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Terrestrial ecosystems worldwide are receiving increasing amounts of biologically reactive nitrogen (N) as a consequence of anthropogenic activities. This intended or unintended fertilization can have a wide‐range of impacts on biotic communities and hence on soil respiration. Reduction in below‐ground carbon (C) allocation induced by high N availability has been assumed to be a major mechanism determining the effects of N enrichment on soil respiration. In addition to increasing available N, however, N enrichment causes soil acidification, which may also affect root and microbial activities. The relative importance of increased N availability vs. soil acidification on soil respiration in natural ecosystems experiencing N enrichment is unclear. We conducted a 12‐year N enrichment experiment and a 4‐year complementary acid addition experiment in a semi‐arid Inner Mongolian grassland. We found that N enrichment had contrasting effects on root and microbial respiration. N enrichment significantly increased root biomass, root N content and specific root respiration, thereby promoting root respiration. In contrast, N enrichment significantly suppressed microbial respiration likely by reducing total microbial biomass and changing the microbial community composition. The effect on root activities was due to both soil acidity and increased available N, while the effect on microbes primarily stemmed from soil acidity, which was further confirmed by results from the acid addition experiment. Our results indicate that soil acidification exerts a greater control than soil N availability on soil respiration in grasslands experiencing long‐term N enrichment. These findings suggest that N‐induced soil acidification should be included in predicting terrestrial ecosystem C balance under future N deposition scenarios.

Aims Specific root respiration (RR S ) is a key root trait, determining i.e. nutrient foraging and uptake efficiencies. However, a considerable uncertainty exists regarding the effects of storage time and conditions on RR S measurements. Methods Fine root CO 2 efflux rates of three plant types (tree seedling Carpinus betulus , legume Pisum sativum , grass Lolium perenne ) were measured as depending on storage time (30–1440 min post-rinsing) and conditions (i.e. attached to plant, warm and cold water storage, and storage under dry conditions). Results Short-term storage conditions (30 min) had a significant effect on measured RR S rates, in specific, RR S rates of all three species were significantly lower under dry storage. Irrespective of plant species or temperature, storage of excised roots in water did not affect RR S for 300 min,. RR S measurements remained stable for 1 day if roots were stored cold. Conclusions Our results have important implications on measurement routines of RR S —a generally understudied root trait. Henceforth it seems reasonable to collect roots in the field and transport them, hydrated but even uncooled, to the laboratory for subsequent measurements for at least 300 min post-rinsing.

Defining root death in studies of root dynamics is problematic because cell death occurs gradually and the resulting effects on root function are not well understood. In this study, metabolic activity of grape roots of different ages was assessed by excised root respiration and tetrazolium chloride reduction. We investigated changes in metabolic activity and patterns of cell death occurring with root age and changes in root pigmentation. Tetrazolium chloride reduction of roots of different ages was strongly correlated to respiration (R2 = 0.786). As roots aged, respiration and tetrazolium chloride reduction declined similarly, with minimum metabolic activity reached at six weeks. Tetrazolium chloride reduction indicated that the onset of root browning corresponded to a 77% reduction in metabolic activity (P < 0.001). Anatomical examination of roots at each pigmentation stage showed that even though some cells in brown roots were still alive, these roots were functionally dead. The effect of using different definitions of root death in relation to root survivorship was determined in a study of ‘Concord’ grapes with two pruning treatments, using three criteria for root death: browning, blackening or shriveling, and disappearance. There was no effect of vine pruning on root life span when life span was defined as the time from first appearance to the onset of browning. However, if death was judged as the point when roots either became black or shriveled or disappeared, vine pruning decreased root life span by 34% and 40%, respectively (P < 0.001), and also increased the decay constant for root decomposition by about 45% (P < 0.001). We conclude that the discrepancy among determinations of root life span assessed with different definitions of death might be partly caused by the latter evaluations of root life span incorporating a portion of root decomposition in definitions of root death.

