Consequences of liming and gypsum top-dressing on nitrogen and carbon dynamics in acid forest soils with different humus forms

The aim of this study was to understand the effects of lime and gypsum on nitrogen and carbon turnover of the soil. A pot experiment was conducted in parallel with a field experiment which was set up in 1989 in a declining forest of the French Ardennes. A dystric cambisol, associated with a moder and mull humus separately, was used to study changes in the soil chemistry as a result of added lime and gypsum top-dressing.The lime was applied to the surface of an acid mull humus of an oak (Quercus petraea) stand and of a moder humus of a spruce (Picea abies) stand. A quantity of 2.8 t ha^-1 equivalent CaO was supplied as CaCO₃, CaCO₃+MgO and CaSO₄.2H₂O. The experiment was installed in an open-air nursery for 20 months, during which the organic carbon and nitrogen in the solution were analysed monthly. They were analysed in the solid phase after 20 months. At the end of this period the changes in the soil and leachate depended mainly on the type of the material added.The leachate was enriched with nitrogen from the third month of the experiment under lime treatments and in the control. The same pattern was found under the two humus types but the magnitude was higher in soil with a mull humus. The nitrogen was mostly leached as NO₃ ^--N in the carbonate treatments and in the control, whereas it was predominantly NH₄ ⁺-N under gypsum. The NO₃ ^--N was 50% higher than NH₄ ⁺-N in the control and CaCO₃, CaCO₃+MgO treatments. In the CaSO₄ treatment this phenomenon was reversed. The leaching of organic carbon was greater under gypsum than under the other treatments whatever the humus.In the solid phase of the soil (organic layers) the organic carbon and nitrogen concentration decreased significantly after liming, especially in the mull humus. Consequently it induced a decrease in C:N ratio of about 18% with respect to the control.     

The influence of omnivorous elaterid larvae on the microbial carbon cycle in different forest soils

Summary Data are presented on the influence of Athous subfuscus larvae (Coleoptera, Elateridae) on the microbial carbon cycle in the biotically most active horizons of three contrasting beech forest soils: the A_h horizon of a mull soil on limestone (Göttinger Wald, FRG), the F/H horizon of a moder soil on new red sandstone (Solling area, FRG) and in the F/H horizon of a lime ameliorated area close to the second site. Gut content analyses demonstrated that the larvae of A. subfuscus are humiphagous and that this unspecific feeding behaviour is widely independent of soil conditions. Differences in ¹⁴C incorporation demonstrated that only the larvae in the F/H horizon of the limed moder soil directly affected primary decomposer organisms. However, the burrowing activity of the larvae in the topsoil indirectly modified the time course of beech leaf-litter decomposition in the litter layer of all three soils. The microflora of the mull soil contained 2.6%, that of the moder soil 0.7% and that of the limed moder soil 2.2% of total C. The metabolic quotient (qCO₂, 10°C) of the soil microflora was 0.0010 (mgCO₂-C·mg^-1 biomass-C·h^-1) in the mull soil, 0.0034 in the moder soil and 0.0012 in the limed moder soil. The A. subfuscus larvae generally reduced the size of the microbial C pool (<-30%) and increased the metabolic quotient of the microflora (>+50%). Considering these soil-independent effects of A. subfuscus on the C turnover of the soil microflora, the burrowing activity of humiphagous soil arthropods may generally increase nutrient availability to primary producers. The results of this study reveal that some of the micro- and mesoscale effects of humiphagous arthropods on the microbial carbon turnover in beech forest soils are surprisingly similar, even under very different soil conditions. The long-term modification of the time course of leaf litter decomposition, in contrast, indicates that the influence of humiphagous arthropods on the formation of the humus layer is soil-specific. There are profound differences in the role of humiphagous arthropods in limed moder soils and in naturally base-rich soils. It is concluded that liming increases competition within the microfloral population due to accelerated humification. The negative effect of A. subfuscus on ¹⁴C mineralization in the limed substrate could thus be explained by its effects on a microflora that was strongly limited by the availability of carbon.     

