Foliar Nitrogen Dynamics and Nitrogen Resorption of a Sandy Shrub Salix gordejevii in Northern China

Resorption of nitrogen (N) from senescing leaves is an important conservation mechanism that allows plants to use the same N repeatedly. Seasonal variations in leaf nitrogen of mature green and senescing leaves and N resorption in Salix gordejevii Chang, a sandy shrub in northern China, were studied. Our objective was to compare N resorption of this Salix species that successfully occupy different habitats (shifting sandland, fixed sandland and lowland) with differences in soil N availability and moisture. Nitrogen concentrations in green and senescing leaves were higher in June and July. N resorption efficiency (percentage reduction of N between green and senescing leaves) was highest at shifting sandland, intermediate at fixed sandland, and lowest at lowland. There was a clear seasonal variation in N-resorption efficiency, with a lower value at the early growing season and a higher value during summer. N resorption efficiency was lower at the sites with higher soil N availability, suggesting that the efficiency of the resorption process is determined by the availability of the nutrient in the soil. Resorption from senescing leaves may play an important role in the nitrogen dynamics of sandy plants and reduce the nitrogen requirements for plant growth. We conclude that N resorption from senescing leaves in S. gordejevii was correlated to soil characteristics and higher N resorption on poor soils is a phenotypic adjustment by this species to maximize N-use at low availability.     

Non-Additive Effects of Water and Nitrogen Addition on Ecosystem Carbon Exchange in a Temperate Steppe

Changes in precipitation and nitrogen (N) deposition can influence ecosystem carbon (C) cycling and budget in terrestrial biomes, with consequent feedbacks to climate change. However, little is known about the main and interactive effects of water and N additions on net ecosystem C exchange (NEE). In a temperate steppe of northern China, a field-manipulated experiment was conducted to evaluate the responses of NEE and its components to improve N and water availability from 2005 to 2008. The results showed that both water and N additions stimulated gross ecosystem productivity (GEP), ecosystem respiration (ER), and NEE. Water addition increased GEP by 17%, ER by 24%, and NEE by 11% during the experimental period, whereas N addition increased GEP by 17%, ER by 16%, and NEE by 19%. The main effects of both water and N additions changed with time, with the strongest water stimulation in the dry year and a diminishing N stimulation over time. When water and N were added in combination, there were non-additive effects of water and N on ecosystem C fluxes, which could be explained by the changes in species composition and the shifts of limiting resources from belowground (water or N) to aboveground (light). The positive water and N additions effects indicate that increasing precipitation and N deposition in the future will favor C sequestration in the temperate steppe. The non-additive effects of water and N on ecosystem C fluxes suggest that multifactor experiments are better able to capture complex interactive processes, thus improving model simulations and projections.     

Effects of Multi-nutrient Additions on GHG Fluxes in a Temperate Grassland of Northern China

Human activities have substantially enhanced the availability of important nutrient elements such as nitrogen (N), phosphorus (P), and potassium (K) in ecosystems worldwide. However, how the concurrent increase in all of these nutrients will affect greenhouse gas (that is, CO₂, N₂O, CH₄) levels remains unknown. In a temperate steppe of northern China, a 2-year field experiment was conducted to examine the effects of multi-nutrient additions on GHG fluxes from 2009 to 2010. Four levels of annual nutrient loads were mimicked: 0 g NPK (control), 15.5 g P m^?2 and 19.5 g K m^?2 as KH₂PO₄ (PK), 10 g N m^?2 as NH₄NO₃ plus PK (10N + PK), and 20 g N m^?2 plus PK (20N + PK) per year. The results show that multi-nutrient additions led to significant increases in net primary production (NPP) and soil temperature (ST), a significant decrease in soil moisture (SM) in 2010, and no significant changes in other soil parameters. Seasonal patterns differed greatly for different GHG fluxes in response to different nutrient treatments, largely as a result of differences in influential factors. The 10N + PK treatment significantly increased CO₂ uptake, whereas the 20N + PK treatment significantly decreased CO₂ uptake. The application of P and K without additional N significantly enhanced CH₄ uptake, whereas the two N + PK treatments significantly enhanced N₂O emissions. Significant positive, linear relationships were found between cumulative CO₂ uptake and soil total nitrogen (TN), microbial biomass carbon, and microbial biomass nitrogen, whereas significant negative, linear relationships were found with NPP, SM, and the C/N ratio. Significant positive, linear relationships were found between cumulative N₂O emission and ST, TN, NPP, and total organic carbon, whereas no relationships were found between cumulative CH₄ uptake and any soil parameters. CO₂ flux was related to N₂O flux temporally, to a certain extent, for all the treatments. In the control, N₂O flux showed a negative, linear relationship with CH₄ flux, whereas no regular relationships were detected between CO₂ and CH₄ fluxes in any treatment. Our findings imply that increasing nutrient deposition will change the magnitude, patterns, and relationships among GHG uptakes and emissions in the future.     

