Soil nitrogen mineralization in a coastal fynbos succession

Patterns of net nitrogen mineralization and nitrification in 0–7.5 cm deep mineral soils of different stages (seral ages 1, 6 and 20 years) of a post-fire coastal fynbos succession were assayed using laboratory andin situ incubations. No evidence of increasing allelopathic inhibition of nitrification with successional development was found as NO₃−N was the predominant product at all seral stages and the NO₃−N∶NH₄−N ratio remained constant. Rather the results of field incubations of soils beneathProtea repens stands of different successional ages showed that increased mineralization and nitrification appeared to be associated with increased soil total N content rather than with successional age. Further, the incubation of soilsin situ during the dry summer months showed that NO₃−N production appears to be closely related to temperature and soil moisture content, both of which are variables that vary throughout succession due to the changing structure of the vegetation.     

Nitrification and nitrogen mineralization in a lowland rainforest succession in Costa Rica,Central America

Summary Nitrogen availability is a critical component of productivity in successional lowland rainforests, and nitrogen losses from a given system may largely depend on rates of nitrification in soils of the system. Two hypotheses were tested in a study of a 6-point secondary rainforest sere in the coastal lowlands of Costa Rica: that nitrification and N mineralization change in a directed fashion in lowland rainforest successions, and that nitrification is regulated by ammonium availability at all points along the sere. Nitrate and mineral N production were measured in short-term laboratory incubations of soils from different stages of secondary succession corresponding to 0, 3, 8, 16, 31 and 60 + years following disturbance. Results indicate that nitrification increases through the first 4 successional stages and then declines somewhat before leveling off. In soil from all sites, most of the N mineralized was nitrified, and added NH₄Cl strikingly stimulated net nitrate production. Added NaH₂PO₄, CaCO₃, and CaSO₄ did not stimulate net nitrate production or did not result in a greater proportion of nitrate than in controls. These results suggest that nitrification and N mineralization may tend to increase through secondary rainforest succession and that ammonium availability along the sere regulates rates of nitrification.     

Field observed relationships between biodiversity and ecosystem functioning during secondary succession in a tropical lowland rainforest

The relationship between biodiversity and ecosystem functioning (BEF) is one of the most concerned topics in ecology. However, most of the studies have been conducted in controlled experiments in grasslands, few observational field studies have been carried out in forests. In this paper, we report variations of species diversity, functional diversity and aboveground biomass (AGB) for woody plants (trees and shrubs) along a chronosequence of four successional stages (18-year-old fallow, 30-year-old fallow, 60-year-old fallow, and old-growth forest) in a tropical lowland rainforest recovered after shifting cultivation on Hainan Island, China. Fifty randomly selected sample plots of 20 m × 20 m were investigated in each of the four successional stages. Four functional traits (specific leaf area, wood density, maximum species height and leaf dry matter content) were measured for each woody plants species and the relationships between species/functional diversity and AGB during secondary succession were explored. The results showed that both plant diversity and AGB recovered gradually with the secondary succession. AGB was positively correlated with both species and functional diversity in each stage of succession. Consistent with many controlled experimental results in grasslands, our observational field study confirms that ecosystem functioning is closely related to biodiversity during secondary succession in species rich tropical forests.     

子午岭次生林植被演替过程的土壤抗冲性

在水土流失极为严重的黄土高原地区,土壤抗冲性决定着土壤的可蚀性.于2004年5月在黄土高原惟一的次生林区——子午岭林区,通过原状土冲刷实验对不同植被演替阶段下的土壤抗冲性进行了研究.结果表明： （1）随植被的正向演替,表层土壤（0～15cm）的抗冲性明显增大,但亚表层（15～30cm）和底层（30～50cm）土壤抗冲性则没有太大的变化;（2）植物根系能显著的增强土壤抗冲性,土壤抗冲系数与单位土体根系表面积具有极显著的（p＜0.001）线性相关关系;（3）土壤抗冲系数随土壤中水稳性团聚体含量和微生物量的增加而增大,且其相关关系极显著（p＜0.001）.综合根系（x1）、水稳性团聚体（x2）以及微生物（x3）对土壤抗冲性的影响,建立黄土高原地区土壤抗冲性方程： y=-4.89＋1.27x1＋0.079x2＋1.94E-3x3 （R^2=0.914 p＜0.001）.     

