_a was lower for soils with more labile C, consistent with the ‘Carbon quality-temperature’ hypothesis. Soil pH explained 67% of the variation in E_a. Although there are strong environmental correlates with the E_a of SOC decomposition at landscape scales, the amount of variation within landscapes could confound regional- to global-scale predictions of the response of soil C to warming.     

Estimating Rainy Season Nitrous Oxide and Methane Fluxes Across Forest and Pasture Landscapes in Costa Rica

The objectives of this research were to estimate exchanges of CH4 and N2O, both radiatively active gases, between soil and atmosphere on hilltop, slope and swale hillslope positions of northeastern Costa Rica; and to assess the importance of accounting for topography in making areal estimates across hilly terrain. Emission rates from soils were measured during the rainy season on three hillslope positions of both actively grazed pastures and primary forests. Emission rates from pasture and forest sites were significantly different for both gases. Differences between slope positions, though notable, were not significantly different for CH4, but were significantly different for N2O. The forest landscape was partitioned with GIS methods into hilltop, slope and swale topographic positions. The calculated areas for each of these were multiplied by their respective emission rates to calculate overall flux from the entire forested area of 618 ha. Nitrous oxide flux ranged from 16,181 to 17,100 g N d-1. Similarly, CH4 flux ranged from -6,201 to -6,658 g CH4d-1. Errors associated with both estimating mean emission rates for each hillslope position and judgmental errors in partitioning the landscape into hillslope positional classes are important to making landscape-scale estimates of flux.     

MINErosion 4: Using measurements from a tilting flume-rainfall simulator facility to predict erosion rates from post-mining catchments/landscapes in Central Queensland,Australia

The use of draglines to remove overburden in Queensland opencut mines, results in landscapes that consist of long parallel tertiary overburden spoil-piles that are generally highly saline, dispersive, and highly erodible. The height of these spoil-piles may exceed 50–60 m above the original landscapes and the slopes are at the angle of repose of around 75% or 37°. Legislation and public opinion require that these highly disturbed open-cut post-mining landscapes should be satisfactorily rehabilitated into an approved post-mining land use with acceptable erosion rates. Therefore, these slopes must be reduced before the landscape can be rehabilitated. The most expensive component of the rehabilitation process is the re-shaping and preparation of the overburden to create a suitable landscape for vegetation growth. As soils and overburden varies greatly in their erodibilities, the extent and cost of earthworks can be minimized, and rehabilitation failures avoided, if soil erosion from designed landscapes can be predicted using laboratory-based parameters prior to construction of these landscapes. This paper describes the development of a model for that purpose.A catchment or landscape erosion model MINErosion 4 was developed by upscaling the existing hillslope model MINErosion 3 (So, et al., 2018) and integrate it with both ESRI ArcGIS 10.3 or QGIS 3.16 (freeware), to predict event based and mean annual erosion rate from a postmining catchment or landscape. MINErosion 3 is a model that can be used to predict event and annual erosion rates from field scale hillslopes using laboratory measured erodibility parameters or routinely measured soil physical and chemical properties, and to derive suitable landscape design parameters (slope gradient, slope length and vegetation cover) that will result in acceptable erosion rates. But it cannot be used to predict the sediment delivery from catchments or landscapes. MINErosion 4 was validated against data collected on three instrumented catchments (up to 0.91 ha in size) on the Curragh mine site in Central Queensland. The agreement between predicted (Y) and measured (X) values were very good with the regression equation of Y = 0.92X and an R² value of 0.81 for individual storm events, and Y = 1.47X and an R² value of 0.73 for the average annual soil loss. This is probably the first time that a catchment scale erosion is successfully predicted from laboratory measured erodibility parameters.     

