Environmental controls and the influence of tree species on temporal variation in soil respiration in subtropical China

Background and aims

The knowledge of individual tree species impacts on soil respiration based on rigorous experimental designs is limited, but is crucial to help guide selection of species for reforestation and carbon (C) management purposes.

Methods

We assessed monthly soil respiration and its components, litterfall input, fine root production and mortality under 19-year-old native coniferous Cunninghamia lanceolata and broadleaved Mytilaria laosensis plantations in sub-tropical China.

Results

Total soil respiration from October 2011 to March 2013 was significantly lower under the C. lanceolata than the M. laosensis plantation. The difference in respiration rates derived from fine roots and the litter layer explained much of the variation of total soil respiration between the two tree species. We used an exponential equation and base temperature (10 °C) to normalize soil respiration rate and its components (R₁₀) and determined the correlation between R₁₀ and soil moisture. Although soil moisture had a positive relationship with R₁₀ derived from roots or litter under both C. lanceolata and M. laosensis forests, these positive correlations were masked by negative relationships between soil moisture and R₁₀ derived from root-free soil, which resulted in a neutral correlation between total R₁₀ and soil moisture under C. lanceolata forests. Monthly litterfall input was associated with variation in concurrent total soil respiration rate under the M. laosensis plantation and respiration rate lagging 3 months behind under the C. lanceolata plantation, which may suggest that litterfall input from M. laosensis can more rapidly produce C substrates for microbial respiration than litterfall from C. lanceolata.

Conclusions

This study highlighted that tree species-induced variation in the quality and quantity of fine roots and litterfall can impact not only the soil respiration rate but also the seasonal variation model of forest soil respiration.     

Effect of the replacement of tropical forests with tree plantations on soil organic carbon levels in the Jomoro district,Ghana

Background and aims

In the Jomoro district in Ghana, tree plantations were the first cause of deforestation in the past, drastically reducing the area occupied by primary forests. The aim of this study was to quantify soil organic carbon (SOC) losses due to a change in land use from primary forest to tree plantations (cocoa, coconut, rubber, oil palm) on the different substrates of the district. Secondary forests and mixed plantations were also included in the study.

Methods

Soils were sampled at different depths up to 100 cm along a series of chronosequences in each of the three substrates (Granite, Lower Birrimian and Tertiary Sands) present in the area.

Results

The highest SOC losses in the 0–30 cm layer were caused by the conversion of primary forests to tree plantations: cocoa ?61 % of the original SOC stock, coconut ?55 %, rubber ?35 % and oil palm 28 %, while mixed plantations and secondary forests showed a loss of 23 % and 21 % of the original SOC stock, respectively. C losses were less apparent from the entire profile (to a depth of 100 cm).

Conclusions

All conversions to tree plantations caused substantial SOC losses, comparable to the conversion of forests to agricultural systems. Secondary forests and mixed plantations were the only sustainable land uses that restricted SOC losses considerably.     

Harvest residue management effects on tree growth and ecosystem carbon in a Chinese fir plantation in subtropical China

Aims

This study aimed to determine the influence of different harvest residue management strategies on tree growth, soil carbon (C) concentrations, soil nitrogen (N) availability and ecosystem C stocks 15 years after replanting second rotation Chinese fir (Cunninghamia lanceolata), an important plantation species in subtropical China. Such information is needed for designing improved management strategies for reforestation programmes in subtropical environments aimed at mitigating CO₂ emissions.

Methods

Four harvest residue management treatments including slash burning, whole tree, stem-only and double residue retention were applied to sixteen 20 m?×?30 m plots in a randomized complete block design with four replicates. Tree growth was measured annually and soil properties were measured at 3 year intervals over a 15 year period after re-planting.

Results

Cumulative diameter growth at age 15 years was significantly smaller in the slash burning than the whole tree and double residue harvest treatments. Hot water extractable N concentrations increased with the increased organic residue retention levels and significant differences were observed between double residue and slash burning treatments. Harvest residue management had no significant effect on the soil C concentrations to 40 cm depth. ANOVA showed that harvest residue management had no significant effect on total biomass carbon at age 15, but the plantation ecosystem (soil C at 0–40 cm depth plus forest biomass C) had significantly lower C mass in the slash burning treatment compared with whole tree, stem only harvest and double residue harvest treatments.

