Biology and Fertility of Soils最新文献

Conventional agriculture has been suggested to promote less mutualistic arbuscular mycorrhizal fungi (AMF). The main aim of this study was to test this assumption by a detailed functional analysis of the plant mycorrhizal benefits and costs. A cross-inoculation experiment was established with Plantago lanceolata as a host plant and inocula of AMF sourced from four pairs of conventionally managed arable fields and neighbouring grasslands. Mycorrhizal effects were determined for a range of plant parameters including fluxes of isotopically labelled phosphorus (P), nitrogen (N) and carbon (C), and related to root colonization and composition of the different AMF communities.

The association of P. lanceolata with arable-field inocula was less beneficial in terms of plant growth promotion and it also led to more pronounced P accumulation in plant biomass, as compared to grassland inocula. Furthermore, arable-field AMF increased ¹⁵N depletion in soil and ¹⁵N transfer to shoots, and induced higher ¹³C drain to soil. These differential functional parameters were related to consistent compositional differences between arable-soil and grassland AMF communities in the roots. Differential effects of the AMF inocula on N and C partitioning in the soil–plant system suggest faster foraging for nutrients by arable-soil AMF and higher demand for C, which are characteristics associated with ruderal AMF. This implies that arable-soil AMF may be less beneficial in conditions of plant growth limitation by C than the grassland AMF.

Denitrification, the microbial process (and its subprocesses) of reducing nitrogenous oxides to gaseous nitrogen, is usually modelled using the relevant scale, i.e. microscopic, laboratory, field, or landscape scale. It is shown that a newly developed model can simulate several experiments with a denitrifying strain of bacteria at the microscopic scale with different initial oxygen and nitrate concentrations all at once. It is shown that for this, a new approach for the onset of denitrification is needed. It will then be investigated whether the model can be transferred from the microscopic scale to the laboratory scale to simulate an experimental setup with sintered glass beads that mimic hot spots in the soil. For this, the reaction from the batch experiment model is not changed, but diffusion of the components is added. While the spatially resolved model seems to incorporate the spatial structure correctly, shown by the good agreement between simulation and experiment under purely oxic conditions, there is a structural mismatch between the simulation and the experiments with denitrification.

Subtropical forests are significant contributors to N₂O emissions with consequences for climate regulation. Biochar application has emerged as a promising strategy to mitigate soil N₂O emissions, yet its effects and the underlying mechanisms under nitrogen (N) deposition in subtropical forests remain poorly understood. A comprehensive 3-year field study within a subtropical forest reveals that N deposition led to a significant increase in soil N₂O emissions by 14.6–25.1% annually. However, biochar application resulted in a substantial reduction of these emissions, ranging from 8.0–20.8% each year. Notably, the mitigation effect of biochar was particularly pronounced when N deposition was occurring, leading to an even greater reduction in N₂O emissions by 14.2–22.0% annually. This mitigation effect is attributed to biochar’s capacity to lower the nitrification and denitrification rates of soil via reducing levels of ammonium N and water-soluble organic N. Additionally, biochar decreased the abundance of critical microbial genes, including AOAamoA, nirK and nirS, and reduced the activity of key enzymes such as nitrate and nitrite reductase. These findings highlight the potential of straw biochar to effectively mitigate soil N₂O emissions in subtropical forests experiencing N deposition, offering important insights for supporting ecosystem sustainability under global climate change.

The special issue summarises and highlights key findings of the research unit DASIM funded by the German Research Foundation (DFG) on the process of denitrification. Progress was made in several areas including the development of new and advanced methods to quantify N₂ fluxes such as a new ¹⁵N gas flux method, enhanced Raman spectroscopy and a new incubation system to study plant-soil interactions in He-O₂ atmosphere. Understanding of denitrification in disturbed and structured soil was gained by combining X-ray CT scanning and microbial ecology methods. High resolution models developed as part of DASIM were able to successfully simulate experimental data and provide valuable insights for the improvement of existing ecosystem models. Improved ¹⁵N tracing tools for the analysis of ¹⁵N tracing data in soil-plant systems have been developed that are extensively used by associated partners. DASIM brought together an interdisciplinary network of researchers interested in analytical but also modelling aspects. This includes close collaboration with the FAO/IAEA centre of Nuclear Techniques in Food and Agriculture of the United Nations which resulted in an open access book that describes the methods used in DASIM. The impact of the DASIM research unit on the scientific community is manifold and will most likely have a lasting impact on the understanding of nitrogen cycling in terrestrial ecosystems.