磷肥减施对玉米根系生长及根际土壤磷组分的影响

【目的】 我国农业过量和不合理施用磷肥现象普遍存在,导致磷资源的浪费,对环境也造成潜在威胁。研究减少磷肥用量对玉米产量、根系形态及根际中磷转化特征的影响,为集约化农业生产体系中磷肥合理施用提供技术基础。 【方法】 在河北省衡水小麦玉米轮作体系下连续三年进行了田间试验,在冬小麦季设置4个P₂O₅用量处理:0、112.5、150.0、187.5 kg/hm2,收获后在原处理小区免耕播种夏玉米。利用WinRHIZO根系分析系统分析获取根长、直径等数据,测定玉米籽粒产量、生物量和地上部磷含量及根际土壤中磷形态等指标。 【结果】 与农民习惯磷肥用量(P₂O₅187.5 kg/hm2)相比,3年磷肥用量减施20%~40%处理(P₂O₅150和112.5 kg/hm2),玉米籽粒产量、根系长度与直径和土壤有效磷含量尚未发生明显变化。但3年不施磷处理,根际土壤有效形态磷含量和玉米籽粒产量开始出现下降趋势。2009年和2010年玉米收获期,不施磷肥处理根际土壤有机磷含量低于非根际土壤。2008年玉米苗期和收获期土壤有机磷分组中,中等活性有机磷含量最高;磷肥减施20%~40%处理苗期根际中中等活性有机磷含量显著低于非根际土壤。土壤无机磷形态分组研究发现:从玉米苗期到收获期,各磷肥处理根际和非根际土壤中Ca₂-P下降明显;而不同磷肥处理间土壤中Ca₁₀-P、Ca₈-P、O-P (闭蓄态磷)、Al-P和Fe-P含量差异不显著。减施磷肥处理2008年玉米苗期根际土壤微生物量P含量较非根际土壤高;与习惯施肥量相比,磷肥减施未明显降低根际土壤微生物量磷。 【结论】 在华北小麦玉米轮作种植体系下,在土壤肥力水平较高地区,连续3年将小麦季磷肥的习惯用量减少20%~40%,对夏玉米产量、根系形态以及根际土壤无机磷、有机磷、微生物量磷含量影响尚不明显,因此,该地区磷肥施用量可从习惯用量的P₂O₅180 kg/hm2减至112.5 kg/hm2。     

Contribution of dissolved organic matter to carbon storage in forest mineral soils

Dissolved organic matter (DOM) is often considered the most labile portion of organic matter in soil and to be negligible with respect to the accumulation of soil C. In this short review, we present recent evidence that this view is invalid. The stability of DOM from forest floor horizons, peats, and topsoils against microbial degradation increases with advanced decomposition of the parent organic matter (OM). Aromatic compounds, deriving from lignin, likely are the most stable components of DOM while plant‐derived carbohydrates seem easily degradable. Carbohydrates and N‐rich compounds of microbial origin produced during the degradation of DOM can be relatively stable. Such components contribute much to DOM in the mineral subsoil. Sorption of DOM to soil minerals and (co‐)precipitation with Al (and probably also with Fe), especially of the inherently stable aromatic moieties, result in distinct stabilization. In laboratory incubation experiments, the mean residence time of DOM from the Oa horizon of a Haplic Podzol increased from <30 y in solution to >90 y after sorption to a subsoil. We combined DOM fluxes and mineralization rate constants for DOM sorbed to minerals and a subsoil horizon, and (co‐)precipitated with Al to estimate the potential contribution of DOM to total C in the mineral soil of a Haplic Podzol in Germany. The contribution of roots to DOM was not considered because of lack of data. The DOM‐derived soil C ranges from 20 to 55 Mg ha^–1 in the mineral soil, which represents 19%–50% of the total soil C. The variation of the estimate reflects the variation in mineralization rate constants obtained for sorbed and (co‐)precipitated DOM. Nevertheless, the estimates indicate that DOM contributes significantly to the accumulation of stable OM in soil. A more precise estimation of DOM‐derived C in soils requires mineralization rate constants for DOM sorbed to all relevant minerals or (co‐)precipitated with Fe. Additionally, we need information on the contribution of sorption to distinct minerals as well as of (co‐)precipitation with Al and Fe to DOM retention.     

“MIRSED” towards an MIR approach to modelling hillslope soil erosion at the national scale

This paper reports a new methodology for assessing regional and national patterns of hillslope scale soil erosion rates in the UK using a MIR (minimum information requirement) version of WEPP (Water Erosion Prediction Project) known as MIRSED. WEPP is parameterised using a national coverage, environmental database containing topographic, soil, land management and climate variables for all hillslopes within each grid cell to be modelled. The MIRSED matrix summarises the behaviour of WEPP in a multi-dimensional parameter space, allowing results to be queried using a subset of key, spatially variable parameters to produce an averaged hillslope soil erosion response from each 1 km² grid cell. The approach is demonstrated for the Great Ouse catchment, Cambridgeshire, UK and highlights highest hillslope erosion rates of 2.2 t ha⁻¹ year⁻¹ associated with steepest slopes, erodible soils and management practices that leave the soil exposed for critical times of the year. A mean soil erosion rate of 0.4 t ha⁻¹ year⁻¹ is predicted from hillslopes across the catchment which compares well with observed data collated at different scales, using contrasting measurement techniques.     