Soil nitrogen dynamics along a gradient of long-term soil development in a Hawaiian wet montane rainforest

I analyzed the rates of net N mineralization and nitrification of soils from seven sites in a Hawaiian wet montane forest. The sites differ in age, ranging from 400 to 4,100,000 yr, but are comparable in other variables (all at 1200 miasl with 4000 mm or more mean annual rainfall), and the chronosequence simulated a development of soils from basaltic lava. Soils were incubated for 20 days at 17.5 °C, which is nearly equivalent to a mean field air temperature of the sites, and at an elevated temperature of 25.5 °C under three treatments: 1) field-wet without amendments, 2) air dried to a permanent wilting point, and 3) fertilized with phosphate (NaH₂PO₄) at the rate of 50 g P per g dry soil. Both mineralization and nitrification rates varied significantly among the sites at the field temperature (p<.00001). Fractions of the mineralized organic matter (indexed by the N produced per g organic C) increased sharply from the youngest to the 5000-yr site before declining abruptly to a near constant value from the 9000 to the 1,400,000-yr sites. Total organic C in the top soils (<15 cm deep) increased almost linearly with age across the sites. Consequently, net NH₄- and NO₃-N produced on an area basis (g m^-2 20 d^-1) increased sharply from 0.2 in the youngest site to 1.2 in the 5000-yr site, then both became depressed once but steadily increased again. The fraction of organic matter mineralized, and the net N turnover rates were outstandingly high in the oldest site where a large amount of organic matter was observed; the topsoil organic matter which was used in this analysis appeared to be highly labile, whereas the subsurface organic matter could be relatively recalcitrant. As suggested by earlier workers, the initial increase in N turnover seemed to correspond to the increasing quantity of N in the soils through atmospheric deposition and biological fixation. The later decline in fraction of organic matter mineralized seemed to relate to increasing soil C/N ratios, increasingly recalcitrant organic matter, and poorer soil drainage with age. The elevated temperature treatment produced significantly higher amounts of N mineralization, except for the youngest site where N was most limiting, and for two sites where soil waterlogging might be severe. P fertilization invariably resulted in slower N turnovers, suggesting that soil microbes responded to added P causing N immobilization. The youngest site did not significantly respond to added P. The magnitude of immobilization was higher in older than in younger soils, suggesting that P more strongly limits microbial populations in the older soils.     

Effect of a legume on soil nitrogen mineralisation and percentage nitrogen in grasses

Summary The percentage nitrogen in three mature grasses,Pennisetum purpureum, Tripsacum laxum andPanicum maximum was significantly increased when grown in association with a legumeDesmodium uncinatum. When the grasses were grazed and cut back a similar but non-significant effect occurred in the regrowth material. Soil analyses showed that the legume effectively increased the surface organic horizon and the amount of nitrogen mineralised in the underlying soil. The surface layer also produced large amounts of nitrate on mineralisation. Both with the soil and the organic surface mineralisation was greatest at the start of the rains when the influence of the legume on the percentage nitrogen in the grass was most apparent.     

Root-induced nitrogen mineralisation: A nitrogen balance model

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of nitrogen (N) to plants by stimulating microbial activity. Two models are described, both of which assume that C released from roots is used by bacteria to mineralise and immobilise soil organic N and that immobilised N released when bacteria are grazed by bacterial-feeding nematodes or protozoa is taken up by the plant. The first model simulates the individual transformations of C and N and indicates that root-induced N mineralisation could supply only up to 10% of the plant's requirement, even if unrealistically ideal conditions are assumed. The other model is based on evidence that about 40% of immobilised N is subsequently taken up by the plant. A small net gain of N by the plant is shown (i.e. the plant takes up more N than it loses through exudation), although with exudate of up to C:N 33:1 less than 6% of the plant's requirement is supplied by root-induced N mineralisation. It is argued, however, that rhizosphere bacteria do not use plant-derived C to mineralise soil organic N to any great extent and that in reality root-induced N mineralisation is even less important than these models indicate.     

