Nutrient stocks and phosphorus fractions in mountain soils of Southern Ecuador after conversion of forest to pasture

Understanding pasture degradation processes is the key for sustainable land management in the tropical mountain rainforest region of the South Ecuadorian Andes. We estimated the stocks of total carbon and nutrients, microbial biomass and different P fractions along a gradient of land-uses that is typical of the eastern escarpment of the Cordillera Real i.e., old-growth evergreen lower montane forest, active pastures (17 and 50 years-old), abandoned pastures 10 and 20 years old with bracken fern or successional vegetation. Conversion of forest to pasture by slash-and-burn increased the stocks of SOC, TN, P and S in mineral topsoil of active pasture sites. Microbial growth in pasture soils was enhanced by improved availability of nutrients, C:N ratio, and increased soil pH. Up to 39 % of the total P in mineral soil was stored in the microbial biomass indicating its importance as a dynamic, easily available P reservoir at all sites. At a 17 years-old pasture the stock of NH₄F extractable organic P, which is considered to be mineralisable in the short-term, was twice as high as in all other soils. The importance of the NaOH extractable organic P pool increased with pasture age. Pasture degradation was accelerated by a decline of this P stock, which is essential for the long-term P supply. Stocks of microbial biomass, total N and S had returned to forest levels 10 years after pasture abandonment; soil pH and total P 20 years after growth of successional bush vegetation. Only the C:N ratio increased above forest level indicating an ongoing loss of N after 20 years. Soil nutrient depletion and microbial biomass decline enforced the degradation of pastures on the investigated Cambisol sites.     

Seedling assemblages and the alternative successional pathways experienced by Atlantic forest fragments

Background: Habitat loss and fragmentation have been argued to drastically alter the composition of tree assemblages inhabiting small forest fragments but the successional trajectory experienced by such edge-affected habitats remains controversial.

Aims: Here we examine whether small fragments (3.4–91.2 ha) support seedling assemblages more similar to those in 10–70-year-old secondary forests than to those in mature forests, in order to infer to what extent fragments move toward early successional systems.

Methods: Using 59 0.1-ha plots distributed in a fragmented landscape of Brazilian Atlantic forest, we evaluated species richness and functional and taxonomic composition of seedling assemblages in 20 small forest fragments, 19 stands of secondary forest and 20 stands of mature forests in the interior of an exceptionally large fragment (ca. 3500 ha).

Results: Small fragments presented the least species-rich seedling assemblages (17.2 ± 5.7 species), followed by secondary (22.5 ± 5.3), and mature forest (28.4 ± 5.3). Small fragments had seedling assemblages with functional and taxonomic composition more similar to those in secondary than in mature forest. Small fragments had a greater relative richness and abundance of pioneer trees (ca. 40% more), vertebrate-dispersed (6–25%), and those bearing medium-sized seeds (30–70%), while large-seeded species and individuals were reduced (>50% decrement) in comparison to seedling assemblages in mature forest.

Conclusions: By comparing seedlings across a wide range of successional habitats we offer evidence that small forest fragments are experiencing an alternative successional pathway towards an early-successional system with reduced plant diversity.     

Amino acid uptake in deciduous and coniferous taiga ecosystems

We measured in situ uptake of amino acids and ammonium across deciduous and coniferous taiga forest ecosystems in interior Alaska to examine the idea that late successional (coniferous) forests rely more heavily on dissolved organic nitrogen (DON), than do early successional (deciduous) ecosystems. We traced ¹⁵N-NH₄⁺ and ¹³C-¹⁵N-amino acids from the soil solution into plant roots and soil pools over a 24 h period in stands of early successional willow and late successional black spruce. Late successional soils have much higher concentrations of amino acid in soil solution and a greater ratio of DON to dissolved inorganic N (DIN) (ammonium plus nitrate) than do early successional soils. Moreover, late successional coniferous forests exhibit higher rates of soil proteolytic activity, but lower rates of inorganic N turnover. Differences in ammonium and amino acid uptake by early successional willow stands were insignificant. By contrast, the in situ uptake of amino acid by late successional black spruce forests were approximately 4-fold greater than ammonium uptake. The relative difference in uptake of ammonium and amino acids in these forests was approximately proportional to the relative difference of these N forms in the soil solution. Thus, we suggest that differences in uptake of different N forms across succession in these boreal forests largely reflect edaphic variation in available soil N (composition), rather than any apparent physiological specialization to absorb particular forms of N. These finding are relevant to our understanding of how taiga ecosystems may respond to increases in temperature, fire frequency, N deposition, and other potential consequences of global change.     

