Responses of fine roots and soil N availability to short-term nitrogen fertilization in a broad-leaved Korean pine mixed forest in northeastern China

Fine-Root Distribution and Soil Physicochemical Property Variations in Four Contrasting Urban Land-Use Types in South Korea

Urbanization and associated forest conversions have given rise to a continuum of native (forest fragments) and modified (artificial grasslands and perennial ecosystems) land-use types. However, little is known about how these shifts affect soil and fine-root compartments that are critical to a functioning carbon and nutrient circulation system. In this study, soil physicochemical properties, fine-root mass, and vertical distribution patterns were investigated in four representative urban land-use types: grassland (ZJ), perennial agroecosystem (MP), broadleaf deciduous forest patch (QA), and coniferous evergreen forest patch (PD). We quantified the fine-root mass in the upper 30 cm vertical profile (0-30 cm) and at every 5 cm depth across three diameter classes (<2 mm, 2-5 mm, and <5 mm). Soil physicochemical properties, except for phosphorus, nitrogen, ammonium nitrogen, and sodium cations, varied significantly across land-use types. The total root biomass (<5 mm) decreased in the order of QA (700.3 g m-2) > PD (487.2 g m-2) > ZJ (440.1 g m-2) > MP (98.3 g m-2). The fine-root mass of ZJ and MP was correlated with soil nutrients, which was attributed to intensive management operations, while the fine-root mass of QA and PD had a significant relationship with soil organic matter due to the high inputs from forest litter. Very fine roots (<2 mm) presented a distinct decremental pattern with depth for all land-use types, except for MP. Very fine roots populated the topmost 5 cm layer in ZJ, QA, and PD at 52.1%, 49.4%, and 39.4%, respectively. Maintaining a woody fine-root system benefits urban landscapes by promoting soil stabilization, improving ground infiltration rates, and increasing carbon sequestration capacity. Our findings underscore the importance of profiling fine-root mass when assessing urban expansion effects on terrestrial ecosystems.

Responses of microbial function, biomass and heterotrophic respiration, and organic carbon in fir plantation soil to successive nitrogen and phosphorus fertilization

Carbon dioxide (CO₂) emissions from forest ecosystems originate largely from soil respiration, and microbial heterotrophic respiration plays a critical role in determining organic carbon (C) stock. This study investigated the impacts of successive nitrogen (N) and phosphorus (P) fertilization after 9 years on soil organic C stock; CO₂ emission; and microbial biomass, community, and function in a Chinese fir plantation. The annual fertilization rates were (1) CK, control without N or P fertilization; (2) N50, 50 kg N ha^-1; (3) N100, 100 kg N ha^-1; (4) P50, 50 kg P ha^-1; (5) N50P50, 50 kg N ha^-1 + 50 kg P ha^-1; and (6) N100P50, 100 kg N ha^-1 + 50 kg P ha^-1. The N100P50 treatment had the highest cumulative soil CO₂ emissions, but the CK treatment had the lowest cumulative soil CO₂ emissions among all treatments. The declines of soil organic C (SOC) after successive 9-year fertilization were in the order of 100 kg N ha^-1 year^-1 > 50 kg N ha^-1 year^-1 > CK. Compared to the CK treatment, successive N fertilization significantly changed soil microbial communities at different application rates and increased the relative gene abundances of glycoside hydrolases, glycosyl transferases, carbohydrate-binding modules, and polysaccharide lyases at 100 kg N ha^-1 year^-1. Relative to P fertilization alone (50 kg P ha^-1 year^-1), combined N and P fertilization significantly altered the soil microbial community structure and favored more active soil microbial metabolism. Microbial community and metabolism changes caused by N fertilization could have enhanced CO₂ emission from heterotrophic respiration and eventually led to the decrease in organic C stock in the forest plantation soil. KEY POINTS: • N fertilization, alone or with P, favored more active microbial metabolism genes. • 100 kg N ha^-1 fertilization significantly changed microbial community and function. • N fertilization led to a "domino effect" on the decrease of soil C stock.

