Temperature sensitivity of gross N transformation rates in an alpine meadow on the Qinghai-Tibetan Plateau

Purpose

Alpine ecosystems on the Qinghai-Tibetan Plateau are sensitive to global climatic changes. However, the effects of temperature change resulting from global warming or seasonal variation on soil N availability in those ecosystems are largely unknown.

Materials and methods

We therefore conducted a ¹⁵N tracing study to investigate the effects of various temperatures (5–35 °C) on soil gross N transformation rates in an alpine meadow (AM) soil on the Qinghai-Tibetan Plateau. A natural secondary coniferous forest (CF) soil from the subtropical region was chosen as a reference to compare the temperature sensitivity of soil gross N transformation rates between alpine meadow and coniferous forest.

Results and discussion

Our results showed that increasing temperature increased gross N mineralization and NH₄ ⁺ immobilization rates and overall enhanced N availability for plants in both soils. However, both rates in the CF soil were less sensitive to a temperature change from 5 to 15 °C compared to the AM soil. In both soils, different N retention mechanisms could have been operating with respect to changing temperatures in the different climatic regions. In the CF soil, the absence of NO₃ ^? production at all incubation temperatures suggests that in the subtropical soil which is characterized by high rainfall, an increase in N availability due to increasing temperature could be completely retained in soils. In contrast, the AM soil may be vulnerable to N losses with respect to temperature changes, in particular at 35 °C, in which higher nitrification rates were coupled with lower NH₄ ⁺ and NO₃ ^? immobilization rates.

Conclusions

Our results suggest that increased soil temperature arising from global warming and seasonal variations will most likely enhance soil N availability for plants and probably increase the risk of N losses in the alpine meadow on the Qinghai-Tibetan Plateau.

     

Nitrogen addition impacts on the emissions of greenhouse gases depending on the forest type: a case study in Changbai Mountain,Northeast China

Purpose

Anthropogenic-induced greenhouse gas (GHG) emission rates derived from the soil are influenced by long-term nitrogen (N) deposition and N fertilization. However, our understanding of the interplay between increased N load and GHG emissions among soil aggregates is incomplete.

Materials and methods

Here, we conducted an incubation experiment to explore the effects of soil aggregate size and N addition on GHG emissions. The soil aggregate samples (0–10 cm) were collected from two 6-year N addition experiment sites with different vegetation types (mixed Korean pine forest vs. broad-leaved forest) in Northeast China. Carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) production were quantified from the soil samples in the laboratory using gas chromatography with 24-h intervals during the incubation (at 20 °C for 168 h with 80 % field water capacity).

Results and discussion

The results showed that the GHG emission/uptake rates were significantly higher in the micro-aggregates than in the macro-aggregates due to the higher concentration of soil bio-chemical properties (DOC, MBC, NO₃ ^?, NH₄ ⁺, SOC and TN) in smaller aggregates. For the N addition treatments, the emission/uptake rates of GHG decreased after N addition across aggregate sizes especially in mixed Korean pine forest where CO₂ emission was decreased about 30 %. Similar patterns in GHG emission/uptake rates expressed by per soil organic matter basis were observed in response to N addition treatments, indicating that N addition might decrease the decomposability of SOM in mixed Korean pine forest. The global warming potential (GWP) which was mainly contributed by CO₂ emission (>98 %) decreased in mixed Korean pine forest after N addition but no changes in broad-leaved forest.

Conclusions

These findings suggest that soil aggregate size is an important factor controlling GHG emissions through mediating the content of substrate resources in temperate forest ecosystems. The inhibitory effect of N addition on the GHG emission/uptake rates depends on the forest type.

     

The properties and functions of biochars in forest ecosystems

Purpose

The production of large quantities of biochar from natural fires has been a part of human history for millennia, causing CO₂ emissions to the atmosphere and exerting long-term effects on soil processes. Despite its potential importance and recent work reflecting the wide interest in biochar, a general review of our deep understanding of biochar functions within forest soils is currently lacking. Gaps in research knowledge in this field are identified in this paper.

Materials and methods

This paper summarizes recent research to provide a better understanding of the concentrations, distribution, and characteristics of biochar produced from forest wildfire and its influences on soil processes. Perspectives and recommendations for future research on biochar in post-fire forest soils are also discussed.