Characterization of dissolved organic matter in decomposing Norway spruce and silver birch litter

Dissolved organic matter (DOM) plays an important role in transport, storage and cycling of carbon (C) and nitrogen (N) in forest soils where litter is one of the main sources. The aim was to study the amount and characteristics of DOM leached from freshly fallen litters of silver birch (Betula pendula Roth.), Norway spruce (Picea abies (L.) Karst.) and their mixture during decomposition. DOM was collected after irrigation on eight occasions during 252 days incubation in the laboratory at about 18°C, including one freeze‐thaw cycle. During the incubation about 33–35% of C from birch and spruce litter and 40% of C from their mixture was lost. The total cumulative flux of dissolved organic carbon (DOC) from the mixture of litters was approximately 40% larger than that from single litters. The flux of DOC, DON, phenolic compounds and proteins followed a two‐stage pattern during decomposition. In the first stage the initially large fluxes decreased gradually. In the second stage, after freezing and thawing, the fluxes tended to increase again. Mixing birch and spruce litters and a freeze‐thaw cycle seems to increase the decomposition of litter and result in the increased flux of DOC, DON and phenolic compounds. The flux of hemicelluloses and the degradability of DOM were large at the first leaching occasion and decreased during the incubation. Birch had a 40% larger total flux of easily degradable DOM than spruce, supporting the previous consistent signs of greater microbial biomass and activities related to C and N cycling in soil under birch than under spruce. It is known that recalcitrant DOM might be stabilized whereas labile DOM may promote microbial activity and nutrient cycling. We conclude that the storage and cycling of C and N is affected by both tree species and degradation stage of litter in forest soils.     

Distribution of soil organic carbon and phosphorus on an eroded hillslope of the rangeland in the northern Tibet Plateau, China

Some studies on the relationship between soil erosion and subsequent redeposition of eroded soils in the same field and soil quality have been conducted in croplands, yet few studies have revealed this relationship in rangelands. We selected a toposequence with a slope of 30% and a horizontal length of 342 m from the rangeland in the northern Tibet Autonomous Region, China (31°16′N, 92°09′E) to determine the relationship between soil erosion, soil organic carbon (SOC) content and available P patterns within a hillslope landscape. Soil samples for the determination of ¹³⁷Cs as well as SOC, available P and particle‐size fractions were collected at 20 m intervals along a transect of this hillslope. Soil redistribution was caused primarily by wind erosion at toe‐slope positions, but primarily by water erosion at the hillslope positions above the toe‐slope. In upper‐ and mid‐slope portions (0 m to 244 m horizontal length), SOC content is closely correlated to ¹³⁷Cs concentration (r = 0.74, P < 0.01, n= 15), suggesting that SOC distribution along the slope was similar to ¹³⁷Cs distribution, which itself was dependent on topographic changes. However, SOC contents in toe‐slope portions are less than those above the toe‐slope (i.e. upper‐ and mid‐slope portions), and the correlation between ¹³⁷Cs and SOC in the toe‐slope portion is weaker than that above the toe‐slope. A highly significant correlation (r = 0.72, P < 0.001, n= 20) between ¹³⁷Cs concentration and available P was found within the whole hillslope landscape, implying the distribution pattern of available P was somewhat different from that of SOC. We suggest that the distribution of SOC within the hillslope landscape is also affected by factors such as assimilation rates due to difference in grassland productivity at different points and different biological oxidation rates of carbon related to patterns of moisture distribution.     

Variation in soil chemical properties along toposequences in an arid region of the Levant

