N-limited Forests Research Articles

Spatial-temporal patterns of foliar and bulk soil 15N isotopic signatures across a heterogeneous landscape: Linkages to soil N status, nitrate leaching, and N2O fluxes

The natural abundance of plant and bulk soil 15N isotopic signatures provides valuable insights into the magnitude of nitrogen cycling and loss processes within terrestrial ecosystems. However, 15N isotopic signatures are highly variable in space due to natural and anthropogenic factors affecting N cycling processes and losses. To date, most studies on foliar and bulk soil 15N isotopic signatures have focused on N-limited forest ecosystems at relatively large spatial scales, while similar studies in N-enriched ecosystems at finer spatial scales are lacking. To address this gap and evaluate links between soil 15N isotopic signatures and ecosystem N cycling and loss processes (plant N uptake, N leaching, and gaseous loss), this study quantified foliar and bulk soil 15N isotopic signatures, soil physicochemical parameters, gaseous (N2O), and hydrological (NO3) N losses at 80 sites distributed across a heterogeneous landscape (∼5.8 km2). To account for the spatial-temporal heterogeneity, the measurements were performed in four campaigns (March, June, September 2022, and March 2023) at sites that considered different land uses, soil types, and topography. Results indicated that foliar and bulk soil 15N isotopic signatures were significantly (P < 0.05) more enriched in arable and grassland ecosystems than forests, suggesting a more open N cycle with significant N cycling and losses due to higher N inputs from fertilizers. Similar to soil inorganic N, N2O fluxes, and NO3 leaching rates, landscape-scale foliar and soil 15N isotopic signatures varied widely spatially, particularly at grassland and arable land (−3 to 9.0‰), with bivariate and multivariate analyses also showing significant relationships between landscape-scale soil 15N isotopic signatures and the aforementioned parameters (r2: 0.29 to 0.82). Based on these relationships, our findings suggested that foliar and bulk 15N isotopic signatures may capture fine-scale areas with persistently high and low environmental N losses (N2O fluxes and NO3 leaching) within a heterogeneous landscape.

Soil solution chemistry weak response to long-term N addition points towards a strong resilience of northeastern American forests to past and future N deposition

Northern temperate and boreal forests are large biomes playing crucial ecological and environmental roles, such as carbon sequestration. Despite being generally remote, these forests were exposed to anthropogenic nitrogen (N) deposition over the last two centuries and may still experience elevated N deposition as human activities expand towards high latitudes. However, the impacts of long-term high N deposition on these N-limited forest ecosystems remain unclear. For 18 years, we simulated N deposition by chronically adding ammonium nitrate at rates of 3 (LN treatment) and 10 (HN treatment) times the ambient N deposition estimated at the beginning of the experiment at a temperate sugar maple and a boreal balsam fir forest site, both located in northeastern America. LN and HN treatments corresponded respectively to addition of 26 kgN·ha−1·yr−1 and 85 kgN·ha−1·yr−1 at the temperate site and 17 kgN·ha−1·yr−1 and 57 kgN·ha−1·yr−1 at the boreal site. Between 2002 and 2018, soil solution was collected weekly during summer and concentrations of NO3−, NH4+, Ca2+ and pH were measured, totalling ~12,700–13,500 observations per variable on the study period. N treatments caused soil solution NO3−, NH4+ and Ca2+ concentrations to increase while reducing its pH. However, ion responses manifested through punctual high concentration events (predominantly on the HN plots) that were very rare and leached N quantity was extremely low at both sites. Therefore, N addition corresponding to 54 years (LN treatment) and 180 years (HN treatment) of accelerated ambient N deposition had overall small impacts on soil solution chemistry. Our results indicate an important N retention of northeastern American forests and an unexpected strong resilience of their soil solution chemistry to long-term simulated N deposition, potentially explained by the widespread N-limitation in high latitude ecosystems. This finding can help predict the future productivity of N-limited forests and improve forest management strategies in northeastern America.

What happens to trees and soils during five decades of experimental nitrogen loading?

