Mineralization rate of C-labelled dissolved organic matter from leaf litter in soils of a weathering chronosequence

During the processes of primary succession and soil development, large stocks of organic C with very long residence times accumulate in many soils. Soluble organic C adsorbed by soils may contribute to the stock of organic C accumulating during soil development. We determined whether the mineralization rate of water-soluble organic C and the insoluble residue from ¹⁴C-labelled leaf litter added to soils from a weathering chronosequence decrease as soil age and adsorption capacity increase. The soils were formed on mudflows of andesitic material deposited about 75, 255, 616 y ago, and another older but undetermined time before this study. The percentage of the DOC adsorbed by the soils increased with age. After 1 year of incubation there were no significant differences in the mineralization rates of DOC added to soils of different ages. The DOC appeared to be comprised of two fractions, one that comprises about 32% of the total that mineralized with a half decay time of 0.02 y (7 d) and a second fraction comprising 68% with a half decay time of about 1.6 y. Consequently, the slowly mineralized fraction of the soluble C contributed to the accumulation of slowly mineralized C in the soil. Both the slowly and rapidly mineralized fractions of the insoluble residue decomposed more slowly than the corresponding fractions in DOC. We found no support for the idea that increased adsorption capacity due to weathering resulted in protection of soluble organic C from microbial mineralization.     

长期处于植被条件下香港土壤中氮的矿化

This research examined nitrogen mineralization in the top 10 cm of soils with a vegetation gradient in Hong Kong at sites where fire has been absent for 0, 1, 3, 6 and 17 years (at the time of the study), and the relationships between N mineralization and successional development of vegetation in the absence of fire. The sites including a newly burnt area (S1), short grassland (S2), tall grassland (S3), mixed tall grassland and shrubland (S4), and woodland (S5) were selected, with the in situ core incubation method used to estimate nitrogen mineralization. Throughout the 60-day incubation in four periods, more nitrogen was mineralized at the S3 and S4 sites, the predominantly grassland sites, which contained the highest levels of soil organic matter (SOM) and total Kjeldahl nitrogen (TKN), than the S1 site, while immobilization occurred at the S2 and S5 sites. Leaching loss decreased with successional development of the vegetation, in the order of S1 > S2 > S3 > S4 > S5. The pattern of nitrogen uptake with ecological succession was less conspicuous, being complicated by the immediate effect of fire and possibly the ability of the woodland species to extract nitrogen from the deeper ground. In the absence of fire for 3 to 6 years, the build-up of SOM and TKN was accompanied by active mineralization, thus paving the way for the invasion of shrub and tree species. A close relationship existed between nitrogen mineralization and ecological succession with this vegetation gradient. Inherent mechanisms to preserve nitrogen in a fire-prone environment including immobilization and uptake and the practical relevance of nitrogen mineralization to reforestation are discussed.     

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.     

添加生物质炭对红壤水稻土有机碳矿化和微生物生物量的影响

通过室内培育试验,研究了添加生物质炭对江西红壤水稻土有机碳矿化和微生物生物量碳、氮含量的影响。结果表明:红壤有机碳矿化速率在培育第2天达最大值后迅速降低,培养7天后下降缓慢并趋于平稳;添加生物质炭降低了土壤有机碳的矿化速率和累积矿化量,培养结束时,不加生物质炭的对照处理中有机碳的累积矿化量分别比添加0.5%和1.0%生物质炭的处理高10.0%和10.8%。此外,生物质炭的加入显著提高了土壤微生物生物量,添加0.5%生物质炭处理的土壤微生物生物量碳、氮含量分别比对照高111.5%～250.6%和11.6%～97.6%,添加1.0%生物质炭处理的土壤微生物生物量碳、氮含量分别比对照高58.9%～243.6%和55.9%～110.4%。相同处理中,干旱的水分条件下(40%田间持水量)微生物生物量要高于湿润的水分条件(70%田间持水量)。同时,添加0.5%和1.0%的生物质炭使土壤代谢熵分别降低2.4%和26.8%,微生物商减少了43.7%和31.7%。     

