Litter type control on soil C and N stabilization dynamics in a temperate forest

While plant litters are the main source of soil organic matter (SOM) in forests, the controllers and pathways to stable SOM formation remain unclear. Here, we address how litter type (¹³C/¹⁵N‐labeled needles vs. fine roots) and placement‐depth (O vs. A horizon) affect in situ C and N dynamics in a temperate forest soil after 5 years. Litter type rather than placement‐depth controlled soil C and N retention after 5 years in situ, with belowground fine root inputs greatly enhancing soil C (x1.4) and N (x1.2) retention compared with aboveground needles. While the proportions of added needle and fine root‐derived C and N recovered into stable SOM fractions were similar, they followed different transformation pathways into stable SOM fractions: fine root transfer was slower than for needles, but proportionally more of the remaining needle‐derived C and N was transferred into stable SOM fractions. The stoichiometry of litter‐derived C vs. N within individual SOM fractions revealed the presence at least two pools of different turnover times (per SOM fraction) and emphasized the role of N‐rich compounds for long‐term persistence. Finally, a regression approach suggested that models may underestimate soil C retention from litter with fast decomposition rates.     

Long term decomposition: the influence of litter type and soil horizon on retention of plant carbon and nitrogen in soils

How plant inputs from above- versus below-ground affect long term carbon (C) and nitrogen (N) retention and stabilization in soils is not well known. We present results of a decade-long field study that traced the decomposition of ¹³C- and ¹⁵N-labeled Pinus ponderosa needle and fine root litter placed in O or A soil horizons of a sandy Alfisol under a coniferous forest. We measured the retention of litter-derived C and N in particulate (>2 mm) and bulk soil (<2 mm) fractions, as well as in density-separated free light and three mineral-associated fractions. After 10 years, the influence of slower initial mineralization of root litter compared to needle litter was still evident: almost twice as much root litter (44% of C) was retained than needle litter (22–28% of C). After 10 years, the O horizon retained more litter in coarse particulate matter implying the crucial comminution step was slower than in the A horizon, while the A horizon retained more litter in the finer bulk soil, where it was recovered in organo-mineral associations. Retention in these A horizon mineral-associated fractions was similar for roots and needles. Nearly 5% of the applied litter C (and almost 15% of the applied N) was in organo-mineral associations, which had centennial residence times and potential for long-term stabilization. Vertical movement of litter-derived C was minimal after a decade, but N was significantly more mobile. Overall, the legacy of initial litter quality influences total SOM retention; however, the potential for and mechanisms of long-term SOM stabilization are influenced not by litter type but by soil horizon.     

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.     

Flow of Deposited Inorganic N in Two Gleysol-dominated Mountain Catchments Traced with <Superscript>15</Superscript>NO<Subscript>3</Subscript><Superscript>−</Superscript> and <Superscript>15</Superscript>NH<Subscript>4</Subscript><Superscript>+</Superscript>

In two mountain ecosystems at the Alptal research site in central Switzerland, pulses of ¹⁵NO₃ and ¹⁵NH₄ were separately applied to trace deposited inorganic N. One forested and one litter meadow catchment, each approximately 1600 m², were delimited by trenches in the Gleysols. K¹⁵NO₃ was applied weekly or fortnightly over one year with a backpack sprayer, thus labelling the atmospheric nitrate deposition. After the sampling and a one-year break, ¹⁵NH₄Cl was applied as a second one-year pulse, followed by a second sampling campaign. Trees (needles, branches and bole wood), ground vegetation, litter layer and soil (LF, A and B horizon) were sampled at the end of each labelling period. Extractable inorganic N, microbial N, and immobilised soil N were analysed in the LF and A horizons. During the whole labelling period, the runoff water was sampled as well. Most of the added tracer remained in both ecosystems. More NO₃⁻ than NH₄⁺ tracer was retained, especially in the forest. The highest recovery was in the soil, mainly in the organic horizon, and in the ground vegetation, especially in the mosses. Event-based runoff analyses showed an immediate response of ¹⁵NO₃⁻ in runoff, with sharp ¹⁵N peaks corresponding to discharge peaks. NO₃⁻ leaching showed a clear seasonal pattern, being highest in spring during snowmelt. The high capacity of N retention in these ecosystems leads to the assumption that deposited N accumulates in the soil organic matter, causing a progressive decline of its C:N ratio.     

Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter

The mechanisms behind the ¹³C enrichment of organic matter with increasing soil depth in forests are unclear. To determine if ¹³C discrimination during respiration could contribute to this pattern, we compared δ¹³C signatures of respired CO₂ from sieved mineral soil, litter layer and litterfall with measurements of δ¹³C and δ¹⁵N of mineral soil, litter layer, litterfall, roots and fungal mycelia sampled from a 68-year-old Norway spruce forest stand planted on previously cultivated land. Because the land was subjected to ploughing before establishment of the forest stand, shifts in δ¹³C in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As ¹³C-depleted organic matter accumulated in the upper soil, a 1.0‰ δ¹³C gradient from −28.5‰ in the litter layer to −27.6‰ at a depth of 2–6 cm was formed. This can be explained by the 1‰ drop in δ¹³C of atmospheric CO₂ since the beginning of reforestation together with the mixing of new C (forest) and old C (farmland). However, the isotopic change of the atmospheric CO₂ explains only a portion of the additional 1.0‰ increase in δ¹³C below a depth of 20 cm. The δ¹³C of the respired CO₂ was similar to that of the organic matter in the upper soil layers but became increasingly ¹³C enriched with depth, up to 2.5‰ relative to the organic matter. We hypothesise that this ¹³C enrichment of the CO₂ as well as the residual increase in δ¹³C of the organic matter below a soil depth of 20 cm results from the increased contribution of ¹³C-enriched microbially derived C with depth. Our results suggest that ¹³C discrimination during microbial respiration does not contribute to the ¹³C enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when natural variations in δ¹³C of respired CO₂ are used to separate different components of soil respiration or ecosystem respiration.     

滇中高原云南松林枯落物输入对土壤碳氮储量及其分布格局的影响

为了更好地理解土壤碳氮对枯落物输入变化的响应,通过枯落物添加与去除实验（DIRT）对滇中高原云南松林枯落物输入变化对土壤碳氮储量及其分布格局的影响进行了研究。2018年3月至2019年2月分别设置6种枯落物输处理,分别为对照（CO）、去除枯落物（NL）、双倍枯落物（DL）、去除根系（NR）、无输入（NI）以及去除有机层与A层（O/A-Less）,研究了不同处理条件下土壤剖面上碳氮储量的分布规律。研究结果表明：（1）不同处理全碳储量为134.49-170.92 t/hm²,全碳储量在不同处理间表现为：S_C（NL）=170.92 t/hm² > S_C（CO）=168.10 t/hm² > S_C（NR）=153.26 t/hm² > S_C（NI）=147.20 t/hm² > S_{C（O/A-Less）}=143.54 t/hm² > S_C（DL）=134.49 t/hm²,不同处理0-20 cm土层全碳储量占0-60 cm土层全碳储量的40.86%-53.56%;不同处理全氮储量表现为：S_N（CO）=11.83 t/hm² > S_N（NL）=9.70 t/hm² > S_N（DL）=8.70 t/hm² > S_N（NR）=8.35 t/hm² > S_{N（O/A-Less）}=8.21 t/hm² > S_N（NI）=8.09 t/hm²。不同处理0-20 cm土层的全氮储量占0-60 cm土层全氮储量的39.28%-46.04%。云南松林地枯落物添加去除实验发现去除枯落物短期内可以增加土壤碳储量,其他处理均在一定程度上减少了土壤碳氮储量。（2）地上枯落物输入对表层（0-20 cm）土壤碳氮影响显著,根系输入对深层（20-40 cm）土壤碳氮影响显著;（3）土壤C、N存在耦合关系,不同处理土壤全碳含量与全氮含量极显著正相关,并且土壤全碳含量与土壤各化学计量比均呈极显著正相关关系;土壤容重与土壤碳氮含量具有极显著负相关关系。     

15N abundance of surface soils,roots and mycorrhizas in profiles of European forest soils

