Effects of nitrogen deposition on carbon cycle in terrestrial ecosystems of China: A meta-analysis

Effects of microplastics and nitrogen deposition on soil multifunctionality, particularly C and N cycling

Both nitrogen deposition (ND) and microplastics (MPs) pose global change challenges. The effects of MPs co-existing with ND on ecosystem functions are still largely unknown. Herein, we conducted a 10-month soil incubation experiment to explore the effects of polyethylene (PE) and polylactic acid (PLA) MPs on soil multifunctionality under different ND scenarios. We found that the interactions between ND and MPs affected soil multifucntionality. FAPROTAX function prediction indicated that both ND and MPs affected C and N cycling. ND increased some C-cycling processes, such as cellulolysis, ligninolysis, and plastic degradation. MPs also showed stimulating effects on these processes, particularly in the soil with ND. ND significantly decreased the abundance of functional genes NifH, amoA, and NirK, leading to inhibited N-fixation, nitrification, and denitrification. The addition of MPs also modified N-cycling processes: 0.1% PE enriched the bacterial groups for nitrate reduction, nitrate respiration, nitrite respiration, and nitrate ammonification, and 1% PLA MPs enriched N-fixation bacteria at all ND levels. We found that ND caused lower soil pH but higher soil N, decreased bacterial diversity and richness, and changed the composition and activity of functional bacteria, which explains why ND changed soil functions and regulated the impact of MPs.

Simulated nitrogen deposition promotes the carbon assimilation of shrubs rather than tree species in an evergreen broad-leaved forest

Although understory addition of nitrogen (UAN) is commonly used to simulate nitrogen deposition in field studies in forest ecosystems, it ignores the effects of atmospheric nitrogen deposition on the canopy. We studied the effects of nitrogen deposition simulated by UAN and by canopy addition of nitrogen (CAN) on leaf structure, chemical properties, Calvin cycle, and photosynthate distribution strategy of representative woody plant species in a subtropical evergreen broadleaved forest in South China. The results showed that maximum photosynthetic rate (A_max) of shrub species Blastus cochinchinensis and Ardisia quinquegona under CAN treatments was significantly higher than that of UAN treatments at the same N addition concentration. The concentrations of intermediates (PGK, DPGA and G3P) in Calvin cycle of B. cochinchinensis and A. quinquegona, and Castanea henryi were significantly increased with CAN treatments, but the opposite was true with UAN treatments. CAN25 significantly increased starch concentrations of shrub species Lasianthus chinensis and B. cochinchinensis, and significantly decreased sucrose concentrations of shrub species A. quinquegona and tree species C. henryi. Correlation analyses showed that nitrogen application amount under different modes helped explain the changes in A_max and Calvin cycle intermediates. In summary, nitrogen deposition may promote the A_max and Calvin cycle of shrub species, and the adaptability of shrub species to nitrogen deposition is higher than that of tree species, which may help to explain the degradation of subtropical evergreen broad-leaved forest.

Soil bacterial community is more sensitive than fungal community to canopy nitrogen deposition and understory removal in a Chinese fir plantation

Soil microorganisms are key regulators for plant growth and ecosystem health of forest ecosystem. Although previous research has demonstrated that soil microorganisms are greatly affected by understory nitrogen (N) addition, little is known about the effects of canopy N addition (CNA) and understory management on soil microorganisms in forests. In this study, we conducted a full designed field experiment with four treatments: CNA (25 kg N ha-1 year-1), understory removal (UR), canopy N addition, and understory removal (CNAUR) (25 kg N ha-1 year-1), and control in a Chinese fir plantation. High-throughput sequencing and qPCR techniques were used to determine the abundance, diversity, and composition of bacterial and fungal communities in three soil layers. Our results showed that CNA increased bacterial diversity in the 10-20 cm soil layer but decreased bacterial abundance in the 20-40 cm soil layer and fungal diversity in the 0-10 cm soil layer. UR increased bacterial abundance only in the 20-40 cm soil layer. CNA, not UR significantly altered the compositions of soil bacterial and fungal community compositions, especially in the 0-20 cm soil layer. CNA sharply reduced the relative abundance of copiotrophic taxa (i.e., taxa in the bacterial phylum Proteobacteria and the orders Eurotiales and Helotiales in the fungal phylum Ascomycota) but increased the relative abundance of oligotrophic taxa (i.e., in the bacterial phylum Verrucomicrobia). RDA analysis revealed that soil pH, DON, and DOC were the main factors associated with the variation in bacterial and fungal communities. Our findings suggest that short-term CNA changes both soil bacterial and fungal communities, with stronger responses in the surface and middle soil than in the deep soil layer, and that UR may enhance this effect on the soil bacterial abundance. This study improves our understanding of soil microorganisms in plantations managed with understory removal and that experience increases in N deposition.

High Level of Ammonium Nitrogen Increases Net Ecosystem Productivity in a Quercus liaotungensis Forest in Northern China

Forest ecosystems are vital to the terrestrial ecosystem’s carbon (C) cycle. The effects of nitrogen (N) addition on C sequestration in forest ecosystems are critical for better understanding C dynamics when facing an increase in N availability. We conducted a six-year field experiment to examine the effects of N addition on C sequestration and net ecosystem productivity (NEP) in a Quercus liaotungensis forest in northern China. N addition resulted in a significant increase in biomass C storage (17.54–48.62%) and changed the distribution patterns of above and belowground biomass C storage, resulting in a 9.64 to 23.23% reduction in the proportion of belowground biomass C compared with the control. The annual average heterotrophic respiration was significantly increased by the additional N (by 0.06–0.94 Mg C ha−1 yr1). In comparison with the control, the C sequestration efficiency driven by N addition ranged from 7.12 to 33.50 kg C/kg N. High-level N addition exerted stronger effects on ecosystem C sequestration than low-level N addition. NH4+-N, rather than NO3−-N, dominated the increase in ecosystem C sequestration. We found that Q. liaotungensis forest acted as a C sink. The increase in NEP in the study forest in northern China was mainly due to an increase in net primary productivity (NPP) caused by N addition. Atmospheric N deposition increased the C sequestration efficiency depending on the rate and form of N deposition.