Detailed characterization of soils and their variation along different topography positions has not been investigated in depth for Mediterranean arid regions. There is a need to accurately understand the variation and the spatial distribution of soil properties within dry region of the Levant. Such understanding is required to optimize the use and management of scarce land and water resources. The objective of this study was to examine the effect of hillslope characteristics on the variation of selected soil chemical properties in an arid Mediterranean climate. At each of five selected transects four sites were chosen to represent four different topographic positions: summit, shoulder, backslope and toeslope. A soil profile was examined at each site and a representative sample from each horizon was withdrawn for chemical analyses. The analyses indicated that generally, the carbonate contents of the surface horizons decreased from higher to lower positions of the toposequence, the carbonate content increased with depth for profiles occupying the lower positions. This suggests more intense leaching within soil at lower positions. The effect of steepness and curvature on controlling the variation of soil properties was obvious at the summit and shoulder positions. Leaching process seems to hinder the effect of steepness and curvature for soils at lower positions. Lower positions receive runoff water and organic matter from upper positions, which complicate the relationship between landform shape and organic matter content. Continuous tillage resulted in lower organic matter contents for soils at lower positions. Higher pH values were reported for soils down along the transect due to the movement of soil material from upper hillslope positions. Electrical conductivity, exchangeable calcium and potassium contents decreased downslope due to higher moisture accumulation. However, no obvious relationship was found between the variation of pH or EC in one hand and the variation of steepness and curvature on the other. Potassium content was variable due to its greater mobility. The analyses indicated that variation in the soil CEC is governed by two factors: the leaching pattern, which is controlled by hillslope position, and the accumulation of Eolian carbonates at the soil surface. The distribution of iron oxides and types of clay minerals indicated more weathering in a descending direction and with soil depth, which is attributed to higher availability of soil moisture along the same direction. These relationships suggested systematic variation of chemical properties along toposequences in this arid environment.     

The response of organic matter mineralisation to nutrient and substrate additions in sub-arctic soils

Global warming in the Arctic may alter decomposition rates in Arctic soils and therefore nutrient availability. In addition, changes in the length of the growing season may increase plant productivity and the rate of labile C input below ground. We carried out an experiment in which inorganic nutrients (NH₄NO₃ and NaPO₄) and organic substrates (glucose and glycine) were added to soils sampled from across the mountain birch forest-tundra heath ecotone in northern Sweden (organic and mineral soils from the forest, and organic soil only from the heath). Carbon dioxide production was then monitored continuously over the following 19 days. Neither inorganic N nor P additions substantially affected soil respiration rates when added separately. However, combined N and P additions stimulated microbial activity, with the response being greatest in the birch forest mineral soil (57% increase in CO₂ production compared with 26% in the heath soil and 8% in the birch forest organic soil). Therefore, mineralisation rates in these soils may be stimulated if the overall nutrient availability to microbes increases in response to global change, but N deposition alone is unlikely to enhance decomposition. Adding either, or both, glucose and glycine increased microbial respiration. Isotopic separation indicated that the mineralisation of native soil organic matter (SOM) was stimulated by glucose addition in the heath soil and the forest mineral soil, but not in the forest organic soil. These positive ‘priming’ effects were lost following N addition in forest mineral soil, and following both N and P additions in the heath soil. In order to meet enhanced microbial nutrient demand, increased inputs of labile C from plants could stimulate the mineralisation of SOM, with the soil C stocks in the tundra-heath potentially most vulnerable.     

Effect of litterbags on rate of organic substrate decomposition along soil depth and geomorphic gradients

Purpose

The effect of litterbags on rate of organic substrate decomposition and how the magnitude of this effect varies depending on different soil and environmental variables remains poorly quantified. In this work, I determine the effect of mesh litterbags on decomposition of organic substrate at different landform positions and soil depths in a small, naturally eroding watershed.

Materials and methods

Results presented here are derived from an 18-month field decomposition experiment using a common substrate, wood. I monitored the rate of mass loss of wood as is and the same type of wood enclosed in litterbags constructed from nylon mesh material with 1-mm wide openings. The study was carried out in three to four depths each at four geomorphologically distinct landform positions along an undisturbed hillslope in Northern California.

Results and discussion

Enclosure of wood inside mesh litterbags significantly changed its rate of decomposition overall and the percent C and C/N ratio of the decomposing substrate, at an inconsistent rate at the different depths and landform positions. The highest change in mass loss due to bagging was observed on the surface of the soil, on the summit part of the hillslope where there was up to 80-times more difference in decay rate of bagged vs. not-bagged substrate. At depth, the effect of bagging on rate of mass loss reached a maximum of 10 times at 50 cm depth in the alluvial/colluvial plain at the base of the study hillslope.

Conclusions

Results from this study underscore the need to exercise caution when inferring rates of organic substrate decomposition in dynamic hillslope environments from litterbag decomposition experiments conducted on the surface of soil or flat landscapes.     

