Do soil aggregates really protect encapsulated organic matter against microbial decomposition?

Soil aggregates can provide an effective protection of organic matter against microbial decomposition as reported by several macroaggregate disruption studies. However, research on the role of aggregation for carbon mineralization was mainly focused on arable soils. In the present study we aim to clarify the impact of aggregation on organic matter protection by measuring carbon mineralization in terms of microbial respiration rates of intact macroaggregates (2–4 and 4–8 mm) and corresponding crushed aggregates from seven topsoil horizons from both arable and forest sites. For two arable and one forest soil we found a significantly (P < 0.001) lower carbon mineralization from intact aggregates as compared to the corresponding crushed material. The portion of aggregate protected carbon reached up to 30% for a grassland soil. For the other arable and forest soils no significant effect of aggregation was found. Similarly, no clear trend could be found for the protective capacity of different size fractions. We conclude that protection by aggregation is effective primarily for soils with a large pool of labile organic matter regardless of their usage as arable land or forest.     

A time for every season: soil aggregate turnover stimulates decomposition and reduces carbon loss in grasslands managed for bioenergy

A primary goal of many next‐generation bioenergy systems is to increase ecosystem services such as soil carbon (C) storage and nutrient retention. Evaluating whether bioenergy management systems are achieving these goals is challenging in part because these processes occur over long periods of time at varying spatial scales. Investigation of microbially mediated soil processes at the microbe scale may provide early insights into the mechanisms driving these long‐term ecosystem services. Furthermore, seasonal fluctuations in microbial activity are rarely considered when estimating whole ecosystem functioning, but are central to decomposition, soil structure, and realized C storage. Some studies have characterized extracellular enzyme activity within soil structures (aggregates); however, seasonal variation in decomposition at the microscale remains virtually unknown, particularly in managed ecosystems. As such, we hypothesize that temporal variation in aggregate turnover is a strong regulator of microbial activity, with important implications for decomposition and C and nitrogen (N) storage in bioenergy systems. We address variation in soil microbial extracellular enzyme activity spatially across soil aggregates and temporally across two growing seasons in three ecosystems managed for bioenergy feedstock production: Zea mays L. (corn) agroecosystem, fertilized and unfertilized reconstructed tallgrass prairie. We measured potential N‐acetyl‐glucosaminidase (NAG), β‐glucosidase (BG), β‐xylosidase (BX), and cellobiohydrolase (CB) enzyme activity. Aggregate turnover in prairie systems was driven by precipitation events and seasonal spikes in enzyme activity corresponded with aggregate turnover events. In corn monocultures, soil aggregates turned over early in the growing season, followed by increasing, albeit low, enzyme activity throughout the growing season. Independent of management system or sampling date, NAG activity was greatest in large macroaggregates (>2000 μm) and CB activity was greatest in microaggregates (<250 μm). High microbial activity coupled with greater aggregation in prairie bioenergy systems may reduce loss of soil organic matter through decomposition and increase soil C storage.     

Protection of soil organic C and N in temperate and tropical soils: effect of native and agroecosystems

Soil carbon sequestration is a viable short-term option to mitigate increased atmospheric CO₂. In agriculture, strategies to increase the soil carbon (C) sink include no-tillage, cover crops, and improved crop rotation. The objective of this study was to determine the influence of tillage systems on SOC and total N, soil aggregation and aggregate associated C and N in three soil types: Oxisol (Brazil), Vertisol (Argentina), and Mollisol (USA). Long-term tillage experiments included tilled (T) and no-till (NT) systems. A native grassland was included for comparison in each site. Soil samples were taken at 0–5, 0–15, and 15–30 cm depths. Water-stable aggregates (WSA) were separated using a wet-sieving method. Total C and total N were determined by dry combustion. A shift from native grassland to an agroecosystem decreased microbial biomass, but this decrease was less pronounced under NT. Cultivation reduced the mass of macroaggregates and the concentration associated C and N; however among agroecosystems, NT, regardless soil type, tended to be more similar to the native grassland sites. Agroecosystems reduced TOC and total N stocks, regardless of soil type, compared to the native grassland. This effect followed: Mollisol > Oxisol > Vertisol, and was more pronounced at the 0–5 cm soil depth than at deeper depths. This loss of C and N was associated with the decrease in the mass of macroaggregates and lower C and N concentrations of the aggregates. Macroaggregation was related to TOC and microbial biomass in the Mollisol, suggesting that the biological process of aggregate formation is the principal mechanism of C protection in these soils. The relationship between TOC and large macroaggregates showed lower values for the Oxisol and Vertisol, indicating that in these soils TOC has a complementary role in macroaggregation.     

