Precipitation Pattern Regulates Soil Carbon Flux Responses to Nitrogen Addition in a Temperate Forest

Ecosystems - Changes in precipitation frequency and intensity are predicted to be more intense and frequent accompanying climate change and may have immediate or potentially prolonged effects on...     

Contrasting Responses of Soil Nematode Trophic Groups to Long-Term Nitrogen Addition

Ecosystems - Increases in nitrogen (N) deposition greatly affect soil processes and functions, but there is limited information about the effects of long-term N deposition on soil nematodes. A...     

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.     

Seasonal Patterns of Soil Respiration and Related Soil Biochemical Properties under Nitrogen Addition in Winter Wheat Field

Understanding the changes of soil respiration under increasing N fertilizer in cropland ecosystems is crucial to accurately predicting global warming. This study explored seasonal variations of soil respiration and its controlling biochemical properties under a gradient of Nitrogen addition during two consecutive winter wheat growing seasons (2013–2015). N was applied at four different levels: 0, 120, 180 and 240 kg N ha^-1 year^-1 (denoted as N0, N12, N18 and N24, respectively). Soil respiration exhibited significant seasonal variation and was significantly affected by soil temperature with Q₁₀ ranging from 2.04 to 2.46 and from 1.49 to 1.53 during 2013–2014 and 2014–2015 winter wheat growing season, respectively. Soil moisture had no significant effect on soil respiration during 2013–2014 winter wheat growing season but showed a significant and negative correlation with soil respiration during 2014–2015 winter wheat growing season. Soil respiration under N24 treatment was significantly higher than N0 treatment. Averaged over the two growing seasons, N12, N18 and N24 significantly increased soil respiration by 13.4, 16.4 and 25.4% compared with N0, respectively. N addition also significantly increased easily extractable glomalin-related soil protein (EEG), soil organic carbon (SOC), total N, ammonium N and nitrate N contents. In addition, soil respiration was significantly and positively correlated with β-glucosidase activity, EEG, SOC, total N, ammonium N and nitrate N contents. The results indicated that high N fertilization improved soil chemical properties, but significantly increased soil respiration.     

Response of Fine Root Carbohydrate Content to Soil Nitrogen Addition and Its Relationship with Soil Factors in a Schrenk (Picea schrenkiana) Forest

Fine roots are vital for water and nutrient uptake in plants. Understanding the responses of metabolic traits to changing environmental conditions is critical, but little is known. In this study, the carbohydrate content of fine roots was characterized for six soil layers and three diameter classes in a Schrenk spruce (Picea schrenkiana) forest on the Tianshan Mountains of China. Soil nitrogen addition can influence carbohydrate content, but the degree is related to soil layer and fine root diameter. Specifically, as nitrogen (N) levels increased, the soluble sugar, cellulose, lignin and structural carbohydrate content (SC) all increased and then decreased; the starch and non-structural carbohydrate content (NSC) decreased gradually. In addition, N addition had significant effects on starch and cellulose content, as well as total carbohydrates. The interaction between root diameter and N addition also influenced soluble sugars, cellulose and NSC, while the three-way interaction among N addition, soil layer and diameter only affected cellulose content and NSC/SC. In the control treatment (no nitrogen addition), the following soil factors influenced fine root metabolism in order of decreasing importance: C:N, C:P, N:P, C, N, water content, bulk density, pH, conductivity, and P. In the N addition treatments, the soil conductivity, pH, and N content were strongly correlated with root metabolic characteristics.

     

氮添加对温带森林细根长期分解的影响

细根分解是森林生态系统碳循环的重要过程之一,其分解速率受到大气氮沉降增加的潜在影响。利用长期模拟氮沉降样地（2009年至今）,采用凋落物分解袋方法,研究了氮添加对温带常见的5个森林树种长期细根分解的影响。结果表明：细根分解呈现先快后慢的趋势,在分解第516天质量损失达30%~50%,之后质量残留率变化较为平缓。总体上,渐近线分解模型可以更准确的反应各处理细根分解速率。氮添加对细根分解具有阶段性影响,分解前期促进细根分解,分解后期抑制分解。在细根分解后期氮添加减缓分解速率,一方面是因为木质素等较难分解的物质所占比例升高所带来的直接影响,另一方面,是因为氮添加改变了微生物活动所带来的间接影响。     