Conclusions

These observations suggest that organic residue retention during the harvesting could improve the growth and ecosystem C stocks of Chinese fir in second rotation forest plantations in subtropical China and highlight the importance of viewing the ecosystem as a whole when evaluating the impact of harvest residue management on C stocks.     

Effect of monospecific and mixed Cunninghamia lanceolata plantations on microbial community and two functional genes involved in nitrogen cycling

Conversion of native broadleaf forest (NF) and introduction of broadleaf species into monospecific Cunninghamia lanceolata plantations are silvicultural practices driven by the increasing demand for timber production. This study was conducted to evaluate the impacts of successive planting of C. lanceolata and mixed plantations (C. lanceolata-Michelia macclurei, CFM; C. lanceolata-Alnus cremastogyne, CFA; C. lanceolata-Kalopanax septemlobus, CFK) on microbial community diversity. Microbial biomass (MBC) was assessed using chloroform fumigation-extraction. Using denaturing gradient gel electrophoresis (DGGE), we examined the biodiversity within eubacterial (16S rRNA gene) and fungal (28S rRNA gene) species and two genes involved in N cycling: nifH and amoA. Microbial community diversities and microbial biomass decreased as NF was substituted by successive plantings of C. lanceolata plantations, whereas the trend reversed after introducing the broadleaf, M. macclurei, into pure C. lanceolata plantations. A strong positive correlation between MBC changes and total organic C (TOC), total organic N (TON), available N and extractable C (C_ext) were seen, which suggests that MBC was tightly coupled with the content of soil organic matter. The Shannon index showed that bacterial diversity and two functional genes (nifH and amoA) diversities associated with monospecific C. lanceolata plantations were lower than that of NF or mixed C. lanceolata plantations, such as CFM and CFA, whereas the opposite was seen for fungal diversity. Bacterial diversity was positively correlated with pH, TOC, TON, C_ext and NH ₄ ⁺ -N; while fungal diversity was positively correlated with C/N ratio and negatively correlated with pH. Both nitrogen fixing and ammonia oxidizing bacterial diversities were positively correlated with pH. Thus, soil pH was not only significantly positively correlated with bacterial diversity (r?=?0.502, P?<?0.05), nifH gene diversity (r?=?0.564, P?<?0.01) and amoA gene diversity (r?=?0.659, P?<?0.001), but also negatively correlated with fungal diversity (r?=?? 0.505, P?<?0.05), which seemed to be responsible for the discrimination of the soil microbial communities among these plantations. These findings suggest that different silvicultural practices have significant impacts on the soil microbial community through influences on soil chemical properties.     

Standing fine root mass and production in four Chinese subtropical forests along a succession and species diversity gradient

Background and aims

The influences of succession and species diversity on fine root production are not well known in forests. This study aimed to investigate: (i) whether fine root biomass and production increased with successional stage and increasing tree species diversity; (ii) how forest type affected seasonal variation and regrowth of fine roots.

Methods

Sequential coring and ingrowth core methods were used to measure fine root production in four Chinese subtropical forests differing in successional stages and species diversity.

Results

Fine root biomass increased from 262 g·m^?2 to 626 g·m^?2 with increasing successional stage and species diversity. A similar trend was also found for fine root production, which increased from 86 to 114 g·m^?2 yr ^?1 for Cunninghamia lanceolata plantation to 211–240 g·m^?2 yr ^?1 for Choerospondias axillaries forest when estimated with sequential coring data. Fine root production calculated using the ingrowth core data ranged from 186 g·m^?2 yr ^?1 for C. lanceolata plantation to 513 g·m^?2 yr ^?1 for Lithocarpus glaber – Cyclobalanopsis glauca forest.

Conclusions

Fine root biomass and production increased along a successional gradient and increasing tree species diversity in subtropical forests. Fine roots in forests with higher species diversity exhibited higher seasonal variation and regrowth rate.     

