Phosphorus Transformations Research Articles

Co-inoculation of phosphate-solubilizing bacteria and phosphate accumulating bacteria in phosphorus-enriched composting regulates phosphorus transformation by facilitating polyphosphate formation

This study aimed to explore the impact of co-inoculating phosphate-solubilizing bacteria (PSB) and phosphate accumulating bacteria (PAB) on phosphorus forms transformation, microbial biomass phosphorus (MBP) and polyphosphate (Poly-P) accumulation, bacterial community composition in composting, using high throughput sequencing, PICRUSt 2, network analysis, structural equation model (SEM) and random forest (RF) analysis. The results demonstrated PSB-PAB co-inoculation (T1) reduced Olsen-P content (1.4 g) but had higher levels of MBP (74.2 mg/kg) and Poly-P (419 A.U.) compared to PSB-only (T0). The mantel test revealed a significantly positive correlation between bacterial diversity and both bioavailable P and MBP. Halocella was identified as a key genus related to Poly-P synthesis by network analysis. SEM and RF analysis showed that pH and bacterial community had the most influence on Poly-P synthesis, and PICRUSt 2 analysis revealed inoculation of PAB increased ppk gene abundance in T1. Thus, PSB-PAB co-inoculation provides a new idea for phosphorus management.

Migration and transformation of soil phosphorus by organic acids: a global meta-analysis

Migration and transformation of soil phosphorus by organic acids: a global meta-analysis

Hydrothermal carbonization of milk/dairy processing sludge: Fate of plant nutrients

Dairy processing sludge (DPS) is a byproduct generated in wastewater treatment plants located in dairy (milk) processing companies (waste activated sludge). DPS presents challenges in terms of its management (as biosolids) due to its high moisture content, prolonged storage required, uncontrolled nutrient loss and accumulation of certain substances in soil in the proximity of dairy companies. This study investigates the potential of hydrothermal carbonization (HTC) for recovery of nutrients in the form of solid hydrochar (biochar) produced from DPS originating from four different dairy processing companies. The HTC tests were carried out at 160 °C, 180 °C, 200 °C and 220 °C, and a residence time of 1h. The elemental properties of hydrochars (biochars), the content of primary and secondary nutrients, as well as contaminants were examined. The transformation of phosphorus in DPS during HTC was investigated. The fraction of plant available phosphorus was determined. The properties of hydrochar (biochar) were compared against the European Union Fertilizing Products Regulation. The findings of this study demonstrate that the content of nutrient in hydrochars (biochars) meet the requirements for organo-mineral fertilizer with nitrogen and phosphorus as the declared nutrients (13.9–26.7%). Further research on plant growth and field tests are needed to fully assess the agronomic potential of HTC hydrochar (biochar).

Simultaneous immobilizations of soluble phosphorus and fluorine in phosphogypsum by milling with calcareous and aluminiferous samples: Reaction products and immobilization performances

Phosphogypsum (PG) is a by-product of phosphorus chemical industry, which contains a large amount of soluble phosphorus and fluorine impurities to pose a great ecological risk to the environment and human health. In this study, we proposed one-step mechanical ball milling as a cleaner operation for simultaneous immobilization of phosphorus and fluorine in PG waste with different calcareous and aluminiferous immobilization reagents. The results show that, the remaining soluble PO43− and F− concentrations could reduce to 0.056 and 0.99 mg/L in PG-Ca(OH)2–Al(OH)3 system, respectively, when the total mass ratio at 5% of Ca(OH)2 and Al(OH)3 was added to PG. Furthermore, mineralogical and chemical evidence indicated that, the main immobilization mechanisms of soluble phosphorus and fluorine in PG are as follows: a) the transformation of co-crystal phosphorus (Ca((SO4)x,(HPO)4)1-x), CSHP) into insoluble phosphorus during milling process, b) the exchange of F− by OH− in Katoite formed by Ca(OH)2 and Al(OH)3, c) the formation of calcium aluminum fluorosilicate and hydroxyapatite due to the increased activity of silicate. Additionally, when using kaolinite and calcium carbonate slag as calcareous and aluminiferous alternative reagents, the concentrations of PO43− and F− were 0.056 and 7.42 mg/L in leaching solution, within the permitted level for the GB8978-1996 (0.5 mg/L for PO43−, 10.0 mg/L for F−). The application of calcium base, aluminum base solid wastes could provide a potential solution for large-capacity disposal of industrial waste and sustainable utilization of PG.