Results and discussion

The concentration, distribution, and characteristics of biochar produced from forest wildfire largely depend on forest landscapes, regional climates, and mostly its feedstock and fire history, like, its duration and severity. The influences of biochar on soil processes, particularly carbon and nitrogen transformations and cycling, like, nitrification and nitrous oxide emissions reduction (Clough and Condron, J Environ Qual 39:1218–1223, 2010), are also determined mainly by the fire temperature and raw materials. Mechanisms can be attributed to the adsorption of organic compounds and nutrients or changed microenvironment, termed as charsphere, by biochar. We also identify the microbial mechanisms involved in the biochar-containing soils.

     

Relationships between soil–litter interface enzyme activities and decomposition in <Emphasis Type="Italic">Pinus massoniana</Emphasis> plantations in China

Purpose

Enzyme activities in decomposing litter are directly related to the rate of litter mass loss and have been widely accepted as indicators of changes in belowground processes. Studies of variation in enzyme activities of soil–litter interface and its effects on decomposition are lacking. Evaluating enzyme activities in this layer is important to better understand energy flow and nutrient cycling in forest ecosystems.

Materials and methods

Litter decomposition and the seasonal dynamics of soil–litter enzyme activities were investigated in situ in 20- (younger) and 46-year-old (older) Pinus massoniana stands for 540 days from August 2010 to March 2012 by litterbag method. We measured potential activities of invertase, cellulase, urease, polyphenol oxidase, and peroxidase in litter and the upper mineral soils, and evaluated their relationships with the main environment factors.

Results and discussion

Remaining litter mass was 57.6 % of the initial weights in the younger stands and 61.3 % in the older stands after 540-day decomposition. Levels of enzyme activity were higher in the litter layer than in the soil layer. Soil temperature, litter moisture, and litter nitrogen (N) concentration were the most important factors affecting the enzyme activities. The enzyme activity showed significantly seasonal dynamics in association with the seasonal variations in temperature, water, and decomposition stages. Remaining litter dry mass was found to be significantly linearly correlated with enzyme activities (except for litter peroxidase), which indicates an important role of enzyme activity in the litter decomposition process.

Conclusions

Our results indicated the important effects of biotic (litter N) and abiotic factors (soil temperature and litter moisture) on soil–litter interface enzyme activities. Overall significant linear relationship between remaining dry mass and enzyme activities highlighted the important role of enzyme activity in affecting litter decomposition processes, which will further influence nutrient cycling in forest ecosystems. Our results contributed to the better understanding of the mechanistic link between upper soil–litter extracellular enzyme production and litter decomposition in forest ecosystems.

     

Alpine meadow restorations by non-dominant species increased soil nitrogen transformation rates but decreased their sensitivity to warming

Purpose

Alpine meadow soils are large carbon (C) and nitrogen (N) pools correlated significantly with global C and N cycling. Soil N transformations, including nitrification and N mineralization, are key processes controlling N availability. Alpine meadow degradations are common worldwide, and vegetation restorations have been widely implemented. However, grass species used for restorations may alter soil N transformations or their response to warming and N deposition due to divergent plant traits and their different effects on soil characteristic. To understand the effects of meadow restorations by non-historically dominant species on N transformations, we measured N transformation rates in restored soils and control soils under the context of warming and N deposition.

Materials and methods

We collected soils from plots restored by dominant (Miscanthus floridulus) and non-dominant species (including Carex chinensis and Fimbristylis dichotoma) and non-restored plots in alpine meadows of Wugong Mountain, China. We measured nitrification and N mineralization rates when soils were incubated at different temperature (15 or 25 °C) and N additions (control vs. 4 g m^?2) to examine their responses to restoration species, warming, and N.

Results and discussion

Vegetation restored soils differed substantially from non-restored bare soils. Total N, total organic C, pH, and dissolved organic C contributed the most to the separation. Restoration altered soil N transformations substantially, even though the effects varied among restoration species. Specifically, non-historically dominant species accelerated N transformations, while the originally dominant species decreased N transformations. In addition, sensitivity of nitrification to warming in restored soils was decreased by restorations. Soils restored by originally dominant species were higher in sensitivity of N transformations to warming than those restored by the other two species. Warming increased nitrification rates by 45.5 and 17.4 % in bare soils and restored soils, respectively. Meanwhile, N mineralization rates were increased by 52.8 and 21.9 %, respectively.

