Purpose

The objectives of the study were (1) to quantify the long-term effects of nitrogen-phosphorus fertilizer (NP) and a combination of nitrogen-phosphorus with organic manure (NPM) on total soil organic carbon (SOC) and total soil inorganic carbon (SIC), (2) to identify the changes of SOC and SIC in soil particle-size fractions, and (3) to investigate the relationship between SOC and SIC.

Materials and methods

Two long-term field experiments (sites A and B) were performed in 1984 (site A) and 1979 (site B) in the North China Plain. The soil samples were collected in 2006 and separated for clay, silt and sand size particle fractions and then determined for SOC and SIC.

Results and discussion

The long-term fertilization significantly increased SOC in 0–20 cm soil layer by 9–68% but significantly decreased or had no effect on SIC. In total, soil carbon storage was little affected by NP, but significantly increased by NPM application (p < 0.05). Fertilization affected both SOC and SIC in sand- and silt-sized particles but not in clay-size fraction. Both NP and NPM increased SOC in sand- and silt-sized particles by 8.7–123.9% in the 0–20 cm layer but decreased SIC up to 80.4% in the 40–60 cm layer. The SOC concentration in the particle-size fractions was negatively correlated with SIC concentration, which may imply an antagonistic interaction between organic and inorganic carbon levels.

Conclusions

These results illustrate the importance of soil inorganic carbon pool in evaluating soil total carbon pool in semi-arid farmlands. Previous assessments of the effects of fertilizers on the soil carbon pool, using only SOC determinations, require re-evaluation with the inclusion of SIC determinations.

Purpose

Chemical protection facilitates soil organic carbon (SOC) sequestration and stabilisation due to a strong chemical binding with mineral surfaces and metal ions (e.g. iron [Fe], aluminium [Al] and calcium [Ca]). However, there is not much information regarding the role of chemical protection in SOC stabilisation in paddy soils, particularly in terms of the specific forms of organo-mineral complexes such as Fe-, Al- and Ca-bonded OC.

Materials and methods

We sampled paddy soils at the 0–20 cm soil layer from a long-term field experiment (initiated in 1981) conducted under humid subtropical conditions in China, which has five fertilisation treatments (i.e. control treatment without fertiliser [CK], chemical fertiliser only [CF], green manure [GM], Straw and Manure) with equivalent nutrient inputs (i.e. N, P₂O₅ and K₂O at the rates of 135–67.5–135 kg ha^?1, respectively, for both early and late rice) except CK. We determined the chemical binding forms of SOC and the associated soil properties in the particulate fraction (PF, >53 μm) and the mineral-associated fraction (MAF, <53 μm), which were obtained using a low-energy ultrasonic dispersion procedure, of a paddy soil in the long-term fertilisation experiment.

Results and discussion

Iron- and Al-bonded OC (Fe/Al-OC) was the dominant fraction and made up 55–70% of the total SOC in the paddy soil, while Ca-bonded OC (Ca-OC) was only a minor fraction (<4%). The Fe/Al-OC was mainly allocated in the MAF (52–67%), indicating that the chemical protection of SOC occurred mostly in the finer particle fractions. Long-term application of organic amendments increased the contents of bulk SOC by 27–34% (P < 0.05), of Fe/Al-OC by 9–16% and of Ca-OC by 35–83% (P < 0.05), whereas the sole application of chemical fertiliser had no significant effects on SOC contents of the paddy soil compared with the treatment without fertiliser inputs. Both amorphous Fe and Al extracted by ammonium oxalate (Fe_ox and Al_ox) showed significant correlations with Fe/Al-OC (r = 0.52 and 0.78, respectively), but Al_ox appeared to have a greater influence on C stabilisation in the paddy soil.

Conclusions

These results demonstrated that the dominant chemical binding forms of SOC in the paddy soils were Fe/Al-OC and amorphous Fe/Al oxyhydrates, especially amorphous Al, contributed mostly to the chemical stabilisation of SOC.

Purpose

Soils of tidal marshes play an important role in regional carbon (C) cycles as they are able to store considerable amounts of organic carbon (OC). However, the C dynamics of marsh soils of the Elbe estuary have not been investigated so far. Therefore, the aim of this study was to identify the sources and distribution of soil organic carbon (SOC) and the factors influencing the SOC pools of tidal marshes of the study region.

