Contrast effect of long-term fertilization on SOC and SIC stocks and distribution in different soil particle-size fractions

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.

     

Changes in organic carbon content and its physical and chemical distribution in paddy soils cultivated under different fertilisation practices

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.

     

Plant–soil interaction affects the mineralization of soil organic carbon: evidence from 73-year-old plantations with three coniferous tree species in subtropical Australia

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.

     

Land use affects soil organic carbon of paddy soils: empirical evidence from 6280 years BP to present

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.

     

Changes in the pH,EC, available P,SOC and TN stocks in a single rice paddy after long-term application of inorganic fertilizers and organic matters in a cold temperate region of Japan

Purpose

The objective of this study was to determine the changes in the main soil chemical properties including pH, electrical conductivity (EC), available phosphorus (P), soil organic carbon (SOC) and total nitrogen (TN) stocks after long-term (31 years) additions of two types of organic matters—rice straw and rice straw compost, combined with NPK fertilizers in single rice paddy in a cold temperate region of Japan.

Materials and methods

A long-term experiment on combined inorganic fertilizers and organic matters in paddy rice cultivation began in May 1982 in Yamagata, northeastern Japan. After the 31st harvest, soil samples were collected from five treatments [(1) PK, (2) NPK, (3) NPK + 6 Mg ha^?1 rice straw (RS), (4) NPK + 10 Mg ha^?1 rice straw compost (CM1), and (5) NPK + 30 Mg ha^?1 rice straw compost (CM3)] at five soil depths (0–5, 5–10, 10–15, 15–20, and 20–25 cm). Soil chemical properties of pH, EC, available P, SOC, and TN were analyzed.

Results and discussion

The pH decreased significantly only at the higher compost rate of 30 Mg ha^?1, while EC increased in all the organic matter treatments. Available P significantly increased in the CM1 and CM3 treatments by 55.1 and 86.4 %. The amounts of SOC stock increased by 67.2, 21.4, and 8.6 %, and soil TN stock by 64.1, 20.2, and 8.5 % in CM3, RS, and CM1, respectively, compared to NPK treatment.

Conclusions

Significant changes in soil properties were observed after 31 years of organic matter applications with reference to PK- and NPK-fertilized rice paddy soils. A significant decrease in pH was observed with the application of a high rate (30 Mg ha^?1) of rice straw compost but not with the conventional rate of 10 Mg ha^?1. However, EC increased significantly relative to that of the PK- and NPK-fertilized plots in all the organic matter treatments. Available P significantly increased in the CM1 and CM3 treatments by 55.1 and 86.4 %. The amounts of SOC stock expressed as a percentage of total C applied to the soil were higher from 10 Mg ha^?1 compost (28.7 %) than that from 6 Mg ha^?1 rice straw (17.4 %), indicating a more effective soil organic C accumulation from rice straw compost than that from original rice straw.

     

Improved nutrient status affects soil microbial biomass,respiration, and functional diversity in a Lei bamboo plantation under intensive management

Purpose

Intensive management, such as fertilization and organic mulching, is applied frequently in Lei bamboo (Phyllostachys praecox) plantations to achieve higher production in subtropical China. However, responses as well as key impact factors of soil microbial properties under such management remain uncertain. We analyzed the relationships between nutrient changes and microbial properties and assessed the main factors determining microbial biomass, activity, and functional diversity in soils under intensive management in a Lei bamboo plantation.

Materials and methods

Soil samples of treatments of no fertilization (control), chemical fertilization (CF), and chemical and organic fertilization combined with organic mulching (CFOM + M) were taken before mulching. The soil organic carbon (SOC), dissolved organic carbon, and total and available nitrogen (N), phosphorus (P), and potassium (K) were measured. Microbial biomass carbon (MBC), basal respiration, and mineralization were also analyzed. Community level of physiological profile (CLPP) of microorganisms was analyzed by BIOLOG method to estimate the functional diversity and carbon (C) source utilization patterns of microbes. Principal component analysis (PCA), principal response curve (PRC), correlation analysis, regression analysis, and redundancy analysis (RDA) were performed to clarify changes in variables and determine the factors influencing microbial properties.