Conclusions

Vegetation restoration of the degraded meadows impacted N transformations and their sensitivity to warming. The effects varied with identity of the restoration species, suggesting that grass species should be considered in future restorations of degraded meadows in terms of their divergent effects on N transformations and sensitivity to warming.

     

How environmental and vegetation factors affect spatial patterns of soil carbon and nitrogen in a subtropical mixed forest in Central China

Purpose

Soil properties are highly heterogeneous in forest ecosystems, which poses difficulties in estimating soil carbon (C) and nitrogen (N) pools. However, little is known about the relative contributions of environmental factors and vegetation to spatial variations in soil C and N, especially in highly diverse mixed forests. Here, we examined the spatial variations of soil organic carbon (SOC) and total nitrogen (TN) in a subtropical mixed forest in central China, and then quantified the main drivers.

Materials and methods

Soil samples (n = 972) were collected from a 25-ha forest dynamic plot in Badagonshan Nature Reserve, central China. All trees with diameter at breast height (DBH) ≥1 cm and topography data in the plot were surveyed in detail. Geostatistical analyses were used to characterize the spatial variability of SOC and TN, while variation partitioning combined with Mantel’s test were used to quantify the relative contribution of each type of factors.

Results and discussion

Both surface soil (0–10 cm) and subsurface soil (10–30 cm) exhibited moderate spatial autocorrelation with explainable fractions ranged from 31 to 47 %. The highest contribution to SOC and TN variation came from soil variables (including soil pH and available phosphorus), followed by vegetation and topographic variables. Although the effect of topography was weak, Mantel’s test still showed a significant relationship between topography and SOC. Strong interactions among these variables were discovered. Compared with surface soil, the explanatory power of environmental variables was much lower for subsurface soil.

Conclusions

The differences in relative contributions between surface and subsurface soils suggest that the dominating ecological process are likely different in the two soil depths. The large unexplained variation emphasized the importance of fine-scale variations and ecological processes. The large variations in soil C and N and their controlling mechanisms should be taken into account when evaluating how forest managements may affect C and N cycles.

     

Mechanisms behind the stimulation of nitrification by N input in subtropical acid forest soil

Purpose

Input of N as NH₄ ⁺ is known to stimulate nitrification and to enhance the risk of N losses through NO₃ ^? leaching in humid subtropical soils. However, the mechanisms responsible for this stimulation effect have not been fully addressed.

Materials and methods

In this study, an acid subtropical forest soil amended with urea at rates of 0, 20, 50, 100 mg N kg^?1 was pre-incubated at 25 °C and 60 % water-holding capacity (WHC) for 60 days. Gross N transformation rates were then measured using a ¹⁵N tracing methodology.

Results and discussion

Gross rates of mineralization and nitrification of NH₄ ⁺-N increased (P?<?0.05), while gross rate of NO₃ ^? immobilization significantly decreased with increasing N input rates (P?<?0.001). A significant relationship was established between the gross nitrification rate of NH₄ ⁺ and the gross mineralization rate (R ²?=?0.991, P?<?0.01), so was between net nitrification rate of NH₄ ⁺ and the net mineralization rate (R ²?=?0.973, P?<?0.05).

Conclusions

Stimulation effect of N input on the gross rate of nitrification of NH₄ ⁺-N in the acid soil, partially, resulted from stimulation effect of N input on organic N mineralization, which provides pH-favorable microsites for the nitrification of NH₄ ⁺ in acid soils (De Boer et al., Soil Biol Biochem 20:845–850, 1988; Prosser, Advan Microb Physiol 30:125–181, 1989). The stimulated gross nitrification rate with the decreased gross NO₃ ^? immobilization rate under the elevated N inputs could lead to accumulation of NO₃ ^? and to enhance the risk of NO₃ ^? loss from humid forest soils.