Effects on nutrient cycling of conifer restoration in a degraded tropical montane forest

Background and aims

Exotic coniferous species have been used widely in restoration efforts in tropical montane forests due to their tolerance to adverse conditions and rapid growth, with little consideration given to the potential ecological benefits provided by native tree species. The aim of this study was to elucidate differences in litterfall and nutrient flow between a montane oak forest (Quercus humboldtii Bonpl.) and exotic coniferous plantations of pine (Pinus patula Schltdl. & Cham.) and cypress (Cupressus lusitanica Mill.) in the Colombian Andes.

Methods

Litter production, litter decomposition rate, and element composition of leaf litter were monitored during 3 years.

Results

Litter production in the oak forest and pine plantation was similar, but considerably lower in the cypress plantation . Similar patterns were observed for nutrient concentrations in litterfall, with the exception of Ca which was three times higher in the cypress plantation. The annual decay rate of litter was faster in the montane oak forest than in either of the exotic coniferous plantations. The potential and net return of nutrients to the forest floor were significantly higher in oak forest than in the exotic coniferous plantations.

Conclusions

Future restoration programs should consider new species that can emulate the nutrient flow of native broadleaf species instead of exotic species that tend to impoverish soil nutrient stocks in tropical montane forests.     

Spatial and temporal patterns of carbon storage from 1992 to 2002 in forest ecosystems in Guangdong, Southern China

Aims

The overall goal of this study was to examine the spatial and temporal patterns of C storage from 1992 to 2002 in forest ecosystems in Guangdong, China.

Methods

We used 2237, 2103, and 1978 plot data from three continuous forest inventory in 1992, 1997, and 2002, respectively, four TM images and one soil survey data in Guangdong to examine the spatial and temporal patterns of C storage in forest ecosystems during 1992–2002. The uncertainty analysis of forest C storage in Guangdong in 1992 and 2002 was also conducted to provide the range of estimations.

Results

The forest coverage percent in Guangdong increased from 37.1 % in 1992 to 57.2 % in 2002 while the total forest C storage in Guangdong increased from 144.73?±?6.20 Tg in 1992 to 215.03?±?8.48 Tg in 2002. The order of average forest biomass C storage increase during 1992–2002 among the four regions is Western Guangdong (GW)?>?Eastern Guangdong (GE)?>?the Pearl River Delta (PRE)?>?Northern Guangdong (GN). The factors including tree species and altitude and slope aspect can explain 58–67 % variation of Veg C storage multiple regression model in Guangdong. However, the multiple regression model for SOC storage can only explain about 18–39 % variation of SOC storage in Guangdong. Total ecosystem C is mainly determined by SOC storage.

Conclusions

The total forest C storage in Guangdong increased about 49 % from 1992 to 2002 partially due to the increasing forest coverage percent over the period. The spatial distribution of forest C storage in Guangdong was uneven and this pattern reflects differences in forest management and economic and social development. Future forest management should focus on the selection of tree species, management of forest stand structures and implementation of sustainable practices so that Veg C sequestration potentials can be maximized.     

Converting paddy fields to Lei bamboo (Phyllostachys praecox) stands affected soil nutrient concentrations, labile organic carbon pools, and organic carbon chemical compositions

Background and aims

Land-use change often markedly alters soil carbon (C) and nitrogen (N) pool sizes with implications for climate change and soil sustainability. The objective of this research was to study the effect of converting paddy fields to Lei bamboo (Phyllostachys praecox) stands on soil C and N and other nutrient pools as well as the chemical structure of soil organic C (SOC) in the soil profile.

Methods

Soils (Anthrosols) from four adjacent paddy field–bamboo forest pairs with a known land-use history were sampled from Lin’an County, Zhejiang Province. Soil water soluble organic C (WSOC), hot water soluble organic C (HWSOC), microbial biomass C (MBC), readily oxidizable C (ROC), water soluble organic N (WSON), and other soil chemical and physical properties were determined. Soil organic C functional group compositions were determined by ¹³C-nuclear magnetic resonance (NMR).

Results

Concentrations of soil available P, available K, and different N forms increased (P?<?0.05) by the land-use conversion. Higher concentrations of SOC and total N (TN) were observed in the subsoil (20–40 and 40–60 cm soil layers) but not in the topsoil (0–20 cm layer) in the bamboo stands than in the paddy fields. The storage of SOC and TN in the entire soil profile (0–60 cm) increased by 56.7 and 70.7 %, respectively, after the land-use change. The increases in the SOC stock of the three soil layers were 11.0, 14.3, and 9.5 Mg C ha^?1, respectively. The conversion decreased WSOC concentrations in the subsoil but increased the ROC concentration in the topsoil. Solid-state NMR spectroscopy of soil samples showed that the conversion increased (P?<?0.05) the O-alkyl C content while decreased the aromatic C content, alkyl C to O-alkyl C ratio (A/O-A), and aromaticity of SOC.

