Shuijin Hu

Lead immobilization by geological fluorapatite and fungus Aspergillus niger

Phosphate solubilizing fungi have high ability to secrete organic acids. In this study, fungus Aspergillus niger and geological fluorapatite were applied in lead remediation in aqueous solution. Formation and morphology of the lead minerals, e.g., pyromorphite and lead oxalate, were investigated by SEM, XRD, and ATR-IR. The total quantity of organic acids reached the maximum at the sixth day, which improved the concentration of soluble P up to ∼370mg/L from ∼0.4mg/L. The organic acids, especially the oxalic acid, enhance the solubility of fluorapatite significantly. The stable fluoropyromorphite [Pb5(PO4)3F] is precipitated with the elevated solubility of fluorapatite in the acidic environment. Furthermore, A. niger grows normally with the presence of lead cations. It is shown that >99% lead cations can be removed from the solution. However, immobilization caused by the precipitation of lead oxalate cannot be ignored if the fungus A. niger was cultured in the Pb solution. This study elucidates the mechanisms of lead immobilization by FAp and A. niger, and sheds its perspective in lead remediation, especially for high Pb concentration solution.

A study of organic acid production in contrasts between two phosphate solubilizing fungi: Penicillium oxalicum and Aspergillus niger.

Phosphate solubilizing fungi (PSF) have huge potentials in enhancing release of phosphorus from fertilizer. Two PSF (NJDL-03 and NJDL-12) were isolated and identified as Penicillium oxalicum and Aspergillus niger respectively in this study. The quantification and identification of organic acids were performed by HPLC. Total concentrations of organic acids secreted by NJDL-03 and NJDL-12 are ~4000 and ~10,000u2009mg/L with pH values of 3.6 and 2.4 respectively after five-days culture. Oxalic acid dominates acidity in the medium due to its high concentration and high acidity constant. The two fungi were also cultured for five days with the initial pH values of the medium varied from 6.5 to 1.5. The biomass reached the maximum when the initial pH values are 4.5 for NJDL-03 and 2.5 for NJDL-12. The organic acids for NJDL-12 reach the maximum at the initial pHu2009=u20095.5. However, the acids by NJDL-03 continue to decrease and proliferation of the fungus terminates at pHu2009=u20092.5. The citric acid production increases significantly for NJDL-12 at acidic environment, whereas formic and oxalic acids decrease sharply for both two fungi. This study shows that NJDL-12 has higher ability in acid production and has stronger adaptability to acidic environment than NJDL-03.

Mechanisms of biochar assisted immobilization of Pb2+ by bioapatite in aqueous solution

Bioapatite (BAp) is regarded as an effective material to immobilize lead (Pb2+) via the formation of stable pyromorphite. However, when applied in contaminated soil, due to its low surface area and low adsorption capacity, BAp might not sufficiently contact and react with Pb2+. Biochar, a carbon storage material, typically has high surface area and high adsorption capacity. This study investigated the feasibility of using biochar as a reaction platform to enhance BAp immobilization of Pb2+. An alkaline biochar produced from wheat straw pellets (WSP) and a slightly acidic biochar produced from hardwood (SB) were selected. The results of aqueous adsorption showed the combination of biochar (WSP or SB) and BAp effectively removed Pb2+ from the aqueous solution containing 1000xa0ppm Pb2+. XRD, ATR-IR, and SEM/EDX results revealed the formation of hydroxypyromorphite on both biochars surfaces. This study demonstrates that biochars could act as an efficient reaction platform for BAp and Pb2+ in aqueous solution due to their high surface area, porous structure, and high adsorption capacity. Therefore, it is mechanistically feasible to apply biochar to enhance BAp immobilization of Pb2+ in contaminated soil.

Taxonomic resolution is a determinant of biodiversity effects in arbuscular mycorrhizal fungal communities

Summary nArbuscular mycorrhizal fungi (AMF) are key regulators of ecosystem processes, yet how their biodiversity works in ecosystems remains poorly understood. nWe documented the extent to which taxonomic resolution influenced the effect of biodiversity of AMF taxa on plant performance (growth, nutrient uptake and stress tolerance) in a meta-analysis of 902 articles. nWe found that the effect of biodiversity of AMF taxa depended on taxonomic resolution. Plant performance was positively promoted by AMF family richness, while no effect was found for fungal species richness. In addition, negative effect was found between AMF phylogenetic diversity and plant growth. This pattern can be explained by functional conservatism within AMF families and functional differentiation among AMF families. nSynthesis. Conservation of AMF communities to maintain a full complement of ecosystem functions requires the presence of diverse families and not simply diverse species within a family. This finding may be of key importance for the function of ecosystems under various environmental perturbations to which AMF families may respond differently.