     

Changes in soil bacterial communities in an evergreen broad-leaved forest in east China following 4 years of nitrogen addition

Purpose

Evergreen broad-leaved forest ecosystems are common in east China, where they are both ecologically and economically important. However, nitrogen (N) addition over many years has had a detrimental effect on these ecosystems. The objective of this research was to evaluate the effect of 4 years of N addition on microbial communities in an evergreen broad-leaved forest in southern Anhui, China.

Materials and methods

Allochthonous N in the form of aqueous NH₄NO₃ and phosphorus (P) in the form of Ca(H₂PO₄)₂·H₂O were applied at three doses with a control (CK, stream water only without fertilizer): low-N (50 kg N ha^?1 year^?1), high-N (100 kg N ha^?1 year^?1) and high-N+P (100 kg N ha^?1 year^?1 + 50 kg P ha^?1 year^?1). Quantitative PCR analysis of microbial community size and Illumina platform-based sequencing analysis of the V3-V4 16S rRNA gene region were performed to characterize soil bacterial community abundance, structure, and diversity.

Results and discussion

Bacterial diversity was increased in low-N and high-N treatments and decreased in the high-N+P treatment, but α-diversity indices were not significantly affected by N additions. Proteobacteria, Acidobacteria, and Actinobacteria were the predominant phyla in all treatments, and the relative abundance of different genera varied among treatments. Only soil pH (P = 0.051) showed a weak correlation with the bacterial community in CK and low-N treatment.

Conclusions

The composition of the bacterial community and the abundance of different phyla were significantly altered by N addition. The results of the present study indicate that soil bacterial communities in subtropical evergreen broad-leaved forest are, to a certain extent, resilient to changes derived from N additions.

     

Organic carbon and nitrogen dynamics in different soil fractions between broad-leaved Korean pine forests and aspen–birch forests in northeastern China

Purpose

Both overharvesting and climate changes have greatly altered forest composition in northeastern China; however, forest-specific effects on soil organic carbon (SOC), N, and compositional features in different soil fractions have not yet been defined.

Materials and methods

By sampling from broad-leaved Korean pine forest (the climax vegetation) and aspen–birch forest (the secondary forest), five soil fractions were separated by a physicochemical soil fractionation method, and Fourier transform infrared spectroscopy, X-ray diffraction analysis, and X-ray photoelectron spectrometry were used for functional groups, mineral diffraction, and elemental composition determination together with SOC and N measurements.

Results and discussion

Aspen–birch forests tended to sequestrate more SOC in the slow fractions (sand and aggregate [SA] and easily oxidized fractions) and more N in the sensitive fractions (particulate and soluble fractions), indicating that in aspen–birch forests, high SOC sequestration (1.26-fold) coincided with the active and rapid N supply. Much higher percentages (13.1–40.5 %) of O–H and N–H stretching, O–H bending, and C=O, COO–, and C–H stretching, and also the much lower quartz grain size and mineral diffraction peaks in SA and acid-insoluble fraction (over 85 % of total soil mass), in aspen–birch forests were possibly associated with the 1.17- to 1.53-fold higher SOC compared to broad-leaved Korean pine forest. However, elemental composition on soil particles might marginally contribute to the SOC and N forest-dependent differences.

Conclusions

Considering the increase of aspen–birch forests and the decrease of broad-leaved Korean pine forests in historical and future scenarios in northeastern China, more SOC has been and also will sequestrate in intact soils and stable soil fractions, with more N in sensitive fractions, and these should be highlighted in evaluating forest C and N dynamics during forest successions in this region.

     

Changes in forest soil organic matter quality affected by windstorm and wildfire

Purpose

Windthrows and fires are major natural disturbances in forest ecosystems, which can affect organic matter in the surface and the mineral layer of forest soil. The main aim of this study was to evaluate the changes occurring in the structure and properties of humic acid (HA) in the lands where windthrows and wildfires occurred.

Materials and methods

In November 2004, the forest in the area of 12,000 ha in the Tatra National Park, Slovakia, was seriously damaged by northern wind gusts exceeding 200 km/h. In July 2005, a wildfire broke out in a 220 ha of wind-damaged area. The HAs have been isolated from four research plots: (a) the area where the fallen trees were removed (EXT); (b) an area after windstorm covered by wood from struck trees (NEX), left for spontaneous succession; (c) an area after extracted timber, damaged by the surface wildfire (FIR); and (d) a reference intact spruce forest area (REF). Changes in the chemical structure of the HAs isolated from the research plots were determined on the basis of elemental analysis and UV-Vis, EPR, IR, and ¹³C NMR spectroscopy.

