Forest systems cover more than 4.1 x 10(9) hectares of the Earth's land area. Globally, forest vegetation and soils contain about 1146 petagrams of carbon, with approximately 37 percent of this carbon in low-latitude forests, 14 percent in mid-latitudes, and 49 percent at high latitudes. Over two-thirds of the carbon in forest ecosystems is contained in soils and associated peat deposits. In 1990, deforestation in the low latitudes emitted 1.6 +/- 0.4 petagrams of carbon per year, whereas forest area expansion and growth in mid- and high-latitude forest sequestered 0.7 +/- 0.2 petagrams of carbon per year, for a net flux to the atmosphere of 0.9 +/- 0.4 petagrams of carbon per year. Slowing deforestation, combined with an increase in forestation and other management measures to improve forest ecosystem productivity, could conserve or sequester significant quantities of carbon. Future forest carbon cycling trends attributable to losses and regrowth associated with global climate and land-use change are uncertain. Model projections and some results suggest that forests could be carbon sinks or sources in the future.

Only very limited information exists on the plasticity in size and structure of fine root systems, and fine root morphology of mature trees as a function of environmental variation. Six northwest German old-growth beech forests (Fagus sylvatica L.) differing in precipitation (520 – 1030 mm year^–1) and soil acidity/fertility (acidic infertile to basic fertile) were studied by soil coring for stand totals of fine root biomass (0–40 cm plus organic horizons), vertical and horizontal root distribution patterns, the fine root necromass/biomass ratio, and fine root morphology (root specific surface area, root tip frequency, and degree of mycorrhizal infection). Stand total of fine root biomass, and vertical and horizontal fine root distribution patterns were similar in beech stands on acidic infertile and basic fertile soils. In five of six stands, stand fine root biomass ranged between 320 and 470 g m^–2; fine root density showed an exponential decrease with soil depth in all profiles irrespective of soil type. An exceptionally small stand fine root biomass (<150 g="">^–2) was found in the driest stand with 520 mm year^–1 of rainfall. In all stands, fine root morphological parameters changed markedly from the topsoil to the lower profile; differences in fine root morphology among the six stands, however, were remarkably small. Two parameters, the necromass/biomass ratio and fine root tip density (tips per soil volume), however, were both much higher in acidic than basic soils. We conclude that variation in soil acidity and fertility only weakly influences fine root system size and morphology of F. sylvatica, but affects root system structure and, probably, fine root mortality. It is hypothesized that high root tip densities in acidic infertile soils compensate for low nutrient supply rates, and large necromasses are a consequence of adverse soil chemical conditions. Data from a literature survey support the view that rainfall is another major environmental factor that influences the stand fine root biomass of F. sylvatica.

细根在森林生态系统地下碳循环过程中具有核心地位.2007年11月—2009年11月，对华西雨屏区苦竹人工林进行了模拟氮沉降试验.氮沉降水平分别为对照(CK,0 g N·m^-2·a^-1)、低氮(5 g N·m^-2·a^-1)、中氮(15 g N·m^-2·a^-1)和高氮(30 g N·m^-2·a^-1)处理，研究氮沉降对苦竹人工林细根和土壤根际呼吸的影响.结果表明：不同处理氮沉降下，<1  mm和1～2 mm细根特性差异较大，与< 1 mm细根相比，1～2 mm细根的木质素、磷和镁含量更高，而纤维素、钙含量更低；氮沉降显著增加了<2 mm细根生物量,对照、低氮、中氮和高氮处理的细根生物量分别为(533±89)、(630±140)、(632±168)和(820±161) g·m^-2,氮、钾、镁元素含量也明显增加；苦竹林各处理年均土壤呼吸速率分别为(5.85±0.43)、(6.48±0.71)、(6.84±0.57)和(7.62±0.55) t C·hm^-2·a^-1，氮沉降对土壤呼吸有明显的促进作用；苦竹林的年均土壤呼吸速率与<2 mm细根生物量和细根N含量呈极显著线性相关.氮沉降使细根生物量和代谢强度增加，并通过增加微生物活性促进了根际土壤呼吸.

