Effects of six years of simulated N deposition on gross soil N transformation rates in an old-growth temperate forest

Elevated atmospheric nitrogen(N) deposition has been detected in many regions of China, but its effects on soil N transformation in temperate forest ecosystems are not well known. We therefore simulated N deposition with four levels of N addition rate(N0, N30, N60, and N120) for6 years in an old-growth temperate forest in Xiaoxing'an Mountains in Northeastern China. We measured gross N transformation rates in the laboratory using ~(15)N tracing technology to explore the effects of N deposition on soil gross N transformations taking advantage of N deposition soils. No significant differences in gross soil N transformation rates were observed after 6 years of N deposition with various levels of N addition rate. For all N deposition soils, the gross NH_4~+ immobilization rates were consistently lower than the gross N mineralization rates,leading to net N mineralization. Nitrate(NO_3~-) was primarily produced via oxidation of NH_4~+(i.e., autotrophic nitrification), whereas oxidation of organic N(i.e., heterotrophic nitrification) was negligible. Differences between the quantity of ammonia-oxidizing bacteria and ammonia-oxidizing archaea were not significant for any treatment, which likely explains the lack of a significant effect on gross nitrification rates. Gross nitrification rates were much higher than the total NO_3~- consumption rates,resulting in a build-up of NO_3~-, which highlights the high risk of N losses via NO_3~- leaching or gaseous N emissions from soils. This response is opposite that of typical N-limited temperate forests suffering from N deposition,suggesting that the investigated old-growth temperate forest ecosystem is likely to approach N saturation.     

Nitrification and denitrification in subalpine coniferous forests of different restoration stages in western Sichuan, China

Nitrification is the biological conversion of organic or inorganic nitrogen compounds from a reduced to a more oxidized state. Denitrification is generally referred to as the microbial reduction of nitrate to nitrite and further gaseous forms of nitric oxide, nitrous oxide and molecular nitrogen. They are functionally interconnected processes in the soil nitrogen cycle that are involved in the control of long-term nitrogen losses in ecosystems through nitrate leaching and gaseous N losses. In order to better understand how nitrification and denitrification change during the process of ecosystem restoration and how they are affected by various controlling factors, gross nitrification rates and denitrification rates were determined using the barometric process separation (BaPS) technique in subalpine coniferous forests of different restoration stages. The results showed that forest restoration stage had no significant effects on gross nitrification rates or denitrification rates (One-way ANOVA (analysis of variance), p < 0.05). There was no significant difference in the temperature coefficient (Q ₁₀) for gross nitrification rate among all the forest sites (One-way ANOVA, p < 0.05). Gross nitrification rates were positively correlated with water content (p < 0.05), but not with soil pH, organic matter, total nitrogen, or C/N ratios. Denitrification rates in all the forest soils were low and not closely correlated with water content, soil pH, organic matter, or total nitrogen. Nevertheless, we found that C/N ratios obviously affected denitrification rates (p < 0.05). Results from this research suggest that gross nitrification is more responsible for the nitrogen loss from soils compared with denitrification. Translated from Journal of Plant Ecology, 2006, 30(1): 90–96 [译自: 植物生态学报]     

