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1. Long-term fertilization increases soil but not plant or microbial N in a Chihuahuan Desert grassland

   https://doi.org/10.5194/bg-21-2655-2024  

   Mendoza-Martinez, Violeta; Collins, Scott L; McLaren, Jennie R (June 2024, Biogeosciences)

   Abstract. Although the negative consequences of increased nitrogen (N) supply for plant communities and soil chemistry are well known, most studies have focused on mesic grasslands, and the fate of added N in arid and semi-arid ecosystems remains unclear. To study the impacts of long-term increased N deposition on ecosystem N pools, we sampled a 26-year-long fertilization (10 g N m−2 yr−1) experiment in the northern Chihuahuan Desert at the Sevilleta National Wildlife Refuge (SNWR) in New Mexico. To determine the fate of the added N, we measured multiple soil, microbial, and plant N pools in shallow soils at three time points across the 2020 growing season. We found small but significant increases with fertilization in soil-available NO3--N and NH4+-N, yet the soil microbial and plant communities do not appear to be taking advantage of the increased N availability, with no changes in biomass or N content in either community. However, there were increases in total soil N with fertilization, suggesting increases in microbial or plant N earlier in the experiment. Ultimately, the majority of the N added in this multi-decadal experiment was not found in the shallow soil or the microbial or plant community and is likely to have been lost from the ecosystem entirely. 

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2. Long-term nitrogen deposition enhances microbial capacities in soil carbon stabilization but reduces network complexity

   https://doi.org/10.1186/s40168-022-01309-9  

   Ma, Xingyu; Wang, Tengxu; Shi, Zhou; Chiariello, Nona R.; Docherty, Kathryn; Field, Christopher B.; Gutknecht, Jessica; Gao, Qun; Gu, Yunfu; Guo, Xue; et al (December 2022, Microbiome)

   Abstract Background Anthropogenic activities have increased the inputs of atmospheric reactive nitrogen (N) into terrestrial ecosystems, affecting soil carbon stability and microbial communities. Previous studies have primarily examined the effects of nitrogen deposition on microbial taxonomy, enzymatic activities, and functional processes. Here, we examined various functional traits of soil microbial communities and how these traits are interrelated in a Mediterranean-type grassland administrated with 14 years of 7 g m −2 year −1 of N amendment, based on estimated atmospheric N deposition in areas within California, USA, by the end of the twenty-first century. Results Soil microbial communities were significantly altered by N deposition. Consistent with higher aboveground plant biomass and litter, fast-growing bacteria, assessed by abundance-weighted average rRNA operon copy number, were favored in N deposited soils. The relative abundances of genes associated with labile carbon (C) degradation (e.g., amyA and cda ) were also increased. In contrast, the relative abundances of functional genes associated with the degradation of more recalcitrant C (e.g., mannanase and chitinase ) were either unchanged or decreased. Compared with the ambient control, N deposition significantly reduced network complexity, such as average degree and connectedness. The network for N deposited samples contained only genes associated with C degradation, suggesting that C degradation genes became more intensely connected under N deposition. Conclusions We propose a conceptual model to summarize the mechanisms of how changes in above- and belowground ecosystems by long-term N deposition collectively lead to more soil C accumulation. 

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3. Stronger fertilization effects on aboveground versus belowground plant properties across nine U.S. grasslands

   https://doi.org/10.1002/ecy.3891  

   Keller, Adrienne B.; Walter, Christopher A.; Blumenthal, Dana M.; Borer, Elizabeth T.; Collins, Scott L.; DeLancey, Lang C.; Fay, Philip A.; Hofmockel, Kirsten S.; Knops, Johannes M.; Leakey, Andrew D.; et al (October 2022, Ecology)

