Microorganisms play an important role in soil phosphorus (P) cycling and regulation of P availability in agroecosystems. However, the responses of the functional and ecological traits of ...P-transformation microorganisms to long-term nutrient inputs are largely unknown. This study used metagenomics to investigate changes in the relative abundance of microbial P-transformation genes at four long-term experimental sites that received various inputs of N and P nutrients (up to 39 years). Long-term P input increased microbial P immobilization by decreasing the relative abundance of the P-starvation response gene (phoR) and increasing that of the low-affinity inorganic phosphate transporter gene (pit). This contrasts with previous findings that low-P conditions facilitate P immobilization in culturable microorganisms in short-term studies. In comparison, long-term nitrogen (N) input significantly decreased soil pH, and consequently decreased the relative abundances of total microbial P-solubilizing genes and the abundances of Actinobacteria, Gammaproteobacteria, and Alphaproteobacteria containing genes coding for alkaline phosphatase, and weakened the connection of relevant key genes. This challenges the concept that microbial P-solubilization capacity is mainly regulated by N:P stoichiometry. It is concluded that long-term N inputs decreased microbial P-solubilizing and mineralizing capacity while P inputs favored microbial immobilization via altering the microbial functional profiles, providing a novel insight into the regulation of P cycling in sustainable agroecosystems from a microbial perspective.
A comprehensive understanding of the rhizosphere priming effect (RPE) on the decomposition of soil organic carbon (SOC) requires an integration of many factors. It is unclear how N form-induced ...change in soil pH affects the RPE and SOC sequestration.
This study compared the change in the RPE under supply of NO3-N and NH4-N. The effect of the RPE on the mineralization of soil N and hence its availability to plant and microbes was also examined using a 15N-labelled N source.
The supply of NH4-N decreased rhizosphere pH by 0.16-0.38 units, and resulted in a decreased or negative RPE. In contrast, NO3-N nutrition increased rhizosphere pH by 0.19-0.78 units, and led to a persistently positive RPE. The amounts of rhizosphere-primed C were positively correlated with rhizosphere pH. Rhizosphere pH affected the RPE mainly through influencing microbial biomass, activity and utilization of root exudates, and the availability of SOC to microbes. Furthermore, the amount of rhizosphere primed C correlated negatively with microbial biomass atom% 15N (R2 0.77-0.98, n = 12), suggesting that microbes in the rhizosphere acted as the immediate sink for N released from enhanced SOC decomposition via the RPE.
N form was an important factor affecting the magnitude and direction of the RPE via its effect on rhizosphere pH. Rhizosphere pH needs to be considered in SOC and RPE modelling.
Elevated atmospheric CO2 (eCO2) concentration can increase root exudation into soils, which improves plant tolerance to abiotic stresses. This review used a meta-analysis to assess effect sizes of ...eCO2 on both efflux rates and total amounts of some specific root exudates, and dissected whether eCO2 enhances plant's resistance to nutrient deficiency and ion toxicity via root exudates. Elevated CO2 did not affect efflux rates of total dissolved organic carbon, a measure of combined root exudates per unit of root biomass or length, but increased the efflux amount of root systems per plant by 31% which is likely attributed to increased root biomass (29%). Elevated CO2 increased efflux rates of soluble-sugars, carboxylates, and citrate by 47%, 111%, and 16%, respectively, but did not affect those of amino acids and malate. The increased carbon allocation to roots, increased plant requirements of mineral nutrients, and heightened detoxification responses to toxic ions under eCO2 collectively contribute to the increased efflux rates despite lacking molecular evidence. The increased efflux rates of root exudates under eCO2 were closely associated with improved nutrient uptake whilst less studies have validated the associations between root exudates and resistance to toxic ions of plants when grown under eCO2. Future studies are required to reveal how climate change (eCO2) affect the efflux of specific root exudates, particularly organic anions, the corresponding nutrient uptake and toxic ion resistance from plant molecular biology and soil microbial ecology perspectives.