Experimental Warming Has Not Affected the Changes in Soil Organic Carbon During the Growing Season in an Alpine Meadow Ecosystem on the Qinghai-Tibet Plateau

The effects of climate warming and season on soil organic carbon (SOC) have received widespread attention, but how climate warming affects the seasonal changes of SOC remains unclear. Here, we established a gradient warming experiment to investigate plant attributes and soil physicochemical and microbial properties that were potentially associated with changes in SOC at the beginning (May) and end (August) of the growing season in an alpine meadow ecosystem on the Qinghai-Tibet Plateau. The SOC of August was lower than that of May, and the storage of SOC in August decreased by an average of 18.53 million grams of carbon per hectare. Warming not only failed to alter the content of SOC regardless of the season but also did not affect the change in SOC during the growing season. Among all the variables measured, microbial biomass carbon was highly coupled to the change in SOC. These findings indicate that alpine meadow soil is a source of carbon during the growing season, but climate warming has no significant impact on it. This study highlights that in the regulation of carbon source or pool in alpine meadow ecosystem, more attention should be paid to changes in SOC during the growing season, rather than climate warming.

Variations and controlling factors of soil denitrification rate

The denitrification process profoundly affects soil nitrogen (N) availability and generates its byproduct, nitrous oxide, as a potent greenhouse gas. There are large uncertainties in predicting global denitrification because its controlling factors remain elusive. In this study, we compiled 4301 observations of denitrification rates across a variety of terrestrial ecosystems from 214 papers published in the literature. The averaged denitrification rate was 3516.3 ± 91.1 µg N kg^-1  soil day^-1 . The highest denitrification rate was 4242.3 ± 152.3 µg N kg^-1  soil day^-1 under humid subtropical climates, and the lowest was 965.8 ± 150.4 µg N kg^-1 under dry climates. The denitrification rate increased with temperature, precipitation, soil carbon and N contents, as well as microbial biomass carbon and N, but decreased with soil clay contents. The variables related to soil N contents (e.g., nitrate, ammonium, and total N) explained the variation of denitrification more than climatic and edaphic variables (e.g., mean annual temperature (MAT), soil moisture, soil pH, and clay content) according to structural equation models. Soil microbial biomass carbon, which was influenced by soil nitrate, ammonium, and total N, also strongly influenced denitrification at a global scale. Collectively, soil N contents, microbial biomass, pH, texture, moisture, and MAT accounted for 60% of the variation in global denitrification rates. The findings suggest that soil N contents and microbial biomass are strong predictors of denitrification at the global scale.

Effect of atmospheric nitrogen deposition and its components on carbon flux in terrestrial ecosystems in China

Long-term atmospheric nitrogen (N) deposition increases bioavailable N in terrestrial ecosystems, thereby influencing ecosystem productivity. However, how N deposition and its components (i.e., NO₃^--N and NH₄⁺-N) influence the spatial pattern of productivity in terrestrial ecosystems in China remains unknown. Here, we utilize published data including carbon (C) fluxes from eddy flux tower (gross ecosystem productivity, ecosystem respiration, and net ecosystem productivity) and the corresponding climate and N deposition data for 60 typical ecosystems in China. The objective was to investigate the effect of N deposition on ecosystem productivity and explore the variations of N use efficiency (NUE). Our results reveal that atmospheric total N deposition is significantly correlated with C fluxes of terrestrial ecosystems in China. Ecosystems respond variably to different components of N deposition. In detail, forest ecosystem marginally correlated with NO₃^--N and wet deposition, while grassland ecosystem significantly correlated with NH₄⁺-N and dry deposition. NUE of productivity induced by N deposition in Chinese terrestrial ecosystems was 53.95 ± 40.30 g C g^-1 N, and it was influenced by precipitation and aridity index. This study quantifies the contribution of total N deposition and its associated components to productivity in terrestrial ecosystems in China, offering vital information for regional C and N management.

Meta-analysis of the effects of liming on soil pH and cadmium accumulation in crops

Increasing cadmium (Cd) contamination in agricultural fields has resulted in a higher risk of Cd accumulation in the food chain. Lime addition can mitigate soil acidification and reduce Cd accumulation in crops cultured in Cd-contaminated soil. To determine key factors controlling the outcomes of liming in reducing Cd accumulation and enhancing soil pH, we performed a meta-analysis using previously published data from field and pot experiments. The results indicated that the liming showed positive effect sizes on the soil pH but negative effect sizes on Cd accumulation in crops, indicating the addition of different lime materials could enhance soil pH and reduce Cd accumulation in crops. The effect sizes of liming on soil pH under pot experimental conditions were higher than that under field experimental conditions, however, the effect sizes of application types and amount of limes on soil pH did not significantly differ between their individual different levels. Under a low background value of soil pH, SOM, CEC and clay, the addition of limes showed a significantly higher effect size on soil pH when compared to their individual higher soil background value, suggesting that the lower background values of soil pH, SOM, CEC and clay might facilitate the outcomes of liming to enhance soil pH. The experiment patterns, crop types and lime application amounts showed a limit effect on the outcomes of liming to reduce the shoot and grain Cd concentrations in crops. The lime types only showed a significant effect size on the shoot Cd accumulation but not on the grain Cd accumulation, in which the CaCO₃ had the highest effect size (absolute value, the same below) followed by Ca(OH)₂ and CaO. The low soil background values of total Cd concentration and CEC content, but a high soil SOM background content might facilitate the outcomes of liming to reduce the shoot Cd concentration in crops. However, only the background value of soil clay content showed a significantly negative effect size on the grain Cd accumulation, where a high soil clay content had a higher effect size than a low soil clay content. These findings provide useful knowledge about the effects of experiment patterns, crop types, soil conditions, lime types and lime addition amounts on the efficiency of liming in enhancing soil pH and decrease crop Cd concentration.