Warming and Nitrogen Addition Increase Litter Decomposition in a Temperate Meadow Ecosystem

Background

Litter decomposition greatly influences soil structure, nutrient content and carbon sequestration, but how litter decomposition is affected by climate change is still not well understood.

Methodology/Principal Findings

A field experiment with increased temperature and nitrogen (N) addition was established in April 2007 to examine the effects of experimental warming, N addition and their interaction on litter decomposition in a temperate meadow steppe in northeastern China. Warming, N addition and warming plus N addition reduced the residual mass of L. chinensis litter by 3.78%, 7.51% and 4.53%, respectively, in 2008 and 2009, and by 4.73%, 24.08% and 16.1%, respectively, in 2010. Warming, N addition and warming plus N addition had no effect on the decomposition of P. communis litter in 2008 or 2009, but reduced the residual litter mass by 5.58%, 15.53% and 5.17%, respectively, in 2010. Warming and N addition reduced the cellulose percentage of L. chinensis and P. communis, specifically in 2010. The lignin percentage of L. chinensis and P. communis was reduced by warming but increased by N addition. The C, N and P contents of L. chinensis and P. communis litter increased with time. Warming and N addition reduced the C content and C:N ratios of L. chinensisand P. communis litter, but increased the N and P contents. Significant interactive effects of warming and N addition on litter decomposition were observed (P<0.01).

Conclusion/Significance

The litter decomposition rate was highly correlated with soil temperature, soil water content and litter quality. Warming and N addition significantly impacted the litter decomposition rate in the Songnen meadow ecosystem, and the effects of warming and N addition on litter decomposition were also influenced by the quality of litter. These results highlight how climate change could alter grassland ecosystem carbon, nitrogen and phosphorus contents in soil by influencing litter decomposition.     

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。     

Age-Related Modulation of the Nitrogen Resorption Efficiency Response to Growth Requirements and Soil Nitrogen Availability in a Temperate Pine Plantation

Nitrogen (N) resorption is a key strategy for conserving N in forests, and is often affected by soil nutrient condition and N sink strength within the plant. However, our understanding of the age-related pattern of N resorption and how increasing N deposition will affect this pattern is limited. Here, we investigated N resorption along a chronosequence of stands ranging in age from 2 to 100 years old, and conducted a 4-year exogenous N input experiment in stands at age class 11, 20, and 45 in a Larix Principis-rupprechtii plantation in north China. We found a logarithmic increase in leaf N resorption efficiency (NRE) and green leaf N concentration, and a logarithmic decrease in senesced-leaf N concentration along the stand-age chronosequence. Leaf NRE was negatively correlated with plant-available N concentration. Stand-level N resorption was positively correlated with the annual N requirement for tree growth. N resorption contributed to 45, 62, and 68% of the annual N supply in the 11-, 20-, and 45-year-old stands, respectively. Our exogenous N input experiment showed that leaf NRE in the 11- and 20-year-old stands decreased 17 and 12% following a 50-kg N ha^?1 y^?1 input. However, leaf NRE was not affected in the 45-year-old stand. The increases in leaf NRE and the contribution of N resorption to annual N supply along stand ages suggested that, with stand development, tree growth depends more on N resorption to supply its N need. Furthermore, the leaf NRE of mature stand was not decreased under exogenous N input, suggesting that mature stands can be stronger sinks for N deposition than young stands due to their higher capacity to retain the deposited N within plants via internal cycle. Ignoring age-related N use strategies can lead to a bias in N cycle models when evaluating forest net primary production under increasing global N deposition.     

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...     