High deposition of nitrogen was postulated to drive losses of NO3- and nutrient base cations, causing soil acidification, nutrient deficiencies reducing tree growth and ultimately tree mortality. We tested these predictions in a uniquely long-term study involving three NH4NO3 addition treatments (N1-N3) in a boreal Pinus sylvestris forest. The lowest level (N1), 30 kg N ha−1 yr−1 was applied during 50 years. Twice this rate (N2) was added 38 years, followed by 12 years of recovery, while thrice this rate (N3) was added 20 years followed by 30 years of recovery. We compared tree growth, changes in foliar and soil chemistry among treatments including control plots without N additions. As predicted, the N treatments lowered soil pH and reduced soil base saturation by around 50 %. They also lowered foliar levels of Ca, Mg, K, P and B initially, but after 50 years only Ca and Mg remained lower than in the control. Lack of B motivated a single addition of 2.5 kg ha−1 after ten years of N treatment. Tree stem growth became and then remained higher in N1 than in the other treatments through the 50 years of treatments. In N2 and N3, foliar δ15N increased during the N-loading phase, but declined during the recovery phase, indicating a return of ectomycorrhizal fungi and their role in tightening the N cycle in N-limited forests. In the terminated, initially highest N treatments, N2 and N3, the trees even show signs of returning to N-limitation. In these treatments, the soil base saturation remains lower, while the pH was only lower at 0–10 depth in the mineral soil, but not in the 10–20 cm depth horizon or in the superficial organic mor-layer. Accurately documenting the effect of N additions on forest growth required a long-term approach, where reasonable rates of application could be compared with extreme rates. Such long-term experiments are necessary to support forest management in achieving goals for developing future forests as they shift in response to major, global-scale changes.

No impact of nitrogen fertilization on carbon sequestration in a temperate Pinus densiflora forest

Carbon (C) sequestration capacity in forest ecosystems is generally constrained by soil nitrogen (N) availability. Consequently, N fertilization is seen as a promising tool for enhancing ecosystem-level C sequestration in N-limited forests. We examined the responses of ecosystem C (vegetation and soil) and soil N dynamics to 3 years of annual nitrogen-phosphorus-potassium (N3P4K1 = 11.3 g N, 15.0 g P, 3.7 g K m−2 year−1) or PK fertilization (P4K1), observed over 4 years in a 40-year-old Pinus densiflora forest with poor N nutrition in South Korea. PK fertilization without N was performed to test for PK limitation other than N. Neither tree growth nor soil C fluxes responded to annual NPK or PK fertilization despite an increase in soil mineral N fluxes following NPK fertilization. NPK fertilization increased the rate of N immobilization and 80% of the added N was recovered from mineral soil in the 0–5 cm layer, suggesting that relatively little of the added N was available to trees. These results indicate that N fertilization does not always enhance C sequestration even in forests with poor N nutrition and should therefore be applied with caution.

Response of litter decomposition and the soil environment to one-year nitrogen addition in a Schrenk spruce forest in the Tianshan Mountains, China

Human activities have increased the input of nitrogen (N) to forest ecosystems and have greatly affected litter decomposition and the soil environment. But differences in forests with different nitrogen deposition backgrounds. To better understand the response of litter decomposition and soil environment of N-limited forest to nitrogen deposition. We established an in situ experiment to simulate the effects of N deposition on soil and litter ecosystem processes in a Picea schrenkiana forest in the Tianshan Mountains, China. This study included four N treatments: control (no N addition), low N addition (LN: 5 kg N ha−1 a−1), medium N addition (MN: 10 kg N ha−1 a−1) and high N addition (HN: 20 kg N ha−1 a−1). Our results showed that N addition had a significant effect on litter decomposition and the soil environment. Litter mass loss in the LN treatment and in the MN treatment was significantly higher than that in the control treatment. In contrast, the amount of litter lost in the HN treatment was significantly lower than the other treatments. N application inhibited the degradation of lignin but promoted the breakdown of cellulose. The carbon (C), N, and phosphorus (P) contents of litter did not differ significantly among the treatments, but LN promoted the release of C and P. Our results also showed that soil pH decreased with increasing nitrogen application rates, while soil enzyme activity showed the opposite trend. In addition, the results of redundancy analysis (RDA) and correlation analyses showed that the soil environment was closely related to litter decomposition. Soil enzymes had a positive effect on litter decomposition rates, and N addition amplified these correlations. Our study confirmed that N application had effects on litter decomposition and the soil environment in a N-limited P. schrenkiana forest. LN had a strong positive effect on litter decomposition and the soil environment, while HN was significantly negative. Therefore, increased N deposition may have a negative effect on material cycling of similar forest ecosystems in the near future.