¹⁵N natural abundances of soil total N, roots and mycorrhizas were studied in surface soil profiles in coniferous and broadleaved forests along a transect from central to northern Europe. Under conditions of N limitation in Sweden, there was an increase in ¹⁵N of soil total N of up to 9% from the uppermost horizon of the organic mor layer down to the upper 0–5 cm of the mineral soil. The ¹⁵N of roots was only slightly lower than that of soil total N in the upper organic horizon, but further down roots were up to 5% depleted under such conditions. In experimentally N-enriched forest in Sweden, i.e. in plots which have received an average of c. 100 kg N ha^–1 year^–1 for 20 years and which retain less than 50% of this added N in the stand and the soil down to 20 cm depth, and in some forests in central Europe, the increase in ¹⁵N with depth in soil total N was smaller. An increase in ¹⁵N of the surface soil was even observed on experimentally N-enriched plots, although other data suggest that the N fertilizer added was depleted in¹⁵N. In such cases roots could be enriched in¹⁵N relative to soil total N, suggesting that labelling of the surface soil is via the pathway: — available pools of N-plant N-litter N. Under N-limiting conditions roots of different species sampled from the same soil horizon showed similar ¹⁵N. By contrast, in experimentally N-enriched forest ¹⁵N of roots increased in the sequence: ericaceous dwarf shrubs15N enriched compounds in fungal material, which could contribute to explain the observed ¹⁵N profiles if fungal material is enriched, because it is a precursor of stable organic matter and recalcitrant N. This could act in addition to the previous explanation of the isotopically lighter soil surface in forests: plant uptake of ¹⁵N-depleted N and its redeposition onto the soil surface by litter-fall.     

The distribution of tree and grass roots in savannas in relation to soil nitrogen and water

Here we describe the fine root distribution of trees and grasses relative to soil nitrogen and water profiles. The primary objective is to improve our understanding of edaphic processes influencing the relative abundance of trees and grasses in savanna systems. We do this at both a mesic (737 mm MAP) site on sandy-loam soils and at an arid (547 mm MAP) site on clay rich soils in the Kruger National Park in South Africa. The proportion of tree and grass fine roots at each soil depth were estimated using the δ¹³C values of fine roots and the δ¹³C end members of the fine roots of the dominant trees and grasses at our study sites. Changes in soil nitrogen concentrations with depth were indexed using total soil nitrogen concentrations and soil δ¹⁵N values. Soil water content was measured at different depths using capacitance probes. We show that most tree and grass roots are located in the upper layers of the soil and that both tree and grass roots are present at the bottom of the profile. We demonstrate that root density is positively related to the distribution of soil nitrogen and negatively related to soil moisture. We attribute the negative correlation with soil moisture to evaporation from the soil surface and uptake by roots. Our data is a snapshot of a dynamic process, here the picture it provides is potentially misleading. To understand whether roots in this system are primarily foraging for water or for nitrogen future studies need to include a dynamic component.     

Determination of root biomasses of three species grown in a mixture using stable isotopes of carbon and nitrogen

A method is evaluated that employs variation in stable C and N isotopes from fractionations in C and N acquisition and growth to predict root biomasses of three plant species in mixtures. Celtis laevigata Willd. (C₃), Prosopis glandulosa Torr. (C₃, legume) and Schizachyrium scoparium (Michx.) Nash (C₄), or Gossypium hirsutum L. (C₃), Glycine max (L.) Merr. (C₃ legume), and Sorghum bicolor (L.) Moench (C₄) were grown together in separate, three-species combinations. Surface roots (0–10 cm depth) of each species from each of the two combinations were mixed in various proportions, and the relative abundances of ¹⁵N and ¹⁴N and ¹³C and ¹²C in prepared mixtures, surface roots of single species, and roots extracted from the 80-cm soil profile in which each species combination was grown were analyzed by mass spectrometry. An algebraic determination which employed the δ ¹³C, % ¹⁵N, and C and N concentrations of root subsamples of individual species accounted for more than 95% of the variance in biomass of each species in prepared mixtures with G. max, G. hirsutum, and S. bicolor. A similar analysis demonstrated species-specific differences in rooting patterns. Root biomasses of the C₄ monocots in each combination, S. scoparium and S. bicolor, were concentrated in the upper 20 cm of soil, while those of G. hirsutum and the woody P. glandulosa were largest in lower soil strata. Analyses of stable C and N isotopes can effectively be used to distinguish roots of species which differ in ratios of ¹⁵N to ¹⁴N and ¹³C to ¹²C and thus to study belowground competition between or rooting patterns of associated species with different C and N isotope signatures. The method evaluated can be extended to quantify aboveground and belowground biomasses of component species in mixtures with isotopes of other elements or element concentrations that differ consistently among plants of interest.     