不同利用年限红壤水稻土有机碳和养分含量的粒级分布变化

通过田间采样结合沉降法分级提取,研究了不同利用年限红壤水稻土有机碳和养分含量的粒级分布变化特征。结果表明,红壤水稻土有机碳和养分含量随土壤颗粒粒径的增大而下降,但在各粒级中的分布比例存在显著差异。<0.002mm、0.002～0.02mm、0.02～0.05mm、>0.05mm粒级的有机碳占全土有机碳的比例分别是29.2%、30.7%、11.9%、15.4%,氮的相应数值为36.7%、31.9%、10.2%、14.0%,磷为49.2%、26.5%、11.1%、12.4%,钾为36.9%、33.4%、12.9%、20.0%。总体来说,黏粒和粉粒中有机碳和养分的分布比例较高。红壤水稻土有机碳和养分含量及分布比例还随利用年限而有明显变化。开垦利用不到10a的水田土壤,有机碳和养分含量较低且主要集中在<0.002mm粒级中;而利用超过10a的水稻土,有机碳和养分在粉粒中(0.002～0.05mm)的比例大于50%。各利用年限的红壤水稻土多以0.02～0.05mm粒级的C/N为最高,并随利用年限延长而下降。红壤水稻土各粒级有机碳和养分含量及分布状况随利用年限的变化反映了土壤肥力熟化和养分有效性的提高过程。     

不同水分条件下添加白云石对酸性水稻土有机碳矿化的影响

施用石灰改良酸性土壤是常用的农艺措施之一。施用石灰影响土壤理化性质,进而影响土壤有机碳（Soil Organic Carbon,SOC）矿化。而SOC矿化与土壤肥力保持和有机碳库的大小存在紧密联系。因此,明晰施用石灰对酸性土壤有机碳矿化的影响具有重要的理论和现实意义。该研究以2种母质的酸性水稻土为对象,在50%、90%和130%土壤最大田间持水量（Water Holding Capacity,WHC）条件下添加和不添加白云石,再进行为期45 d的室内培养试验,探讨白云石和水分对SOC矿化的影响。研究结果表明,添加白云石显著影响2种土壤有机碳矿化速率,但白云石添加和水分的交互作用不显著。土壤含水量较低时（50% WHC）,2种土壤有机碳矿化均受到抑制。在较高土壤含水量情况下（90%～130% WHC）,白云石添加和水分的共同作用对SOC矿化的影响因土壤质地不同而异,淹水条件下（130% WHC）棕红壤有机碳矿化量高于湿润条件（90% WHC）,而红壤中的情况正好相反。白云石添加和水分均显著影响SOC累计矿化量,但二者交互作用仅在棕红壤中显著。添加白云石后,2种土壤pH值随着水分含量的增加而提高;土壤含水量较低时（50% WHC）,土壤pH值即可达到或接近目标值（pH值6.5）。这些结果表明,在评估施用白云石对SOC矿化的影响时,需要考虑土壤含水量和土壤本身的性质,以便为农业生产实践中合理施用白云石提供指导和建议。     

Bacteriological studies on the mineralization of organic nitrogen in paddy soils

In this report we studied the effects of mechanical grinding of paddy soils on nitrogen mineralization and bacterial number when soils were incubated under a submerged condition after grinding.

Nitrogen mineralization was increased by grinding soil samples as compared with those without grinding. The value of (Nd-Nu)/Nu, where Nd is the amount of ammonia-nitrogen formed by incubation of disrupted samples and Nu those of the undisrupted soils, was in good correlation with clay/humus ratio.

Although significant difference was not observed between the number of aerobic bacteria with undisrupted and disrupted soil samples, higher anaerobic bacterial numbers were found with disrupted than with undisrupted ones.

A significant correlation was also obtained between the amounts of nitrogen mineralization increased by the grinding of soil and the numbers of anaerobic bacteria.     

Cycling downwards – dissolved organic matter in soils

Dissolved organic matter has been recognized as mobile, thus crucial to translocation of metals, pollutants but also of nutrients in soil. We present a conceptual model of the vertical movement of dissolved organic matter with soil water, which deviates from the view of a chromatographic stripping along the flow path. It assumes temporal immobilization (sorptive or by co-precipitation), followed by microbial processing, and re-release (by desorption or dissolution) into soil water of altered compounds. The proposed scheme explains well depth trends in age and composition of dissolved organic matter as well as of solid-phase organic matter in soil. It resolves the paradox of soil organic matter being oldest in the youngest part of the soil profile – the deep mineral subsoil.     

