Measuring Fine Root Turnover in Forest Ecosystems

Development of direct and indirect methods for measuring root turnover and the status of knowledge on fine root turnover in forest ecosystems are discussed. While soil and ingrowth cores give estimates of standing root biomass and relative growth, respectively, minirhizotrons provide estimates of median root longevity (turnover time) i.e., the time by which 50% of the roots are dead. Advanced minirhizotron and carbon tracer studies combined with demographic statistical methods and new models hold the promise of improving our fundamental understanding of the factors controlling root turnover. Using minirhizotron data, fine root turnover (y⁻¹) can be estimated in two ways: as the ratio of annual root length production to average live root length observed and as the inverse of median root longevity. Fine root production and mortality can be estimated by combining data from minirhizotrons and soil cores, provided that these data are based on roots of the same diameter class (e.g., < 1 mm in diameter) and changes in the same time steps. Fluxes of carbon and nutrients via fine root mortality can then be estimated by multiplying the amount of carbon and nutrients in fine root biomass by fine root turnover. It is suggested that the minirhizotron method is suitable for estimating median fine root longevity. In comparison to the minirhizotron method, the radio carbon technique favor larger fine roots that are less dynamics. We need to reconcile and improve both methods to develop a more complete understanding of root turnover.     

Fine root growth phenology,production, and turnover in a northern hardwood forest ecosystem

A large part of the nutrient flux in deciduous forests is through fine root turnover, yet this process is seldom measured. As part of a nutrient cycling study, fine root dynamics were studied for two years at Huntington Forest in the Adirondack Mountain region of New York, USA. Root growth phenology was characterized using field rhizotrons, three methods were used to estimate fine root production, two methods were used to estimate fine root mortality, and decomposition was estimated using the buried bag technique. During both 1986 and 1987, fine root elongation began in early April, peaked during July and August, and nearly ceased by mid-October. Mean fine root ( 3 mm diameter) biomass in the surface 28-cm was 2.5 t ha^–1 and necromass was 2.9 t ha^–1. Annual decomposition rates ranged from 17 to 30% beneath the litter and 27 to 52% at a depth of 10 cm. Depending on the method used for estimation, fine root production ranged from 2.0 to 2.9 t ha^–1, mortality ranged from 1.8 to 3.7 t ha^–1 yr^–1, and decomposition was 0.9 t ha^–1 yr^–1. Thus, turnover ranged from 0.8 to 1.2 yr^–1. The nutrients that cycled through fine roots annually were 4.5–6.1 kg Ca, 1.1–1.4 kg Mg, 0.3–0.4 kg K, 1.2–1.7 kg P, 20.3–27.3 kg N, and 1.8–2.4 kg S ha^–1. Fine root turnover was less important than leaf litterfall in the cycling of Ca and Mg and was similar to leaf litterfall in the amount of N, P, K and S cycled.     

Net mineralization of N at deeper soil depths as a potential mechanism for sustained forest production under elevated [CO2]

Elevated atmospheric carbon dioxide concentrations [CO₂] is projected to increase forest production, which could increase ecosystem carbon (C) storage. This study contributes to our broad goal of understanding the causes and consequences of increased fine‐root production and mortality under elevated [CO₂] by examining potential gross nitrogen (N) cycling rates throughout the soil profile. Our study was conducted in a CO₂‐enriched sweetgum (Liquidambar styraciflua L.) plantation in Oak Ridge, TN, USA. We used ¹⁵N isotope pool dilution methodology to measure potential gross N cycling rates in laboratory incubations of soil from four depth increments to 60 cm. Our objectives were twofold: (1) to determine whether N is available for root acquisition in deeper soil and (2) to determine whether elevated [CO₂], which has increased inputs of labile C resulting from greater fine‐root mortality at depth, has altered N cycling rates. Although gross N fluxes declined with soil depth, we found that N is potentially available for roots to access, especially below 15 cm depth where rates of microbial consumption of mineral N were reduced relative to production. Overall, up to 60% of potential gross N mineralization and 100% of potential net N mineralization occurred below 15 cm depth at this site. This finding was supported by in situ measurements from ion‐exchange resins, where total inorganic N availability at 55 cm depth was equal to or greater than N availability at 15 cm depth. While it is likely that trees grown under elevated [CO₂] are accessing a larger pool of inorganic N by mining deeper soil, we found no effect of elevated [CO₂] on potential gross or net N cycling rates. Thus, increased root exploration of the soil volume under elevated [CO₂] may be more important than changes in potential gross N cycling rates in sustaining forest responses to rising atmospheric CO₂.     