Disentangling the relative importance of precipitation, biocrust succession, and shrub cover in mediating soil phoD-harbouring communities and organic phosphorus mineralisation

Biological soil crusts (biocrusts) are crucial components of dryland ecosystems worldwide; however, their roles in phosphorus (P) transformation and associated functional communities have received less scientific attention compared with carbon (C) and nitrogen (N) cycling. Currently, little is known about the relative importance of precipitation, biocrust succession, and shrub cover in mediating soil P transformation. Here, we conducted a broad field survey across deserts in Northern China to investigate the roles of these factors in soil organic P (Po) mineralisation. We examined various P mineralisation parameters, including soil C:P ratio, soil Po content, bioavailable P, alkaline phosphatase (ALP)-encoding gene phoD abundance, and ALP activity. Additionally, we investigated the diversity and composition of soil phoD-harbouring communities and their relationships with Po mineralisation. Overall, the P mineralisation parameters increased with biocrust succession, and their responses to precipitation levels varied and were contingent upon the biocrust successional stages, while less significant differences were observed between the presence and absence of shrub cover. Most P mineralisation parameters were positively correlated within individual deserts or across all deserts studied. Moreover, the beta diversity, rather than the alpha diversity, of soil phoD-harbouring communities predominantly drove Po mineralisation. The abundance of certain genera (Actinoplanes and Micromonospora) that were relatively less dominant but showed significant positive correlations with P mineralisation parameters. By highlighting biocrust succession as the key determinant of soil Po mineralisation, the significance of precipitation and biocrust succession on phoD-harbouring communities, as well as the prevalence of community beta diversity in driving soil Po mineralisation, this study provides novel insights into the mechanisms underlying the transformation of Po into bioavailable P in biocrust systems.

Stronger effects of simultaneous warming and precipitation increase than the individual factor on soil bacterial community composition and assembly processes in an alpine grassland.

Composition and traits of soil microbial communities that closely related to their ecological functions received extensive attention in the context of climate changes. We investigated the responses of soil bacterial community structure, traits, and functional genes to the individual warming, precipitation increases, and the combination of warming and precipitation increases in an alpine grassland in the Qinghai-Tibet Plateau that is experiencing warming and wetting climate change. Soil properties, plant diversity and biomass were measured, and the ecological processes and environmental factors driving bacterial community changes were further explored. Results indicated that the Shannon diversity of soil bacterial communities decreased significantly only under the combination treatment, which might due to the decreased plant diversity. Soil bacterial community composition was significantly correlated with soil pH, and was affected obviously by the combination treatment. At the taxonomic classification, the relative abundance of Xanthobacteraceae and Beijerinckiaceae increased 127.67 and 107.62%, while the relative abundance of Rubrobacteriaceae and Micromonosporaceae decreased 78.29 and 54.72% under the combination treatment. Functional genes related to nitrogen and phosphorus transformation were enhanced in the combination treatment. Furthermore, weighted mean ribosomal operon copy numbers that positively correlated with plant aboveground biomass increased remarkably in the combination treatment, indicating a trend of life-history strategies shift from oligotrophic to copiotrophic. Stochastic processes dominated soil bacterial community, and the proportion of stochasticity increased under the combination treatment. Our study highlights the significant effects of simultaneous warming and precipitation increase on soil bacterial community.