Nutrient limitation to primary productivity and other biological processes is widespread in terrestrial ecosystems, and nitrogen (N) and phosphorus (P) are the most common limiting elements, both individually and in combination. Mechanisms that drive P limitation, and their interactions with the N cycle, have received less attention than mechanisms causing N limitation. We identify and discuss six mechanisms that could drive P limitation in terrestrial ecosystems. The best known of these is depletion-driven limitation, in which accumulated P losses during long-term soil and ecosystem development contribute to what Walker and Syers termed a &amp;quot;terminal steady state&amp;quot; of profound P depletion and limitation. The other mechanisms are soil barriers that prevent access to P; transactional limitation, in which weathering of P-containing minerals does not keep pace with the supply of other resources; low-P parent materials; P sinks; and anthropogenic changes that increase the supply of other resources (often N) relative to P. We distinguish proximate nutrient limitation (which occurs where additions of a nutrient stimulate biological processes, especially productivity) from ultimate nutrient limitation (where additions of a nutrient can transform ecosystems). Of the mechanisms that drive P limitation, we suggest that depletion, soil barriers, and low-P parent material often cause ultimate limitation because they control the ecosystem mass balance of P. Similarly, demand-independent losses and constraints to N fixation can control the ecosystem-level mass balance of N and cause it to be an ultimate limiting nutrient.

根系具有高度的形态和生理功能异质性, 在森林生态系统碳和养分循环中起重要作用。根系分枝的顺序构成根序,是根系最基本的构型特征, 根序代表根系不同的发育阶段。然而, 目前直接测定不同根序细根生理功能的研究很少。以落叶松(Larix gmelinii)和水曲柳(Fraxinus mandshurica)的细根为研究对象, 使用气相氧电极测定不同根序细根的呼吸速率, 探讨根系呼吸速率与其形态、结构和组织氮浓度的关系。结果表明: 落叶松和水曲柳细根的直径、根长和维管束直径均随着根序的增加(1–5级)而增加, 而比根长、组织氮浓度和呼吸速率随着根序的增加而降低, 各根序之间差异显著(P < 0.05); 1级根比根长最大、皮层组织发达、组织氮浓度最高且呼吸速率也最高, 其呼吸速率分别为17.57 nmolO₂·g^–1·s^–1(落叶松)和18.80 nmolO₂·g^–1·s^–1(水曲柳), 比5级根分别高148%(落叶松)和124%(水曲柳); 并且, 落叶松根的呼吸速率几乎有96%与根系组织氮浓度相关, 而水曲柳根的呼吸速率则有89%与根系组织氮浓度相关。上述结果说明, 细根的形态和生理功能异质性是紧密相连的, 低级根的形态、结构决定其功能是吸收养分和水, 而高级根的形态、结构决定其功能是运输和贮存养分。