Effects of elevated atmospheric carbon dioxide on amino acid and NH4+-N cycling in a temperate pine ecosystem

Rising atmospheric carbon dioxide (CO₂) is expected to increase forest productivity, resulting in greater carbon (C) storage in forest ecosystems. Because elevated atmospheric CO₂ does not increase nitrogen (N) use efficiency in many forest tree species, additional N inputs will be required to sustain increased net primary productivity (NPP) under elevated atmospheric CO₂. We investigated the importance of free amino acids (AAs) as a source for forest N uptake at the Duke Forest Free Air CO₂ Enrichment (FACE) site, comparing its importance with that of better‐studied inorganic N sources. Potential proteolytic enzyme activity was monitored seasonally, and individual AA concentrations were measured in organic horizon extracts. Potential free AA production in soils ranged from 190 to 690 nmol N g⁻¹ h⁻¹ and was greater than potential rates of soil NH₄⁺ production. Because of this high potential rate of organic N production, we determined (1) whether intact AA uptake occurs by Pinus taeda L., the dominant tree species at the FACE site, (2) if the rate of cycling of AAs is comparable with that of ammonium (NH₄⁺), and (3) if atmospheric CO₂ concentration alters the aforementioned N cycling processes. A field experiment using universally labeled ammonium (¹⁵NH₄⁺) and alanine (¹³C₃H₇¹⁵NO₂) demonstrated that ¹⁵N is more readily taken up by plants and heterotrophic microorganisms as NH₄⁺. Pine roots and microbes take up on average 2.4 and two times as much NH₄^{+ 15}N compared with alanine ¹⁵N 1 week after tracer application. N cycling through soil pools was similar for alanine and NH₄⁺, with the greatest ¹⁵N tracer recovery in soil organic matter, followed by microbial biomass, dissolved organic N, extractable NH₄⁺, and fine roots. Stoichiometric analyses of ¹³C and ¹⁵N uptake demonstrated that both plants and soil microorganisms take up alanine directly, with a ¹³C : ¹⁵N ratio of 3.3 : 1 in fine roots and 1.5 : 1 in microbial biomass. Our results suggest that intact AA (alanine) uptake contributes substantially to plant N uptake in loblolly pine forests. However, we found no evidence supporting increased recovery of free AAs in fine roots under elevated CO₂, suggesting plants will need to acquire additional N via other mechanisms, such as increased root exploration or increased N use efficiency.     

Evaluating minirhizotron estimates of fine root longevity and production in the forest floor of a temperate broadleaf forest

The minirhizotron technique (MR) for in situ measurement of fine root dynamics offers the opportunity to obtain accurate and unbiased estimates of root production in perennial vegetation only if MR tubes do not affect the longevity of fine roots. Assuming fine root biomass is near steady-state, fine root production (g m^–2 yr^–1) can be estimated as the ratio of fine root biomass (g m^–2) to median fine root longevity (yr). This study evaluates the critical question of whether MR access tubes affect the longevity of fine roots, by comparing fine root survivorship obtained using MR with those from a non-intrusive in situ screen method in the forest floor horizons of a northern hardwood forest in New Hampshire, USA. Fine root survivorship was measured in 380 root screens during 1993–1997 and in six horizontal minirhizotron tubes during 1996–1997. No statistically significant difference was found between estimates of survivorship of fine roots (<1 mm dia.) at this site from MR versus from in situ screens, suggesting that MR tubes do not substantially affect fine root longevity in the forest floor of this northern hardwood forest and providing greater confidence in measurements of fine root production using the MR technique. Furthermore, the methodology for estimating fine root production from MR longevity data was evaluated by comparison of fine root longevity and production estimates made using single vs. multiple root cohorts, and using root-number, root-length, and root-mass weighted methods. Our results indicate that fine root-length longevity estimates based on multiple root cohorts throughout the year can be used to approximate fine root biomass production. Using this method, we estimated fine root longevity and production in the forest floor at this site to be 314 days (or 0.86 yr) and 303 g m^–2 yr^–1, respectively. Fine root production in this northern hardwood forest is approximately equivalent to standing biomass and was previously underestimated by root in-growth cores. We conclude that the use of MR to estimate fine root longevity and production as outlined here may result in improved estimates of fine root production in perennial vegetation.     