Nitrogen-15 signals of leaf-litter-soil continuum as a possible indicator of ecosystem nitrogen saturation by forest succession and N loads

Understanding forest carbon cycling responses to atmospheric N deposition is critical to evaluating ecosystem N dynamics. The natural abundance of ¹⁵N (??¹⁵N) has been suggested as an efficient and non-invasive tool to monitor N pools and fluxes. In this study, three successional forests in southern China were treated with four levels of N addition. In each treatment, we measured rates of soil N mineralization, nitrification, N₂O emission and inorganic N leaching as well as N concentration and ?? ¹⁵N of leaves, litters and soils. We found that foliar N concentration and ??¹⁵N were higher in the mature broadleaf forest than in the successional pine or mixed forests. Three-year continuous N addition did not change foliar N concentration, but significantly increased foliar ?? ¹⁵N (p < 0.05). Also, N addition decreased the ?? ¹⁵N of top soil in the N-poor pine and mixed forests and significantly increased that of organic and mineral soils in N-rich broadleaf forests (p < 0.05). In addition, the soil N₂O emission flux and inorganic N leaching rate increased with increasing N addition and were positively correlated with the ¹⁵N enrichment factor (?? _p/s) of forest ecosystems. Our study indicates that ?? ¹⁵N of leaf, litter and soil integrates various information on plant species, forest stand age, exogenous N input and soil N transformation and loss, which can be used to monitor N availability and N dynamics in forest ecosystems caused by increasing N deposition in the future.     

Nitrogen oligotrophication in northern hardwood forests

While much research over the past 30 years has focused on the deleterious effects of excess N on forests and associated aquatic ecosystems, recent declines in atmospheric N deposition and unexplained declines in N export from these ecosystems have raised new concerns about N oligotrophication, limitations of forest productivity, and the capacity for forests to respond dynamically to disturbance and environmental change. Here we show multiple data streams from long-term ecological research at the Hubbard Brook Experimental Forest in New Hampshire, USA suggesting that N oligotrophication in forest soils is driven by increased carbon flow from the atmosphere through soils that stimulates microbial immobilization of N and decreases available N for plants. Decreased available N in soils can result in increased N resorption by trees, which reduces litterfall N input to soils, further limiting available N supply and leading to further declines in soil N availability. Moreover, N oligotrophication has been likely exacerbated by changes in climate that increase the length of the growing season and decrease production of available N by mineralization during both winter and spring. These results suggest a need to re-evaluate the nature and extent of N cycling in temperate forests and assess how changing conditions will influence forest ecosystem response to multiple, dynamic stresses of global environmental change.     

Increasing phosphorus limitation along three successional forests in southern China

Background and Aims

Phosphorus (P) is commonly one of most limiting nutrients in tropical and subtropical forests, but whether P limitation would be exacerbated during forest succession remains unclear.

Methods

Soil phosphatase activity is often used as an indicator of P limitation. Here we examined soil acid phosphatase activity (APA) underneath tree species in pine forest (PF), mixed pine and broadleaf forest (MF) and monsoon evergreen broadleaf forest (MEBF) which represented the early, middle and late successional stages of subtropical forests in China, respectively. We also analyzed other indicators of P status (soil available P and N and P stoichiometry of the tree species).

Results

APA or APA per unit soil organic carbon under tree species was relatively low in the early successional forest. Different from PF and MF, soil available P beneath the tree species was lower than in the bulk soils in MEBF. Soil APA was closely related to N:P ratios of tree species across all three forests.

Conclusions

Our results imply that P limitation increases during forest succession at our site. The dominant tree species with low soil APAs in MEBF are likely more P-limited than other tree species.     