Successive mineral nitrogen or phosphorus fertilization alone significantly altered bacterial community rather than bacterial biomass in plantation soil

Bacteria play determining roles in forest soil environment and contribute to essential functions in the cycling of nitrogen (N) and phosphorus (P). Understanding the effects of different fertilizer applications, especially successive fertilization, on soil properties and bacterial community could reveal the impacts of fertilization on forest soil ecology and shed light on the nutrient cycling in forest system. This study aimed to evaluate the impacts of successive mineral N (NH₄NO₃) and P (NaH₂PO₄) fertilization at different rates, alone or together, on soil bacterial biomass and communities at 0-5, 5-10, and 10-20 cm. Compared with the control, N fertilization decreased soil pH, but P alone or with N fertilization had negligibly negative impacts on soil pH. Different mineral fertilizer applications, alone or together, showed no significant effects on soil organic matter contents, relative to the control treatment. Bacterial biomass remained stable to different fertilizations but decreased with sampling depths. Sole N or P fertilization, rather than combined fertilizations, significantly changed soil bacterial community structures. Our results demonstrated that mineral N or P fertilization alone significantly affected bacterial community structures rather than biomass in the plantation soils. KEY POINTS: • Impacts of successive mineral fertilization on soil bacteria were determined. • Mineral fertilization showed negligible impacts on bacterial biomass. • N additions stimulated Chloroflexi relative abundances. • Mineral N or P fertilization significantly altered bacterial community structure.

The Dynamics of Living and Dead Fine Roots of Forest Biomes Across the Northern Hemisphere

Research Highlights: A detailed picture of the seasonality in fine root biomass (FRB), necromass (FRN), and the biomass/necromass ratio (FRBN) throughout the whole year is crucial to uncover profound effects of long-term environmental changes on fine root dynamics. Materials and Methods: We used meta-analysis to characterize the variability of FRB, FRN and FRBN, and determined their relations with climatic (monthly versus annual), edaphic and geomorphic factors for tropical, temperate and boreal forest biomes across the Northern Hemisphere. Results: Boreal forests exhibited the highest FRB and FRN, while tropical forests yielded the lowest FRN, and thus the greatest FRBN. FRB and FRN significantly decreased with sampling depth, but increased with soil organic carbon content and elevation, while an opposite pattern was found for FRBN. Temperature and precipitation at different time scales (monthly versus annual) and latitude had varying influences on fine roots. High FRB and FRN were observed during dry season for tropical forests, but in the late growing season for temperate forests. The three forest biomes exhibited the high root activity (measured as FRBN) in June or July. Conclusions: It is crucial to realize the universal and specific responses of fine roots to multiple environmental factors when attempting to incorporate these parameters into fine root monthly dynamic models in forest ecosystems. The biome-specific fluctuation of fine roots contributes to identify the influence factors on fine root seasonal patterns throughout the whole year. Our analysis is expected to improve the understanding of the key role of fine roots at monthly level in modeling and predicting carbon budget of various forest biomes under future climate change.

Simulated nitrogen deposition significantly reduces soil respiration in an evergreen broadleaf forest in western China

Soil respiration is the second largest terrestrial carbon (C) flux; the responses of soil respiration to nitrogen (N) deposition have far-reaching influences on the global C cycle. N deposition has been documented to significantly affect soil respiration, but the results are conflicting. The response of soil respiration to N deposition gradients remains unclear, especially in ecosystems receiving increasing ambient N depositions. A field experiment was conducted in a natural evergreen broadleaf forest in western China from November 2013 to November 2015 to understand the effects of increasing N deposition on soil respiration. Four levels of N deposition were investigated: control (Ctr, without N added), low N (L, 50 kg N ha⁻¹·a⁻¹), medium N (M, 150 kg N ha⁻¹·a⁻¹), and high N (H, 300 kg N ha⁻¹·a⁻¹). The results show that (1) the mean soil respiration rates in the L, M, and H treatments were 9.13%, 15.8% (P < 0.05) and 22.57% (P < 0.05) lower than that in the Ctr treatment (1.56 ± 0.13 μmol·m⁻²·s⁻¹), respectively; (2) soil respiration rates showed significant positive exponential and linear relationships with soil temperature and moisture (P < 0.01), respectively. Soil temperature is more important than soil moisture in controlling the soil respiration rate; (3) the Ctr, L, M, and H treatments yielded Q₁₀ values of 2.98, 2.78, 2.65, and 2.63, respectively. N deposition decreased the temperature sensitivity of soil respiration; (4) simulated N deposition also significantly decreased the microbial biomass C and N, fine root biomass, pH and extractable dissolved organic C (P < 0.05). Overall, the results suggest that soil respiration declines in response to N deposition. The decrease in soil respiration caused by simulated N deposition may occur through decreasing the microbial biomass C and N, fine root biomass, pH and extractable dissolved organic C. Ongoing N deposition may have significant impacts on C cycles and increase C sequestration with the increase in global temperature in evergreen broadleaf forests.