Effects of land use on 15N natural abundance of soils in Ethiopian highlands

We used the natural abundance of ¹⁵N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in ¹³C natural abundance, patterns of δ¹⁵N and δ¹³C values were compared to determine if shifts of ¹⁵N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ¹⁵N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth. The low δ¹⁵N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ¹³C of the surface horizon and the high δ¹³C of the lower soil horizons is clear evidence of a long phase of C₄ grass dominance or cultivation of C₄ crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ¹³C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ¹⁵N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ¹⁵N values of surface mineral soils. The high δ¹⁵N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil micro-habitat effects on fine roots of Chamaecyparis obtusa Endl.: A field experiment using root ingrowth cores

Ingrowth cores in the field were used to compare fine root characteristics of hinoki cypress (Chamaecyparis obtusa) among rooting substrate in the form of needle leaf litter, decomposing organic material, and mineral soil. Fine root growth, morphology, arbuscular mycorrhizal (AM) associations, and tissue C and N concentration were determined. The inorganic N leaching from each soil substrate was taken as a measure of N availability. Although there was no significant difference in total N leaching among substrates, more NH ₊ ⁴ -N leached from the decomposing organic material than other substrates. Rapid fine root production was observed in the organic material, whereas root production in the litter substrate was suppressed. Annual net fine root productions in litter, organic material, and mineral soil were 51, 193, and 132 g m⁻², respectively. In the leaf litter substrate, AM colonization was suppressed and specific root length was higher than in the other substrates, indicating severe nutrient limitation in the litter. These responses of hinoki cypress roots seemed to be a soil exploitation pattern whereby absorptive fine roots were arranged to maximize nutrient acquisition.     

Mycorrhizal Hyphal Turnover as a Dominant Process for Carbon Input into Soil Organic Matter

The atmospheric concentration of CO₂ is predicted to reach double current levels by 2075. Detritus from aboveground and belowground plant parts constitutes the primary source of C for soil organic matter (SOM), and accumulation of SOM in forests may provide a significant mechanism to mitigate increasing atmospheric CO₂ concentrations. In a poplar (three species) plantation exposed to ambient (380 ppm) and elevated (580 ppm) atmospheric CO₂ concentrations using a Free Air Carbon Dioxide Enrichment (FACE) system, the relative importance of leaf litter decomposition, fine root and fungal turnover for C incorporation into SOM was investigated. A technique using cores of soil in which a C₄ crop has been grown (δ¹³C −18.1‰) inserted into the plantation and detritus from C₃ trees (δ¹³C −27 to −30‰) was used to distinguish between old (native soil) and new (tree derived) soil C. In-growth cores using a fine mesh (39 μm) to prevent in-growth of roots, but allow in-growth of fungal hyphae were used to assess contribution of fine roots and the mycorrhizal external mycelium to soil C during a period of three growing seasons (1999–2001). Across all species and treatments, the mycorrhizal external mycelium was the dominant pathway (62%) through which carbon entered the SOM pool, exceeding the input via leaf litter and fine root turnover. The input via the mycorrhizal external mycelium was not influenced by elevated CO₂, but elevated atmospheric CO₂ enhanced soil C inputs via fine root turnover. The turnover of the mycorrhizal external mycelium may be a fundamental mechanism for the transfer of root-derived C to SOM.     

Incorporation of atmospheric 15NO2-nitrogen into free amino acids by Norway spruce Picea abies (L.) Karst.