Nitrate attenuation in a small temperate wetland following forest harvest

Elevated nitrate concentrations in streams and groundwater are frequently observed following forest harvest. In addition to depleting nutrients available for forest regeneration, elevated nitrate export following harvest can have deleterious effects on downstream aquatic ecosystems. As part of a forest harvest experiment conducted at the Turkey Lakes Watershed, Ontario, Canada, stable isotope techniques were employed to investigate nitrate attenuation in a natural wetland receiving high concentrations of nitrate as a result of clear-cutting in the catchment. Isotopic analysis of nitrate (δ¹⁸O, δ¹⁵N) and vegetation (δ¹⁵N) demonstrated that both denitrification and plant uptake of nitrate resulted in significantly lower nitrate concentrations in wetland outflow compared to incoming stream water. Although the 0.2-ha forested swamp (4% of catchment by area) was too small to be featured on standard topographic maps, the wetland remove 65–100% of surface water nitrate inputs, thereby protecting downstream aquatic habitats from the full effect of N release from forest harvest. The δ¹⁵N enrichment factor associated with nitrate attenuation in wetland surface water was lower than typically observed during denitrification in groundwaters, suggesting that nitrate removal is complete in some areas of the wetland. Plant assimilation of nitrate was also partially responsible for the low observed enrichment factor. Wetland plants recorded the high δ¹⁵N associated with denitrification activity in portions of the wetland. Apportionment of nitrate sources using δ¹⁸O–NO₃⁻ at the outlet weir was unaffected by the wetland nitrate attenuation under pre- and post-harvest conditions due to the mid-catchment position of the wetland. Future forest management practices designed to recognize and preserve small wetlands could reduce the potentially detrimental effects of forest harvest on aquatic systems.     

Selective logging of lowland evergreen rainforests in Chiloé Island, Chile: Effects of changing tree species composition on soil nitrogen transformations

Lowland evergreen rainforests in southern Chile growing on highly productive soils and accessible sites have been subjected to traditional and industrial logging of valuable timber trees. Old-growth rain forests in this area are characterized by highly conservative N cycles, which results in an efficient N use of ecosystems. We hypothesize that different logging practices, by changing forest structure and species composition, can alter the quantity and quality (i.e. C/N ratio) of litterfall and soil organic matter and soil microbial processes that determine N storage and availability. To test this hypothesis we investigated chemical properties, microbial N transformations, N fluxes and N storage in soils of lowland evergreen rainforests of Chiloé Island after 10 years since industrial selective logging (ISL) and in stands subjected to traditional selective logging (TSL) by landowners in small properties. We compared them to reference unlogged old-growth stands (OG) in the same area. Tree basal area was more reduced in the stands subjected to ISL than to TSL. Litterfall inputs were similar in both logging treatments as in OG stands. This was due to greater biomass of understory species after logging. In TSL understory tree species determined a higher litterfall C/N ratio than ISL. We found higher soil N availability and content of base cations in surface soils of logged forests than in OG. The litter horizon of OG forest had significantly higher rates of non-symbiotic N fixation than logged forests. In the ISL treatment there was a trend toward increasing soil denitrification and significantly higher NO₃–N/N_t ratio in spring waters, which led to a stronger δ¹⁵N signal in surface and deep soils. We conclude that massive understory occupation by the shade-intolerant native bamboo Chusquea quila in ISL led to enhanced litter quality (lower C/N ratios) relaxing the tightness of the N cycle, which increased soil N availability leading to a higher proportion of nitrate in spring waters and higher gaseous N losses. In contrast, under TSL a higher litterfall C/N ratio slowed decomposition and net N mineralization rates thus reducing the chances for N losses, and enhancing C and N storage in soil. We suggest that sustainable logging practices in these rain forests should be based on lower rates of canopy removal to enhance colonization of the understory by shade-tolerant trees, which are associated with a more efficient N cycle.     

Advances of study on atmospheric methane oxidation （consumption） in forest soil

Next to CO2, methane (CH4) is the second important contributor to global warming in the atmosphere and global atmospheric CH4 budget depends on both CH4 sources and sinks. Unsaturated soil is known as a unique sink for atmospheric CH4 in terrestrial ecosystem. Many comparison studies proved that forest soil had the biggest capacity of oxidizing atmospheric CH4 in various unsaturated soils. However, up to now, there is not an overall review in the aspect of atmospheric CH4 oxidation (consumption) in forest soil. This paper analyzed advances of studies on the mechanism of atmospheric CH4 oxidation, and related natural factors (Soil physical and chemical characters, temperature and moisture, ambient main greenhouse gases concentrations, tree species, and forest fire) and anthropogenic factors (forest clear-cutting and thinning, fertilization, exogenou saluminum salts and atmospheric deposition, adding biocides, and switch of forest land use) in forest soils. It was believed that OH4 consumption rate by forest soil was limited by diffusion and sensitive to changes in water status and temperature of soil.CH4 oxidation was also particularly sensitive to soil C/N, Ambient CO2, CH4 and N2O concentrations, tree species and forest fire.In most cases, anthropogenic disturbances will decrease atmospheric CH4 oxidation, thus resulting in the elevating of atmos-pheric CH4. Finally, the author pointed out that our knowledge of atmospheric CH4 oxidation (consumption) in forest soil was insufficient. In order to evaluate the contribution of forest soils to atmospheric CH4 oxidation and the role of forest played in the process of global environmental change, and to forecast the trends of global warming exactly, more researchers need to studiesfurther on CH4 oxidation in various forest soils of different areas.     

