Structures of intransitive competition network affect functional attributes of plant community under nitrogen enrichment

Atmospheric nitrogen (N) deposition is a potential danger factor for grassland ecology, and will cause unpredictable consequences to plant communities. However, how plant species interactions response to N enrichment and then affect ecological functions are not fully known. We investigated how intransitive competition network was related to the functional attributes of plant community under a 13-years N-deposition experiment. Results showed that intransitive competition network was not a single structure, but a complexly interwoven structure of various simple structures. Nested work was more common, accounting for 76.96%, and gained new species at a higher colonization rate than short network did. The network had a long-term mechanism to maintain the small-scale Alpha diversity, and a significant lag effect on the large-scale Gamma diversity. Under the conditions of N ≥ 2 g N·m-2·year-1, without mowing and under high fertilization frequency, the increase of network complexity significantly decreased plot biomass gradually. The relationship between biomass and network complexity is quadratic curves, also between abundancy and the complexity, but with the opposite bending directions, which indicated that biomass and abundance were complementary to each other, which may be a mechanism of maintaining the relative balance of species competition. In addition, the decrease of species asynchronism changing with the increase of N-enrichment gradually destroyed ecosystem stability. However, at medium N enrichment, intransitive network counteracted the negative effects of N enrichment and maintained or even improved the biomass ecosystem stability. Our results suggested that intransitive competition network is an internal mechanism of self-restoration of a grassland ecosystem. Under nitrogen enrichment conditions, competitive networks complexity is reduced, leading to a reduction in species diversity. These analyses emphasize the important role of intransitive network structure to stabilize grassland ecosystem. In order to achieve sustainable development of grassland, it is indispensable to control nitrogen addition rate.

Soil Nitrate Mediates the Responses of Plant Community Production to the Frequency of N Addition in a Temperate Grassland: A Decadal Field Experiment

PurposeNitrogen (N) enrichment through either artificial N application or atmospheric N deposition often increases ecosystem aboveground net primary productivity (ANPP). Therefore, results from N addition experiments have been used to assess the effects of atmospheric N deposition on ecosystems. However, the frequency of atmospheric N deposition is higher than that of artificial N addition. Whether the frequency of N addition alters the long-term response of ecosystem ANPP remains unclear. MethodsWe conducted a N addition frequency experiment from 2010 in a temperate grassland, northern China. Plant community ANPP was collected in 2019 and 2020, and soil physicochemical properties were measured in 2020. ResultsPlant community ANPP was significantly enhanced by N addition, whereas these increments declined with the frequency of N addition. The responses of the grasses ANPP to the frequency of N addition were similar to those of the plant community ANPP. Forbs ANPP was not significantly altered by the frequency of N addition. Meanwhile, soil ammonium and nitrate (NO3−–N) concentrations decreased with increasing N addition frequency, while the soil water content (SWC) and pH were similar among the frequencies of N addition. Moreover, SWC and soil NO3−–N jointly promoted grasses ANPP, ultimately increasing the plant community ANPP. ConclusionOur findings extend the water and N co-limitation hypothesis by specifying the preference for NO3−–N in arid/semi-arid regions. This study also illustrates that a higher frequency of N addition is more suitable for assessing the long-term impacts of atmospheric N deposition on ecosystems.

Monitoring transformation of two tropical lignocellulosics and their lignins after residence in Benin soils

Thermally assisted Hydrolysis and Methylation (THM), and 2D-heteronuclear single quantum coherence nuclear magnetic resonance (2D HSQC NMR) spectroscopy were used to monitor the transformation of ramial chipped wood (RCW) from Gmelina arborea and Sarcocephalus latifolius, together with their organosolv lignins, following soil incubation in Benin (West Africa). Mesh litterbags containing RCW were buried in soils (10 cm depth) and were retrieved after 0, 6, 12 and 18 months of field incubation. Chemical analysis showed that total carbohydrate content decreased, while total lignin content increased as RCW decomposition progressed. Ash and mineral content of RCW increased significantly after 18 months of decomposition in soil. Significant N-enrichment of the RCW was determined following 18 months incubation in soils, reaching 2.6 and 1.9 times the initial N-content for G. arborea and S. latifolius. Results of THM showed that the S + G sum, corresponding to lignins, increased with RCW residence time in the soils, in contrast to the response of compounds derived from carbohydrates, the sum of which decreased. Remarkably, lignin interunit linkages, most notably β-O-4′ aryl ethers, β-β′ resinol, β-5′ phenylcoumaran and p-PCA p-coumarate, survived after 18 months in the soil, despite their gradual decrease over the duration of the experiment.