Microbial characteristics of soils on a latitudinal transect in Siberia

Soil microbial properties were studied from localities on a transect along the Yenisei River, Central Siberia. The 1000 km‐long transect, from 56°N to 68°N, passed through tundra, taiga and pine forest characteristic of Northern Russia. Soil microbial properties were characterized by dehydrogenase activity, microbial biomass, composition of microbial community (PLFAs), respiration rates, denitrification and N mineralization rates. Relationships between vegetation, latitude, soil quality (pH, texture), soil organic carbon (SOC) and the microbial properties were examined using multivariate analysis. In addition, the temperature responses of microbial growth (net growth rate) and activity (soil respiration rate) were tested by laboratory experiments. The major conclusions of the study are as follows: 1. Multivariate analysis of the data revealed significant differences in microbial activity. SOC clay content was positively related to clay content. Soil texture and SOC exhibited the dominant effect on soil microbial parameters, while the vegetation and climatic effects (expressed as a function of latitude) were weaker but still significant. The effect of vegetation cover is linked to SOC quality, which can control soil microbial activity. 2. When compared to fine‐textured soils, coarse‐textured soils have (i) proportionally more SOC bound in microbial biomass, which might result in higher susceptibility of SOC transformation to fluctuation of environmental factors, and (ii) low mineralization potential, but with a substantial part of the consumed C being transformed to microbial products. 3. The soil microbial community from the northernmost study region located within the permafrost zone appears to be adapted to cold conditions. As a result, microbial net growth rate became negative when temperature rose above 5 °C and C mineralization then exceeded C accumulation.     

Carbon mineralization in the southern Sonoran Desert

We measured carbon mineralization in four different desert habitats (Arroyos, Hillsides, Canopies-Plains and Open-Plains) and the separate effect of litter addition from annual and perennial plants on soil microbial respiration using two laboratory soil incubation experiments. The differences in total aboveground phytomass among habitats correlates with soil nutrient content, soil particulate organic matter (POM) and consequently, C mineralization. The Arroyos habitat with the highest perennial plant phytomass and litter production, had the highest soil nutrient content, soil POM and C mineralization. Litter from annual plants had twice the P concentration than litter from the perennials, but only half the N concentration. Soil microbial respiration was higher with annual plant litter than with perennial plant litter in the Hillsides and Canopies-Plains, suggesting that microbial activity in both habitats was improved by litter with a higher C quality. In contrast, in the poorest habitat, the Open-Plains, the better response to the addition of perennial plant litter suggests that microbial activity may have been constrained by N input.     

Variables of Microbial Response in Natural Soil Aggregates for Soil Characterization in Different Fluvial Land Shapes

The main objective of this study was to determine changes in microbial response in natural soil aggregates for soil characterization in different fluvial land shapes. This study was carried out in fluvial lands formed on accumulated sediment depositions carried by K?z?l?rmak River. The majority soils of the study area were classified as Typic Ustifluvent and Typic Haplustept in Soil Taxonomy. It was found that macroaggregates (especially >6300 μm and 2000–4750 μm diameters) of all soil samples were higher than microaggregate of soils. In addition, it was determined that the C_org content varies between 0.41–0.91% in soil samples. C_mic content was also found higher level in aggregates involved <250 and 250–425 μm diameters as compared to other aggregate size classes. Moreover, we detected that C_org:C_mic ratio was much higher in macroaggregates than in microaggregate fractions. BR levels were also greater in macroaggregates of >6,300, 4,750–6,300 and 2,000–4,750 μm than in the other macroaggregates sizes and microaggregates. Consequently, macroaggregates have relatively more C_org level than the C_org level in microaggregates, even if the absolute values of C_mic were the lower. This study thus evidenced contrasting microbial habitats and their response in different soil aggregate size formed in various developed soils.     