Three-Year Soil Carbon and Nitrogen Responses to Sugarcane Straw Management

Green harvest sugarcane management has increased soil organic C and N stocks over time. However, emerging sugarcane straw removal to meet increasing bioenergy demands has raised concerns about soil C and N depletions. Thus, we conducted a field study in southeast Brazil over nearly three years (1100 days) for assessing soil C and N responses to increasing sugarcane straw removal rates. In order to detect the C input as a function of the different amounts of straw over three years, a field simulation was performed, where the original soil layer (0–0.30 m) was replaced by another from an adjacent area with low total C and δ¹³C. The treatments tested were as follows: (i) 0 Mg ha^?1 year^?1 (i.e., 100% removal), (ii) 3.5 Mg ha^?1 year^?1 (i.e., 75% removal), (iii) 7.0 Mg ha^?1 year^?1 (i.e., 50% removal), (iv) 14.0 Mg ha^?1 year^?1 (i.e., no removal), and (v) 21.0 Mg ha^?1 year^?1 (i.e., no removal + extra 50% of the straw left on the field). The results showed that sugarcane straw removal affected the soil C and total N pools. In the first 45 days of straw decomposition, a small but important straw-derived C portion enters into the soil as dissolved organic carbon (DOC). The lower the straw removal rate, the higher was straw-derived DOC content found into the soil, down to 0.50 m depth. After 3 years of management, keeping sugarcane straw on soil surface significantly increased C and N stocks within surface soil layer (0–0.025 m). Our findings suggest that under no straw removal management (i.e., 14 Mg ha^?1), approximately 364 kg ha^?1 of C and 23 kg ha^?1 of N are annually stored into this low-C soil. The contribution of the straw-derived C (C-C4) to the total soil C increases over time, which accounted for about 60% under no straw removal rate. The greatest contribution of the C storage preferentially occurs into the fraction of organic matter (<?0.53 μm) associated with soil clay minerals. We concluded that indiscriminate sugarcane straw removal to produce cellulosic ethanol or bioelectricity depletes soil C stocks and reduces N cycling in sugarcane fields, impairing environmental gains associated with bioenergy production. Therefore, this information, linked with other agronomic and environmental issues, should be taken into account towards a more sustainable straw removal management for bioenergy production in Brazil.     

A New Conceptual Model of Nitrogen Saturation Based on Experimental Nitrogen Addition to an Oak Forest

The dominant conceptual model of nitrogen (N) saturation in forests predicts the temporal patterns of key N cycling indicators as an initially N-limited forest is progressively enriched in N. We present the results from a long-term N addition experiment in an oak forest in southeastern New York State, USA, which do not conform to the predictions of the conceptual model in several ways. In contrast to the predictions of the conceptual model, the foliar N concentrations in the N-treated stands of our study increased to about 20% above the levels in the control stands and then remained essentially constant, and nitrogen leaching from the treated stands increased almost immediately after the start of the experiment, prior to the onset of elevated nitrification. Concentrations of N in soil solution of the N-treated stands peaked at over 150-fold greater than the concentrations in the control stands. There were no significant changes in potential net N mineralization. Tree mortality increased in the treated stands, but the tree mortality did not appear to be the primary cause of the excess nitrate leaching. Based on these results and those of other recent studies, we present a new conceptual model of the N saturation process focused on the mass balance of N rather than the temporal dynamics of N cycling indicators. The mass balance is characterized by inputs of N from atmospheric deposition and fertilization, internal sinks in the vegetation and soils, and outputs to leaching and gaseous losses. The key points of the conceptual model are (1) added N can flow simultaneously to all sinks and losses in the system, (2) the fate of the added N and the temporal patterns of flow of N depend on the strength of the sinks and the factors that control them, and (3) the movement of N to the various sinks determines how N saturation is manifested in the ecosystem. We distinguish capacity N saturation, in which the sinks in the vegetation and soil are zero or negative, from kinetic N saturation, in which the sinks are positive but lower than the N input rate. The sink strengths in the vegetation and soil have two components, one due to carbon (C) accumulation in the system and the other due to change in the stoichiometry (C:N ratio) of the pool. Further work quantifying the magnitudes and controlling factors for the N sinks will allow better prediction of the dynamics of N saturation in different types of forested ecosystems.     