Conclusions

Conversion of paddy fields to bamboo stands increased soil nutrient availability, and SOC and TN stocks. Effects of land-use change on C pools and C chemistry of SOC varied among different soil layers in the profile. The impact of the land-use conversion on soil organic C pools was not restricted to the topsoil, but changes in the subsoil were equally large and should be accounted for.     

Biomass allocation patterns in forests growing different climatic zones of China

Key message

Temperature generally explained variation in branch and leaf biomasses, whereas stem and root biomasses–temperature relationships restricted certain age stages may not hold at broader age ranges.

Abstract

In this study, biomass data for alpine temperate Larix forest, alpine Picea-Abies forest, temperate typical deciduous broadleaved forest, temperate Pinus tabulaeformis forest, temperate mixed coniferous-broadleaved forest, montane Populus-Betula deciduous forest, subtropical evergreen broadleaved forest, subtropical montane Cupressus and Sabina forest, subtropical Pinus massoniana forest and subtropical Cunninghamia lanceolata forest were used to examine the effect of temperature on biomass allocations between organs. The data of the ten forests were classified as ≤30, 31–60 and >60 years, to test whether biomass allocations of these age group forests vary systematically in their responses to temperature. With increasing mean annual temperature, branch and leaf biomasses significantly increased in ≤30, 31–60 and >60 years and all age groups; stem biomass significantly increased in ≤30-, 31–60- and >60-year groups, but no significant trend in all age groups; Root biomass significantly increased in 31–60, >60 years and all age groups, but had no response to mean annual temperature in the 30-year group, which suggest that root biomass allocation in response to temperature is dependent upon forest age. We conclude that temperature generally explained variation in branch and leaf biomasses, whereas stem and root biomasses–temperature relationships restricted certain age stages may not hold at broader age ranges.

     

Causes of variation in mineral soil C content and turnover in differently managed beech dominated forests

Background and aims

Forest soils are important carbon stores and considered as net CO₂ sinks over decadal to centennial time scales. Intensive forest management is thought to reduce the carbon sequestration potential of forest soils. Here we study the effects of decades of forest management (as unmanaged forest, forest under selection cutting, forest under age class management) on the turnover of mineral associated soil organic matter (MOM) in German beech (Fagus sylvatica L.) dominated forests.

Methods

Radiocarbon contents were determined by accelerator mass spectrometry (AMS) in 79 Ah horizon MOM fractions of Cambisols (n?=?13), Luvisols (n?=?51) and Stagnosols (n?=?15). Mean residence times (MRTs) for soil organic carbon (SOC) were estimated with a 2-pool model using the litter input derived from a forest inventory.

Results

MOM fractions from Ah horizons contained 64?±?8.8 % of the bulk SOC. The radiocarbon content of MOM fractions in Ah horizons, expressed as Δ¹⁴C, ranged between ?2.8?‰ and 114?‰ for the three soil groups. Almost all samples contained a detectable proportion of ‘bomb’ carbon fixed from the atmosphere since 1963. Under the assumption that depending on the soil texture between 19 % and 24 % of the SOC from the labile pool is transferred to the stable SOC pool, the corresponding MRTs ranged between 72 and 723 years, with a median of 164 years.

Conclusions

Our results indicate that the MOM fraction of Ah horizons from beech forests contained a high proportion of young carbon, but we did not find a significant decadal effect of forest management on the radiocarbon signature and related turnover times. Instead, both variables were controlled by clay contents and associated SOC concentrations (p?<?0.01). This underlines the importance of pedogenic properties for SOC turnover in the MOM fraction.     