土地利用对石漠化地区土壤团聚体有机碳分布及保护的影响

对贵州省关岭县石漠化地区不同土地利用方式下的土壤团聚体的稳定性、有机碳分布以及大团聚体有机碳矿化进行了研究,探讨了大团聚体对有机碳的保护作用,以期为选择合理的石漠化治理措施提供科学依据。选取了当地主要的4种土地利用方式,分别为水田(水旱轮作)、旱地、花椒林和火龙果林;其中花椒林和火龙果林位于石漠化治理区内。采用湿筛法分离出各级土壤团聚体并结合室内恒温培养法测定原状和破碎大团聚体中有机碳的矿化动态变化,其中大团聚体保护性碳含量为破碎与原状大团聚体有机碳在42 d内累积矿化量的差值。结果表明:土地利用方式对土壤团聚体稳定性具有显著影响。水田土壤团聚体稳定性要明显优于旱地、花椒林和火龙果林,且后3种土地利用方式间也存在显著差异。土壤有机碳也受到土地利用方式的影响,水田和旱地土壤有机碳含量要明显高于火龙果林和花椒林。各粒级团聚体有机碳含量在土地利用方式间具有较大差异,2 5 mm、0.25 2 mm和<0.25 mm团聚体中有机碳含量按水田、火龙果林、旱地和花椒林依次下降,5 8 mm团聚体中有机碳含量则以花椒林最高,其次是水田和火龙果林,旱地最低。但是就各粒径团聚体的有机碳库而言,<0.25 mm团聚体是土壤有机碳的主要载体。花椒林、旱地、火龙果和水田的大团聚体保护性碳含量分别为83.37、78.86、73.81\,61.04 mg/kg,其差异表明花椒林土壤大团聚体对有机碳的保护作用最强,其次是旱地和火龙果林,水田最弱。因此,在该地区种植花椒林和火龙果林可以改善其土壤质量,其可能机理是通过增加土壤中大团聚体含量,同时增强大团聚体对有机碳的保护作用。     

草甸草原土壤碳/氮矿化潜力及土壤微生物水分敏感性对极端干旱的响应

土壤碳、氮矿化是生态系统养分循环的关键过程,受到水分供给的强烈影响。本研究对极端干旱处理(连续3年生长季减少66%降水)的内蒙古草甸草原野外取土,采用超低温冻干后再调节土壤水分至3%、8%、13%、18%、25%和35% 6个水平进行室内培养,研究极端干旱处理后土壤碳/氮矿化潜力以及土壤微生物对水分变化的敏感性。结果表明: 与对照(自然降雨)相比,极端干旱处理后,6个培养水平的平均土壤氮矿化潜力显著提高14.2%,但未显著影响土壤碳矿化潜力。极端干旱显著提高土壤微生物生物量氮和土壤可溶性有机碳26.8%和26.9%。无论是对照还是极端干旱处理,土壤氮矿化潜力、碳矿化潜力和微生物生物量碳和氮均随着土壤含水量增加而增加,而可溶性有机碳从较低水分的培养水平(3%和8%)到较高水分的培养水平(>13%)显著降低,表明底物的扩散起到重要作用。极端干旱处理显著提高了碳矿化初始脉冲强度,表明极端干旱提高了土壤微生物对水分的敏感性。极端干旱显著降低了土壤碳矿化潜力/氮矿化潜力的比值,表明长期干旱可能会降低土壤碳、氮循环过程的耦合作用。极端干旱对土壤碳矿化和氮矿化过程的影响存在差异,激发了土壤微生物对水分的敏感性,弱化了碳、氮循环过程的耦合关系,并进一步影响中国北方草甸草原生态系统的生物地球化学循环过程及草地生产力。     

