Alkalinity exacerbates phosphorus deficiency in subtropical red soils: Insights from phosphate-solubilizing fungi

Red soils in subtropical regions are often low in available phosphorus (P), a vital plant nutrient. Phosphate-solubilizing microorganisms (PSMs) can release P from phosphate reservoir, making it accessible to plants. However, the complex interactions between PSMs and minerals in red soils are not yet fully understood. In this study, we investigated the effects of Aspergillus niger, a typical phosphate-solubilizing fungus (PSF), on phosphate dissolution in two representative red soils – an acidic soil and an alkaline soil. In the acidic red soil, the fungal abundance reached 3.01 × 10 ⁷ cfu g⁻¹ after a 28-day incubation period, with respiration of ~2000 mg C kg⁻¹. The secretion of oxalic acid promoted P release from inorganic phosphate (from ~1 to 187 mg kg⁻¹). Additionally, the contents of amorphous Fe/Al oxides decreased, which otherwise could have contributed to P sorption in the soil. In contrast, P availability declined in the alkaline red soil after the addition of A. niger, regardless of the P source (inorganic or organic phosphate). Meanwhile, the fungal respiration decreased to ~780 mg C kg⁻¹. Therefore, alkaline red soils with abundant carbonates are susceptible to P deficiency due to both the diminished function of PSMs and strong soil buffering. These findings have important implications for sustainable agriculture on alkaline red soils, as they suggest that the use of PSMs to improve P availability may be limited.     

Redox reactions and phosphorus release in re-flooded soils of an altered wetland

Phosphorus loss from land can be a major factor affecting surface water quality. We studied P‐release mechanisms in wetland soils that had been drained and cultivated for four decades and then re‐flooded. We measured redox, pH and solution composition in two sites in the field and in four peat and calcareous soils incubated in biogeochemical microcosms. The redox and pH measurements during the 120 days of incubation and the resulting soil solution composition indicated that the main process leading to P release is reductive dissolution of ferric hydroxides on which P was adsorbed and in which P was occluded. The molar Fe:P ratio increased with period of reduction from below 1 in the first week of re‐flooding to 15–60 after 120 days. This suggests an increased P‐retention capacity upon reoxidation of the soil solution, whether within the soil profile or in the drainage canals. Prolonged flooding of the calcite‐poor, gypsum‐rich peat soils increased the oversaturation of soil solutions with respect to hydroxyapatite and occasionally β‐Ca₃(PO₄)₂(c), indicating that in spite of the large Ca concentration, the rate of Ca‐P precipitation was insufficient to maintain the saturation status of the Ca‐P system. In the calcareous soils the Ca‐P system effectively controlled the P activity in soil solution throughout the incubation period. In both cases the precipitation of Ca‐P minerals could be an important P‐retention mechanism.     

Forms of phosphorus in bacteria and fungi isolated from two Australian soils

To understand the origin of organic and condensed forms of phosphorus (P) in soils, detailed information about P forms in microorganisms is required. We isolated 7 bacteria and 8 fungi from two Australian soils and analyzed the P forms in their pure cultures by extraction with NaOH-EDTA followed by ³¹P solution nuclear magnetic (NMR) spectroscopy. The bacteria belonged to the actinobacteria and the fungi to the ascomycota, as determined by rDNA sequencing. The proportions of broad forms of P were significantly different between the bacterial and fungal isolates (analysis of similarities, p = 0.001). Ortho-, pyro- and polyphosphate were present in higher proportions in fungi, while monoester and diester P were present in higher proportions in bacteria. Spectral deconvolution of the monoester region revealed 15 distinct resonances. The three major ones, which were identified by spiking experiments as glycerol 1-phosphate, glycerol 2-phosphate and adenosine-5′-monophosphate (AMP), comprised 56–74% of P in the monoester region. Ordination by principal component analysis and testing for treatment effects using analysis of similarities showed significant separation of P distribution in the monoester region between bacterial and fungal isolates (p = 0.007). However, neither group of microorganisms had a specific single P form which might be considered characteristic. As such, it may be difficult to distinguish soil P from bacterial or fungal origins, with the possible exception of a predominantly fungal origin of pyro- and polyphosphate. The identification of three major resonances in the monoester region of microorganisms is important, since the same resonances are found in ³¹P NMR spectra of soil extracts.     