Nitrogen fertilization impact on soil carbon pools and their stratification and lability in subtropical wheat-mungbean-rice agroecosystems

Nitrogen (N) is the prime nutrient for crop production and carbon-based functions associated with soil quality. The objective of our study (2012 to 2019) was to evaluate the impact of variable rates of N fertilization on soil organic carbon (C) pools and their stocks, stratification, and lability in subtropical wheat (Triticum aestivum)-mungbean (Vigna radiata)-rice (Oryza sativa L) agroecosystems. The field experiment was conducted in a randomized complete block design (RCB) with N fertilization at 60, 80, 100, 120, and 140% of the recommended rates of wheat (100 kg/ha), mungbean (20 kg/ha), and rice (80 kg/ha), respectively. Composite soils were collected at 0-15 and 15-30 cm depths from each replicated plot and analyzed for microbial biomass (MBC), basal respiration (BR), total organic C (TOC), particulate organic C (POC), permanganate oxidizable C (POXC), carbon lability indices, and stratification. N fertilization (120 and 140%) significantly increased the POC at both depths; however, the effect was more pronounced in the surface layer. Moreover, N fertilization (at 120% and 140%) significantly increased the TOC and labile C pools when compared to the control (100%) and the lower rates (60 and 80%). N fertilization significantly increased MBC, C pool (CPI), lability (CLI), and management indices (CMI), indicating improved and efficient soil biological activities in such systems. The MBC and POC stocks were significantly higher with higher rates of N fertilization (120% and 140%) than the control. Likewise, higher rates of N fertilization significantly increased the stocks of labile C pools. Equally, the stratification values for POC, MBC, and POXC show evidence of improved soil quality because of optimum N fertilization (120-140%) to maintain and/or improve soil quality under rice-based systems in subtropical climates.

Three-year-period nitrogen additions did not alter soil organic carbon content and lability in soil aggregates in a tropical forest

Soil immobilizes a considerable proportion of carbon (C) as organic matter in terrestrial ecosystems and is thus critical to stabilize the global climate system. Atmospheric nitrogen (N) deposition could influence soil C storage and stabilization, but how N deposition changes soil organic C (SOC) fractions and lability remains elusive. We investigated the effects of 3-year-period N inputs on SOC fractions and lability along three soil depths (0-10, 10-20, and 20-40 cm) in a tropical forest of southern China. Results showed that N additions did not significantly change contents of SOC fractions and the C lability, either in bulk or aggregate-based soils at any of the three depths, and it showed no significant interaction with soil aggregate or soil depth. The SOC content was 43.7 ± 1.5, 18.2 ± 1.0, and 10.7 ± 0.4 mg g^-1 at the three soil layers downwards, with the non-readily oxidizable SOC (NROC) contributing over 70% while the remaining SOC consisting of readily oxidizable SOC at each soil layer. Moreover, contents of SOC and NROC were consistently higher in small soil aggregates, but the C decrement with increasing size of soil aggregates declined along soil profile downwards. This scenario suggests that physical protection of the small soil aggregate is limited, but its greater specific surface area could obviously contribute to the SOC pattern among soil aggregates. These results indicate that the highly developed forests could be resistant to short-term N deposition, even with a high load, to maintain its SOC stabilization.

Climate and atmospheric deposition drive the inter-annual variability and long-term trend of dissolved organic carbon flux in the conterminous United States

The lateral flux of dissolved organic carbon (DOC) from soils to inland waters and ultimately to the ocean represents a fundamental component of the global carbon cycle. To estimate the DOC flux, we developed an empirical terrestrial-aquatic DOC fluxes model (TAF-DOC). TAF-DOC incorporates various environmental factors (e.g., meteorology, sulfur, and nitrogen deposition) that to-date have not been comprehensively considered or well-represented in existing modeling frameworks. TAF-DOC was applied to estimate spatial-temporal patterns of DOC flux and potential fates across the conterminous United States during the 1985 to 2018 time period. Our results suggest that TAF-DOC successfully characterized spatial-temporal of DOC flux. As expected, the interannual pattern of DOC flux was strongly regulated by precipitation, but the long-term trend was significantly influenced by the rate of atmospheric wet sulfur deposition. From 1985 to 2018, TAF-DOC estimated DOC loading from terrestrial to aquatic ecosystems in the conterminous United States to be 33.5 ± 2.2 TgC per year, which was roughly 0.39-0.49% of total soil organic carbon stock estimates. The dominant fate of terrestrially-derived DOC was delivery to the coastal ocean in riverine export (41%), with another 21% buried in sediment and the remaining 12.8 ± 0.4 TgC per year (38%) returned to the atmosphere through outgassing from inland waters. Assuming the quantities of DOC sediment burial and export to the ocean as an annual sink of terrestrially-derived carbon, budget inventories and models that do not account for DOC flux in the conterminous United States will underestimate the net annual carbon sink by as much as 5.5-6.4%.

Nitrogen addition stimulates soil aggregation and enhances carbon storage in terrestrial ecosystems of China: A meta-analysis

China is experiencing a high level of atmospheric nitrogen (N) deposition, which greatly affects the soil carbon (C) dynamics in terrestrial ecosystems. Soil aggregation contributes to the stability of soil structure and to soil C sequestration. Although many studies have reported the effects of N enrichment on bulk soil C dynamics, the underlying mechanisms explaining how soil aggregates respond to N enrichment remain unclear. Here, we used a meta-analysis of data from 76N manipulation experiments in terrestrial ecosystems in China to assess the effects of N enrichment on soil aggregation and its sequestration of C. On average, N enrichment significantly increased the mean weight diameter of soil aggregates by 10%. The proportion of macroaggregates and silt-clay fraction were significantly increased (6%) and decreased (9%) by N enrichment, respectively. A greater response of macroaggregate C (+15%) than of bulk soil C (+5%) to N enrichment was detected across all ecosystems. However, N enrichment had minor effects on microaggregate C and silt-clay C. The magnitude of N enrichment effect on soil aggregation varied with ecosystem type and fertilization regime. Additionally, soil pH declined consistently and was correlated with soil aggregate C. Overall, our meta-analysis suggests that N enrichment promotes particulate organic C accumulation via increasing macroaggregate C and acidifying soils. In contrast, increases in soil aggregation could inhibit microbially mediated breakdown of soil organic matter, causing minimal change in mineral-associated organic C. Our findings highlight that atmospheric N deposition may enhance the formation of soil aggregates and their sequestration of C in terrestrial ecosystems in China.

High nitrogen addition decreases the ozone flux by reducing the maximum stomatal conductance in poplar saplings

Ground-level ozone (O₃) and nitrogen (N) deposition are major environmental pollutants, often occurring concurrently. Ozone exposure- and flux-response relationships for tree biomass are used for regional O₃ risk assessment. In order to investigate whether soil N addition affects stomatal O₃ uptake of poplar, poplar saplings were exposed to treatment combinations of five O₃ levels and four N addition levels. High N addition treatment reduced the accumulated stomatal O₃ uptake in the leaf due to reduced maximum stomatal conductance (g_s). Nitrogen addition also significantly reduced the steady-state light-saturated g_s in August and September. Elevated O₃ significantly reduced and N addition increased total plant biomass; however, there were no significant O₃ × N interactions. The slopes of biomass-based O₃ exposure- and flux-response relationships did not differ significantly among N treatments. The critical levels for a 5% biomass reduction were estimated at 15.4 ppm h and 17.1 mmol O₃ m^-2 projected leaf area (PLA) for Accumulated O₃ exposure Over an hourly Threshold of 40 ppb (AOT40) and Phytotoxic Ozone Dose above a threshold 1 nmol O₃ m^-2 PLA s^-1 (POD₁). These results can facilitate the evaluations of O₃ effect on the carbon-sink capacity and productivity of forest.