Soil microbial nitrogen (N) immobilizations are important processes of biogeochemical cycles. How the soil N immobilizations change with increasing N inputs, especially in the subsoil, is not clear. Based on a long-term field manipulative experiment in an alpine meadow, we evaluated changes of soil gross NH₄⁺ immobilization rate (GAIR) and NO₃^‒ immobilization rate (GNIR) under six N addition rates at 0–10, 10–20 and 20–40 cm soil depths. The corresponding biotic and abiotic mechanisms were also explored. The results showed that GAIR negatively correlated with N addition rate, but GNIR followed the unimodal response (increase first and then drop down) at all the three soil depths. The decrease in substrate supply by mineralization contributed to the decrease of GAIR with increasing N additions at the three soil depths. The changes of substrate supply by nitrification influenced the response of GNIR in the topsoil, but the changes of fungal abundance mediated the responses of GNIR in the subsoil. The increase in GNIR reduced denitrification derived N₂O emission and contributed to retain NO₃^‒, benefitting to the environmental protection. These different responses of GAIR and GNIR to increasing N additions and the different mechanisms underlying the responses from topsoil to subsoil should be considered in biogeochemical models and land management.

Yield losses caused by Fusarium wilt pose a risk to global food security. Nitrogen fertilizer regime affected the soil bacterial community and could reduce the occurrence of diseases. However, there are unresolved questions regarding the effects of single or combined applications of different nitrogen forms on disease development. Here, using the split-root system, we explored the impact of two forms of nitrogen (nitrate and ammonium) on the cucumber’s resistance to Fusarium. We found that nitrate supply altered the rhizosphere bacterial taxa, which could inhibit the Fusarium. Moreover, metabolomic analysis demonstrated that rhizosphere bacterial taxa gradients along the lateral distance from the root are associated with the release of root exudates. Our research revealed that ammonium-induced root exudates included several compounds, specifically gluconic acid, sorbitol, and sorbose, which were shown to be preferred by pathogen. These metabolites might negatively affect the growth of beneficial bacterial taxa. We found that nitrate enhanced the release of root exudates, such as guanidinosuccinic acid and behenic acid, that inhibited pathogen growth and recruited beneficial bacterial taxa. In summary, our results highlighted that nitrate supply can shape the spatial patterns of the rhizosphere microbial community by regulating the composition of root exudates to inhibit the growth of the pathogen, thereby reducing disease occurrence. This study provides a novel insight into how nitrogen forms affect rhizosphere microbial assembly to promote plant health.

Little is known about the path of root-derived carbon (C) into soil microbial communities in response to arbuscular mycorrhizal fungi (AMF) and nitrogen (N) fertilization. A mycorrhiza defective mutant of tomato (reduced mycorrhizal colonization: rmc) and its mycorrhizal wild type progenitor (MYC) were used to control for the formation of AMF. 16-week continuous ¹³CO₂ labeling was performed to quantify the photosynthetic C allocation in active microorganisms via ¹³C profiles of neutral (NLFAs) and phospholipid fatty acids (PLFAs). The ¹³C incorporation into fungal biomarker (the sum of PLFA 16:1ω5c, NLFA 16:1ω5c, PLFA 18:2ω6,9) increased with time over 16 weeks, and 4.62% of totally assimilated C was incorporated into AMF. More ¹³C was allocated into AMF storage compounds (NLFA 16:1ω5c, 3.1–4.1%) than hyphal biomass (PLFA 16:1ω5c, 0.12–0.25%). Furthermore, AMF symbiosis shifted microbial community composition, resulting in a lower ¹³C incorporation into bacteria and saprotrophic fungi compared to rmc plants. This suggests a lower use of root-derived C by bacteria and saprotrophic fungi but preference to older C compounds as energy sources. However, N fertilization decreased AMF abundance and subsequently less root-derived C was incorporated into PLFA and NLFA 16:1ω5c in relative to unfertilized soils, due to less C allocation caused by an increased C immobilization in the aboveground biomass. Our findings suggested that root-derived C can be sequestered by AMF through storage in their reproductive organs, but the preferential C allocation to AMF might be at the expense of C flow to other microbial groups. Overall, our results confirmed that mycorrhizal plants exert a greater influence on C incorporation into bacteria and saprotrophic fungi, which, however, is highly dependent on N fertilization.