为揭示全球变暖背景下杉木人工林幼苗与其伴生的其它植物间的对土壤养分的竞争关系和适应性，本研究采用埋设加热电缆进行土壤增温（+5℃）技术，在福建省三明市陈大国有采育场内建立杉木（Cunninghamia lanceolata）幼苗试验小区，包括对照（NW）与增温（WNW）处理（均不除草）。采用内生长环法与土钻法相结合，测定增温对杉木幼苗及伴生的其他植物（主要为山油麻Helicteres angustifolia、东南野桐Mallotus lianus）等细根生物量、呼吸、形态、及根组织氮浓度的短期影响。结果表明，（1）增温显著降低了杉木 < 1mm细根生物量，而显著增加了其他植物 < 1mm细根生物量。增温显著提高了其他植物 < 1mm细根的氮浓度，显著降低了其比根长（SRL）和比表面积（SRA）；同时降低了比根呼吸（参比温度18℃，SRR₁₈），表明细根呼吸对增温产生了驯化现象。而增温对杉木细根的氮浓度没有显著影响，却显著提高了<1mm细根比表面积；同时增温对杉木SRR₁₈没有显著影响，表明杉木细根呼吸没有产生驯化现象。（2）SRR₁₈与比根长间的关系受到增温的显著影响，但树种以及增温×树种的交互作用没有显著影响，表明杉木和其他植物细根竞争能力与维持成本间的平衡关系均受到增温的共同影响。综上结果显示，相较于杉木，伴生的其他植物在增温环境中对地下资源的竞争具有更强的优势，能通过增加细根生物量迅速抢夺吸收因增温而加速矿化的土壤养分，同时通过生理和形态的调整，减少根系单位质量的维持成本，从而提高其对全球变暖的适应性；而杉木在增温条件下面临其他植物的强烈竞争，细根生物量降低，处于不利地位，为了满足生长所需，需增大比根长和比根表面积，且因细根呼吸没有产生驯化现象，从而增加了细根单位质量的维持成本，说明杉木对全球变暖的适应性低于其他植物。该研究结果对于全球变暖下杉木人工林的管理具有重要意义。

细根对氮沉降的生理生态响应将显著影响森林生态系统的生产力和碳吸存。为了揭示氮沉降对杉木细根的生理生态影响，对一年生杉木（Cunninghamia lanceolata）幼苗进行了模拟氮沉降试验，并测定施氮1年后杉木幼苗细根生物量、细根形态学特征（比根长、比表面积）、元素化学计量学指标（C、N、P、C/N、C/P、N/P）、细根代谢特征（细根比呼吸速率、非结构性碳水化合物）。结果表明：（1）杉木细根生物量随氮添加水平的升高而显著降低，尤其是0-1 mm细根生物量；细根比根长和比表面积随氮添加水平升高而显著增大。（2）氮添加后杉木细根C含量、C/N、C/P显著降低，高氮添加导致1-2 mm细根N含量和N/P显著升高，而低氮添加导致1-2 mm细根P含量显著升高、N/P显著降低，而0-1 mm细根的N、P含量则保持相对稳定。（3）氮添加后杉木细根比呼吸速率无显著变化，细根可溶性糖含量随氮添加增加而显著增加，而淀粉含量和NSC显著降低。综合以上结果表明：氮添加后用于细根形态构建的碳分配减少，这可能会减少土壤中有机碳的保留，0-1 mm细根的形态更易发生变化，但是其内部N、P养分含量相对更稳定以维持生理活动，细根NSC对氮添加的响应表明施氮可能导致细根受光合产物的限制。

Root respiration is closely related to root morphology, yet it is unclear precisely how to distinguish respiration-related root physiological functions within the branching fine root system. Root respiration and tissue N concentration were examined for different N fertilization treatments, sampling dates, branch orders and temperatures of larch (Larix gmelinii L.) and ash (Fraxinus mandshurica L.) using the excised roots method. The results showed that N fertilization enhanced both root respiration and tissue N concentration for all five branch orders. The greatest increases in average root respiration for N fertilization treatment were 13.30% in larch and 18.25% in ash at 6 degrees C. However, N fertilization did not change the seasonal dynamics of root respiration. Both root respiration and root tissue N concentration decreased with increase in root branch order. First-order (finest) roots exhibited the highest respiration rates and tissue N concentrations out of the five root branch orders examined. There was a highly significant linear relationship between fine root N concentration and root respiration rate. Root N concentration explained >60% of the variation in respiration rate at any given combination of root order and temperature. Root respiration showed a classical exponential relationship with temperature, with the Q(10) for root respiration in roots of different branching orders ranging from 1.62 to 2.20. The variation in root respiration by order illustrates that first-order roots are more metabolically active, suggesting that roots at different branch order positions have different physiological functions. The highly significant relationship between root respiration at different branch orders and root tissue N concentration suggests that root tissue N concentration may be used as a surrogate for root respiration, simplifying future research into the C dynamics of rooting systems.