Canopy mitigates the effects of nitrogen deposition on soil carbon-related processes in a subtropical forest

The rapid increases in atmospheric nitrogen (N) deposition have greatly affected the carbon (C) cycles of terrestrial ecosystems. Most studies concerning on the effects of N deposition have simulated N deposition by directly applying N to the understory and have therefore not accounted for the possibility of N absorption, retention, and transformation by the canopy. In this study, we compared the effects of understory addition of N (UN), canopy addition of N (CN) at 25 and 50 kg N ha^-1 yr^-1, and ambient addition of N (CK) on soil carbon-related processes in a subtropical forest. After seven years of addition, the contribution of new C from litter (F_new) was more than 2× greater with UN treatments than with CN treatments. UN treatments significantly increased the activity of β-1,4-glucosidase (BG) but reduced the activities of β-1,4-N-acetylglucosaminidase (NAG), polyphenol oxidase (PPO), and peroxidase (PER). CN treatments, in contrast, did not alter the activities of extracellular enzyme. Compared to CN, UN treatments significantly enhanced soil organic carbon (SOC) and mean weight diameter (MWD, represents soil aggregate stability). Differences in the responses of SOC and MWD to CN and UN treatments were positively correlated with F_new but negatively correlated with the activities of PPO and PER. The results imply that forest canopy mitigates the effects of atmospheric N inputs on SOC, and that conventional understory N addition might overestimate the positive effects of N deposition on forest soil C-related processes. We suggest that CN rather than UN should be used to simulate the effects of atmospheric N deposition on soil C dynamics in subtropical forests.

Shoot and root biomass production in semi-arid shrublands exposed to long-term experimental N input

Anthropogenic nitrogen (N) deposition has affected the primary production of terrestrial ecosystems worldwide; however, ecosystem responses often vary over time because of transient responses, interactions between N, precipitation, and/or other nutrients, and changes in plant species composition. Here we report N-induced changes in above- and below-ground standing crop and production over an 11-year period for two semi-arid shrublands, chaparral and coastal sage scrub (CSS), of Southern California. Shrubs were exposed to 50 kgN ha^-1 in the fall of each year to simulate the accumulation of dry N deposition, and shoot and root biomass and leaf area index (LAI) were measured every 3 months to assess how biomass production responded to chronic, dry N inputs. N inputs significantly altered above- and below-ground standing crop, production, and LAI; however, N impacts varied over time. For chaparral, N inputs initially increased root production but suppressed shoot production; however, over time biomass partitioning reversed and plants exposed to N had significantly more shoot biomass. In CSS, N inputs caused aboveground production to increase only during wet years, and this interaction between added N and precipitation was due in part to a highly flexible growth response of CSS shrubs to increases in N and water availability and to a shift from slower-growing native shrubs to fast-growing introduced annuals. Together, these results indicate that long-term N inputs will lead to complex, spatially and temporally variable growth responses for these, and similar, Mediterranean-type shrublands.

Effects of nitrogen deposition and litter layer management on soil CO2, N2O, and CH4 emissions in a subtropical pine forestland

Forestland soils play vital role in regulating global soil greenhouse gas (GHG) budgets, but the interactive effect of the litter layer management and simulated nitrogen (N) deposition on these GHG flux has not been elucidated clearly in subtropical forestland. A field trial was conducted to study these effects by using litter removal method under 0 and 40 kg N ha⁻¹ yr⁻¹ addition in a subtropical forestland in Yingtan, Jiangxi Province, China. Soil CO₂ emission was increased by N addition (18–24%) but decreased by litter removal (24–32%). Litter removal significantly (P < 0.05) decreased cumulative N₂O emission by 21% in treatments without N addition but only by 10% in treatments with 40 kg N ha⁻¹ yr⁻¹ addition. Moreover, litter-induced N₂O emission under elevated N deposition (0.094 kg N₂O-N ha⁻¹) was almost the same as without N addition (0.088 kg N₂O-N ha⁻¹). Diffusion of atmospheric CH₄ into soil was facilitated by litter removal, which increased CH₄ uptake by 55%. Given that the increasing trend of atmospheric N deposition in future, which would reduce litterfall in subtropical N-rich forest, the effect of surface litter layer change on soil GHG emissions should be considered in assessing forest GHG budgets and future climate scenario modeling.

Global response patterns of plant photosynthesis to nitrogen addition: A meta-analysis

A mechanistic understanding of plant photosynthetic response is needed to reliably predict changes in terrestrial carbon (C) gain under conditions of chronically elevated atmospheric nitrogen (N) deposition. Here, using 2,683 observations from 240 journal articles, we conducted a global meta-analysis to reveal effects of N addition on 14 photosynthesis-related traits and affecting moderators. We found that across 320 terrestrial plant species, leaf N was enhanced comparably on mass basis (N_mass , +18.4%) and area basis (N_area , +14.3%), with no changes in specific leaf area or leaf mass per area. Total leaf area (TLA) was increased significantly, as indicated by the increases in total leaf biomass (+46.5%), leaf area per plant (+29.7%), and leaf area index (LAI, +24.4%). To a lesser extent than for TLA, N addition significantly enhanced leaf photosynthetic rate per area (A_area , +12.6%), stomatal conductance (g_s , +7.5%), and transpiration rate (E, +10.5%). The responses of A_area were positively related with that of g_s , with no changes in instantaneous water-use efficiency and only slight increases in long-term water-use efficiency (+2.5%) inferred from ¹³ C composition. The responses of traits depended on biological, experimental, and environmental moderators. As experimental duration and N load increased, the responses of LAI and A_area diminished while that of E increased significantly. The observed patterns of increases in both TLA and E indicate that N deposition will increase the amount of water used by plants. Taken together, N deposition will enhance gross photosynthetic C gain of the terrestrial plants while increasing their water loss to the atmosphere, but the effects on C gain might diminish over time and that on plant water use would be amplified if N deposition persists.