Nutrient and carbon budgets in forest soils as decision support in sustainable forest management

Knowledge about the nutrient and carbon budgets in forest soils is essential to maintain sustainable production, but also in several environmental issues, such as acidification, eutrophication and climate change. The budgets are strongly influenced by atmospheric deposition as well as forestry. This study demonstrates how budget calculations for nitrogen (N), carbon (C) and base cations (BC) can be used as a basis for policy decisions on a regional level in Sweden.The study was based on existing nutrient and C budget calculations on a regional scale in Sweden. The nutrient budgets have been calculated for each square in a national 5 km × 5 km net by means of mass balances including deposition, harvest losses, leaching, weathering (BC) and fixation (N). Scenarios with different deposition and forestry intensity have been run and illustrated on maps. A simplified C budget has been estimated by multiplying the N accumulation with the C/N ratio in the organic layer, based on the assumption that the C/N ratio in the accumulating organic matter is equal to the ratio in the soil organic matter pool. The budget approaches differ from earlier budget studies since they involve regional high resolution data, combine deposition and forestry scenarios and integrate different environmental aspects.The results indicate that whole-tree harvesting will cause net losses of N and base cations in large parts of Sweden, which means that forestry will not be sustainable unless nutrients are added through compensatory fertilization. To prevent net losses following whole-tree harvesting, compensatory fertilization of base cations would be required in almost the whole country, whereas N fertilization would be needed mainly in the northern half of Sweden. The results further suggest that today's recommendations for N fertilization should be revised in southern Sweden by applying the southwest–northeast gradient of the N budget calculations. The C and N accumulation calculations show that C sequestration in Swedish forest soils is not an effective or sustainable way to decrease the net carbon dioxide emissions. A better way is to apply whole-tree harvesting and use the branches, tops and needles as biofuel replacing fossil fuels. This could reduce the present carbon dioxide emissions from fossil fuels substantially.The study shows that high resolution budget calculations that illuminate different aspects of sustainability in forest ecosystems are important tools for identifying problem areas, investigating different alternatives through scenario analyses and developing new policies. Cooperation with stakeholders increases the probability that the research will be useful.     

氮磷沉降对森林土壤生化特性影响研究进展

人类活动引起的氮磷沉降给土壤带来严重影响。外源性氮可以直接增加土壤全氮和碱解氮的含量,提高土壤磷的有效性,减少全磷含量;外源性磷可以促进树木对土壤氮的吸收,可能造成土壤全氮含量下降。外源性氮对微生物群落存在促进、抑制和没有影响3种情况。外源性磷通常可增加微生物生物量,改变原有的森林微生物群落组成;氮沉降可以提高、降低或无影响土壤酶的活性。氮磷沉降对不同土壤酶种类的影响效果各异。氮沉降的影响也与土壤深度及酶的种类有关。对磷沉降影响土壤酶的研究甚少。未来氮磷沉降的研究热点包括热带氮磷沉降、土壤氮磷比和氮磷沉降交互作用对土壤的影响,不同地形、森林类型、林龄条件下氮磷沉降的对比分析,生态系统中缓冲氮磷沉降作用的关键因子及全球气候变化下氮磷沉降对土壤的影响。     

The relation of harvesting intensity to changes in soil, soil water, and stream chemistry in a northern hardwood forest, Catskill Mountains, USA

