Long-term Nitrogen Inputs Research Articles

Biochar amendment alleviates soil microbial nitrogen and phosphorus limitation and increases soil heterotrophic respiration under long-term nitrogen input in a subtropical forest

Nitrogen (N) and carbon (C) inputs substantially affect soil microbial functions. However, the influences of long-term N and C additions on soil microbial resource limitation and heterotrophic respiration—fundamental microbial functional traits—remain unclear, impeding the understanding of how soil C dynamics respond to global change. In this study, the responses of soil microbial resource limitation and heterotrophic respiration (Rh) to 7-year N and biochar (BC) additions in a subtropical Moso bamboo (Phyllostachys edulis) plantation were investigated. We used eight treatments: Control, no N and BC addition; N30, 30 kg N (ammonium nitrate)·hm−2·a−1; N60, 60 kg N·hm−2·a−1; N90, 90 kg N·hm−2·a−1; BC20, 20 t BC (originating from Moso bamboo chips) hm−2; N30 + BC20, 30 kg N·hm−2·a−1 + 20 t BC hm−2; N60 + BC20, 60 kg N·hm−2·a−1 + 20 t BC hm−2; and N90 + BC20, 90 kg N·hm−2·a−1 + 20 t BC hm−2. Soil microbes were co-limited by N and phosphorus (P) and not limited by C in the control treatments. Long-term N addition enhanced soil microbial N and P limitation but significantly reduced soil Rh by 15.1 %–20.0 % relative to that in the control treatments. BC amendment alleviated soil microbial N and P limitation and significantly decreased C use efficiency by 10.9 %–42.1 % but increased Rh by 33.6 %–91.6 % in the long-term N-free and N-supplemented treatments (P < 0.05). Soil C- and N-acquisition enzyme activities were the dominant drivers of soil microbial resource limitation. Furthermore, microbial resource limitation was a more reliable predictor of Rh than soil resources or microbial biomass. The results suggested that long-term N and BC additions affect Rh by regulating microbial resource limitation, highlighting its significance in understanding soil C cycling under environmental change.

Long-term nitrogen input reduces soil bacterial network complexity by shifts in life history strategy in temperate grassland.

We investigated soil bacterial and fungal communities, constructed co-occurrence networks, and estimated bacterial traits along a gradient of nitrogen (N) input. The results showed that soil bacterial co-occurrence networks complexity decreased with increasing N input. The ratio of negative to positive cohesion decreased with increasing N input, suggesting the declined competitive but strengthened cooperative interactions. However, soil fungal network complexity did not change under N enrichment. In addition, N input stimulated the copiotroph/oligotroph ratio, ribosomal RNA operon (rrn) copy number, and guanine-cytosine (GC) content of soil bacteria, shifting bacterial life history strategy toward copiotroph with increased r-/K-strategy ratio. Piecewise structural equation modeling results further revealed that the reduction in bacterial co-occurrence network complexity was directly regulated by the increased bacterial r-/K-strategy ratio, rather than reduced bacterial richness. Our study reveals the mechanisms through which microbial traits regulate interactions and shape co-occurrence networks under global changes.

Chemotaxis mediates nitrogen acquisition of maize under long-term nitrogen input

Chemotaxis is a process that enables motile bacteria to move along nutrient gradients, thereby exploring nutrient hotspots, and potentially playing important biogeochemical roles in agricultural ecosystems. In this study, a 36-year field experiment of nitrogen (N) input was conducted, along with the use of a synthetic community (SynCom) inoculation, to confirm the role of chemotaxis in the cycling of N. The chemotaxis genes abundances of N+ treatments were 97.2% and 95.3% higher than that of N- treatments in bulk soil and rhizosphere, respectively, indicating a significant increase in chemotaxis genes under long-term N input. Positive correlations between chemotaxis genes and N metabolism genes in N+ treatments suggested an important role of chemotaxis in N cycling. The chemotaxis genes abundance was 3.6 times higher in rhizosphere than in bulk soil, and chemotaxis genes showed the most complex correlations with N metabolism genes in rhizosphere under N input, suggesting a key role of chemotaxis in plant N uptake. To assess the potential function of chemotactic bacteria, a SynCom consisting of 10 bacterial strains isolated from in situ N-input soils and capable of chemotaxis, was applied to the maize rhizosphere. The promotion of N acquisition in maize plants through inoculation was confirmed by about 30% greater N content in the shoots of SynCom inoculated soil than in the control. Long-term N input enhanced the functions related to metabolite transport and energy metabolism in the bacterial community, particularly in the rhizosphere. Thus, plants may provide bacteria that migrate to the rhizosphere via chemotaxis with more root metabolites as nutrients. In summary, this study provides novel ecological and molecular insights into chemotaxis-mediated biogeochemical cycling in agricultural ecosystems.