Disentangling thermal acclimation and substrate limitation effects on C and N cycling in peatlands

Temperature and substrate availability are among the key factors controlling microbial metabolism. The relative importance of these two drivers on soil organic matter turnover is, however, hotly debated. In this study, we investigated the effect of temperature changes on the potential enzyme activities involved in C (phenol-oxidase) and N (protease and amidase) cycling by incubating peat soils collected in winter and summer at the two typical temperatures recorded in the field during these two distinct periods (4 and 19 °C, respectively). In addition, to evaluate the effect of substrate limitations, we also compared the respiration rates of the thermally adapted soils with and without plant litter additions. Results showed that both collection season and incubation temperature had a significant effect on the two enzymes involved in N-cycling, with summer and increasing temperatures having detrimental effects on the potential activities of protease and amidase, whereas none of these factors affected phenol-oxidase activity. Furthermore, while adding readily decomposable substrate accelerated decomposition rates, CO₂ flux rates were similar for all temperature conditions. Interestingly, the greatest contribution of litter to CO₂ emissions occurred in the summer samples incubated at the lower temperature, whereas for the winter samples the stimulating effect on soil respiration was observed under warmer conditions. These findings suggest that the responses of soil microbial communities to temperature and substrate availability seem to strongly depend on the long-term temperature conditions and its interaction with substrate availability.     

Land Use and Soil Organic Carbon in China’s Village Landscapes

Village landscapes, which integrate small-scale agriculture with housing, forestry, and a host of other land use practices, cover more than 2 million square kilometers across China. Village lands tend to be managed at very fine spatial scales (≤ 30 m), with managers both adapting their practices to existing variation in soils and terrain (e.g., fertile plains vs. infertile slopes) and also altering soil fertility and even terrain by terracing, irrigation, fertilizing, and other land use practices. Relationships between fine-scale land management patterns and soil organic carbon (SOC) in the top 30 cm of village soils were studied by sampling soils within fine-scale landscape features using a regionally weighted landscape sampling design across five environmentally distinct sites in China. SOC stocks across China’s village regions (5 Pg C in the top 30 cm of 2 × 10 6 km 2 ) represent roughly 4% of the total SOC stocks in global croplands. Although macroclimate varied from temperate to tropical in this study, SOC density did not vary significantly with climate, though it was negatively correlated with regional mean elevation. The highest SOC densities within landscapes were found in agricultural lands, especially paddy, the lowest SOC densities were found in nonproductive lands, and forest lands tended toward moderate SOC densities. Due to the high SOC densities of agricultural lands and their predominance in village landscapes, most village SOC was found in agricultural land, except in the tropical hilly region, where forestry accounted for about 45% of the SOC stocks. A surprisingly large portion of village SOC was associated with built structures and with the disturbed lands surrounding these structures, ranging from 18% in the North China Plain to about 9% in the tropical hilly region. These results confirmed that local land use practices, combined with local and regional variation in terrain, were associated with most of the SOC variation within and across China’s village landscapes and may be an important cause of regional variation in SOC.     

Temperature and soil moisture effects on dissolved organic matter release from a moorland Podzol O horizon under field and controlled laboratory conditions

Organic upland soils store large amounts of humified organic matter. The mechanisms controlling the leaching of this C pool are not completely understood. To examine the effects of temperature and microbial cycling on C leaching, we incubated five unvegetated soil cores from a Podzol O horizon (from NE Scotland), over a simulated natural temperature cycle for 1 year, whilst maintaining a constant soil moisture content. Soil cores were leached with artificial rain (177 mm each, monthly) and the leachates analysed for dissolved organic carbon (DOC) and their specific C‐normalized UV absorbance determined (SUVA, 285 nm). Monthly values of respiration of the incubated soils were determined as CO₂ efflux. To examine the effects of vegetation C inputs and soil moisture, in addition to temperature, we sampled O horizon pore waters in situ and collected five additional field soil cores every month. The field cores were leached under controlled laboratory conditions. Hysteresis in the monthly amount of DOC leached from field cores resulted in greater DOC on the rising, than falling temperature phases. This hysteresis suggested that photosynthetic C stimulated greater DOC losses in early summer, whereas limitations in the availability of soil moisture in late summer suppressed microbial decomposition and DOC loss. Greater DOC concentrations of in‐situ pore waters than for any core leachates were attributed to the effects of soil drying and physico‐chemical processes in the field. Variation in the respiration rates for the incubated soils was related to temperature, and respiration provided a greater pathway of C loss (44 g C m⁻² year⁻¹) than DOC (7.2 g C m⁻² year⁻¹). Changes in SUVA over spring and summer observed in all experimental systems were related to the period of increased temperature. During this time, DOC became less aromatic, which suggests that lower molecular weight labile compounds were not completely mineralized. The ultimate DOC source appears to be the incomplete microbial decomposition of recalcitrant humified C. In warmer periods, any labile C that is not respired is leached, but in autumn either labile C production ceases, or it is sequestered in soil biomass.     