Fine root dynamics in a loblolly pine forest are influenced by free-air-CO2-enrichment: a six-year-minirhizotron study

Efforts to characterize carbon (C) cycling among atmosphere, forest canopy, and soil C pools are hindered by poorly quantified fine root dynamics. We characterized the influence of free‐air‐CO₂‐enrichment (ambient +200 ppm) on fine roots for a period of 6 years (Autumn 1998 through Autumn 2004) in an 18‐year‐old loblolly pine (Pinus taeda) plantation near Durham, NC, USA using minirhizotrons. Root production and mortality were synchronous processes that peaked most years during spring and early summer. Seasonality of fine root production and mortality was not influenced by atmospheric CO₂ availability. Averaged over all 6 years of the study, CO₂ enrichment increased average fine root standing crop (+23%), annual root length production (+25%), and annual root length mortality (+36%). Larger increase in mortality compared with production with CO₂ enrichment is explained by shorter average fine root lifespans in elevated plots (500 days) compared with controls (574 days). The effects of CO₂‐enrichment on fine root proliferation tended to shift from shallow (0–15 cm) to deeper soil depths (15–30) with increasing duration of the study. Diameters of fine roots were initially increased by CO₂‐enrichment but this effect diminished over time. Averaged over 6 years, annual fine root NPP was estimated to be 163 g dw m^?2 yr^?1 in CO₂‐enriched plots and 130 g dw m^?2 yr^?1 in control plots (P= 0.13) corresponding to an average annual additional input of fine root biomass to soil of 33 g m^?2 yr^?1 in CO₂‐enriched plots. A lack of consistent CO₂× year effects suggest that the positive effects of CO₂ enrichment on fine root growth persisted 6 years following minirhizotron tube installation (8 years following initiation of the CO₂ fumigation). Although CO₂‐enrichment contributed to extra flow of C into soil in this experiment, the magnitude of the effect was small suggesting only modest potential for fine root processes to directly contribute to soil C storage in south‐eastern pine forests.     

Fine root demography in alfalfa (Medicago sativa L.)

In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality which, in turn, could affect deposition of fixed N in soil. Our objective was to determine the extent and patterns of fine-diameter root production and loss during the year of alfalfa stand establishment. The experiment was conducted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA, using horizontally installed minirhizotrons placed directly under the seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N₂ fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercially-available cultivar; Ineffective Agate, which is a non-N₂-fixing near isoline of Agate; a new germplasm that has few fibrous roots and strong tap-rooted traits; and a new germplasm that has many fibrous roots and a strongly branched root system architecture. Video images collected biweekly throughout the initial growing season were processed using C-MAP-ROOTS software.More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root production was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than other germplasms. In all germplasms, about 7% of the fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). Survival of contemporaneous root cohorts was not related to soil depth in a simple fashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. Median life spans of these early-season cohorts ranged from 58 to 131 days, based on fitted exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter median life spans than early-season cohorts. Similar patterns of fine root mortality did not occur at the second harvest. Germplasms differed in the pattern, but not the ultimate extent, of fine root mortality. Fine root turnover during the first year of alfalfa establishment in this experiment released an estimated 830 kg C ha^–1 and 60 kg N ha^–1, with no differences due to N₂ fixation capacity or root system architecture.     