Materials and methods

In this study, SOC pools were determined in different salinity zones and elevation classes of the estuarine marshes. The amount of initial allochthonous OC was derived from the OC content in fresh sediments. The difference to the recent OC content in the soils was interpreted as autochthonous accumulation or mineralization by microorganisms.

Results and discussion

Young, low marshes of the study sites seem to be predominantly influenced by allochthonous OC deposition whereas the older, high marshes show autochthonous OC accumulation in the topsoils (0–30 cm) and mineralization in the subsoils (30–70 cm). SOC pools of the whole profile depth (0–100 cm) did not significantly differ between elevation classes, but decreased significantly with increasing salinity from 28.3 kg m^?2 in the most upstream site of the oligohaline zone to 9.7 kg m^?2 in the most downstream site of the polyhaline zone. Even though the areal extent of the investigated salinity zones was similar, the SOC mass within 100 cm soil depth decreased from 0.62 Tg (1 Tg = 10¹² g) in the oligohaline zone to 0.18 Tg in the polyhaline zone.

Conclusions

Elevation was found to be one factor influencing the SOC pools of tidal marshes. However, salinity seems to be an even stronger influencing factor reducing the above-ground biomass and, accordingly, the autochthonous OC input as well as the allochthonous input by enhanced mineralization of OC along the course of the estuary. An upstream shift of the salinity zones by sea level rise could, therefore, lead to a reduction of the SOC storage of the estuarine marshes.

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.

Purpose

Biochar application is deemed to modify soil properties, but current research has been mostly conducted on the degraded land in tropical regions. Using six consecutive years of biochar field trial, we investigated effects of biochar on soil aggregates, structural stability, and soil organic carbon (SOC) and black C (BC) concentrations in aggregate fractions. The findings have important implications in managing soil structure and SOC sequestration in high fertility soils of the temperate areas.

Materials and methods

The study had four treatments: control; biochar rate at 4.5 (B4.5) and biochar rate at 9.0 t ha^?1 year^?1 (B9.0); and straw return (SR). Soil samples were collected from 0–10-cm layer, and aggregate size distribution was determined with the wet-sieving method. Then, the mean weight diameter (MWD) of aggregates and the aggregate ratio (AR), i.e., the ratio of the >250 μm to the 53–250 μm size were calculated to assess the structural stability. Total SOC and BC concentrations in bulk soil (<2 mm) and separated fractions (i.e., >2000, 250–2000, 53–250, and <53 μm) were measured.

Results and discussion

The B4.5 and B9.0 significantly increased macroaggregate (250–2000 μm) and MWD and AR indices relative to the control. Comparing to the SR, the improvements in soil aggregation under biochar treatments were limited. Additionally, more SOC in larger fractions (>2000, 250–2000, and 53–250 μm) and BC in extracted fractions under biochar soils were observed. These results implied that biochar addition enhanced both native SOC and BC physical protection by aggregation.

Conclusions

Biochar application is effective in mediating soil aggregation, and thus improves both native SOC and BC stabilization in an intensive cropping system of North China.

Purpose

It has been widely recognized that land use changes can cause significant alterations of soil organic matter (SOM) of various ecosystems. Forest conversion, a common land use change, and its effects on SOM have been a hot research topic during the past two decades. However, the mechanisms of the effects of forest conversion on SOM dynamics, particularly in deep soils, largely remain uncertain. This study aimed to examine the impacts of forest conversion on SOM stabilization through the analysis of soil aggregate and density fractionation, microbial composition, and functions in deep soils.

Materials and methods

Soil C and microbes were sampled in soil layers of 0–20 and 60–80 cm under broadleaved secondary forest and two coniferous plantations (Cunninghamia lanceolata and Pinus massoniana). Aggregate and density fractionation techniques were used to analyze C accumulation in non-protected, physically, chemically, and biochemically protected C fractions. A 90-day laboratory mineralization incubation experiment with and without 400-mg C kg^?1 soil glucose and phenol was conducted to determine the potential mineralizable C, utilization of substrate capacity, and metabolic quotient (qCO₂).