细根的生态化学计量特征承载着植物生存环境的变化信息，从而为探索全球变化对植物内在机制的影响提供理论依据。以江西武夷山国家级自然保护区内五个不同海拔梯度（1200、1400、1600、1800、2000 m）的黄山松为对象，运用挖掘法采样后测定细根C、N、P含量及化学计量比特征，研究不同的海拔下细根对土壤养分变化的适应规律。结果表明：（1）黄山松细根C含量年平均值为（486.27&#177;64.32）mg/g，海拔对其没有显著的影响，与土壤养分之间不存在显著的相关关系。（2）细根N含量年平均值为（9.26&#177;2.09）mg/g，海拔对其没有显著的影响，但与土壤C含量存在显著的正相关关系。（3）细根P含量年平均值为（0.39&#177;0.13）mg/g，与海拔梯度及土壤P含量均存在极显著正相关关系，而与土壤碳氮比呈显著负相关关系。（4）细根氮磷比为26.94&#177;12.51，与海拔梯度、土壤P含量及土壤碳氮比均显著负相关。因此，黄山松细根吸收N是以消耗C为代价；细根P主要受土壤P供应量的限制；武夷山地区N沉降将进一步增加植物的氮磷比，加剧黄山松生长的P限制。

Wolbachia pipientis bacteria are common endosymbionts of insects that are best known for their ability to increase their prevalence in populations by manipulating host reproductive systems. However, there are examples of Wolbachia that exist in nature that seem to induce no reproductive parasitism trait and yet are able to invade populations. We demonstrate a fitness benefit for Wolbachia-infected insects that may explain this paradox. Drosophila melanogaster flies infected with Wolbachia are less susceptible to mortality induced by a range of RNA viruses. The antiviral protection associated with Wolbachia infection might be exploited in future strategies to reduce transmission of pathogens by insects.

Two methods of estimating fine root production and turnover are compared for 13 forest ecosystems exhibiting a wide range in form (NH₄⁺ vs. NO₃^-) and quantity of available nitrogen. The two methods are by comparison of seasonal maxima and minima in biomess and by nitrogen budgeting. Both methods give similar results for stands with low rates of nitrification. The budgeting method predicts higher fine root turnover and productivity than the max-min method for systems with significant rates of nitrification.

We measured respiration of 20-year-old Pinus radiata D. Don trees growing in control (C), irrigated (I), and irrigated + fertilized (IL) stands in the Biology of Forest Growth experimental plantation near Canberra, Australia. Respiration was measured on fully expanded foliage, live branches, boles, and fine and coarse roots to determine the relationship between CO(2) efflux, tissue temperature, and biomass or nitrogen (N) content of individual tissues. Efflux of CO(2) from foliage (dark respiration at night) and fine roots was linearly related to biomass and N content, but N was a better predictor of CO(2) efflux than biomass. Respiration (assumed to be maintenance) per unit N at 15 degrees C and a CO(2) concentration of 400 micro mol mol(-1) was 1.71 micro mol s(-1) mol(-1) N for foliage and 11.2 micro mol s(-1) mol(-1) N for fine roots. Efflux of CO(2) from stems, coarse roots and branches was linearly related to sapwood volume (stems) or total volume (branches + coarse roots) and growth, with rates for maintenance respiration at 15 degrees C ranging from 18 to 104 micro mol m(-3) s(-1). Among woody components, branches in the upper canopy and small diameter coarse roots had the highest respiration rates. Stem maintenance respiration per unit sapwood volume did not differ among treatments. Annual C flux was estimated by summing (1) dry matter production and respiration of aboveground components, (2) annual soil CO(2) efflux minus aboveground litterfall, and (3) the annual increment in coarse root biomass. Annual C flux was 24.4, 25.3 and 34.4 Mg ha(-1) year(-1) for the C, I and IL treatments, respectively. Total belowground C allocation, estimated as the sum of (2) and (3) above, was equal to the sum of root respiration and estimated root production in the IL treatment, whereas in the nutrient-limited C and I treatments, total belowground C allocation was greater than the sum of root respiration and estimated root production, suggesting higher fine root turnover or increased allocation to mycorrhizae and root exudation. Carbon use efficiency, the ratio of net primary production to assimilation, was similar among treatments for aboveground tissues (0.43-0.50). Therefore, the proportion of assimilation used for construction and maintenance respiration on an annual basis was also similar among treatments.