During spring and autumn 1991, potted 6-yearold spruce trees (Picea abies (L.) Karst.) were fumigated with 60 nl·1^{–1 15}NO₂ for 4 days under controlled conditions in constant light. Current and previous flush needles, the bark and the fine roots were analysed for total ¹⁵N content and incorporation of ¹⁵N into the -amino nitrogen of free amino acids. In addition, in vitro nitrate reductase activity and stomatal conductance of the needles were measured. Nitrate reductase activity was significantly higher in the needles of fumigated trees compared to control trees exposed to filtered air. With an average of 9.1% ¹⁵N, free glutamate was the pool with the most label. Taking into account the time-course of the labelling of this pool, this figure can be taken as an estimate of the minimum contribution of NO₂ to the N nutrition of the needles. ¹⁵N-labelled amino acids were also detected in the bark and the roots, indicating export from the needles.     

Assimilation dynamics of soil carbon and nitrogen by wheat roots and Collembola

It has been demonstrated that plant roots can take up small amounts of low-molecular weight (LMW) compounds from the surrounding soil. Root uptake of LMW compounds have been investigated by applying isotopically labelled sugars or amino acids but not labelled organic matter. We tested whether wheat roots took up LMW compounds released from dual-labelled (¹³C and ¹⁵N) green manure by analysing for excess ¹³C in roots. To estimate the fraction of green manure C that potentially was available for root uptake, excess ¹³C and ¹⁵N in the primary decomposers was estimated by analysing soil dwelling Collembola that primarily feed on fungi or microfauna. The experimental setup consisted of soil microcosm with wheat and dual-labelled green manure additions. Plant growth, plant N and recoveries of ¹³C and ¹⁵N in soil, roots, shoots and Collembola were measured at 27, 56 and 84 days. We found a small (<1%) but significant uptake of green manure derived ¹³C in roots at the first but not the two last samplings. About 50% of green manure C was not recovered from the soil-plant system at 27 days and additional 8% was not recovered at 84 days. Up to 23% of C in collembolans derived from the green manure at 56 days (the 27 days sampling was lost). Using a linear mixing model we estimated that roots or root effluxes provided the main C source for collembolans (54−79%). We conclude that there is no solid support for claiming that roots assimilated green manure derived C due to very small or no recoveries of excess ¹³C in wheat roots. During the incubation the pool of green manure derived C available for root uptake decreased due to decomposition. However, the isotopic composition in Collembola indicated that there was a considerable fraction of green manure derived C in the decomposer system at 56 days thus supporting the premise that LMW compounds containing C from the green manure was released throughout the incubation. Responsible Editor: A. C. Borstlap.     

Carbon and nitrogen isotope ratios in different compartments of a healthy and a declining Picea abies forest in the Fichtelgebirge,NE Bavaria

Summary Natural carbon and nitrogen isotope ratios were measured in different compartments (needles and twigs of different ages and crown positions, litter, understorey vegetation, roots and soils of different horizons) on 5 plots of a healthy and on 8 plots of a declining Norway spruce (Picea abies (L.) Karst.) forest in the Fichtelgebirge (NE Bavaria, Germany), which has recently been described in detail (Oren et al. 1988a; Schulze et al. 1989). The ¹³C values of needles did not differ between sites or change consistently with needle age, but did decrease from the sun-to the shade-crown. This result confirms earlier conclusions from gas exchange measurements that gaseous air pollutants did no long-lasting damage in an area where such damage was expected. Twigs (¹³C between-25.3 and-27.8) were significantly less depleted in ¹³C than needles (¹³C between-27.3 and-29.1), and ¹³C in twigs increased consistently with age. The ¹⁵N values of needles ranged between-2.5 and-4.1 and varied according to stand and age. In young needles ¹⁵N decreased with needle age, but remained constant or increased in needles that were 2 or 3 years old. Needles from the healthy site were more depleted in ¹⁵N than those from the declining site. The difference between sites was greater in old needles than in young ones. This differentiation presumably reflects an earlier onset of nitrogen reallocation in needles of the declining stand. ¹⁵N values in twigs were more negative than in needles (-3.5 to-5.2) and showed age- and stand-dependent trends that were similar to the needles. ¹⁵N values of roots and soil samples increased at both stands with soil depth from-3.5 in the organic layer to +4 in the mineral soil. The ¹⁵N values of roots from the mineral soil were different from those of twigs and needles. Roots from the shallower organic layer had values similar to twigs and needles. Thus, the bulk of the assimilated nitrogen was presumably taken up by the roots from the organic layer. The problem of separation of ammonium or nitrate use by roots from different soil horizons is discussed.     