Coexistence of multiple leaf nutrient resorption strategies in a single ecosystem

Leaf resorption is critical for considerations of how plants use and recycle nutrients, but fundamental unknowns remain regarding the controls over plant nutrient resorption. Empirical studies suggest at least three basic types of resorption control, including (i) stoichiometric control, (ii) nutrient limitation control, and (iii) nutrient concentration control strategies. However, which strategies are adopted in given conditions and whether multiple strategies coexist in an ecosystem are still open questions. To address these unknowns, leaf nitrogen (N) and phosphorus (P) resorption efficiency (NRE and PRE) and proficiency were measured for seven woody species at a nutrient-rich but potentially N-limited secondary forest and a nutrient-poor and potentially P-limited secondary forest. NRE was higher in the N-limited forest while PRE was higher in the P-limited forest, suggesting that plants responded to nutrient limitation with preferential resorption of the more limiting nutrient. NRE:PRE was positively related to leaf N:P ratios within each forest, demonstrating a role for stoichiometric control. Nutrient concentration controls were also found, with higher nutrient resorption proficiency in the nutrient-poor forest than in the nutrient-rich forest. The controls of stoichiometry and nutrient concentration were community-wide, but the nutrient limitation control was species-specific. Our results highlight the coexistence of multiple nutrient resorption strategies in a single ecosystem, and suggest these strategies are scale-dependent.

Soil C:N ratio is the main driver of soil δ15N in cold and N-limited eastern Canadian forests

Significant relationships have been observed between soil N isotopic natural abundance (δ15N) and both climate and soil characteristics across a large range of ecosystems over the globe, suggesting strong and consistent effects of these variables on N cycling. However, whether the strength and the nature of these relationships vary at regional scales and with soil depth is less documented, especially in northern cold and N-limited forest ecosystems. In this study, we analyzed δ15N in soil horizons at 21 forest sites in eastern Quebec along a gradient of concomitant decreasing N deposition and temperature (MAAT) and increasing precipitation (MAP). We hypothesized that both soil δ15N and the magnitude of increase in soil δ15N would decrease along this gradient, in accordance with relationships reported at a global scale. The data show an increase in δ15N with soil depth, although it remained constant or sharply decreased between the B- and the C-horizon at most sites. The natural abundance of 15N in the forest floor (FF), in the B-horizon and in the C-horizon averaged 2.2 ± 0.9‰, 6.2 ± 1.2‰ and 4.9 ± 2.0‰, respectively while total soil profile δ15N ranged from 3.8‰ to 7.4‰. Contrary to our hypothesis, soil δ15N was poorly correlated with climate, vegetation and most soil metrics. As a consequence, there was no spatial gradient in soil δ15N values and in the magnitude of increase in δ15N with soil depth across the study area. Soil C:N ratio was the only variable significantly correlated with soil δ15N. Multivariate models including the C:N ratio explained 47%, 60% and 36% of the inter-sites δ15N variation in B-horizon, C-horizon and total soil, respectively. In contrast with global scale studies, which have reported higher soil δ15N at sites with low soil C:N ratio, the relationship between these two variables was positive across the study area. The possible influence of ecto-myccorhizal association on this pattern is discussed. Overall, our data show that soil δ15N is controlled by complex mechanisms influenced by several variables with potential antagonist effects. Climate and most soil metrics appear to have no direct influence in the cold and N-limited forest ecosystems studied here and soil C:N ratio can affect soil δ15N in an opposite manner to what has been commonly observed.