Microbial biomass and its activities in salt-affected coastal soils

Seasonal fluctuations in salinity are typical in coastal soils due to the intrusion of seawater in the groundwater. We studied the effect of salinity on the microbial and biochemical parameters of the salt-affected soils of the coastal region of Bay of Bengal, Sundarbans, India. The average pH values and average organic C (OC) contents of soils from nine different sites cultivated with rice (Oryza sativa) ranged from 4.8 to 7.8 and from 5.2 to 14.1 g kg⁻¹, respectively. The average electrical conductivity of the saturation extract (EC_e) during the summer season was about five times higher than that during the monsoon season. Within the nine sites, three soils (S₃, S₄, and S₅) were the most saline. The average microbial biomass C (MBC), average basal soil respiration (BSR), and average fluorescein diacetate hydrolyzing activity (FDHA) were lowest during the summer season, indicating a negative influence of soil salinity. About 59%, 50%, and 20% variation in MBC/OC, FDHA/OC, and BSR/MBC (metabolic quotient, qCO₂), respectively, which are indicators of environmental stress, could be explained by the variation in EC_e. The decrease in MBC and microbial activities with a rise in salinity is probably one of the reasons for the poor crop growth in salt-affected coastal soils.     

Quantifying the contribution of dissolved organic matter to soil nitrogen cycling using N isotopic pool dilution

Dissolved organic matter (DOM) has been recognised as a key carbon and nitrogen (N) pool involved with soil-plant-microbe interactions. Yet few studies have quantified this contribution in agricultural soils. In this study we leached DOM from a sandy loam and sandy clay loam soil under either grassland or arable cropping. Two weeks after DOM removal microbial respiration from soils was not altered. However, a significant (P<0.05) decline in microbial biomass-N, potentially mineralizable-N, gross N mineralization and gross nitrification occurred after leaching. This data illustrate that whilst DOM is a small component of the soil OM it contributed up to 25% of microbial N supply within these agricultural soils.     

玉米植株不同部位还田土壤活性碳、氮的动态变化

探讨玉米植株不同部位腐解对还田土壤活性碳、 氮动态变化的影响。采用室内培养方法,通过动态监测土壤微生物量碳（SMBC）、微生物量氮（SMBN）、可溶性碳（DOC）和矿质氮含量,研究等量玉米根茬、秸秆、茎及叶4个部位在连续7季还田（秸秆+根茬还田）和不还田土壤（仅根茬还田）中的腐解转化特征。结果表明,秸秆腐解的最初 7 d是土壤活性碳、 氮动态变化的高峰期;腐解期间（62 d）SMBC、SMBN含量表现为添加秸秆始终高于根茬,叶分别在前28 d、14 d内高于茎,后期则低于茎,秸秆介于茎、叶之间;土壤DOC、矿质氮含量为叶＞秸秆＞茎＞根茬;培养结束时,各处理SMBC和矿质氮含量均较起始（0 d）显著提高,DOC含量基本保持不变,SMBN含量显著下降。与不还田土壤相比,还田土壤对新鲜残体的腐解影响不显著,且两者间土壤活性氮组分的差异较碳组分明显。腐解期间土壤活性碳、 氮的动态变化主要取决于各器官碳、 氮等化学组分的差异性,等量秸秆较根茬更有利于补充土壤活性碳、氮数量,土壤活性氮组分对还田土壤的响应较碳组分灵敏。     

Detecting microbial N-limitation in tussock tundra soil: Implications for Arctic soil organic carbon cycling