Results and discussion

All used analytical methods showed a decrease in the humification degree of the humic acids extracted from the soils where the spruce forest has been affected by a wildfire and a windthrow. In the case of the control sample HA (REF), the calculated atomic H/C and O/C ratios and the degree of aromaticity (α) calculated from the ¹³C NMR spectra were higher, indicating higher aromaticity of HA from the REF area. The more complex and developed structure of REF HA was confirmed by the higher value of E^1%₆ and the lower E4/E6 ratio obtained using UV-Vis spectroscopy. Also, the higher g-parameters determined from the EPR spectra of the stable radicals present in HA confirmed the lower aromaticity on the plots that have been subjected to the calamities. The ¹³C NMR spectra and the elemental analysis show that the structure of the HA extracted from the NEX plot is the closest to the REF.

Conclusions

The results of the systematic research showed significant changes in the structure of HA taken from spruce forest soils that were subjected to windstorms and fires. An enrichment of the HAs in aliphatic carbon and so a lower humification degree of the organic matter in the areas calamity-affected were observed. The results clearly indicate that the HAs extracted from the disturbed plots of the spruce forest are not as stable as those extracted taken from the control plot.

     

Cellulose decomposed faster in fallow soil than in meadow soil due to a shorter lag time

Purpose

The study aimed at comparing organic matter decomposition in two semi-natural agrobiocenozes, namely fallows and meadows, with similar plant biomass but differing in plant community composition and diversity and in succession stage.

Materials and methods

The decomposition rate of a standard material (cellulose) was measured in soils from six fallows and six meadows spanning a few kilometres apart. The mathematical model was fitted to the data.

Results and discussion

The model showed a significantly longer lag-time in cellulose decomposition in the meadows. Despite the delayed start of decomposition in the meadows, the estimated decomposition rates were similar in both ecosystem types, once the decay started.

Conclusions

The faster start of decomposition in fallows seems to be promoted by higher contents of nitrates and phosphates in the fallow soils. The fallows, as younger ecosystems, may have faster C turnover than older grasslands due to remains of fertilisers on these ex-arable fields.

     

Impact of climate and lithology on soil phytolith-occluded carbon accumulation in eastern China

Purpose

Occlusion of carbon (C) within phytoliths, biogenic silica deposited in plant tissues and returned to the soil, is an important mechanism for long-term terrestrial biogeochemical C sequestration and might play a significant role in mitigating climate change.

Materials and methods

Subtropical and tropical soil profiles (to 100 cm depth) developed on granite and basalt were sampled using a mass-balance approach to explore the influence of climate and lithology on soil phytolith-occluded carbon (PhytOC) accumulation.

Results and discussion

Soil PhytOC storage in the subtropics was significantly greater than in the tropics, with the soil profiles developed on granite storing greater PhytOC than soils derived on basalt. Phytolith and PhytOC content decreased with depth in all soil profiles. Phytolith content showed a positive correlation with the soil bio-available silicon in the soil profiles developed on basalt, while a negative correlation was observed in soil profiles developed on granite.

Conclusions

Climate and lithology have a significant impact on soil PhytOC sequestration. The management of forests (e.g., afforestation and reforestation) and external silicon amendments (e.g., basalt powder amendment) in soils, especially those developed on granite, have the potential to enhance PhytOC accumulation in forest ecosystems.

     

Topography-soil relationships in a hilly evergreen broadleaf forest in subtropical China

Purpose

Topography-soil relationships usually vary with climate, vegetation type, degree of human disturbance, type of parent material, and the scale being studied. In this paper, we studied the topography-soil relationship in a hilly forest in subtropical China.

Materials and methods

The influence of topography on soil properties (soil moisture, organic carbon (C), total nitrogen (N) and total phosphorus contents, C:N ratio, and pH) was evaluated using a recursive partitioning conditional inference tree (CIT) as well as a multiple linear regression (MLR) method.

