Does background nitrogen deposition affect the response of boreal vegetation to fertilization?

N addition alters growth, non-structural carbohydrates, and C:N:P stoichiometry of Reaumuria soongorica seedlings in Northwest China

Reaumuria soongorica is an important biological barrier for ecological protection in the Gobi Desert in northwestern China, where soil nitrogen availability is low. N deposition has recently increased significantly in Gobi Desert, and the responses of R. soongorica to N enrichment may become a problem for ecological restoration and protection. However, little is known about the effects of N addition on the biomass, non-structural carbohydrates (NSC), and carbon:nitrogen:phosphorus (C:N:P) stoichiometry of R. soongorica in this region. Here, we examined changes in biomass, NSC and C:N:P ratios of different organs of R. soongorica seedlings in four N addition treatments: 0 (N₀), 4.6 (N₁), 9.2 (N₂), and 13.8 (N₃) g m⁻² year⁻¹. N addition up to 9.2 g m⁻² year⁻¹ significantly increased the biomass of different organs, simultaneously increasing the belowground: aboveground ratio of R. soongorica seedlings. Root NSC concentrations significantly increased under all N addition treatments, but leaf and stem NSC concentrations only increased under the N₁ and N₂ addition treatments. Nitrogen addition enhanced the soluble sugar concentrations (SSC) of leaves and roots, and reduced starch concentrations (SC) of all organs. Stem and root N concentrations significantly increased under the N₂ and N₃ treatments, and leaf N concentrations only increased under the N₃ treatment, but N addition had no significant effect on plant C and P concentrations. Leaf and stem C:N ratios decreased significantly under the N₂ and N₃ treatments, but root C:N decreased significantly in all N addition treatments. The N₃ treatment significantly increased the N:P ratio of all organs. N addition significantly enhanced available N (AN), available P (AP) and total phosphorus (TP) in rhizosphere soil. Our results suggest that N addition alters the biomass, NSC, N concentrations, C:N and N:P ratios of all plant organs, but roots responded more strongly than stems or leaves to N addition, potentially allowing the plants to absorb more water from the arid soil in this region ensuring the survival of R. soongorica seedlings. Rhizosphere soil AP, AN and TP concentrations were important factors affecting the NSC concentrations and stoichiometric characteristics of R. soongorica.

Levels and variations of soil bioavailable nitrogen among forests under high atmospheric nitrogen deposition

To examine the perturbation of atmospheric nitrogen (N) deposition on soil N status and the biogeochemical cycle is meaningful for understanding forest function evolution with environmental changes. However, levels of soil bioavailable N and their environmental controls in forests receiving high atmospheric N deposition remain less investigated, which hinders evaluating the effects of enhanced anthropogenic N loading on forest N availability and N losses. This study analyzed concentrations of soil extractable N, microbial biomass N, net rates of N mineralization and nitrification, and their relationships with environmental factors among 26 temperate forests under the N deposition rates between 28.7 and 69.0 kg N ha^-1 yr^-1 in the Beijing-Tianjin-Hebei (BTH) region of northern China. Compared with other forests globally, forests in the BTH region showed higher levels of soil bioavailable N (NH₄⁺, 27.1 ± 0.8 mg N kg^-1; NO₃^-, 7.0 ± 0.8 mg N kg^-1) but lower net rates of N mineralization and nitrification (0.5 ± 0.1 mg N kg^-1 d^-1 and 0.4 ± 0.1 mg N kg^-1 d^-1, respectively). Increasing N deposition levels increased soil nitrification and NO₃^- concentrations but did not increase microbial biomass N and N mineralization among the study forests. Soil moisture and C availability were found as dominant factors influencing microbial N mineralization and bioavailable N. In addition, by budgeting the differences in soil total N densities between the 2000s and 2010s, atmospheric N inputs to the forests were more retained in soils than lost proportionally (84% vs. 16%). We concluded that the high N deposition enriched soil N without stimulating microbial N mineralization among the study forests. These results clarified soil N status and the major controlling factors under high anthropogenic N loading, which is helpful for evaluating the fates and ecological effects of atmospheric N pollution.