Multiyear fate of a 15N tracer in a mixed deciduous forest: retention,redistribution, and differences by mycorrhizal association

The impact of atmospheric nitrogen deposition on forest ecosystems depends in large part on its fate. Past tracer studies show that litter and soils dominate the short‐term fate of added ¹⁵N, yet few have examined its longer term dynamics or differences among forest types. This study examined the fate of a ¹⁵N‐ tracer over 5–6 years in a mixed deciduous stand that was evenly composed of trees with ectomycorrhizal and arbuscular mycorrhizal associations. The tracer was expected to slowly mineralize from its main initial fate in litter and surface soil, with some ¹⁵N moving to trees, some to deeper soil, and some net losses. Recovery of added ¹⁵N in trees and litterfall totaled 11.3% both 1 and 5–6 years after the tracer addition, as ¹⁵N redistributed from fine and especially coarse roots into cumulative litterfall and small accumulations in woody tissues. Estimates of potential carbon sequestration from tree ¹⁵N recovery amounted to 12–14 kg C per kg of N deposition. Tree ¹⁵N acquisition occurred within the first year after the tracer addition, with no subsequent additional net transfer of ¹⁵N from detrital to plant pools. In both years, ectomycorrhizal trees gained 50% more of the tracer than did trees with arbuscular mycorrhizae. Much of the ¹⁵N recovered in wood occurred in tree rings formed prior to the ¹⁵N addition, demonstrating the mobility of N in wood. Tracer recovery rapidly decreased over time in surface litter material and accumulated in both shallow and deep soil, perhaps through mixing by earthworms. Overall, results showed redistribution of tracer ¹⁵N through trees and surface soils without any losses, as whole‐ecosystem recovery remained constant between 1 and 5–6 years at 70% of the ¹⁵N addition. These results demonstrate the persistent ecosystem retention of N deposition even as it redistributes, without additional plant uptake over this timescale.     

The change of soil carbon stocks and fine root dynamics after land use change from a native pasture to a pine plantation

A published meta-analysis of worldwide data showed soil carbon decreasing following land use change from pasture to conifer plantation. A paired site (a native pasture with Themeda triandra dominant, and an adjacent Pinus radiata plantation planted onto the pasture 16 years ago) was set up as a case study to assess the soil carbon reduction and the possible reason for the reduction under pine, including the change in fine root (diameter <2 mm) dynamics (production and mortality). Soil analysis confirmed that soil carbon and nitrogen stocks to 100 cm under the plantation were significantly less than under the pasture by 20 and 15%, respectively. A 36% greater mass of fine root was found in the soil under the pasture than under the plantation and the length of fine root was about nine times greater in the pasture. Much less fine root length was produced and roots died more slowly under the plantation than under the pasture based on observations of fine root dynamics in minirhizotrons. The annual inputs of fine root litter to the top 100 cm soil, estimated from soil coring and minirhizotron observations, were 6.3 Mg dry matter ha⁻¹ year⁻¹ (containing 2.7 Mg C and 38.9 kg N) under the plantation, and 9.7 Mg ha⁻¹ year⁻¹ (containing 3.6 Mg C and 81.4 kg N) under the pasture. The reduced amount of carbon, following afforestation of the pasture, in each depth-layer of the soil profile correlated with the lower length of dead fine roots in the layer under the plantation compared with the pasture. This correlation was consistent with the hypothesis that the soil carbon reduction after land use change from pasture to conifer plantation might be related to change of fine root dynamics, at least in part.     

Estimation of fine root production, mortality and turnover with Minirhizotron in Larix gmelinii and Fraxinus mandshurica plantations