Dynamics of the internal soil nitrogen cycles under moder and mull forest floor types on a slope in a <Emphasis Type="Italic">Cryptomeria japonica</Emphasis> D. Don plantation

To examine the influence of microbial carbon (C) availability on the internal soil nitrogen (N) cycles under moder and mull forest floor types within the same slope sequence, surface mineral soils (0–5cm depth) taken at upper (moder-type forest floor) and lower (mull-type forest floor) positions on a slope in a Cryptomeria japonica D. Don plantation were incubated for 300days. During the incubation, changes in net and gross N transformations, the organic C and N pools, and microbial respiration were monitored. Despite relatively small differences in net N mineralization in both soils, very rapid rates of gross N transformations were found in mull soil during the initial 15days of the experiment. A rapid net nitrification occurred after days 150 and 100 in moder and mull soils, respectively, presumably because of decreased microbial C availability. However, a rapid net nitrification also occurred in the mull soil during the initial 15days when microbial C availability was high, and gross nitrification was detected in both soils, except at day 0 in the moder soil. Changes in gross N transformations and in organic C and N pools over the experiment suggested that the start of rapid net nitrification might be influenced not only by microbial C availability, but also by the microbial availability of N relative to C.     

15N natural abundance of foliage and soil across boreal forests of Finland

This study presents the latitudinal variation (from 60° 30′ N to 68° 2′ N latitude) of natural abundances of ¹⁵N in the foliage, humus and soils of boreal forests of Finland. Our results clearly showed that N concentration of the foliage did not change significantly with latitudes but their ¹⁵N values were significantly higher in higher latitude sites relative to that of the mid and lower latitude sites, indicating the different forms of N uptake at the higher latitudes compared to the lower latitudes. We assume that the higher foliage δ¹⁵N values of the higher latitudes trees might be due to either more openness of N cycle (greater proportional N losses) in these latitudes compared to the sites of southern latitudes (lower N losses) or the differences in their mycorrhizal associations. Regression analysis showed that the temperature was the main factor influencing the ¹⁵N natural abundance of humus and soil of all forest ecosystems, both before and after clear-cut, whereas rainfall was the main controlling factor to the foliage ¹⁵N. Possible reasons behind the increasing δ¹⁵N natural abundances of plants and soils with increasing latitudes are discussed in this paper. The clear-cut did not show any specific trend on the ¹⁵N fractionation in humus and soil, i.e. both ¹⁵N-enrichment and -depletion occurred after clear-cut.     

Nitrogen cycling in canopy soils of tropical montane forests responds rapidly to indirect N and P fertilization

Although the canopy can play an important role in forest nutrient cycles, canopy‐based processes are often overlooked in studies on nutrient deposition. In areas of nitrogen (N) and phosphorus (P) deposition, canopy soils may retain a significant proportion of atmospheric inputs, and also receive indirect enrichment through root uptake followed by throughfall or recycling of plant litter in the canopy. We measured net and gross rates of N cycling in canopy soils of tropical montane forests along an elevation gradient and assessed indirect effects of elevated nutrient inputs to the forest floor. Net N cycling rates were measured using the buried bag method. Gross N cycling rates were measured using ¹⁵N pool dilution techniques. Measurements took place in the field, in the wet and dry season, using intact cores of canopy soil from three elevations (1000, 2000 and 3000 m). The forest floor had been fertilized biannually with moderate amounts of N and P for 4 years; treatments included control, N, P, and N + P. In control plots, gross rates of NH₄⁺ transformations decreased with increasing elevation; gross rates of NO₃^? transformations did not exhibit a clear elevation trend, but were significantly affected by season. Nutrient‐addition effects were different at each elevation, but combined N + P generally increased N cycling rates at all elevations. Results showed that canopy soils could be a significant N source for epiphytes as well as contributing up to 23% of total (canopy + forest floor) mineral N production in our forests. In contrast to theories that canopy soils are decoupled from nutrient cycling in forest floor soil, N cycling in our canopy soils was sensitive to slight changes in forest floor nutrient availability. Long‐term atmospheric N and P deposition may lead to increased N cycling, but also increased mineral N losses from the canopy soil system.     