Biodegradability of dissolved organic matter extracted from a chronosequence of forest‐floor materials

Dissolved organic matter (DOM) is an important part of the microbially utilizable organic matter in soils. This paper describes an incubation experiment by which the lability and mobility of DOM extracted from forest‐floor materials are related to their age and degree of degradation. DOM extracted from a chronosequence of decomposing leaves (green leaves, fresh fallen leaves, litter [1 y weathering], fibric matter [2–3 y], hemic matter [>3 y], or peat [100+ y]) was applied to intact soil cores, and the CO₂ produced over 5 d was measured to determine biodegradability. Biodegradability of DOM varied somewhat along the chronosequence, with the green leaves yielding more CO₂ on an equal C basis than the older substrates and equivalent to glucose which was used as a reference substrate. Thus, the DOM extracts of the older substrates contain some refractory components. The humic‐acid content (or that of its precursors) of the extracts from older substrates relates to the lowered lability. The hydrophobic and hydrophilic content of DOM extracted from substrates was not related to CO₂ production in the incubation studies conducted.     

Bacteriological studies on the mineralization of soil organic nitrogen in paddy soils

Anaerobes were isolated from both disrupted and undisrupted soil samples incubated under submerged condition. All isolates were divided into 9 groups based on their gram stain and morphological characteristics. Correlation coefficients between the amount of nitrogen mineralized and the number of each group were calculated. A significant correlation was observed only between the amount of nitrogen mineralized and the number of gram positive-rod shaped anaerobic microorganisms (r=0.768 at P=0.05). This result indicates that in this study system, sorne particular anaerobes may play an important role in the mineralization of soil organic nitrogen.     

施肥和秸秆还田对红壤水稻土有机碳分布变异及其矿化特性的影响

利用始于1990年的湖南桃园红壤水稻土长期定位试验,研究了长期施肥和秸秆还田对红壤水稻土剖面有机碳含量和δ13C值分布,有机碳密度和变异幅度,以及有机碳矿化特性的影响.结果表明,长期施肥和秸秆还田使红壤稻田土壤表层有机碳含量显著升高,秸秆还田对有机质的累积作用明显优于仅施化肥处理.施肥或秸秆还田使0～25 cm和0～ 50 cm土层的有机碳密度均明显提高,且有机碳变异幅度也明显增大.0～10cm表层土壤δ13C值与对照的差异最大,而施化肥或秸秆还田处理之间的差异不大.各施肥处理土壤有机碳在培养初期的矿化速率较大,并在1～3周内迅速降低且逐渐达到稳定状态.秸秆还田使各处理有机碳矿化速率的提高较明显,各处理的平均矿化速率为CO247.75～31.16 ml/(kg.d),稳定矿化速率为CO240～60ml/(kg·d),12周培养期内累积矿化量大小为CK+C＞NP+C＞N+C＞NPK+C＞NP＞NPK＞N＞NK＞CK.长期施化肥和秸秆还田,均使土壤有机碳的累积矿化量提高,而秸秆还田作用更明显.因此,长期施肥和秸秆还田作为土壤有机碳累积的途径,有利于提高红壤水稻土的养分供应能力,并且能够维持稻田生态系统有机碳库的缓冲性和长期稳定性.     

Impact of organic matter addition on pH change of paddy soils

Purpose

The objective of the present study was to explore the effect of initial pH on the decomposition rate of plant residues and the effect of residue type on soil pH change in three different paddy soils.

Materials and methods

Two variable charge paddy soils (Psammaquent soil and Plinthudult soil) and one constant charge paddy soil (Paleudalfs soil) were used to be incubated at 45 % of field capacity for 105 days at 25 °C in the dark after three plant residues (Chinese milk vetch, wheat straw, and rice straw) were separately added at a level of 12 g?kg^?1 soil. Soil pH, CO₂ escaped, DOC, DON, MBC, MBN, NH ₄ ⁺ , and NO ₃ ^? during the incubation period were dynamically determined.