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•Elevated CO2 (eCO2) increases total efflux of root exudates from root systems.•Elevated CO2 increases efflux rates of soluble sugars, carboxylates and citrate.•Increased root exudation enhances nutrient uptake under eCO2.•Contributions of root exudates to toxic-ion resistance under eCO2 need further validation.
In rotations, cereals after legumes often have higher P uptake than after cereals, and it has been suggested that legume residues may play an important role in this effect by mobilising soil P in the ...legume phase and by the P added with the residues. However, little is known about the changes in P pools during legume residue decomposition. Residues from faba bean, white lupin or chickpea (shoots or roots) with varying P concentrations were added to a loamy sand soil with a low available P concentration, and the concentration of various soil P pools were assessed by soil P fractionation on days 0, 14, 28 and 56. Residue addition significantly increased cumulative respiration which was positively correlated with amount of C added with residues (r = 0.54, p < 0.05), and negatively correlated with the C/P ratio (r = −0.58, p < 0.05). The size of the P pools changed over time and was affected by both residue P concentration and plant part (root or shoot). In the first two weeks, microbial P increased and resin P and NaHCO3–Pi decreased with low-P residues (0.6–1.8 mg kg−1) while the reverse was true for high-P residues (6.5–8.3 mg kg−1). In medium-P residues (2.9–3.3 mg kg−1), there was a balance between mineralisation and immobilisation. Decreases of NaOH–Po occurred earlier with low-P and medium-P residues (d0 to d14) than with high-P residues (d14–d28). The increase in residual P with all residues indicated that part of mineralised P was converted into stable P within 14d; but later (d28–d56), the concentration of residual P strongly decreased. In the period from d28 to d56, there was an increase in NaOH–Po and HCl–P with all residues, indicating net conversion of P into stable organic and inorganic P. Changes in P pools between roots and shoots occurred mainly in the initial phase. The concentration of NaOH–Po increased from d0 to d14, but then decreased from d14 to d28 with addition of shoot residues whereas the reverse was found with roots. These changes were generally more pronounced in low-P than in medium-P residues. This study demonstrates that changes and transformations in soil P pools over time were dependant on residue P concentration and plant part.
► Legume residues with varying P concentrations were added to a sandy loam soil. ► P dynamics over 56 days were dependant on residue P concentration and plant part. ► Differences among residues on soil P pools were greatest in the first 14 days. ► Roots and shoots had opposite effects on P transformations. ► Part of mineralised P was converted into stable P within 14 days.
Heavy-metal contamination in soil has long been a persistent challenge and the utilization of agricultural waste for in-situ stabilization remediation presents a promising approach to tackle this ...problem. Agricultural wastes exhibit promising potential in the remediation of contaminated land and modification could improve the adsorption performance markedly. Citric acid and Fe3O4 treated sugarcane bagasse adsorbed more heavy metals than raw materials in the aqueous system, employing these materials for heavy metal remediation in soil holds significant implications for broadening the raw material source of passivators and enhancing waste utilization efficiency. In this paper, a 120-day soil incubation study was conducted to compare the effects of pristine sugarcane bagasse (SB), citric-acid modified (SSB1, SSB2 and SSB3 with increasing proportion of citric acid) and citric-acid/Fe3O4 modified (MSB1, MSB4 and MSB7 with increasing proportion of Fe3O4) sugarcane bagasse at 1 % addition rate on cadmium (Cd) and copper (Cu) passivation. The SB, SSB1 and MSB1 did not always decrease the content of CaCl2-extractable Cd while all the seven amendments decreased the CaCl2-extractable Cu during the experiment period. Among all materials, SSB3 and MSB7 exhibited the highest efficiency in reducing the concentrations of CaCl2-extractable Cd and Cu. At Day 120, SB, SSB3 and MSB7 reduced the content of CaCl2-extractable Cd by 8 %, 18 % and 24 %, and of CaCl2-extractable Cu by 25 %, 50 % and 61 %, respectively. The efficiency of Cd and Cu immobilization was associated positively with the pH, functional groups and H-bonds of the amendments. The results suggest that the efficiency of sugarcane bagasse in heavy-metal passivation can be largely enhanced through chemical modifications using high proportions of citric acid and Fe3O4.
