Phosphorus Speciation Suggests Contrasting P Supply Mechanisms Across Aggregates in Subtropical Forest Soils With Different Lithology

To reveal the differences in soil microbial nutrient limitation and nutrient availability at the aggregate scale of soil types in a subtropical forest in China, the O/A horizon （0-3 cm and 0-7 cm, respectively） and AB horizon （below O/A horizon to 20 cm depth） of typical acidic red soil and neutral limestone soil in Northern Guangxi were studied. The activities of carbon-acquiring enzymes （sucrase, amylase, β-1,4-glucosidase （BG））, nitrogen-acquiring enzymes （urease, β-1,4-N-acetyl-glucosaminidase （NAG）, leucine amino peptidase （LAP））, and phosphorus-acquiring enzymes （phytase and acid phosphatase （AP）） in different aggregate sizes were determined to reveal the variation in soil enzyme stoichiometry characteristics. The results showed that the limestone soil had higher nitrogen-acquiring enzyme activity compared to that in the red soil, with urease, NAG, and LAP activities being 38.84%, 123.89%, and 4.06% higher, respectively. The differences in carbon- and phosphorus-acquiring enzyme activities between the two soils were not significant. The overall soil enzyme activities in the O/A horizon were higher than in the AB horizon for both red soil and limestone soil. Soil total organic carbon and pH were identified as key factors influencing soil enzyme activities. Most enzyme activities （BG, urease, NAG, and phytase） were higher in micro-aggregates （0.1-0.25 mm and <0.1 mm）, emphasizing the role of micro-aggregates in promoting organic matter decomposition and accelerating soil nutrient cycling. Compared to those in red soil, limestone soil exhibited greater differences in NAG and phytase activities among aggregates. The mean values of ln（BG）∶ln（NAG+LAP）∶ln（AP） of red soil and limestone soil as well as their aggregates were 1.02∶1∶1.04 and 0.95∶1∶0.93, respectively, generally conforming to the global average ratio of 1∶1∶1. The ln（BG）∶ln（NAG+LAP） and vector lengths were higher in red soil than in limestone soil, indicating stronger carbon limitation in the red soil. The ln（NAG+LAP）∶ln（AP） ratio was lower in red soil than in limestone soil, with the former having a larger vector angle, suggesting stronger phosphorus limitation. Soil pH and total phosphorus were identified as the primary influencing factors of enzyme activity stoichiometry characteristics, suggesting that the bioavailability of organic matter and phosphorus in acidic soil was lower than in limestone soil, prompting microbes to secrete carbon and phosphorus enzymes to enhance acquisition efficiency. The high pH of limestone soil may lead to nitrogen limitation due to nitrate leaching caused by higher mineralization and nitrification rates. Microbes in the large aggregates （1-2 mm and 0.5-1 mm） of both soils experienced stronger phosphorus limitation, whereas carbon limitation was stronger in micro-aggregates, especially in the 0.1-0.25 mm aggregates. The microbial nutrient pattern in limestone soil aggregates shifted from phosphorus limitation to nitrogen limitation from large to micro-aggregates. This study reveals the influence of soil type and aggregate on the soil microbial nutrient limitation patterns of soil microbial elements in subtropical forest soils of China.

Phosphorus (P) is one of the key limiting factors for the growth of forests and their net primary productivity in subtropical forest ecosystems. Phosphorus leaching of the forest soil to the catchment and groundwater in karst region is the main source of water eutrophication. Strong P sorption capacity of minerals is generally assumed to be a key driver of P leaching in subtropical ecosystems which varies among different soil types. Here, we estimated P adsorption capacity of the O/A and AB horizon in both limestone soil and red soil of subtropical forests by fitting the Langmuir and Freundlich isotherm to investigate the potential environmental risks of P. The maximum P sorption capacity ([Formula: see text]), P sorption constant ([Formula: see text]), P sorption index (PSI), degree of P saturation (DPS), and maximum buffer capacity (MBC) were calculated. The results indicate that [Formula: see text] of the O/A horizon in both soils were similar. Comparing these two soils, the red soil had a higher [Formula: see text] and MBC in the AB horizon; [Formula: see text] of limestone soil was larger but [Formula: see text] was lower, indicating that the adsorption capacity of limestone soil was weaker and MBC was lower. There was no significant difference in PSI between the two soils. The DPS values of both soils were below 1.1%, indicating that P saturation is low in both subtropical forest soils due to the lack of marked anthropogenic disturbance. In the O/A horizon, P saturation associated with available P (DPS M3 and DPS Olsen ) and that associated with P in the Fe-Al bound state (DPS citrate ) were higher in the red soil than in the limestone soil. DPS did not differ significantly in the AB horizon, except for higher DPS M3 and DPS citrate in the red soil. The findings highlight the influence of the soil types on P adsorption. The P adsorption and buffering of red soils were higher than those of limestone soils, indicating a lower risk of P leaching in red subtropical forest soils.