Potential transition in the effects of atmospheric nitrogen deposition in China

Nitrogen (N) deposition in China may increase due to urbanization and economic growth. Current research has considered the ecological significance under the assumption of increasing N deposition. Atmospheric N deposition tending toward levelling or declining has been observed in China. Such potential recovery and responses of high N loads ecosystems under decreasing atmospheric N deposition scenarios have yet to be adequately investigated. This work reviews existing literature to consider possible responses of carbon (C) sequestration, biodiversity and species composition, soil acidification, and greenhouse emissions in ecosystems responding to recent patterns of N deposition. Potential effects of N composition and internal ratios may be further explored through state-of-the-art N addition experiments and model development.

Litter Management as a Key Factor Relieves Soil Respiration Decay in an Urban-Adjacent Camphor Forest under a Short-Term Nitrogen Increment

Increases in bioavailable nitrogen (N) level can impact the soil carbon (C) sequestration in many forest ecosystems through its influences on litter decomposition and soil respiration (Rs). This study aims to detect whether the litter management can affect the influence of N addition on Rs. We conducted a one-year field experiment in a camphor forest of central-south China to investigate the responses of available N status and soil Rs to N addition and litter manipulation. Four N addition plots (NH4NO3; 0, 5, 15, 30 g N m−2 year−1 as N0, N1, N2, N3, respectively) were established with three nested litter treatments: natural litter input (CK), double litter input (LA), and non-litter input (LR). We found a short-lived enhancement effect of N addition on soil (NO3-N) and net nitrification (RN), but not on (NH4-N), net ammonification (RA), or mineralization (RM). N addition also decreased Rs in CK spots, but not in LA or LR spots, in which the negative effects of N additions on Rs were alleviated by either litter addition or reduction. A priming effect was also observed in LA treatments. A structural equation modeling analysis showed that litter treatments had direct positive effects on soil available N contents and Rs, which suggested that litter decomposition may benefit from litter management when N is not a limiting factor in subtropical forests.

Intensified vegetation water use under acid deposition

Despite the important role vegetation plays in the global water cycle, the exact controls of vegetation water use, especially the role of soil biogeochemistry, remain elusive. In this study, we reveal a new mechanism of soil biogeochemical control of large-scale vegetation water use. Nitrate and sulfate deposition from fossil fuel burning have caused substantial soil acidification, leading to the leaching of soil base cations. Of these, calcium has a unique role in plant cells by regulating stomatal aperture, thus affecting vegetation water use. We hypothesized that the leaching of the soil calcium supply, induced by acid deposition, would increase large-scale vegetation water use. We present evidence from a long-term whole watershed acidification experiment demonstrating that the alteration of the soil calcium supply by acid deposition can significantly intensify vegetation water use (~10% increase in evapotranspiration) and deplete available soil water. These results are critical to understanding future water availability, biogeochemical cycles, and surface energy flux and to help reduce uncertainties in terrestrial biosphere models.

Differences in main processes to transform phosphorus influenced by ammonium nitrogen in flooded intensive agricultural and steppe soils

Exogenous ammonium nitrogen (AN) fertilization/deposition and the subsequently induced soil acidification, might exacerbate soil phosphorus (P) transformation and mobility, whereas poor understanding in abiotic and biotic processes still existed between intensively cultivated soil with high proportion of legacy inorganic P (P_i) in comparison to natural steppe soil with high proportion of organic P (P_o). Column batch flooded experiment using vegetable (VEG) and steppe (STE) soils was conducted to investigate the P transformation and mobility influenced by 60-day intermittent elution with an (1500 mg N kg^-1) solution or an acid sulfate (107 mmol H⁺ kg^-1) solution. The results showed that AN elution significantly decreased the contents of all P_i fractions and strengthened P_i leaching, whereas acid elution increased the contents of Al-P_i, Fe-P_i, and reductant-soluble P_i and reduced P leaching in VEG soil. In contrast, AN elution increased the contents of soluble-P_o and Al-(P_i + P_o) and decreased Ca-P and Fe-P_i, no effects on P leaching, whereas acid elution increased the contents of soluble-(P_i + P_o), Al-(P_i + P_o), Fe-(P_i + P_o) and reduced P leaching in STE soil. Variances analysis showed that pH and microbial biomass carbon were the most important factors to predict the P composition of the VEG and the STE soil, respectively. This indicated that AN elution released the soil mineral-bound phosphate with ammonia oxidation coupled with Fe(III) reduction, besides the same chemical influences on Fe/Al oxides as acid elution in VEG soil; while predominantly affected biochemical/biological processes of soil P by changing microbial biomass and enzyme activities in STE soil.

Warming but Not Nitrogen Addition Alters the Linear Relationship Between Microbial Respiration and Biomass

Soil contains a large amount of organic matter, which constitutes the largest terrestrial carbon pool. Heterotrophic or microbial respiration (R_h) that results from microbial decomposition of soil organic carbon (SOC) constitutes a substantial proportion of soil C efflux. Whether soil microbial biomass is of primary importance in controlling R_h remains under debate, and the question of whether the microbial biomass-decomposition relationship changes with warming and nitrogen (N) deposition has rarely been assessed. We conducted an incubation experiment to test the relationship between R_h and the size of soil microbial communities in two layers of soil collected from a natural subtropical forest and to examine whether the relationship was affected by changes in temperature and by added N in different forms. The results showed that regardless of the added N species, the N load did not significantly affect R_h or the size of the soil microbial communities. These results could be due to a long-term N-rich soil condition that acclimates soil microbial communities to resist N inputs into the studied forest; however, warming may significantly stimulate SOC decomposition, reducing soil microbial biomass under high temperatures. A significant linear soil microbial biomass-decomposition relationship was observed in our study, with the coefficients of determination ranging from 54 to 70%. Temperature rather than N additions significantly modified the linear relationship between soil microbial biomass and respiration. These results suggest that warming could impose a more substantial impact than N addition on the relationship between soil microbial biomass and SOC decomposition.