   Increased nutrient inputs due to anthropogenic activity are expected to increase primary productivity across terrestrial ecosystems, but changes in allocation aboveground versus belowground with nutrient addition have different implications for soil carbon (C) storage. Thus, given that roots are major contributors to soil C storage, understanding belowground net primary productivity (BNPP) and biomass responses to changes in nutrient availability is essential to predicting carbon–climate feedbacks in the context of interacting global environmental changes. To address this knowledge gap, we tested whether a decade of nitrogen (N) and phosphorus (P) fertilization consistently influenced aboveground and belowground biomass and productivity at nine grassland sites spanning a wide range of climatic and edaphic conditions in the continental United States. Fertilization effects were strong aboveground, with both N and P addition stimulating aboveground biomass at nearly all sites (by 30% and 36%, respectively, on average). P addition consistently increased root production (by 15% on average), whereas other belowground responses to fertilization were more variable, ranging from positive to negative across sites. Site-specific responses to P were not predicted by the measured covariates. Atmospheric N deposition mediated the effect of N fertilization on root biomass and turnover. Specifically, atmospheric N deposition was positively correlated with root turnover rates, and this relationship was amplified with N addition. Nitrogen addition increased root biomass at sites with low N deposition but decreased it at sites with high N deposition. Overall, these results suggest that the effects of nutrient supply on belowground plant properties are context dependent, particularly with regard to background N supply rates, demonstrating that site conditions must be considered when predicting how grassland ecosystems will respond to increased nutrient loading from anthropogenic activity. 

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4. Sensitivity of grassland carbon pools to plant diversity, elevated CO ₂ , and soil nitrogen addition over 19 years

   https://doi.org/10.1073/pnas.2016965118  

   Pastore, Melissa A.; Hobbie, Sarah E.; Reich, Peter B. (April 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Whether the terrestrial biosphere will continue to act as a net carbon (C) sink in the face of multiple global changes is questionable. A key uncertainty is whether increases in plant C fixation under elevated carbon dioxide (CO 2 ) will translate into decades-long C storage and whether this depends on other concurrently changing factors. We investigated how manipulations of CO 2 , soil nitrogen (N) supply, and plant species richness influenced total ecosystem (plant + soil to 60 cm) C storage over 19 y in a free-air CO 2 enrichment grassland experiment (BioCON) in Minnesota. On average, after 19 y of treatments, increasing species richness from 1 to 4, 9, or 16 enhanced total ecosystem C storage by 22 to 32%, whereas N addition of 4 g N m −2 ⋅ y −1 and elevated CO 2 of +180 ppm had only modest effects (increasing C stores by less than 5%). While all treatments increased net primary productivity, only increasing species richness enhanced net primary productivity sufficiently to more than offset enhanced C losses and substantially increase ecosystem C pools. Effects of the three global change treatments were generally additive, and we did not observe any interactions between CO 2 and N. Overall, our results call into question whether elevated CO 2 will increase the soil C sink in grassland ecosystems, helping to slow climate change, and suggest that losses of biodiversity may influence C storage as much as or more than increasing CO 2 or high rates of N deposition in perennial grassland systems. 

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5. Plant use of water across soil depths regulates species dominance under nitrogen addition

   https://doi.org/10.1016/j.pld.2025.02.006  

   Huang, Fu-Qiang; Peñuelas, Josep; Sardans, Jordi; Collins, Scott L; Yu, Kai-Liang; Liu, Man-Qiong; Pei, Jiu-Ying; Ke, Wen-Bin; Ye, Jian-Sheng (May 2025, Plant Diversity)

   The primary mechanism driving plant species loss after nitrogen (N) addition has been often hypothesized to be asymmetric competition for light, resulting from increased aboveground biomass. However, it is largely unknown whether plants’ access to soil water at different depths would affect their responses, fate, and community composition under nitrogen addition. In a semiarid grassland exposed to 8-years of N addition, we measured plant aboveground biomass and diversity under four nitrogen addition rates (0, 4, 10, and 16 g m 2 year 1), and evaluated plant use of water across the soil profile using oxygen isotope. Aboveground biomass increased significantly, but diversity and shallow soil-water content decreased, with increasing rate of nitrogen addition. The water isotopic signature for both plant and soil water at the high N rate indicated that Leymus secalinus (a perennial grass) absorbed 7% more water from the subsurface soil layer (20e100 cm) compared to Elymus dahuricus (a perennial grass) and Artemisia annua (an annual forb). L. secalinus thus had a significantly larger biomass and was more abundant than the other two species at the high N rate but did not differ significantly from the other two species under ambient and the low N rate. Species that could use water from deeper soil layers became dominant when water in the shallow layers was insufficient to meet the demands of increased aboveground plant biomass. Our study highlights the importance of water across soil depths as key driver of plant growth and dominance in grasslands under N addition. 

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