Root respiration rates have been shown to be correlated with temperature and root N concentration in studies of individual forest types or species, but it is not known how universal these relationships are across forest species adapted to widely different climatic and edaphic conditions. In order to test for broad, cross-species relationships, we measured fine root respiration, as O₂ consumption, over a range of temperatures on excised root samples from ten forested study sites across North America in 1997. Significant differences existed among study sites in root respiration rates, with patterns among sites in respiration rate at a given temperature corresponding to differences among sites in fine root N concentrations. Root respiration rates were highly correlated with root N concentrations at all measurement temperatures (r ²&gt;0.81, P&lt;0.001, for 6, 18 and 24°C). Lower root respiration rates in gymnosperms than in angiosperms were largely explained by lower fine root N concentrations in gymnosperms, and root N concentrations and respiration rates (at a given temperature) tended to be lower at warm sites (New Mexico, Florida, and Georgia) than at cool sites with short growing seasons (Michigan and Alaska). Root respiration rates increased exponentially with temperature at all sites. The Q ₁₀ for root respiration ranged from 2.4 to 3.1, but there were no significant differences among the forest types. The average Q ₁₀s for gymnosperms (Q ₁₀=2.7) and angiosperms (Q ₁₀=2.6) were almost identical, as were the average Q ₁₀s for roots of ectomycorrhizal species (Q ₁₀=2.7) and arbuscular mycorrhizal species (Q ₁₀=2.6). In 1998, fine root respiration at the study sites was measured in the field as CO₂ production at ambient soil temperature. Respiration rates under field conditions were dependent on both ambient soil temperature and root N concentration. Relationships between respiration (adjusted for temperature) and root N concentration for the field measurements were similar to those observed in the 1997 laboratory experiments. For root respiration in tree species, it appears that basic relationships with temperature and nitrogen exist across species and biomes.

The influence of external nitrogen (N) on carbon (C) allocation and processes related to phosphorus (P) metabolism were studied in monoxenic arbuscular mycorrhiza (AM) cultures of Daucus carota. Fungal hyphae of Glomus intraradices proliferated from colonized roots growing on solid medium into C-free liquid minimal medium with two different N and P levels. Furthermore, we exposed the colonized roots to high or low N availability and then studied the mycelial development. Roots were provided with (13)C-glucose in order to follow the C allocation. The mycelium was analysed for phosphatase activity and transcription levels of two nutrient regulated genes. High N availability to the monoxenic AM root reduced the C allocation to the AM fungus while N availability to the mycelium was important for the upregulation of the fungal inorganic phosphorus (Pi)-transporter GiPT. We found that N availability can regulate nutritional processes in arbuscular mycorrhiza. We conclude that negative impacts of N on AM abundance are caused by reduced C allocation from the plant. Upregulation of the fungal Pi-transporter GiPT indicated that increased N availability might induce P limitation in the mycelium.

Nutrient limitation to primary productivity and other biological processes is widespread in terrestrial ecosystems, and nitrogen (N) and phosphorus (P) are the most common limiting elements, both individually and in combination. Mechanisms that drive P limitation, and their interactions with the N cycle, have received less attention than mechanisms causing N limitation. We identify and discuss six mechanisms that could drive P limitation in terrestrial ecosystems. The best known of these is depletion-driven limitation, in which accumulated P losses during long-term soil and ecosystem development contribute to what Walker and Syers termed a &amp;quot;terminal steady state&amp;quot; of profound P depletion and limitation. The other mechanisms are soil barriers that prevent access to P; transactional limitation, in which weathering of P-containing minerals does not keep pace with the supply of other resources; low-P parent materials; P sinks; and anthropogenic changes that increase the supply of other resources (often N) relative to P. We distinguish proximate nutrient limitation (which occurs where additions of a nutrient stimulate biological processes, especially productivity) from ultimate nutrient limitation (where additions of a nutrient can transform ecosystems). Of the mechanisms that drive P limitation, we suggest that depletion, soil barriers, and low-P parent material often cause ultimate limitation because they control the ecosystem mass balance of P. Similarly, demand-independent losses and constraints to N fixation can control the ecosystem-level mass balance of N and cause it to be an ultimate limiting nutrient.