半干旱黄土丘陵区纯林土壤腐殖质异化特征及与其他性质的关系

纯林长期生长或多代连栽必然会导致土壤腐殖质含量和构成发生异化,探究这种异化特征及其与土壤其他性质的关系可以为纯林管理或混交改造提供科学依据。通过对半干旱黄土丘陵区南泥湾林场8种典型纯林土壤腐殖质及其他性质进行系统检测,结果表明:(1)侧柏林土壤腐殖质含量最高(34.61 g/kg),腐殖化程度和稳定性一般;白榆和白桦林土壤的腐殖质含量中等(19.69—23.58 g/kg)、腐殖化程度和稳定性最佳;茶条槭和小叶杨林土壤的腐殖质含量(20.59—22.53 g/kg)和构成均为中等水平;油松、沙棘和刺槐林土壤的腐殖质质量较低(11.77—13.81 g/kg),且腐殖化程度较低,稳定性相对最差;(2)与胡敏酸含量存在显著相互促进作用(P0.05)的土壤性质为CEC、N、微生物量和蛋白酶活性(相关系数0.769—0.926,下同),存在显著相互抑制作用的为有效Cu(-0.793);与富啡酸存在显著相互促进作用的为N、CEC、微生物量、蔗糖酶和磷酸酶活性(0.836—0.955),存在显著相互抑制作用的为有效Cu(-0.822);与胡敏素存在显著相互促进作用的为N、CEC、微生物量、磷酸酶活性和有效Zn(0.766—0.951),存在显著相互抑制作用的为脱氢酶活性(-0.784)。(3)腐殖质构成与其他性质的相关性均不显著(P0.05),其中相对有利于提高胡敏酸/腐殖酸含量之比的土壤性质为蛋白酶、蔗糖酶和过氧化氢酶活性,而不利的是脱氢酶活性;相对有利于提高胡敏酸/富啡酸含量之比的为速效K、CEC和脲酶活性,而不利的是脱氢酶活性。(4)总体而言土壤腐殖质含量较之腐殖质构成与其他性质之间具有更大的相关性;向土壤增施N肥可以促进腐殖质的形成,增加K肥则有利于腐殖质构成的改善,而通过混交改造或增加林下植被是促进纯林土壤腐殖质化过程和解决土壤退化的根本措施。     

川西亚高山不同森林生态系统碳氮储量及其分配格局

森林采伐和恢复是影响森林碳氮储量的重要因素。以川西亚高山岷江冷杉原始林、粗枝云杉阔叶林、天然次生林和粗枝云杉人工林为研究对象,采用样地调查和生物量实测的方法,研究了不同森林生态系统各组分碳、氮储量及其分配特征。结果表明岷江冷杉原始林、粗枝云杉阔叶林、天然次生林和粗枝云杉人工林生态系统碳储量分别为611.18、252.31、363.07 tC/hm~2和239.06 tC/hm~2;氮储量分别为16.44、12.11、15.48 tN/hm~2和8.92 tN/hm~2。恢复林分与原始林碳储量在土壤—植被的分配格局发生了变化,而氮储量未发生变化。岷江冷杉原始林以植被碳储量为主,恢复林分以土壤为主,氮储量均以土壤为主。乔木层碳储量分别占生态系统总储量的56.65%、17.63%、13.57%和22.05%,土壤层(0—80 cm)分别占32.03%、69.87%、76.20%和72.12%;土壤层氮储量占生态系统总储量的76.80%—92.58%。植物残体碳氮储量分别占生态系统总储量的4.40%—9.83%和2.94%—7.08%,林下植被所占比例最小。空间格局上,岷江冷杉原始林植被部分具有较高的碳储量,应进行保护。3种恢复林分具有较高的碳汇潜力,且地上/地下碳储量较低,表明其碳汇潜力尤其表现在地上部分。天然次生林利于土壤有机碳的积累,而人工林乔木层碳储量较高。     

The soil food web of two beech forests (<Emphasis Type="Italic">Fagus sylvatica</Emphasis>) of contrasting humus type: stable isotope analysis of a macro- and a mesofauna-dominated community