Results and discussion

Conversion of secondary forests into coniferous plantations significantly decreased bulk soil C, especially in the deep soils. Forest conversion significantly decreased non-protected, physically, and chemically protected C fractions in both topsoil and deep soil and biochemically protected C fraction in deep soils. The soil organic carbon (SOC) of topsoils was dominated by non-protected fraction while in deep soil which was dominated by protected fraction. Compared with the topsoils, soil microbes in the deep soils tend to preferentially use labile soil organic matter with lower substrate use efficiency (higher values of qCO₂), which indicates that a r-strategy dominates of microbes. The increased respiration rate in the deep soils caused by forest conversion, when normalized to soil C, indicates that deep SOM may be more prone to decomposition and destabilization than top SOM.

Conclusions

Forest conversion can cause a significant alteration of SOC stabilization through the changes of physically, chemically, and biochemically protected SOC fractions. The mechanisms for the changes in non-protected or/and protected SOC fractions may be associated with the redistribution of r-strategy- and K-strategy-dominated microbes due to changes in litter inputs and priming effects.

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.

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.

Purpose

Phosphorus (P) in soil particulate fraction (PF; >53 μm) is suggested to have a significant importance in soil P cycling. However, the effects of continuous fertilization on P-PF and its association with soil organic carbon (SOC) in paddy soils have not been well studied.

Materials and methods

We sampled paddy soils at 0–20 cm from a long-term field experiment (initiated in 1981) conducted under humid subtropical conditions in China, which has five fertilization treatments with equivalent P input (135 kg P₂O₅?ha^?1 year^?1) except the control treatment (CK). Changes in total P (Pt), inorganic P (Pi), organic P (Po), and SOC under different fertilization managements were evaluated in the whole soil, in the PF, and in the mineral-associated fraction (MAF; <53 μm).

Results and discussion

Continuous fertilization increased the contents of SOC and P in all soil fractions. Both Po and organic carbon in PF were the most sensitive variables to fertilization, indicating that they constitute a useful tool to detect the effects of management practices. Among the fertilization treatments, organic amendments significantly increased Po-PF contents more than chemical fertilizer applied only (p?<?0.05), although they had equivalent P input. The paddy soil without fertilization showed a more significant decrease in Pi compared with Po. The SOC/Po ratios were significantly lower in fertilization treatments (especially those with manure or straw incorporation) than in CK and decreased from PF to MAF. A significant relationship was found between Po-PF contents and rice P uptake during the growing season.

Conclusions

These results demonstrate that Po-PF may also play a significant role in P cycling of paddy soil, and thus, it would be better to consider Po-PF in soil diagnosis to promote P management of paddy soil, especially for that under long-term organic amendments.

Purpose

Biochemical protection is an important mechanism for maintaining the long-term stability of the soil carbon (C) pool. The labile and recalcitrant pools of soil organic matter (SOM) play different roles in regulating C and N dynamics; however, few studies have characterized the capacity of soil C sequestration while considering the biochemical quality of SOM. The aim of the present study was to assess the changes in the soil organic carbon (SOC) and nitrogen (N) pools during a traditional rotation period (25 years) of a Chinese fir (Cunninghamia lanceolata) plantation with an emphasis on SOM biochemical quality.

Materials and methods

Three different forest stand development stages—young (6 years old), middle-aged (16 years old) and mature (25 years old)—were selected for soil sampling to a depth of 100 cm. Total C and total N of the soil was analysed to determine the changes in the SOC and N stocks among the three development stages using an equivalent soil mass (ESM) approach. Bulk soils were fractionated into labile and recalcitrant fractions using the acid hydrolysis method to identify the quality of SOM.

Results and discussion

The mineral soil organic carbon pool at a 1-m depth slightly decreased from the young stand to the middle-aged stand and rapidly increased by 28 % to reach a maximum in the mature stand. SOC accumulation in the surface soil predominated the changes in total SOC stocks in all three stands. The increased N was reflected in the entire depth, and the highest soil N accumulation was in the mature stand. The recalcitrant C concentration and SOC were positively correlated. The non-hydrolysable C proportion was lower in the middle-aged stand versus the young stand (8.69 % loss), while the labile C percentage was higher (13.89 % gain). In the mature stand, the recalcitrant C index increased to 39.84 %. The recalcitrant index of C decreased with an increasing soil depth, whereas the recalcitrant index of N dramatically increased.