More than a third of the global soil organic carbon (SOC) pool is estimated to be stored in northern latitudes. While the primary regulators of microbially-mediated decomposition in physically unprotected organic soils are typically attributed to abiotic factors ( e.g. temperature and moisture), in extremely nutrient-poor environments such as the Alaskan Arctic tussock tundra, evidence from field studies suggests that low N-availability may also strongly limit microbial growth, and thus the rate of SOC decomposition. However, there have been few direct tests of microbial nutrient-limitation, particularly in Arctic systems. We predicted that during the Arctic summer growing season, when both plants and microbes are competing for mineralized nutrients, N-availability in tussock tundra soil is so low that it will limit microbial biomass production, and thus decomposition potential. We tested this prediction by adding N and C to tussock tundra organic soil and tracking microbial responses to these additions. We used a combination of approaches to identify microbial N-limitation, including changes in microbial biomass, C-mineralization, substrate use efficiency, and extracellular enzyme activity. The Arctic soil's microbial community demonstrated strong signals of N-limitation, with N-addition increasing all aspects of decomposition tested, including extracellular enzyme activity, the rate-limiting step in decomposition. The corresponding C-addition experiment did not similarly influence the microbial activity of the tundra soil. These results suggest that tundra SOC decomposition is at least seasonally constrained by N-availability through microbial N-limitation. Therefore, explicitly including N as a regulator of microbial growth in this N-poor system is critical to accurately modeling the effects of climatic warming on Arctic SOC decomposition rates.     

Effects of tree species and topography on soil chemistry, litter quality, and decomposition in Northeast Turkey

Leaf litters from beech (Fagus orientalis Lipsky.) and oak (Quercus robur L.), and needle litters from fir (Abies nordmanniana Spach.) and pine (Pinus sylvestris L.) trees were collected from north-facing site and south-facing site and at three slope positions (top, middle and bottom) on each aspect that varied in soil chemical characteristics (soil pH, cation exchange capacity and base saturation). The litters were analysed for initial total carbon, nitrogen, acid detergent fibre, lignin and cellulose concentrations. Nitrogen, acid detergent fibre and lignin concentrations and carbon:nitrogen and lignin:nitrogen ratios varied significantly within and between species according to soil chemical characteristics on aspects and slope positions. Litter decomposition was studied in the field using the litterbag technique. The litters were placed on two aspects and at three slopes on each aspect in October 2001, and were sampled every 6-month for 2 years. The main effects of aspect, species and slope position on decomposition rates were all statistically significant. Oak leaf litter showed highest decomposition rates, followed by pine, fir and beech litter, and the litters placed on north-facing site decomposed faster than those on the south-facing site. The litters placed at the top slope position decomposed slower than at those at either the bottom or middle positions. Initial lignin concentrations explained most of the variation in decomposition rates between species, and within species for the aspects and the slope positions, but the explained variance showed differences between aspects and slope positions. This result illustrates the important point that litter quality may define the potential rates of microbial decomposition but these are significantly influenced by the biotic and abiotic environment in which decomposition takes place.     

Effects of changed litter inputs on soil labile carbon and nitrogen pools in a eucalyptus-dominated forest of southeast Queensland,Australia

Purpose

The quantity and quality of litter inputs to forest soils are likely to be changed as a result of the climate change and human disturbances. However, the effects of changed litter inputs on soil labile carbon (C) and nitrogen (N) pools still remain unclear.

Materials and methods

A 15-month in situ field experiment was conducted within both high and low litter quality site in a eucalyptus-dominated native forest of Queensland, Australia. Three rates of litter inputs were applied, including (i) no litter (NL); (ii) single litter (SL), representing the average condition of the surrounding forest floor; and (iii) double litter (DL). Water-extractable organic C (WEOC) and total N (WETN), hot water-extractable organic C (HWEOC) and total N (HWETN), microbial biomass C (MBC), and N (MBN) were analyzed in the 0–5-cm soil layer seasonally.

Results and discussion

Litter input rates had no significant effects on litter decomposition at both sites (P?>?0.05). After 15-month of decomposition, mean litter mass loss was 46.3% and 31.2% at the HQ and LQ sites, respectively. Changed litter quantity had no significant effects on any of the soil labile C and N pools, regardless of litter quality. However, soil labile C and N pools significantly varied with sampling times, and the samples of different sampling times were clearly separated at both sites according to the redundancy analysis (RDA). WEOC peaked in summer, declined in autumn and winter, and increased again in spring, while the concentrations of HWEOC and MBC peaked in the winter period. The seasonal trends of MBN were opposite to the trends of WETN, which might be due to the temporal partitioning of N between plants and microbes.

Conclusions

The findings indicated that soil labile C and N pools in the eucalyptus-dominated forest of subtropical Australia were resistant to a short-term change in aboveground litter inputs. Future research should expand on these findings by keeping observing over a longer time period and considering the influence of changed belowground litter inputs.