Comparison of ammonium lactate,sodium bicarbonate and double calcium lactate methods for extraction of phosphorus from wetland peat soils

Procedures for routine analysis of soil phosphorus (P) have been used for assessment of P status, distribution and P losses from cultivated mineral soils. No similar studies have been carried out on wetland peat soils. The objective was to compare extraction efficiency of ammonium lactate (P-AL), sodium bicarbonate (P-Olsen), and double calcium lactate (P-DCaL) and P distribution in the soil profile of wetland peat soils. For this purpose, 34 samples of the 0–30, 30–60 and 60–90 cm layers were collected from peat soils in Germany, Israel, Poland, Slovenia, Sweden and the United Kingdom and analysed for P. Mean soil pH (CaCl₂, 0.01 M) was 5.84, 5.51 and 5.47 in the 0–30, 30–60 and 60–90 cm layers, respectively. The P-DCaL was consistently about half the magnitude of either P-AL or P-Olsen. The efficiency of P extraction increased in the order P-DCaL<P-AL≤P-Olsen, with corresponding means (mg kg^?1) for all soils (34 samples) of 15.32, 33.49 and 34.27 in 0–30 cm; 8.87, 17.30 and 21.46 in 30–60 cm; and 5.69, 14.00 and 21.40 in 60–90 cm. The means decreased with depth. When examining soils for each country separately, P-Olsen was relatively evenly distributed in the German, UK and Slovenian soils. P-Olsen was linearly correlated (r=0.594, P=0.0002) with pH, whereas the three P tests (except P-Olsen vs P-DCaL) significantly correlated with each other (P=0.0178≤0.0001). The strongest correlation (r=0.617, P=0.0001) was recorded for P-AL vs P-DCaL) and the two methods were inter-convertible using a regression equation: P-AL=?22.593+5.353 pH+1.423 P-DCaL, R ² =0.550.     

Effects of sulfate amendments on mineralization and phosphorus release from South Florida (USA) wetland soils under anaerobic conditions

We investigated the potential effects of elevated water-column sulfate (SO₄) levels on heterotrophic microbial respiration and net phosphorus (P) release for soils collected from impacted and unimpacted Everglades wetlands in south Florida. Soils from three sites, ranging from low P and low SO₄ to high P and high SO₄ environments, were examined under controlled laboratory conditions. The soils were subjected to anaerobic incubations to evaluate net P release and organic matter decomposition in response to SO₄ amendments of 32 or 96 mg l⁻¹ (0.33 and 1.0 mM).Three processes have been described in the literature to explain why SO₄ enrichment may lead to P release from soils under anaerobic conditions. First, alkalinization can lead to a more favorable pH environment for decomposition. For the soils examined here, alkalinization due to the hydrogen ion-consuming reaction of SO₄ reduction was not a prominent mechanism. We found that pH decreased in the incubation vessels, and that increases in alkalinity were more likely attributable to calcium carbonate dissolution than SO₄ reduction. Moreover, all the soils exhibited near circum-neutral pH levels, with moderate to high concentrations of native alkalinity.Second, formation of iron sulfide (FeS_x) compounds has been shown to mobilize iron (Fe)-associated P. Soils from only one of the study sites had Fe concentrations that would be expected to be high enough to influence P mobility. Relatively high porewater Fe:soluble reactive P (SRP) ratios (>83:1) were observed at this site, which suggests that Fe could theoretically exert control over the release of P from the soil. However, soil P levels at this site were too low to measure any substantial influence of Fe on net P mobilization.Finally, availability of electron acceptors such as SO₄ is a major determinant of decomposition rate, and thus rate of organic P release. Amending the soils with SO₄ did not result in either more heterotrophic microbial respiration as measured by carbon dioxide (CO₂) and methane (CH₄) production, or increased net P mobilization. In two of the SO₄-amended soils where post-incubation total sulfide concentrations were as high as 23.4 mg l⁻¹, SO₄ addition reduced production of respiratory carbon end products, suggesting hydrogen sulfide inhibition. Moreover, limitations imposed by substrate quality and low P contributed to the lack of meaningful enhanced decomposition of organic matter with the addition of 32 or 96 mg SO₄ l⁻¹ to the oligotrophic wetland soils. Even though P release did occur under anaerobic conditions for the more enriched site, addition of SO₄ did not enhance P release.     