Although fine roots are important components of the global carbon cycle, there is limited understanding of root structure-function relationships among species. We determined whether root respiration rate and decomposability, two key processes driving carbon cycling but always studied separately, varied with root morphological and chemical traits, in a coordinated way that would demonstrate the existence of a root economics spectrum (RES). Twelve traits were measured on fine roots (diameter 2mm) of 74 species (31 graminoids and 43 herbaceous and dwarf shrub eudicots) collected in three biomes. The findings of this study support the existence of a RES representing an axis of trait variation in which root respiration was positively correlated to nitrogen concentration and specific root length and negatively correlated to the root dry matter content, lignin:nitrogen ratio and the remaining mass after decomposition. This pattern of traits was highly consistent within graminoids but less consistent within eudicots, as a result of an uncoupling between decomposability and morphology, and of heterogeneity of individual roots of eudicots within the fine-root pool. The positive relationship found between root respiration and decomposability is essential for a better understanding of vegetation-soil feedbacks and for improving terrestrial biosphere models predicting the consequences of plant community changes for carbon cycling.

Root respiration at the level of a forest stand, an important component of ecosystem carbon balance, has been estimated in the past using various methods, most of them being indirect and relying on soil respiration measurements. On a 3-yr-old Eucalyptus stand in Congo-Brazzaville, a method involving the upscaling of direct measurements made on roots in situ, was compared with an independent approach using soil respiration measurements conducted on control and trenched plots (i.e. without living roots). The first estimation was based on the knowledge of root-diameter distribution and on a relationship between root diameter and specific respiration rates. The direct technique involving the upscaling of direct measurements on roots resulted in an estimation of 1.53 μmol m⁻² s⁻¹, c. 50% higher than the mean estimation obtained with the indirect technique (1.05 μmol m⁻² s⁻¹). Monte-Carlo simulations showed that the results carried high uncertainty, but this uncertainty was no higher for the direct method than for the trenched-plot method. The reduction of the uncertainties on upscaled results requires more extensive knowledge of temperature sensitivity and more confidence and precision on the respiration rates and biomasses of fine roots.

Background Soil salt stress is a problem in the world, which turns into one of the main limiting factors hindering maize production. Salinity significantly affects root physiological processes in maize plants. There are few studies, however, that analyses the response of maize to salt stress in terms of the development of root anatomy and respiration. Results We found that the leaf relative water content, photosynthetic characteristics, and catalase activity exhibited a significantly decrease of salt stress treatments. However, salt stress treatments caused the superoxide dismutase activity, peroxidase activity, malondialdehyde content, Na + uptake and translocation rate to be higher than that of control treatments. The detrimental effect of salt stress on YY7 variety was more pronounced than that of JNY658. Under salt stress, the number of root cortical aerenchyma in salt-tolerant JNY658 plants was significantly higher than that of control, as well as a larger cortical cell size and a lower root cortical cell file number, all of which help to maintain higher biomass. The total respiration rate of two varieties exposed to salt stress was lower than that of control treatment, while the alternate oxidative respiration rate was higher, and the root response of JNY658 plants was significant. Under salt stress, the roots net Na + and K + efflux rates of two varieties were higher than those of the control treatment, where the strength of net Na + efflux rate from the roots of JNY658 plants and the net K + efflux rate from roots of YY7 plants was remarkable. The increase in efflux rates reduced the Na + toxicity of the root and helped to maintain its ion balance. Conclusion These results demonstrated that salt-tolerant maize varieties incur a relatively low metabolic cost required to establish a higher root cortical aerenchyma, larger cortical cell size and lower root cortical cell file number, significantly reduced the total respiration rate, and that it also increased the alternate oxidative respiration rate, thereby counteracting the detrimental effect of oxidative damage on root respiration of root growth. In addition, Na + uptake on the root surface decreased, the translocation of Na + to the rest of the plant was constrained and the level of Na + accumulation in leaves significantly reduced under salt stress, thus preempting salt-stress induced impediments to the formation of shoot biomass.

Drought affects the carbon (C) source and sink activities of plant organs, with potential consequences for belowground C allocation, a key process of the terrestrial C cycle. The responses of belowground C allocation dynamics to drought are so far poorly understood. We combined experimental rain exclusion with¹³C pulse labelling in a mountain meadow to analyse the effects of summer drought on the dynamics of belowground allocation of recently assimilated C and how it is partitioned among different carbohydrate pools and root respiration. Severe soil moisture deficit decreased the ecosystem C uptake and the amounts and velocity of C allocated from shoots to roots. However, the proportion of recently assimilated C translocated belowground remained unaffected by drought. Reduced root respiration, reflecting reduced C demand under drought, was increasingly sustained by C reserves, whilst recent assimilates were preferentially allocated to root storage and an enlarged pool of osmotically active compounds. Our results indicate that under drought conditions the usage of recent photosynthates is shifted from metabolic activity to osmotic adjustment and storage compounds.