Fine root turnover is a major pathway for carbon and nutrient cycling in forest ecosystems. However, to estimate fine root turnover, it is important to first understand the fine root dynamic processes associated with soil resource availability and climate factors. The objectives of this study were: (1) to examine patterns of fine root production and mortality in different seasons and soil depths in the Larix gmelinii and Fraxinus mandshurica plantations, (2) to analyze the correlation of fine root production and mortality with environmental factors such as air temperature, precipitation, soil temperature and available nitrogen, and (3) to estimate fine root turnover. We installed 36 Minirhizotron tubes in six mono-specific plots of each species in September 2003 in the Mao’ershan Experimental Forest Station. Minirhizotron sampling was conducted every two weeks from April 2004 to April 2005. We calculated the average fine root length, annual fine root length production and mortality using image data of Minirhizotrons, and estimated fine root turnover using three approaches. Results show that the average growth rate and mortality rate in L. melinii were markedly smaller than in F. mandshurica, and were highest in the surface soil and lowest at the bottom among all the four soil layers. The annual fine root production and mortality in F. mandshurica were significantly higher than in L. gmelinii. The fine root production in spring and summer accounted for 41.7% and 39.7% of the total annual production in F. mandshurica and 24.0% and 51.2% in L. gmelinii. The majority of fine root mortality occurred in spring and summer for F. mandshurica and in summer and autumn for L. gmelinii. The turnover rate was 3.1 a⁻¹ for L. gmelinii and 2.7 a⁻¹ for F. mandshurica. Multiple regression analysis indicates that climate and soil resource factors together could explain 80% of the variations of the fine root seasonal growth and 95% of the seasonal mortality. In conclusion, fine root production and mortality in L. gmelinii and F. mandshurica have different patterns in different seasons and at different soil depths. Air temperature, precipitation, soil temperature and soil available nitrogen integratively control the dynamics of fine root production, mortality and turnover in both species. Transtlated from Journal of Plant Ecology, 2007, 31(2): 333–342 [译自: 植物生态学报]     

Intact amino acid uptake by northern hardwood and conifer trees

Empirical and modeling studies of the N cycle in temperate forests of eastern North America have focused on the mechanisms regulating the production of inorganic N, and assumed that only inorganic forms of N are available for plant growth. Recent isotope studies in field conditions suggest that amino acid capture is a widespread ecological phenomenon, although northern temperate forests have yet to be studied. We quantified fine root biomass and applied tracer-level quantities of U–¹³C₂–¹⁵N-glycine, ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ in two stands, one dominated by sugar maple and white ash, the other dominated by red oak, beech, and hemlock, to assess the importance of amino acids to the N nutrition of northeastern US forests. Significant enrichment of ¹³C in fine roots 2 and 5 h following tracer application indicated intact glycine uptake in both stands. Glycine accounted for up to 77% of total N uptake in the oak–beech–hemlock stand, a stand that produces recalcitrant litter, cycles N slowly and has a thick, amino acid-rich organic horizon. By contrast, glycine accounted for only 20% of total N uptake in the sugar maple and white ash stand, a stand characterized by labile litter and rapid rates of amino acid production and turnover resulting in high rates of mineralization and nitrification. This study shows that amino acid uptake is an important process occurring in two widespread, northeastern US temperate forest types with widely differing rates of N cycling.     

Assessing nitrogen fluxes from roots to soil associated to rhizodeposition by apple (<Emphasis Type="Italic">Malus domestica</Emphasis>) trees

The mass transfer from root to soil by means of rhizodeposition has been studied in grasses and forest trees, but its role in fruit trees is still unknown. In this study, N fluxes from roots to soil were estimated by applying a ¹⁵N mass balance technique to the soil–tree system. Apple (Malus domestica) trees were pre-labelled with ¹⁵N and then grown outdoors in 40 L pots for one vegetative season in (1) a coarse-textured, low organic matter soil, (2) a coarse-textured, high organic matter soil, and (3) a fine-textured, high organic matter soil. At tree harvest the ¹⁵N abundance of the soils was higher than at transplanting, but the total amount of ¹⁵N present in the tree–soil system was similar at transplanting and tree harvest. The soils had a strong effect on N fluxes from and to the soil. In the fine-textured soil, 11% of the total plant-derived nitrogen was transferred to the soil, compared with 2–5% in the two coarse-textured soils. Rhizodeposition was higher in the fine soil (18% of the primary production) than in the coarse-textured soils, whereas higher soil organic matter depressed rhizodeposition. Nitrogen uptake was almost double in the coarse-textured, high organic matter soil versus the other soils. Our results indicate that belowground primary productivity is significantly underestimated if based on root production data only. Rhizodeposition represents a major process, whose role should not be underestimated in carbon and nitrogen cycles in orchard ecosystems.     