Denitrification potential of intermittently saturated floodplain soils from a subtropical perennial stream and an ephemeral tributary

Denitrification has the potential to remove excess nitrogen from groundwater passing through riparian buffers, thus improving water quality downstream. In regions with markedly seasonal precipitation, transient stream flow events may be important in saturating adjacent floodplain soils and intermittently providing the anaerobic conditions necessary for denitrification to occur. In two experiments we characterised the denitrification potential of soils from two contrasting floodplains that experience intermittent saturation. We quantified under controlled laboratory conditions: 1) potential rates of denitrification in these soils with depth and over time, for a typical period of saturation; and 2) the influences on rates of nitrate and organic carbon. Treatments differed between experiments, but in each case soil-water slurries were incubated anaerobically with differing amendments of organic carbon and nitrate; denitrification rates were measured at selected time intervals by the acetylene-block technique; and slurry filtrates were analysed for various chemical constituents. In the first experiment (ephemeral tributary), denitrification was evident in soils from both depths (0-0.3 m; 0.3-1.1 m) within hours of saturation. Before Day 2, mean denitrification rates at each depth were generally comparable, irrespective of added substrates; mean rates (Days 0 and 1) were 5.2 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0-0.3 m) and 1.6 ± 0.2 mg N kg dry soil⁻¹ day⁻¹ (0.3-1.1 m). Rates generally peaked on Days 2 or 3. The availability of labile organic carbon was a major constraint on denitrification in these soils. Acetate addition greatly increased rates, reaching a maximum in ephemeral floodplain soils of 17.4 ± 1.8 mg N kg dry soil⁻¹ day⁻¹ on Day 2: in one deep-soil treatment (low nitrate) this overcame differences in rates observed with depth when acetate was not added, although the rate increase in the other deep-soil treatment (high nitrate) was significantly less (P ≤ 0.01). Without acetate, peak denitrification rates in this experiment were 6.9 ± 0.4 and 2.8 ± 0.2 mg N kg dry soil⁻¹ day⁻¹ in surface and deep soils, respectively. Differences in rates were observed with depth on all occasions, despite similar initial concentrations of dissolved organic carbon (DOC) at both depths. Levels of substrate addition in the second experiment (perennial stream) more closely reflected natural conditions at the site. Mean denitrification rates were consistently much higher in surface soil (P ≤ 0.001), while the source of water used in the slurries (surface water or groundwater from the site) had little effect on rates at any depth. Mean rates when all treatments retained nitrate were: 4.5 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0-0.3 m depth); 0.8 ± 0.3 mg N kg dry soil⁻¹ day⁻¹ (0.3-1.0 m); and 0.6 ± 0.1 mg N kg dry soil⁻¹ day⁻¹ (1.8-3.5 m). For comparable treatments and soil depths, denitrification potentials at both sites were similar, apart from higher initial rates in the ephemeral floodplain soils, probably associated with their higher DOC content and possibly also their history of more frequent saturation. The rapid onset of denitrification and the rates measured in these soils suggest there may be considerable potential for nitrate removal from groundwater in these floodplain environments during relatively short periods of saturation.     

Organo-mineral complexes in soil colloids: Implications for carbon storage in saline-alkaline paddy soils from an eight-year field experiment

The combination of organic carbon(OC) and reactive minerals is a crucial mechanism of soil carbon(C) storage, which is regulated by the formation of organo-mineral complexes on the surface of soil colloids. The effect of organic fertilizer on the storage mechanism of OC in soil colloids was studied through an 8-year field experiment, which included four treatments: i) no fertilization(control, CK), ii) only mineral N, P, and K fertilization(NPK), iii) NPK plus a low level(450 kg C ha^-1     

Metal uptake by wetland plants: implications for phytoremediation and restoration

Purpose

This study was undertaken to determine the feasibility of using three aquatic macrophytes, Phragmites australis, Juncus effusus and Iris pseudacorus, to phytoextract potentially toxic elements (PTEs) from a contaminated area by mining activities.