Results and discussion

Addition of the residues increased soil pH by 0.1–0.8 U, and pH reached a maximum in the Psammaquent and Plinthudult soils with low initial pH at day 105 but at day 3 in the Paleudalfs soil with high initial pH. Incorporation of Chinese milk vetch which had higher concentration of alkalinity (excess cations) and nitrogen increased soil pH more as compared with incorporation of rice and wheat straws. Microbial activity was the highest in Chinese milk vetch treatment, which resulted in the highest increase of soil pH as compared with addition of rice and wheat straws. However, nitrification seemed to be inhibited in the variable charge soils of Psammaquent and Plinthudult but not in the constant charge soil of Paleudalfs.

Conclusions

The effectiveness of increasing soil pH after incorporation of the plant materials would be longer in low initial pH soils of Psammaquent and Plinthudult than in high initial pH soil of Paleudalfs. In order to achieve the same degree of pH improvement, higher amounts of plant residues should be applied in constant charge soils than in variable charge soils.     

Mobilization and immobilization of dissolved organic matter in forest soils

Field and laboratory studies combined with destructive and nondestructive analytical methods were used to characterize dissolved organic matter (DOM) in acid forest soils. DOM is produced in significant amounts in the forest canopy and in the forest floor. A major part of the organic solutes are lignocellulose-degradation products being strongly microbially altered in the course of ligninolysis. The release of lignin-derived moieties into the soil solution is controlled by their degree of biooxidation. Microorganisms contribute also directly to the organic solutes through the release of microbial metabolites. DOM released from the forest floor passes the upper mineral soil almost conservatively, whereas in the subsoil most DOM is removed from solution. Immobilization of DOM is mainly due to sorption on Fe and Al oxides. The highly oxidized lignin-derived moieties are preferentially removed from the soil solution whereas the saccharides are relatively enriched. We conclude that DOM in the forest soil output to the hydrosphere is a result of (1) the release of microbially degraded lignocellulose compounds and of microbial metabolites into the forest floor solution and (2) selective sorptive removal of the lignin-derived constituents in the subsoil.     

Biochemical properties and biodegradation of dissolved organic matter from soils

Various biologically mediated processes are involved in the turnover of dissolved organic matter (DOM) in soil; however, relatively little is known about the dynamics of either the microbial community or the individual classes of organic molecules during the decomposition of DOM. We examined the net loss of DOC, the mineralisation of C to CO₂ and the degradation of DOC from six different soils by soil microorganisms. We also quantified the changes in the concentrations of protein, carbohydrate and amino acid C during microbial biodegradation. Over a 70-day incubation period at 20°C, the mineralisation of DOC to CO₂ was described by a double exponential model with a labile pool (half-life, 3–8 days) and a stable pool (half-life, 0.4–6 years). However, in nearly all cases, the mass loss of DOC exceeded the C released as CO₂ with significant deviations from the double exponential model. Comparison of mass DOC loss, CO₂ production and microbial cell counts, determined by epifluorescence microscopy, showed that a proportion of the lost DOC mass could be accounted for by microbial assimilation. Carbohydrate and protein C concentrations fluctuated throughout the incubation with a net change of between 3 to 13 and −30 to 22.4% initial DOC, respectively. No amino acid C was detected during the incubation period (level of detection, 0.01 mg C l⁻¹).     

Tracking litter-derived dissolved organic matter along a soil chronosequence using 14C imaging: Biodegradation,physico-chemical retention or preferential flow?