Conclusions

These results highlighted the significant effect of the stand age and the soil depth on the storage and biochemical availability of SOM in Chinese fir plantations of southern China. The recalcitrant index of C changed with the change in SOC concentration, indicating that biochemical protection mechanism plays an important role in soil C sequestration. In addition, more attention should be paid to subsoil C protection in the management of Chinese fir plantations because of low biochemical stability.

Purpose

Submerged rice cultivation has been practiced in China for 7000 years. Empirical evidence on changes of soil organic carbon (SOC) contents in paddy soils over this historical time period is scarce. Therefore, a field study was conducted to investigate the effect of submerged rice cultivation on the accumulation and preservation of SOC in paddies.

Materials and methods

Two buried ancient paddy profiles (6280 years BP, named P-01 and P-03) in the Yangtze Delta of eastern China were excavated to illustrate the development of SOC contents in soils during the evolution of paddies under anthropogenic land use and environmental changes from the prehistoric period to the present time. Trends in SOC concentrations, total nitrogen concentrations, and stable carbon isotope ratio were identified for different points in time.

Results and discussion

Accumulation of organic carbon was found in the paddy soil layers of P-01 at 100–174 cm depth. This site was taken under submerged rice cultivation in about 6280 years BP. The average SOC concentration in the prehistoric paddy topsoil in 100–130 cm depth was 1.27 %, which is seven times higher than that in the adjacent uncultivated land at 103–130 cm depth of P-03. This implies that the paddy soil has experienced substantial CO₂ sequestration under submerged management during that time. By about 3320 years BP, organic carbon contents were halved, potentially due to marine inundation by sea level rise. Up to the year 2003, the SOC contents in all horizons in the present time paddy soil have increased, especially in the surface layer, indicative of continuous rice cultivation. However, due to rapid urbanization and industrialization, the cultivation of paddies in eastern China has gradually been discontinued leading to the loss of SOC stocks of approximately 10 % in a 6-year interval (from 2003 to 2009). A significant relationship between SOC and rice phytolith contents was found in the paddy soil horizons of P-01 (r?=?0.71, p?<?0.01) and P-03 (r?=?0.72, p?<?0.01), suggesting that phytolith-occluded organic carbon could be used as a biomarker to ascertain the development of SOC in the evolution of rice paddies over the past 6000 years.

Conclusions

Submerged rice cultivation led to a noticeable accumulation of SOC in paddies. Phytolith-occluded organic carbon could be used as a biomarker to monitor changes of OC contents in paddy soils.

Purpose

Application of functional organisms in soil organic amendments has the potential to accelerate organic matter decomposition and stimulate C cycling. In this study, a short-term (a year) field experiment was conducted to investigate the collaborative effects of earthworms and phosphate-solubilizing bacteria on C accumulation in pig manure-amended soil.

Materials and methods

A field experiment was conducted with six treatments established. The first three treatments, including control (CK), pig manure (Pm), and pig manure?+?slurry (Pm?+?S), were set up to evaluate the influences of pig manure on soil C accumulation. The other three treatments, including manure?+?slurry?+?earthworms (Te), manure?+?slurry?+?phosphate-solubilizing bacteria (Tb), and manure?+?slurry?+?earthworms?+?bacteria (T(e?+?b)), were set up to investigate the collaborative effects of functional organisms on soil C cycling. The Pm?+?S treatment was chosen as the control (T) for this purpose.

Results and discussion

The results showed that the soil C pools did not increase significantly under the manure treatment. In contrast, an integrated application of manure, slurry, earthworms, and bacteria significantly increased the various C fractions, such as SOC and humin, indicating a rapid and positive effect of earthworms and bacteria on C accumulation. Besides, C sequestration by the integrated application was as high as 1.35 Mg C ha^?1 soil, half of which was stabilized.

Conclusions

The T(e?+?b) was an efficient strategy to sequestrate and stabilize SOC in arid hillside soils. The bacteria increased the labile OC, especially microbial biomass C, while the earthworms were apparently essential for the increase in stable OC.

Purpose

Soil organic carbon (SOC) and its labile fractions are strong determinants of physical, chemical and biological properties. The objective of the present work was to evaluate the effects of organic amendments (technosol made of wastes and biochar) and Brassica juncea L. on the soil C fractions in a reclaimed mine soil.