Results and discussion

SOC and total and available N, P, and K increased as follows: CFOM + M > CF > control. However, C/P and N/P ratios showed an opposite trend. MBC and respiration were not affected, but microbial quotient and metabolic quotient declined under intensive management. McIntosh diversity index was much higher in CFOM + M. The PCA showed that microorganisms in CFOM + M had a stronger ability to use most C sources. Weaker utilization of serine indicated an alleviation of nutrient deficiency in CFOM + M. PRC of CLPP showed a significant treatment effect and that utilization of serine sensitively responded to nutrient status over the whole incubation time. RDA showed that total and available N, total K, and C/P were the main factors influencing utilization of C sources by microbial communities.

Conclusions

Fertilization combined with organic mulching increased soil nutrients, microbial biomass, and respiration in a Lei bamboo plantation. Abundant nutrients also increased C source use efficiency of microorganisms under intensive management. Changes of N and K and C/P might have led to a shift in microorganisms toward a different life strategy and determined the change in C source utilization patterns of microbial communities.

     

Inherent interreplicate variability during small-scale rainfall simulations

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.

     

Stability of soil organic carbon and potential carbon sequestration at eroding and deposition sites

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.

     

Decomposition of soil organic carbon influenced by soil temperature and moisture in Andisol and Inceptisol paddy soils in a cold temperate region of Japan

Purpose

Understanding the effects of temperature and moisture on soil organic carbon (SOC) dynamics is crucial to predict the cycling of C in terrestrial ecosystems under a changing climate. For single rice cropping system, there are two contrasting phases of SOC decomposition in rice paddy soils: mineralization under aerobic conditions during the off-rice season and fermentation under anaerobic conditions during the growth season. This study aimed to investigate the effects of soil temperature and moisture on SOC decomposition under the aerobic and subsequently anaerobic conditions.

Materials and methods

Two Japanese paddy soils (Andisol and Inceptisol) were firstly incubated under four temperatures (±5, 5, 15, and 25°C) and two moisture levels (60 and 100% water-filled pore space (WFPS)) under aerobic conditions for 24 weeks. Then, these samples were incubated for 4 weeks at 30°C and under anaerobic conditions. Carbon dioxide (CO₂) and methane (CH₄) productions were measured during the two incubation stages to monitor the SOC decomposition dynamics. The temperature sensitivity of SOC was estimated by calculation of the Q₁₀ parameter.

Results and discussion

The total CO₂ production after the 24-week aerobic incubation was significantly higher in both soils for increasing soil temperature and moisture (P < 0.01). During the subsequent anaerobic incubation, total decomposed C (sum of CO₂ and CH₄ productions) was significantly lower in samples that had been aerobically incubated at higher temperatures (15 and 25°C). Moreover, CH₄ production was extremely low in all soil samples. Total decomposed C after the two incubation stages ranged from 256.8 to 1146.1 mg C kg^?1 in the Andisol and from 301.3 to 668.8 mg C kg^?1 in the Inceptisol. However, the ratios of total decomposed C to SOC ranged from 0.29 to 1.29% in the Andisol and from 2.21 to 4.91% in the Inceptisol.

Conclusions

Both aerobic and anaerobic decompositions of SOC in two paddy soils were significantly affected by soil temperature and moisture. Maintaining optimal soil temperature and medium moisture during the off-rice season might be an appropriate agricultural management to mitigate CH₄ emission in the following rice growth season. Although it is high in SOC content, Andisol has less biodegradable components compared to Inceptisol and this could be a probable reason for the distinct difference in temperature sensitivity of SOC decomposition between two paddy soils.

     

Phosphine-induced phosphorus mobilization in the rhizosphere of rice seedlings

Purpose

The aim of this study was to evaluate the role of phosphine in the mobilization of phosphorus in the rhizosphere soil of rice seedlings and to determine the relative efficiency of phosphine in plant P acquisition.

Materials and methods

An indoor simulation experiment was conducted and the matrix-bound phosphine (MBP), phosphorus fractions, and phosphatase activity in the rhizosphere soil samples from rice cultivation, biomass, the plant P, and the root system activity were measured under different phosphine concentrations (0, 1.4, 4.2, and 7.0 mg m^?3) for a period of 30 days.