Optimizing rice plant photosynthate allocation reduces N2O emissions from paddy fields

Rice paddies are a major source of anthropogenic nitrous oxide (N2O) emissions, especially under alternate wetting-drying irrigation and high N input. Increasing photosynthate allocation to the grain in rice (Oryza sativa L.) has been identified as an effective strategy of genetic and agronomic innovation for yield enhancement; however, its impacts on N2O emissions are still unknown. We conducted three independent but complementary experiments (variety, mutant study, and spikelet clipping) to examine the impacts of rice plant photosynthate allocation on paddy N2O emissions. The three experiments showed that N2O fluxes were significantly and negatively correlated with the ratio of grain yield to total aboveground biomass, known as the harvest index (HI) in agronomy (Pu2009<u20090.01). Biomass accumulation and N uptake after anthesis were significantly and positively correlated with HI (Pu2009<u20090.05). Reducing photosynthate allocation to the grain by spikelet clipping significantly increased white root biomass and soil dissolved organic C and reduced plant N uptake, resulting in high soil denitrification potential (Pu2009<u20090.05). Our findings demonstrate that optimizing photosynthate allocation to the grain can reduce paddy N2O emissions through decreasing belowground C input and increasing plant N uptake, suggesting the potential for genetic and agronomic efforts to produce more rice with less N2O emissions.

Nitrogen:phosphorous supply ratio and allometry in five alpine plant species

Abstract In terrestrial ecosystems, atmospheric nitrogen (N) deposition has greatly increased N availability relative to other elements, particularly phosphorus (P). Alterations in the availability of N relative to P can affect plant growth rate and functional traits, as well as resource allocation to above‐ versus belowground biomass (M A and M B). Biomass allocation among individual plants is broadly size‐dependent, and this can often be described as an allometric relationship between M A and M B, as represented by the equation MA=αMBβ, or log M A = logα + βlog M B. Here, we investigated whether the scaling exponent or regression slope may be affected by the N:P supply ratio. We hypothesized that the regression slope between M A and M B should be steeper under a high N:P supply ratio due to P limitation, and shallower under a low N:P supply ratio due to N limitation. To test these hypotheses, we experimentally altered the levels of N, P, and the N:P supply ratio (from 1.7:1 to 135:1) provided to five alpine species representing two functional groups (grasses and composite forbs) under greenhouse conditions; we then measured the effects of these treatments on plant morphology and tissue content (SLA, leaf area, and leaf and root N/P concentrations) and on the scaling relationship between M A and M B. Unbalanced N:P supply ratios generally negatively affected plant biomass, leaf area, and tissue nutrient concentration in both grasses and composite forbs. High N:P ratios increased tissue N:P ratios in both functional groups, but more in the two composite forbs than in the grasses. The positive regression slopes between log M A and log M B exhibited by plants raised under a N:P supply ratio of 135:1 were significantly steeper than those observed under the N:P ratio of 1.7:1 and 15:1. Synthesis: Plant biomass allocation is highly plastic in response to variation in the N:P supply ratio. Studies of resource allocation of individual plants should focus on the effects of nutrient ratios as well as the availability of individual elements. The two forb species were more sensitive than grasses to unbalanced N:P supplies. To evaluate the adaptive significance of this plasticity, the effects of unbalanced N:P supply ratio on individual lifetime fitness must be measured.