Soil Carbon and Nitrogen Responses to Nitrogen Fertilizer and Harvesting Rates in Switchgrass Cropping Systems

The environmental sustainability of bioenergy cropping systems depends upon multiple factors such as crop selection, agricultural practices, and the management of carbon (C), nitrogen (N), and water resources. Perennial grasses, such as switchgrass (Panicum virgatum L.), show potential as a sustainable bioenergy source due to high yields on marginal lands with low fertilizer inputs and an extensive root system that may increase sequestration of C and N in subsurface soil horizons. We quantified the C and N stocks in roots, free particulate, and mineral-associated soil organic matter pools in a 4-year-old switchgrass system following conversion from row crop agriculture at the W.K. Kellogg Biological Station in southwest Michigan. Crops were fertilized with nitrogen at either 0, 84, or 196 kg N ha^?1 and harvested either once or twice annually. Twice-annual harvesting caused a reduction of C and N stocks in the relatively labile roots and free-particulate organic matter pools. Nitrogen fertilizer significantly reduced total soil organic C and N stocks, particularly in the stable, mineral-associated C and N pools at depths greater than 15 cm. The largest total belowground C stocks in biomass and soil occurred in unfertilized plots with annual harvesting. These findings suggest that fertilization in switchgrass agriculture moderates the sequestration potential of the soil C pool.     

Plant and Soil Responses to Created Microtopography and Soil Treatments in Bottomland Hardwood Forest Restoration

Although microtopographic heterogeneity is common in bottomland hardwood forests, it is rarely considered in bottomland restoration efforts. The objective of this study was to determine the responses of hydrologic condition, soil physiochemical properties, and introduced and colonizing vegetation to created microtopography and soil treatments at a landfill borrow pit in northern Texas. A series of mounds and pools were created and planted with fast‐growing pioneer species as well as more desirable, later‐successional species. Erosion control mats were installed on half the plots as a source of organic matter. Erosion control mats had little influence on introduced seedling survival or colonizing species abundance, but microtopography strongly influenced hydrologic condition, soil properties, seedling survival and growth, and colonizing species abundance and distribution. Pools were flooded during much of the summer months and had significantly higher nitrate and total nitrogen concentrations than mounds. Topographic position had little effect on survival of pioneer species, but mortality of most later‐successional species was highest in pools. Colonizing species distribution and abundance were also strongly related to topographic position. Despite differences in soil nutrient concentration among topographic zones, hydrologic condition likely had the strongest influence on growth and survival of planted species and distribution of colonizing species. Creating microtopography resulted in a spatially heterogeneous system that reflected variations in natural bottomlands, and introducing a mix of species (pioneer and later‐successional) across topographic and hydrologic gradients may improve the establishment and survival of a diverse community when hydrologic condition is highly variable or difficult to predict.     

Soil Bacterial Community Structure Responses to Precipitation Reduction and Forest Management in Forest Ecosystems across Germany

Soil microbial communities play an important role in forest ecosystem functioning, but how climate change will affect the community composition and consequently bacterial functions is poorly understood. We assessed the effects of reduced precipitation with the aim of simulating realistic future drought conditions for one growing season on the bacterial community and its relation to soil properties and forest management. We manipulated precipitation in beech and conifer forest plots managed at different levels of intensity in three different regions across Germany. The precipitation reduction decreased soil water content across the growing season by between 2 to 8% depending on plot and region. T-RFLP analysis and pyrosequencing of the 16S rRNA gene were used to study the total soil bacterial community and its active members after six months of precipitation reduction. The effect of reduced precipitation on the total bacterial community structure was negligible while significant effects could be observed for the active bacteria. However, the effect was secondary to the stronger influence of specific soil characteristics across the three regions and management selection of overstorey tree species and their respective understorey vegetation. The impact of reduced precipitation differed between the studied plots; however, we could not determine the particular parameters being able to modify the response of the active bacterial community among plots. We conclude that the moderate drought induced by the precipitation manipulation treatment started to affect the active but not the total bacterial community, which points to an adequate resistance of the soil microbial system over one growing season.     

Responses of Soil,Heterotrophic, and Autotrophic Respiration to Experimental Open-Field Soil Warming in a Cool-Temperate Deciduous Forest