Experimental addition of nitrogen to a whole forest ecosystem at Gårdsjön, Sweden (NITREX): Nitrate leaching during 26 years of treatment

Chronic high deposition of nitrogen (N) to forest ecosystems can lead to increased leaching of inorganic N to surface waters, enhancing acidification and eutrophication. For 26 years nitrogen has been added as ammonium nitrate (NH4NO3) at 40 kg N ha−1 yr−1 to a whole forested catchment ecosystem at Gårdsjön, Sweden, to experimentally simulate the transition from a N-limited to N-rich state. Over the first 10 years of treatment there was an increasing amount of nitrate (NO3−) and to a lesser extent ammonium (NH4+) lost in runoff, but then N leaching stabilised, and for the subsequent 16 years the fraction of N added lost in runoff remained at 9%. NO3− concentrations in runoff were low in the summer during the first years of treatment, but now are high throughout the year. High frequency sampling showed that peaks in NO3− concentrations generally occurred with high discharge, and were enhanced if high discharge coincided with occasions of N addition. Approximately 50% of the added N has gone to the soil. The added N is equivalent to 140 years of ambient N deposition. At current ambient levels of N deposition there thus appears to be no immediate risk of N saturation at this coniferous forest ecosystem, and by inference to other such N-limited forests in Scandinavia.

Strong root respiration response to nitrogen and phosphorus addition in nitrogen-limited temperate forests

Nutrient availability is one of the key regulator of the global forest carbon balance. The use of fossil fuels and fertilizers has increased the amount of biologically reactive nitrogen (N) in recent decades and N fertilization also changes the availability of other nutrients such as phosphorous (P) and potassium (K). The increased soil nutrient availability is known to stimulate forest growth, but we currently lack comprehensive understanding of the response of soil respiration and its three components (roots, microbes, and ectomycorrhizal (ECM) fungi) to the increased soil N, P and K availability. We conducted a 4-year field fertilization experiment with N, P and K addition in an N-limited temperate forest and separated ECM fungi respiration (Rm), root respiration (Rr) and heterotrophic microbial respiration (Rh) from total soil respiration. Our results showed that Rr increased with N and P addition while Rh and Rm did not respond to nutrient addition. Rm, Rr and Rh varied substantially from year to year, but their responses to nutrient addition did not fluctuate in different years. Our results indicate that in N-limited forest ecosystems, Rh and Rm may not respond substantially to future changes in nutrient addition and that inter-annual variation in climate may be the determinant of soil CO2 efflux in response to global changes.

Seasonal variation in N uptake strategies in the understorey of a beech-dominated N-limited forest ecosystem depends on N source and species.

In forest ecosystems, species use different strategies to increase their competitive ability for nitrogen (N) acquisition. The acquisition of N by trees is regulated by tree internal and environmental factors including mycorrhizae. In this study, we investigated the N uptake strategies of three co-occurring tree species [European beech (Fagus sylvatica L.), sycamore maple (Acer pseudoplatanus L.) and Norway maple (Acer platanoides L.)] in the understorey of a beech-dominated, N-limited forest on calcareous soil over two consecutive seasons. For this purpose, we studied (15)N uptake capacity as well as the allocation to N pools in the fine roots. Our results show that European beech had a higher capacity for both inorganic and organic N acquisition throughout the whole growing season compared with sycamore maple and Norway maple. The higher capacity of N acquisition in beech indicates a better adaption of beech to the understorey conditions of beech forests compared with the seedlings of other tree competitors under N-limited conditions. Despite these differences, all three species preferred organic over inorganic N sources throughout the growing season and showed similar seasonal patterns of N acquisition with an increased N uptake capacity in summer. However, this pattern varied with N source and year indicating that other environmental factors not assessed in this study further influenced N acquisition by the seedlings of the three tree species.

Long-term nitrogen deposition linked to reduced water use efficiency in forests with low phosphorus availability.