不同退化沙地土壤碳的矿化潜力

通过实验室土壤培养试验 ,研究了科尔沁退化沙质草地不同生境 (流动沙地 ,半固定沙地 ,固定沙地和丘间低地 )下土壤碳的矿化潜力及不同凋落物在沙地土壤中的分解。经 33d的室内培养 ,不同生境土壤 CO2 - C的释放有极显著的差异 ,与生境植被盖度 ,凋落物积累 ,土壤沙化程度 ,土壤有机碳和全氮含量的分布有显著相关。流动沙地土壤有极低的土壤有机碳和氮的含量及其微弱的土壤微生物呼吸 ,表明土地沙漠化不仅导致土壤有机碳库衰竭 ,也使土壤微生物活性丧失。在有机质含量很低的流动沙地和半固定沙地土壤中 ,含氮量高的小叶锦鸡儿 (Caragana microphylla)凋落物比含氮量低、C/N比高的差巴嘎蒿(Artemisia halodendron)和 1年生植物凋落物有较快的分解。在沙漠化的演变中 ,土壤的粗粒化 ,有机物质和养分及微生物活性的丧失制约着凋落物在土壤中的矿化潜力。灌木的存在使更多的有机物质和养分积聚在灌丛下 ,形成灌丛肥岛 ,因而显著贡献于碳的固存。     

不同施肥处理黑土覆膜后秸秆碳氮在团聚体中的固存特征

地膜覆盖是提高作物产量的重要措施,理解覆膜条件下黑土团聚体中外源碳和氮的固存特征,为深刻认识地膜覆盖措施的可持续应用提供理论依据。选取长期定位试验站(29年)不施肥(CK)、单施化肥(NPK)和有机肥配施化肥(MNPK)3个典型施肥处理,表层土壤(0—20 cm)添加¹³C¹⁵N双标记玉米秸秆后设置裸地和覆膜的田间原位微区培养试验,探讨不同施肥处理结合覆膜黑土团聚体中有机碳和全氮对秸秆来源碳和氮的响应。结果表明,与裸地相比,所有处理覆膜后微团聚体(<0.25 mm)中秸秆来源碳和氮的含量平均降低了26.49%和32.05%。覆膜MNPK与裸地处理相比大团聚体(>0.25 mm)中秸秆来源碳和氮的含量显著降低了35.58%和15.97%,但大团聚体中原土壤有机碳的含量提高了9.16%。在CK和NPK处理微团聚体中,秸秆来源碳占该粒级团聚体有机碳的比例表现为覆膜>裸地,而在MNPK处理各粒级团聚体中则表现为裸地>覆膜。无论覆膜与否,秸秆来源碳对团聚体有机碳和秸秆来源氮对团聚体全氮的贡献率受施肥处理的影响表现为CK>N...     

Field measurement of nitrogen mineralization using soil core incubation and acetylene inhibition of nitrification

A technique for measuring net rates of mineralization under field conditions is described. Soil cores were incubated in the field in sealed containers with acetylene to inhibit nitrification and thereby minimize losses of N through denitrification. Mineralization was estimated as the difference between the mineral N content after a 14-d incubation and that determined from soil samples taken at the start of incubation. Mineralization in the spring and summer in unfertilized plots in the field amounted to 90 and 70 kg N ha⁻¹ in S.E. England under grass and grass/clover swards, respectively, and 40 kg N ha⁻¹ under a grass sward in S.W. England. Daily rates of mineralization ranged from 0.02 to 1.90 kg N ha⁻¹, with peak values related to re-wetting of the soil after dry weather. Laboratory incubation of soil showed that neither the low concentration of acetylene (2% v/v) adopted for field incubation, nor the accumulation of mineral N during incubation was likely to affect the total measurement, but that frequent and regular soil sampling was necessary to minimize the effects of changes in soil water content. Estimates for mineralization over the whole growing season (180 d) were obtained for two years from extrapolation of the early season field measurements and were, on average, 50% higher than predictions based on a chemical extraction index of potentially mineralizable N.     