How global warming will affect soil respiration (R _S) and its source components is poorly understood despite its importance for accurate prediction of global carbon (C) cycles. We examined the responses of R _S, heterotrophic respiration (R _H), autotrophic respiration (R _A), nitrogen (N) availability, and fine-root biomass to increased temperature in an open-field soil warming experiment. The experiment was conducted in a cool-temperate deciduous forest ecosystem in northern Japan. As this forest is subjected to strong temporal variation in temperature, on scales ranging from daily to seasonal, we also investigated the temporal variation in the effects of soil warming on R _S, R _H, and R _A. Soil temperature was continuously elevated by about 4.0°C from 2007 to 2014 using heating wires buried in the soil, and we measured soil respiratory processes in all four seasons from 2012 to 2014. Soil warming increased annual R _S by 32–45%, but the magnitude of the increase was different between the components: R _H and R _A were also stimulated, and increased by 39–41 and 17–18%, respectively. Soil N availability during the growing season and fine-root biomass were not remarkably affected by the warming treatment. We found that the warming effects varied seasonally. R _H increased significantly throughout the year, but the warming effect showed remarkable seasonal differences, with the maximum stimulation in the spring. This suggests that warmer spring temperature will produce a greater increase in CO₂ release than warmer summer temperatures. In addition, we found that soil warming reduced the temperature sensitivity (Q ₁₀) of R _S. Although the Q ₁₀ of both R _H and R _A tended to be reduced, the decrease in the Q ₁₀ of R _S was caused mainly by a decrease in the response of R _A to warming. These long-term results indicate that a balance between the rapid and large response of soil microbes and the acclimation of plant roots both play important roles in determining the response of R _S to soil warming, and must be carefully considered to predict the responses of soil C dynamics under future temperature conditions.     

Land Use History Mediates Soil Biogeochemical Responses to Drought in Temperate Forest Ecosystems

Ecosystems - Terrestrial ecosystems are experiencing increasing frequency and intensity of droughts as a result of climate change. Despite a wealth of previous studies investigating soil responses...     

Biodegradation of Diesel Fuel in Soil Under Various Nitrogen Addition Regimes

Bioremediation is a growing technology for treating fuel-contaminated soils. Many biological, physical, and chemical parameters control the rate and efficiency of this process, including type and concentration of contaminants, temperature, oxygen content, and nutrient status. This study investigated the effect that nitrogen sources and concentrations had on the degradation rate of diesel fuel in nutrient limited soil at two carbon-to-nitrogen ratios. The different sources of nitrogen studied were ammonium nitrate, ammonium sulfate, potassium nitrate, urea, and urea oligomers (control release fertilizer). Laboratory experiments were conducted on field-contaminated soil using sealed bioreactors at a controlled temperature of 25°C. For both carbon-to-nitrogen ratios tested, hydrocarbon degradation rates were the highest for the ammonium sulfate (20:1 at 0.032?d^?1; 40:1 at 0.019d^?1) and urea treatments (20:1 at 0.025?d^?1; 40:1 at0.011?d^?1). A degradation rate correlation as a function of nitrate and ammonia concentrations was developed. The correlation suggests the occurrence of nitrate inhibition at elevated nitrate concentrations.     

不同降水条件下荒漠草原植物的养分含量及回收对增温和氮素添加的响应

为了探讨荒漠草原植物养分回收特征对长期增温和氮素添加的响应以及自然降水变异对其的调控作用,该研究依托实施12年的模拟增温和氮素添加实验平台,在相对多雨的2016年(超过长期均值52%)和相对少雨的2017年(低于长期均值16%),以常见C_3植物银灰旋花(Convolvulus ammannii)和C_4植物木地肤(Kochia prostrata)为研究对象,测定分析绿叶和枯叶的氮磷含量及回收效率。结果表明:(1)在相对多雨年(2016年),增温使2种植物的绿叶氮、枯叶氮、绿叶磷、枯叶磷含量分别增加了14.32%、25.45%、17.97%和46.47%,氮、磷回收效率分别显著减小了9.41%和16.99%(P0.05);氮素添加使2种植物的绿叶氮、枯叶氮、绿叶磷、枯叶磷含量分别提高了17.32%、25.62%、20.21%和51.41%,而氮、磷回收效率显著降低了9.33%和18.89%(P0.05);增温+氮素添加共同处理显著增加了植物氮磷含量、降低了氮磷回收效率。(2)在相对少雨年(2017年),增温、氮素添加、增温+氮素添加处理对植物叶片氮磷含量、回收效率均无显著影响。(3)叶片氮磷含量在物种间差异极显著(P0.000 1),而氮磷回收效率在物种间无显著差异。(4)回归分析表明,植物叶片氮磷含量随着土壤无机氮、有效磷及含水量的增加而增加,植物氮磷回收效率则随着土壤养分和水分的可利用性的增加而降低。研究认为,荒漠草原植物养分回收对全球变化的响应受自然降水变异的调控。     

Nitrogen Addition Regulates Soil Nematode Community Composition through Ammonium Suppression