The impact of long-term nitrogen (N) deposition is under-studied in phosphorus (P)-limited subtropical forests. We exploited historically collected herbarium specimens to investigate potential physiological responses of trees in three subtropical forests representing an urban-to-rural gradient, across which N deposition has probably varied over the past six decades. We measured foliar [N] and [P] and stable carbon (δ(13) C), oxygen (δ(18) O) and nitrogen (δ(15) N) isotopic compositions in tissue from herbarium specimens of plant species collected from 1947 to 2014. Foliar [N] and N:P increased, and δ(15) N and [P] decreased in the two forests close to urban centers. Consistent with recent studies demonstrating that N deposition in the region is (15) N-depleted, these data suggest that the increased foliar [N] and N:P, and decreased [P], may be attributable to atmospheric deposition and associated enhancement of P limitation. Estimates of intrinsic water use efficiency calculated from foliar δ(13) C decreased by c. 30% from the 1950s to 2014, contrasting with multiple studies investigating similar parameters in N-limited forests. This effect may reflect decreased photosynthesis, as suggested by a conceptual model of foliar δ(13) C and δ(18) O. Long-term N deposition may exacerbate P limitation and mitigate projected increases in carbon stocks driven by elevated CO2 in forests on P-limited soils.

模拟N沉降对太岳山油松人工林和天然林草本群落的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 模拟N沉降对太岳山油松人工林和天然林草本群落的影响 DOI: 10.5846/stxb201307141892 作者: 作者单位: 北京林业大学森林资源与生态系统过程北京市重点实验室,北京林业大学森林资源与生态系统过程北京市重点实验室,北京林业大学森林资源与生态系统过程北京市重点实验室,北京林业大学森林资源与生态系统过程北京市重点实验室,北京林业大学森林资源与生态系统过程北京市重点实验室,北京林业大学森林资源与生态系统过程北京市重点实验室 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目(31340022);国家林业公益性行业科研专项(201104009) Effect of simulation N deposition on herbaceous vegetation community in the plantation and natural forests of Pinus tabulaeformis in the Taiyue Mountain Author: Affiliation: Beijing Forestry University Key Laboratory for Forest Resources Ecosystem Processes of Beijing Beijing 100083,Beijing Forestry University Key Laboratory for Forest Resources Ecosystem Processes of Beijing Beijing 100083,Beijing Forestry University Key Laboratory for Forest Resources Ecosystem Processes of Beijing Beijing 100083,Beijing Forestry University Key Laboratory for Forest Resources Ecosystem Processes of Beijing Beijing 100083,Beijing Forestry University Key Laboratory for Forest Resources Ecosystem Processes of Beijing Beijing 100083,Beijing Forestry University Key Laboratory for Forest Resources Ecosystem Processes of Beijing Beijing 100083 Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:由于人类活动氮沉降呈逐年增加的趋势,进而增加了陆地生态系统氮的输入,从而影响陆地生态系统多样性、物种组成和功能。为揭示氮沉降增加对油松林草本群落的影响,于2009年7月在太岳山油松人工林和天然林,设计4个施氮水平:对照(CK,0 kg N hm-2a-1),低氮(LN,50 kg N hm-2 a-1),中氮(MN,100 kg N hm-2 a-1)和高氮(HN,150 kg N hm-2 a-1),研究草本群落的生物多样性、生物量以及草本元素含量对模拟N沉降的响应。研究结果表明:模拟N沉降未能显著影响人工林草本群落的生物多样性(P > 0.05),而中氮、高氮显著降低了天然林草本群落的生物多样性(P < 0.05);从Jaccard指数和Sorensen指数分析得出人工林不同氮水平之间草本群落差异性较小,而天然林不同氮水平之间草本群落差异性较大。模拟N沉降没有显著改变人工林草本群落生物量 (P > 0.05),而高氮明显促进天然林草本群落生物量的增加(P < 0.05)。与对照相比,模拟N沉降提高了人工林和天然林羊胡子苔草叶根中的全N含量(P < 0.05),而降低了全Mg的含量(P < 0.05),并且根部元素含量变化与土壤养分含量变化较为一致。施氮提高了N/K、N/Ca、N/Mg(P < 0.05)的比值。说明油松林下草本群落对氮沉降的响应因林分土壤N饱和程度以及林地利用历史的不同而产生差异,其中天然林响应最为敏感。 Abstract:Human activities have significantly altered global and regiona1 cycles of nitrogen (N) in the last several decades due to combustion of fossil fuels, intensive agriculture and stock breeding. The amount of human-made reactive N which is more than the natural production, released into the atmosphere. Nitrogen (N) deposition has been widely considered as an environmental problem that leads to destruction of ecological balance such as biodiversity loss, base cation loss, soil acidification, but increases net primary productivity of N-limited forest ecosystems. Greater N deposition has a significantly effect on productivity, plant diversity and biomass allocation of forest ecosystem. To determine the influence of N deposition on species diversity, biomass and nutrients of herbaceous vegetation community, for this:12, 20 m×20 m plots, respectively, distributed in the plantation and natural forest of Pinus tabulaeformis were established in the Taiyue Mountain, Shanxi Province, since July, 2009. Within each stand, equal numbers of 20 m×20 m plots were treated with four nitrogen fertilization levels: control (CK, 0 kg N hm-2a-1), low nitrogen (LN, 50 kg N hm-2 a-1), moderate nitrogen (MN, 100 kg N hm-2 a-1) and high nitrogen (HN, 150 kg N hm-2 a-1), the different levels of N were added at the middle of each month in growing season Since 2009. Herb samples were collected and investigated in August in 2012, and the soil samples were collected in August of 2009 and 2012. The coverage, diversity indices, above-ground biomass, underground biomass, total biomass and element contents of herbaceous vegetation community and soil physical and chemical characteristics of different nitrogen levels were analyzed. Results showed that: Nitrogen additions did not significantly (P > 0.05) affect diversity indices of herbaceous vegetation community in the plantation forest, however, MN and HN treatments significantly decreased the diversity indices (P < 0.05) of herbaceous vegetation community in the natural forest. Jaccard and Sorensen index indicated that there were significant differences in community composition of herbaceous vegetation between different nitrogen levels in the natural forest, while that of plantation forest was not significant. The herb biomass showed no significant (P > 0.05) response to nitrogen deposition in the plantation forest, while HN treatment significantly increased herb biomass (P < 0.05) in the natural forest. The variations of nutrient in root of Carex callitrichos were closely related to soil physical and chemical characteristics. Nitrogen addition increased total N contents (P < 0.05), but decreased total Mg contents (P < 0.05) in leaves and roots of C.callitrichos, and nitrogen deposition significantly increased the ratio of N/K, N/Ca, N/Mg (P < 0.05) when compared to the control in the plantation and natural forest of P. tabulaeformis. It was suggested that external nitrogen addition would affect the species diversity, productivity and elements uptake of herb community in the P. tabulaeformis forests, however, the impact of N enrichment on the herbaceous vegetation community may vary with different soil nitrogen saturation and land-use history, and natural forests may be more sensitive to N deposition. 参考文献 相似文献 引证文献