Nitrogen Addition and Warming Independently Influence the Belowground Micro-Food Web in a Temperate Steppe

Climate warming and atmospheric nitrogen (N) deposition are known to influence ecosystem structure and functioning. However, our understanding of the interactive effect of these global changes on ecosystem functioning is relatively limited, especially when it concerns the responses of soils and soil organisms. We conducted a field experiment to study the interactive effects of warming and N addition on soil food web. The experiment was established in 2006 in a temperate steppe in northern China. After three to four years (2009–2010), we found that N addition positively affected microbial biomass and negatively influenced trophic group and ecological indices of soil nematodes. However, the warming effects were less obvious, only fungal PLFA showed a decreasing trend under warming. Interestingly, the influence of N addition did not depend on warming.Structural equation modeling analysis suggested that the direct pathway between N addition and soil food web components were more important than the indirect connections through alterations in soil abiotic characters or plant growth. Nitrogen enrichment also affected the soil nematode community indirectly through changes in soil pH and PLFA. We conclude that experimental warming influenced soil food web components of the temperate steppe less than N addition, and there was little influence of warming on N addition effects under these experimental conditions.     

长期氮添加对东北地区兴安落叶松人工林土壤酶的影响

全球氮沉降持续增加导致的氮沉降量上升,已成为森林中物质循环的重要生态因子。为探讨氮沉降增加对东北地区兴安落叶松人工林土壤酶的影响,对长期人工添加无机氮(NH4NO3)和未添加无机氮的兴安落叶松人工林土壤酶活性进行了比较分析。实验分为2个处理,分别为对照处理(CK:0kg·N·hm^-2·a^-1)和无机氮处理(IN:10kg·N·hm^-2·a^-1),每个处理含3个重复。结果表明,在连续8年添加无机氮的情况下,β-1,4-糖苷酶(BG)、酸性磷酸酶(AP)和过氧化物酶(PER)活性上升(5.56%、8.66%和0.61%);纤维素二糖水解酶(CBH)、亮氨酸氨基肽酶(LAP)和多酚氧化酶(POX)活性下降(10.71%、3.85%和12.30%)。与之相对应的土壤碳、氮和磷元素含量也有变化(0.80%、12.66%和-2.25%)。因此,在氮沉降量逐年增加的情况下,研究土壤酶活性变化,能更好的解释森林生态系统物质循环特性,理解生物地球物质循环机理并预测未来土壤碳库变化。     

Effects of Irrigation and Nitrogen on the Performance of Aerobic Rice in Northern China

Aerobic rice is a new production system in which specially-developed varieties are grown under non-flooded, non-puddled, and non-saturated soil conditions. In 2003-2004, irrigation x Nitrogen experiments were carried out near Beijing using variety HD297. Water treatments included four irrigation levels, and Nitrogen treatments included different fertilizer N application rates and different numbers of N splits. The highest yields were 4460 kg/ha with 688 mm of total (rain plus irrigation) water input in 2003 and 6 026 kg/ha with 705 mm of water input in 2004. Because of the quite even distribution of rainfall in both years, the four irrigation treatments did not result in large differences of soil water conditions. There were few significant effects of irrigation on biomass accumulation, but yield increased with the total amount of water applied. High yields coincided with high harvest index and high percentages of grain filling. The application of fertilizer N either reduced biomass and yield or kept it at the same level as 0 N and consistently reduced the percentage grain filling and 1 000-grain weight. With the highest water application, five splits of N gave higher yield than three splits, whereas three splits gave higher yield than five splits with lower water applications.     

Influence of Earthworm Invasion on Redistribution and Retention of Soil Carbon and Nitrogen in Northern Temperate Forests

We analyzed soil organic matter distribution and soil solution chemistry in plots with and without earthworms at two sugar maple (Acer saccharum)–dominated forests in New York State, USA, with differing land-use histories to assess the influence of earthworm invasion on the retention or loss of soil carbon (C) and nitrogen (N) in northern temperate forests. Our objectives were to assess the influence of exotic earthworm invasion on (a) the amount and depth distribution of soil C and N, (b) soil ¹³C and ¹⁵N, and (c) soil solution chemistry and leaching of C and N in forests with different land-use histories. At a relatively undisturbed forest site (Arnot Forest), earthworms eliminated the thick forest floor, decreased soil C storage in the upper 12 cm by 28%, and reduced soil C:N ratios from 19.2 to 15.3. At a previously cultivated forest site with little forest floor (Tompkins Farm), earthworms did not influence the storage of soil C or N or soil C:N ratios. Earthworms altered the stable isotopic signature of soil at Arnot Forest but not at Tompkins Farm; the alteration of stable isotopes indicated that earthworms significantly increased the loss of forest floor C but not N from the soil profile at Arnot Forest. Nitrate (NO₃^–) concentrations in tension and zero-tension lysimeters were much greater at Tompkins Farm than Arnot Forest, and earthworms increased NO₃^– leaching at Tompkins Farm. The results suggest that the effect of earthworm invasion on the distribution, retention, and solution chemistry of soil C and N in northern temperate forests may depend on the initial quantity and quality of soil organic matter at invaded sites.     

