Effects of different types of N deposition on the fungal decomposition activities of temperate forest soils

Distinct effects of canopy vs understory and organic vs inorganic N deposition on root resource acquisition strategies of subtropical Moso bamboo plants

Atmospheric nitrogen (N) deposition inevitably alters soil nutrient status, subsequently prompting plants to modify their root morphology (i.e., adopting a do-it-yourself strategy), mycorrhizal symbioses (i.e., outsourcing strategy), and root exudation (i.e., nutrient-mining strategy) linking with resource acquisition. However, how N deposition influences the integrated pattern of these resource-acquisition strategies remains unclear. Furthermore, most studies in forest ecosystems have focused on understory N and inorganic N deposition, neglecting canopy-associated processes (e.g., N interception and assimilation) and the impacts of organic N on root functional traits. In this study, we compared the effects of canopy vs understory, organic vs inorganic N deposition on eight root functional traits of Moso bamboo plants. Our results showed that N deposition significantly decreased arbuscular mycorrhizal fungi (AMF) colonization, altered root exudation rate and root foraging traits (branching intensity, specific root area, and length), but did not influence root tissue density and N concentration. Moreover, the impacts of N deposition on root functional traits varied significantly with deposition approach (canopy vs. understory), form (organic vs. inorganic), and their interaction, showing variations in both intensity and direction (positive/negative). Furthermore, specific root area and length were positively correlated with AMF colonization under canopy N deposition and root exudation rate in understory N deposition. Root trait variation under understory N deposition, but not under canopy N deposition, was classified into the collaboration gradient and the conservation gradient. These findings imply that coordination of nutrient-acquisition strategies dependent on N deposition approach. Overall, this study provides a holistic understanding of the impacts of N deposition on root resource-acquisition strategies. Our results indicate that the evaluation of N deposition on fine roots in forest ecosystems might be biased if N is added understory.

Atmospheric wet organic nitrogen deposition in China: Insights from the national observation network

Organic nitrogen (N) is an important component of atmospheric reactive N deposition, and its bioavailability is almost as important as that of inorganic N. Currently, there are limited reports of national observations of organic N deposition; most stations are concentrated in rural and urban areas, with even fewer long-term observations of natural ecosystems in remote areas. Based on the China Wet Deposition Observation Network, this study regularly collected monthly wet deposition samples from 43 typical ecosystems from 2013 to 2021 and measured related N concentrations. The aim was to provide a more comprehensive assessment of the multi-component characteristics of atmospheric wet N deposition and reveal the influencing factors and potential sources of wet dissolved organic N (DON) deposition. The results showed that atmospheric wet deposition fluxes of NO3-, NH4+, DON and dissolved total N (DTN) were 4.68, 5.25, 4.32, and 13.05 kg N ha-1 yr-1, respectively, and that DON accounted for 30 % of DTN deposition (potentially up to 50 % in remote areas). Wet DON deposition was related to anthropogenic emissions (agriculture, biomass burning, and traffic), natural emissions (volatile organic compound emissions from vegetation), and precipitation processes. The wet DON deposition flux was higher in South, Central, and Southwest China, with more precipitation and intensive agricultural activities or more vegetation cover, and lower in Northwest China and Inner Mongolia, with less precipitation and human activities or vegetation cover. DON was the main contributor to DTN deposition in remote areas and was possibly related to natural emissions. In rural and urban areas, DON may have been more influenced by agricultural activities and anthropogenic emissions. This study quantified the long-term spatiotemporal patterns of wet N deposition and provides a reference for future N addition experiments and N cycle studies. Further consideration of DON deposition is required, especially in the context of anthropogenic control of NO2 and NH3.

Effects of Short-Term Nitrogen Addition on Soil Fungal Community Increase with Nitrogen Addition Rate in an Alpine Steppe at the Source of Brahmaputra

The soil fungal community plays a crucial role in terrestrial decomposition and biogeochemical cycles. However, the responses of the soil fungal community to short-term nitrogen addition and its related dominant drivers still remain unclear. To address this gap, we conducted an experiment to explore how different levels of nitrogen addition (five levels: 0, 2.5, 5, 10, and 20 g N m-2 y-1) affected the soil fungal community in an alpine steppe at the source of Brahmaputra. Results showed that the reduced magnitudes of soil fungal species and phylogenetic α-diversity increased with the increasing nitrogen addition rate. Nitrogen addition significantly changed the community composition of species, and the dissimilarity of the soil fungal community increased with the increasing nitrogen addition rate, with a greater dissimilarity observed in the superficial soil (0-10 cm) compared to the subsurface soil (10-20 cm). Increases in the soil nitrogen availability were found to be the predominant factor in controlling the changes in the soil fungal community with the nitrogen addition gradient. Therefore, short-term nitrogen addition can still cause obvious changes in the soil fungal community in the alpine grassland at the source of Brahmaputra. We should not underestimate the potential influence of future nitrogen deposition on the soil fungal community in the high-altitude grassland of the Qinghai-Tibet Plateau. Adverse effects on the soil fungal community should be carefully considered when nitrogen fertilizer is used for ecosystem restoration of the alpine grassland of the Qinghai-Tibet Plateau.