Irrigation and weed control alter soil microbiology and nutrient availability in North Carolina Sandhill peach orchards

Orchard management practices such as weed control and irrigation are primarily aimed at maximizing fruit yields and economic profits. However, the impact of these practices on soil fertility and soil microbiology is often overlooked. We conducted a two-factor experimental manipulation of weed control by herbicide and trickle irrigation in a nutrient-poor peach (Prunus persica L. cv. Contender) orchard near Jackson Springs, North Carolina. After three and eight years of treatments, an array of soil fertility parameters were examined, including soil pH, soil N, P and cation nutrients, microbial biomass and respiration, N mineralization, and presence of arbuscular mycorrhizal fungi (AMF). Three general trends emerged: 1) irrigation significantly increased soil microbial biomass and activity, 2) infection rate of mycorrhizal fungi within roots were significantly higher under irrigation than non-irrigation treatments, but no significant difference in the AMF community composition was detected among treatments, 3) weed control through herbicides reduced soil organic matter, microbial biomass and activity, and mineral nutrients, but had no significant impacts on root mycorrhizal infection and AMF communities. Weed-control treatments directly decreased availability of soil nutrients in year 8, especially soil extractable inorganic N. Weed control also appears to have altered the soil nutrients via changes in soil microbes and altered net N mineralization via changes in soil microbial biomass and activity. These results indicate that long-term weed control using herbicides reduces soil fertility through reducing organic C inputs, nutrient retention and soil microbes. Together, these findings highlight the need for alternative practices such as winter legume cover cropping that maintain and/or enhance organic inputs to sustain the soil fertility.

Long-term N and P additions alter the scaling of plant nitrogen to phosphorus in a Tibetan alpine meadow

Nitrogen and phosphorus are two important nutrient elements for plants. The current paradigm suggests that the scaling of plant tissue N to P is conserved across environments and plant taxa because these two elements are coupled and coordinately change with each other following a constant allometric trajectory. However, this assumption has not been vigorously examined, particularly in changing N and P environments. We propose that changes in relative availability of N and P in soil alter the N to P relationship in plants. Taking advantage of a 4-yr N and P addition experiment in a Tibetan alpine meadow, we examined changes in plant N and P concentrations of 14 common species. Our results showed that while the scaling of N to P under N additions was similar to the previously reported pattern with a uniform 2/3 slope of the regression between log N and log P, it was significantly different under P additions with a smaller slope. Also, graminoids had different responses from forbs. These results indicate that the relative availability of soil N and P is an important determinant regulating the N and P concentrations in plants. These findings suggest that alterations in the N to P relationships may not only alter plant photosynthate allocation to vegetative or reproductive organs, but also regulate the metabolic and growth rate of plant and promote shifts in plant community composition in a changing nutrient loading environment.

Effects of elevated atmospheric CO2 on dissolution of geological fluorapatite in water and soil

Most of phosphorus (P) is present as insoluble phosphorus-bearing minerals or organic forms in soil. Geological fluorapatite (FAp) is the dominant mineral-weathering source of P. In this study, FAp was added into water and soil under elevated CO2 to investigate the pathway of P release. Two types of soils (an acidic soil from subtropical China and a saline-alkali soil from Tibet Plateau, China) with similar total P content were studied. In the solution, increased CO2 in air enhanced the dissolution of FAp, i.e., from 0.04 to 1.18ppm for P and from 2.48 to 13.61ppm for Ca. In addition, release of Ca and P from FAp reached the maximum (2.14ppm for P and 13.84ppm for Ca) under the combination of elevated CO2 and NaCl due to the increasing ion exchange. Consistent with the results from the solution, CO2 elevation promoted P release more significantly (triple) in the saline-alkali soil than in the acidic soil. Therefore, saline-alkali soils in Tibet Plateau would be an important reservoir of available P under the global CO2 rise. This study sheds the light on understanding the geological cycle of phosphorus.

Reconciling multiple impacts of nitrogen enrichment on soil carbon: plant, microbial and geochemical controls

Impacts of reactive nitrogen (N) inputs on ecosystem carbon (C) dynamics are highly variable, and the underlying mechanisms remain unclear. Here, we proposed a new conceptual framework that integrates plant, microbial and geochemical mechanisms to reconcile diverse and contrasting impacts of N on soil C. This framework was tested using long-term N enrichment and acid addition experiments in a Mongolian steppe grassland. Distinct mechanisms could explain effects of N on particulate and mineral-associated soil C pools, potentially explaining discrepancies among previous N addition studies. While plant production predominated particulate C changes, N-induced soil acidification strongly affected mineral-associated C through decreased microbial growth and pH-sensitive associations between iron and aluminium minerals and C. Our findings suggest that effects of N-induced acidification on microbial respiration and geochemical properties should be included in Earth system models that predict ecosystem C budgets under future N deposition/input scenarios.