Microbial C, N and P in soils of Mediterranean oak forests: influence of season, canopy cover and soil depth

In Mediterranean ecosystems the effect of aboveground and belowground environmental factors on soil microbial biomass and nutrient immobilization-release cycles may be conditioned by the distinctive seasonal pattern of the Mediterranean-type climates. We studied the effects of season, canopy cover and soil depth on microbial C, N and P in soils of two Mediterranean forests using the fumigation-extraction procedure. Average microbial values recorded were 820 μg C g^?1, 115 μg N g^?1 and 19 μg P g^?1, which accounted for 2.7, 4.7 and 8.8% of the total pools in the surface soil, respectively. Microbial N and P pools were about 10 times higher than the inorganic N and P fractions available for plants. Microbial C values differed between forest sites but in each site they were similar across seasons. Both microbial and inorganic N and P showed maximum values in spring and minimum values in summer, which were positively correlated with soil moisture. Significant differences in soil microbial properties among canopy cover types were observed in the surface soil but only under favourable environmental conditions (spring) and not during summer. Soil depth affected microbial contents which decreased twofold from surface to subsurface soil. Microbial nutrient ratios (C/N, C/P and N/P) varied with seasons and soil depth. Soil moisture regime, which was intimately related to seasonality, emerged as a potential key factor for microbial biomass growth in the studied forests. Our research shows that under a Mediterranean-type climate the interaction among season, vegetation type and structure and soil properties affect microbial nutrient immobilization and thus could influence the biogeochemical cycles of C, N and P in Mediterranean forest ecosystems.     

Belowground insights into nutrient limitation in northern hardwood forests

Soil nutrient environments are changing in forests of the northeastern United States due to decades of anthropogenic nitrogen (N) emissions and acidic deposition, causing N enrichment and cation depletion, and possibly alleviating N limitation to forest growth. We asked whether biotic demand for phosphorus (P) or calcium (Ca) exceeded that for N and used an ingrowth core approach to test belowground responses to different nutrients. We tested fine root foraging for nutrients (N, P, or Ca) in three mid-age (26–30 years) and mature (≥100 years) northern hardwood forest stands in the Bartlett Experimental Forest (BEF), NH, and in one mature forest stand in the Hubbard Brook Experimental Forest (HBEF), NH, USA. Fine root colonization of cores responded clearly to Ca in mature forest at HBEF, responded to P in mid-age forests at BEF, and responded primarily to N in mature forests at BEF. Net N mineralization potential was higher in soils of mid-age than mature forests at BEF, with roots responding to N where N availability was low and to P or Ca where N availability was high. Nutrients elicited no responses from either fungi or phosphatase activity in mid-age forests, but in mature forests at BEF, N enhanced phosphatase activity. While no straightforward pattern emerged among the different mechanisms of nutrient acquisition that we tested, our results do suggest that P and Ca can be important limiting nutrients in these northern hardwood forests when N availability is relatively high. We hypothesize that the interacting effects of disturbance by forest harvest and N deposition can cause a transient P limitation to forest growth, and that other nutrients become more limiting as forests age.     

Influence of habitat,litter type,and soil invertebrates on leaf-litter decomposition in a fragmented Amazonian landscape

Amazonian forest fragments and second-growth forests often differ substantially from undisturbed forests in their microclimate, plant-species composition, and soil fauna. To determine if these changes could affect litter decomposition, we quantified the mass loss of two contrasting leaf-litter mixtures, in the presence or absence of soil macroinvertebrates, and in three forest habitats. Leaf-litter decomposition rates in second-growth forests (>10 years old) and in fragment edges (<100 m from the edge) did not differ from that in the forest interior (>250 m from the edges of primary forests). In all three habitats, experimental exclusion of soil invertebrates resulted in slower decomposition rates. Faunal-exclosure effects were stronger for litter of the primary forest, composed mostly of leaves of old-growth trees, than for litter of second-growth forests, which was dominated by leaves of successional species. The latter had a significantly lower initial concentration of N, higher C:N and lignin:N ratios, and decomposed at a slower rate than did litter from forest interiors. Our results indicate that land-cover changes in Amazonia affect decomposition mainly through changes in plant species composition, which in turn affect litter quality. Similar effects may occur on fragment edges, particularly on very disturbed edges, where successional trees become dominant. The drier microclimatic conditions in fragment edges and second-growth forests (>10 years old) did not appear to inhibit decomposition. Finally, although soil invertebrates play a key role in leaf-litter decomposition, we found no evidence that differences in the abundance, species richness, or species composition of invertebrates between disturbed and undisturbed forests significantly altered decomposition rates.     