Fine root vertical distribution and temporal dynamics in mature stands of two enset (Enset ventricosum Welw Cheesman) clones

Quantification of the role of fine roots in the biological cycle of nutrients necessitates understanding root distribution, estimating root biomass, turnover rate and nutrient concentrations, and the dynamics of these parameters in perennial systems. Temporal dynamics, vertical distribution, annual production and turnover, and nitrogen use of fine roots (≤2 mm in diameter) were studied in mature (5-year-old) stands of two enset (Ensete ventricosum) clones using the in-growth bag technique. Live fine root mass generally decreased with increasing depth across all seasons except the dry period. Except for the dry period, more than 70% of the fine root mass was in the above 0-20 cm depth, and the fine root mass in the upper 0–10 cm depth was significantly higher than in the lowest depth (20–30 cm). Live fine root mass showed a seasonal peak at the end of the major rainy season but fell to its lowest value during the dry or short rainy season. The difference between the peak and low periods were significant (p ≤ 0.05). Fine root nitrogen (N) use showed significant seasonal variation where the mean monthly fine root N use was highest during the major rainy season. There were significant effects on N use due to depths and in-growth periods, but not due to clones. Enset fine root production and turnover ranged from 2,339 to 2,451 kg ha⁻¹ year⁻¹ and from 1.55 to 1.80 year⁻¹, respectively. Root N return, calculated from fine root turnover, was estimated at 64–65 kg ha⁻¹ year⁻¹. Fine root production, vertical distribution and temporal dynamics may be related to moisture variations and nutrient (N) fluxes among seasons and along the soil depth. The study showed that fine root production and turnover can contribute considerably to the carbon and nitrogen economy of mature enset plots.     

Links between Belowground and Aboveground Resource-Related Traits Reveal Species Growth Strategies that Promote Invasive Advantages

Belowground processes are rarely considered in comparison studies of native verses invasive species. We examined relationships between belowground fine root production and lifespan, leaf phenology, and seasonal nitrogen dynamics of Lonicera japonica (non-native) versus L. sempervirens (native) and Frangula alnus (non-native) versus Rhamnus alnifolia (native), over time. First and second order fine roots were monitored from 2010 to 2012 using minirhizotron technology and rhizotron windows. ¹⁵N uptake of fine roots was measured across spring and fall seasons. Significant differences in fine root production across seasons were seen between Lonicera species, but not between Frangula and Rhamnus, with both groups having notable asynchrony in regards to the timing of leaf production. Root order and the number of root neighbors at the time of root death were the strongest predictors of root lifespan of both species pairs. Seasonal ¹⁵N uptake was higher in spring than in the fall, which did not support the need for higher root activity to correspond with extended leaf phenology. We found higher spring ¹⁵N uptake in non-native L. japonica compared to native L. sempervirens, although there was no difference in ¹⁵N uptake between Frangula and Rhamnus species. Our findings indicate the potential for fast-growing non-native Lonicera japonica and Frangula alnus to outcompete native counterparts through differences in biomass allocation, root turnover, and nitrogen uptake, however evidence that this is a general strategy of invader dominance is limited.     

Insights into root growth, function, and mycorrhizal abundance from chemical and isotopic data across root orders

Background and aims

Detailed analyses of root chemistry by branching order may provide insights into root function, root lifespan and the abundance of root-associated mycorrhizal fungi in forest ecosystems.

Methods

We examined the nitrogen and carbon stable isotopes (δ¹⁵N and δ¹³C) and concentration (%N and %C) in the fine roots of an arbuscular mycorrhizal tree, Fraxinus mandshurica, and an ectomycorrhizal tree, Larix gmelinii, over depth, time, and across five root branching orders.