Previous studies have shown that clearcutting of northern hardwood forests mobilizes base cations, inorganic monomeric aluminum (Al_im), and nitrate (NO₃⁻-N) from soils to surface waters, but the effects of partial harvests on NO₃⁻-N have been less frequently studied. In this study we describe the effects of a series of partial harvests of varying proportions of basal area removal (22%, 28% and 68%) on Al_im, calcium (Ca²⁺), and NO₃⁻-N concentrations in soil extracts, soil water, and surface water in the Catskill Mountains of New York, USA. Increases in NO₃⁻-N concentrations relative to pre-harvest values were observed within a few months after harvest in soils, soil water, and stream water for all three harvests. Increases in Al_im and Ca²⁺ concentrations were also evident in soil water and stream water over the same time period for all three harvests. The increases in Al_im, Ca²⁺, and NO₃⁻-N concentrations in the 68% harvest were statistically significant as measured by comparing the 18-month pre-harvest period with the 18-month post-harvest period, with fewer significant responses in the two harvests of lowest intensity. All three solutes returned to pre-harvest concentrations in soil water and stream water in the two lowest intensity harvests in 2-3 years compared to a full 3 years in the 68% harvest. When the results of this study were combined with those of a previous nearby clearcut and 40% harvest, the post-harvest increases in NO₃⁻-N concentrations in stream water and soil water suggest a harvesting level above which the relation between concentration and harvest intensity changes; there was a greater change in concentration per unit change in harvest intensity when basal area removal was greater than 40%. These results indicate that the deleterious effects on aquatic ecosystems previously demonstrated for intensive harvests in northern hardwood forests of northeastern North America that receive high levels of atmospheric N deposition can be greatly diminished as harvesting intensity decreases below 40-68%. These results await confirmation through additional incremental forest harvest studies at other locations throughout the world that receive high levels of atmospheric N deposition.     

Soil N cycling in old-growth forests across an Andosol toposequence in Ecuador

Nitrogen (N) deposition in the tropics is predicted to increase drastically in the next decades. The sparse information on N cycling in tropical forests revealed that the soil N status of an ecosystem is the key to analyze its reactions to projected increase in N input. Our study was aimed at (1) comparing the soil N availability of forest sites across an Ecuadorian Andosol toposequence by quantifying gross rates of soil N cycling in situ, and (2) determining the factors controlling the differences in soil N cycling across sites. The toposequence was represented by five old-growth forest sites with elevations ranging from 300 m to 1500 m. Our results provide general insights into the role of elevation-mediated factors (i.e. degree of soil development and temperature) in driving patterns of soil N cycling. Gross rates of N transformations, microbial N turnover time, and δ¹⁵N signatures in soil and leaf litter decreased with increasing elevation, signifying a decreasing N availability across the toposequence. This was paralleled by a decreasing degree of soil development with increasing elevation, as indicated by declining clay contents, total C, total N, effective cation exchange capacity and increasing base saturation. Soil N-cycling rates and δ¹⁵N signatures were highly correlated with mean annual temperature but not with mean annual rainfall and soil moisture which did not systematically vary across the toposequence. Microbial immobilization was the largest fate of produced NH₄⁺ across all sites, and nitrification activity was only 5–11% of gross NH₄⁺ production. We observed a fast reaction of NO₃⁻ to organic N and its role for N retention deserves further attention. If projected increase in N deposition will occur, the timing and magnitude of gaseous N losses may follow the pattern of N availability across this Andosol toposequence.     

Soil Carbon and Nitrogen Dynamics Followed by a Forest-to-pasture Conversion in Western Mexico

Gains and losses of soil carbon (C), have been reported when tropical forests are converted to pastures. Regional studies are crucial for setting regional baselines and explaining each particular trend, in order to solve this controversy. Tropical deciduous forest (TDF) is under high deforestation pressure, mainly for conversion to pastures. The present study compared soil organic C (SOC) and nitrogen (SON) in the surface layer (0–5 cm) of forest and pasture soils in a TDF of western Mexico. SOC and SON concentrations were 18 and 60% lower in pasture soils than in forest soils, and C:N ratio increased in pasture soils. Furthermore, pasture soils had lower labile C and available inorganic nitrogen (N) than forest soils. These results can be explained as a reduction in C inputs to pasture soils and management-induced disruption of soil aggregates. In forest soils, macroaggregates (> 250 μm) were predominant (85%), whereas in pasture soils they were reduced to 35% of dry sand-free soil mass. The estimated SOC and SON losses from the top 5 cm of soil were 3 Mg C ha⁻¹ and 0.9 Mg N ha⁻¹, respectively.     

