Cascading effects of N fertilization activate biologically driven mechanisms promoting P availability in a semi‐arid grassland ecosystem

Different microbial functional traits drive bulk and rhizosphere soil phosphorus mobilization in an alpine meadow after nitrogen input

Enhanced nitrogen (N) input is expected to influence the soil phosphorus (P) cycling through biotic and abiotic factors. Among these factors, soil microorganisms play a vital role in regulating soil P availability. However, the divergent contribution of functional microorganisms to soil P availability in the rhizosphere and bulk soil under N addition remains unclear. We conducted an N addition experiment with four N input rates (0, 5, 10, and 15 g N m-2 year-1) in an alpine meadow over three years. Metagenomics was employed to investigate the functional microbial traits in the rhizosphere and bulk soil. We showed that N addition had positive effects on microbial functional traits related to P-cycling in the bulk and rhizosphere soil. Specifically, high N addition significantly increased the abundance of most microbial genes in the bulk soil but only enhanced the abundance of five genes in the rhizosphere soil. The soil compartment, rather than the N addition treatment, was the dominant factor explaining the changes in the diversity and network of functional microorganisms. Furthermore, the abundance of functional microbial genes had a profound effect on soil available P, particularly in bulk soil P availability driven by the ppa and ppx genes, as well as rhizosphere soil P availability driven by the ugpE gene. Our results highlight that N addition stimulates the microbial potential for soil P mobilization in alpine meadows. Distinct microbial genes play vital roles in soil P availability in bulk and rhizosphere soil respectively. This indicates the necessity for models to further our knowledge of P mobilization processes from the bulk soil to the rhizosphere soil, allowing for more precise predictions of the effects of N enrichment on soil P cycling.

Nitrogen addition regulates the effects of variation in precipitation patterns on plant biomass formation and allocation in a Leymus chinensis grassland of northeast China

Global warming is predicted to change precipitation amount and reduce precipitation frequency, which may alter grassland primary productivity and biomass allocation, especially when interact with other global change factors, such as nitrogen deposition. The interactive effects of changes in precipitation amount and nitrogen addition on productivity and biomass allocation are extensively studied; however, how these effects may be regulated by the predicted reduction in precipitation frequency remain largely unknown. Using a mesocosm experiment, we investigated responses of primary productivity and biomass allocation to the manipulated changes in precipitation amount (PA: 150 mm, 300 mm, 450 mm), precipitation frequency (PF: medium and low), and nitrogen addition (NA: 0 and 10 g N m-2 yr-1) in a Leymus chinensis grassland. We detected significant effects of the PA, PF and NA treatments on both aboveground biomass (AGB) and belowground biomass (BGB); but the interactive effects were only significant between the PA and NA on AGB. Both AGB and BGB increased with an increment in precipitation amount and nitrogen addition; the reduction in PF decreased AGB, but increased BGB. The reduced PF treatment induced an enhancement in the variation of soil moisture, which subsequently affected photosynthesis and biomass formation. Overall, there were mismatches in the above- and belowground biomass responses to changes in precipitation regime. Our results suggest the predicted changes in precipitation regime, including precipitation amount and frequency, is likely to alter primary productivity and biomass allocation, especially when interact with nitrogen deposition. Therefore, predicting the influence of global changes on grassland structure and functions requires the consideration of interactions among multiple global change factors.

Microbial network structure, not plant and microbial community diversity, regulates multifunctionality under increased precipitation in a cold steppe

It is known that the dynamics of multiple ecosystem functions (i. e., multifunctionality) are positively associated with microbial diversity and/or biodiversity. However, how the relationship between microbial species affects ecosystem multifunctionality remains unclear, especially in the case of changes in precipitation patterns. To explore the contribution of biodiversity and microbial co-occurrence networks to multifunctionality, we used rainfall shelters to simulate precipitation enhancement in a cold steppe in Northeast China over two consecutive growing seasons. We showed that an increased 50% precipitation profoundly reduced bacterial diversity and multidiversity, while inter-annual differences in precipitation did not shift microbial diversity, plant diversity, or multidiversity. Our analyses also revealed that increased annual precipitation significantly increased ecosystem, soil, nitrogen, and phosphorous cycle multifunctionality. Neither increased precipitation nor inter-annual differences in precipitation had a significant effect on carbon cycle multifunctionality, probably due to the relatively short period (2 years) of our experiment. The co-occurrence network of bacterial and fungal communities was the most dominant factor affecting multifunctionality, the numbers of negative interactions but not positive interactions were linked to multifunctionality. In particular, our results provided evidence that microbial network topological features are crucial for maintaining ecosystem functions in grassland ecosystems, which should be considered in related studies to accurately predict the responses of ecosystem multifunctionality to predicted changes in precipitation patterns.

Suppression of methane uptake by precipitation pulses and long-term nitrogen addition in a semi-arid meadow steppe in northeast China