Effect of conventional versus mechanized sugarcane cropping systems on soil organic carbon stocks and labile carbon pools in Mauritius as revealed by 13C natural abundance

Aims

Maintenance of adequate levels of soil organic carbon (SOC) is crucial for the biological, chemical and physical functioning of soils. This study was conducted (i) to determine the impact of long-term sugarcane monoculture on total SOC stocks and on its labile fractions and (ii) to quantify the loss of original SOC and the accretion of sugarcane-derived C following the adoption of new management practices namely de-rocking/land grading and mechanized harvesting.

Methods

Five study sites representing the five major soil groups under sugarcane in Mauritius were selected with a classical “paired-plot” design adopted. In this design, two sites with similar initial conditions were developed in different ways over time. One represents the reference soil (virgin land with predominantly C₃ type vegetation) and the other represents one of the following cropping treatments: (i) fields continuously cultivated with sugarcane for more than 25 or 50 years without de-rocking or land grading, (ii) fields under long-term sugarcane but having undergone de-rocking and land grading for mechanized harvesting in the last 3 years. Soil samples were taken to a depth of 50 cm and analysed for total organic C, labile C, ¹³C natural abundance, bulk density and stone content.

Results

Changes in SOC stock in the 0–50 cm profile following >50 years of cane cropping were not significant (P?>?0.05) compared to virgin land at any site. Soil δ¹³C values revealed that long-term sugarcane cultivation resulted in a depletion of original SOC by 34 to 70 %. However, this loss was fully compensated by C input from sugarcane residues at all sites studied resulting in no net change in SOC stock. Adoption of mechanized harvest did not have any detrimental effect on SOC stocks due to C inputs from crop residues. However, long-term sugarcane cultivation resulted in significant decline in a labile C (KMnO₄-oxidizable) fraction.

Conclusion

Despite the large losses of original C following conversion from forest to sugarcane, long-term sugarcane cultivation resulted in sequestration of sugarcane-derived C which adequately compensated these losses. Moreover, intensive de-rocking and land grading preceding mechanized harvesting did not have any detrimental effect on SOC stocks. However, the quality of sugarcane soils, as indicated by a decline in labile C, could be degraded.     

Changes in soil organic carbon and total nitrogen stocks after conversion of meadow to cropland in Northeast China

Aims

Grassland conversion to cropland (GCC) may result in loss of a large amount of soil organic carbon (SOC). However, the assessment of such loss of SOC still involves large uncertainty due to shallow sampling depth, soil bulk density estimation and spatial heterogeneity. Our objectives were to quantify changes in SOC, soil total nitrogen (STN) and C:N ratio in 0–100 cm soil profile after GCC and to clarify factors influencing the SOC change.

Methods

A nest-paired sampling design was used in six sites along a temperature gradient in Northeast China.

Results

SOC change after GCC ranged from ?17 to 0 Mg ha^?1 in 0–30 cm soil layer, recommended by IPCC, across the six sites, but ranged from ?30 to 7 Mg ha^?1 when considering 0–100 cm. We found a linear relationship between SOC change in 30–100 cm and that in 0–30 cm profile (ΔC_30?100?=?0.35ΔC_0?30, P?<?0.001), suggesting that SOC change in 0–100 cm was averagely 35 % higher than that in 0–30 cm. The change in STN showed a similar pattern to SOC, and soil C:N ratio did not change at most of sites. On the other hand, SOC loss after GCC was greater in soils with higher initial SOC content or in croplands without applying chemical fertilizers. Furthermore, SOC loss after GCC decreased with falling mean annual temperature (MAT), and even vanished in the coldest sites.

Conclusions

The magnitude of SOC loss following GCC in Northeast China is lower than the global average value, partly due to low MAT here. However, the current low SOC loss can be intensified by remarkable climate warming in this region.     

Effects of experimental warming and nitrogen fertilization on soil microbial communities and processes of two subalpine coniferous species in Eastern Tibetan Plateau,China

Aim

This study aimed at predicting how sub-alpine coniferous ecosystems respond to global changes in the Eastern Tibetan Plateau by understanding soil microbial communities and activities, as well as variation in the quality and quantity of soil organic matter.

Methods

An experiment was conducted to examine soil microbial communities and their related soil processes in rhizospheric soil of two coniferous species that were exposed to two levels of temperature (unwarmed and infrared heater warming) and two levels of nitrogen (unfertilized and 25 g N m^?2 a^?1) from April 2007.