Materials and methods

An artificial pond was constructed with two topsoils (yellow and black samples) collected from Portman Bay. In order to simulate the mixing with carbonate materials, which naturally occurs in this area, a stabilisation approach was applied by mixing with 30 % of limestone filler. Three replicates of each type of soil have been prepared in pots for the selected species. The total PTEs content (arsenic, cadmium, copper, iron, lead and zinc) was determined and the bioconcentration factor (BCF) and transfer factor (TF) calculated.

Results and discussion

Soil samples showed high PTEs content as a result of mining activities. As regards the root contents, the PTEs is higher in yellow samples (YS) than in black ones, because in these samples the PTEs content that could be mobilised is higher. The BCF results were higher than unity for arsenic, copper, lead and cadmium for I. pseudacorus and P. australis growing on YS soil. Overall, copper and manganese showed a larger number of plants with BCF higher than unity. The PTEs content in leaves is low, and the TF results are lower than unity in almost all samples.

Conclusions

The results indicate that it is possible to use the selected species for phytostabilisation of soils contaminated with PTEs. J. effusus, P. australis and I. pseudacorus could be considered as tolerant, and natural or artificial wetlands containing these species could be used for remediation purposes.

     

The response of finely textured and organic soils to lime and phosphorus application: Results from an incubation experiment

A soil's responses to phosphorus (P) input differs based on its chemical composition. Soil acidity and the presence of metallic cations dictate a soil's chemical composition. Currently, soil P application recommendations are universal and do not account for differing soil composition. A targeted soil-specific approach is required to optimize P efficiency and availability. A pot incubation experiment was established to explore the effects of contrasting lime and P application rates across a range of soils (25), characterized by fine particle size and high levels of soil organic matter. Three contrasting rates of P were applied (0, 50, and 150 kg P ha⁻¹) both with and without ground lime (CaCO₃) at 5 tonne ha⁻¹ over a 140-day incubation period. The addition of lime buffered the soil, increasing nutrient availability and reducing P fixation. The 50 kg P ha⁻¹ treatment rate was required to achieve sufficient plant available P in both mineral soil textural classes. Current legislative recommendations however do not allow the application of such rates, which has an impact on agronomic performance. Loam soils experienced a greater increase in M3 soil P in comparison to clay and organic soils. Organic soils posed a major threat to water quality due to poor P retention. A re-evaluation of P recommendations is required to account for soil variability as current P allowances are insufficient on these particular soils.     

Quality indicators in subtropical soils of Formosa,Argentina: Changes for agriculturization process

The agriculturization process has been defined as the advance of annual crops in different environments, in competition with traditional land uses such as agriculture rotations with pastures. In Argentina and other countries agriculturization has different degrees of impact on natural resources. In the northeast region of the province of Formosa, Argentina, agriculturization includes deforestation (clear cutting, slash burning and plowing), technological improvements and changes in land use. Because of these alterations, it is necessary to define the state of the soil to evaluate its sustainability. This can be done by means of indicators, which are not universal; they differ according to the use, management and type of soils, weather conditions and ecosystems.The objectives of this paper are: (1) To identify quality indicators for subtropical Argiudolls and Hapludolls; (2) To determine which indicators related to organic matter are most affected during agriculturization.The changes produced in the Typic Hapludolls and Typic Argiudolls after 25 years of continuously using native forests, agriculture, fruit plantations and pastures were analyzed. These changes were in pH, electrical conductivity, total organic carbon, particulate organic carbon, total nitrogen, structural stability, hydraulic conductivity, respiration and dehydrogenase and urease and enzyme activity. Variables with significant differences between diverse uses were evaluated by multivariate methods, Principal Component Analysis, and Correlation Analysis. The results of this study showed that total organic carbon, particulate organic carbon, structural stability and dehydrogenase activity are the quality indicators most affected by agriculturization. All are related to organic matter.     