The potential bias of nitrogen deposition effects on primary productivity and biodiversity

Atmospheric nitrogen (N) deposition is composed of both inorganic nitrogen (IN) and organic nitrogen (ON), and these sources of N may exhibit different impacts on ecosystems. However, our understanding of the impacts of N deposition is largely based on experimental gradients of INs or more rarely ONs. Thus, the effects of N deposition on ecosystem productivity and biodiversity may be biased. We explored the differential impacts of N addition with different IN:ON ratios (0:10, 3:7, 5:5, 7:3, and 10:0) on aboveground net primary productivity (ANPP) of plant community and plant diversity in a typical temperate grassland with a long-term N addition experiment. Soil pH, litter biomass, soil IN concentration, and light penetration were measured to examine the potential mechanisms underlying species loss with N addition. Our results showed that N addition significantly increased plant community ANPP by 68.33%-105.50% and reduced species richness by 16.20%-37.99%. The IN:ON ratios showed no significant effects on plant community ANPP. However, IN-induced species richness loss was about 2.34 times of ON-induced richness loss. Soil pH was positively related to species richness, and they exhibited very similar response patterns to IN:ON ratios. It implies that soil acidification accounts for the different magnitudes of species loss with IN and ON additions. Overall, our study suggests that it might be reasonable to evaluate the effects of N deposition on plant community ANPP with either IN or ON addition. However, the evaluation of N deposition on biodiversity might be overestimated if only IN is added or underestimated if only ON is added.

Nitrogen and phosphorus enrichment accelerates soil organic carbon loss in alpine grassland on the Qinghai-Tibetan Plateau

Anthropogenic activities have substantially increased soil nutrient availability, which in turn affects ecosystem processes and functions, especially in nutrient-limited ecosystems such as alpine grasslands. Although considerable efforts have been devoted to understanding the responses of plant productivity and community composition to nitrogen (N) and phosphorus (P) enrichment, the nutrient enrichment effects on soil organic carbon (SOC) and microbial functions are not well understood. A four-year field experiment was established to evaluate the influence of continuous N and P enrichment on plant growth and SOC content in an alpine grassland of the Qinghai-Tibetan Plateau. The study included four treatments: Control without addition, N addition, P addition, and N plus P addition. N addition strongly increased aboveground plant biomass and decreased species richness by promoting growth of the dominant grasses species. In contrast, N and P enrichment significantly decreased SOC, especially the recalcitrant organic C content in the surface layer (0-10 cm) by reducing the slow C pool and enlarging the active C pool. Microbial biomass and activities of C-degrading enzymes (β-glucosidase, cellulase and polyphenol oxidase) and an N-degrading enzyme (chitinase) increased with nutrient inputs. The CO₂ emissions during a 300 d incubation period were positively correlated with the cellulase and chitinase activities, while the slow C pool was negatively correlated with the cellulase and polyphenol oxidase activities. Consequently, N and P enrichment accelerated decomposition of the recalcitrant C by stimulating microbial growth and increasing enzyme activities, leading to negative impacts on soil C sequestration. Overall, the results indicate that alpine grassland soils of the Qinghai-Tibetan Plateau may be changing from a C sink to a C source under increasing N and P availability, and improvement of alpine grassland management through nutrient inputs should consider not only the aboveground biomass for grazing, but also the soil C sequestration and ecosystem functioning.

Increasing atmospheric deposition nitrogen and ammonium reduced microbial activity and changed the bacterial community composition of red paddy soil

Atmospheric deposition nitrogen (ADN) increases the N content in soil and subsequently impacts microbial activity of soil. However, the effects of ADN on paddy soil microbial activity have not been well characterized. In this study, we studied how red paddy soil microbial activity responses to different contents of ADN through a 10-months ADN simulation on well managed pot experiments. Results showed that all tested contents of ADN fluxes (27, 55, and 82kgNha^-1 when its ratio of NH₄⁺/NO₃^--N (R_N) was 2:1) enhanced the soil enzyme activity and microbial biomass carbon and nitrogen and 27kgNha^-1 ADN had maximum effects while comparing with the fertilizer treatment. Generally, increasing of both ADN flux and R_N (1:2, 1:1 and 2:1 with the ADN flux of 55kgNha^-1) had similar reduced effects on microbial activity. Furthermore, both ADN flux and R_N significantly reduced soil bacterial alpha diversity (p<0.05) and altered bacterial community structure (e.g., the relative abundances of genera Dyella and Rhodoblastus affiliated to Proteobacteria increased). Redundancy analysis demonstrated that ADN flux and R_N were the main drivers in shaping paddy soil bacteria community. Overall, the results have indicated that increasing ADN flux and ammonium reduced soil microbial activity and changed the soil bacterial community. The finding highlights how paddy soil microbial community response to ADN and provides information for N management in paddy soil.