A sand tank experimental study of distribution, migration and transformation mechanism of iron and phosphorus species under redox fluctuation in a simulated riparian zone

The coupling transformation of iron (Fe) and phosphorous (P) in riparian zones is significant to their biogeochemical cycle in the interaction between surface water and groundwater systems. However, the spatiotemporal distribution and biochemical transformation within the river bank media are not well elucidated in the literature. In this study, we used a sand tank experiment to investigate the migration and transformation of iron and phosphorus under redox fluctuation forced by the variability in river stage in the riparian zone. Results show that there is a significant positive correlation between the trend of iron/aluminum coupled phosphorus (Fe/Al coupled P) and amorphous total iron (TFe). Additionally, more attention should be paid to the effect of dissolved organic carbon (DOC) rather than dissolved oxygen on the redox condition in the underground environment. For example, with the organic carbon decreasing, the inverse crystalline transformation process of Fe species (i.e. conversion of high crystalline Fe species into the low ones) gradually changes to the crystallization process, resulting in the immobilization and release of the aqueous phosphorus (PO43–). Furthermore, due to the adsorption and oxidation process, the migration of aqueous iron (Fe2+) was seriously inhibited, while the distribution and transformation of solid Fe species was similar at different locations. On the other hand, PO43− was observed to accumulate in the transition area of the riparian zone, and was hard to migrate through the river regardless of the recharge or discharge relationship between the river and the groundwater. Overall, this study elucidates the distribution, migration and transformation mechanism of Fe and P in a typical fine sandy riparian zone, and first provides theoretical support for tracing and controlling the source of phosphorus pollution in riparian aquifers.

Effects of different remediation methods on phosphorus transformation and availability

The effects of different heavy metal pollution remediation methods on soil nutrient transformation and soil health remain unclear. In this study, the effects of phytoextraction (PE) and passivation remediation (PR) on Cd-polluted soil phosphorus transformation and availability were compared by pot experiment. The results showed that PE significantly reduced the concentrations of total and available Cd (both H2O-Cd and DTPA-Cd) in soil, PR also decreased available Cd content but had no significant effect on total Cd content. PE slightly increased soil pH and NH4+-N content, while PR significantly increased soil pH, NO3−-N and AK content. PE promoted the conversion of stable P (including HCl-Pi and residual-Pt), and increased the content of labile P (including H2O-Pi, NaHCO3-Pi and NaHCO3-Po) and the proportion of moderately labile P (including NaOH-Pi and NaOH-Po), while PR showed the opposite trend. PE showed a higher soil phoC gene abundance and acid phosphatase (ACP) activity, while PR showed a higher phoD gene copies and alkaline phosphatase (ALP) activity. Soil bacteria and phoD-harboring bacteria community was significantly affected by remediation methods and soil types. Compared with PR, PE reduced phoD-harboring bacterial diversity but significantly increased the abundance of genera associated with P dissolution (Streptomyces) and P conversion (Bradyrhizobium and Frankia), both of which were significantly positively correlated with labile P or moderately labile P. In general, compared with PR, PE can effectively remove soil Cd pollution, while maintaining a higher content of labile P and a higher proportion of moderately labile P, which can be considered as a green and sustainable remediation strategy conducive to soil quality.

Phosphorus availability and fractions responses to chemodiversity of exogenous dissolved organic matter in lime concretion black soil

AbstractThe chemodiversity of exogenous dissolved organic matter (DOM) is significantly increased during the composting process, yet its influence on soil phosphorus (P) transformation is uncertain. A 60‐day incubation experiment was conducted to assess the availability and fractions of P in lime concretion black soil in response to exogenous DOM input, with three treatments: control (CK), soil with DOM from 30 days decomposed straw (DOM1) and soil with DOM from 90 days decomposed straw (DOM2). The results demonstrated that the addition of DOM caused an increase in the content of available P in the soil, particularly the addition of DOM1 with more protein‐like substances and a lower degree of humification. Compared to the control, DOM1 and DOM2 treatments showed an increase in available P content of 17.3%–54.3% and 0.6%–18.2%, respectively. However, the response of the soil's physicochemical and biological properties varied considerably. DOM1 treatment caused a marked decrease in soil pH compared to DOM2 treatment, which then led to the release of Ca2+ and Mg2+ and improved the solubility of inactive P. Moreover, DOM1 treatment was found to significantly raise the content of microbial biomass P (MBP), particularly at 60 days. The Mantel test and random forest analysis revealed that the strongest correlation was between MBP and available P. Furthermore, DOM1 treatment had a stimulating effect on the growth of phosphate‐solubilizing microorganisms, including Arthrobacter, Penicillium and Mortierella. This investigation could provide insights into how the chemodiversity of exogenous DOM affects the activation of legacy‐P in soil, thus enhancing the soil P supply capacity in agricultural systems.