PurposeThe purpose of this paper is to study the corrosion behavior of Q235 steel in saturated acidic red and yellow soils.Design/methodology/approachThe corrosion behavior of Q235 steel in saturated red and yellow soils was compared by weight-loss, SEM/EDS, 3D ultra-depth microscopy and electrochemical measurements.FindingsRp of the steel gradually increases and icorr gradually decreases in both the red and yellow soils with time. The Rp of the steel in the red soil is lower, but its icorr is higher than that in the yellow soil. The uniform corrosion rate, diameter and density of the corrosion pit on the steel surface in the red soil are greater than those in the yellow soil. Lower pH, higher contents of corrosive anions and high-valence Fe oxides in the red soil are responsible for its higher corrosion rates and local corrosion susceptibility.Originality/valueThis paper investigates the difference in corrosion behavior of carbon steel in saturated acidic red and yellow soils, which can help to understand the mechanism of soil corrosion.

Phosphorus (P) is a limited nutrient in highly weathered subtropical or tropical forest soils and it is unclear how both the cycling and availability of P will respond to future global warming and nitrogen (N) deposition. This study aims to evaluate the influence of soil warming and N deposition on soil P fractions in relation to plant growth and P uptake. We carried out a field mesocosm bioassay experiment with a soil warming and N addition treatment in a factorial design and measured soil Hedley P fraction and P uptake of the Chinese fir (Cunninghamia lanceolata) seedlings. Soil warming significantly increased soil easily-available and moderately-available inorganic phosphorus (Pi) but had no effect on soil easily-available and moderately-available organic P (Po), or on plant growth or plant biomass P, and decreased soil microbial biomass P (MBP). Nitrogen addition increased plant growth and leaf biomass P, but had no effect on soil Pi and MBP, and decreased soil moderately-available Po and leaf P concentration. There were no interactions between soil warming and nitrogen on Hedley P fractions. None of the Hedley P fractions had correlated with plant height and diameter growth or with plant P uptake. Our results suggest that the Hedley P fractions do not reflect soil P availability and that the growth of the Chinese fir is not limited by soil P availability in this subtropical Ultisol, rather it is most likely limited by N and water availability based on the results from this bioassay experiment.

Differences in available phosphorus in temperate and subtropical forest soils

Soil aggregates are the basic units of soil, which regulate soil carbon cycling and nutrient availability through the protective effect of soil aggregates on soil organic matter. It is still uncertain whether larger aggregates are more conducive to soil nutrient availability in red soil. This study explored the regulation of soil aggregates on soil nutrient availability by studying the distribution of soil aggregates, nutrient concentrations, nutrient availability and organo-mineral bonds in soil aggregates in a low-productivity Chinese fir forest, in south China. We sampled the 0–10 cm soil with nine repeated plots and analyzed the soil aggregate structure, total nutrients, available nutrients and organo-mineral bonds of soil aggregates. The results showed that the contribution of >2 mm soil aggregates to soil nutrients was highest, because the mass of >2 mm soil aggregates accounted for about 50% of the total mass of aggregates and was much higher than that of other aggregates. The availability (available nutrient/total nutrient) of nitrogen, phosphorus and potassium increased with decreases in soil aggregate size, indicating that soil aggregates with a larger particle size were more averse to nutrient availability. Strong organo-mineral bonds accounted for more than 80% of the total organo-mineral bonds in the soil aggregates of each size, and the proportion of weak organo-mineral bonds in the soil aggregate increased with decreases in the soil aggregate size. There was a significant negative correlation between the size of soil aggregates and the proportion of weak organo-mineral bonds in soil aggregates. The availability of carbon, nitrogen, phosphorus and potassium in soil aggregates was positively correlated with the proportion of weak organo-mineral bonds. These results suggest that Fe/Al oxides may play an important role in regulating nutrient availability, especially in red soil. A higher proportion of strong organo-mineral bonds in larger soil aggregates may exert a stronger negative effect on the accessibility of microorganisms to organic matter and result in a lower nutrient availability. In conclusion, this study shows that larger-sized soil macroaggregates may exert a negative effect on nutrient availability, owing to a higher proportion of strong binding bonds, which can better prevent microorganisms from mineralizing organic matter into effective nutrients in red soil.