Nitrification and denitrification as sources of gaseous nitrogen emission from different forest soils in Changbai Mountain

The contributions of nitrification and denitrification to N₂O and N₂ emissions from four forest soils on northern slop of Changbai Mountain were measured with acetylene inhibition methods. In incubation experiments, 0.06% and 3% C₂H₂ were used to inhibit nitrification and denitrification in these soils, respectively. Both nitrification and denitification existed in these soils except tundra soil, where only denitrification was found. The annually averaged rates of nitrification and denitrification in mountain dark brown forest soil were much higher than that in other three soils. In mountain brown coniferous soil, contributions of different processes to gaseous nitrogen emissions were Denitrification N₂O>nitrification N₂O>Denitrification N₂. The same sequence exists in mountain soddy soil as that in the mountain brown coniferous soil. The sequence in mountain tundra soil was Denitrification N₂O>Denitrification N₂. Foundation item: This paper was supported by the National Natural Science Foundation of China (No.49701016) and the “Hundred Scientists” Project of Chinese Academy of Sciences. Biography: XU Hui (1967-), male, Ph. Doctor, associate research fellow in Laboratory of Ecological Process of Trace Substance in Terrestrial Ecosystem, Institute of Applied Ecology, Chinese Academy of sciences, Shenyang 110015, P. R. China. Responsible editor: Song Funan     

土壤硝化和反硝化作用及影响因素研究进展

土壤硝化和反硝化作用是生态系统中氮循环的两个重要环节,是氮素损失的潜在途径,土壤硝化和反硝化作用可向大气中释放温室气体,由此带来环境危害。本文综述了国内外学者对土壤硝化和反硝化作用的研究现状,总结了土壤硝化和反硝化作用的研究方法及其影响因子。土壤硝化和反硝化作用是两个非常复杂的生态学过程,针对研究工作中存在的不足,提出建议:1)改进实验方法、加强对总硝化作用的研究;2)进一步探索森林生态系统中硝化和反硝化作用规律;3)注重对土壤中硝化和反硝化作用微生物学机理的研究。     

Denitrification of an Upland Forest Site

Rates of nitrogen loss through denitrification were monitoredfor standing forest and adjacent clear-felled areas locatedon a peaty-gley soil at Kershope Forest in the north of England,in two year-long studies. The rates of denitrification in soilcores brought back to the laboratory were determined using theacetylene (C₂H₂) block technique. An equation relating denitrificationto temperature was applied to derive an estimate for the monthlyloss of nitrogen via denitrification from the sites. In an additional study, half of the cores were incubated inthe absence of C₂H₂, so that an estimate of the ratio of emissionof N₂O/N₂ could be made. An annual loss of 1–3 kg N ha^–1 y^–1 was estimatedfor the standing forest while losses from the clearfelled siteswere estimated at 10–40 kg N ha^–1 y^–1 duringthe first 2 years after felling. This loss returned to pre-fellinglevels 4 years after felling. The results are discussed in relation to other studies of denitrificationin forest soils and to the rates of N₂O being lost to the atmosphereby UK forests.     