Effects of previous nitrogen fertilization on soil-solution chemistry after final felling and soil scarification at two nitrogen-limited forest sites

Nitrogen (N) fertilization and soil scarification are common measures used in commercial forestry in the boreal zone. This study was performed to investigate how previous N fertilization in two N-limited Scots pine (Pinus sylvestris L.) stands affected the soil-solution chemistry after final felling and also to determine the effect of subsequent soil scarification. Nitrogen had been applied to study plots at different intervals, resulting in total applications of 0, 450, 900, or 1800 kg N·ha−1. Soil-solution samples were collected before and after whole-tree harvesting of the P. sylvestris stand, from undisturbed soil and also after harvesting from soil below furrows, tilts, and areas between furrows created by disc trenching. After harvesting, the K+ concentration was lower at higher N fertilization intensities. No overall effect on the N concentrations was detected. Electrical conductivity and the concentrations of Na+, K+, Mg2+, Ca2+, Cl−, NO3−–N, total N, and total C were all affected by soil scarification. The highest concentrations of these variables were found below tilts and the lowest concentrations below furrows. The experiment was repeated, at a lower monitoring intensity, at a site where the previous total N application amounted to 0 and 450 kg N·ha−1. Here the NO3−–N concentration responded to disc trenching in a similar way to that observed in the main experiment. The study shows that previous N fertilization of N-limited forest does not necessarily affect the soil-solution chemistry significantly after whole-tree harvesting.

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.