Results and discussion

The CIT models generally performed better than MLR in describing the topography-soil relationships. Topographic parameters chosen by the CIT models, which indicate the mechanisms at play for the spatial variation of the soil properties, varied with the soil property of concern. The soil moisture, organic C, and total N models contained only primary terrain attributes, the soil C:N ratio and pH models contained both primary and secondary terrain attributes, while the total phosphorus model contained mostly secondary terrain attributes.

Conclusions

The CIT method worked well for exploring the topography-soil relationships in the studied undisturbed hilly forest. We conclude that (1) soil moisture, organic C, and total N were strongly affected by location-specific topographic features such as gravitational potential, the amount of precipitation, temperature, and vegetation type; (2) total phosphorus was affected by catchment-related hydrological activities and soil C:N ratio; and (3) pH was affected by location-specific topographic features and catchment-related hydrological activities.

     

Soil microbial abundance and community structure vary with altitude and season in the coniferous forests,China

Purpose

Soil microbes control the bioelement cycles and energy transformation in forest ecosystems, and are sensitive to environmental change. As yet, the effects of altitude and season on soil microbes remain unknown. A 560 m vertical transitional zone was selected along an altitude gradient from 3023, 3298 and 3582 m, to determine the potential effects of seasonal freeze-thaw on soil microbial community.

Materials and methods

Soil samples were collected from the three elevations in the growing season (GS), onset of freezing period (FP), deeply frozen period (FPD), thawing period (TP), and later thawing period (TPL), respectively. Real-time qPCR and polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) were used to measure the abundance and structure of soil microbial community.

Results and discussion

The bacterial, archaeal, and fungal ribosomal DNA (rDNA) copy numbers decreased from GS to freezing stage (FP and FPD) and then increased in thawing stage (TP and TPL). Similarly, the diversity of microbial community varied with seasonal freeze-thaw processes. The diversity index (H) of the bacterial and archaeal communities decreased from GS to FP and then increased to TPL. The fungal community H index increased in the freezing process.

Conclusions

Our results suggested that abundance and structure of soil microbial community in the Tibetan coniferous forests varied by season and bacterial and archaeal communities respond more promptly to seasonal freeze-thaw processes relative to fungal community. This may have important implications for carbon and nutrient cycles in alpine forest ecosystems. Accordingly, future warming-induced changes in seasonal freeze-thaw patterns would affect soil nutrient cycles via altering soil microbial properties.

     

Nitrogen transformation rates and N<Subscript>2</Subscript>O producing pathways in two pasture soils

Purpose

Better understanding of N transformations and the regulation of N₂O-related N transformation processes in pasture soil contributes significantly to N fertilizer management and development of targeted mitigation strategies.

Materials and methods

¹⁵N tracer technique combined with acetylene (C₂H₂) method was used to measure gross N transformation rates and to distinguish pathways of N₂O production in two Australian pasture soils. The soils were collected from Glenormiston (GN) and Terang (TR), Victoria, Australia, and incubated at a soil moisture content of 60% water-filled pore space (WFPS) and at temperature of 20 °C.

Results and discussion

Two tested pasture soils were characterized by high mineralization and immobilization turnover. The average gross N nitrification rate (n_tot) was 7.28 mg N kg^?1 day^?1 in TR soil () and 5.79 mg N kg^?1 day^?1 in GN soil. Heterotrophic nitrification rates (n_h), which accounting for 50.8 and 41.9% of n_tot, and 23.4 and 30.1% of N₂O emissions in GN and TR soils, respectively, played a role similar with autotrophic nitrification in total nitrification and N₂O emission. Denitrification rates in two pasture soils were as low as 0.003–0.004 mg N kg^?1 day^?1 under selected conditions but contributed more than 30% of N₂O emissions.

Conclusions

Results demonstrated that two tested pasture soils were characterized by fast N transformation rates of mineralization, immobilization, and nitrification. Heterotrophic nitrification could be an important NO₃^?–N production transformation process in studied pasture soils. Except for autotrophic nitrification, roles of heterotrophic nitrification and denitrification in N₂O emission in two pasture soils should be considered when developing mitigation strategies.