Recovery of carbon and nutrient pools in a northern forested wetland 11 years after harvesting and site preparation

We measured the change in above- and below-ground carbon and nutrient pools 11 years after the harvesting and site preparation of a histic-mineral soil wetland forest in the Upper Peninsula of Michigan. The original stand of black spruce (Picea mariana), jack pine (Pinus banksiana) and tamarack (Larix laricina) was whole-tree harvested, and three post-harvest treatments (disk trenching, bedding, and none) were randomly assigned to three Latin square blocks (n = 9). Nine control plots were also established in an adjoining uncut stand. Carbon and nutrients were measured in three strata of above-ground vegetation, woody debris, roots, forest floor, and mineral soil to a depth of 1.5 m. Eleven years following harvesting, soil C, N, Ca, Mg, and K pools were similar among the three site preparation treatments and the uncut stand. However, there were differences in ecosystem-level nutrient pools because of differences in live biomass. Coarse roots comprised approximately 30% of the tree biomass C in the regenerated stands and 18% in the uncut stand. Nutrient sequestration, in the vegetation since harvesting yielded an average net ecosystem gain of 332 kg N ha⁻¹, 110 kg Ca ha⁻¹, 18 kg Mg ha⁻¹, and 65 kg K ha⁻¹. The likely source for the cations and N is uptake from shallow groundwater, but N additions could also come from non-symbiotic N-fixation and N deposition. These are the only reported findings on long-term effects of harvesting and site preparation on a histic-mineral soil wetland and the results illustrate the importance of understanding the ecohydrology and nutrient dynamics of the wetland forest. This wetland type appears less sensitive to disturbance than upland sites, and is capable of sustained productivity under these silvicultural treatments.     

森林土壤酶活性对氮沉降的响应

近年来,大气氮沉降日益增加,已对森林生态系统产生了不可忽视的影响,而土壤酶活性反映了土壤肥力及土壤环境质量,因而可以用来评价氮沉降对森林土壤造成的影响。关于氮沉降对森林生态系统酶活性的影响已开展了一系列的研究,然而尚缺少系统总结。文中从森林土壤酶种类和林分类型角度总结了氮沉降对土壤酶活性的影响,并从氮沉降水平、氮种类形态、氮沉降与环境的交互作用等方面探讨了土壤酶活性对氮沉降的响应机制,提出未来研究热点是氮沉降对不同类型的森林土壤酶影响、不同森林类型土壤酶的氮沉降临界值、氮沉降对土壤酶活性影响的长期定位研究以及氮沉降与CO₂浓度、温度、降雨、磷添加的交互作用对土壤酶活性影响,以期为未来森林土壤管理提供参考。     

Nitrogen transformations and soil processes in a wastewater-irrigated,mature Appalachian hardwood forest

Wastewater bioremediation has been practised successfully in several forests without significant adverse effect on water quality of adjacent aquatic systems. However, long-term success of wastewater irrigation systems depends on an overall positive response of the forest ecosystem to substantial amounts of added water and nutrients over time. Municipal wastewater irrigation effects on the fate of added nitrogen in a mature Appalachian hardwood forest were investigated during the first 2 years of irrigation. Wastewater was secondarily treated, chlorinated, and sprayed on the study site at five rates. Forest litter N decreased on irrigated sites due to increased litter decomposition rates. Nitrogen mineralization potential (N₀) decreased greatly in soils irrigated at a rate of 140 cm year⁻¹ for 2 years. Net nitrification and relative nitrification (the amount of NO₃⁻-N as a proportion of the total mineral N) increased proportionally with irrigation rate. The highest irrigation rates increased denitrification activity and contributed significantly to the bioremediation process by removing nitrate that otherwise would have been subject to leaching. The increase in NO₃⁻ production in the soil and limited N sequestration by the forest system nevertheless resulted in a net loss of N via leaching. Nitrate concentrations of soil water increased owing to irrigation, with the highest rate at 11 mg 1⁻¹ on sites receiving 70 cm year⁻¹. During the 2-year period, the forest ecosystem experienced a net leaching loss of N that ranged from 14.8 to 105 kg N ha⁻¹ year⁻¹, depending on the application rate. It is likely that this mature hardwood forest will continue to lose N, and that little or no additional N will be sequestered.     

The role of fire and nutrient loss in the genesis of the forest soils of Tasmania and southern New Zealand

The dominant soil patterns in forested or previously forested landscapes in southern New Zealand and Tasmania are described. Soil properties on adjacent sunny and shady aspects in hill country of the South Island of New Zealand are compared to soil properties under adjacent ‘dry’ and ‘wet’ eucalypt forest in Tasmania.