Nitrogen mineralisation and plant nitrogen acquisition in a nitrogen-limited calcareous grassland

A field study measured the rate of soil mineral N supply and its effects on plant biomass and N accumulation in a 13-year-old, naturally regenerating, calcareous grassland. Gross rates of N mineralisation (2 μg g⁻¹ day⁻¹, i.e. 0.69 kg ha⁻¹ day⁻¹), assessed using ¹⁵N pool dilution, were at the lower end of the range previously reported for grasslands. Weekly additions of liquid N fertiliser ([NH₄]₂SO₄, NH₄NO₃ or KNO₃) and, to a lesser extent the addition of water, increased plant growth substantially, demonstrating that the primary constraint to plant growth was low N availability. In plants that had received NO₃⁻, the activity of the inducible enzyme nitrate reductase in shoots initially increased in proportion to the amount of NO₃⁻ supplied. However, as above-ground herbage accumulated, nitrate reductase activity declined to similar low levels in all treatments, despite the continuance of the constant NO₃⁻ additions. The decline in NR specific activity reflected declining tissue NO₃⁻ concentrations, although total plant NRA may have remained constant during the period of study. The study has shown that plant growth is limited by low N mineralisation rates and indeed the soil is a sink for much added N. Low water availability provides an additional constraint on N mineralisation in this calcareous grassland soil. Any disturbances in the N cycle which increase the availability of mineral N will result in a substantial increase in plant growth within this ecosystem.     

Effects of nitrogen limitation on species replacement dynamics during early secondary succession on a semiarid sagebrush site

Summary A soil nitrogen (N) availability gradient was induced on a disturbed sagebrush site in northwestern Colorado by fertilizing with nitrogen (high available N), applying sucrose (low available N), and applying neither nitrogen nor sucrose (control). Species composition was studied for 3 years. At the end of the study, N concentration of aboveground tissue of 3 major species was determined. The rate of species replacement was most rapid on plots receiving the sucrose treatment and was slowest on plots receiving the N treatment. Early-seral dominats had greater tissue N concentrations when availability of the resource was high but lower tissue N concentrations when available soil N became limited. Midseral dominants displayed the opposite pattern. These results suggest that the supply of available soil N, and therefore the dynamics of N incorporation in perennial plant tissue, is a primary mechanism in controlling the rate of secondary succession within this semiarid ecosystem.     

Root-induced nitrogen mineralisation: A theoretical analysis

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of inorganic nitrogen (N) to plants by stimulating microbial activity. The release of soluble C compounds from roots is assumed to occur by one of two general processes: cortical cell death or exudation from intact cells. On the basis of several assumptions chosen to allow maximal amounts of N mineralisation to be calculated, greater amounts of net N mineralisation are theoretically possible at realistic soil C:N ratios of bacteria are grazed by predators such as protozoa, than if bacteria alone are active. More N is mineralised when the substrate released from the root has a high C:N ratio (as in cell death) than when it is relatively N-rich. The amounts of N that a root might realistically cause to be mineralised are unlikely to account entirely for high nitrate inflow rates that have been measured experimentally. However there are circumstances in which the loss of C from roots is essential if any N is to be mineralised and obtained by plants.     

Effects of nitrogen and phosphorus fertilization in a lowland evergreen rainforest

A nutrient addition experiment was set up in August 1993 in a species-rich primary lowland dipterocarp forest in Barito Ulu, Central Kalimantan, Indonesia. The following treatments were applied: control, +N, +P and +NP. There were five blocks of four 50 m x 50 m plots with a separate treatment for each plot. Fine litterfall was measured on all the plots from 1 May 1994 for 12 months. Litterfall mass and phosphorus concentrations were significantly higher in all the fertilizer treatments compared with the controls. All trees (> or = 10 cm dbh) were measured in August 1993 and in August 1998, and there was no significant girth increment response to fertilization in dipterocarps or non-dipterocarps. Dipterocarps of the red meranti group showed a doubling of girth increment in the +NP treatment, however, the difference from the control fell short of significance.     