The structure of the soil food web in two beech (Fagus sylvatica) forests, the Göttinger Wald and the Solling forest (Northern Germany), was investigated using variations in tissue ¹⁵N concentrations of animal species or taxa. The Göttinger Wald is located on a limestone plateau and characterized by mull humus with high macrofauna activity, particularly of Lumbricidae, Diplopoda and Isopoda. In contrast, the Solling forest is located on a sandstone mountain range and characterized by moder humus. The soil fauna of this forest is dominated by mesofauna, particularly by Collembola, Enchytraeidae and Oribatida. In June 1995 soil fauna was sampled using heat extraction. Three soil layers were analysed at each of the sites. ¹⁵N/¹⁴N ratios of bulk material increased strongly with soil depth in both forests. This also applied to the water-soluble fraction at the Göttinger Wald, but not at the Solling. Generally, the water-soluble fraction was more enriched in ¹⁵N than the bulk materials. For most animals studied ¹⁵N/¹⁴N ratios varied little with soil depth. In both forests soil animals could be classified either as saprophages, including microphytophages, or predators. On average, the δ¹⁵N of predatory taxa (Chilopoda, Araneida, Gamasina, Staphylinidae) exceeded that of saprophagous or microphytophagous taxa (Lumbricidae, Isopoda, Diplopoda, Collembola, Oribatida, Enchytraeidae) by 4.4 and 3.9‰ for the Göttinger Wald and the Solling, respectively. We assume that most of the saprophagous or microphytophagous taxa studied consist of primary and secondary decomposers and hypothesize that predators prey more on secondary than primary decomposers. Generally, average δ¹⁵N values differed little between saprophagous (Lumbricidae, Diplopoda, Isopoda) and microphytophagous taxa (Collembola, Oribatida). The variations in δ¹⁵N values of species within these taxa consistently exceeded the variation between them, indicating that the species of each of these taxa form a continuum from primary to secondary decomposers. Also, variations in δ¹⁵N values within predatory taxa in most cases exceeded that between taxa excluding top predators like Sorex. We conclude that using higher taxonomic units in soil food web analysis is problematic and in general not consistent with nature. Higher taxonomic units may only be useful for depicting very general trophic groupings such as predators or microbi-detritivores.     

Nitrogen addition reduces soil respiration in a mature tropical forest in southern China

Response of soil respiration (CO₂ emission) to simulated nitrogen (N) deposition in a mature tropical forest in southern China was studied from October 2005 to September 2006. The objective was to test the hypothesis that N addition would reduce soil respiration in N saturated tropical forests. Static chamber and gas chromatography techniques were used to quantify the soil respiration, following four‐levels of N treatments (Control, no N addition; Low‐N, 5 g N m^?2 yr^?1; Medium‐N, 10 g N m^?2 yr^?1; and High‐N, 15 g N m^?2 yr^?1 experimental inputs), which had been applied for 26 months before and continued throughout the respiration measurement period. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates found in the warm and wet growing season (April–September) and the lowest rates in the dry dormant season (December–February). Soil respiration rates showed a significant positive exponential relationship with soil temperature, whereas soil moisture only affect soil respiration at dry conditions in the dormant season. Annual accumulative soil respiration was 601±30 g CO₂‐C m^?2 yr^?1 in the Controls. Annual mean soil respiration rate in the Control, Low‐N and Medium‐N treatments (69±3, 72±3 and 63±1 mg CO₂‐C m^?2 h^?1, respectively) did not differ significantly, whereas it was 14% lower in the High‐N treatment (58±3 mg CO₂‐C m^?2 h^?1) compared with the Control treatment, also the temperature sensitivity of respiration, Q₁₀ was reduced from 2.6 in the Control with 2.2 in the High‐N treatment. The decrease in soil respiration occurred in the warm and wet growing season and were correlated with a decrease in soil microbial activities and in fine root biomass in the N‐treated plots. Our results suggest that response of soil respiration to atmospheric N deposition in tropical forests is a decline, but it may vary depending on the rate of N deposition.     