好气与淹水条件下水稻土各粒级团聚体有机碳矿化量

采用室内恒温培养法观测了好气和淹水处理下水稻土不同粒级团聚体中有机碳矿化的动态变化.结果表明：两种处理下,各粒级团聚体中有机碳矿化量都表现为培养前期快速下降,培养后期保持相对稳定的趋势.不同粒级团聚体之间有机碳矿化速率存在明显差异,在整个培养过程中,均以1～2 mm粒级团聚体最高,以＜0.053 mm粒级团聚体最低.统计分析表明,不同粒级团聚体中有机碳矿化量变化与有机碳和微生物生物量碳含量呈显著线性相关.好气和淹水处理下对土壤总有机碳累计矿化量贡献最大的是0.25～1 mm粒级团聚体,分别达41.77%和34.11%;好气处理下贡献最小的是0.053 mm粒级团聚体,淹水处理下贡献最小的是1~2 mm粒级团聚体,分别仅为7.8%和6.6%.     

Tropical forest and savanna ecosystems show differential impact of N and P additions on soil organic matter and aggregate structure

Landscape transformation and atmospheric nutrient depositions, important global change drivers, are affecting the vegetation and soil properties of natural dry tropical forest and derived savanna ecosystems in India. This study assessed the effect of continuous N and P additions for 6 years on the size distribution and properties of soil aggregates in forest–ecotone–savanna gradient. Addition of N significantly increased the proportion of macroaggregates in forest and ecotone, whereas the same input significantly decreased their proportion in the savanna. Consequently, the stability of soil aggregates increased significantly in forest and ecotone, whereas it decreased significantly in the savanna. The effect of P addition on soil aggregate stability was marginal. N addition also altered the biological and chemical qualities of soil aggregates. It caused increase in microbial biomass C (MBC) associated with macroaggregates in forest and ecotone; however, in savanna, MBC increased in the microaggregates. P addition did not affect the amount of MBC in both types of soil aggregates. Because of rapid accumulation of applied N and P in the microbial biomass, the ratios of MBC to microbial biomass nitrogen (MBN) as well as microbial biomass phosphorous (MBP) were decreased in both aggregates. Overall, the effect of N addition was more marked than that of P addition, suggesting that N is more limiting than P in these dry tropical ecosystems. In the current scenario of N loading, continued soil N loading in forest may lead to increased macroaggregates with associated MBC and MBN and greater aggregate stability. In contrast, the extensively distributed savannas may show the reverse trend leading to a decrease in soil fertility.     

Rates of net nitrogen mineralization in disturbed and undisturbed soils

Quantification of net nitrogen mineralization (NNM) in soils is indispensable in order to optimize N fertilization of crops. Two long-term laboratory incubation methods were applied to determine rates of net nitrogen mineralization (r_NNM) of soils from two sites of arable land (sandy loam soil, silty loam soil) at four temperature levels (2°C, 8°C, 14°C, 21°C). Since variability within replicates was small, the modified 12-week incubation method of Stanford and Smith (1972) using disturbed soils allowed to establish reliable Arrhenius functions with reasonable expenditure. The fit of the functions derived from the 5-month incubation of 23 undisturbed soil columns (4420 cm³) was worse. This was caused by greater variability and less differentiation between temperature levels. Results of both experiments could be described best by zero-order kinetics. Mean mineralization rates of disturbed samples were approximately twice as high than those of undisturbed samples. The suitability of both methods for the prediction of NNM at site conditions is discussed. Actual respiration (AR) at incubation temperatures and substrate induced respiration (SIR) were measured at the end of the incubation of undisturbed soil columns. The results presented reveal that soil microbial communities develop in a different manner during long-term incubation at different temperatures. This behavior offends the underlying assumption that soil microbes remain in steady-state during incubation and that rising rates are physiological reactions to temperature enhancement. Therefore soil microbial biomass (SMB) dynamics during the experiment has to be accounted for when rates of NNM and Arrhenius functions are established. R Merck Section editor     

Environmental factors affect the response of microbial extracellular enzyme activity in soils when determined as a function of water availability and temperature