Nitrogen (N) enrichment resulting from anthropogenic activities has greatly changed the composition and functioning of soil communities. Nematodes are one of the most abundant and diverse groups of soil organisms, and they occupy key trophic positions in the soil detritus food web. Nematodes have therefore been proposed as useful indicators for shifts in soil ecosystem functioning under N enrichment. Here, we monitored temporal dynamics of the soil nematode community using a multi-level N addition experiment in an Inner Mongolia grassland. Measurements were made three years after the start of the experiment. We used structural equation modeling (SEM) to explore the mechanisms regulating nematode responses to N enrichment. Across the N enrichment gradient, significant reductions in total nematode abundance, diversity (H' and taxonomic richness), maturity index (MI), and the abundance of root herbivores, fungivores and omnivores-predators were found in August. Root herbivores recovered in September, contributing to the temporal variation of total nematode abundance across the N gradient. Bacterivores showed a hump-shaped relationship with N addition rate, both in August and September. Ammonium concentration was negatively correlated with the abundance of total and herbivorous nematodes in August, but not in September. Ammonium suppression explained 61% of the variation in nematode richness and 43% of the variation in nematode trophic group composition. Ammonium toxicity may occur when herbivorous nematodes feed on root fluid, providing a possible explanation for the negative relationship between herbivorous nematodes and ammonium concentration in August. We found a significantly positive relationship between fungivores and fungal phospholipid fatty acids (PLFA), suggesting bottom-up control of fungivores. No such relationship was found between bacterivorous nematodes and bacterial PLFA. Our findings contribute to the understanding of effects of N enrichment in semiarid grassland on soil nematode trophic groups, and the cascading effects in the detrital soil food web.     

Long-Term Nitrogen Addition Leads to Loss of Species Richness Due to Litter Accumulation and Soil Acidification in a Temperate Steppe

Background

Although community structure and species richness are known to respond to nitrogen fertilization dramatically, little is known about the mechanisms underlying specific species replacement and richness loss. In an experiment in semiarid temperate steppe of China, manipulative N addition with five treatments was conducted to evaluate the effect of N addition on the community structure and species richness.

Methodology/Principal Findings

Species richness and biomass of community in each plot were investigated in a randomly selected quadrat. Root element, available and total phosphorus (AP, TP) in rhizospheric soil, and soil moisture, pH, AP, TP and inorganic N in the soil were measured. The relationship between species richness and the measured factors was analyzed using bivariate correlations and stepwise multiple linear regressions. The two dominant species, a shrub Artemisia frigida and a grass Stipa krylovii, responded differently to N addition such that the former was gradually replaced by the latter. S. krylovii and A. frigida had highly-branched fibrous and un-branched tap root systems, respectively. S. krylovii had higher height than A. frigida in both control and N added plots. These differences may contribute to the observed species replacement. In addition, the analysis on root element and AP contents in rhizospheric soil suggests that different calcium acquisition strategies, and phosphorus and sodium responses of the two species may account for the replacement. Species richness was significantly reduced along the five N addition levels. Our results revealed a significant relationship between species richness and soil pH, litter amount, soil moisture, AP concentration and inorganic N concentration.

Conclusions/Significance

Our results indicate that litter accumulation and soil acidification accounted for 52.3% and 43.3% of the variation in species richness, respectively. These findings would advance our knowledge on the changes in species richness in semiarid temperate steppe of northern China under N deposition scenario.     

酸与酸肥混合处理对瑞典中部里兹堡松林地上部分的影响

在实验处理期间，瑞典Ｌｉｓｅｌｂｏ地区实验松林受益于硫酸的施用和酸肥的混合使用．１９７１～１９７７年间，酸处理的松树林分基径面积的增长比对照提高了４％；１９７６年酸处理结束，但其潜在的正效应在１９７８～１９８５年间得到增强，基径面积的增长比对照提高了２０％，随后的某一阶段开始下降，１９９３～１９９４为１１％．酸肥混合处理的松树林分的基径面积增长在１９７１～１９７７年比对照提高了３８％；１９７８～１９８５年略有下降，为３０％；１９９３～１９９４年间为－１６％，提示酸肥混合处理对松林基径面积的促进作用至少在１９８５年处理结束后开始减弱，最后表现为负的生长效应．１９９４年，和对照、酸处理的松林样地比，酸肥处理的松林地上部分具有较高的生物现存量，较低的针叶生产力水平．然而，酸处理的松林样地的地上部分现存量相对高于对照，明显低于酸肥混合处理，其针叶生产力水平则相对比对照的高，显著高于酸肥混合处理的松林样地