15N-ammonium and 15N-nitrate uptake of a 15-year-old Picea abies plantation

Throughfall nitrogen of a 15-year-old Picea abies (L.) Karst. (Norway spruce) stand in the Fichtelgebirge, Germany, was labeled with either ¹⁵N-ammonium or ¹⁵N-nitrate and uptake of these two tracers was followed during two successive growing seasons (1991 and 1992). ¹⁵N-labeling (62 mg ¹⁵N m^-2 under conditions of 1.5 g N m^-2 atmospheric nitrogen deposition) did not increase N concentrations in plant tissues. The ¹⁵N recovery within the entire stand (including soils) was 94%±6% of the applied ¹⁵N-ammonium tracer and 100%±6% of the applied ¹⁵N-nitrate tracer during the 1st year of investigation. This decreased to 80%±24% and 83%±20%, respectively, during the 2nd year. After 11 days, the ¹⁵N tracer was detectable in 1-year-old spruce needles and leaves of understory species. After 1 month, tracer was detectable in needle litter fall. At the end of the first growing season, more than 50% of the ¹⁵N taken up by spruce was assimilated in needles, and more than 20% in twigs. The relative distribution of recovered tracer of both ¹⁵N-ammonium and ¹⁵N-nitrate was similar within the different foliage age classes (recent to 11-year-old) and other compartments of the trees. ¹⁵N enrichment generally decreased with increasing tissue age. Roots accounted for up to 20% of the recovered ¹⁵N in spruce; no enrichment could be detected in stem wood. Although ¹⁵N-ammonium and ¹⁵N-nitrate were applied in the same molar quantities (¹⁵NH ₄ ⁺ : ¹⁵NO ₃ ^- =1:1), the tracers were diluted differently in the inorganic soil N pools (¹⁵NH ₄ ⁺ /NH ₄ ⁺ : ¹⁵NO ₃ ^- /NO ₃ ^- =1:9). Therefore the measured ¹⁵N amounts retained by the vegetation do not represent the actual fluxes of ammonium and nitrate in the soil solution. Use of the molar ammonium-to-nitrate ratio of 9:1 in the soil water extract to estimate ¹⁵N uptake from inorganic N pools resulted in a 2–4 times higher ammonium than nitrate uptake by P. abies.     

Hyphal N transport by a vesicular-arbuscular mycorrhizal fungus associated with cucumber grown at three nitrogen levels

Cucumis sativus L. cv. Aminex (F1 hybrid) was grown alone or in symbiosis with Glomus intraradices Schenck and Smith in containers with two hyphal compartments (HC_A and HC_B) on either side of a root compartment (RC) separated by fine nylon mesh. Plants received a total of either 100, 200 or 400 mg N which were applied gradually to the RC during the experiment. ¹⁵N was supplied to HC_A 42 d after plating, at 50 mg ¹⁵NH₄ ⁺-N kg^–1 soil. Lateral movement of the applied ¹⁵N towards the roots was minimized by using a nitrification inhibitor and a hyphal buffer compartment.Non-mycorrhizal controls contained only traces of ¹⁵N after a 27 d labelling period irrespective of the amount of N supplied to the RC. In contrast, 49, 48 and 27% of the applied ¹⁵N was recovered in mycorrhizal plants supplied with 100, 200 and 400 mg N, respectively. The plant dry weight was increased by mycorrhizal colonization at all three levels of N supply, but this effect was strongest in plants of low N status. The results indicated that this increase was due partly to the improved inflow of N via the external hyphae. Root colonization by G. intraradices was unaffected by the amount of N supplied to the RC, while hyphal length increased in HC_A compared to HC_B. Although a considerable ¹⁵N content was detected in mycorrhizal roots adjacent to HC_B, only insignificant amounts of ¹⁵N were found in the external hyphae in HC_B. The external hyphae depleted the soil of inorganic N in both HC_A and HC_B, while the concentration of soil mineral N was still high in non-mycorrhizal containers at harvest. An exception was plants supplied with 400 mg N, where some inorganic N was present at 5 cm distance from the RC in HC_A. The possibility of a regulation mechanism for hyphal transport of N is discussed.     