Foliar and soil 15N natural abundances provide field evidence on nitrogen dynamics in temperate and boreal forest ecosystems

The natural abundance of 15N (δ15N) in plants and soils is an ideal tool for assessing ecosystem N dynamics. However, many of the mechanisms driving the variability of foliar and soil δ15N values within and across ecosystems are still unclear. In this study, we analyzed the patterns of N concentrations and δ15N values in leaves, bulk soils and soil mineral N as well as soil N turnover rates across four temperate and boreal forest ecosystems along a mountain transect. The results showed that plant species and soil properties directly controlled soil δ15N patterns and climate factors (air temperature and precipitation) indirectly affected foliar δ15N patterns. Foliar N concentrations varied consistently with the concentrations of soil available N and soil NO3−-N, whereas foliar δ15N was most closely associated with the δ15N of soil NH4+, the most abundant form of N in soil solution. 15N enrichment in surface mineral soil in high elevation forests was mainly attributed to 15N-enriched organic N accumulation. Furthermore, the foliar enrichment factor (ep/s = δ15Nfoliage−δ15Nsoil) was significantly correlated with N transformation and loss rates, and was negatively correlated with the ratio of NH4+ to total inorganic N. These results suggest that foliar δ15N value and foliar N concentration together accurately reflect the N availability of forest ecosystems. Foliar ep/s can act as an integrated proxy to reflect the status of N cycling within or across forest ecosystems. Soil nitrification and species’ NH4+ to NO3− uptake ratios are key processes controlling foliar δ15N patterns in N-limited forest ecosystems. Our findings improve the mechanistic understanding of the commonly observed variability in foliar and soil δ15N within and across forest ecosystems.

The natural abundance of 15N in plant and soil‐available N indicates a shift of main plant N resources to NO from NH along the N leaching gradient

To investigate which of ammonium (NH(4)(+)) or nitrate (NO(3)(-)) is used by plants at gradient sites with different nitrogen (N) availability, we measured the natural abundance of (15)N in foliage and soil extractable N. Hinoki cypress (Chamaecyparis obtusa Endlicher) planted broadly in Japan was selected for use in this study. We estimated the source proportion of foliar N (NH(4)(+) vs. NO(3)(-)) quantitatively using mass balance equations. The results showed that C. obtusa used mainly NH(4)(+) in N-limited forests, although the dependence of C. obtusa on NO(3)(-) was greater in other NO(3)(-)-rich forests. We regarded dissolved organic N (DON) as a potential N source because a previous study demonstrated that C. obtusa can take up glycine. Thus we added DON to our mass balance equations and calculated the source proportion using an isotope-mixing model (IsoSource model). The results still showed a positive correlation between the calculated plant N proportion of NO(3)(-) and the NO(3)(-) pool size in the soil, indicating that high NO(3)(-) availability increases the reliance of C. obtusa on NO(3)(-). Our data suggest the shift of the N source for C. obtusa from NH(4)(+) to NO(3)(-) according to the relative availability of NO(3)(-). They also show the potential of the foliar delta(15)N of C. obtusa as an indicator of the N status in forest ecosystems with the help of the delta(15)N values of soil inorganic and organic N.

Chronic nitrogen addition causes a reduction in soil carbon dioxide efflux during the high stem-growth period in a tropical montane forest but no response from a tropical lowland forest on a decadal time scale

Abstract. Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We studied the response of soil carbon dioxide (CO2) efflux to long-term experimental N addition (125 kg N ha−1 yr−1) in mature lowland and montane forests in Panama. In the lowland forest, on soils with high nutrient-supplying and buffering capacity, fine litterfall and stem-growth were neither N- nor phosphorus-limited. In the montane forest, on soils with low nutrient supplying capacity and an organic layer, fine litterfall and stem-growth were N-limited. Our objectives were to 1) explore the influence of soil temperature and moisture on the dynamics of soil CO2 efflux and 2) determine the responses of soil CO2 efflux from an N-rich and N-limited forest to elevated N input. Annual soil CO2-C efflux was larger in the lowland (15.44 ± 1.02 Mg C ha−1) than in the montane forest (9.37 ± 0.28 Mg C ha−1). In the lowland forest, soil moisture explained the largest fraction of the variance in soil CO2 efflux while soil temperature was the main explanatory variable in the montane forest. Soil CO2 efflux in the lowland forest did not differ between the control and 9–11 yr N-addition plots, suggesting that chronic N input to nutrient-rich tropical lowland forests on well-buffered soils may not change their C balance on a decadal time scale. In the montane forest, first year N addition did not affect soil CO2 efflux but annual CO2 efflux was reduced by 14% and 8% in the 2nd and 3rd year N-addition plots, respectively, compared to the control. This reduction was caused by a decrease in soil CO2 efflux during the high stem-growth period of the year, suggesting a shift in carbon partitioning from below- to aboveground in the N-addition plots in which stem diameter growth was promoted.