Results and conclusions

Larix δ¹⁵N increased by 2.3?‰ from 4th order to 1st order roots, reflecting the increased presence of ¹⁵N-enriched ECM fungi on the lower root orders. In contrast, arbuscular mycorrhizal Fraxinus only increased by 0.7?‰ from 4th order to 1st order roots, reflecting the smaller ¹⁵N enrichment and lower fungal mass on arbuscular mycorrhizal fine roots. Isotopic and anatomical mass balance calculations indicate that first, second, and third order roots in ectomycorrhizal Larix averaged 36 %, 23 %, and 8 % fungal tissue by mass, respectively. Using literature values of root production by root branching order, we estimate that about 25 % of fine root production in ECM species like Larix is actually of fungal sheaths. In contrast to %N, %C, and δ¹⁵N, δ¹³C changed minimally across depth, time, and branching order. The homogeneity of δ¹³C suggests root tissues are constructed from a large well-mixed reservoir of carbon, although compound specific δ¹³C data is needed to fully interpret these patterns. The measurements developed here are an important step towards explicitly including mycorrhizal production in forest ecosystem carbon budgets.     

AM真菌对紫花苜蓿细根生长及其生物量动态特征的影响

细根对植物功能的发挥和土壤碳库及全球碳循环具有重要意义。采用容器法和微根管法于2013年6~10月整个生长季内对紫花苜蓿的细根生物量、生产以及周转规律进行研究。结果表明:(1)紫花苜蓿活细根现存生物量平均值以接种摩西球囊霉(Gm)处理最高(12.46g·m-2),未接种对照最低(7.31g·m-2),并且活细根现存量在9月中旬达到峰值;死细根现存生物量呈先增加后降低再增加的变化趋势,在整个生长过程中未接种处理高于接种处理,接种根内球囊霉(Gi)处理死细根现存平均生物量(3.11g·m-2)又较接种组其他处理低。(2)苜蓿植株细根生长量以接种幼套球囊霉(Ge)处理最大(0.045 mm·cm-2·d-1),接种Gm处理和未接种对照最低(均为0.027mm·cm-2·d-1);而未接菌植株细根死亡量(0.044mm·cm-2·d-1)显著高于接种植株,接种组又以Gi处理最低(0.021mm·cm-2·d-1)。(3)紫花苜蓿在生长季节内细根生产和死亡的高峰分别出现在8月底和10月份,低谷出现在9月底到10月中旬和6月底到8月;接种地表球囊霉(Gv)后细根现存量和年生长量显著高于对照和接种其他菌种处理,细根的周转以对照组最大,而接种Gv和Gm处理较低。研究发现,通过接种丛植菌根真菌可以提高苜蓿细根生物量,降低细根的死亡,增加细根寿命。     

Direct flux and 15N tracer methods for measuring denitrification in forest soils

Estimates of denitrification are one of the key uncertainties in the terrestrial nitrogen (N) cycle, primarily because reliable measurements of this highly variable process—especially the production of its terminal product (N₂)—are difficult to obtain. We evaluated the ability of gas-flow soil core and ¹⁵N tracer methods to provide reliable estimates of denitrification in forest soils. Our objectives were to: (1) describe and present typical results from new gas-flow soil core and in situ ¹⁵N tracer methods for measuring denitrification, (2) discuss factors that affect the relevance of these methods to actual in situ denitrification, and (3) compare denitrification estimates produced by the two methods for a series of sites in a northern hardwood forest ecosystem. Both methods were able to measure accumulations of N₂ over relatively short (2–5 h) incubations of either unamended or tracer-amended intact soils. Denitrification rates measured by the direct flux soil core method were very sensitive to incubation oxygen (O₂) concentration and decreased with increased O₂ levels. Denitrification rates measured by the in situ ¹⁵N tracer method were very sensitive to the ¹⁵N content of the nitrate (NO₃ ^?) pool undergoing denitrification, which limits the applicability of this method for quantifying denitrification in N-poor ecosystems. While its ability to provide accurate estimates of denitrification was limited, the ¹⁵N tracer method provided estimates of the short-term abiotic and biotic transformations of atmospheric N deposition to gas. Furthermore, results suggest that denitrification is higher and that N₂O:N₂ ratios are lower (<0.02) than previously thought in the northern hardwood forest and that short-term abiotic and biotic transformations of atmospheric N deposition to gas are significant in this ecosystem.     

