Too Much of a Good Thing? Inorganic Nitrogen (N) Inhibits Moss-Associated N2 Fixation But Organic N Can Promote It

Moss-associated nitrogen (N2) fixation is one of the main inputs of new N in pristine ecosystems that receive low amounts of atmospheric N deposition. Previous studies have shown that N2 fixation is inhibited by inorganic N (IN) inputs, but if N2 fixation in mosses is similarly affected by organic N (ON) remains unknown. Here, we assessed N2 fixation in two dominant mosses in boreal forests (Pleurozium schreberi and Sphagnum capillifolium) in response to different levels of N, simulating realistic (up to 4 kg N ha−1 yr−1) and extreme N deposition rates in pristine ecosystems (up to 20 kg N ha−1 yr−1) of IN (NH4NO3) and ON (alanine and urea). We also assessed if N2 fixation can recover from the N additions. In the realistic scenario, N2 fixation was inhibited by increasing NH4NO3 additions in P. schreberi but not in S. capillifolium, and alanine and urea stimulated N2 fixation in both moss species. In contrast, in the extreme N additions, increasing N inputs inhibited N2 fixation in both moss species and all N forms. Nitrogen fixation was more sensitive to N inputs in P. schreberi than in S. capillifolium and was higher in the recovery phase after the realistic compared to the extreme N additions. These results demonstrate that N2 fixation in mosses is less sensitive to organic than inorganic N inputs and highlight the importance of considering different N forms and species-specific responses when estimating the impact of N inputs on ecosystem functions such as moss-associated N2 fixation.

Effects of Nitrogen Additions on Soil Respiration in an Asian Tropical Montane Rainforest

Understanding the impacts of nitrogen (N) addition on soil respiration (RS) and its temperature sensitivity (Q10) in tropical forests is very important for the global carbon cycle in a changing environment. Here, we investigated how RS respond to N addition in a tropical montane rainforest in Southern China. Four levels of N treatments (0, 25, 50, and 100 kg N ha−1 a−1 as control (CK), low N (N25), moderate N (N50), and high N (N100), respectively) were established in September 2010. Based on a static chamber-gas chromatography method, RS was measured from January 2015 to December 2018. RS exhibited significant seasonal variability, with low RS rates appeared in the dry season and high rates appeared in the wet season regardless of treatment. RS was significantly related to the measured soil temperature and moisture. Our results showed that soil RS increased after N additions, the mean annual RS was 7% higher in N25 plots, 8% higher in N50 plots, and 11% higher in N100 plots than that in the CK plots. However, the overall impacts of N additions on RS were statistically insignificant. For the entire study period, the CK, N25, N50, and N100 treatments yielded Q10 values of 2.27, 3.45, 4.11, and 2.94, respectively. N addition increased the temperature sensitivity (Q10) of RS. Our results suggest that increasing atmospheric N deposition may have a large impact on the stimulation of soil CO2 emissions from tropical rainforests in China.

Long-Term Nitrogen Addition Decreases Soil Carbon Mineralization in an N-Rich Primary Tropical Forest

Anthropogenic elevated nitrogen (N) deposition has an accelerated terrestrial N cycle, shaping soil carbon dynamics and storage through altering soil organic carbon mineralization processes. However, it remains unclear how long-term high N deposition affects soil carbon mineralization in tropical forests. To address this question, we established a long-term N deposition experiment in an N-rich lowland tropical forest of Southern China with N additions such as NH4NO3 of 0 (Control), 50 (Low-N), 100 (Medium-N) and 150 (High-N) kg N ha−1 yr−1, and laboratory incubation experiment, used to explore the response of soil carbon mineralization to the N additions therein. The results showed that 15 years of N additions significantly decreased soil carbon mineralization rates. During the incubation period from the 14th day to 56th day, the average decreases in soil CO2 emission rates were 18%, 33% and 47% in the low-N, medium-N and high-N treatments, respectively, compared with the Control. These negative effects were primarily aroused by the reduced soil microbial biomass and modified microbial functions (e.g., a decrease in bacteria relative abundance), which could be attributed to N-addition-induced soil acidification and potential phosphorus limitation in this forest. We further found that N additions greatly increased soil-dissolved organic carbon (DOC), and there were significantly negative relationships between microbial biomass and soil DOC, indicating that microbial consumption on soil-soluble carbon pool may decrease. These results suggests that long-term N deposition can increase soil carbon stability and benefit carbon sequestration through decreased carbon mineralization in N-rich tropical forests. This study can help us understand how microbes control soil carbon cycling and carbon sink in the tropics under both elevated N deposition and carbon dioxide in the future.