Warming and Nitrogen Addition Alter Photosynthetic Pigments,Sugars and Nutrients in a Temperate Meadow Ecosystem

Global warming and nitrogen (N) deposition have an important influence on terrestrial ecosystems; however, the influence of warming and N deposition on plant photosynthetic products and nutrient cycling in plants is not well understood. We examined the effects of 3 years of warming and N addition on the plant photosynthetic products, foliar chemistry and stoichiometric ratios of two dominant species, i.e., Leymus chinensis and Phragmites communis, in a temperate meadow in northeastern China. Warming significantly increased the chlorophyll content and soluble sugars in L. chinensis but had no impact on the carotenoid and fructose contents. N addition caused a significant increase in the carotenoid and fructose contents. Warming and N addition had little impact on the photosynthetic products of P. communis. Warming caused significant decreases in the N and phosphorus (P) concentrations and significantly increased the carbon (C):P and N:P ratios of L. chinensis, but not the C concentration or the C:N ratio. N addition significantly increased the N concentration, C:P and N:P ratios, but significantly reduced the C:N ratio of L. chinensis. Warming significantly increased P. communis C and P concentrations, and the C:N and C:P ratios, whereas N addition increased the C, N and P concentrations but had no impact on the stoichiometric variables. This study suggests that both warming and N addition have direct impacts on plant photosynthates and elemental stoichiometry, which may play a vital role in plant-mediated biogeochemical cycling in temperate meadow ecosystems.     

Enhancement of Carbon Sequestration in Soil in the Temperature Grasslands of Northern China by Addition of Nitrogen and Phosphorus

Increased nitrogen (N) deposition is common worldwide. Questions of where, how, and if reactive N-input influences soil carbon (C) sequestration in terrestrial ecosystems are of great concern. To explore the potential for soil C sequestration in steppe region under N and phosphorus (P) addition, we conducted a field experiment between 2006 and 2012 in the temperate grasslands of northern China. The experiment examined 6 levels of N (0–56 g N m^-2 yr^-1), 6 levels of P (0–12.4 g P m^-2 yr^-1), and a control scenario. Our results showed that addition of both N and P enhanced soil total C storage in grasslands due to significant increases of C input from litter and roots. Compared with control plots, soil organic carbon (SOC) in the 0–100 cm soil layer varied quadratically, from 156.8 to 1352.9 g C m^-2 with N addition gradient (R² = 0.99, P < 0.001); and logarithmically, from 293.6 to 788.6 g C m^-2 with P addition gradient (R² = 0.56, P = 0.087). Soil inorganic carbon (SIC) decreased quadratically with N addition. The net C sequestration on grassland (including plant, roots, SIC, and SOC) increased linearly from -128.6 to 729.0 g C m^-2 under N addition (R² = 0.72, P = 0.023); and increased logarithmically, from 248.5 to 698 g C m^-2under P addition (R² = 0.82, P = 0.014). Our study implies that N addition has complex effects on soil carbon dynamics, and future studies of soil C sequestration on grasslands should include evaluations of both SOC and SIC under various scenarios.     

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.

     

Effects of Long-Term Nitrogen Addition and Atmospheric Nitrogen Deposition on Carbon Accumulation in Picea sitchensis Plantations

This study aimed to assess the combined effects of long-term nitrogen (N) supply and nitrogen deposition (N _dep) on carbon (C) accumulation within Sitka spruce [Picea sitchensis (Bong.) Carr.] plantations in Scotland. Six study sites established from 1970 to 1982 were periodically N-fertilized, monitored over time and commonly surveyed in 2010. Soil, aboveground biomass, and ground vegetation C stock changes were analyzed; aboveground C stocks were correlated with total additional N experienced at each site, that is, the sum of experimental N supply (N _add) and site-specific accumulated N _dep from 1900 to 2010. Results showed a positive N effect on aboveground tree C stock and no decline in tree growth was observed either during fertilization or after the latest N addition. The amount of C in litter was significantly higher in experimentally N-treated plots, whereas the amount of C in understory vegetation was higher in control plots. Pooling all the compartments (that is, understory vegetation, litter, soil, and tree biomass) the total ecosystem C content was estimated for each site, and at most sites a higher C stock was estimated for N-treated plots. Differences in aboveground C accumulation rates between treated and control plots were lower at sites with high levels of accumulated N _dep. Our results indicate that site-specific accumulated N _dep should be considered to understand tree growth responses to N fertilization.     