Land reclamation increased labile and moderately labile P fractions and strengthened co‐occurrence network ofgcdcommunity in calcareous soils

AbstractLand reclamation significantly impacts the transformation and availability of phosphorus (P) in soils. P‐solubilizing microorganisms (PSMs) play key roles in activating soil P. However, the responses of PSMs to the buildup of P under land reclamation and the relationship between different P fractions and PSMs have not been clearly clarified. Here, an investigation was carried out containing three treatments: (i) vegetable field (VF), (ii) cropland (CF), and (iii) uncultivated land (UL). Different P fractions were measured; the abundance of stable oxygen isotope in phosphate of D.HCl‐Pi was determined. The diversity, composition, and co‐occurrence network of P‐solubilizing glucose dehydrogenase community (gcd) were analyzed. Compared with the UL treatment, soil total P in the treatments of VF and CF was significantly increased by 1.5 and 1.2 times, respectively. The proportions of labile and moderately labile P in the treatments of VF and CF were 151% and 58% higher than in the UL treatment. The α‐diversity ofgcdcommunity (i.e., Shannon and Chao1 indices) in the VF and CF treatments was significantly greater than in the UL treatment. The change ingcdcommunity structure was closely associated with resin‐Pi, NaHCO3‐Pi, and NaOH‐Pi. The nodes and edges in the co‐occurrence networks of VF and CF treatments were higher than in the UL treatment. Overall, a substantial amount of P was accumulated after land reclamation, which was more pronounced by labile and moderately labile P than non‐labile P; the complexity and robustness ofgcdcommunity's co‐occurrence network were notably strengthened by land reclamation.

Influence of dissolved oxygen on phosphorus removal by polyphosphate-accumulating organisms biofilm: Performance and metabolic response

The present study focused on investigating the effect of dissolved oxygen (DO) concentration on phosphorus (P) transformation and metabolism in the polyphosphate-accumulating organisms (PAOs)-biofilm system. Two biofilm sequencing batch reactors (BSBRs) with high-DO (6 mg/L, BSBR-H) versus low-DO (2 mg/L, BSBR-L) concentrations were typically operated for 90 days. Collectively, the high DO concentration in aerobic phase would force PAOs to preferentially and rapidly perform phosphate metabolism and be beneficial for them to establish the dominant ecological niche. Clear differences in stoichiometry and kinetics indicated a glycolytic pathway in BSBR-L, while it shifted to polyphosphate hydrolysis in BSBR-H. The expression of functional genes suggested that the high-affinity P transport system was mainly responsible for P transformation in BSBR-H while the low-affinity P transport system was accountable for BSBR-L. Moreover, the BSBR-H showed relatively higher abundances of Pseudomonas while Tetrasphaera were the most dominant PAOs in BSBR-L. Dechloromonas contributed substantially to poly-P metabolism by occupying the largest proportion of ppk1/2 and ppx genes in both BSBRs.

Post-agricultural succession affects the accumulation and enzymatic transformation of organic phosphorus in a karst area, southwest China

Post-agricultural succession affects the accumulation and enzymatic transformation of organic phosphorus in a karst area, southwest China

Nutrient-cycling functional gene diversity mirrors phosphorus transformation during chicken manure composting

Elucidating ecological mechanism underlying phosphorus transformation mediated by phosphate-solubilizing bacteria (PSB) during manure composting is an important but rarely investigated subject. The research objective is to disentangle ecological functions of the inoculation of PSB Pseudomonas sp. WWJ-22 during chicken manure composting based on gene quantification and amplicon sequencing. There are large dynamic changes in phosphorus fractions, gene abundances, and bacterial community structure. The PSB addition notably increased available phosphorus from 0.29–0.89 g kg−1 to 0.49–1.39 g kg−1 and significantly affected phosphorus fractionation. The PSB inoculation significantly affected composition of nutrient-cycling functional genes (NCFGs), and notably influenced bacterial community composition and function. Compost bacteria showed significant phylogenetic signals in response to phosphorus fractions, and stochastic processes dominated bacterial community assembly. Results emphasized that PSB addition increased functional redundancy, phylogenetic conservatism, and stochasticity-dominated assembly of bacterial community. Overall, findings highlight NCFG diversity can be a bio-indicator to mirror phosphorus transformation.