Microorganisms govern soil carbon cycling with critical effects at local and global scales. The activity of microbial extracellular enzymes is generally the limiting step for soil organic matter mineralization. Nevertheless, the influence of soil characteristics and climate parameters on microbial extracellular enzyme activity (EEA) performance at different water availabilities and temperatures remains to be detailed. Different soils from the Iberian Peninsula presenting distinctive climatic scenarios were sampled for these analyses. Results showed that microbial EEA in the mesophilic temperature range presents optimal rates under wet conditions (high water availability) while activity at the thermophilic temperature range (60°C) could present maximum EEA rates under dry conditions if the soil is frequently exposed to high temperatures. Optimum water availability conditions for maximum soil microbial EEA were influenced mainly by soil texture. Soil properties and climatic parameters are major environmental components ruling soil water availability and temperature which were decisive factors regulating soil microbial EEA. This study contributes decisively to the understanding of environmental factors on the microbial EEA in soils, specifically on the decisive influence of water availability and temperature on EEA. Unlike previous belief, optimum EEA in high temperature exposed soil upper layers can occur at low water availability (i.e., dryness) and high temperatures. This study shows the potential for a significant response by soil microbial EEA under conditions of high temperature and dryness due to a progressive environmental warming which will influence organic carbon decomposition at local and global scenarios.     

东北黑土水稳性团聚体及其结合碳分布特征

以东北黑土区32对自然和耕作黑土为研究对象,对比研究了两种土壤水稳性团聚体及其结合碳的分布特征.结果表明：自然土壤0～30 cm水稳性大团聚体（>0.25 mm）质量分数及其结合碳均高于微团聚体;随着土层深度的增加,大团聚体及其结合碳逐渐降低,而微团聚体（<0.25 mm）及其结合碳显著增加（P<0.01）.耕作土壤团聚体分布特征与自然土壤相反,与自然土壤相比,耕作土壤大团聚体数量及其结合碳急剧降低,且>1 mm的大团聚体降低幅度远大于其他粒级团聚体.自然土壤大团聚体质量分数与土壤有机碳（SOC）存在明显的正相关关系（P<0.01）,尤其是>1 mm大团聚体;耕作土壤>1 mm大团聚体质量分数与土壤总SOC相关不显著;自然与耕作土壤总SOC与各级团聚体结合碳呈极显著正相关（P<0.01）.与自然土壤相比,耕作土壤大团聚体结合碳明显降低,而微团聚体结合碳增加了37.1%,总SOC含量下降了29.5%,表明大团聚体在总SOC变化中起主要作用.水稳性大团聚体对管理措施响应迅速,可以作为评价农业管理措施转变对土壤肥力和土壤质量影响的指标.     

马尾松人工林土壤和团聚体有机碳矿化对穿透雨减少的响应

为研究气候变化背景下降水格局对森林土壤碳排放机制的影响,在南亚热带马尾松人工林中模拟穿透雨减少50%,采用室内恒温培养法研究减水处理对土壤和不同粒级团聚体有机碳矿化的影响.结果表明: 1～2 mm团聚体有机碳累积矿化量高于其他粒级团聚体.干湿季减雨样地表层土壤含水量分别是对照样地的82.1%和82.7%,而其0.106～0.25 mm土壤团聚体质量分数分别比对照增加1.8%和4.2%.与对照相比,穿透雨减少显著降低了干季土壤和微团聚体易矿化碳库的矿化速率(k₁),增加了土壤和<1 mm团聚体难分解碳库的分解速率(k₂),但对土壤有机碳累积矿化量无显著影响.相关分析表明,土壤和微团聚体k₁呈显著正相关,土壤和0.25～1 mm团聚体k₂呈显著正相关.受团聚体结构、水分条件和土壤有机碳含量的影响,穿透雨减少对干季土壤易矿化和难分解有机碳的矿化分别起抑制和促进作用.     