Phosphorus cycling in UK agriculture and implications for phosphorus loss from soil

Abstract. Phosphorus (P) use in UK agriculture is reviewed and a P balance sheet presented. The productive grassland and arable area has accumulated an average P surplus of c. 1000 kg ha^–1 over the last 65 years. Over the period 1935–1970, the annual P surplus more than doubled due to an increase in animal numbers and associated requirements for inorganic fertilizers and livestock feeds. Since 1970, surplus P has declined by c . 40% as crop yields and P offtake have continued to increase while fertilizer and manure P inputs have remained relatively constant. In 1993, P use efficiency (P imports/P exports) in UK agriculture was estimated at 25% leading to an average annual surplus of 15 kg P ha^–1 yr^–1, although the latter has since decreased slightly due to reduced fertilizer use. Intensification and specialization of agriculture has also increased the range in P surpluses that are likely between livestock and arable dominated systems. The largest P surpluses occur in the relatively limited areas of arable soils which receive manure from intensive pig and poultry units, whilst farms without manure inputs generate only small surpluses, or are in balance. The cumulative P surplus has led to a build-up of soil total and easily-exchangeable P, especially in areas receiving both fertilizers and manures. Fundamental differences in P use efficiency, surplus P accumulation and the potential for P loss to water, exist between arable and grassland farms and it is important to separate these, due to the marked regionalization of UK agriculture. More judicial use of feeds and fertilizers is required to further reduce the P surplus and minimize the long-term risk of water eutrophication.     

土壤磷素形态及其分级方法研究进展

基于对土壤磷素相关文献的全面梳理与系统总结,明确国内外土壤磷素研究的现状、热点与未来发展趋势。以Web of Science核心数据库中1990 ~ 2021年间8444篇与土壤磷素相关的研究论文为数据源,利用CiteSpace软件对发文国家、机构、关键词及其聚类等进行可视化分析。结果表明：土壤磷素研究的发文数量从2017年开始快速增长。美国、中国和澳大利亚发文数量位列前三,其中美国和澳大利亚中介中心性较高。研究机构合作网络显示中介中心性较高的机构为中国农业科学院、阿德莱德大学和拉筹伯大学,其分别以农田磷素可持续管理、植物-微生物介导磷素利用效率、土壤磷组分和磷循环为研究重点。文献共被引分析表明,磷素的资源短缺、遗留效应、径流迁移特征和限制机制是该领域研究的重点内容。关键词共现网络、聚类和突现性分析表明,土壤氮磷互作、磷素-植物效应、磷素有效性、动态变化等是该研究领域的热点。研究趋势集中于微生物（群落）、养分限制（化学计量比）和气候变化等方面对土壤磷有效性和磷周转的影响。未来充分挖掘微生物资源、缓解土壤养分限制将为土壤磷素的可持续利用提供科学措施。     

Development of an alternative to the Olsen bicarbonate-extraction test for determining plant-available phosphorus in basaltic soils

Abstract. Recent work has demonstrated that the Olsen test for phosphorus (P) is an unreliable predictor of plant-available P in soils derived from basalt parent material in Northern Ireland. The present study was conducted to develop a more reliable soil-P test for these soils by regressing P fractions removed from soil by various chemical extractants against herbage P indices calculated from plant tissue test data using a diagnosis and recommendation integrated system. The degree of P saturation of the soil P sorption capacity, based on ammonium oxalate extractable P, Al and Fe, provided a better prediction of P available to swards on basaltic soils than either the Olsen test or a number of other well-known soil-P test procedures. The superiority of the degree of P saturation test on basaltic soils was attributed to the fact that it simultaneously takes account of both P quantity and P buffering capacity factors in predicting P availability. The Olsen-P test, which accounts for the P quantity factor alone, was only reliable for non-basaltic soils. Re-classifying the P fertility status of basaltic soils according to the degree of P saturation test could result in considerably less P being recommended for these soils with possible consequential benefits to water quality.     