Mangrove Forest and Soil Development on a Rapidly Accreting Shore in New Zealand

Mangrove forests are rapidly expanding their distribution in New Zealand, which is at the southern limit of their range. We investigated how these expanding mangrove forests develop through time. We assessed patterns in forest structure and function at the Firth of Thames, which is a rapidly accreting mangrove site in New Zealand where 1 km of mangrove of Avicennia marina has established seaward since the 1950s. Across the intertidal region, mangrove forest structure was highly variable. We used bomb-pulse radiocarbon dating to age the forest. Two major forest establishment events were identified; one in 1978–1981 and another in 1991–1995. These events coincided with sustained El Niño activity and are likely the result of reduced wind and wave energy at the site during these periods. We used the two forests of different ages to assess whether mangroves in New Zealand mature at similar rates as other mangroves and whether they conform to classic models of succession. The timing of forest maturation is similar in New Zealand as in more tropical locations with trees exhibiting features of mature forests as they age from about 10 to about 30 years. In older forest (~30 years old) trees become larger and stands more homogenous than in the younger forest (~10 years old). Carbon and nutrient concentrations in soils increased and soils become more aerobic in older forest compared to younger forest. Additionally, using fertilization experiments, we established that despite reduced growth rates in older forests, nitrogen remained limiting to growth in both older and young forests. However, in contrast to classic successional models leaf tissue nutrient concentrations and nutrient conservation (nutrient resorption from senescence leaf tissue) were similar in forests of differing ages and did not vary with fertilization. We conclude that mangrove forest expansion in New Zealand is influenced by climatic factors. Mangrove forests mature rapidly, even at the limits of their range and they satisfy many of the successional patterns predicted by Odum (1969) for the early stages of forest succession.     

The value of the trackway system within a lowland plantation forest for ground-active spiders

European forest management guidelines include conservation and enhancement of biodiversity. Within plantation forestry, trackways provide contiguous permanent open-habitat with potential to enhance biodiversity. We examined the ground-active spider assemblage in the trackway network of Thetford Forest, Eastern England, the largest lowland conifer forest in the UK, created by afforestation of heathland and farmland. Results are relevant to other forests in heath regions across Europe. We used pitfall trapping to sample the spider assemblage of trackways within thicket-aged stands (n = 17), mature stands (n = 13) and heathland reference sites (n = 9). A total of 9,314 individuals of 71 species were recorded. Spider assemblages of the trackway network were distinct from those of the heathland reference sites; however, trackways were found to support specialist species associated with grass-heath habitats, including nationally scarce species. Richness of grass-heath species was similar for trackways in thicket-aged forest and heathland reference sites, although the abundance of individuals was three times greater in the reference sites. Trackways in mature stands had lower grass-heath species richness and abundance than both thicket trackways and heath reference sites. Wide trackways within thicket stands contained greater richness and abundance of specialist xeric species than narrower trackways. However, fewer xeric individuals were found in trackways compared to heathland reference sites. Either inferior habitat quality in trackways or poor dispersal ability of specialist xeric species may largely restrict these to relict areas of heathland. Targeted widening of trackways to allow permanent unshaded habitat and creating early successional stages by mechanical disturbance regimes could improve trackway suitability for specialist species, helping to restore connectivity networks for grass-heath biodiversity.     

Soil Solution Nitrogen Losses During Clearing of Lowland Amazon Forest for Pasture