Vertical Redistribution of Soil Organic Carbon Pools After Twenty Years of Nitrogen Addition in Two Temperate Coniferous Forests

Nitrogen (N) inputs from atmospheric deposition can increase soil organic carbon (SOC) storage in temperate and boreal forests, thereby mitigating the adverse effects of anthropogenic CO₂ emissions on global climate. However, direct evidence of N-induced SOC sequestration from low-dose, long-term N addition experiments (that is, addition of < 50 kg N ha^-1 y^-1 for > 10 years) is scarce worldwide and virtually absent for European temperate forests. Here, we examine how tree growth, fine roots, physicochemical soil properties as well as pools of SOC and soil total N responded to 20 years of regular, low-dose N addition in two European coniferous forests in Switzerland and Denmark. At the Swiss site, the addition of 22 kg N ha^-1 y^-1 (or 1.3 times throughfall deposition) stimulated tree growth, but decreased soil pH and exchangeable calcium. At the Danish site, the addition of 35 kg N ha^-1 y^-1 (1.5 times throughfall deposition) impaired tree growth, increased fine root biomass and led to an accumulation of N in several belowground pools. At both sites, elevated N inputs increased SOC pools in the moderately decomposed organic horizons, but decreased them in the mineral topsoil. Hence, long-term N addition led to a vertical redistribution of SOC pools, whereas overall SOC storage within 30 cm depth was unaffected. Our results imply that an N-induced shift of SOC from older, mineral-associated pools to younger, unprotected pools might foster the vulnerability of SOC in temperate coniferous forest soils.

Six-Year Nitrogen-Water Interaction Shifts the Frequency Distribution and Size Inequality of the First-Order Roots of Fraxinus mandschurica in a Mixed Mature Pinus koraiensis Forest

The variation in fine root traits in terms of size inequality at the individual root level can be identified as a strategy for adapting to the drastic changes in soil water and nutrient availabilities. The Gini and Lorenz asymmetry coefficients have been applied to describe the overall degree of size inequality, which, however, are neglected when conventional statistical means are calculated. Here, we used the Gini coefficient, Lorenz asymmetry coefficient and statistical mean in an investigation of Fraxinus mandschurica roots in a mixed mature Pinus koraiensis forest on Changbai Mountain, China. We analyzed 967 individual roots to determine the responses of length, diameter and area of the first-order roots and of branching intensity to 6 years of nitrogen addition (N), rainfall reduction (W) and their combination (NW). We found that first-order roots had a significantly greater average length and area but had smaller Gini coefficients in NW plots compared to in control plots (CK). Furthermore, the relationship between first-order root length and branching intensity was negative in CK, N, and W plots but positive in NW plots. The Lorenz asymmetry coefficient was >1 for the first-order root diameter in NW and W plots as well as for branching intensity in N plots. The bimodal frequency distribution of the first-order root length in NW plots differed clearly from the unimodal one in CK, N, and W plots. These results demonstrate that not only the mean but also the variation and the distribution mode of the first-order roots of F. mandschurica respond to soil nitrogen and water availability. The changes in size inequality of the first-order root traits suggest that Gini and Lorenz asymmetry coefficients can serve as informative parameters in ecological investigations of roots to improve our ability to predict how trees will respond to a changing climate at the individual root level.

Soil biochemical responses to nitrogen addition in a secondary evergreen broad-leaved forest ecosystem

In order to investigate the effects of N deposition on soil biochemistry in secondary forests, one N addition experiment was conducted in a secondary evergreen broad-leaved forest in the western edge of Sichuan Basin, with the highest level of background N deposition (about 95 kg N ha-1 yr-1) in China. Three N treatment levels (+0, +50, +150 kg N ha-1 yr-1) were monthly added to soil surface in this forest beginning in April 2013. Soil biochemistry and root biomass of the 0-10 cm soil horizon were measured from May 2014 to April 2015. Soil respiration was measured for two years (September 2013 to August 2015). It was showed that N additions were correlated to significantly lower soil pH, microbial biomass C (MBC) concentration, MBC/microbial biomass N (MBN) ratio, root biomass, and soil respiration rate, and significantly higher concentrations of ammonium (NH4+) and nitrate (NO3-). These results indicate that N additions had a significant effect on the size of soil microbial community. In addition, soil C storage may potentially increase due to the dropped soil C release under N addition.