Fertilizing-induced changes in the nitrifying microbiota associated with soil nitrification and crop yield

Ammonia oxidizing archaea (AOA) and bacteria (AOB), nitrite-oxidizing bacteria (NOB), and comammox Nitrospira (CMX) play pivotal roles in global nitrogen-cycling network. Despite its importance, the driving forces for niche specialization of these nitrifiers, as well as their relative contributions to nitrification and crop yield have not been fully understood. Here, we investigated the niche specialization and environmental prevalence of nitrifying communities, and their importance for the nitrification rate and crop yield across a gradient of nitrogen inputs in a two-decade old field experiment. The results of 15N-tracer and quantitative PCR revealed that AOB and NOB jointly determined the gross nitrification rates across mineral fertilizer treatments, whereas AOA and AOB contributed more than other nitrifiers to nitrification under with organic fertilizer amendments. Linear regression model revealed that crop yield could be linked with AOB and NOB under inorganic farming but closely associated with CMX under organic management. Amplicon sequencing of these functional genes further demonstrated that mineral and organic fertilizers have distinct influences on the β-diversity and niche breadth of these nitrifying communities, indicating that fertilization triggered niche specialization of nitrifying guilds in agricultural soils. Notably, organic fertilization enhanced the network complexity of these nitrifiers by harboring keystone taxa. Random forest analysis provide robustly evidence for the hypothesis that abundance of functional genes contributed more than a- and β-diversity of these nitrifiers for driving nitrification rates and crop yields. Collectively, these findings provide the empirical evidence for the environmental adaptation and niche specialization of nitrifying communities, and their contributions in nitrification and crop yield when confronted with long-term nitrogen inputs.

Long-term nitrogen addition does not sustain host tree stem radial growth but doubles the abundance of high-biomass ectomycorrhizal fungi.

Global change has altered nitrogen availability in boreal forest soils. As ectomycorrhizal fungi play critical ecological functions, shifts in their abundance and community composition must be considered in the response of forests to changes in nitrogen availability. Furthermore, ectomycorrhizas are symbiotic, so the response of ectomycorrhizal fungi to nitrogen cannot be understood in isolation of their plant partners. Most previous studies, however, neglect to measure the response of host trees to nitrogen addition simultaneously with that of fungal communities. In addition to being one-sided, most of these studies have also been conducted in coniferous forests. Deciduous and "dual-mycorrhizal" tree species, namely those that form ecto- and arbuscular mycorrhizas, have received little attention despite being widespread in the boreal forest. We applied nitrogen (30kgha-1 year-1 ) for 13years to stands dominated by aspen (Populus tremuloides Michx.) and hypothesized that tree stem radial growth would increase, ectomycorrhizal fungal biomass would decrease, ectomycorrhizal fungal community composition would shift, and the abundance of arbuscular mycorrhizal (AM) fungi would increase. Nitrogen addition initially increased stem radial growth of aspen, but it was not sustained at the time we characterized their mycorrhizas. After 13years, the abundance of fungi possessing extramatrical hyphae, or "high-biomass" ectomycorrhizas, doubled. No changes occurred in ectomycorrhizal and AM fungal community composition, or in ecto- and AM abundance measured as root colonization. This dual-mycorrhizal tree species did not shift away from ectomycorrhizal fungal dominance with long-term nitrogen input. The unexpected increase in high-biomass ectomycorrhizal fungi with nitrogen addition may be due to increased carbon allocation to their fungal partners by growth-limited trees. Given the focus on conifers in past studies, reconciling results of plant-mycorrhizal fungal relationships in stands of deciduous trees may demand a broader view on the impacts of nitrogen addition on the structure and function of boreal forests.

Niche Differentiation of Bacterial Versus Archaeal Soil Nitrifiers Induced by Ammonium Inhibition Along a Management Gradient.