Losses of nitrogen (N) often follow severe disturbance of forest ecosystems. In tropical forests, losses of N associated with the disturbance of clearing may be particularly important because rates of soil N cycling are high and forest clearing now occurs on a large scale. We measured soil solution inorganic N concentrations and fluxes for 1 year in an intact forest in the Brazilian Amazon state of Rondônia and in an adjacent 3-ha forest plot that was cleared for pasture by cutting, burning and planting pasture grass and in established cattle pastures on the same soils that were 5 and 22 years old. The cleared forest had higher soil solution NO ₃ ^? concentrations than the intact forest, but the difference between the cleared and control forests declined with time after the start of the first post-clearing rainy season. Established pastures had much lower solution NH ₄ ⁺ and NO ₃ ^? concentrations than forest or cleared forest. Estimated annual dissolved inorganic solution N fluxes to below 1 m during the first year after clearing were 2.5 kg ha^?1 in forest and 24.4 kg ha^?1 in newly cleared forest compared with only 0.5–1.2 kg ha^?1 in established pastures. The solution fluxes from cleared forest during the first year after clearing were approximately 7 times greater than gaseous N oxide (N₂O+NO) losses estimated for the same time. These results were consistent with the characterization of moist tropical forests on weathered soils as N-rich and likely to respond to disturbances that elevate soil N availability with increased loss to both soil solution and the atmosphere. These results also suggest that the relative increase in N oxide loss is substantially less than the increase solution inorganic N loss.     

Nitrogen cycling in two riparian forest soils under different geomorphic conditions

At the floodplain scale, spatial pattern and successional development of riparian vegetation are under the control of geomorphic processes. The geomorphic and hydraulic characteristics of stream channels affect the sorting of organic material and inorganic sediment through erosion/sedimentation during floods. In turn, the proportion of fine sediments fractions differs by location within a given community of riparian forest succession. In this paper we tested the effect of geomorphic features of floodplains, through soil grain size sorting, on the nitrogen cycling in riparian forest soils. Two typical riparian forests formed by vertical accretion deposits from repeated addition of sediments from overbank flow have been chosen along the River Garonne, southwest France. These riparian forests had equivalent vegetation, flood frequency and duration, differing only in soil grain size composition: one riparian forest had sandy soils and the other had loamy soils. The evolution of the main soil physical and chemical parameters as well as denitrification (DNT), N uptake (N _U ) and mineralization (N _M ) rates were measured monthly over a period of 13 months in the two study sites. The loamy riparian forest presented a better physical retention of suspended matter during floods. Moreover,in situ denitrification rates (DNT) and N uptake by plants (N _U ) measured in the loamy riparian forest soils were significantly greater than in the sandy soils. Although DNT and N _U could be in competition for available nitrogen, the peak rates of these two processes did not occur at the same period of the year, N _U being more important during the dry season when DNT was minimum, while DNT rates were maximum following the spring floods. N retention by uptake (N _U ) and loss by DNT represented together the equivalent of 32% of total organic nitrogen deposited during floods on the sandy riparian forest soils and 70% on the loamy ones. These significant differences between the two sites show that, at the landscape level, one should not estimate the rates of N _U and DNT, in riparian forests soils only on the basis of vegetation, but should take also into account the geomorphic features of the floodplain.     

中亚热带丘陵红壤区森林演替典型阶段土壤氮磷有效性

在中亚热带典型丘陵红壤区选取裸露地、马尾松(Pinus massoniana)林地、针阔混交林地、常绿阔叶林地为研究对象,开展土壤氮(N)、磷(P)供应、有效性及其耦合过程的研究.结果表明,土壤有机C、全N、净矿化速率、中性磷酸酶活性表现为随森林演替进展呈现逐步提高的变化趋势;而土壤全P、C/N、C/P、氨化速率、硝化速率、树脂P、NaHCO3-P、NaOH-P、声波P、酸性P、总有效P、酸性磷酸酶活性未表现出此趋势;但反映N、P有效供应的指标,除氨化速率、树脂P和酸性磷酸酶外,在常绿阔叶林中均为最高.相关分析表明大部分N、P供应指标之间存在显著相关性(P<0.05).丘陵红壤区森林演替初级阶段P的限制性明显强于N,土壤N、P供应在森林演替进展过程中可以逐步得到优化而实现协调供应.以常绿阔叶林为中亚热带丘陵红壤区植被恢复的最终目标是可行和理想的.     