Nitrogen deposition accelerates soil carbon sequestration in tropical forests

Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO2 and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems.

Biochar Amendment Alters The Nutrient-Use Strategy of Moso Bamboo Under N Additions

Background: While we know that N and biochar fertilizers affect soil nutrient concentrations and plant nutrient uptake, our understanding of how combined applications of N and biochar affect plant nutrient resorption in plantations is largely inadequate. A field experiment was conducted to investigate the effects of N (0, 30, 60, and 90 kg N ha-1 yr-1 or N0, N30, N60, and N90), in combination with biochar (0, 20, and 40 t biochar ha-1 or BC0, BC20, and BC40) on N and P resorption by young and mature bamboo plants as well as the relationship between nutrient resorption and leaf nutrient and soil concentrations. Fresh and senescent leaf samples were collected in July 2016 and March 2017, respectively.Results: Young bamboo showed significantly greater foliar N resorption efficiency (NRE) and P resorption efficiency (PRE) than mature bamboo. N additions alone significantly increased the N resorption proficiency (NRP) and P resorption proficiency (PRP) but decreased the NRE and PRE of both young and mature bamboo. In both the N-free (control) and N addition treatments, biochar amendments significantly reduced the foliar NRE and PRE of young bamboo but had the opposite effect on mature bamboo. Foliar NRE and PRE were significantly correlated with fresh leaf N and P concentrations and soil total P concentration. Conclusion: Our findings suggest that N addition inhibits plant nutrient resorption and alters the nutrient-use strategy of young and mature bamboo from “conservative consumption” to “resource spending.” Furthermore, biochar amendment enhanced the negative priming effect of N addition on nutrient resorption of young bamboo but reduced the negative effect on that of mature bamboo. This study provides new insights into the combined effects of N and biochar additions on the nutrient resorption of Moso bamboo and may assist in improving fertilization strategies in Moso bamboo plantations.

Fungi Outcompete Bacteria for Straw and Soil Organic Matter Mineralization

Background: Understanding the effects of straw return and nitrogen (N) fertilization on soil organic matter (SOM) transformations will help to mitigate climate change and maintain crop production and soil function. A 100-day soil incubation experiment was conducted using a two-factorial design with three fertilization levels and four 13C-labeled maize straw and N addition treatments. The competition and contributions of the bacterial and fungal communities were assessed with relation to straw mineralization.Results: Mineral fertilizer alone and with straw increased straw decomposition by 59% and 55% and SOM mineralization by 27% and 37%, respectively, compared with the unfertilized soil, due to raised β-N-acetylglucosaminidase and cellobiohyrolase activities. Conversely, priming effect was decreased by 59% and 39%, respectively. Priming effect increased with higher N additions and decreased with lower N additions because an improved C:N ratio for microorganisms. Straw additions increased bacterial and fungal abundance by 1.4 and 4.9 times. Fungal diversity decreased with N fertilization because lower C:N ratios increased the bacterial competition. Bacterial abundance decreased but diversity increased with the duration of incubation as bacteria preferred to utilize labile organic compounds abundant in the initial stages. Along with labile organic compounds depletion, fungal abundance was increased. Firmicutes, Actinobacteria, and Proteobacteria bacterial as well as Ascomycota, Basidiomycota, and Mucoromycota fungi dominated straw and SOM decomposition. Firmicutes were mostly involved in straw and SOM mineralization on day one because of their capacity for labile compound decomposition. Integrated co-occurrence networks revealed that fungal taxa had a stronger correlation with straw decomposition than bacterial groups. Straw and N addition increased the number of negative edges among bacterial taxa but these decreased within fungal groups when compared to trials without straw and N. The ratio for pairwise correlations between abundant fungal taxa, straw, and SOM mineralization (29.9%) was greater than with bacteria (1.2%).Conclusions: Straw with low N additions increased soil C sequestration by decreasing priming effect. Straw alone and with N addition decreased competition for C and N among fungal groups, but increased competition within bacterial taxa. Fungi outcompete bacteria for straw and soil organic matter mineralization in long-term fertilized soils.