In karst landscapes, soil CO2 is a key factor in weathering processes and carbon cycling, where its distribution and migration characteristics directly affect fluxes in carbon source–sink dynamics. We measured the CO2 emission and dissolution rates of carbonate tablets in calcareous soil developed from limestone and red soil developed from clastic rock, in karst and non-karst subtropical forests, in Guilin, southwest China between 2015 and 2018, to analyze their CO2 transfer characteristics and source–sink effects. The results showed similar average soil respiration rates between calcareous soil and red soil, with an average CO2 emission flux of 1305 and 1167 t C km−2 a−1, respectively. Carbonate tablet dissolution rates were bidirectional with increasing depth and were greater in red soil than calcareous soil, averaging 13.88 ± 5.42 and 7.20 ± 2.11 mg cm−2 a−1, respectively. CO2 concentration was bidirectional with increasing soil depth, reaching a maximum at the base of the soil–atmosphere interface (50–60 cm), and the bidirectional gradient was more distinctive in red soil. Change in the carbon isotope value of soil CO2 was also bidirectional in calcareous soils, for which the overall average was 0.87‰ heavier in calcareous than red soil. The carbon sink in calcareous soil in karst regions was estimated to be 11.97 times that of red soil in non-karst regions, whereas its role as a carbon source is just 1.12 times that of red soil, thus indicating the key role of karst soil in the reduction of atmospheric CO2.

Tobacco (Nicotiana tabacum) root affects the yield and quality of tobacco leaves. To gain insight into the responses of the tobacco root system to different soil types, we integrated morphological characteristics, the physiological index, the metabolic pathways of the root system, and the aboveground biomass of tobacco cultivated in limestone soil (LS), paddy soil (PS), and red soil (RS). Compared with plants growing in LS and PS, the chemical composition of tobacco leaves in RS tended to be coordinated. Red soil facilitated the accumulation of aboveground and belowground biomass of flue-cured tobacco and had the most significant effect on the dry matter quality of the roots. In addition, it promoted an increased root length, root surface area (RSA), root volume, and a higher number of root forks and improved root vigor and nitrate reductase (NR) activity; however, the activities of superoxide dismutase (SOD) and peroxidase (POD) were decreased. We studied differentially the abundant proteins (DAPs) of the flue-cured tobacco roots cultivated in different soil types by isobaric tags for the relative and absolute quantification (iTRAQ) of the proteomic profiles of cultivar. In total, 699, 650, and 569 differentially abundant proteins (DAPs) were identified from limestone soil (LS) vs. PS, LS vs. RS, and PS vs. RS, respectively, including 412/287, 291/359, and 323/246 up-/downregulated proteins, respectively. These DAPs were mainly involved in starch and sucrose metabolism, phenylalanine metabolism, the biosynthesis of secondary metabolites, microbial metabolism in different environments, and ribosomes. The parallel reaction monitoring (PRM) and quantitative reverse transcription PCR (qRT-PCR) analysis showed that the results of the iTRAQ proteomics were reliable. Overall, our study facilitates a new understanding of the responses of tobacco roots to different soil types at the protein level.

Syringic acid (SA) is a novel biological nitrification inhibitor (BNIs) discovered in rice root exudates with significant inhibition of Nitrosomonas strains. However, the inhibitory effect of SA on nitrification and nitrous oxide (N2O) emissions in different soils and the environmental factors controlling the degree of inhibition have not been studied. Using 14-day microcosm incubation, we investigated the effects of different concentrations of SA on nitrification activity, abundance of ammonia-oxidizing microorganisms, and N2O emissions in three typical agricultural soils. The nitrification inhibitory efficacy of SA was strongest in acidic red soil, followed by weakly acidic paddy soil, with no significant effect in an alkaline calcareous soil. Potential nitrification activity (PNA) were also greatly reduced by SA additions in paddy and red soil. Pearson correlation analysis showed that the inhibitory efficacy of SA might be negatively correlated with soil pH and positively correlated with clay percentage. SA treatments significantly reduced N2O emissions by 69.1-79.3% from paddy soil and by 40.8%-46.4% from red soil, respectively, but no effect was recorded in the calcareous soil. SA addition possessed dual inhibition of both ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) abundance in paddy and red soil. Structural equation modelling revealed that soil ammonium (NH4 +) and dissolved organic carbon content (DOC) were the key variables explaining AOA and AOB abundance and subsequent N2O emissions. Our results support the potential for the use of the BNI SA in mitigating N2O emissions and enhancing N utilization in red and paddy soils.