Results

Four-year night warming alone slightly affected the phospholipid fatty acid contents of the microbial community. However, the combination of nitrogen addition and soil warming significantly affected soil microbial composition while reducing the biomass of major microbial groups and the activities of most enzymes, especially in Abies faxoniana plots. The combination of warming and nitrogen addition increased soil labile C and N pools in Picea asperata plots and was beneficial for soil recalcitrant C, as well as for labile and total C and N pools in A. faxoniana plots.

Conclusion

Results indicated that future warming will slightly affect soil microbial communities and their related soil processes. However, warming combined with high nitrogen deposition will significantly constrain soil microbial biomass and enzyme activities, consequently increasing soil C and N pools in sub-alpine coniferous forests of this region.     

Effect of wildfires on soil respiration in three typical Mediterranean forest ecosystems in Madrid, Spain

Background and Aims

Mediterranean forests are vulnerable to numerous threats including wildfires due to a combination of climatic factors and increased urbanization. In addition, increased temperatures and summer drought lead to increased risk of forest fires as a result of climate change. This may have important consequences for C dynamics and balance in these ecosystems. Soil respiration was measured over 2 successive years in Holm oak (Quercus ilex subsp. ballota; Qi); Pyrenean Oak (Quercus pyrenaica Willd; Qp); and Scots pine (Pinus sylvestris L.; Ps) forest stands located in the area surrounding Madrid (Spain), to assess the long term effects of wildfires on C efflux from the soil, soil properties, and the role of soil temperature and soil moisture in the variation of soil respiration.

Methods

Soil respiration, soil temperature, soil moisture, fine root mass, microbial biomass, biological and chemical soil parameters were compared between non burned (NB) and burned sites (B).

Results

The annual C losses through soil respiration from NB sites in Qi, Qp and Ps were 790, 1010, 1380 gCm^?2?yr^?1, respectively, with the B sites emitting 43 %, 22 % and 11 % less in Qi, Qp and Ps respectively. Soil microclimate changed with higher soil temperature and lower soil moisture in B sites after fire. Exchangeable cations and the pH also decreased. The total SOC stocks were not significantly altered, but 6–8 years after wildfires, there was still measurably lower fine root and microbial biomass, while SOC quality changed, indicated by lower the C/N ratio and the labile carbon and a relative increase in refractory SOC forms, which resulted in lower Q₁₀ values.

Conclusions

We found long term effects of wildfires on the physical, chemical and biological soil characteristics, which in turn affected soil respiration. The response of soil respiration to temperature was controlled by moisture and changed with ecosystem type, season, and between B and NB sites. Lower post-burn Q₁₀ integrated the loss of roots and microbial biomass, change in SOC quality and a decrease in soil moisture.     

Selenium fertilization strategies for bio-fortification of food: an agro-ecosystem approach

Background and aims

Although numerous studies have quantified the effects of land-use changes on soil organic carbon (SOC) stocks, few have examined simultaneously the weight of carbon (C) inputs vs. outputs in shaping these changes. We quantified the relative importance of soil C inputs and outputs in determining SOC changes following the conversion of natural ecosystems to pastures or tree plantations, and evaluated them in light of variations in biomass production, its quality (C:N) and above/belowground allocation patterns.

Methods

We sampled soils up to one-meter depth under native grasslands or forests and compared them to adjacent sites with pastures or plantations to estimate the proportion of new SOC (SOC_new) retained in the soil and the decomposition rates of old SOC (k _SOC-old ) based on δ ¹³C shifts. We also analyzed these changes in the particulate organic matter fraction (POM) and estimated above and belowground net primary production (ANPP and BNPP) from satellite images, as well as changes in vegetation and soil’s C:N ratios.

Results

The conversion of grasslands to tree plantations decreased total SOC contents while the conversion of forests to pastures increased SOC contents in the topsoil but decreased them in deep layers, maintaining similar soil stocks up to 1 m. Changes in POM were less important and occurred only in the topsoil after cultivating pastures, following SOC changes. Surprisingly, both land-use trajectories showed similar decomposition rates in the topsoil and therefore overall SOC changes were not correlated with C outputs (k _SOC-old ) but were significantly correlated with C inputs and their stabilization as SOC_new (similar results were obtained for the POM fraction). Pastures although decreased ANPP (as compared to forest) they increased belowground allocation and C:N ratios of their inputs to the soil, probably favoring the retention and stabilization of their new C inputs. In contrast, tree plantations increased ANPP but decreased BNPP (as compared to grasslands) and scarcely accumulated SOC_new probably as a result of the high C retention in standing biomass.