Agriculture is going through a profound revolution worldwide due to increasing world demand for food, higher costs of energy and other inputs, environmental pollution problems, and instability of cropping systems. In this context, knowledge of factors that affect root development is fundamental to improving nutrient cycling and uptake in soil-plant systems. Roots are important organs that supply water, nutrients, hormones, and mechanical support (anchorage) to crop plants and consequently affect economic yields. In addition, roots improve soil organic matter (OM) by contributing to soil pools of organic carbon (C), nitrogen (N), and microbial biomass. Root-derived soil C is retained and forms more stable soil aggregates than shoot-derived soil C. Although roots normally contribute only 10-20% of the total plant weight, a well-developed root system is essential for healthy plant growth and development. Root growth of plants is controlled genetically, but it is also influenced by environmental factors. Mineral nutrition is an important factor influencing the growth of plant roots, but detailed information on nutritional effects is limited, primarily because roots are half-hidden organs that are very difficult to separate from soil. As a result, it is difficult to measure the effect of biotic and abiotic factors on root growth under field conditions. Root growth is mainly measured in terms of root density, length, and weight. Root dry weight is often better related to crop yields than is root length or density. The response of root growth to chemical fertilization is similar to that of shoot growth; however, the magnitude of the response may differ. In nutrient-deficient soils, root weight often increases in a quadratic manner with the addition of chemical fertilizers. Increasing nutrient supplies in the soil may also decrease root length but increase root weight in a quadratic fashion. Roots with adequate nutrient supplies may also have more root hairs than nutrient-deficient roots. This may result in greater uptake of water and nutrients by roots well supplied with essential plant nutrients, compared with roots grown in nutrient-deficient soils. Under favorable conditions, a major part of the root system is usually found in the top 20 cm of soil. Maximum root growth is generally achieved at flowering in cereals and at pod-setting in legumes. Genotypic variations are often found in the response of root growth to nutrient applications, and the possibility of modifying root system response to soil properties offers exciting prospects for future improvements in crop yields. Rooting pattern in crop plants is under multi- or polygenic control, and breeding programs can be used to improve root system properties for environments where drought is a problem. The use of crop species and cultivars tolerant to biotic and abiotic stresses, as well as the use of appropriate cultural practices, can improve plant root system function under favorable and unfavorable environmental conditions.

Fine roots play an important role in the overall functions of individual plants. Previous studies showed that fertilization and available soil resources have a notably profound effect on fine root, but there is lack of study centered on how fine root morphology, physiology, and chemistry respond to biochar with N additions. Different levels of biochar (0, 10, 15, and 20 g) and N (0, 2, 4 and 6 g) were applied to Acer mono seedling plants in a field nursery. The root system morphology and root chemistry and physiology were evaluated in line with root length, root diameter, SRL, N and N: C and root respiration. Biochar and N significantly affected root morphology, chemistry and root respiration. Morphological, chemical and physiological parameters were found to be at their maximum with 20 g biochar and 6 g N; however, no significant effect was noted on fourth- and fifth-order roots. Furthermore, a significant increase in root respiration was recognized with the increase in root tissue N concentration and the negative relationship of root respiration with higher branch order. Thus, overall, study parameters indicate that biochar and nitrogen positively influence the Acer mono fine root, and therefore should be used to improve fine root health.