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•Novel sugarcane bagasse (SB)-based amendments immobilize Cd and Cu in soil.•The amendments are more effective to immobilize Cu than Cd.•SSB3 and MSB7 exhibit the best performance in passivating both Cd and Cu.•CaCl2-extractable Cd is the main component of acid-soluble Cd.•Cd and Cu immobilization results from high pH, functional groups and H-bonding.
Soil pH and the availability of carbon (C) substrate and nutrients to microorganisms are well recognized to influence C priming. However, the mechanisms underpinning such interplay so far remain ...elusive. Given that liming acid soils, residue retention and fertilization are common agricultural practices, small changes in SOC content by these practices could have a big impact on the global C budget. This study aimed to gain insight into the impact of initial pH and mineral N availability on the priming effect of two C substrates with contrasting biodegradability. Stable 13C-labelled substrates, glucose and lignocellulose, were applied at the rate of 0.5 mg C g−1 soil with or without NH4NO3 to the same soil matrix with three different initial pH levels; pH 4.1 (strongly acidic), 4.7 (moderately acidic) and 6.6 (slightly acidic). The N treatment was based on a C:N of 10 of the added substrate (0.05 mg N g−1 soil) to ensure N was non-limiting. Interestingly, the priming effect was not linearly related to soil pH; greatest at pH 4.1, followed by pH 6.6 and lowest at pH 4.7. The greater net increase in microbial biomass upon C supply in strongly acidic soils compared to the moderately and slightly acidic soils would have enhanced co-metabolic decomposition of native soil organic C (SOC). The cumulative amount of primed SOC during the 30-day incubation period was greater in glucose- (21 μg C g−1) than lignocellulose-amended soils (13 μg C g−1). Nitrogen application reduced the C priming effect of both C substrates at all pH levels. This reduction was more prominent with lignocellulose and in the moderately acidic soils. The results suggest that maintaining optimal soil pH for nutrient availability and N application that exceeds the microbial N requirements in agricultural fields may minimize SOC loss via the priming effect in the short term.
•The carbon priming effect was lowest in the moderately acidic soils.•Glucose amendment induced greater priming effect than lignocellulose.•Nitrogen decreased priming more with lignocellulose than glucose.•Reduced priming by nitrogen was greatest in the moderately acidic soils.
Background and aims
Crop species differ in phosphorus (P) acquisition in P-limiting environments. However, it is not fully understood how elevated atmospheric CO
2
concentrations affects these P ...acquisition mechanisms and the plant's ability to acquire P from soil. This study aimed to investigate the effect of elevated CO
2
on P acquisition in crop species with contrasting P acquisition mechanisms.
Methods
White lupin, faba bean, canola and near-isogenic wheat lines with and without citrate efflux were grown for 70 days in a P-deficient Chromosol soil under ambient (400 ppm) and elevated (800 ppm) CO
2
. Plant P uptake and P transformation in the rhizosphere were determined.
Results
Elevated CO
2
promoted total P uptake in white lupin and canola by 84% and 48%, respectively, and decreased the P uptake in the non-citrate-exuding wheat (by 24%) but not the exuding wheat. In white lupin, elevated CO
2
enhanced phosphatase activity and depletion of organic P in the rhizosphere. Elevated CO
2
increased P uptake by increasing root length which allowed canola to exploit a greater volume of soil for P. In the rhizosphere of faba bean, NaOH-extractable inorganic P was greater under elevated CO
2
.
Conclusion
Crops which rely on organic acid exudation and phosphatases appear to be better adapted to acquiring P under elevated CO
2
.
Effects of rhizosphere properties on the rhizosphere priming effect (RPE) are unknown. This study aimed to link species variation in RPE with plant traits and rhizosphere properties.
Four C3 species ...(chickpea, Cicer arietinum; field pea, Pisum sativum; wheat, Triticum aestivum; and white lupin, Lupinus albus) differing in soil acidification and root exudation, were grown in a C4 soil. The CO2 released from soil was trapped using a newly developed NaOH-trapping system.