Coupling tree-ring 13C and 15N to test the effect of fertilization on mature Douglas-fir (Pseudotsuga menziesii var. glauca) stands across the Interior northwest, USA

Nitrogen (N) fertilization causes long-term increases in biomass production in many N-limited forests around the world, but the mechanistic basis underlying the increase is often unclear. One possibility, especially in summer-dry climates, is that N fertilization increases the efficiency with which a finite water supply is consumed to support photosynthesis. This increase is achieved by a reduction in the canopy-integrated concentration of internal CO(2) and thus discrimination against (13)C. We used stable isotopes of carbon (delta(13)C) in tree rings to experimentally test the physiological impact of N fertilization on mature Douglas-fir (Pseudotsuga menziesii Franco var. glauca) stands across the geographic extent of the Intermountain West, USA. The concentration and the stable isotopes of N (delta(15)N) in tree rings were also used to assess the presence and activity of fertilizer N. We hypothesized that N fertilization would (i) increase delta(15)N and N concentration of stemwood relative to non-fertilized stands and (ii) increase stemwood delta(13)C as photosynthetic gas exchange responded to the additional N. This experiment included two rates of urea addition, 178 kg ha(-1) (low) and 357 kg ha(-1) (high), which were applied twice over a 6-year interval bracketed by the 18 years of wood production measured in this study. Foliar N concentrations measured the year after each fertilization treatment suggest that the fertilizer N had been assimilated by the trees (P < 0.001). The N fertilization significantly enriched stemwood delta(15)N by 1.3 per thousand at the low fertilization rate and by 2.4 per thousand at the high rate (P < 0.001) despite variation in soil N between sites. However, we found no significant effect of the N fertilizer on delta(13)C of the annual rings (P = 0.76). These data lead us to suggest that alternative mechanisms underlie the growth response to fertilizer, i.e., increase in canopy area and shifts in biomass allocation.

Nitrogen balance in forest soils: nutritional limitation of plants under climate change stresses

Forest ecosystems with low soil nitrogen (N) availability are characterized by direct competition for this growth-limiting resource between several players, i.e. various components of vegetation, such as old-growth trees, natural regeneration and understorey species, mycorrhizal fungi, free-living fungi and bacteria. With the increase in frequency and intensity of extreme climate events predicted in current climate change scenarios, also competition for N between plants and/or soil microorganisms will be affected. In this review, we summarize the present understanding of ecosystem N cycling in N-limited forests and its interaction with extreme climate events, such as heat, drought and flooding. More specifically, the impacts of environmental stresses on microbial release and consumption of bioavailable N, N uptake and competition between plants, as well as plant and microbial uptake are presented. Furthermore, the consequences of drying-wetting cycles on N cycling are discussed. Additionally, we highlight the current methodological difficulties that limit present understanding of N cycling in forest ecosystems and the need for interdisciplinary studies.

Controls of nitrogen isotope patterns in soil profiles

To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (δ15N) data, patterns of 15N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum 15N enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 ± 0.4‰ in ectomycorrhizal systems and 4.6 ± 0.5‰ in arbuscular mycorrhizal systems. The 15N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling 15N in soil profiles, fractionation against 15N during N transfer by mycorrhizal fungi to host plants, leads to 15N-depleted plant litter at the soil surface and 15N-enriched nitrogen of fungal origin at depth. The preferential preservation of 15N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large 15N enrichments with depth in less N-limited forests and may account for soil profiles where maximum δ15N is at intermediate depths. Mixing among soil horizons should also decrease differences among soil horizons. We suggest that dynamic models of isotope distributions within soil profiles that can incorporate multiple processes could provide additional information about the history of nitrogen movements and transformations at a site.