Rainfall reduction amplifies the stimulatory effect of nitrogen addition on N2O emissions from a temperate forest soil

Soil is a significant source of atmospheric N₂O, and soil N₂O emissions at a global scale are greatly affected by environment changes that include continuous deposition of atmospheric nitrogen and changing precipitation distribution. However, to date, field simulations of multiple factors that control the interaction between nitrogen deposition and precipitation on forest soil N₂O emissions are scarce. In this study, we conducted a 2-year continuous assessment of N₂O emissions from November 2012 to October 2014 at a nitrogen addition and rainfall reduction manipulation platform in an old broad-leaved Korean pine mixed forest at Changbai Mountain in northeastern China. We found that N₂O emissions from control plots were 1.25 ± 0.22 kg N₂O-N ha^-1 a^-1. Nitrogen addition significantly increased N₂O emissions, with the emission factor of 1.59%. A 30% reduction in rainfall decreased N₂O emissions by 17-45%. However, in combination, nitrogen addition and rainfall reduction increased N₂O emissions by 58-140%, with the emission factor of 3.19%, and had a larger promotional effect than the addition of nitrogen alone. Our results indicated that drought slightly decreases forest soil N₂O emission; however, with increasing deposition of atmospheric N in temperate forest soils, the effect of drought might become altered to increase N₂O emission.

Combined effects of nitrogen addition and organic matter manipulation on soil respiration in a Chinese pine forest

The response of soil respiration (Rs) to nitrogen (N) addition is one of the uncertainties in modelling ecosystem carbon (C). We reported on a long-term nitrogen (N) addition experiment using urea (CO(NH₂)₂) fertilizer in which Rs was continuously measured after N addition during the growing season in a Chinese pine forest. Four levels of N addition, i.e. no added N (N0: 0 g N m^-2 year^-1), low-N (N1: 5 g N m^-2 year^-1), medium-N (N2: 10 g N m^-2 year^-1), and high-N (N3: 15 g N m^-2 year^-1), and three organic matter treatments, i.e. both aboveground litter and belowground root removal (LRE), only aboveground litter removal (LE), and intact soil (CK), were examined. The Rs was measured continuously for 3 days following each N addition application and was measured approximately 3-5 times during the rest of each month from July to October 2012. N addition inhibited microbial heterotrophic respiration by suppressing soil microbial biomass, but stimulated root respiration and CO₂ release from litter decomposition by increasing either root biomass or microbial biomass. When litter and/or root were removed, the "priming" effect of N addition on the Rs disappeared more quickly than intact soil. This is likely to provide a point of view for why Rs varies so much in response to exogenous N and also has implications for future determination of sampling interval of Rs measurement.

Urbanization in China drives soil acidification of Pinus massoniana forests

Soil acidification instead of alkalization has become a new environmental issue caused by urbanization. However, it remains unclear the characters and main contributors of this acidification. We investigated the effects of an urbanization gradient on soil acidity of Pinus massoniana forests in Pearl River Delta, South China. The soil pH of pine forests at 20-cm depth had significantly positive linear correlations with the distance from the urban core of Guangzhou. Soil pH reduced by 0.44 unit at the 0–10 cm layer in urbanized areas compared to that in non-urbanized areas. Nitrogen deposition, mean annual temperature and mean annual precipitation were key factors influencing soil acidification based on a principal component analysis. Nitrogen deposition showed significant linear relationships with soil pH at the 0–10 cm (for ammonium N (-N), P < 0.05; for nitrate N (-N), P < 0.01) and 10–20 cm (for -N, P < 0.05) layers. However, there was no significant loss of exchangeable non-acidic cations along the urbanization gradient, instead their levels were higher in urban than in urban/suburban area at the 0–10 cm layer. Our results suggested N deposition particularly under the climate of high temperature and rainfall, greatly contributed to a significant soil acidification occurred in the urbanized environment.