Immobilization, stabilization and remobilization of nitrogen in forest soils at elevated CO2: a 15N and 13C tracer study

The fate of immobilized N in soils is one of the great uncertainties in predicting C sequestration at increased CO₂ and N deposition. In a dual isotope tracer experiment (¹³C, ¹⁵N) within a 4‐year CO₂ enrichment (+200 ppm_v) study with forest model ecosystems, we (i) quantified the effects of elevated CO₂ on the partitioning of N; (ii) traced immobilized N into physically separated pools of soil organic matter (SOM) with turnover rates known from their ¹³C signals; and (iii) estimated the remobilization and thus, the bio‐availability of newly sequestered C and N. (1) CO₂ enrichment significantly decreased NO₃^? concentrations in soil waters and export from 1.5 m deep lysimeters by 30–80%. Consequently, elevated CO₂ increased the overall retention of N in the model ecosystems. (2) About 60–80% of added ¹⁵NH₄¹⁵NO₃ were retained in soils. The clay fraction was the greatest sink for the immobilized ¹⁵N sequestering 50–60% of the total new soil N. SOM associated with clay contained only 25% of the total new soil C pool and had small C/N ratios (<13), indicating that it consists of humified organic matter with a relatively slow turn over rate. This implies that added ¹⁵N was mainly immobilized in stable mineral‐bound SOM pools. (3) Incubation of soils for 1 year showed that the remobilization of newly sequestered N was three to nine times smaller than that of newly sequestered C. Thus, inorganic inputs of N were stabilized more effectively in soils than C. Significantly less newly sequestered N was remobilized from soils previously exposed to elevated CO₂. In summary, our results show firstly that a large fraction of inorganic N inputs becomes effectively immobilized in relative stable SOM pools and secondly that elevated CO₂ can increase N retention in soils and hence it may tighten N cycling and diminish the risk of nitrate leaching to groundwater.     

How much carbon is stored in the terrestrial ecosystems of the Chilean Patagonia?

We estimated the amount of carbon (C) stored in terrestrial ecosystems of the Chilean Patagonia and the proportion within protected areas. We used existing public databases that provide information on C stocks in biomass and soils. Data were analysed by ecosystem and forest type in the case of native forests. Our results show that some ecosystems have been more extensively studied both for their stocks in biomass and soils (e.g. forests) compared with others (e.g. shrublands). Forests and peatlands store the largest amount of C because of their large stocks per hectare and the large area they cover. The total amount of C stored per unit area varies from 261.7 to 432.8 Mg C ha⁻¹, depending on the published value used for soil organic C stocks in peatlands, highlighting the need to have more precise estimates of the C stored in this and other ecosystems. The mean stock in national parks (508 Mg C ha⁻¹) is almost twice the amount stored in undisturbed forests in the Amazon. State and private protected areas contain 58.9% and 2.1% of the C stock, respectively, playing a key role in protecting ecosystems in this once pristine area.     

Soil macro-invertebrates and litter disappearance in a Japanese mixed deciduous forest and comparison with European deciduous forests and tropical rainforests

Soil macro-invertebrates and rate of litter disappearance were studied in a ridge plot with moder (mor) humus and a bottom plot with mull humus on a slope in a temperate mixed deciduous forest in Kyoto, Japan (J). The results were compared with those from two German beech forests (G) representative of European deciduous forest mor and mull. Between-plot differences in biomass of total saprophagous animals was much smaller in J than in G, which is dominated by earthworms. Susceptibility to soil acidity and zoogeographical distribution of earthworms were suggested to be related to this situation. Biomass of soil macro-invertebrates and litter turnover rate were compared among J, G and three types of tropical rainforests in Malaysia (M) in relation to climatic conditions. Taking into account among-site differences in temperature and moisture, which affect microbial activity and in biomass of saprophagous macro-invertebrates especially earthworms, the following order of importance of soil macro-invertebrates in determining the rate of litter disappearance was suggested: G>J>M. Based on the comparison of biomass of earthworms among European deciduous forests, Japanese deciduous forests and tropical rainforests, as well as on the presence or absence of anecic earthworms in these forests, it was suggested that this ranking could be generalized to European deciduous forests > Japanese deciduous forests > tropical rainforests. It was pointed out that this order was the opposite of the gradient in evapotranspiration rate existing among these regions.     