The objective of this study is to examine the impact of various oyster shell soil conditioners, which are primarily composed of oyster shells, on the growth of tomatoes in acidic soil. Moreover, the aim of this investigation is to analyze the variety and structure of soil bacterial populations in close proximity to tomato roots while also contributing to the understanding of the physical, chemical, and biological mechanisms of oyster shell soil conditioners. Tomato plants were grown in acidic red soil in three groups: a control group and a treatment group that used two types of oyster shell soil conditioners, OS (oyster shell powder) and OSF (oyster shell powder with organic microbial fertilizer). A range of soil physicochemical properties were measured to study differences in inter-soil physicochemical parameters and the growth of tomato plantings. In addition, this study utilized the CTAB (Cetyltrimethylammonium Bromide) technique to extract DNA from the soil in order to investigate the effects of oyster shell soil conditioner on the composition and diversity of bacterial populations. Utilizing high-throughput sequencing technologies and diversity index analysis, the composition and diversity of bacterial populations in the soil adjacent to plant roots were then evaluated. Ultimately, correlation analysis was used in this study to explore the relationship between environmental factors and the relative abundance of soil bacteria in the inter-root zone of tomato plants. The findings indicated that the oyster shell soil conditioners were capable of modifying the physicochemical characteristics of the soil. This was evidenced by significant increases in soil total nitrogen (16.2 and 59.9%), soil total carbon (25.8 and 27.7%), pH (56.9 and 55.8%), and electrical conductivity (377.5 and 311.7%) in the OS and OSF groups, respectively, compared to the control group (p < 0.05). Additionally, data pertaining to tomato seed germination and seedling growth biomass demonstrated that both oyster shell soil conditioners facilitated the germination of tomato seeds and the growth of seedlings in an acidic red clay soil (p < 0.05). On the other hand, the application of two oyster shell soil conditioners resulted in a modest reduction in the diversity of inter-root soil bacteria in tomato plants. Specifically, the group treated with OSF exhibited the most substantial fall in the diversity index, which was 13.6% lower compared to the control group. The investigation carried out on the soil between tomato plant roots yielded findings about the identification of the ten most abundant phyla. These phyla together represented 91.00-97.64% of the overall abundance. In the inter-root soil of tomatoes, a study identified four major phyla, namely Proteobacteria, Bacteroidetes, Acidobacteria, and Actinobacteria, which collectively accounted for up to 85% of the total abundance. At the general level, the relative abundance of Massilia increased by 2.18 and 7.93%, Brevundimonas by 5.43 and 3.01%, and Lysobacter by 3.12 and 7.49% in the OS and OSF groups, respectively, compared to the control group. However, the pathogenic bacteria unidentified_Burkholderiaceae decreased by 5.76 and 5.05%, respectively. The correlation analysis yielded conclusive evidence indicating that, which involved the use of CCA (Canonical Correlation Analysis) graphs and Spearman correlation coefficients, pH exhibited a positive correlation (p < 0.05) with Shewanella and a negative correlation (p < 0.05) with Bradyrhizobium. The relative abundance of Lysobacter and Massilia exhibited a positive correlation with the levels of total soil nitrogen. The utilization of oyster shell soil conditioner on acidic red soil resulted in several positive effects. Firstly, it raised the pH level of the inter-root soil of tomato plants, which is typically acidic. This pH adjustment facilitated the germination of tomato seeds and promoted the growth of seedlings. In addition, the application of oyster shell soil conditioner resulted in changes in the structure of the bacterial community in the inter-root soil, leading to an increase in the relative abundance of Proteobacteria and Bacteroidetes and a decrease in the relative abundance of Acidobacteria. Furthermore, this treatment fostered the proliferation of genera of beneficial bacteria like Massilia, Brevundimonas, and Lysobacter, ultimately enhancing the fertility of the red soil.

Phosphorus addition accelerates soil organic carbon mineralization by desorbing organic carbon and increasing microbial activity in subtropical forest soils

Evaluating short-term effects of rice straw management on carbon fractions, composition and stability of soil aggregates in an acidic red soil with a vegetable planting history

Analysis of the intra-aggregate pore structures in three soil types using X-ray computed tomography

The phosphorus (P) distribution in the sediments of Haizi Lake from the middle reach of the Yangtze River region, China, was investigated using a sequential chemical extraction procedure. P forms and concentrations of sediment samples taken at 25 sites over the whole lake were measured. The relationships between various forms of P in sediments and dissolved P in the overlying water were also discussed. Results showed that the concentrations of total P (TP) in the sediments ranged from 404 to 670 mg kg⁻¹, with an average of 503 mg kg⁻¹. The exchangeable P (Ex-P), Al-bound P (Al-P), Fe-bound P (Fe-P), occluded P (Oc-P), authigenic carbonate fluorapatite + biogenic apatite + CaCO₃-associated P (ACa-P), detrital apatite + other inorganic P (De-P) and organic P (Or-P) accounted for, on average, 0.52, 0.04, 10.9, 32.0, 7.4, 20.1 and 29.0% of TP, respectively. Relevance analysis indicated that Oc-P, ACa-P and De-P, as the majority forms of inorganic P, were less correlated to others. The significant correlations between Ex-P, Al-P, Fe-P, Or-P and TP suggested the probability of reciprocal transformation. It was suggested that Ex-P, Al-P, Fe-P, Or-P and TP in the sediments might be released easily to the water interface, resulting in sustained lake eutrophication.

Long-term throughfall exclusion decreases soil organic phosphorus associated with reduced plant roots and soil microbial biomass in a subtropical forest