Fine root decay rates vary widely among lowland tropical tree species

Prolific fine root growth coupled with small accumulations of dead fine roots indicate rapid rates of fine root production, mortality and decay in young tree plantations in lowland Costa Rica. However, published studies indicate that fine roots decay relatively slowly in tropical forests. To resolve this discrepancy, we used the intact-core technique to quantify first-year decay rates of fine roots in four single-species plantations of native tree species. We tested three hypotheses: first, that fine roots from different tree species would decay at different rates; second, that species having rapid fine root growth rates would also have rapid rates of fine root decay; and third, that differences in fine root decay among species could be explained by fine root chemistry variables previously identified as influencing decay rates. Fine roots in Virola koschnyi plantations decayed very slowly (k = 0.29 ± 0.15 year⁻¹); those of Vochysia guatemalensis decayed seven times faster (k = 2.00 ± 0.13 year⁻¹). Decay rates of the remaining two species, Hieronyma alchorneoides and Pentaclethra macroloba, were 1.36 and 1.28 year⁻¹, respectively. We found a positive, marginally significant correlation between fine root decay rates and the relative growth rates of live fine roots (R = 0.93, n = 4, P = 0.072). There was a highly significant negative correlation between fine root decay and fine root lignin:N (R = 0.99, P = 0.01), which supports the use of lignin:N as a decay-controlling factor within terrestrial ecosystem models. The decay rates that we observed in this single study location encompassed the entire range of fine root decay rates previously observed in moist tropical forests, and thus suggest great potential for individual tree species to alter belowground organic matter and nutrient dynamics within a biotically rich rainforest environment. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Denitrification measurements in aquatic sediments: A comparison of three methods

Measurements of denitrification using the acetylene inhibition,¹⁵N isotope tracer, and N₂ flux methods were carried out concurrently using sediment cores from Vilhelmsborg sø, Denmark, in an attempt to clarify some of the limitations of each technique. Three experimental treatments of overlying water were used: control, nitrate enriched, and ammonia enriched water. The N₂ flux and¹⁵N tracer experiments showed high rates of coupled nitrification/denitrification in the sediments. The acetylene inhibition method did not capture any coupled nitrification/denitrification. This could be explained by acetylene inhibition of nitrification. A combined¹⁵N tracer/acetylene inhibition experiment demonstrated that acetylene inhibition of N₂O reduction was incomplete and the method, therefore, only measured approximately 50% of the denitrification due to nitrate from the overlying water. Similar rates of denitrification due to nitrate in the overlying water were measured by the N₂ flux method and the acetylene inhibition method, after correcting for the 50% efficiency of acetylene inhibition. Rates of denitrification due to nitrate from the overlying water measured by the¹⁵N tracer method, however, were only approximately 35% or less of those measured by the acetylene inhibition or N₂ flux methods.     

Nitrogen deposition and drought events have non-additive effects on plant growth – Evidence from greenhouse experiments

Climate change and nitrogen deposition affect biodiversity and ecosystem functioning, but interactive effects of these global change drivers are poorly understood. We analysed single and interactive effects of nitrogen (N) fertilisation and drought on the growth performance of Calluna vulgaris. We measured biomass production and allocation, tissue nutrient (N, phosphorus (P) and carbon (C)) concentrations, N allocation patterns (using ¹⁵N tracer) and plant's water status (using δ ¹³C signatures) as response variables in a 2-year greenhouse experiment. N fertilisation increased biomass production and biomass shoot:root ratios. ¹⁵N allocation patterns indicated an increasing aboveground N allocation following N fertilisation. Tissue δ ¹³C signatures were higher in N-fertilised plants. Plant responses to drought were weak. We found strong antagonistic interaction effects of N fertilisation and drought for biomass production. δ ¹³C values peaked when N-fertilised plants were subjected to drought, indicating that N fertilisation increased the evaporative demands of Calluna plants, likely due to increased biomass shoot:root ratios, which in turn resulted in higher drought susceptibility. As an important consequence, even slight drought events may weaken the competitiveness of Calluna when interacting with enhanced airborne N loads. Single-factor studies, thus, need to be complemented by multi-factor analyses to assess conceivable impacts of co-occurring global change drivers.