Effects of plant diversity, functional group composition, and fertilization on soil microbial properties in experimental grassland

BACKGROUND

Loss of biodiversity and increased nutrient inputs are two of the most crucial anthropogenic factors driving ecosystem change. Although both received considerable attention in previous studies, information on their interactive effects on ecosystem functioning is scarce. In particular, little is known on how soil biota and their functions are affected by combined changes in plant diversity and fertilization.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the effects of plant diversity, functional community composition, and fertilization on the biomass and respiration of soil microbial communities in a long-term biodiversity experiment in semi-natural grassland (Jena Experiment). Plant species richness enhanced microbial basal respiration and microbial biomass, but did not significantly affect microbial specific respiration. In contrast, the presence of legumes and fertilization significantly decreased microbial specific respiration, without altering microbial biomass. The effect of legumes was superimposed by fertilization as indicated by a significant interaction between the presence of legumes and fertilization. Further, changes in microbial stoichiometry (C-to-N ratio) and specific respiration suggest the presence of legumes to reduce N limitation of soil microorganisms and to modify microbial C use efficiency.

CONCLUSIONS/SIGNIFICANCE

Our study highlights the role of plant species and functional group diversity as well as interactions between plant community composition and fertilizer application for soil microbial functions. Our results suggest soil microbial stoichiometry to be a powerful indicator of microbial functioning under N limited conditions. Although our results support the notion that plant diversity and fertilizer application independently affect microbial functioning, legume effects on microbial N limitation were superimposed by fertilization, indicating significant interactions between the functional composition of plant communities and nutrient inputs for soil processes.

Pulse increase of soil N2O emission in response to N addition in a temperate forest on Mt Changbai, northeast China

Nitrogen (N) deposition has increased significantly globally since the industrial revolution. Previous studies on the response of gaseous emissions to N deposition have shown controversial results, pointing to the system-specific effect of N addition. Here we conducted an N addition experiment in a temperate natural forest in northeastern China to test how potential changes in N deposition alter soil N2O emission and its sources from nitrification and denitrification. Soil N2O emission was measured using closed chamber method and a separate incubation experiment using acetylene inhibition method was carried out to determine denitrification fluxes and the contribution of nitrification and denitrification to N2O emissions between Jul. and Oct. 2012. An NH4NO3 addition of 50 kg N/ha/yr significantly increased N2O and N2 emissions, but their "pulse emission" induced by N addition only lasted for two weeks. Mean nitrification-derived N2O to denitrification-derived N2O ratio was 0.56 in control plots, indicating higher contribution of denitrification to N2O emissions in the study area, and this ratio was not influenced by N addition. The N2O to (N2+N2O) ratio was 0.41-0.55 in control plots and was reduced by N addition at one sampling time point. Based on this short term experiment, we propose that N2O and denitrification rate might increase with increasing N deposition at least by the same fold in the future, which would deteriorate global warming problems.

Soil nematode responses to increases in nitrogen deposition and precipitation in a temperate forest

The environmental changes arising from nitrogen (N) deposition and precipitation influence soil ecological processes in forest ecosystems. However, the corresponding effects of environmental changes on soil biota are poorly known. Soil nematodes are the important bioindicator of soil environmental change, and their responses play a key role in the feedbacks of terrestrial ecosystems to climate change. Therefore, to explore the responsive mechanisms of soil biota to N deposition and precipitation, soil nematode communities were studied after 3 years of environmental changes by water and/or N addition in a temperate forest of Changbai Mountain, Northeast China. The results showed that water combined with N addition treatment decreased the total nematode abundance in the organic horizon (O), while the opposite trend was found in the mineral horizon (A). Significant reductions in the abundances of fungivores, plant-parasites and omnivores-predators were also found in the water combined with N addition treatment. The significant effect of water interacted with N on the total nematode abundance and trophic groups indicated that the impacts of N on soil nematode communities were mediated by water availability. The synergistic effect of precipitation and N deposition on soil nematode communities was stronger than each effect alone. Structural equation modeling suggested water and N additions had direct effects on soil nematode communities. The feedback of soil nematodes to water and nitrogen addition was highly sensitive and our results indicate that minimal variations in soil properties such as those caused by climate changes can lead to severe changes in soil nematode communities.