Global biogeochemical models have improved dramatically in the last decade in their representation of the biosphere. Although leaf area data are an important input to such models and are readily available globally, global root distributions for modeling water and nutrient uptake and carbon cycling have not been available. This analysis provides global distributions for fine root biomass, length, and surface area with depth in the soil, and global estimates of nutrient pools in fine roots. Calculated root surface area is almost always greater than leaf area, more than an order of magnitude so in grasslands. The average C:N:P ratio in living fine roots is 450:11:1, and global fine root carbon is more than 5% of all carbon contained in the atmosphere. Assuming conservatively that fine roots turn over once per year, they represent 33% of global annual net primary productivity.

We estimated carbon allocation to belowground processes in unfertilized and fertilized red pine (Pinus resinosa Ait.) plantations in northern Wisconsin to determine how soil fertility affects belowground allocation patterns. We used soil CO(2) efflux and litterfall measurements to estimate total belowground carbon allocation (root production and root respiration) by the carbon balance method, established root-free trenched plots to examine treatment effects on microbial respiration, estimated fine root production by sequential coring, and developed allometric equations to estimate coarse root production. Fine root production ranged from 150 to 284 g m(-2) year(-1) and was significantly lower for fertilized plots than for unfertilized plots. Coarse root production ranged from 60 to 90 g m(-2) year(-1) and was significantly lower for fertilized plots than for unfertilized plots. Annual soil CO(2) fluxes ranged from 331 to 541 g C m(-2) year(-1) and were significantly lower for fertilized plots than for unfertilized plots. Annual foliage litterfall ranged from 110 to 187 g C m(-2) year(-1) and was significantly greater for fertilized plots than for unfertilized plots. Total belowground carbon allocation ranged from 188 to 395 g C m(-2) year(-1) and was significantly lower for fertilized than for unfertilized plots. Annual soil CO(2) flux was lower for trenched plots than for untrenched plots but did not differ between fertilized and unfertilized trenched plots. Collectively, these independent estimates suggest that fertilization decreased the relative allocation of carbon belowground.

The response of belowground biological processes to soil N availability in Larix gmelinii (larch) and Fraxinus mandshurica (ash) plantations was studied. Soil and root respiration were measured with Li-Cor 6400 and gas-phase O₂ electrodes, respectively. Compared with the control, N fertilization induced the decreases of fine root biomass by 52% and 25%, and soil respiration by 30% and 24% in larch and ash plantations, respectively. The average soil microbial biomass C and N were decreased by 29% and 42% under larch stand and 39% and 47% under ash stand, respectively. While the fine root tissue N concentration under fertilized plots was higher 26% and 12% than that under control plots, respectively, the average fine root respiration rates were increased by 10% and 13% in larch and ash stands under fertilized plot, respectively. Soil respiration rates showed significantly positive exponential relationships with soil temperature, and a seasonal dynamic. These findings suggest that N fertilization can suppress fine root biomass at five branch orders (<2 mm in diameter), soil respiration, and soil microbial biomass C and N, and alter soil microbial communities in L. gmelinii and F. mandshurica plantations.

A pot experiment was conducted in a glass-house to investigate the correlation between specific fine root length (SFRL) and root colonization (RC) of maize inoculated with six arbuscular mycorrhizal fungi (AMF) in three soil types. The results showed that six AMF associated with maize presented different abilities in RC and effects on SFRL. In addition, there was a significant correlation between SFRL and RC of arbuscular mycorrhizal maize in Beijing soil (Cinnamon soil), but no significant correlation in Hubei soil (Brunisolic soil) and Guangdong soil (Red soil). It is concluded that mycorrhizal colonization decreased the SFRL of maize, and the correlation between SFRL and RC of mycorrhizal maize depended on soil type.