Nitrogen and phosphorus enrichment accelerates soil organic carbon loss in alpine grassland on the Qinghai-Tibetan Plateau

Anthropogenic activities have substantially increased soil nutrient availability, which in turn affects ecosystem processes and functions, especially in nutrient-limited ecosystems such as alpine grasslands. Although considerable efforts have been devoted to understanding the responses of plant productivity and community composition to nitrogen (N) and phosphorus (P) enrichment, the nutrient enrichment effects on soil organic carbon (SOC) and microbial functions are not well understood. A four-year field experiment was established to evaluate the influence of continuous N and P enrichment on plant growth and SOC content in an alpine grassland of the Qinghai-Tibetan Plateau. The study included four treatments: Control without addition, N addition, P addition, and N plus P addition. N addition strongly increased aboveground plant biomass and decreased species richness by promoting growth of the dominant grasses species. In contrast, N and P enrichment significantly decreased SOC, especially the recalcitrant organic C content in the surface layer (0-10 cm) by reducing the slow C pool and enlarging the active C pool. Microbial biomass and activities of C-degrading enzymes (β-glucosidase, cellulase and polyphenol oxidase) and an N-degrading enzyme (chitinase) increased with nutrient inputs. The CO₂ emissions during a 300 d incubation period were positively correlated with the cellulase and chitinase activities, while the slow C pool was negatively correlated with the cellulase and polyphenol oxidase activities. Consequently, N and P enrichment accelerated decomposition of the recalcitrant C by stimulating microbial growth and increasing enzyme activities, leading to negative impacts on soil C sequestration. Overall, the results indicate that alpine grassland soils of the Qinghai-Tibetan Plateau may be changing from a C sink to a C source under increasing N and P availability, and improvement of alpine grassland management through nutrient inputs should consider not only the aboveground biomass for grazing, but also the soil C sequestration and ecosystem functioning.

Responses of sap flux and intrinsic water use efficiency to canopy and understory nitrogen addition in a temperate broadleaved deciduous forest

Increasing atmospheric nitrogen (N) deposition could profoundly impact structure and functioning of forest ecosystems. Therefore, we conducted a two-year (2014-2015) experiment to assess the responses of tree sap flux density (J_s) and intrinsic water use efficiency (WUE_i) of dominant tree species (Liquidambar formosana, Quercus acutissima and Quercus variabilis) to increased N deposition at a manipulative experiment with canopy and understory N addition in a deciduous broadleaved forest. Five treatments were administered including N addition of 25 kg ha^-1 year^-1 and 50 kg ha^-1 year^-1 onto canopy (C25 and C50) and understory (U25 and U50), and control treatment (CK, without N addition). Our results showed neither canopy nor understory N addition had an impact on leaf N content and C:N ratio (P > 0.05). Due to the distinct influencing ways, canopy and understory N addition generated different impacts on J_s and WUE_i of the dominant tree species. Canopy N addition increased WUE_i of Q. variabilis, whereas understory addition treatment had a minimal impact on WUE_i. Both N additions did not exert impacts on WUE_i of L. formosana and Q. acutissima. Canopy N addition exerted negative impacts on J_s and its sensitivity to micrometeorological factors of Q. acutissima and Q. variabilis in 2014, while understory addition showed no effect. Neither canopy nor understory N addition had an influence on J_s of L. formosana in 2014. Probably owing to the increased soil acidification as the experiment proceeded, J_s of L. formosana and Q. variabilis was decreased by understory N addition while canopy addition had a minimal effect in 2015. Thus, the traditional understory addition approach could not fully reflect the effects of increased N deposition on the canopy-associated transpiration process indicated by the different responses of J_s and WUE_i to canopy and understory N addition, and exaggerated its influences induced by the variation of soil chemical properties.

Increased nitrogen supply promoted the growth of non-N-fixing woody legume species but not the growth of N-fixing Robinia pseudoacacia

Nitrogen (N) is an essential macronutrient for plant development and growth, and the deposition of N has increased in recent decades. Legumes that fix N can also provide N for nearby species. However, N in soil inhibits N fixation. We tested the effects of N fertilisation on one N-fixing (Robinia pseudoacacia) and two non-N-fixing (Sophora japonica and Senna surattensis) woody legume species, which were subjected to five different N levels (0, 1.5, 2.9, 5.9 and 11.4 mg N per plant day^-1) under greenhouse conditions. The growth of the two non-N-fixing species was promoted by N supply, while that of R. pseudoacacia was unaffected. Among the three species, R. pseudoacacia had the largest specific leaf area and chlorophyll concentration, S. japonica had the largest root-to-shoot ratio and main root-to-lateral root ratio, and S. surattensis had the largest leaf N and phosphorus concentrations. The N-fixing species was mostly unaffected by N supply. The growth, leaf chlorophyll concentration, and leaf number in the non-N-fixing species were promoted by N supply. The N-fixing species showed better growth in low-N environments, while under increased N deposition, its growth was similar to that of the non-N-fixing species.

Responses of forest ecosystems to increasing N deposition in China: A critical review

China has been experiencing a rapid increase in nitrogen (N) deposition due to intensified anthropogenic N emissions since the late 1970s. By synthesizing experimental and observational data taken from literature, we reviewed the responses of China's forests to increasing N deposition over time, with a focus on soil biogeochemical properties and acidification, plant nutrient stoichiometry, understory biodiversity, forest growth, and carbon (C) sequestration. Nitrogen deposition generally increased soil N availability and soil N leaching and decreased soil pH in China's forests. Consequently, microbial biomass C and microbial biomass N were both decreased, especially in subtropical forests. Nitrogen deposition increased the leaf N concentration and phosphorus resorption efficiency, which might induce nutrient imbalances in the forest ecosystems. Although experimental N addition might not affect plant species richness in the overstorey, it did significantly alter species composition of understory plants. Increased N stimulated tree growth in temperate forests, but this effect was weak in subtropical and tropical forests. Soil respiration in temperate forests was non-linearly responsive to N additions, with an increase at dosages of <60 kg N ha^-1 yr^-1 and a decrease at dosages of >60 kg N ha^-1 yr^-1. However, it was consistently decreased by increased N inputs in subtropical and tropical forests. In light of future trends in the composition (e.g., reduced N vs. oxidized N) and the loads of N deposition in China, further research on the effects of N deposition on forest ecosystems will have critical implications for the management strategies of China's forests.