Effects of land use on 15N natural abundance of soils in Ethiopian highlands

We used the natural abundance of ¹⁵N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in ¹³C natural abundance, patterns of δ¹⁵N and δ¹³C values were compared to determine if shifts of ¹⁵N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ¹⁵N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth. The low δ¹⁵N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ¹³C of the surface horizon and the high δ¹³C of the lower soil horizons is clear evidence of a long phase of C₄ grass dominance or cultivation of C₄ crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ¹³C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ¹⁵N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ¹⁵N values of surface mineral soils. The high δ¹⁵N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Impacts of climate and land use on N2O and CH4 fluxes from tropical ecosystems in the Mt. Kilimanjaro region,Tanzania

In this study, we quantify the impacts of climate and land use on soil N₂O and CH₄ fluxes from tropical forest, agroforest, arable and savanna ecosystems in Africa. To do so, we measured greenhouse gases (GHG) fluxes from 12 different ecosystems along climate and land‐use gradients at Mt. Kilimanjaro, combining long‐term in situ chamber and laboratory soil core incubation techniques. Both methods showed similar patterns of GHG exchange. Although there were distinct differences from ecosystem to ecosystem, soils generally functioned as net sources and sinks for N₂O and CH₄ respectively. N₂O emissions correlated positively with soil moisture and total soil nitrogen content. CH₄ uptake rates correlated negatively with soil moisture and clay content and positively with SOC. Due to moderate soil moisture contents and the dominance of nitrification in soil N turnover, N₂O emissions of tropical montane forests were generally low (<1.2 kg N ha^?1 year^?1), and it is likely that ecosystem N losses are driven instead by nitrate leaching (~10 kg N ha^?1 year^?1). Forest soils with well‐aerated litter layers were a significant sink for atmospheric CH₄ (up to 4 kg C ha^?1 year^?1) regardless of low mean annual temperatures at higher elevations. Land‐use intensification significantly increased the soil N₂O source strength and significantly decreased the soil CH₄ sink. Compared to decreases in aboveground and belowground carbon stocks enhanced soil non‐CO₂ GHG emissions following land‐use conversion from tropical forests to homegardens and coffee plantations were only a small factor in the total GHG budget. However, due to lower ecosystem carbon stock changes, enhanced N₂O emissions significantly contributed to total GHG emissions following conversion of savanna into grassland and particularly maize. Overall, we found that the protection and sustainable management of aboveground and belowground carbon and nitrogen stocks of agroforestry and arable systems is most crucial for mitigating GHG emissions from land‐use change.     

六盘山四种森林生态系统的碳氮储量、组成及分布特征

碳和氮是森林生态系统的重要组成元素,其含量有很大时空差异,并和立地及森林特征关系很大,需做大量的积累性调查才能得到其变化规律,尤其是加强在过去较少研究的西北地区的调查。在宁夏六盘山区选择华北落叶松(Larix principisrupprechtii)人工林、华山松(Pinus armandii)次生林、桦木(Betula platyphylla)次生林和野李子(Prunus salicina)灌丛4种典型森林,测定了乔木层(分不同器官)、灌木层、草本层、枯落物层、根系层(0—100 cm土壤)的碳、氮含量,分析了生态系统的碳、氮储量及成分组成和层次分布特征。结果表明,碳含量在不同乔木树种及其不同器官之间的差异不明显;但氮含量存在显著的树种差别和器官差异,以树叶的最高、树干的最低。灌木层和草本层的碳氮含量均表现为地上部分地下部分。各森林样地的乔木层、灌木层、草本层的碳含量依次降低,但氮含量依次增高;枯落物层的碳含量低于各植被层,但氮含量高于各植被层;根系层土壤的碳、氮含量则随土层增深而递减。包括活植被层、枯落物层和根系层土壤在内的华北落叶松人工林、华山松次生林、桦木次生林、野李子灌丛的生态系统碳储量依次为364.56、450.98、640.02、196.55 t/hm2,氮储量依次为27.86、36.19、47.02、15.99 t/hm2。所有4种森林生态系统的根系层土壤的碳氮储量均占整个生态系统总储量的绝大部分,其比例对碳储量为84.69%—93.92%,氮储量为98.09%—98.64%。从乔木层、灌木层、草本层、枯落物层到根系层(土壤),呈现出C/N比依次减小的趋势;根系层土壤和整个生态系统的C/N比分别为华北落叶松林的11.84和13.12、华山松林的10.76和12.56、桦木林的12.48和13.52、野李子灌丛的11.70和12.29。     