Mesocosm experimental study on sustainable riparian restoration using sediment-modified planting eco-concrete

The continued deterioration of riparian ecosystems is a worldwide concern, which can lead to soil erosion, plant degradation, biodiversity loss, and water quality decline. Here, taking into account waste resource utilization and eco-environmental friendliness, the sediment-modified planting eco-concrete with both H. verticillata and T. orientalis (SEC-H&T) was prepared and explored for the first time to achieve sustainable riparian restoration. Concrete mechanical characterizations showed that the compressive strength and porosity of SEC with 30% sediment content could reach up to 15.8 MPa and 21.25%, respectively. The mechanical properties and the sediment utilization levels of SEC were appropriately balanced, and potentially toxic element leaching results verified the environmental safety of eco-concrete modified with dredged sediments. Plant physiological parameters of both aquatic plants (biomass, chlorophyll, protein and starch) were observed to reach the normal levels in SEC during the 30-day culture period, and T. orientalis seemed better adapted to SEC environment than H. verticillate. Importantly, compared to SEC-H and SEC-T, SEC-H&T could effectively reduce the concentrations of COD, TN and TP by 58.59%, 74.00% and 79.98% in water, respectively. Notably, water purification mechanisms by SEC-H&T were further elucidated from the perspective of microbial community responses. Shannon index of bacterial diversity and proliferation of specific populations dominating nutrient transformation (such as Bacillus and Nitrospira) was increased under the synergy of SEC and aquatic plants. Correspondingly, functional genes involved in nitrogen and phosphorus transformation (such as nosZ and phoU) were also enriched. Our study can not only showcase an effective and flexible approach to recycle dredged sediments into eco-concrete with low environment impacts, but also provide a promising alternative for sustainable riparian restoration.

Linking soil phosphorus availability and phosphatase functional genes to coastal marsh erosion: Implications for nutrient cycling and wetland restoration

Accelerated marsh erosion caused by climate change and human activity may have important implications for nutrient cycling and availability. However, how erosion affects phosphorus (P) transformation and microbial function in subtropical coastal marshes remains largely unknown. Here we assessed soil P fractions, availability and the phoD-harboring bacterial community along a marsh erosion gradient (non-eroded, lightly eroded, and heavily eroded). We showed that marsh erosion caused a shift in P fractions, leading to a decrease in P availability and a reduction in concentrations of labile P, moderately labile P, and stable P by 20 %, 9 %, and 17 % respectively. The abundance and diversity of phoD phosphatase genes decreased dramatically along the erosion gradient and were lower at heavily eroded sites than at non-eroded sites. Marsh erosion reshaped phoD gene community composition, and Corallococcus, Amycolatopsis, and Phaeobacter were identified as the dominant phoD-harboring microbes. Notably, marsh erosion reduced the complexity and stability of the phoD-harboring bacterial network, and heavily eroded sites have fewer network edges and nodes than non-eroded sites. The dynamics of soil P fractions, availability, and phoD-harboring bacterial communities driven by marsh erosion are largely shaped by substrate availability and soil properties (e.g., nutrients, pH, electrical conductivity, and moisture). Additionally, strong linkages between P availability and the abundance and diversity of phosphatase genes following erosion, suggest that phosphatase drives P mineralization and dissolution, and erosion weakens the regulation of P transformation by reshaping the phoD phosphatase gene community. Our findings indicate that marsh erosion alters soil P fractions and phoD-harboring bacterial communities, which weakens microbial regulation of P transformation and availability, thereby significantly reducing soil P pools and availability. Our findings broaden understanding of the impacts of coastal erosion on nutrient balance and ecosystem function, offering valuable perspectives that could inform wetland restoration and coastal management strategies.