A soil contrast index or SCI is defined for comparing soil contrasts on parent materials of different absolute nutrient contents. Three soil groups are defined using the SCI. Group 1 soil pairs are stable New Zealand soils in which exchangeable Ca + Mg + K values are higher on drier sunny aspects than on moister shady aspects. Group 2 soil pairs are New Zealand soils in which soils on sunny aspects display evidence of topsoil erosion by wind; consequently some soil pairs on dry (sunny) aspects have lower levels of exchangeable Ca + Mg + K than soils on moister (shady) aspects. Group 3 soil pairs are Tasmanian. Soils on drier sites (under dry eucalypt forest) invariably have lower exchangeable Ca + Mg + K values than soils on moister sites (under wet eucalypt forest), which is the reverse of the pattern in SCI Group 1 soils in New Zealand.

Except on clay-rich parent materials, Tasmanian soils under dry forest generally have texture-contrast profiles and a mean C/N ratio in topsoils (A1 horizons) of 29. Soils under wet forest generally have uniform or gradational texture profiles and a mean topsoil C/N ratio of 15. The texture-contrast soils show strong clay eluviation with sand or sandy loam textures in upper horizons and clayey textures in lower horizons. However, in New Zealand texture-contrast soils are all but absent, and do not occur in the previously forested areas described in this paper. Topsoils (Ah horizons and soils sampled to 7.5 cm depth) in New Zealand areas sampled in this study have a mean C/N ratio of 15, regardless of whether they occur on sunny or shady aspects.

We propose that the frequency and spatial occurrence of fire are the dominant processes causing: (1) the marked difference in levels of nutrients and different topsoil C/N ratios in soils of Tasmania; (2) the development of texture-contrast soils under dry forests in Tasmania; and (3) the difference between soil patterns in New Zealand and Tasmania. Fire depletes nutrients in forests by causing losses to the atmosphere, losses by runoff, and losses by leaching. Nutrient loss by fire encourages fire-tolerant vegetation adapted to lower soil nutrient status, so frequent fire is a feedback mechanism that causes progressive soil nutrient depletion. By destroying organic matter and diminishing organic matter supply to the soil surface fire inhibits clay–organic matter linkages and soil faunal mixing and promotes clay eluviation. Fire frequency is likely to have increased markedly with the arrival of humans at ca. 34 000 years B.P. in Tasmania and ca. 800 years B.P. in New Zealand. We argue that texture-contrast soils have not formed in New Zealand because of the short history of frequent fires in that country. A corollary of this conclusion is that texture-contrast soils in Tasmania are, at least in part, anthropogenic in origin.     

Integrating mycorrhiza in a complex model system: effects on ecosystem C and N fluxes

During the last decades, ectomycorrhiza has been identified to be of major importance for ecosystem carbon (C) and nitrogen (N) cycling and tree growth. Despite this importance, mycorrhiza has largely been neglected in ecosystem models or regarded only implicitly by a static mycorrhiza term. In order to overcome this limitation, we integrated the dynamic mycorrhiza model MYCOFON (Meyer et al. in Plant Soil 327:493–517, 2010a, Plant Soil 327:519, 2010b) into the ecosystem modelling framework MoBiLE (Modular Biosphere simuLation Environment) and coupled it to available forest growth and development process models. Model testing was done for different beech and spruce forest sites in Germany. Simulation results were compared to a standard model set-up, that is, without explicit consideration of mycorrhiza. Parameters were set in order not to violate previous findings about C partitioning into aboveground and belowground biomasses. Nevertheless, the explicit consideration of mycorrhiza let to considerable differences between sites and deposition scenarios with respect to simulated root biomass, plant nitrogen supply, and gaseous soil C and N emissions. The latter was mainly a result of differences in soil N concentration and dynamics. Our simulation results also show that the C supply to mycorrhizal fungi by plants as well as the importance of mycorrhizal fungi for plant N uptake, that is, the allocation of C and N between plants and fungi, depends on the magnitude of N deposition. This effect is neglected by standard model approaches so far. Therefore, explicit consideration of mycorrhiza in ecosystem models has a high potential to improve model simulations of ecosystem C and N cycling and associated biosphere–hydrosphere–atmosphere exchange processes and consequently simulation of soil CO₂ and N trace gas emissions from forest sites.     