Protein turnover is generally regarded as one of the most important maintenance processes in plants in terms of energy requirements. In this study, the contribution of protein turnover to the respiratory costs for maintenance in the roots of two grass species, the fast-growing D. actylis glomerata L. and the slow-growing F. estuca ovina L., is evaluated. Plants were grown under controlled-environment conditions in a nutrient solution to which NO(3)- was added at a relative addition rate of 0.2 and 0.1 mol N mol(-1) N already present in the plant d(-1) for D. glomerata and F. ovina, respectively, so as to obtain a steady exponential growth rate close to the plants' maximum relative growth rate. Pulse-chase labelling with (14)C-leucine was used to determine the rate of protein turnover in the grass roots. The rate of turnover of the total protein pool did not differ significantly between the two species. The protein degradation constant in D. glomerata and F. ovina was 0.156 and 0.116 g protein g(-1) protein d(-1), respectively, which corresponds with a total protein half-life of 4 d and 6 d. Assuming specific respiratory costs for protein turnover of 148 mmol ATP g(-1) protein, the estimated respiratory costs for protein turnover in the roots were 2.8 and 2.4 mmol ATP g(-1) root DM d(-1) in D. glomerata and F. ovina, respectively. Both the fast- and the slow-growing grass spent between 22-30% of their daily ATP production for maintenance on protein turnover, which corresponds to 11-15% of the total root ATP production per day. Note that the data presented in this abstract are based on the assumption that 50% recycling of the (14)C-labelled leucine took place in the roots of both grass species.

Plant respiration is a large, environmentally sensitive component of the ecosystem carbon balance, and net ecosystem carbon flux will change as the balance between photosynthesis and respiration changes. Partitioning respiration into the functional components of construction, maintenance, and ion uptake will aid the estimation of plant respiration for ecosystems. Maintenance respiration is the component most sensitive to changes in temperature, CO₂ , protein concentration and turnover, water stress, and atmospheric pollutants. For a wide variety of plant tissues, maintenance respiration, corrected for temperature, appears to be linearly related to Kjeldahl nitrogen content of live tissue. Total and maintenance respiration may decline under CO₂ enrichment, but the mechanism, independence from changes in protein content, and acclimation are unknown. Response of respiration to temperature can be modelled as a Q₁₀ relationship, if corrections for bias arising from daily and annual temperature amplitude are applied. Occurrence and control of the cyanide-resistant respiratory pathway and acclimation of respiration rates to different climates are poorly understood, but may substantially affect the reliability of model estimates of plant respiration.

Root respiration may account for as much as 60% of total soil respiration. Therefore, factors that regulate the metabolic activity of roots and associated microbes are an important component of terrestrial carbon budgets. Root systems are often sampled by diameter and depth classes to enable researchers to process samples in a systematic and timely fashion. We recently discovered that small, lateral roots at the distal end of the root system have much greater tissue N concentrations than larger roots, and this led to the hypothesis that the smallest roots have significantly higher rates of respiration than larger roots. This study was designed to determine if root respiration is related to root diameter or the location of roots in the soil profile. We examined relationships among root respiration rates and N concentration in four diameter classes from three soil depths in two sugar maple (Acer saccharum Marsh.) forests in Michigan. Root respiration declined as root diameter increased and was lower at deeper soil depths than at the soil surface. Surface roots (0-10 cm depth) respired at rates up to 40% greater than deeper roots, and respiration rates for roots &amp;lt; 0.5 mm in diameter were 2.4 to 3.4 times higher than those for roots in larger diameter classes. Root N concentration explained 70% of the observed variation in respiration across sites and size and depth classes. Differences in respiration among root diameter classes and soil depths appeared to be consistent with hypothesized effects of variation in root function on metabolic activity. Among roots, very fine roots in zones of high nutrient availability had the highest respiration rates. Large roots and roots from depths of low nutrient availability had low respiration rates consistent with structural and transport functions rather than with active nutrient uptake and assimilation. These results suggest that broadly defined root classes, e.g., fine roots are equivalent to all roots &amp;lt; 2.0 mm in diameter, do not accurately reflect the functional categories typically associated with fine roots. Tissue N concentration or N content (mass x concentration N) may be a better indicator of root function than root diameter.