     

The impact of different forest types on phytolith-occluded carbon accumulation in subtropical forest soils

Purpose

Occlusion of carbon in phytoliths is an important biogeochemical carbon sequestration mechanism and plays a significant role in the global biogeochemical carbon cycle and atmospheric carbon dioxide (CO₂) concentration regulation at a millennial scale. However, few studies have focused on the storage of phytolith and phytolith-occluded carbon (PhytOC) in subtropical forest soils.

Materials and methods

Soil profiles with 100-cm depth were sampled from subtropical bamboo forest, fir forest, and chestnut forest in China to investigate the variation of phytoliths and PhytOC storage in the soil profiles based on amass-balance assessment.

Results and discussion

The storage of phytoliths in the top 100 cm of the bamboo forest soil (198.13?±?25.08 t ha^?1) was much higher than that in the fir forest (146.76?±?4.53 t ha^?1) and chestnut forest (170.87?±?9.59 t ha^?1). Similarly, the storage of PhytOC in the bamboo forest soil (3.91?±?0.64 t ha^?1) was much higher than that in the fir forest soil (1.18?±?0.22 t ha^?1) and chestnut forest soil (2.67?±?0.23 t ha^?1). The PhytOC percentage in the soil organic carbon pool increased with soil depth and was the highest (4.29 %) in the bamboo forest soil. Our study demonstrated that PhytOC in soil was significantly influenced by forest type and the bamboo forest ecosystem contributed more significantly to phytolith carbon sequestration than other forest ecosystems.

Conclusions

Different forest types have a significant influence on the soil PhytOC storage. Optimization of bamboo afforestation/reforestation in future forest management plans may significantly enhance the biogeochemical carbon sink in the following centuries.

     

The role of silvicultural systems and forest types in preventing soil erosion processes in mountain forests: a methodological approach using cesium-137 measurements

Purpose

Forests play a key role in providing protection against soil erosion. Particularly, the role of vertical forest structure in increasing rainfall interception capacity is crucial for mitigating raindrop impact and reducing splash and rill erosion. For this reason, studies on the relationships between forest structures, the past management, and the observed rates of soil loss are needed. In the last few decades, importance was given to the use of cesium-137 (¹³⁷Cs) as radioactive tracer to estimate soil erosion rates. The ¹³⁷Cs technique is linked to the global fallout of bomb-derived radiocesium which occurred during a period extending from the mid 1950s to the late 1970s.

Materials and methods

The ¹³⁷Cs technique, providing long-term retrospective estimates, could be related to forest treatments applied during the last decades in different sites, also considering the tree species composition. This approach could be useful to compare the effect of different canopy cover and biomass on soil erosion rates related to different tree species. In the work proposed here, a study area dominated by pine and beech high forests located in the Aspromonte Mountains (Calabria, Italy) was selected. The measurements, related to forest structural traits, focusing on canopy cover and biomass, and also on management approaches and forest types, are compared with rates of soil erosion provided by ¹³⁷Cs.

Results and discussion

The overall results suggest that the minimum values of soil loss are documented in areas with higher canopy cover and biomass evidencing the protective effect provided by forests against soil erosion. Also, techniques based on the use of tracers like ¹³⁷Cs proved to be helpful to select the best forest management options useful to optimize the protective role of forests, with the aim to reduce erosion processes in a long-term perspective.

Conclusions

The experiment indicates that care must be taken when new silviculture treatments are planned. These findings are in agreement with what documented by other authors in similar environments but need further studies to confirm the effectiveness of using ¹³⁷Cs in different forest ecosystems.

     

Soil carbon dynamics in successional and plantation forests in subtropical China

Purpose

The purpose of this study was to better understand how both the content and flux of soil carbon respond to forest succession and anthropogenic management practices in forests in subtropical China.

Materials and methods

We assembled from the literature information on soil organic carbon (SOC) and soil respiration (Rs) covering the forest successional chronosequence from pioneer masson pine (Pinus massoniana) forest (MPF) to medium broadleaf and needleleaf mixed forest (BNMF) and the climax evergreen broadleaf forest (EBF), along with the two major forest plantation types found in subtropical China, Chinese fir (Cunninghamia lanceolata) forest (CFF) and Moso bamboo (Phyllostachys pubescens) forest (MBF).