Long‐term nitrogen addition modifies microbial composition and functions for slow carbon cycling and increased sequestration in tropical forest soil

Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N‐limited temperate forests. In N‐rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old‐growth tropical forest soil responded to 13 years of N addition at four rates: 0 (Control), 50 (Low‐N), 100 (Medium‐N), and 150 (High‐N) kg N ha^?1 year^?1. Soil organic carbon (SOC) content increased under High‐N, corresponding to a 33% decrease in CO₂ efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N₂O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes (narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High‐N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N₂O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems.     

Earthworm invasion into previously earthworm-free temperate and boreal forests

Earthworms are keystone detritivores that can influence primary producers by changing seedbed conditions, soil characteristics, flow of water, nutrients and carbon, and plant–herbivore interactions. The invasion of European earthworms into previously earthworm-free temperate and boreal forests of North America dominated by Acer, Quercus, Betula, Pinus and Populus has provided ample opportunity to observe how earthworms engineer ecosystems. Impacts vary with soil parent material, land use history, and assemblage of invading earthworm species. Earthworms reduce the thickness of organic layers, increase the bulk density of soils and incorporate litter and humus materials into deeper horizons of the soil profile, thereby affecting the whole soil food web and the above ground plant community. Mixing of organic and mineral materials turns mor into mull humus which significantly changes the distribution and community composition of the soil microflora and seedbed conditions for vascular plants. In some forests earthworm invasion leads to reduced availability and increased leaching of N and P in soil horizons where most fine roots are concentrated. Earthworms can contribute to a forest decline syndrome, and forest herbs in the genera Aralia, Botrychium, Osmorhiza, Trillium, Uvularia, and Viola are reduced in abundance during earthworm invasion. The degree of plant recovery after invasion varies greatly among sites and depends on complex interactions with soil processes and herbivores. These changes are likely to alter competitive relationships among plant species, possibly facilitating invasion of exotic plant species such as Rhamnus cathartica into North American forests, leading to as yet unknown changes in successional trajectory.     

Mycorrhizal mediation of plant N acquisition and residue decomposition: Impact of mineral N inputs

Mycorrhizas are ubiquitous plant–fungus mutualists in terrestrial ecosystems and play important roles in plant resource capture and nutrient cycling. Sporadic evidence suggests that anthropogenic nitrogen (N) input may impact the development and the functioning of arbuscular mycorrhizal (AM) fungi, potentially altering host plant growth and soil carbon (C) dynamics. In this study, we examined how mineral N inputs affected mycorrhizal mediation of plant N acquisition and residue decomposition in a microcosm system. Each microcosm unit was separated into HOST and TEST compartments by a replaceable mesh screen that either prevented or allowed AM fungal hyphae but not plant roots to grow into the TEST compartments. Wild oat (Avena fatua L.) was planted in the HOST compartments that had been inoculated with either a single species of AM fungus, Glomus etunicatum, or a mixture of AM fungi including G. etunicatum. Mycorrhizal contributions to plant N acquisition and residue decomposition were directly assessed by introducing a mineral ¹⁵N tracer and ¹³C‐rich residues of a C₄ plant to the TEST compartments. Results from ¹⁵N tracer measurements showed that AM fungal hyphae directly transported N from the TEST soil to the host plant. Compared with the control with no penetration of AM fungal hyphae, AM hyphal penetration led to a 125% increase in biomass ¹⁵N of host plants and a 20% reduction in extractable inorganic N in the TEST soil. Mineral N inputs to the HOST compartments (equivalent to 5.0 g N m^?2 yr^?1) increased oat biomass and total root length colonized by mycorrhizal fungi by 189% and 285%, respectively, as compared with the no‐N control. Mineral N inputs to the HOST plants also reduced extractable inorganic N and particulate residue C proportion by 58% and 12%, respectively, in the corresponding TEST soils as compared to the no‐N control, by stimulating AM fungal growth and activities. The species mixture of mycorrhizal fungi was more effective in facilitating N transport and residue decomposition than the single AM species. These findings indicate that low‐level mineral N inputs may significantly enhance nutrient cycling and plant resource capture in terrestrial ecosystems via stimulation of root growth, mycorrhizal functioning, and residue decomposition. The long‐term effects of these observed alterations on soil C dynamics remain to be investigated.     