White lupin and wheat showed greater positive RPEs, in contrast to the negative RPE produced by chickpea. The greatest RPE of white lupin was in line with its capacity to release root exudates, whereas the negative RPE of chickpea was attributed to its great ability to acidify rhizosphere soil. The enhanced RPE of field pea at maturity might result from high nitrogen deposition and release of structural root carbon components following root senescence. Root biomass and length played a minor role in the species variation in RPE.
Rhizosphere acidification was shown to be an important factor affecting the magnitude and direction of RPE. Future studies on RPE modelling and mechanistic understanding of the processes that regulate RPE should consider the effect of rhizosphere pH.
Nitrification and denitrification are two important processes in the nitrogen (N) cycle. Under heavy-metal pollution with water management of paddy soils, these two processes are not well understood. ...This study aimed to examine the effect of cadmium (Cd) on N transformation under flooding and non-flooding conditions. A paddy soil was incubated under two water regimes (flooding and non-flooding) and four Cd levels (0, 2, 5 and 10 mg kg−1). The availability of Cd was higher in the non-flooding than flooding conditions. Cadmium contamination significantly (p ≤ 0.05) decreased the copy number of archaeal and bacterial amoA genes, bacterial nirS, nirK and nosZ genes under both conditions with the decrease being greater under non-flooding. High level of Cd (10 mg kg−1) was more toxic in non-flooding than flooding conditions to the nitrifiers and denitrifiers, which in turn decreased N transformation through microbially-mediated processes. Its contamination decreased N2O emission initially under both water regimes but the effect was greater under the non-flooding condition. However, the non-significant stimulatory effect of Cd on N2O emission was observed during the late phase. The microbial community structure was changed with time and water regimes. Irrespective of water regime, the dominated fungal phyla were Ascomycota and Basidiomycota while the dominated bacteria phyla were Actinobacteria, Proteobacteria, Firmicutes and Acidobacteria. In summary, water regimes and Cd bioavailability changed soil N transformations via microbial mediated processes.
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•Cadmium decreased the abundance of nitrifiers and denitrifiers in the paddy soil.•Cd contamination decreased N2O emission initially under both water regimes.•The dominated fungal phyla were Ascomycota and Basidiomycota.•Bacterial but not fungal community composition changed by water management practices.•N transformation decreased in Cd polluted soil more in non-flooding than flooding.
Aims
The utilisation of on-farm crop residues to ameliorate soil acidity is thought to be more cost-effective than other organic materials such as animal manures. In addition, using NO
3
−
as the ...form of N can induce rhizosphere alkalinisation due to the excess uptake of anions over cations by plant roots.
Methods
A pot experiment was conducted to evaluate the effectiveness of four commonly-used brown manures, field pea, oats, vetch and wheat in combination with two Ca(NO
3
)
2
levels (64 and 191 mg N kg
− 1
) in improving wheat growth in two acid soils varying in Al
3+
concentration and pH buffer capacity.
Results
All amendments increased plant growth and soil pH, and decreased Al concentration in Sodosol (pH buffer capacity, 23 mmol
c
kg
− 1
pH
− 1
), with legume residues (field pea and vetch) being more effective than cereal residues (oat and wheat). Application of Ca(NO
3
)
2
alone was less effective in ameliorating soil acidity in both Sodosol and Dermosol due to poor root growth and hence lower NO
3
−
-N uptake (< 18 %). However, higher rates of Ca(NO
3
)
2
further increased pH (by 0.12 units) in Sodosol when combined with wheat and oat residues with decreased Al
3+
, increased root growth and NO
3
−
-N uptake.
Conclusions
The capacity of Ca(NO
3
)
2
to ameliorate soil acidity was reduced with legume residues, when it was not the main N source for plant growth. The combined application of Ca(NO
3
)
2
and low-N crop residue (C/N > 52) could act as an alternative to costly lime or off-farm products in ameliorating soil acidity.
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