Conclusions

Our results suggest that SOC changes are mainly controlled by the quantity and quality of C inputs and their retention in the soil, rather than by C outputs in these perennial subtropical ecosystems.

     

Patterns of SOC and soil 13C and their relations to climatic factors and soil characteristics on the Qinghai–Tibetan Plateau

Background and aims

SOC inventory and soil δ¹³C were widely used to access the size of soil C pool and to indicate the dynamics of C input and output. The effects of climatic factors and soil physical characteristics and plant litter input on SOC inventory and soil δ¹³C were analyzed to better understand the dynamics of carbon cycling across ecosystems on the Qinghai-Tibetan Plateau.

Methods

Field investigation was carried out along the two transects with a total of 1,875 km in length and 200 km in width. Sixty-two soil profiles, distributed in forest, meadow, steppe, and cropland, were stratified sampled every 10 cm from 0 to 40 cm.

Results

Our result showed that SOC density in forest and meadows were much higher than in steppe and highland barley. In contrast, δ¹³C in forest and meadow were lower than in steppe and highland barley. Soil δ¹³C tended to enrich with increasing soil depth but SOC decline. SOC and δ¹³C (0–40 cm) were correlated with different climatic factors in different ecosystems, such that SOC correlated negatively with MAT in meadow and positively with MAP in steppe; δ¹³C correlated positively with MAT in meadow and steppe; and δ¹³C also tended to increase with increasing MAT in forest. Of the variation of SOC, 55.15 % was explained by MAP, pH and silt content and 4.63 % was explained by the interaction between MAT and pH across all the ecosystems except for the cropland. Meanwhile, SOC density explained 27.40 % of variation of soil δ¹³C.

Conclusions

It is suggested that different climatic factors controlled the size of the soil C pool in different ecosystems on the Tibetan Plateau. SOC density is a key contributor to the variation of soil δ¹³C.     

Environmental filtering of species with different functional traits into plant assemblages across a tropical coniferous-broadleaved forest ecotone

Background and aims

Ecotones between coniferous and broadleaved forests in tropical regions are poorly understood. Our aim was to understand community assembly across the ecotones by integrating changes in both plant functional traits and environmental factors.

Methods

The coniferous, ecotone, and broadleaved zones along each of the 15 investigation transects were discerned and surveyed. We measured eight functional traits of 174 woody species and 10 environmental factors along transects across the ecotones. We assessed between-site differences by using ANOVA, and correlations between functional traits and the environmental factors by RDA ordination.

Results

With the variation of vegetation zones from coniferous through the ecotone to broadleaved, the functional traits of plants at the community level changed in accordance with the changes in soil and light regimes. The low soil nutrients and low soil water in the coniferous zone were the major constraints for most lowland rain forest species with acquisitive traits, while high soil nutrients, high soil water and low light in the broadleaved zone had strong filtering effects on the conifer and tropical monsoon rainforest species with conservative traits.

Conclusions

The soil and light conditions were the major determinants for the functional community structure of the vegetation types across the tropical coniferous and broadleaved forest ecotone.     

Understory vegetation management affected greenhouse gas emissions and labile organic carbon pools in an intensively managed Chinese chestnut plantation

Background and aims

The impact of understory vegetation control or replacement with selected plant species, which are common forest plantation management practices, on soil C pool and greenhouse gas (GHG, including CO₂, CH₄ and N₂O) emissions are poorly understood. The objective of this paper was to investigate the effects of understory vegetation management on the dynamics of soil GHG emissions and labile C pools in an intensively managed Chinese chestnut (Castanea mollissima Blume) plantation in subtropical China.

Methods

A 12-month field experiment was conducted to study the dynamics of soil labile C pools and GHG emissions in a Chinese chestnut plantation under four different understory management practices: control (Control), understory removal (UR), replacement of understory vegetation with Medicago sativa L. (MS), and replacement with Lolium perenne L. (LP). Soil GHG emissions were determined using the static chamber/GC technique.