Responses of soil organic carbon turnover to nitrogen deposition are associated with nitrogen input rates: Derived from soil 14C evidences

Elevated atmospheric nitrogen (N) deposition has exerted profound influences on ecosystems. Understanding the effects of N deposition on the dynamics of soil organic carbon (SOC) is important in the studies of global carbon cycle. Although many studies have examined the effects of N deposition on SOC turnover using N addition experiments, the effects were reported to be different across studies. Thus, we lack a predictive understanding of how SOC turnover respond to atmospheric N deposition. The inconsistent results could be associated with ecosystem types and N addition rates. This study mainly wants to confirm the argument that the response of SOC turnover to N deposition is related with N input rates. We conducted a field experiment with multiple N addition levels (0, 3, 6, 12, and 24 g N m^-2·yr^-1) in Inner Mongolia Grassland, China. To better reveal the responses of SOC turnover to N enrichment, this study measured the soil ¹⁴C contents, because it can indicate SOC turnover directly. Compared with the control treatment (0 g N m^-2·yr^-1), N addition inhibits SOC turnover at the addition rate of 3 g N m^-2·yr^-1, whereas SOC turnover is not affected when N addition rate was 6, 12, and 24 g N m^-2·yr^-1. Our results suggest that N input rates affect the responses of SOC turnover to N enrichment. Thus, this study can confirm the argument mentioned above. Based on this study, it should be considered in the climate prediction model that varied atmospheric N deposition levels across regions may have different impacts on local SOC turnover. In addition, we also carried out a soil incubation to compare between the results obtained in incubation and that in ¹⁴C measurements. Two results are found to be inconsistent with each other. This indicates that soil respiration from incubation experiments could not comprehensively assess the effects of N deposition on SOC turnover.

The response of tree growth to nitrogen and phosphorus additions in a tropical montane rainforest

Rapid increase of global nitrogen (N) deposition has greatly altered carbon cycles and functioning of forest ecosystems. Previous studies have focused on changes in carbon dynamics of temperate and subtropical forests through N enrichment experiments; however, the effects of N deposition on tree growth remain inconsistent, especially in tropical forests. Here, we conducted a five-year N addition experiment (0 and 50kgNha^-1yr^-1) in a tropical montane rain forest in Hainan Island, China, to explore the effects of enhanced N deposition on growth of trees. We also set phosphorus (P) treatment (50kgPha^-1yr^-1) and N+P treatment (50kgNha^-1yr^-1+50kgPha^-1yr^-1) to examine potential P limitation driven by N deposition. Our results showed that N addition has not significantly influenced tree growth, while P addition significantly increased the relative growth rate of small (diameter at breast height, DBH≤10cm) and medium (10<DBH≤20cm) trees. The combined N and P addition accelerated the growth of small trees, but did not affect the growth of medium and large (20cm<DBH) trees. These contrasting effects of N and P addition on tree growth indicate that the tropical montane forest is mainly limited by P, which suggests the importance of P in regulating growth of trees in tropical forests.

Effects of short-term warming and nitrogen addition on the quantity and quality of dissolved organic matter in a subtropical Cunninghamia lanceolata plantation

Increasing temperature and nitrogen (N) deposition are two large-scale changes projected to occur over the coming decades. The effects of these changes on dissolved organic matter (DOM) are largely unknown. This study aimed to assess the effects of warming and N addition on the quantity and quality of DOM from a subtropical Cunninghamia lanceolata plantation. Between 2014 and 2016, soil solutions were collected from 0–15, 15–30, and 30–60 cm depths by using a negative pressure sampling method. The quantity and quality of DOM were measured under six different treatments. The spectra showed that the DOM of the forest soil solution mainly consisted of aromatic protein-like components, microbial degradation products, and negligible amounts of humic-like substances. Warming, N addition, and warming + N addition significantly inhibited the concentration of dissolved organic carbon (DOC) in the surface (0–15 cm) soil solution. Our results suggested that warming reduced the amount of DOM originating from microbes. The decrease in protein and carboxylic acid contents was mostly attributed to the reduction of DOC following N addition. The warming + N addition treatment showed an interactive effect rather than an additive effect. Thus, short-term warming and warming + N addition decreased the quantity of DOM and facilitated the migration of nutrients to deeper soils. Further, N addition increased the complexity of the DOM structure. Hence, the loss of soil nutrients and the rational application of N need to be considered in order to prevent the accumulation of N compounds in soil.

Five-year study of the effects of simulated nitrogen deposition levels and forms on soil nitrous oxide emissions from a temperate forest in northern China

Few studies have quantified the effects of different levels and forms of nitrogen (N) deposition on soil nitrous oxide (N₂O) emissions from temperate forest soils. A 5-year field experiment was conducted to investigate the effects of multiple forms and levels of N additions on soil N₂O emissions, by using the static closed chamber method at Xi Mountain Experimental Forest Station in northern China. The experiment included a control (no N added), and additions of NH₄NO₃, NaNO₃, and (NH₄)₂SO₄ that each had two levels: 50 kg N ha⁻¹ yr⁻¹ and 150 kg N ha⁻¹ yr⁻¹. All plots were treated to simulate increased N deposition on a monthly schedule during the annual growing season (March to October) and soil N₂O emissions were measured monthly from March 2011 to February 2016. Simultaneously, the temperature, moisture, and inorganic N contents of soil were also measured to explore how the main factors may have affected soil N₂O emission. The results showed that the types and levels of N addition significantly increased soil inorganic N contents, and the accumulation of soil NO₃^–-N was significantly higher than that of soil NH₄⁺–N due to N addition. The three N forms significantly increased the average N₂O emissions (P < 0.05) in the order of NH₄NO₃ > (NH₄)₂SO₄ > NaNO₃ by 355.95%, 266.35%, and 187.71%, respectively, compared with control. The promotion of N₂O emission via the NH₄⁺–N addition was significantly more than that via the NO₃^––N addition, while N addition at a high level exerted a stronger effect than at the low-level. N addition exerted significantly stronger effects on cumulative N₂O emissions in the initial years, especially the third year when the increased cumulative N₂O emission reached their maximum. In the later years, the increases persisted but were weakened. Increasing inorganic N concentration could change soil from being N-limited to N-rich, and then N-saturated, and so the promotion on soil available N effect increased and then decreased. Moreover, the soil NH₄⁺–N, NO₃^–-N, temperature, and water-filled pore space were all positively correlated with soil N₂O emissions. These findings suggest that atmospheric N deposition can significantly promote soil N₂O emission, and that exogenous NH₄⁺–N and NO₃^–-N inputs into temperate forests can have synergic effects on soil N₂O emission. In future research, both aspects should be better distinguished in the N cycle and balance of terrestrial ecosystems by using ¹⁵N tracer methods.