Effects of phosphorus addition on N2O and NO emissions from soils of an Acacia mangium plantation

Abstract

An incubation experiment was conducted to examine the effects of the phosphorus (P) application on nitrous oxide (N₂O) and nitric oxide (NO) emissions from soils of an Acacia mangium plantation in Indonesia. The soils were incubated with and without the addition of P (Ca[H₂PO₄]₂; 2 mg P g soil)^?1) after adjusting the water-filled pore space (WFPS) to 75% or 100%. The P addition increased N₂O emissions under both WFPS conditions and NO emissions at 75% WFPS. Some possible mechanisms are considered. First, the P addition stimulated nitrogen (N) cycling, and N used for nitrification and/or denitrification also increased. Second, the P addition could have relieved the P shortage for nitrifying and/or denitrifying bacteria, producing N₂O and NO. Our results suggest that the application of P fertilizer has the potential to stimulate N₂O and NO emissions from Acacia mangium plantations, at least when soils are under relatively wet conditions.     

中国长江口潮滩湿地土壤重金属含量的时空变化规律及环境指示意义

The environment of estuarine wetlands has been attracting worldwide attention. To study the spatial distribution of pollutants in the tidal flats of the Yangtze Estuary, Southeast China, the Eastern Tidal Flat of Chongming Island (EC) and the Jiuduansha Shoal (JS) of the estuary were selected as the study sites. At each of the two sites, a cross-transect from land to sea was established and topsoil and soil core samples in the cross-transect were collected spatially and seasonally to determine their contents of heavy metals (Cu, Zn, Pb, Cd, Cr, Ni, Mn, and Fe) and grain-size characteristics. The results showed that the heavy metal loads were commonly higher in the soils of nearshore high tidal flats and had a tendency of decreasing from land to sea at both of the study sites. The contents of heavy metals in the soils of the high and medial tidal flats were mostly higher in April and November but lower in July. Corresponding spatial and seasonal variations in grain size of the intertidal soils were also observed at the two study sites. The soils in the nearshore high tidal flats were finer and gradually got coarser seawards; they were relatively finer in April and November but coarser in July. Furthermore, the contents of heavy metals in the intertidal soils of both the sites EC and JS were significantly positively correlated with the clay (<2 μm) and 2-20 μm fractions, but negatively with the sand (>63 μm) and 20-63 μm fractions, which suggested that the heavy metals in the intertidal soils were primarily combined with the fine particulate fraction (<20 μm), especially clay, and hence the spatial and seasonal variations in heavy metals were actually caused by the change of the grain-size characteristics of the intertidal soils due to the different sedimentary environments in the estuary. The results of this study may also contribute to a better understanding of the soil formation and classification in the tidal flats of the Yangtze Estuary.     

A simplified risk assessment for losses of dissolved reactive phosphorus through drainage pipes from agricultural soils

Abstract

Soil tests with extractions are commonly used for risk assessments of phosphorus (P) leaching. Procedures for routine analysis of crop-available soil P by extraction with acid ammonium lactate (P-AL) have been used for nearly 50 years in Sweden, Norway and several East European countries. Aluminium and iron (Al-AL and Fe-AL) were determined in the same extract for 40 well known clayey, loamy or sandy soils from the Swedish long-term studies. Average outcome was 16.8 and 6.0% for the two elements related to extraction with chelating ammonium oxalate (Al-AO and Fe-AO) and concentrations had a correlation coefficient of 0.947 and 0.891, respectively, when the two extraction agents were compared. On average, P-AL determination using inductive coupled plasma (ICP) resulted in 19% higher soil P concentrations compared to analysis using a colorimetric method based on non-calcareous and calcareous soils from the southern counties in the Swedish soil survey, represented mainly by sandy loam soils. Degree of P saturation on a molar basis in the AL extract (DPS-AL) was determined for 22 Nordic observation fields with drained clayey, loamy and sandy soils. Results were used together with long-term flow-weighed concentration of dissolved reactive P (DRP) concentration in drainage water. These parameters were correlated (r=0.918, p=0.000) and could be fitted to a linear regression model (R²=84.3). In addition, two fields with unusually high DPS-AL values could clearly be identified as those with lowest P sorption index and highest DRP concentrations in drainage water. This demonstrates DPS-AL to have the potential as an environmental risk indicator for Swedish acid soils. A set of 230 non-calcareous soils in the southern counties of Sweden from the Swedish soil survey indicated that 3% of the soils had a high DPS-AL in the topsoil or subsoil, from which high DPS leaching probably occurs.     