Results

Understory management did not change the seasonal pattern of soil GHG emissions; however, as compared with the Control, the UR treatment increased soil CO₂ and N₂O emissions and CH₄ uptake, and the MS and LP treatments increased CO₂ and N₂O emissions and reduced CH₄ uptake (P?<?0.05 for all treatment effects, same below). The total global warming potential (GWP) of GHG emissions in the Control, UR, MS, and LP treatments were 36.56, 39.40, 42.36, and 42.99 Mg CO₂ equivalent (CO₂-e) ha^?1 year^?1, respectively, with CO₂ emission accounting for more than 95 % of total GWP regardless of the understory management treatment. The MS and LP treatments increased soil organic C (SOC), total N (TN), soil water soluble organic C (WSOC) and microbial biomass C (MBC), while the UR treatment decreased SOC, TN and NO₃ ^?-N but had no effect on WSOC and MBC. Soil GHG emissions were correlated with soil temperature and WSOC across the treatments, but had no relationship with soil moisture content and MBC.

Conclusions

Although replacing competitive understory vegetation with legume or less competitive non-legume species increased soil GHG emissions and total GWP, such treatments also increased soil C and N pools and are therefore beneficial for increasing soil C storage, maintaining soil fertility, and enhancing the productivity of Chinese chestnut plantations.     

Differential soil organic carbon storage at forb- and grass-dominated plant communities, 33 years after tallgrass prairie restoration

Background and aims

Dominance of C₄ grasses has been proposed as a means of increasing soil organic carbon (SOC) sequestration in restored tallgrass prairies. However, this hypothesis has not been tested on long time scales and under realistic (e.g. N-limited) environmental conditions. We sampled a restoration in southern Illinois 33 years after establishment to determine the effects of varying plant communities on SOC sequestration in the top 50 cm of soil.

Methods

SOC, total nitrogen (TN), and the stable isotopic composition of SOC (δ¹³C) were used to calculate SOC sequestration rates, N storage, and the relative contributions of C₃ vs. C₄ plant communities as a function of soil depth.

Results

While both a forb-dominated and a mixed forb-grass plant community showed positive sequestration rates (0.56?±?0.13 and 0.27?±?0.10 Mg C ha^?1 yr^?1, respectively), a C₄ grass-dominated community showed SOC losses after 33 years of restoration (?0.31?±?0.08 Mg C ha^?1 yr^?1). Soil δ¹³C values were significantly more negative for forb-dominated plant communities, increasing the confidence that plant communities were stable over time and an important contributor to differences in SOC stocks among transects.

Conclusion

These results suggest that functional diversity may be necessary to sustain sequestration rates on the scale of decades, and that dominance of C₄ grasses, favored by frequent burning, may lead to SOC losses over time.     

Effects of irrigation on the soil CO2 efflux from different poplar clone plantations in arid northwest China

Aims

Soil respiration in forest plantations can be greatly affected by management practices such as irrigation. In northwest China, soil water is usually a limiting factor for the development of forest plantations. This study aims to examine the effects of irrigation intensity on soil respiration from three poplar clone plantations in this arid area.

Methods

The experiment included three poplar clones subjected to three irrigation intensities (without, low and high). Soil respiration was measured using a Li-6400-09 chamber during the growing season in 2007.

Results

Mean soil respiration rates were 2.92, 4.74 and 3.49 μmol m^?2 s^?1 for control, low and high irrigation treatments, respectively. Soil respiration decreased once soil water content was below a lower (14.8 %) or above an upper (26.2 %) threshold. When soil water content ranged from 14.8 % to 26.2 %, soil respiration increased and correlated with soil temperature. Fine root also played a role in the significant differences in soil CO₂ efflux among the three treatments. Furthermore, the three poplar hybrid clones responded differently to irrigation regarding fine root production and soil CO₂ efflux.

Conclusions

Irrigation intensity had a strong impact on soil respiration of the three poplar clone plantations, which was mainly because fine root biomass and microbial activities were greatly influenced by soil water conditions. Our results suggest that irrigation management is a main factor controlling soil carbon dynamics in forest plantation in arid regions.