Abundance of colloidal organic phosphorus in the Taiwan Strait

The Taiwan Strait is a coastal sea with dynamic hydrologic condition and dissolved organic matter (DOM) cycling. To better understand its phosphorus (P) transformation, phosphate and bulk dissolved organic phosphorus (DOP) concentrations were determined in the surface water of the Taiwan Strait during 2010 and 2011 summers. The bulk DOP was further fractionated into the low molecular weight (LMW) DOP and colloidal organic phosphorus (COP) with the application of the cross-flow ultrafiltration technique at a size cut-off of 10 kDa. The surface water in the Taiwan Strait generally showed higher bulk DOP concentration than phosphate excluding estuarine stations. This is similar to the surface waters in the subtropical gyres, implying a P-deficient condition in the Taiwan Strait. The LMW-DOP followed a constant permeation behavior in the ultrafiltration experiments, allowing the quantification of LMW-DOP and COP concentrations with the ultrafiltration permeation model. The average COP concentration was 0.044 ± 0.022 μmol/L and 0.042 ± 0.027 μmol/L during 2010 and 2011 summers, respectively. Accordingly, COP accounted for 35 ± 13% and 34 ± 16% of bulk DOP during 2010 and 2011 summers, respectively, suggesting a substantial occurrence of organic phosphorus in the colloidal fraction. The C/P ratio decreased from LMW-DOM to bulk DOM and to colloidal organic matter (COM), supporting the degradation pathway of bulk DOM from colloidal to LMW fractions and the lability of COP in the Taiwan Strait. The fractionation of C/P ratio between bulk DOM and COM in the Taiwan Strait, denoted by the fractionation factor (FC/D), was comparable to the open oceans, but significantly stronger than rivers and estuaries. It might indicate an autochthonous input and rapid turnover of bulk DOP pool in the Taiwan Strait in the context of low phosphate concentration. The FC/D also showed a spatial variability in the Taiwan Strait, high in the South China Sea Warm Current and upwelling occupied stations and low in inshore areas influenced by terrestrial input. This further supports the FC/D as an indicator of bulk DOM sources besides its application to decipher the diagenetic status of bulk DOM and biogeochemical cycles of bulk DOP in the coastal oceans.

Phosphorus transformation in acid soil as influenced by humic substance

The purpose of the study was to assess the influence of humic substance on the transformation of phosphorus in acid soil, so an experiment was conducted in a lab setting using the same soil (Typic Rhodustalf) collected from the land where the field experiment is expected (Agricultural and Horticultural Research Station, Tarikere taluk, Karnataka India). Humic substance employed in treatment application was extracted from farm yard manure. The study proven that, in contrast to P fertilizers applied without humic substance, application of humic substance at higher doses along with P fertilizers increased the available P status, saloid-P and organic-P fraction over the course of the incubation period, particularly when rock phosphate combined with humic substance @ 60 mg kg−1. However, compared to P fertilizers without humic substance treatments, significant decrease in in Al-P, Fe-P, reductant soluble P, occluded P and Ca-P was found in Rock phosphate + humic substance @ 60 mg kg−1 with lower P fixation (35.31%) and Rock phosphate + humic substance @ 40 mg kg−1 with P fixation of 37.32%, while the absolute control treatment (60.34%) had the maximum P fixation capacity.

Targeting phosphorus transformation to hydroxyapatite through sewage sludge pyrolysis boosted by quicklime toward phosphorus fertilizer alternative with toxic metals compromised

Advancing pyrolysis practice for phosphorus (P) recovery and recycling from sewage sludge arouses intensive research interests due to the forthcoming threat of global P sustainability. However, how to both in situ manipulate P targeted transformation to a designed form and accommodate downstream use while compromise adverse impacts of concomitant toxic metals (TMs) challenges the worldwide initiative of P reclamation from sewage sludge. Here, a P targeted transformation to hydroxyapatite (P2tHAP) technique to bridge the discrepancy of P recovery and subsequent use was developed. Through manipulating calcium-to-P molar ratio (Ca/P) of sludge feedstock by calcium oxide, the sludge P can be targetedly transformed to HAP during pyrolysis, a bioavailable form delivered as sewage sludge char-derived phosphorus fertilizer (SCPF). The P supply competency of the SCPF for Ca/P below or above 1.67 with a risk abatement of TMs was comparable to conventional P fertilizer, while the Ca/P = 1.67 was prone to the occurrence of sludge ash sintering, detrimental to the HAP formation. The lowered bioavailability of TMs was orchestrated by the root-secreted oxalic acid, due to the triggered phosphate starvation response. The random forest analysis indicated the driving force in the evolution of rhizobacterial communities, dominantly arising from those TMs after the SCPF soil application. Thermodynamic and kinetic calculations validated that HAP with the stable molar Gibbs energy gHAP (∼−7.23×103kJ/mol) underpinned the progress of P2tHAP during sludge pyrolysis. The outcome prospects to propel the integrative design of an innovative reclamation of P-rich wastes and simultaneous compromise of concomitant contaminants.