Long term effect of nitrogen addition on understory community in a Chinese boreal forest

Increasing atmospheric nitrogen (N) deposition is an important driver of biodiversity change. By conducting an eight-year N addition experiment (0, 20, 50 and 100 kg N ha^-1 yr^-1), we investigated the long-term effect of simulated N deposition on understory species composition and richness in a boreal forest, northeast China. We found that moss cover decreased significantly with increasing N addition. N addition had no significant effect on vascular plants species richness but changed the plant community composition. The relative coverage of evergreen shrubs decreased, while that of graminoids increased under high-level N addition (100 kg N ha^-1 yr^-1). Under the high-level N treatment, Deyeuxia angustifolia cover increased significantly after 4 years, while that of Vaccinium vitis-idaea decreased significantly after 3 years and almost disappeared after 5 years. The negative effect of N addition on mosses and evergreen shrubs accumulated over time, while the positive effect on graminoids increased during the first 4 years and did not change significantly thereafter. Our results suggest that the effect of N deposition varies across functional groups and shifts over time.

Response of aboveground biomass and diversity to nitrogen addition - a five-year experiment in semi-arid grassland of Inner Mongolia, China

Understanding the response of the plant community to increasing nitrogen (N) deposition is helpful for improving pasture management in semi-arid areas. We implemented a 5-year N addition experiment in a Stipa krylovii steppe of Inner Mongolia, northern China. The aboveground biomass (AGB) and species richness were measured annually. Along with the N addition levels, the species richness declined significantly, and the species composition changed noticeably. However, the total AGB did not exhibit a noticeable increase. We found that compensatory effects of the AGB occurred not only between the grasses and the forbs but also among Gramineae species. The plant responses to N addition, from the community to species level, lessened in dry years compared to wet or normal years. The N addition intensified the reduction of community productivity in dry years. Our study indicated that the compensatory effects of the AGB among the species sustained the stability of grassland productivity. However, biodiversity loss resulting from increasing N deposition might lead the semi-arid grassland ecosystem to be unsustainable, especially in dry years.

Stimulation of terrestrial ecosystem carbon storage by nitrogen addition: a meta-analysis

Elevated nitrogen (N) deposition alters the terrestrial carbon (C) cycle, which is likely to feed back to further climate change. However, how the overall terrestrial ecosystem C pools and fluxes respond to N addition remains unclear. By synthesizing data from multiple terrestrial ecosystems, we quantified the response of C pools and fluxes to experimental N addition using a comprehensive meta-analysis method. Our results showed that N addition significantly stimulated soil total C storage by 5.82% ([2.47%, 9.27%], 95% CI, the same below) and increased the C contents of the above- and below-ground parts of plants by 25.65% [11.07%, 42.12%] and 15.93% [6.80%, 25.85%], respectively. Furthermore, N addition significantly increased aboveground net primary production by 52.38% [40.58%, 65.19%] and litterfall by 14.67% [9.24%, 20.38%] at a global scale. However, the C influx from the plant litter to the soil through litter decomposition and the efflux from the soil due to microbial respiration and soil respiration showed insignificant responses to N addition. Overall, our meta-analysis suggested that N addition will increase soil C storage and plant C in both above- and below-ground parts, indicating that terrestrial ecosystems might act to strengthen as a C sink under increasing N deposition.

Consequence of altered nitrogen cycles in the coupled human and ecological system under changing climate: The need for long-term and site-based research

Anthropogenically derived nitrogen (N) has a central role in global environmental changes, including climate change, biodiversity loss, air pollution, greenhouse gas emission, water pollution, as well as food production and human health. Current understanding of the biogeochemical processes that govern the N cycle in coupled human-ecological systems around the globe is drawn largely from the long-term ecological monitoring and experimental studies. Here, we review spatial and temporal patterns and trends in reactive N emissions, and the interactions between N and other important elements that dictate their delivery from terrestrial to aquatic ecosystems, and the impacts of N on biodiversity and human society. Integrated international and long-term collaborative studies covering research gaps will reduce uncertainties and promote further understanding of the nitrogen cycle in various ecosystems.