Allocation of carbon to shoots,roots, soil and rhizosphere respiration by barrel medic (Medicago truncatula) before and after defoliation

The allocation of carbon to shoots, roots, soil and rhizosphere respiration in barrel medic (Medicago truncatulaGaertn.) before and after defoliation was determined by growing plants in pots in a labelled atmosphere in a growth cabinet. Plants were grown in a ¹⁴CO₂-labelled atmosphere for 30 days, defoliated and then grown in a ¹³CO₂-labelled atmosphere for 19 days. Allocation of ¹⁴C-labelled C to shoots, roots, soil and rhizosphere respiration was determined before defoliation and the allocation of ¹⁴C and ¹³C was determined for the period after defoliation. Before defoliation, 38.4% of assimilated C was allocated below ground, whereas after defoliation it was 19.9%. Over the entire length of the experiment, the proportion of net assimilated carbon allocated below ground was 30.3%. Of this, 46% was found in the roots, 22% in the soil and 32% was recovered as rhizosphere respiration. There was no net translocation of assimilate from roots to new shoot tissue after defoliation, indicating that all new shoot growth arose from above-ground stores and newly assimilated carbon. The rate of rhizosphere respiration decreased immediately after defoliation, but after 8 days, was at comparable levels to those before defoliation. It was not until 14 days after defoliation that the amount of respiration from newly assimilated C (¹³C) exceeded that of C assimilated before defoliation (¹⁴C). This revised version was published online in June 2006 with corrections to the Cover Date.     

Biodegradation of Trichloroacetic Acid in Norway Spruce/Soil System

Trichloroacetic acid (TCA) belongs to secondary atmospheric pollutants affecting the forest health. Distribution of [1,2-¹⁴C]TCA-residues and TCA biodegradation were investigated in 4-year-old nursery-grown trees of Norway spruce [Picea abies (L.) Karst.] in the whole plant/soil system. Radioactivity was monitored in needles, wood, roots and soil as well as in the air. During two weeks of exposure TCA was continuously degraded, especially in the soil. Estimates of radioactivity balance showed loss of radioactivity into the atmosphere in the form of ¹⁴CO₂; unincorporated [1,2-¹⁴C]TCA, chloroform, carbon monoxide and methane were not detected at all. TCA degradation to CO₂ was indicated also in the spruce needles. Moreover, it was found that soil litter contained [1,2-¹⁴C]TCA unavailable to microorganisms.     

Decomposition of leaf and root tissue of three perennial grass species grown at two levels of atmospheric CO2 and N supply

Leaf and root tissue of Lolium perenne L., Agrostis capillaris L. and Festuca ovina L. grown under ambient (350 μl l^-1 CO₂) and elevated (700 μl l^-1) CO₂ in a continuously ¹⁴C-labelled atmosphere and at two soil N levels, were incubated at 14°C for 222 days. Decomposition of leaf and root tissue grown in the low N treatment was not affected by elevated [CO₂], whereas decomposition in the high N treatment was significantly reduced by 7% after 222 days. Despite the increased C/N ratio (g g^-1) of tissue cultivated at elevated [CO₂] when compared with the corresponding ambient tissue, there was no significant correlation between initial C/N ratio and ¹⁴C respired. This finding suggests that the CO₂-induced changes in decomposition rates do not occur via CO₂-induced changes in C/N ratios of plant materials. We combined the decomposition data with data on ¹⁴C uptake and allocation for the same plants, and give evidence that elevated [CO₂] has the potential to increase soil C stores in grassland via increasing C uptake and shifting C allocation towards the roots, with an inherent slower decomposition rate than the leaves. An overall increase of 15% in ¹⁴C remaining after 222 days was estimated for the combined tissues, i.e., the whole plants; the leaves made a much smaller contribution to the C remaining (+6%) than the roots (+26%). This shows the importance of clarifying the contribution of roots and leaves with respect to the question whether grassland soils act as a sink or source for atmospheric CO₂. This revised version was published online in June 2006 with corrections to the Cover Date.