Farms, fires, and forestry: Disturbance legacies in the soils of the Northwest Wisconsin (USA) Sand Plain

We studied the long-term effects of disturbance within the Northwest Wisconsin (USA) Sand Plain (NWSP), an ecoregion that is characterized by very sandy soil and an active disturbance history that includes fire, agriculture and industrial forestry, largely clearcut logging of jack pine (Pinus banksiana) and aspen (Populus spp.). Open “barrens” communities on this landscape were formerly maintained by fire, and are a high conservation priority. Hill's Oak (Quercus ellipsoidalis) can also dominate forest canopies, while blueberry (Vaccinium angustifolium), and sweetfern (Comptonia peregrina) are common shrub species. We structured a field sampling design with a spatial-temporal database built from historic airphotos (1938 and 1997) and fire records to examine whether soil organic matter and nutrients vary with disturbance history in the nonforest habitats of the sand plain. We sampled soils along 83 transects, randomly stratified among five sampled classes: (1) nonforest-farming history; (2) nonforest-fire history; (3) nonforest-clearcut only history; (4) evergreen forest of jack pine and red pine (P. resinosa); and (5) deciduous forest of Hill's oak and aspen. Logging of the original forest took place in the late 1800s–early 1900s. The farms were abandoned between 1938 and 1960, and the most recent fire occurred in 1977. Thus, the duration of the agricultural legacy is approximately 45–65 years while observed fire effects have lasted for 26 years.We observed strong agricultural legacies, including high P and low OM, N and Ca. One possible explanation for the N legacy is that it is tied to soil OM accretion which may be driven by plant growth. We detected no difference in mean values for any of the soil properties between soils from nonforested areas within the Five-Mile fire and soils from nonforested areas with a clearcut-only history. We did observe a fire effect in high variance for soil P. This could have resulted from variations in fire severity and ash convection and deposition.Forest soils generally had lower pH than the nonforest soils, and the deciduous forest soils had the lowest pH and also very low Ca. We also observed high within-transect coefficient of variation for Ca in the forest soils.We conclude that agriculture is a qualitatively different disturbance-type than fire or clearcutting, that disturbance legacies tend to be most persistent with geologically stable elements, such as P, and that management and conservation planning within the NWSP would benefit from site-specific agricultural history, as well as attention to Ca.     

Nitrogen promotes water consumption in seedlings of Cryptomeria japonica but not in Chamaecyparis obtusa

To evaluate the influence of excessive N deposition on the water consumption of a Japanese plantation forest, 1-year-old seedlings of major plantation trees, Cryptomeria japonica (Japanese cedar) and Chamaecyparis obtusa (Hinoki cypress), were treated with combinations of two N levels (Moderate N and High N) and two soil water conditions (Dry and Moist) for 4 months. The High N treatment received five times as much N as in the Moderate N treatment; the total amount of N added in the High N treatment was roughly 25 times the annual N deposition in precipitation. An increase in soil N availability increased the needle transpiration rate, needle biomass, and needle N content of C. japonica under the Moist treatment, whereas those of C. obtusa were not significantly affected by soil N treatment at either soil water level. Needle N content in C. japonica was positively related to needle photosynthetic rate and transpiration rate. Our results suggested that excessive N deposition has the potential to enhance water consumption in C. japonica stands on moist soils. However, the effects of increased N deposition would be insignificant for C. japonica grown on dry sites. Unlike in C. japonica, water consumption in C. obtusa would be unlikely to respond to excess N deposition, regardless of the soil moisture level. Moreover, the significant reduction in the fine root to needle ratio observed with excessive N application in C. japonica under both Dry and Moist treatments suggests that excessive N deposition is likely to cancel out the tree's morphological adaptation to drought.