Changes in Soil Properties, Microbial Quantity and Enzyme Activities in Four Castanopsis hystrix Forest Types in Subtropical China.

It is well established that forest type can have a profound impact on soil physicochemical properties but the associated changes in soil microbial communities and the mechanisms by which soil quality is improved by various plantations are not fully understood. In this study, soil physicochemical properties and microbial and enzyme activities were investigated in four forest types-Castanopsis hystrix pure forests (CHPF), C. hystrix-Pinus elliottii mixed forests (CHPEF), C. hystrix-Michelia macclurei mixed forests (CHMMF), and C. hystrix-Mytilaria laosensis mixed forests (CHMLF) in the subtropical region of China. The purpose of this study was to assess the effects of afforestation types on characteristics of soil-its physical, chemical, and biological properties. The results showed that the contents of soil total organic carbon (TOC), soil total nitrogen (TN), microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN) were significantly improved in both CHMMF and CHMLF mixed forest stands when compared to the CHPF pure stand. Soil enzyme activities were enhanced in the mixed forests. In particular, high phosphatase activity was observed in CHMLF stands, leading to the transformation of soil phosphorus to available phosphorus in this forest type. Our study demonstrated that the broad-leaved mixed forests, but not coniferous mixed forests, could significantly improve soil quality in the study region. Our research provides a scientific insight into the promotion of vegetation restoration and plantation forest management in plantation regions of subtropical areas.

Incorporating biotic phosphorus-acquisition strategies into soil phosphorus transformation under long-term salinization in a tidal wetland

Tidal wetland plants can maintain high primary productivity under salinization, even with low phosphorus (P) availability. However, little is known about how they adapt to salinization-induced ecosystem P limitation over time. We established mesocosms using tidal freshwater wetland soils and the salt-tolerant plant Cyperus malaccensis and subjected them to short-term (6 months) and long-term (3.5 years) salinization. Overall, short-term salinization did not change plant or microbial biomass nitrogen/P ratios, whereas long-term salinization increased both, indicating that short-term salinization did not alter ecosystem P limitation, but long-term salinization exacerbated it. Concurrently, short-term salinization reduced the moderately labile inorganic P (Pi) pool, whereas long-term salinization also reduced the hydrolyzable organic P (Po) and primary mineral P pools. During both short- and long-term salinization, moderately labile Pi mobilization exhibited a negative correlation with Fe sulfide concentration and a positive correlation with Fe(III) concentrations. These results suggested that both short- and long-term salinization obtained P through abiotic P-acquisition strategies. Mobilization of the primary mineral P pool and hydrolyzable Po pools was negatively linked to root arbuscular mycorrhizal fungi (AMF) biomass and soil alkaline phosphomonoesterase (ALP) activity, respectively. This indicated that long-term salinization acquired P via biotic P-acquisition strategies. Specifically, soil microorganisms increased fungi predominance and thereby increased ALP activity to convert hydrolyzable Po, while tidal wetland plants enhanced root AMF associations to release carboxylate to transform primary mineral P. Overall, our results highlight that abiotic P-acquisition strategies could offset the ecosystem P limitation during short-term salinization, whereas biotic P-acquisition strategies play a more important role in soil P transformation under long-term salinization. Through biotic P-acquisition, tidal wetland ecosystems can maintain high plant primary productivity through biotic P-acquisition even under P-limited conditions, exhibiting increased resilience to sea-level rise.