The decomposition of carbon-rich litter in forest ecosystems is thought to regulate critical nutrient cycles, including biological N fixation. However, the dynamics of N fixation and its driving mechanisms during litter decomposition remain elusive. In the present study, we tracked N fixation rate (NFR), diazotrophic community characteristics and associated soil factors during the decomposition of Chinese fir and/or Schima superba leaf litter in a 60-day microcosm experiment. Soil NFR significantly increased as the litter addition and the incubation time, but it was not affected by litter types. Diazotrophic community interactions and key diazotroph species, identified by co-occurrence network, were changed as litter decomposition progressed. NFR was significantly correlated with the richness of putative key diazotrophs, and was mainly mediated by changes in soil NH₄⁺-N, and key C fractions of organic C. Structural equation modeling further revealed that the intensification of soil N fixation functions during litter decomposition was mediated by complex diazotrophic interactions rather than community diversity.

The acquisition of P and N from soil and their exchange for fixed C are key functions of mycorrhizal fungi in their symbiotic relationship with host plants. Additional contribution to plant nutrition is possible when hyphae proliferate into soil space not directly accessible to plant roots or when they locate nutrient-rich patches more effectively than plant roots. We performed a field-based experiment in a seminatural grassland. Community composition, diversity, and root colonisation intensity of mycorrhizal fungi was compared across different types of substrate patches (enriched or not with inorganic N, P or both), between two exposure times, and with unmanipulated soil and patches enriched with plant biomass. Beside evaluating the response of the communities of arbuscular mycorrhizal fungi (G-AMF) and fine root endophytes (M-FRE), we estimated foraging speed and precision of multiple taxa within these two groups. We compared the relative abundance of both groups using molecular barcoding. While G-AMF responded in community composition and diversity to inorganic and organic N enrichment, M-FRE did not discriminate among diferentially nutrient-enriched patches. Individual taxa varied in foraging response, but G-AMF were slower and possibly more discriminatory than M-FRE in occupying patches differing in N and/or P-enrichment. Particularly two virtual taxa of the Rhizophagus irregularis morphospecies of the G-AMF grew preferentially into the N-enriched patches. We thus conclude that there exist important differences in the strategies of soil exploration for nutrients within both fungal groups.

Lime application (liming) has historically been used to ameliorate soil acidity in grasslands. Liming effectively improves soil pH, plant productivity, and soil physicochemical properties, but the long-term impact of acidity control by liming on key microbial nitrogen (N)-cycling genes in semi-natural grasslands is unknown. We investigated the effect of 65 years of liming on N-cycling processes in the limed and control plots of the Ossekampen long-term grassland experiment in the Netherlands. These plots have not received any other fertilizers for 65 years. Soil sampling and nitrous oxide (N₂O) emission measurements were conducted three times in spring and four times in summer, and quantitative real-time PCR was performed to determine the absolute abundances of N-cycling genes, including ammonia-oxidation (amoA-AOB, amoA-AOA, amoA-comammox), denitrification (nirS, nirK, nosZ), nitrate ammonification (nrfA), and N-fixation (nifH) genes. Long-term liming increased the absolute abundances of nitrifiers, denitrifiers, and nitrate ammonifiers. Soil N₂O emissions did not differ significantly between liming and control treatments. Additionally, liming had a buffering effect that maintained the population of N-cycling microbes against seasonal variations in abundance. Our results indicate that improving soil acidity through liming potentially facilitates microbial N-cycling processes without increasing N₂O emissions.