Assessing the microbiological,biochemical, soil-physical and hydrological effects of amelioration of degraded soils in semiarid Spain

The way of improving degraded soils fertility and particularly of improving its microbial activity is to add “young” exogenous organic matter that contribute to provide labile organic matter to stimulate the life of the microorganisms existing in the soil. This organic matter will also improve both the retention and hydraulic characteristics of the degraded soils, all this contributing to soil restoration. In this study, the microbiological, biochemical, soil-physical and hydrological effects of the addition of a municipal solid waste compost to a degraded soil in El Campello, SE Spain were evaluated in a field experiment. Soil samples from experimental plots were analyzed 6 and 18 months after soil amendment. In both sampling time treated plots showed significantly higher microbial biomass carbon and dehydrogenase activity values than control, indicating that soil microbial population’s development and activity were stimulated by compost addition, this effect being not ephemeral but lasting in the time. Soil urease activity was not affected by compost addition while protease hydrolysing N-α-benzoil-L-argininamide (BAA) activity was strongly stimulated by the incorporation of compost into the soils. Phosphatase and β-glucosidase activities were also stimulated by the organic amendment, this stimulation being particularly noticeable 18 months after the compost addition. Nevertheless, this increase in soil microbial populations and activity did not result in an increase in soil aggregation and hydrological parameters. This can be due to the high content of carbonates and Ca²⁺ ions in these calcareous soils, that lead to an initially high content of water-stable macroaggregates. Presented at the International Conference on Bioclimatology and Natural Hazards, Poľana nad Detvou, Slovakia, 17–20 September 2007.     

不同施肥条件下秸秆碳在表层和深层土壤团聚体中的分配与固存

长期施肥引起表层和深层土壤水、热、养分和微生物活性的分异,进而影响秸秆碳在土壤中的分解和周转过程。本研究基于沈阳农业大学长期定位实验站,分别将不同施肥处理表层(0～20 cm)和深层(40～60 cm)土壤与¹³C标记秸秆混合进行田间试验,通过分析团聚体中有机碳含量和δ¹³C值,对比秸秆碳在表层和深层土壤团聚体中分配的差异,探讨施肥对秸秆碳在土壤团聚体中固定的影响。结果表明: 与施肥(包括单施氮肥和有机肥配施氮肥)处理相比,不施肥处理分别使表层土壤<0.053 mm和深层土壤>0.25 mm团聚体中秸秆碳含量增加了106.7%和34.2%;秸秆碳对深层土壤>0.053 mm团聚体有机碳的贡献率约为表层土壤的2.0倍;秸秆碳分配到表层土壤>0.25 mm和<0.25 mm团聚体的比例平均分别为22.6%和11.4%,分配到深层土壤的相应比例分别为29.4%和8.8%。总之,秸秆添加促进了深层土壤大团聚体碳源的更新和固存,提高了深层土壤的固碳潜力。     

川西亚高山不同海拔森林土壤活性氮库及净氮矿化的季节动态

研究了川西理县毕棚沟不同海拔梯度(3600 m、3300 m和3000 m)森林群落土壤活性氮库及土壤净氮矿化速率的季节动态.结果表明: 研究区森林土壤活性氮库(铵态氮、硝态氮、微生物生物量氮和可溶性有机氮)及净氮矿化速率存在明显的季节变化,但不同形态土壤活性氮库的季节动态有一定差异.4个采样时期(非生长季与生长季初期、中期及末期)各海拔土壤硝态氮浓度(8.38～89.60 mg·kg^-1)均显著高于铵态氮浓度(0.44～8.43 mg·kg^-1).生长季初期各海拔梯度的土壤净氮矿化速率均表现为负值(-0.77～-0.56 mg·kg^-1·d^-1),而非生长季、生长季中期和末期均为正值.除硝态氮外,不同海拔的土壤铵态氮、微生物生物量氮和可溶性有机氮浓度的差异极显著,海拔对它们的影响与季节变化有关.该区土壤净氮矿化以硝化为主,且氮矿化过程不受海拔梯度的影响.冬季土壤净氮矿化明显(0.42～099 mg·kg^-1·d^-1),早春高的土壤无机氮可能为植物生长提供基础养分,也可能通过淋溶方式从系统中丢失.