Nitrogen Uptake by Plants May Alleviate N Deposition-induced Increase in Soil N2O Emissions in Subtropical Chinese Fir Plantations

BackgroundContinuous increasing nitrogen (N) deposition interferes with soil nitrogen cycle of forests, which highly impacts soil N2O emissions and accelerates global warming. Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is one of the most widely planted species in southern China which locates in the high N deposition area. However, the impact of N deposition on soil N2O emissions in subtropical Chinese fir plantations and the potential risk of increasing N deposition still remain elusive. Here, we conducted an in situ study in a subtropical Chinese fir plantation at Fengyang Mountain Nature Reserve, China, from 2019-2020 with four different levels of N enrichment: control (CK: ambient N deposition), low-N (LN: 50 kg N ha−1 yr−1), medium-N (MN: 100 kg N ha−1 yr−1), and high-N (HN: 200 kg N ha−1 yr−1). ResultsWe found that soil N2O emission rates increased with N enrichment from an average of 5.89 ± 3.66 to 20.11 ± 3.44 μg N m−2 h−1. The N enrichment in general showed no significant effect on the abundance of nitrate-reducing bacteria, but it tended to raise the abundance of ammonia oxidizing archaea and bacteria, and to decrease the abundance of N2O-reducing bacteria, which likely provided the microbial basis for accelerating soil N2O emissions along with increasing N deposition. However, the relationship of soil N2O emissions with N input did not match an exponential increase, but it matched a logarithmic increase, illustrating that the risk of increasing N deposition on soil N2O emissions was attenuated. It is found that N enrichment significantly decreased soil moisture and tended to increase the fir leaf N concentrations and soil CO2 emission rates. Besides, soil microbial biomass was significantly suppressed by N enrichment during the mid-growing season, while not in end of growing season. These may suggest that N enrichment stimulated plant growth with more N and water uptake, which competed with microorganisms for N and therefore alleviated further increasing N2O emissions under N enrichment. ConclusionThis study deepen our understanding of the impacts of increased N deposition on the greenhouse gas (GHG) balance in the Chinese fir plantations, and highlight that plants need to be incorporated as an important explanatory variable when predicting GHG fluxes in the background of global increasing N deposition.

Effects of nitrogen enrichment on zooplankton biomass and N:P recycling ratios across a DOC gradient in northern-latitude lakes

We used data from whole-lake studies to assess how changes in food quantity (phytoplankton biomass) and quality (phytoplankton community composition, seston C:P and N:P) with N fertilization affect zooplankton biomass, community composition and C:N:P stoichiometry, and their N:P recycling ratio along a gradient in lake DOC concentrations. We found that despite major differences in phytoplankton biomass with DOC (unimodal distributions, especially with N fertilization), no major differences in zooplankton biomass were detectable. Instead, phytoplankton to zooplankton biomass ratios were high, especially at intermediate DOC and after N fertilization, implying low trophic transfer efficiencies. An explanation for the observed low phytoplankton resource use, and biomass responses in zooplankton, was dominance of colony forming chlorophytes of reduced edibility at intermediate lake DOC, combined with reduced phytoplankton mineral quality (enhanced seston N:P) with N fertilization. N fertilization, however, increased zooplankton N:P recycling ratios, with largest impact at low DOC where phytoplankton benefitted from light sufficiently to cause enhanced seston N:P. Our results suggest that although N enrichment and increased phytoplankton biomass do not necessarily increase zooplankton biomass, bottom-up effects may still impact zooplankton and their N:P recycling ratio through promotion of phytoplankton species of low edibility and altered mineral quality.

Neutral responses of plant community Ca concentration to nitrogen enrichment in a semi-arid grassland

Aims Calcium (Ca) is an essential nutrient for plant growth and Ca concentrations in forage have important implications for ruminant diet and health. It remains an open question whether forage Ca concentration would be decreased by increasing nitrogen (N) deposition. Methods We manipulated the increasing rates of N addition (2008-2015) in a semi-arid grassland, northern China. Plant Ca concentrations for all species were examined in each plot under N treatment. The Ca concentrations at functional group and community levels were calculated based on the concentration of each species presented and their relative biomass in each plot. Important findings We found that community-level Ca concentration remained stable across a gradient of wide-ranged N addition rates, although Ca concentration at both species and functional group levels showed negative responses to N enrichment. Given that forbs had higher Ca concentration than grasses, the increasing relative biomass of forbs canceled out the negative responses of species-level and functional group-level Ca concentration. Our results further showed that community Ca pool showed a positive but saturating response to N addition, with a threshold at the rate of 10 g N m -2 yr -1. Our findings highlight the role of changes in plant relative biomass in controlling the responses of forage Ca concentration and stock to N enrichment.