大气CO2浓度升高和氮添加对镉污染下两种树木叶片元素含量及回收率的影响

选择造林树种大叶相思(Acacia auriculiformis)和樟树(Cinnamomum camphora)为对象,采用开顶生长箱(open-top chamber,OTC)构建实验林,探讨镉(Cd)胁迫及其与大气CO2浓度升高、氮(N)添加的复合作用对2种树木叶片N、P养分利用策略和Cd积累特征的影响。试验设5个处理：对照(CK)、加Cd[10 kg Cd/(hm²·a)](Cd)、加Cd与加CO2 (700μmol/mol)(Cd C)、加Cd与加N[100 kg N/(hm²·a)](Cd N)、加Cd加CO₂加N (Cd CN)。处理约2.5 a后,测定两树种的成熟叶和衰老叶中N、磷(P)和Cd含量。结果表明,不同处理对两树种叶片P含量及P回收率均无显著影响,但显著影响叶片N含量及N回收率;Cd N处理下两树种叶片N含量升高;Cd和Cd N处理下樟树叶N回收率显著降低。Cd、Cd C、Cd N和Cd CN处理下两树种叶片Cd积累浓度及樟树成熟叶中的Cd/N和Cd/P升高。大叶相思叶片N、P回收率显著高于樟树...     

Long-Term Nitrogen Additions and Nitrogen Saturation in Two Temperate Forests

This article reports responses of two different forest ecosystems to 9 years (1988–96) of chronic nitrogen (N) additions at the Harvard Forest, Petersham, Massachusetts. Ammonium nitrate (NH₄NO₃) was applied to a pine plantation and a native deciduous broad-leaved (hardwood) forest in six equal monthly doses (May–September) at four rates: control (no fertilizer addition), low N (5 g N m^-2 y^-1), high N (15 g N m^-2 y^-1), and low N + sulfur (5 g N m^-2 y^-1 plus 7.4 g S m^-2 y^-1). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter production, soil C and N content, and concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In the pine stand, nitrate losses were measured after the first year of additions (1989) in the high N plot and increased again in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further increases in 1996. Overall N retention efficiency (percentage of added N retained) over the 9-year period was 97–100% in the control and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground biomass, fine roots, and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots, suggesting that the one major unmeasured pool, soil organic matter, contains the remaining 68–84%. Short-term redistribution of ¹⁵N tracer at natural abundance levels showed similar division between plant and soil pools. Direct measurements of changes in total soil C and N pools were inconclusive due to high variation in both stands. Woody biomass production increased in the hardwood high N plot but was significantly reduced in the pine high N plot, relative to controls. A drought-induced increase in foliar litterfall in the pine stand in 1995 is one possible factor leading to a measured increase in N mineralization, nitrification, and nitrate loss in the pine high N plot in 1996. Received 2 April 1999; Accepted 29 October 1999.     

Fine Root Growth Increases in Response to Nitrogen Addition in Phosphorus-limited Northern Hardwood Forests

Ecosystems - Resource allocation theory posits that increased soil nutrient availability results in decreased plant investment in nutrient acquisition. We evaluated this theory by quantifying fine...     

Nitrogen Fixation and Nitrogen Assimilation in a Temperate Saline Ecosystem

As nitrogen is known to be a limiting factor for plant growth, we were interested in the relationship between soil microbial activity and the nitrogen assimilation of 5 different halophytes from 4 saline sites near the lake “Neusiedlersee”, Austria. The following were studied between May and October 1985: nitrogen fixation (¹⁵N₂ and acetylene reduction): N-mineralization; several soil characteristics and in vivo nitrate reductase activity of roots and shoots of these plants. NO^?₃, org. N- and carboxylate contents of both roots and shoots, as well as the effect of NO^?₃-fertilization on the amounts of these substances, were determined on plants growing in the field during a 3-day period in September 1985. Fertilization led to a decrease in acetylene reduction activity at most sites, and an increase in the nitrate reductase activity of the shoots of all plants. Overall, carboxylate and organic nitrogen contents of these halophytes did not change in response to fertilization. Only in the roots of Aster tripolium and Atriplex hastata was there a marked increase in the nitrate reductase activity in response to fertilization. Species growing at the same site, such as Plantago maritima and Lepidium crassifolium showed contrasting levels of assimilatory activity. Apparent low rates of ammonification and nitrification were detected in soils from the 4 sites. The results are discussed in relation to the nitrogen and carbon economies of the microorganisms and plants.