Effect of prey availability on the abundance of White‐breasted Wood‐Wrens,insectivorous birds of tropical lowland forests

Some understory insectivorous birds manage to persist in tropical forest fragments despite significant habitat loss and forest fragmentation. Their persistence has been related to arthropod biomass. In addition, forest structure has been used as a proxy to estimate prey availability for understory birds and for calculating prey abundance. We used arthropod biomass and forest structural variables (leaf area index [LAI] and aerial leaf litter biomass) to explain the abundance of White‐breasted Wood‐Wrens (Henicorhina leucosticta), tropical understory insectivorous birds, in six forests in the Caribbean lowlands of Costa Rica. To estimate bird abundance, we performed point counts (100‐m radius) in two old‐growth forests, two second‐growth forests, and two selectively logged forests. Arthropod abundance was the best predictor of wood‐wren abundance (wi = 0.75). Wood‐wren abundance increased as the number of arthropods increased, and the estimated range of bird abundance obtained from the model varied from 0.51 (0.28 – 0.93 [95%CI]) to 3.70 (1.68 – 5.20 [95%CI]) within sites. LAI was positively correlated to prey abundance (P = 0.01), and explained part of the variation in wood‐wren abundance. In forests with high LAI, arthropods have more aerial leaf litter as potential habitat so more potential prey are available for wood‐wrens. Forests with a greater abundance of aerial leaf litter arthropods were more likely to sustain higher densities of wood‐wrens in a fragmented tropical landscape.     

Soil phosphorus fractions and tree phosphorus resorption in pine forests along an urban-to-rural gradient in Nanchang, China

Urbanization has been rapid across the world but the responses of phosphorus (P) cycling to urbanization have not been well-investigated. This study was to understand the influences of rapid urbanization on forest P cycling in a developing country. Soil P fractions and P resportion were determined for nine slash pine (Pinus elliottii Engelm.) forests along a 30-km long urban-suburban-rural gradient in Nanchang City, southern China. The total P stocks in the surface soils in urban and suburban forests were 317% and 182% higher, respectively, than levels found in rural forests. The concentrations of soil available P, labile P, slow P, occluded P and total extractable P were also much higher in urban and suburban forests than in rural forests (P?<?0.05). Soil weathered P concentrations were highest in urban forests. Annual mean foliar P concentrations were enhanced in urban and suburban forests compared to rural forests. The P resorption efficiency (PRE) was higher in rural forests than in suburban and urban forests, while the P resorption proficiency (PRP) was lower in rural forests than in suburban and urban forests. Urbanization associated with high extraneous P inputs has altered soil P status and plant P uptake. Foliar P concentration, PRE and PRP were largely dependent on soil P availability in our study forests.     

Effect of conversion of sugarcane plantation to forest and pasture on soil carbon in Hawaii

It is well known that land use change can affect soil C storage of terrestrial ecosystems either by altering the biotic processes involved in carbon cycling or by altering abiotic processes such as carbon adsorption on soil minerals. Relatively few studies, however, have examined the dynamics of soil C pools after conversion of farmland to forest or pasture. We selected three pairs of secondary forests and pastures that originated from the same abandoned sugarcane (interspecific hybrids of Saccharum spp.) land in the wet tropics of Hawaii to examine whether forest or pasture converted from farmland is more effective in sequestering C in soils. We compared the soil C pool, soil chemistry, and stable C isotope ratios between the forests and pastures. We found that total soil C was greater (P?<?0.01) in forests than in the pastures 22 years after land conversion. The percentages of SOC₄ in the pastures were significantly higher than in the secondary forests in both soil layers. The percentages of SOC₃ in the pastures were lower than in the secondary forests in both soil layers. The net SOC₃ increase in the forest soils at 0–10 and 10–25 cm was 28.6?±?5.6 and 43.9?±?3.2 Mg ha^?1 while net SOC₄ increase in pasture soils at these respective depths was 18.8?±?2.2 and 26.1?±?2.7 Mg ha^?1. We found that the net increases of SOC₃ in both soil layers in the forest were greater (P?<?0.01) than the net increases of SOC₄ in the respective soil layers in the pasture. Aluminum saturation was greater (P?<?0.01) in the forests than the pastures in both soil layers. There was no difference in oxalate extractable Fe concentration between the forests and the pastures but oxalate extractable Al concentration in both soil layers was greater (P?<?0.05) in forests than the pastures. Our findings indicated that reforestation of abandoned sugarcane farmland in Hawaii is more effective in soil C increase and stabilization than conversion to pasture.     