Available Phosphorus in Soils Amended with Organic N-Enriched Composts during Periods of Incubation

Inorganic nitrogen (N) fertilizers, microbial inoculum and biologically-active substances are used to fortify composts which characteristically contain low amounts of N. The potentials of organic wastes from agriculture for N enrichment of composts were indicated by significant increase in soil N while there can be fortuitous improvement in the available phosphorus (P) contents on which information about the extent is lacking. In this study, composts: cow dung + sawdust (CDSD) and poultry droppings + sawdust (PDSD) were enriched with meals from bone (BN), blood (BM), hoof (HF) and horn (HN); and neem leaf (NM) and tithonia leaf (TM) to attain 150, 300, 450 and 600 g kg-1 N and the available P was monitored at four-week intervals during 16 weeks of incubation in soil. Available P increased with all N sources and enrichment rates slightly at week 4 but highly from week 8 and for each source, the enrichment to 600 g kg-1 N gave the highest values. The composts enriched to 450 and 600 g kg-1 N gave 15-20 and 20-29 mg kg-1 available P in week 12 and 16 respectively. CDSDBM and PDSDHF enriched to 600 g kg-1 N at week 4 and 8 respectively can be used for short-season crops while PDSDBM and PDSDNM enriched to 600 g kg-1 N in week 12 and 16 respectively would be suitable for long-season crops. The PDSDNM enriched to 600 g kg-1 N gave the highest available P (29.0 mg kg-1) and was followed by PDSDBN and PDSDHF at the same enrichment level, with 28.0 mg kg-1 each. Although the PMSDNM enriched to 600 g kg-1N had the highest available P in week 16, the enriched composts with values exceeding 20 mg kg-1 also have potentials for the cultivation of long-season vegetables.

Drivers of soil respiration in response to nitrogen addition in a Mediterranean mountain forest

Atmospheric nitrogen (N) deposition rates affect soil N dynamics, influencing soil respiration (RS) rates. However, for the Mediterranean region, the effect of changes in atmospheric N deposition on RS are not well constrained yet. We investigated the interplay between increased N deposition and tree species composition on RS at a Scots pine—Pyrenean oak ecotone in Central Spain, and whether the observed responses were mediated by changes on selected soil properties. Throughout 3 years, we simulated two N deposition rates—10 (medium) and 40 kg N ha−1 a−1 (high)—over the background deposition (control) in neighbouring stands in which tree species composition (pine or oak) shapes soil stoichiometry and microbial communities. We monitored RS on a monthly basis during 3 years; in addition, we performed targeted measurements 24 h after the N fertilization events to assess short-term soil responses. During winter and summer, RS did not respond to enhanced N deposition rates. In spring and autumn, higher RS rates were observed in the medium-fertilization, but the size and duration of this effect was tree species dependent. We suggest that climate seasonality modulates the response of RS to N availability, with tree species effects becoming relevant only when environmental conditions are adequate. RS in fertilized plots was larger from February to May and in September under pine, while under oak a response was observed only in April, probably due to differences in native soil stoichiometry under each tree species. Overall, RS showed high stability during 3 years of N enrichment in this Mediterranean ecotone area. However, we observed short-term soil responses after N fertilization events—loss of base cations, soil acidification and reduced microbial biomass—which emphasize the need to investigate consequences for the belowground C and N cycles if chronic N enrichment persists in the long run.

Divergent Accumulation of Microbial Residues and Amino Sugars in Loess Soil after Six Years of Different Inorganic Nitrogen Enrichment Scenarios

Amino sugars are key microbial biomarkers for determining the contribution of microbial residues in soil organic matter (SOM). However, it remains largely unclear as to what extent inorganic nitrogen (N) fertilization can lead to the significant degradation of SOM in alkaline agricultural soils. A six-year field experiment was conducted from 2013 to 2018 to evaluate the effects of chronic N enrichment on microbial residues, amino sugars, and soil biochemical properties under four nitrogen (urea, 46% N) fertilization scenarios: 0 (no-N, control), 75 (low-N), 225 (medium-N), and 375 (high-N) kg N ha−1. The results showed that chronic N enrichment stimulated microbial residues and amino sugar accumulation over time. The medium-N treatment increased the concentration of muramic acid (15.77%), glucosamine (13.55%), galactosamine (18.84%), bacterial residues (16.88%), fungal residues (11.31%), and total microbial residues (12.57%) compared to the control in 2018; however, these concentrations were comparable to the high-N treatment concentrations. The ratio of glucosamine to galactosamine and of glucosamine to muramic acid decreased over time due to a larger increase in bacterial residues as compared to fungal residues. Microbial biomass, soil organic carbon, and aboveground plant biomass positively correlated with microbial residues and amino sugar components. Chronic N enrichment improved the soil biochemical properties and aboveground plant biomass, which stimulated microbial residues and amino sugar accumulation over time.