Results and discussion

The results indicated that phosphine treatments enhanced MBP, inorganic P (resin–P_i, NaHCO₃–P_i, and NaOH–P_i), and phosphatase activity, as well as the root system activity, and the content of P in the rice seedlings was stimulated with increasing phosphine concentrations. However, organic P (NaHCO₃–P_o and NaOH–P_o) accumulation occurred in the rhizosphere of the rice seedlings. In addition, the content of organic P in the soil samples decreased with increased phosphine concentration.

Conclusions

Therefore, relatively high concentrations of phosphine in paddy field could have a positive impact on the effectiveness of phosphorus in rice plants via influencing the rhizosphere properties.

     

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.

     

Long-term fertilization increases soil nutrient accumulations but decreases biological activity in navel orange orchards of subtropical China

Purpose

With its high economic benefits, navel orange (Citrus sinensis) has been widely planted and fertilizer has been increasingly applied in the subtropical China in the last 30 years. Comprehensive assessments are needed to explore the long-term fertilization impacts on soil chemical and biological properties in the navel orange orchards.

Materials and methods

Through a large number of soil and leaf samples from the young, middle-aged, and mature navel orange orchards, this study examined the impacts of stand age (corresponding to the fertilization year using compound chemical fertilizer) on seasonal variations in major soil properties and leaf nutrients in the subtropical China.

Results and discussion

Soil total nitrogen (N) and mineral N were significantly higher in the middle-aged and mature orchards than in the young orchard. Total phosphorus (P), available P, labile P, slow P, occluded P, weathered mineral P, total exactable P, and residual P generally increased with fertilization years (P?<?0.05), and the increasing percentages for soil P fractions were much higher than those for N variables. The total N and P use efficiencies (plant uptake/soil input) were 20–34 and 10–15 %, respectively. Soil microbial biomass, invertase, urease, and acid phosphatase activities showed significant seasonal variations and decreased with fertilization years. Leaf N concentration significantly decreased with fertilization years, but no difference was found for P.

Conclusions

Soil self-fertilization was impeded, and less fertilizer amount should be applied especially in the older navel orange orchards since N and P accumulations do not increase leaf nutrients but worsen soil biological quality.

     

Sensitivity and uncertainty analysis of CENTURY-modeled SOC dynamics in upland soils under different climate-soil-management conditions: a case study in China

Purpose

Process-based models such as CENTURY have been extensively validated for simulating soil organic carbon (SOC) dynamics at the homogeneous plot scale. However, considerable uncertainty may exist when upscaling a simulation from the plot scale to a larger scale because of variation in the model inputs. The objectives of this study were to assess the uncertainty of CENTURY-modeled SOC and to identify the most influential model inputs in various upland regions of China.

Materials and methods

Global sensitivity analysis was used to explore the sensitivity of CENTURY-modeled SOC to seven key model inputs. The uncertainties of the SOC simulated using various model inputs and climate-soil-management conditions were evaluated at 21 long-term monitoring sites located across upland areas in China.

Results and discussion

The identified sensitive model inputs differed among regions and periods due to diverse climate-soil-management conditions; nevertheless, initial SOC content (SOC_i), soil clay content, and crop residue removal rate (Residue_rr) were the most influential inputs. The site-to-region upscaling uncertainties remained moderately large (±42.7, ±49.4, and ±69.3 % at the 90, 95, and 100 % confidence levels, respectively) when currently available observation data were used. Therefore, the collection of detailed information on soil properties and crop residue removal, particularly legacy soil data such as the SOC_i and clay content, is important for reducing the uncertainties in SOC modeling.

Conclusions

Data on SOC_i, Residue_rr, and clay content need to be collected prior to other input data to reduce input-related uncertainty and thus to provide more reliable SOC assessment at the regional or national scale in China.

     

Functional and structural responses of bacterial and fungal communities from paddy fields following long-term rice cultivation

Purpose

Rice paddy soils undergo pedogenesis driven by periodic flooding and drainage cycles that lead to accumulation of organic matter and the stratification of nutrients and oxygen in the soil profile. Here, we examined the effects of continuous rice cultivation on microbial community structures, enzyme activities, and chemical properties for paddy soils along a chronosequence representing 0–700 years of rice cropping in China.

Materials and methods

Changes in the abundance and composition of bacterial and fungal communities were characterized at three depths (0–5, 5–10, and 10–20 cm) in relation to organic carbon, total nitrogen, dissolved organic carbon, microbial biomass carbon/nitrogen, and activities of acid phosphatase, invertase, and urease.