In the context of global change, the frequency of precipitation pulses is expected to decrease while nitrogen (N) addition is expected to increase, which will have a crucial effect on soil C cycling processes as well as methane (CH₄) fluxes. The interactive effects of precipitation pulses and N addition on ecosystem CH₄ fluxes, however, remain largely unknown in grassland. In this study, a series of precipitation pulses (0, 5, 10, 20, and 50 mm) and long-term N addition (0 and 10 g N m^-2 yr^-1, 10 years) was simulated to investigate their effects on CH₄ fluxes in a semi-arid grassland. The results showed that large precipitation pulses (10 mm, 20 mm, and 50 mm) had a negative pulsing effect on CH₄ fluxes and relatively decreased the peak CH₄ fluxes by 203-362% compared with 0 mm precipitation pulse. The large precipitation pulses significantly inhibited CH₄ absorption and decreased the cumulative CH₄ fluxes by 68-88%, but small precipitation pulses (5 mm) did not significantly alter it. For the first time, we found that precipitation pulse size increased cumulative CH₄ fluxes quadratically in both control and N addition treatments. The increased soil moisture caused by precipitation pulses inhibited CH₄ absorption by suppressing CH₄ uptake and promoting CH₄ release. Nitrogen addition significantly decreased the absorption of CH₄ by increasing NH₄ ⁺-N content and NO₃ ^--N content and increased the production of CH₄ by increasing aboveground biomass, ultimately suppressing CH₄ uptake. Surprisingly, precipitation pulses and N addition did not interact to affect CH₄ uptake because precipitation pulses and N addition had an offset effect on pH and affected CH₄ fluxes through different pathways. In summary, precipitation pulses and N addition were able to suppress the absorption of CH₄ from the atmosphere by soil, reducing the CH₄ sink capacity of grassland ecosystems.

Extreme drought exacerbates plant nitrogen‑phosphorus imbalance in nitrogen enriched grassland

The nitrogen‑phosphorus (N-P) imbalance induced by N enrichment has received increasing concerns, because N:P ratios play a critical role in driving many fundamental ecological processes. Given the simultaneous occurrence of different global change drivers, it is important to understand whether and how would such N-induced N-P imbalance would be mediated by other global change factors. We examined the interactive effects of N addition (10 g N m^-2 yr^-1) and extreme drought (-66 % rainfall during the growing season) on species- and community-level N:P ratios in both green and senesced leaves in a temperate grassland of northern China. Extreme drought did not alter soil available N:P ratio under ambient N conditions, but increased that under N enriched conditions. Further, extreme drought did not alter the community-level N:P in both green and senesced leaves under ambient N conditions but significantly enhanced that under N enriched conditions. The drought-induced species turnover made a significant positive contribution to the changes in the community-level N:P ratio under N enriched conditions, but not under ambient N conditions. Our results suggest that the N-induced ecosystem N-P imbalance would be exacerbated by extreme drought event, the frequency of which is predicted to increase across global drylands. Such N-P imbalance would have consequences on litter decomposition, nutrient cycling, and the structures of above- and below-ground food webs. Our findings highlighted the complexity in predicting ecosystem N-P imbalance given the interactions between different global change drivers.

Mowing enhances the positive effects of nitrogen addition on ecosystem carbon fluxes and water use efficiency in a semi-arid meadow steppe

Grasslands are now facing a continuously increasing supply of nitrogen (N) fertilizers, resulting in alterations in ecosystem functioning, including changes in carbon (C) and water cycling. Mowing, one of the most widely used grassland management techniques, has been shown to mitigate the negative impacts of increased N availability on species richness. However, knowledge of how N addition and mowing, alone and/or in combination, affect ecosystem-level C fluxes and water use efficiency (W_N) is still limited. We experimentally manipulated N fertilization (0 and 10 g N m^-2 yr^-1) and mowing (once per year at the end of the growing season) following a randomized block design in a meadow steppe characterized by salinization and alkalinization in northeastern China. We found that, compared to the control plots, N addition, mowing, and their interaction increased net ecosystem CO₂ exchange by 65.1%, 14.7%, and 133%, and W_N by 40.7%, 18.5%, and 96.1%, respectively. Nitrogen enrichment also decreased soil pH, which resulted in greater aboveground biomass (AGB). Moreover, N addition indirectly increased AGB by inducing changes in species richness. Our results indicate that mowing enhances the positive effects of N addition on ecosystem C fluxes and W_N. Therefore, appropriate grassland management practices are essential to improve ecosystem C sequestration, W_N, and mitigate future species diversity declines due to ecosystem eutrophication.

Plant Diversity and Fungal Richness Regulate the Changes in Soil Multifunctionality in a Semi-Arid Grassland

Loss in plant diversity is expected to impact biodiversity and ecosystem functioning (BEF) in terrestrial ecosystems. Soil microbes play essential roles in regulating ecosystem functions. However, the important roles and differences in bacterial and fungal diversity and rare microbial taxa in driving soil multifunctionality based on plant diversity remain poorly understood in grassland ecosystems. Here, we carried out an experiment in six study sites with varied plant diversity levels to evaluate the relationships between soil bacterial and fungal diversity, rare taxa, and soil multifunctionality in a semi-arid grassland. We used Illumina HiSeq sequencing to determine soil bacterial and fungal diversity and evaluated soil functions associated with the nutrient cycle. We found that high diversity plant assemblages had a higher ratio of below-ground biomass to above-ground biomass, soil multifunctionality, and lower microbial carbon limitation than those with low diversity. Moreover, the fungal richness was negatively and significantly associated with microbial carbon limitations. The fungal richness was positively related to soil multifunctionality, but the bacterial richness was not. We also found that the relative abundance of saprotrophs was positively correlated with soil multifunctionality, and the relative abundance of pathogens was negatively correlated with soil multifunctionality. In addition, the rare fungal taxa played a disproportionate role in regulating soil multifunctionality. Structural equation modeling showed that the shift of plant biomass allocation patterns increased plant below-ground biomass in the highly diverse plant plots, which can alleviate soil microbial carbon limitations and enhance the fungal richness, thus promoting soil multifunctionality. Overall, these findings expand our comprehensive understanding of the critical role of soil fungal diversity and rare taxa in regulating soil multifunctionality under global plant diversity loss scenarios.