Soil nitrification, mediated mainly by ammonia oxidizing archaea (AOA) and bacteria (AOB), converts ammonium (NH4+) to nitrite (NO2−) and thence nitrate (NO3−). To better understand ecological differences between AOA and AOB, we investigated the nitrification kinetics of AOA and AOB under eight replicated cropped and unmanaged ecosystems (including two fertilized natural systems) along a long-term management intensity gradient in the upper U.S. Midwest. For five of eight ecosystems, AOB but not AOA exhibited Haldane kinetics (inhibited by high NH4+ additions), especially in perennial and successional systems. In contrast, AOA predominantly exhibited Michaelis-Menten kinetics, suggesting greater resistance to high nitrogen inputs than AOB. These responses suggest the potential for NH4+-induced niche differentiation between AOA and AOB. Additionally, long-term fertilization significantly enhanced maximum nitrification rates (Vmax) in the early successional systems for both AOA and AOB, but not in the deciduous forest systems. This was likely due to pH suppression of nitrification in the acidic forest soils, corroborated by a positive correlation of Vmax with soil pH but not with amoA gene abundance. Results also demonstrated that soil nitrification potentials were relatively stable, as there were no seasonal differences. Overall, results suggest that (1) NH4+ inhibition of AOB but not AOA could be another factor contributing to niche differentiation between AOA and AOB in soil, and (2) nitrification by both AOA and AOB can be significantly promoted by long-term nitrogen inputs.

Positive response of soil microbes to long-term nitrogen input in spruce forest: Results from Gårdsjön whole-catchment N-addition experiment

Chronic nitrogen (N) deposition from anthropogenic emissions alter N cycling of forests in Europe and in other impacted areas. It disrupts plant/microbe interactions in originally N-poor systems, based on a symbiosis of plants with ectomycorrhizal fungi (ECM). ECM fungi that are capable of efficient nutrient mining from complex organics and their long-distance transport play a key role in controlling soil N mineralization and immobilization, and eventual nitrate (NO3−) leaching. Current meta-analyses highlight the importance of ECM biomass in securing the large soil N pool. At the same time, they point to the adverse effect of long-term N input on ECM fungi. The functioning of N-poor and N-overloaded forests is well understood, while the transient stages are much less explored. Therefore, we focused on the spruce-forest dominated catchment at Gårdsjön (Sweden) that received N addition of 40 kg N ha−1yr−1 over 24 years (a cumulative N input of >1200 kg N ha−1) but still loses via runoff only <20% of annual N input (deposition + addition) as NO3−. We found that, compared to the control, the N-addition catchment had a much larger soil microbial biomass. The N addition did not change the fungi/bacteria ratio, but a larger share of the bacterial community was made up of copiotrophs. Furthermore, fungal community composition shifted to more nitrophilic ECM fungi (contact and short exploration type ECM species) and saprotrophs. Such a restructured community has been more active, possessed a higher specific respiration rate, enhanced organic P and C mining through enzymatic production and provided faster net N mineralization and nitrification. These may be early indications of alleviation of N limitation of the system. We observed no signs of soil acidification related to N additions. The larger, structurally and functionally adapted soil microbial community still provides an efficient sink for the added N in the soil and is likely to be one of the explanations for low NO3− leaching that have stabilized in the last decade. Our results suggest that a microbial community can contribute to effective soil N retention in spite of the partial relative retreat (20–30%) of nitrophobic ECM fungi with large external mycelia, provided the fungal biomass remains high because of replacement by other ECM and saprotrophic fungi. Furthermore, we assume that N retention of similar C-rich boreal forests (organic soil molar C/N ~35) is not necessarily threatened by a large cumulative N dose provided N enters at a moderate rate, does not cause acidification and the soil microbial community has time to adapt through structural and functional changes.

Response of ammonium oxidizers to the application of nitrogen fertilizer in an alpine meadow on the Qinghai-Tibetan Plateau

The alpine meadows in Qinghai-Tibetan Plateau is an ecosystem sensitive to environmental changes. As part of a study on global climate change and the effects of N precipitation on soil microbiota, we fingerprinted ammonium oxidizing archaea (AOA) and bacteria (AOB) in relation to the N supplement in an alpine meadow in Qinghai-Tibetan Plateau. The results showed that nutrient content in the studied soil was significantly varied in different months (sampling period) but not influenced by the N supplement rate. Long-term nitrogen input dramatically changed the abundance and community composition of the AOB but exerted no obvious effect on the AOA community in the tested soil. Significant differences in the abundance and composition of AOA were recorded in different months. Our findings implied that 1) the soil fertility and physicochemical features of the studied alpine meadows in Qinghai-Tibetan Plateau were stable even under the long-term high N supply; 2) AOB might be the active nitrifiers responding to the high N supply, while the AOA were the abundant and stable nitrifiers despite the N supply; 3) sampling periods mainly affected the abundance and community composition of the nitrifiers in the tested alpine meadow; and 4) the AOB in the studied area are psychrotrophs. Therefore, despite the enhancement of plant growth, high N supplements in the tested alpine meadow ecosystems might cause more pollution to water and air, which in turn contributes to global climate change.