The cycling of dissolved organic matter (DOM) in soils is controversial. While DOM is believed to be a C source for soil microorganisms, DOM sorption to the mineral phase is regarded as a key stabilization mechanism of soil organic matter (SOM). In this study, we added ¹⁴C-labelled DOM derived from Leucanthemopsis alpina to undisturbed soil columns of a chronosequence ranging from initial unweathered soils of a glacier forefield to alpine soils with thick organic layers. We traced the ¹⁴C label in mineralized and leached DOM and quantified the spatial distribution of DO¹⁴C retained in soils using a new autoradiographic technique. Leaching of DO¹⁴C through the 10 cm-long soil columns amounted up to 28% of the added DO¹⁴C in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-DO¹⁴C as only 2–9% were mineralized, with the highest rates in mature soils. In line with the mass balance of ¹⁴C fluxes, measured ¹⁴C activities in soils indicated that the major part of litter DO¹⁴C was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all DO¹⁴C in the upper 3 cm of the soil columns. In the deeper soil, the ¹⁴C label was concentrated along soil pores and textural discontinuities with similarly high ¹⁴C activities than in the uppermost soil. These findings indicate DOM transport via preferential flow, although this was quantitatively less important than DOM retention in soils. The leaching of DO¹⁴C correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of DO¹⁴C immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization.     

可溶性有机物对土壤中绿麦隆吸附与解吸的影响

A batch equilibrium techniques was used to examine the effect of dissolved organic matter （DOM） extracted from both non-treated sludge （NTS） and heat-expanded sludge （HES） on the sorption and desorption of chlorotoluron （3-（3-chloro-p-tolyl）-1,1-dimethylurea） in two types of soils, a yellow fluvo-aquic and a red soil from China. Without DOM,sorption of chlorotoluron was significantly greater （P 〈 0.05） in the red soil than in the yellow fluvo-aquic soil. However,with DOM the effect was dependent on the soil type and nature of DOM. Chlorotoluron sorption was lower in the yellow fluvo-aquic soil than in the red soil, suggesting that with the same DOM levels the yellow fluvo-aquic soil had a lower sorption capacity for this herbicide. Application of DOM from both NTS and HES led to a general decrease in sorption to the soils and an increase in desorption from the soils. Desorption of chlorotoluron also significantly increased （P 〈 0.05） with an increase in the DOM concentration. Additionally, for sorption and desorption, at each DOM treatment level the NTS treatments were significantly lower （P 〈 0.05） than the HES treatments. This implied that non-treated sludge had a greater effect on the sorption and desorption of chlorotoluron than heat-expanded sludge.     

Cutin and suberin biomarkers as tracers for the turnover of shoot and root derived organic matter along a chronosequence of Ecuadorian pasture soils

Forest‐to‐pasture conversion has been reported to increase soil organic matter (SOM) in mineral topsoils in the tropical mountain rainforest region of south Ecuador, with subsequent decreases following pasture abandonment. Until now the mechanisms behind these changes have not been fully understood. To elucidate their varied preservation patterns, we analysed root‐ and shoot‐derived organic matter and assessed their contribution to the formation of SOM in topsoils (0–5 cm) on a chronosequence of pastures (Setaria sphacelata (Schumach.); C₄) established after slash and burn of the natural forest (diverse C₃ plant species) and an abandoned pasture site invaded by bracken fern (Pteridium arachnoideum (Kaulf.) Maxon.; C₃). Cutin and suberin biomarkers of the two plant species (grass and bracken) and of forest litter were identified after saponification and their contribution to SOM was studied by compound‐specific stable carbon isotope analyses. Our results showed specific root and shoot biomarkers for the two plant species and for forest litter, which often did not correspond to the classification of root‐versus shoot‐specific monomers reported in the literature. This illustrates the importance of direct biomarker determination rather than using results from studies with different plants. Shoot‐ as well as root‐derived OM of forest and grass origin contributed to the stable SOM pool with decadal turnover times. Forest‐derived monomers contributed more to the stable SOM pool compared with grass‐derived monomers. ω‐hydroxy carboxylic acids and α,ω‐alkanedioic acids of forest origin may have been stabilized in these tropical soils by bonding to soil minerals. Rapid degradation of grass‐derived lipids from the same compound classes suggests a saturation of the mineral binding capacity. In pasture soils, the accumulation of SOM was mainly driven by large inputs of root OM. The accumulated SOM during pasture use is, however, lost rapidly after abandonment.     