Results and discussion

Both soil organic carbon and total nitrogen increased over time at all three depths, while pH generally decreased. Microbial abundance (bacteria and fungi) and invertase and urease activity significantly increased with the duration of rice cultivation, especially in the surface layer. Fungal abundance and acid phosphatase activity declined with depth, whereas bacterial abundance was highest at the 5–10-cm soil depth. Profiles of the microbial community based on PCR-DGGE of 16S rRNA indicated that the composition of fungal communities was strongly influenced by soil depth, whereas soil bacterial community structures were similar throughout the profile.

Conclusions

Soil bioactivity (microbial abundance and soil enzymes) gradually increased with organic carbon and total nitrogen accumulation under prolonged rice cultivation. Microbial activity decreased with depth, and soil microbial communities were stratified with soil depth. The fungal community was more sensitive than the bacterial community to cultivation age and soil depth. However, the mechanism of fungal community succession with rice cultivation needs further research.

     

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.

     

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.

     

Impacts of continuous excessive fertilization on soil potential nitrification activity and nitrifying microbial community dynamics in greenhouse system

Purpose

Sampling and analysis of greenhouse soils were conducted in Shouguang, China, to study continuous excessive fertilization effect on nitrifying microbial community dynamics in greenhouse environment.

Materials and methods

Potential nitrification activity (PNA), abundance, and structure of nitrifying microbial communities as well as the correlations with soil properties were investigated.

Results and discussion

Short-term excessive fertilization increased soil nutrient contents and the diversity of nitrifying microbial communities under greenhouse cultivation. However, the abundance and diversity of nitrifying communities decreased greatly due to the increase of soil acidity and salinity after 14 years of high fertilization in greenhouse. There was a significant positive correlation between soil PNA and the abundance of ammonia-oxidizing bacteria (AOB) but not that of ammonia-oxidizing archaea (AOA) in topsoil (0–20 cm) when pH ≥7. Soil PNA and AOB were strongly influenced by soil pH. The groups of Nitrososphaeraceae, Nitrosomonadaceae, and Nitrospiraceae were predominant in the AOA, AOB, and nitrite-oxidizing bacteria (NOB) communities, respectively. Nitrifying community structure was significantly correlated with soil electrical salinity (EC), organic carbon (OC), and nitrate nitrogen (NO₃ ^?–N) content by redundancy analysis (RDA).

Conclusions

Nitrification was predominated by AOB in greenhouse topsoil with high fertilizer loads. Soil salinity, OC, NO₃ ^?–N content, and pH affected by continuous excessive fertilization were the major edaphic factors in shaping nitrifying community structure in greenhouse soils.

     

Factors influencing the organic carbon pools in tidal marsh soils of the Elbe estuary (Germany)

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.

     

Biochar addition drives soil aggregation and carbon sequestration in aggregate fractions from an intensive agricultural system

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.

     

Evaluation of palygorskite for remediation of Cd-polluted soil with different water conditions

Purpose

The study aimed at comparing the effects of different water managements on soil Cd immobilization using palygorskite, which was significant for the selection of reasonable water condition.

Materials and methods

Field experiment was taken to discuss the in situ remediation effects of palygorskite on Cd-polluted paddy soils, under different water managements, using a series of variables, including pH and extractable Cd in soils, plant Cd, enzyme activity, and microorganism number in soils.

Results and discussion

In control group, the pH in continuous flooding was the highest under three water conditions, and compared to conventional irrigation, continuous flooding reduced brown rice Cd by 37.9%, and brown rice Cd in wetting irrigation increased by 31.0%. In palygorskite treated soils, at concentrations of 5, 10, and 15 g kg^?1, brown rice Cd reduced by 16.7, 44.4, and 55.6%; 13.8, 34.5, and 44.8%; and 13.1, 36.8, and 47.3% under continuous flooding, conventional irrigation, and wetting irrigation (p < 0.05), respectively. The enzyme activity and microbial number increased after applying palygorskite to paddy soils.

Conclusions

Continuous flooding was a good candidate as water management for soil Cd stabilization using palygorskite. Rise in soil enzyme activity and microbial number proved that ecological function regained after palygorskite application.