Results and discussion

Both SOC and Rs increased along the forest successional gradient with the climax EBF having both the highest SOC content of 33.1?±?4.9 g C kg^?1(mean?±?standard error) and the highest Rs rate of 46.8?±?3.0 t CO₂?ha^?1 year^?1. It can be inferred that when EBF is converted to any of the other forest types, especially to MPF or CFF, both SOC content and Rs are likely to decline. Stand age did not significantly impact the SOC content or Rs rate in either types of plantation.

Conclusions

Forest succession generally increases SOC content and Rs, and the conversion of natural forests to plantations decreases SOC content and Rs in subtropical China.

     

Soil organic carbon as a key predictor of N in forest soils of Brazil

Purpose

Despite the ancillary knowledge that soil N is chiefly retained as soil organic matter, little is known about how it is affected by other soil and environmental factors, especially in the tropics. In this study, we performed a comprehensive survey of soils under native vegetation in Minas Gerais, Brazil, aiming to (a) measure soil Kjeldahl-N concentrations to a 1-m depth, (b) identify the main affecting factors of soil N retention, and (c) predict N through soil profile based on organic C (SOC) and its main conditioning factors.

Materials and methods

Soils under 36 fragments of native forest and savanna were sampled at five depths (0–10, 10–20, 20–40, 40–60, and 60–100 cm) and characterized by physical and chemical analyses, including total N determined by the micro-Kjeldahl method. Single and multivariate regressions were used to predict N concentrations based on soil properties and climatic factors.

Results and discussion

The average N concentrations ranged between 0.12 and 7.54 g kg^?1, decreasing with depth, and can be predicted using SOC concentrations (R ² = 0.86). Multivariate regressions using more input data, namely texture, cation exchange capacity (CEC), and altitude increased slightly R ² values (0.68–0.90) for separate soil depths, but not for the whole dataset (R ² = 0.85).

Conclusions

We demonstrated that N can be adequately predicted based on SOC concentrations, for any depth and forest type. The implications of the stable SOC/N relation and their coupled cycles and the environmental factors affecting N retention in Brazilian weathered soils are further discussed.

     

Variations in forest aboveground biomass in Miyun Reservoir of Beijing over the past two decades

Purpose

In recent years, climate change, particularly rising carbon dioxide (CO₂) concentration and global warming, has attracted much attention around the world. Forest ecosystems still play a crucial role in global carbon (C) fixation. Quantifying forest aboveground biomass (AGB) and its temporal variations is essential for understanding external impacts (e.g., urbanization, environmental change) as well as assessing the potential of forest ecosystems assimilating the atmospheric CO₂.

Materials and methods

In this study, we established regression models for AGB estimation in the Miyun Reservoir region, based on relationships between Landsat-derived variables and ground truth AGB values, which were obtained from both plot measurements and estimations using Light Detection and Ranging (LiDAR) dataset. The models were applied to calibrated Landsat images acquired in 1990, 2000, and 2010 to track the forest AGB temporal variations and the corresponding spatial distributions for each period. The AGB estimations using LiDAR showed high consistency with values based on the plot measurements, while the established models presented an acceptable accuracy.

Results and discussion

The AGB density in the Miyun Reservoir experienced an overall increase since 1990 and was averaged at 52.20 and 32.12 t ha^?1, for stand forest and shrub in 2010, respectively. Total AGB in 2010 was estimated to be 4.5 × 10⁷ t, which increased by 8% when compared with the level in 1990. Our results are in the similar range of AGB density reported by other studies carried out in Northern China. Ecological programs including Three-North Shelter Forest Project and Returning Farming to Forest promoted the forest expansion and development during this period, while local farming activities exerted certain negative effects on the surrounding forest systems.

Conclusions

Absolute AGB density values indicated that regions with less external interventions present more consistent biomass accumulation. However, the coarse spatial resolution and 10-year interval of the datasets limited detailed analysis of impacts from urbanization of Beijing City. Future studies incorporating sophisticated ecosystem research methods are expected to uncover the mechanisms and key drivers for the observed variations in the AGB in the Miyun Reservoir region of Beijing, China.