热辐射对水稻土有机质组成的影响

本文研究了受不同程度热辐射后的水稻土中有机质组分的变化情况,结果表明:随温度升高,土壤中水溶性有机碳(DOC)含量先增加后减少,200℃变化尤为显著,增加为CK的3.68 ～ 7.45倍,中性糖、糖醛酸与DOC的变化趋势一致.辐射温度不同,土壤中的胡敏酸、富里酸组成和含量亦发生不同程度的变化,300℃胡/富比值最大(1.79 ～ 5.04),土壤腐殖质聚合程度增加;200℃胡敏酸的E4/E6比值最大,反映此温度下土壤芳香缩合度低、具有较多的脂族结构物质、土壤腐殖化程度低.     

Organic carbon mineralization responses to temperature increases in subtropical paddy soils

Purpose

Understanding organic carbon mineralization and its temperature response in subtropical paddy soils is important for the regional carbon balance. There is a growing interest in factors controlling soil organic carbon (SOC) mineralization because of the potential for climate change. This study aims to test the hypothesis that soil clay content impedes SOC mineralization in subtropical paddy soils.

Materials and methods

A 160-day laboratory incubation at temperatures from 10 to 30 °C and 90% water content was conducted to examine the dynamics of SOC mineralization and its temperature response in three subtropical paddy soils with different clay contents (sandy loam, clay loam, and silty clay soils). A three-pool SOC model (active, slow, and resistant) was used to fit SOC mineralization.

Results and discussion

Total CO₂ evolved during incubation following the order of clay loam > silty clay > sandy loam. The temperature response coefficients (Q ₁₀) were 1.92?±?0.39, 2.36?±?0.22, and 2.10?±?0.70, respectively, for the sandy loam soil, clay loam soil, and silty clay soil. But the soil clay content followed the order of silty clay > clay loam > sandy loam. The sandy loam soil neither released larger amounts of CO₂ nor showed higher temperature sensitivity, as expected, even though it contains lower soil clay content among the three soils. It seems that soil clay content did not have a dominant effect which results in the difference in SOC mineralization and its temperature response in the selected three paddy soils. However, dissolved organic carbon (DOC; representing substrate availability) had a great effect. The size of the active C pool ranged from 0.11 to 3.55% of initial SOC, and it increased with increasing temperature. The silty clay soil had the smallest active C pool (1.40%) and the largest Q ₁₀ value (6.33) in the active C pool as compared with the other two soils. The mineralizable SOC protected in the silty clay soil, therefore, had even greater temperature sensitivity than the other two soils that had less SOC stabilization.

Conclusions

Our study suggests that SOC mineralization and its temperature response in subtropical paddy soils were probably not dominantly controlled by soil clay content, but the substrate availability (represented as DOC) and the specific stabilization mechanisms of SOC may have great effects.     

Bioavailability of dissolved organic carbon across a hillslope chronosequence in the Kuparuk River region,Alaska

Dissolved organic matter (DOM) represents an important component of carbon and nutrient cycling in arctic ecosystems. In northern Alaska, DOM production and microbial activity differ among landscapes with varied glaciation histories with lower rates on younger landscapes. In addition, within the region, soil DOM concentrations vary at the scale of hillslope toposequences, with higher concentrations in upslope than streamside positions. However, it is unknown whether variation in DOM production quality among and within landscapes linked to patterns in DOM quality. To answer this question, we conducted a study of DOM biodegradability within and among hillslopes of different landscape age. We examined rates of DOM decomposition and several indices of the quality of water-extracted DOM collected from soils in the summer. A variety of methods indicated that DOM quality generally was consistent across hillslope positions and among landscape ages. For example, DOM fluorescence index, an index of quality for chromophoric DOM, did not vary significantly across all hillslope positions or landscape ages. There were no significant differences among landscape ages or hillslope positions in DOM specific UV absorbance, in rates of DOM mineralization, or in DOM decomposition, indicating that DOM quality was consistent regardless of its source or position along hillslope flow paths. This suggests that despite many potential sources of variation within and among arctic hillslopes linked to differences in vegetation, hydrology, microclimate, and microbial activity, there is little variation in growing-season soil DOM quality. Microbial processing of DOM within arctic hillslopes may lead to a convergence in growing season DOM quality resulting in little spatial variation. Approximately 10–20% of the growing season DOM is labile in tundra soils, slightly higher that the proportion that is labile in arctic rivers during the summer.