黄土高原次生林演替过程土壤氮矿化特征及其影响因素

土壤氮矿化速率限制着森林土壤有效氮素供给能力,但目前关于黄土高原次生林演替过程土壤氮矿化特征的认识还不清楚。选择黄土高原黄龙山次生林区草本阶段、灌木阶段、先锋乔木群落阶段、混交森林群落阶段和顶级森林群落阶段等5个演替阶段的9种群落类型,测定了其林下土壤氮矿化速率,并分析了土壤理化性质、凋落物养分特征和土壤酶活性及其对氮矿化的影响。结果表明:随着演替进展,土壤有机碳和氮不断积累,土壤硝态氮和铵态氮含量逐渐增加,凋落物养分和土壤酶活性也发生了显著变化。土壤硝化速率从草本群落阶段到顶级森林群落阶段显著增加了71.73%;土壤氨化速率一直为负值且持续负向增加;土壤净矿化速率从草本群落阶段的(0.16±0.06) mg kg^-1 d^-1逐渐增加到混交森林阶段的(0.31±0.08)mg kg^-1 d^-1,但从混交林群落到顶级森林群落阶段有所下降。土壤理化性质(路径系数-0.530)和酶活性(路径系数-0.268)对氮矿化速率有显著的直接作用;凋落物养分对氮矿化速率的影响来自于其直接作用(路径系数-0.283)和通过调控土壤理化性质、酶活性产生的间接影响(路径系数-0.315)。结果有助于认识黄土高原植被演替过程中土壤氮循环过程,对森林可持续经营也有一定的意义。     

Wood trait-environment relationships in a secondary forest succession in South-East China

Concerning forest communities, not much is known about the relationship between wood traits and environmental conditions. Using a succession series, we analyzed which wood anatomical traits were correlated with successional stage and asked which traits and which environmental factors were particularly important for the trait–environment relationship. An extensive dataset of 11 groups of wood traits was generated for 93 woody species that occurred in 27 permanent plots in a secondary subtropical secondary broadleaved forest in Zhejiang Province (SE-China) and subjected to Fourth Corner Analyses, using different permutation models. We encountered a strong relationship of wood porosity, visibility of growth rings and vessel arrangement to the successional gradient. Compared to biotic community characteristics such as density of plants, abiotic environmental variables such as soil characteristics, aspect and inclination of the plots showed only marginal correlations to wood anatomical traits. Furthermore, the link between environment and species composition of the forest communities was found to be more important in explaining the trait–environment relationship than between the communities and species wood traits. In addition, our results support the idea that most of the species in the subtropical forest might be functionally equivalent.     

Plant–plant interactions change during succession on nurse logs in a northern temperate rainforest

Plant–plant interactions change through succession from facilitative to competitive. At early stages of succession, early‐colonizing plants can increase the survival and reproductive output of other plants by ameliorating disturbance and stressful conditions. At later stages of succession, plant interactions are more competitive as plants put more energy toward growth and reproduction. In northern temperate rainforests, gap dynamics result in tree falls that facilitate tree regeneration (nurse logs) and bryophyte succession. How bryophyte‐tree seedling interactions vary through log succession remains unclear. We examined the relationships of tree seedlings, bryophyte community composition, bryophyte depth, and percent canopy cover in 166 1.0 m² plots on nurse logs and the forest floor in the Hoh rainforest in Washington, USA, to test the hypothesis that bryophyte‐tree seedling interactions change from facilitative to competitive as the log decays. Tree seedling density was highest on young logs with early‐colonizing bryophyte species (e.g., Rhizomnium glabrescens) and lowest on decayed logs with Hylocomium splendens, a long‐lived moss that reaches depths >20 cm. As a result, bryophyte depth increased with nurse log decay and was negatively associated with tree seedling density. Tree seedling density was 4.6× higher on nurse logs than on the forest floor, which was likely due to competitive exclusion by forest floor plants, such as H. splendens. Nurse logs had 17 species of bryophytes while the forest floor had six, indicating that nurse logs contribute to maintaining bryophyte diversity. Nurse logs enable both tree seedlings and smaller bryophyte species to avoid competition with forest floor plants, including the dominant bryophyte, H. splendens. H. splendens is likely a widespread driver of plant community structure given its dominance in northern temperate forests. Our findings indicate that plant–plant interactions shift with succession on nurse logs from facilitative to competitive and, thus, influence forest community structure and dynamics.     