Materials and methods

The studied soil was from a former copper mine that was subsequently partially reclaimed with vegetation and wastes. A greenhouse experiment was carried out to amend the mine soil with different proportions of technosol and biochar mixture and planting B. juncea. B. juncea plants can tolerate high levels of metals and can produce a large amount of biomass in relatively short periods of time.

Results and discussion

The results showed that with the addition of biochar and wastes, soil pH increased from 2.7 to 6.18, SOC from undetectable to 105 g kg^?1 and soil total nitrogen (TN) from undetectable to 11.4 g kg^?1. Amending with wastes and biochar also increased dissolved organic carbon (DOC) from undetectable to 5.82 g kg^?1, carbon in the free organic matter (FOM) from undetectable to 30.42 g kg^?1, FAP (carbon in fulvic acids removed with phosphoric acid) from undetectable to 24.14 g kg^?1 and also increased the humification ratio, the humification index, the polymerisation rate and the organic carbon in the humified fractions (humic acids, fulvic acids and humin). Soils amended and vegetated with B. juncea showed lower FOM values and higher humification index values than the soils amended only with biochar and wastes.

Conclusions

This study concludes that the combined addition of wastes and biochar has a greater potential for both increasing and improving organic carbon fractions in mine soils. The authors recommend the application of biochar and technosol made of wastes as a soil amendment combined with B. juncea on soils that are deficient in organic matter, since they increased all of the SOC fractions in the studied copper mine soil.

Purpose

Plantation is an important strategy for forest restoration and carbon (C) storage. Plantations with different tree species could significantly affect soil properties, including soil pH, soil nutrient content, soil microbial activities, and soil dissolved organic C. Changes in these abiotic and biotic factors could regulate mineralization of soil organic C (SOC). However, it remains unclear to what extent these factors affect the mineralization of SOC under different tree species plantations.

Materials and methods

Soil was collected at 0–10 cm depth from plantations with Pinus elliottii Engelm. var. elliottii, Araucaria cunninghamii, and Agathis australis, respectively, in southeast Queensland, Australia. Soil samples were assayed for soil organic C; organic N and mineralization of SOC; soil particle size; total C, N, and P; and pH. In addition, a 42-day laboratory incubation with substrate additions was done to examine the influence of different substrates and their combinations on bio-available organic C.

Results and discussion

Our results suggested that SOC mineralization was mainly determined by soil pH and soil C content among plantations with different tree species, whereas SOC mineralization was not correlated with soil N and P contents. These results were further confirmed by the substrate addition experiments. SOC mineralization of soils from slash pine showed greater response to C (glucose) addition than soils from other two plantations, which suggested significant differences in SOC mineralization among plantations with different tree species. However, neither N addition nor P addition had significant effects on SOC mineralization.

Conclusions

Our results indicated that plantations with different tree species substantially affect the mineralization and stability of soil organic C pool mainly by soil pH and soil C content.

Purpose

Soil organic carbon (SOC) stock is one of the most important carbon reservoirs on the earth and plays a vital role in the global climate change. However, research on the carbon sequestration and storage of coastal wetland soil is very scarce. Therefore, a study in the coastal wetland was conducted to investigate the SOC distribution, storage, and variation under the influence of human activities.

Materials and methods

Surface soil samples in different seasons and profile soil samples were collected in the Changyi coastal wetland. SOC content, soil physicochemical properties, and sedimentation rate were determined. Organic carbon storage and burial flux were calculated. On the basis of correlation analysis and comparative study, factors affecting the distribution and storage of SOC were investigated.

Results and discussion

The average SOC content of the surface soil in June and November was 4.65 and 6.13 g kg^?1, respectively. The distribution of surface SOC content was consistent with the distribution of vegetation and was affected by the soil particle size. In plant-covered area, the relationship between SOC content and depth could be expressed by the power function y?=?ax ^b . The contribution of plants to SOC was only significant in the shallow layer. As for the deep layer, the SOC content was higher in the mudflat. The organic carbon storage in the upper 1 m soil was estimated at 1.795 kg m^?2 in average and the total organic carbon storage of Changyi wetland was estimated at 6.373?×?10⁷ kg. The sedimentation rate was very low and the average organic carbon burial flux of the whole wetland was 17.5 g m^?2 a^?1.