Responses of hydrolytic enzyme activities in saline-alkaline soil to mixed inorganic and organic nitrogen addition

The effects of manipulating nitrogen (N) deposition, with the use of a single form of N, on soil enzyme activities have been extensively studied. However, the impacts varying the N type (organic vs. inorganic) on soil hydrolytic enzyme activities have been less studied. We performed a 60 day incubation experiment using saline-alkaline soil. The objectives were to explore how the microbial biomass and enzyme activities respond to a mixed N addition at different inorganic to organic N ratios. The experimental design was full factorial, with two rates of N addition (10 g N m⁻² and 20 g N m⁻²) and four ratios of N addition (inorganic N:organic N = 10:0, 7:3, 3:7, 1:9). The results showed that N addition stimulated enzyme activities involved in C, N and P cycling. Enzyme activities under mixed N addition increased compared to those under single inorganic N addition in most cases. The inorganic to organic N ratios interacted with the N addition rate to affect the enzyme activities. Our results suggest that various N fertilizers, which have different inorganic to organic N ratios, should be applied when evaluating the effects of atmospheric N deposition on the soil microbial enzyme activities and ecosystem structure and function.

Costimulation of soil glycosidase activity and soil respiration by nitrogen addition

Unprecedented levels of nitrogen (N) have been deposited in ecosystems over the past century, which is expected to have cascading effects on microbially mediated soil respiration (SR). Extracellular enzymes play critical roles on the degradation of soil organic matter, and measurements of their activities are potentially useful indicators of SR. The links between soil extracellular enzymatic activities (EEAs) and SR under N addition, however, have not been established. We therefore conducted a meta-analysis from 62 publications to synthesize the responses of soil EEAs and SR to elevated N. Nitrogen addition significantly increased glycosidase activity (GA) by 13.0%, α-1,4-glucosidase (AG) by 19.6%, β-1,4-glucosidase (BG) by 11.1%, β-1,4-xylosidase (BX) by 21.9% and β-D-cellobiosidase (CBH) by 12.6%. Increases in GA were more evident for long duration, high rate, organic and mixed N addition (combination of organic and inorganic N addition), as well as for studies from farmland. The response ratios (RRs) of GA were positively correlated with the SR-RRs, even when evaluated individually for AG, BG, BX and CBH. This positive correlation between GA-RR and SR-RR was maintained for most types of vegetation and soil as well as for different methods of N addition. Our results provide the first evidence that GA is linked to SR under N addition over a range of ecosystems and highlight the need for further studies on the response of other soil EEAs to various global change factors and their implications for ecosystem functions.

Changes of soil bacterial activities and functions after different N additions in a temperate forest

It has been shown that different nitrogen (N) addition led to various influences on soil microbial activities in forest ecosystems; however, the changes of bacteria were still unclear. In this work, inorganic N (NH₄NO₃) and organic N (urea and glycine) were fertilized with different ratios (5:0, 1:4, 3:2, 2:3, and 1:4) on temperate forest soils, while fungicide (cycloheximide) was simultaneously added on half of each treatment to inhibit fungal activities (leaving only bacteria). After a 3-year field experiment, soil samples were harvested, then microbial enzymatic activities involved in carbon (C), and N and phosphorus (P) cycles were determined. Under laboratory conditions, four purified bacteria which were isolated from sample site had been inoculated in sterilized soils under different N types and enzymatic activities were assayed after 90-day incubation. The results showed that cellulase and polyphenol oxidase activities of non-fungicide-added treatments increased after N addition and greater organic N accelerated the increases. However, these enzymatic activities of fungicide-added treatments were not significantly influenced by N addition and N types. It may be due to the insufficient ability of bacteria to synthesize enough enzymes to decompose complex organic C (such as cellulose and lignin) into available compound, although N-limitation was alleviated. Alkaline phosphatase activities increased after N addition in both non-fungicide-added and fungicide-added treatments, and the acceleration on bacterial alkaline phosphatase activities was even greater. Furthermore, organic N showed at least 2.5 times promotion on bacteria alkaline phosphatase than those of inorganic N, which indicated greater alleviation of bacterial P-limitation after the addition of organic N. All the results indicated that soil bacteria may be seriously limited by soil available C but become the dominant decomposer of the complex P compounds after N addition, particularly greater organic N.