Exogenous Nitrogen Addition Reduced the Temperature Sensitivity of Microbial Respiration without Altering the Microbial Community Composition

Atmospheric nitrogen (N) deposition is changing in both load quantity and chemical composition. The load effects have been studied extensively, whereas the composition effects remain poorly understood. We conducted a microcosm experiment to study how N chemistry affected the soil microbial community composition characterized by phospholipid fatty acids (PLFAs) and activity indicated by microbial CO2 release. Surface and subsurface soils collected from an old-growth subtropical forest were supplemented with three N-containing materials (ammonium, nitrate, and urea) at the current regional deposition load (50 kg ha-1 yr-1) and incubated at three temperatures (10, 20, and 30°C) to detect the interactive effects of N deposition and temperature. The results showed that the additions of N, regardless of form, did not alter the microbial PLFAs at any of the three temperatures. However, the addition of urea significantly stimulated soil CO2 release in the early incubation stage. Compared with the control, N addition consistently reduced the temperature dependency of microbial respiration, implying that N deposition could potentially weaken the positive feedback of the warming-stimulated soil CO2 release to the atmosphere. The consistent N effects for the surface and subsurface soils suggest that the effects of N on soil microbial communities may be independent of soil chemical contents and stoichiometry.

Individual size but not additional nitrogen regulates tree carbon sequestration in a subtropical forest

Recent studies have indicated that tree carbon accumulation in subtropical forests has been negatively affected by global change phenomena such as warming and drought. However, the long-term effect of nitrogen addition on plant carbon storage remains poorly understood in these regions. In this study, we conducted a 10-year field experiment examining the effect of experimental N addition on plant growth and carbon storage in a subtropical Chinese fir forest. The N levels were 0 (control), 60, 120, and 240 kg ha⁻¹ yr⁻¹, and the N effects on tree carbon were divided into stand and individual levels. The results indicated that tree carbon storage at the stand scale was not affected by long-term N addition in the subtropical forest. By contrast, significant impacts of different tree size classes on carbon sequestration were found under different N treatments, which indicated that the amount of plant carbon sequestration was significantly enhanced with tree size class. Our findings highlight the importance of community structure and growth characteristics in Chinese fir forests, in which individual size but not additional N regulates tree carbon sequestration in this subtropical forest.

High retention of 15 N-labeled nitrogen deposition in a nitrogen saturated old-growth tropical forest

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem ¹⁵ N labeling experiment performed in a N-rich old-growth tropical forest in southern China. We added ¹⁵ N tracer monthly as ¹⁵ NH₄¹⁵ NO₃ for 1 year to control plots and to N-fertilized plots (N-plots, receiving additions of 50 kg N ha^-1  yr^-1 for 10 years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N-plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N-plots, respectively. Leaching below 20 cm was 50 kg N ha^-1  yr^-1 in the control plots and was close to the N input (51 kg N ha^-1  yr^-1 ), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73 kg N ha^-1  yr^-1 . However, of these only 7 and 23 kg N ha^-1  yr^-1 in the control and N-plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N-saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short-term N retention represent the main difference between temperate forests and the studied tropical forest.

The responses of soil respiration to nitrogen addition in a temperate grassland in northern China

Anthropogenic activities have increased nitrogen (N) inputs to grassland ecosystems. Knowledge of the impact of soil N availability on soil respiration (RS) is critical to understand soil carbon balances and their responses to global climate change. A 2-year field experiment was conducted to evaluate the response of RS to soil mineral N in a temperate grassland in northern China. RS, abiotic and biotic factors, and N mineralization were measured in the grassland, at rates of N addition ranging from 0 to 25gNm(-2)yr(-1). Annual and dormant-season RS ranged from 241.34 to 283.64g C m(-2) and from 61.34 to 83.84g C m(-2) respectively. High N application significantly increased RS, possibly due to increased root biomass and increased microbial biomass. High N treatment significantly increased soil NO3-N and inorganic N content compared with the control. The ratio of NO3-N to NH4-N and the N mineralization rate were significantly positively correlated with RS, but NH4-N was not correlated or negatively correlated with RS during the growing season. The temperature sensitivity of RS (Q10) was not significantly affected by N levels, and ranged from 1.90 to 2.20, but decreased marginally significantly at high N. RS outside the growing season is an important component of annual RS, accounting for 25.0 to 29.6% of the total. High N application indirectly stimulated RS by increasing soil NO3-N and net nitrification, thereby eliminating soil N limitations, promoting ecosystem productivity, and increasing soil CO2 efflux. Our results show the importance of distinguishing between NO3-N and NH4-N, as their impact on soil CO2 efflux differed.

Euedaphic and hemiedaphic Collembola suffer larger damages than epedaphic species to nitrogen input

Wetlands are commonly limited in available nitrogen. But marshes in the Sanjiang Plain, Northeastern China suffer large amounts of exogenous nitrogen from agriculture fertilization after wetland reclamation. This paper focuses on the ecological effects of a short-term increase of nitrogen input on collembolan communities. Our results show a significant decrease in collembolan abundance and Shannon diversity index, and the abundance of euedaphic and hemiedaphic collembolans decreased faster than epedaphic collembolans. These results indicate that euedaphic or hemiedaphic fauna suffer more biodiversity loss caused by nitrogen deposition than epedaphic fauna and call for more researches on trait-based approaches under environmental stress in the future.

Microbial properties explain temporal variation in soil respiration in a grassland subjected to nitrogen addition

The role of soil microbial variables in shaping the temporal variability of soil respiration has been well acknowledged but is poorly understood, particularly under elevated nitrogen (N) deposition conditions. We measured soil respiration along with soil microbial properties during the early, middle, and late growing seasons in temperate grassland plots that had been treated with N additions of 0, 2, 4, 8, 16, or 32 g N m⁻² yr⁻¹ for 10 years. Representing the averages over three observation periods, total (R_s) and heterotrophic (R_h) respiration were highest with 4 g N m⁻² yr⁻¹, but autotrophic respiration (R_a) was highest with 8 to 16 g N m⁻² yr⁻¹. Also, the responses of R_h and R_a were unsynchronized considering the periods separately. N addition had no significant impact on the temperature sensitivity (Q₁₀) for R_s but inhibited the Q₁₀ for R_h. Significant interactions between observation period and N level occurred in soil respiration components, and the temporal variations in soil respiration components were mostly associated with changes in microbial biomass carbon (MBC) and phospholipid fatty acids (PLFAs). Further observation on soil organic carbon and root biomass is needed to reveal the long-term effect of N deposition on soil C sequestration.