Conclusions

Low sedimentation rate, weak downward migration, and high decomposition rate of organic matter caused by poor hydrological condition were the reasons why the SOC storage in Changyi wetland was low. Under intensive human activities, the Changyi wetland was drying and the organic carbon storage was reducing. Strategies were proposed to be taken urgently to restore the wetland for the long-term benefit.

Purpose

The size of soil particles strongly affects the accumulation and adsorption of heavy metals which partly controls the co-transport of heavy metals by soil colloids. However, the effect of the size of soil particles on the accumulation and adsorption of heavy metals in the colloidal dimension has seldom been studied. In this study, variable charge soils were selected and separated into five size fractions to elucidate the effect of the size of soil particles on Cd accumulation and adsorption.

Materials and methods

Five soil particle size fractions (>10, 10–1, 1–0.45, 0.45–0.2 and <0.2 μm) were obtained from Cd-contaminated soil by natural sedimentation and fractional centrifugation. The concentrations and species of Cd were measured in various sized soil particles. Batch adsorption experiments of Cd on the obtained soil particles were conducted under different pH values and concentrations of NaCl.

Results and discussion

Generally, the concentration of Cd increased with decreasing soil particle sizes, and the Cd proportion of exchangeable and carbonate fraction decreased from 43.84 to 17.75% with decreasing particle size. The soil particles with a size of 10–1 and <0.2 μm possessed a stronger adsorption ability than the other fractions in most cases. Moreover, the Cd adsorption capacities of the soil particles increased with increasing pH values and decreasing concentrations of NaCl, especially for soil particles containing more organic matter (OM) and variable charge minerals.

Conclusions

Smaller soil particles are more capable of accumulating Cd and make Cd more stable. The adsorption capability of Cd is negatively related to the particle size and NaCl concentration and is positively related to the pH. The effects of the size of variable charge soil particles on Cd accumulation and adsorption are attributed to the differences in the physicochemical properties among various soil particle size fractions. This study contributes to the understanding of the co-transport of heavy metals in soil by soil colloids.

Purpose

This study aims to explore the dynamics of the factors influencing soil organic carbon (SOC) sequestration and stability at erosion and deposition sites.

Materials and methods

Thermal properties and dissolved aromatic carbon concentration along with Al, Fe concentration and soil specific surface area (SSA) were studied to 1 meter depth at two contrasting sites.

Results and discussion

Fe, Al concentrations and SSA size increased with depth and were negatively correlated with SOC concentration at the erosion site (P?<?0.05), while at the deposition site, these values decreased with increasing depth and were positively correlated with SOC concentration (P?<?0.05). TG mass loss showed that SOC components in the two contrasting sites were similar, but the soils in deposition site contained a larger proportion of labile organic carbon and smaller quantities of stable organic carbon compared to the erosion site. SOC stability increased with soil depth at the erosion site. However, it was slightly variable in the depositional zone. Changes in SUVA₂₅₄ spectroscopy values indicated that aromatic moieties of DOC at the erosion site were more concentrated in the superficial soil layer (0–20 cm), but at the deposition site they changed little with depth and the SUVA₂₅₄ values less than those at the erosion site.

Conclusions

Though large amounts of SOC accumulated in the deposition site, SOC may be vulnerable to severe losses if environmental conditions become more favorable for mineralization in the future due to accretion of more labile carbon. Deep soil layers at the erosion site (>30 cm deep) had a large carbon sink potential.

Purpose

Under a global warming scenario, understanding the response of soil organic carbon fractions and aggregate stability to temperature increases is important not only for better understanding and maintaining relevant ecosystem services like soil fertility and crop productivity, but also for understanding key environmental processes intimately related with the maintenance of other regulatory ecosystem services like global climate change mitigation through carbon sequestration. An increase in temperature would accelerate the mineralization of soil organic carbon. However, the properties of organic carbon remained in soil after mineralization is not well known.

Materials and methods

Mollisol was collected at 0–20-cm depth from maize (Zea mays L.) field in Northeast China. A 180-day incubation experiment was conducted at three different temperatures (10, 30, and 50 °C) under constant soil moisture (60 % water holding capacity). Soil samples were assayed for total organic carbon (TOC), water-soluble organic carbon (WSOC), easily oxidizable organic carbon (EOC), humic fractions carbon, aggregate-associated carbon, and water stability of aggregates. Elemental analysis and solid-state ¹³C nuclear magnetic resonance spectroscopy were used to characterize humic acid and humin fractions.