Litter type control on soil C and N stabilization dynamics in a temperate forest

While plant litters are the main source of soil organic matter (SOM) in forests, the controllers and pathways to stable SOM formation remain unclear. Here, we address how litter type (¹³C/¹⁵N‐labeled needles vs. fine roots) and placement‐depth (O vs. A horizon) affect in situ C and N dynamics in a temperate forest soil after 5 years. Litter type rather than placement‐depth controlled soil C and N retention after 5 years in situ, with belowground fine root inputs greatly enhancing soil C (x1.4) and N (x1.2) retention compared with aboveground needles. While the proportions of added needle and fine root‐derived C and N recovered into stable SOM fractions were similar, they followed different transformation pathways into stable SOM fractions: fine root transfer was slower than for needles, but proportionally more of the remaining needle‐derived C and N was transferred into stable SOM fractions. The stoichiometry of litter‐derived C vs. N within individual SOM fractions revealed the presence at least two pools of different turnover times (per SOM fraction) and emphasized the role of N‐rich compounds for long‐term persistence. Finally, a regression approach suggested that models may underestimate soil C retention from litter with fast decomposition rates.     

亚热带次级森林演替过程中模拟氮磷沉降对土壤微生物生物量及土壤养分的影响

氮（N）沉降深刻影响着森林生态系统的生物多样性、生产力和稳定性。亚热带地区森林土壤磷（P）的有效性较低,N沉降将更突显P的限制作用。N、P输入对亚热带次级森林土壤的影响是否依赖于森林演替阶段知之甚少。选取两种不同演替年龄阶段（年轻林：<40 a;老年林：>85 a）的亚热带常绿阔叶林,设置模拟N和/或P沉降（10 g m^-2 a^-1）4个处理（Ctrl、N、P、NP）,连续处理4.5年后采集表层、次表层和下底层（0-15、15-30、30-60 cm）土壤样品,综合分析了土壤微生物生物量碳（MBC）氮（MBN）和多种土壤养分含量。结果表明,MBC、MBN及土壤养分含量均随土壤深度增加而降低。N添加对两种演替阶段森林土壤中MBC和MBN均无显著影响。施P相关处理（P和NP）对年轻林表层土壤MBC和MBN无显著影响,但显著增加了老年林表层土壤MBC和MBN（P<0.05）,表明老年林可能比年轻林更易受P限制。N添加显著增加了两种演替森林表层土壤可溶性有机氮（DON）、氨态氮（NH₄⁺-N）和硝态氮（NO₃^--N）的含量（P<0.05）;P相关处理（P和NP）显著增加两种演替阶段表层和次表层土壤速效磷（AP）以及表层土壤全磷（TP）的含量（P<0.05）。土壤MBC和MBN与土壤中各养分指标（可溶性有机碳DOC、DON、NH₄⁺-N、NO₃^--N、AP、全碳TC、全氮TN和TP）呈显著正相关关系,土壤TC、TN和DOC是影响土壤微生物生物量的主要因子。研究可为评估和揭示未来全球环境变化背景下不同演替林龄亚热带森林的土肥潜力及土壤质量的演变提供一定的科学理论依据。