Waterlogging tolerance, root porosity and root anatomy were evaluated for 20 Trifolium accessions (species and sub-species, all annuals) selected from the eight Sections of the genus. Nine accessions were sensitive [relative growth rate (RGR) reduced by up to 80%] to waterlogging, nine accessions were tolerant (RGR not reduced), and in two accessions RGR increased (up to 1.9-fold), when compared to drained controls. Growth of the main (i.e. tap) root axis was severely reduced in all accessions when waterlogged. Lateral roots formed the bulk of the root system of tolerant accessions when grown in waterlogged soil. Lengths of the longest lateral roots were up to three-times longer than the main root axis. Root porosity varied from 0.7–12% among accessions when grown in aerated solution and from 1.1–15.5% in plants grown in hypoxic (0.031–0.045 mol O 2m −3) solution. In some accessions aerenchyma formed by cell lysigeny; in others it formed by schizogenous cell separation, or a combination of both processes. O 2consumption rates of expanded lateral root tissues varied by up to 1.7-fold (on a mass basis) among the six accessions tested and was reduced by an average of 24% for roots of plants grown in hypoxic solution prior to measurements. Accessions with the highest root porosity tended to have longer roots when grown in waterlogged soil. Three accessions formed ‘aerotropic roots’ and the lateral root lengths of these plants exceeded those of all other accessions, suggesting enhanced O 2movement to the submerged lateral root axis via the aerotropic roots. Waterlogging-tolerant accessions were identified in seven of the eight Sections in Trifolium, and the tolerant accessions tended to be those with extensive lateral root systems of relatively high porosity.

• The loss of carbon below-ground through respiration of fine roots may be modified by global change. Here we tested the hypothesis that a reduction in N concentration of tree fine-roots grown in an elevated atmospheric CO2 concentration would reduce maintenance respiration and that more energy would be used for root growth and N uptake. We partitioned total fine-root respiration (R T) between maintenance (R M), growth (R G), and N uptake respiration (R N) for loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) forests exposed to elevated CO2. • A substantial increase in fine-root production contributed to a 151% increase in R G for loblolly pine in elevated CO2. Root specific R M for pine was 24% lower under elevated CO2 but when extrapolated to the entire forest, no treatment effect could be detected. • $R_{{\\rm G}}(<10\\%)$ and $R_{{\\rm N}}(<3\\%)$ were small components of R M in both forests. Maintenance respiration was the vast majority of R T, and contributed 92% and 86% of these totals at the pine and sweetgum forests, respectively. • The hypothesis was rejected because the majority of fine-root respiration was used for maintenance and was not reduced by changes in root N concentration in elevated CO2. Because of its large contribution to R T and total soil CO2 efflux, changes in R M caused by warming may greatly alter carbon losses from forests to the atmosphere.

• Plants typically respond to waterlogging by producing new adventitious roots with aerenchyma and many wetland plants form a root barrier to radial O₂ loss (ROL), but it was not known if this was also the case for lateral roots. • We tested the hypothesis that lateral roots arising from adventitious roots can form a ROL barrier, using root-sleeving electrodes and O₂ microsensors to assess ROL of Zea nicaraguensis, the maize (Zea mays ssp. mays) introgression line with a locus for ROL barrier formation (introgression line (IL) #468) from Z. nicaraguensis and a maize inbred line (Mi29). • Lateral roots of Z. nicaraguensis and IL #468 both formed a ROL barrier under stagnant, deoxygenated conditions, whereas Mi29 did not. Lateral roots of Z. nicaraguensis had higher tissue O₂ status than for IL #468 and Mi29. The ROL barrier was visible as suberin in the root hypodermis/exodermis. Modelling showed that laterals roots can grow to a maximum length of 74 mm with a ROL barrier, but only to 33 mm without a barrier. • Presence of a ROL barrier in lateral roots requires reconsideration of the role of these roots as sites of O₂ loss, which for some species now appears to be less than hitherto thought.