Ecosystem controls on nitrogen fixation in boreal feather moss communities

N fixation in feather moss carpets is maximized in late secondary successional boreal forests; however, there is limited understanding of the ecosystem factors that drive cyanobacterial N fixation in feather mosses with successional stage. We conducted a reciprocal transplant experiment to assess factors in both early and late succession that control N fixation in feather moss carpets dominated by Pleurozium schreberi. In 2003, intact microplots of moss carpets (30 cm × 30 cm × 10–20 cm deep) were excavated from three early secondary successional (41–101 years since last fire) forest sites and either replanted within the same stand or transplanted into one of three late successional (241–356 years since last fire) forest sites and the transverse was done for late successional layers of moss. Moss plots were monitored for changes in N-fixation rates by acetylene reduction (June 2003–September 2005) and changes in the presence of cyanobacteria on moss shoots by microscopy (2004). Forest nutrient status was measured using ionic resin capsules buried in the humus layer. Late successional forests exhibit high rates of N fixation and consistently high numbers of cyanobacteria on moss shoots, but low levels of available N. Conversely, early successional forests have higher N availability and have low rates of N fixation and limited presence of cyanobacteria on moss shoots. Transplantation of moss carpets resulted in a significant shift in presence and activity of cyanobacteria 1 year after initiation of the experiment responding to N fertility differences in early versus late successional forests.     

Estimating base cation weathering rates in Canadian forest soils using a simple texture-based model

Mineral weathering is the primary source of long-term buffering capacity in soils and is important for forest nutrient sustainability. Regional assessments of weathering rates in Canada and the U.S. have employed an empirical clay-based Soil Texture Approximation (STA) owing to limited data availability, although the STA is rarely calibrated before application to a study area. Soil weathering rates estimated with the STA at 75 sites in Canadian forests (6–367 eq ha^?1 year^?1) were on average seven times lower than estimates obtained using the PROFILE model and when mineralogy was not available, the Analysis to Mineralogy model and parameter estimation (143–2,119 eq ha^?1 year^?1). Comparison with a catchment mass balance at a subset of sites in Ontario (n = 19) demonstrated the reliability of PROFILE weathering estimates. A revised (generalized) STA model for total base cation weathering was developed at the 75 study sites to incorporate soil silt content (%) and loss-on-ignition (LOI, %) (BC _w = (1.73 + 0.03 · silt ? 0.06 · LOI) · 1,000 · depth). The model performance (R _adj ²  = 73%) and relative bias (?1%) suggested that the revised STA may have broad application to forest soils in Canada but may not necessarily be suited to all soil texture classes.     

Changes in Forest Soil Properties in Different Successional Stages in Lower Tropical China

Background

Natural forest succession often affects soil physical and chemical properties. Selected physical and chemical soil properties were studied in an old-growth forest across a forest successional series in Dinghushan Nature Reserve, Southern China.

Methodology/Principal Findings

The aim was to assess the effects of forest succession change on soil properties. Soil samples (0–20 cm depth) were collected from three forest types at different succession stages, namely pine (Pinus massoniana) forest (PMF), mixed pine and broadleaf forest (PBMF) and monsoon evergreen broadleaf forest (MEBF), representing early, middle and advanced successional stages respectively. The soil samples were analyzed for soil water storage (SWS), soil organic matter (SOM), soil microbial biomass carbon (SMBC), pH, NH₄ ⁺-N, available potassium (K), available phosphorus (P) and microelements (available copper (Cu), available zinc (Zn), available iron (Fe) and available boron (B)) between 1999 and 2009. The results showed that SWS, SOM, SMBC, Cu, Zn, Fe and B concentrations were higher in the advanced successional stage (MEBF stage). Conversely, P and pH were lower in the MEBF but higher in the PMF (early successional stage). pH, NH₄ ⁺-N, P and K declined while SOM, Zn, Cu, Fe and B increased with increasing forest age. Soil pH was lower than 4.5 in the three forest types, indicating that the surface soil was acidic, a stable trend in Dinghushan.

Conclusion/Significance

These findings demonstrated significant impacts of natural succession in an old-growth forest on the surface soil nutrient properties and organic matter. Changes in soil properties along the forest succession gradient may be a useful index for evaluating the successional stages of the subtropical forests. We caution that our inferences are drawn from a pseudo-replicated chronosequence, as true replicates were difficult to find. Further studies are needed to draw rigorous conclusions regarding on nutrient dynamics in different successional stages of forest.