Ecological stoichiometry of carbon, nitrogen, and phosphorus in estuarine wetland soils: influences of vegetation coverage, plant communities, geomorphology, and seawalls

Purpose

Little is known about carbon, nitrogen, and phosphorus stoichiometrical characteristics and influencing factors in estuary wetland soils. The purpose of this work is to study ecological stoichiometric characteristics of carbon, nitrogen, and phosphorus (R _CN, R _CP, and R _NP) in estuarine wetland soils of Shuangtaizi, northeast China and the potential affecting factors like vegetation coverage, plant communities, geomorphology, and seawall.

Materials and methods

During 2008–2010, soil samples in estuarine wetland were collected for soil organic carbon, total nitrogen and phosphorus, and other elements determination. Mole ratios of R _CN, R _CP, and R _NP were calculated.

Results and discussion

As a whole, R _CN was in the range of 8.26～52.97 (mean, 16.15), R _CP was in the range of 23.21～862.53 (mean, 90.66), and R _NP was in the 0.93～29.52 (mean, 5.07). R _CN, R _CP, and R _NP distribution were all with high spatial heterogeneities and significantly affected by vegetation coverage, plant communities, geomorphology, and seawalls. During the typical plant succession sequence of the halophytes–the mesophyte–the hydrophyte in estuarine wetland, P might be the primary limiting elements for nutrients stoichiometrical characteristics. R _CN, R _CP, and R _NP in soils of low-lying areas were all higher than that in highlands. Plant coverage and communities formation would help to reduce restriction from nitrogen, but to increase restrictions from phosphorus meanwhile.

Conclusions

C, N, and P ecological stoichiometry had high complexities. R _CN in estuarine wetland soils were generally high, whereas R _CP and R _NP were comparatively low, indicating that ecosystems in the estuary were limited by nutrients such as N and P, with the latter being the primary factor. Vegetation covers, plant communities, geomorphology, and seawall all affected nutrient stoichiometry in soils.     

Carbon storage in loess derived surface soils from Central Germany: Influence of mineral phase variables

The influence of the soil mineral phase on organic matter storage was studied in loess derived surface soils of Central Germany. The seven soils were developed to different genetic stages. The carbon content of the bulk soils ranged from 8.7 to 19.7 g kg^—1. Clay mineralogy was confirmed to be constant, with illite contents > 80 %. Both, specific surface area (SSA, BET‐N₂‐method) and cation exchange capacity (CEC) of bulk soils after carbon removal were better predictors of carbon content than clay content or dithionite‐extractable iron. SSA explained 55 % and CEC 54 % of the variation in carbon content. The carbon loadings of the soils were between 0.57 and 1.06 mg C m^—2, and therefore in the ”︁monolayer equivalent” (ME) level. The increase in SSA after carbon removal (ΔSSA) was significantly and positively related to carbon content (r² = 0.77). Together with CEC of carbon‐free samples, ΔSSA explained 90 % of the variation in carbon content. Clay (< 2 μm) and fine silt fractions (2—6.3 μm) contained 68—82 % of the bulk soil organic carbon. A significantly positive relationship between carbon content in the clay fraction and in the bulk soil was observed (r² = 0.95). The carbon pools of the clay and fine silt fractions were characterized by differences in C/N ratio, δ¹³C ratio, and enrichment factors for carbon and nitrogen. Organic matter in clay fractions seems to be more altered by microbes than organic matter in fine silt fractions. The results imply that organic matter accumulates in the fractions of smallest size and highest surface area, apparently intimately associated with the mineral phase. The amount of cations adhering to the mineral surface and the size of a certain and specific part of the surface area (ΔSSA) are the mineral phase properties which affect the content of the organic carbon in loess derived arable surface soils in Central Germany most. There is no monolayer of organic matter on the soil surfaces even if carbon loadings are in the ME level.