Aphids decelerate litter nitrogen mineralisation through changes in litter quality

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Patterns of secondary succession in a mangrove forest of Southern Florida

Summary Successional patterns were studied in mangrove forests which had developed recently in response to salinization of areas formerly supporting freshwater marshes along Biscayne Bay in North Miami, Florida. The population structures of these Induced Forests were compared with an adjacent Historical Forest which consisted of a nearly pure stand ofRhizophora mangle. A mixed forest ofRhizophora andLaguncularia racemosa had developed in intertidal areas, while areas above the mean high water elevation supported a scrub community dominated byLaguncularia. Maximum growth of bothRhizophora andLaguncularia occurred in intertidal areas, while both species were stunted and had sparse, poorly formed canopies in drier environments above the mean high water level. Analysis of population structure suggests that Induced Forests in intertidal areas are undergoing succession to a stand ofRhizophora. Laguncularia is unable to compete effectively withRhizophora in these areas and it is suggested that it eventually will be limited to the drier areas, where competition fromRhizophora will be reduced or absent.formely Kimball     

Inorganic nitrogen assimilation in plants of Austrlian rainforest communities

In a study of the plant communities of two Australian rainforests, it was found that pioner species had high levels of nitrate reductase (EC 1.6.6.1) and were predominantly leaf nitrate assimilators. Under- and over-storey species had low levels of shoot and root nitrate reductase activity, and many of them showed little capacity for nitrate reduction even when nitrate ions were freely available. Although closed-forest species have lower levels of nitrate reductase than those of gaps and forest margins, their total nitrogen contents were similar, suggesting the former utilize nitrogen sources other than nitrate ions. Glutamine synthetase (EC 6.3.1.2) was present in the leaves of all species examined. In the leaves of pioneer species the chloroplastic isoform of glutamine synthetase predominted, while in most of the species typical of closed-forest the cytosolic isoform accounted for at least 40% of total leaf activity. Low levels of chloroplastic glutamine synthetase were correlated with a low capacity for leaf nitrate reduction, and both are characteristic of many species that regenerate and grow for some time in shade. Low levels of chloroplastic glutamine synthetase imply that, in some of these woody plants, photorespiratory ammonia is re-assimilated via cytosolic glutamine synthetase.     

Rates of change in vegetation during secondary succession

Plant Ecology - For a number of experiments on secondary succession on different soils (lasting 9–23 yf) calculations were made of: a) rates of floristic change as measured with the community...     

Early secondary succession in a southeastern U.S. alluvial floodplain

Abstract. Species number, turnover and early successional patterns were examined over the first five years of old-field succession in a former bottomland hardwood forest. Number of species, measured by walk-through surveys, increased up to Year 3, but decreased in Years 4 and 5. Species turnover in 20 quadrats indicated that species’invasions remained high throughout the study, while losses of species were initially high but declined. Detrended Correspondence Analysis (DCA) on species cover in each year showed the existence of a compositional gradient that could be significantly related to relative elevation, an indirect measure of hydrology, in Years 2, 3, 4, and 5. DCA of the pooled five-year species cover data showed greater compositional changes in lower elevation quadrats, and those higher elevation quadrats dominated by the woody vine, Campsis radicans, remained similar over time.