Results and discussion

The contents of soil TOC, EOC, humic fractions carbon, and aggregate-associated carbon decreased with the increase in temperature. The proportion of 2–0.25-mm macroaggregate and the mean weight diameter (MWD) of aggregates also decreased. The C, H, N, S, alkyl C, and O-alkyl C contents of humic acid and humin decreased, whereas the O, aromatic C, and carbonyl C contents increased. The H/C, aliphatic C/aromatic C, and O-alkyl C/aromatic C ratios in humic acid and humin fractions decreased.

Conclusions

The increase in temperature has a negative impact on soil organic carbon content, soil aggregation, and aggregate stability. Moreover, humic acid and humin molecules become less aliphatic and more decomposed with the increase in temperature.

Purpose

The validity of soil erosion data is often questioned because of the variation between replicates. This paper aims to evaluate the relevance of interreplicate variability to soil and soil organic carbon (SOC) erosion over prolonged rainfall.

Materials and methods

Two silty loams were subjected to simulated rainfall of 30 mm h^?1 for 360 min. The entire rainfall event was repeated ten times to enable statistical analysis of the variability of the runoff and soil erosion rates.

Results and discussion

The results show that, as selective removal of depositional particles and crust formation progressively stabilized the soil surface, the interreplicate variability of runoff and soil erosion rates declined considerably over rainfall time. Yet, even after the maximum runoff and erosion rates were reached, the interreplicate variability still remained between 15 and 39 %, indicating the existence of significant inherent variability in soil erosion experiments.

Conclusions

Great caution must be paid when applying soil and SOC erosion data after averaging from a small number of replicates. While not readily applicable to other soil types or rainfall conditions, the great interreplicate variability observed in this study suggests that a large number of replicates is highly recommended to ensure the validity of average values, especially when extrapolating them to assess soil and SOC erosion risk in the field.

Purpose

The impacts of land-use change on dynamics of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in the subsoil (>?30 cm) are poorly understood. This study aims to investigate whether the effects of land-use change on stocks and stoichiometric ratios (R_CN, R_CP, and R_NP) of SOC, TN, and TP can be different between topsoil (0–30 cm) and subsoil (30–60 cm) in the Ili River Valley, northwest China.

Materials and methods

Soil samples (0–10, 10–20, 20–30, 30–40, 40–50, and 50–60 cm) were collected from a pasture (PT), a 27-year-old cropland (CL) converted from PT, and a 13-year-old poplar (Populus tomentosa Carr.) plantation (PP) converted from CL. SOC, TN, and TP concentrations and soil bulk density were determined to calculate stocks and stoichiometric ratios (molar ratios) of SOC, TN, and TP.

Results and discussion

Conversion from PT to CL led to substantial losses in SOC, TN, and TP pools in both topsoil and subsoil, and the reduction rates in subsoil (13.8–24.7%) were higher than those in topsoil (8.5–17.3%), indicating that C, N, and P pools in subsoil could also be depleted by cultivation. Similar to topsoil, significant increases in SOC, TN, and TP stocks were detected after afforestation on CL in subsoil, although the increase rates (31.2–56.2%) were lower than those in topsoil (47.8–69.1%). Soil pH and electrical conductivity (EC), which generally increased after conversion from PT to CL while decreased after CL afforestation, showed significant negative correlations with SOC, TN, and TP, suggesting that cultivation might lead to soil degradation, whereas afforestation contributed to soil restoration in this area. Significant changes in C:N:P ratios in topsoil were only detected for R_NP after conversion from CL to PP. By contrast, land-use change significantly altered both R_CN and R_NP in the subsoil, demonstrating that the impacts of land-use change on R_CN and R_NP were different between topsoil and subsoil. The significant relationship between soil EC and R_NP suggested that R_NP might be a useful indicator of soil salinization.

Conclusions

Stocks of SOC, TN, and TP as well as R_CN and R_NP in subsoil showed different responses to land-use change compared to those in topsoil in this typical agro-pastoral region. Therefore, it is suggested that the effects of land-use change on dynamics of SOC, TN, and TP in subsoil should also be evaluated to better understand the role of land-use change in global biogeochemical cycles.