Root Carbon Research Articles

Sampling volume in root studies: the pitfalls of under‐sampling exposed using accumulation curves

Root systems are important for global models of below-ground carbon and nutrient cycling. Notoriously difficult sampling methods and the fractal distribution of root diameters in the soil make data being used in these models especially susceptible to error resulting from under-sampling. We applied the concept of species accumulation curves to root data to quantify the extent of under-sampling inherent to minirhizotron and soil coring sampling for both root uptake and carbon content studies. Based on differences in sample size alone, minirhizotron sampling missed approximately one third of the root diameters observed by soil core sampling. Sample volumes needed to encounter 90% of root diameters averaged 2481 cm(3) for uptake studies and 5878 cm(3) for root carbon content studies. These results show that small sample volumes encounter a non-representative sample of the overall root pool, and provide future guidelines for determining optimal sample volumes in root studies.

Ecosystem‐level controls on root‐rhizosphere respiration

Recent advances in the partitioning of autotrophic from heterotrophic respiration processes in soils in conjunction with new high temporal resolution soil respiration data sets offer insights into biotic and environmental controls of respiration. Besides temperature, many emerging controlling factors have not yet been incorporated into ecosystem-scale models. We synthesize recent research that has partitioned soil respiration into its process components to evaluate effects of nitrogen, temperature and photosynthesis on autotrophic flux from soils at the ecosystem level. Despite the widely used temperature dependence of root respiration, gross primary productivity (GPP) can explain most patterns of ecosystem root respiration (and to some extent heterotrophic respiration) at within-season time-scales. Specifically, heterotrophi crespiration is influenced by a seasonally variable supply of recent photosynthetic products in the rhizosphere. The contribution of stored root carbon (C) to root respiratory fluxes also varied seasonally, partially decoupling the proportion of photosynthetic C driving root respiration. In order to reflect recent insights, new hierarchical models, which incorporate root respiration as a primary function of GPP and which respond to environmental variables by modifying Callocation belowground, are needed for better prediction of future ecosystem C sequestration.

Carbon stocks in primary and secondary tropical forests in Singapore

Tropical forests contain large reserves of carbon that are vulnerable to perturbation linked to human activities, including deforestation and climate change. Accurate estimates of forest carbon are therefore required urgently to support efforts to conserve tropical forests. We quantified carbon stocks in primary and 60-year-old secondary forest plots located on infertile Ultisols in Bukit Timah Nature Reserve, one of the few remaining areas of forest in Singapore. We used tree census data for 24.2ha of primary forest and 23ha of secondary forest, together with allometric equations, to estimate aboveground and coarse root biomass. Coarse woody debris stocks were censused along 2.44km and 2.12km of transects in primary and secondary forest, respectively. Soil carbon and fine root carbon stocks were assessed from soil samples taken to 3m depth in a 2-ha secondary forest plot and a 2-ha primary forest plot, combined with bulk density measured in a nearby soil profile pit. Total estimated carbon stock in the primary forest, which was located on the hilltop and upper slopes (80–115m elevation), was 337MgCha−1, of which 50% was aboveground biomass, 33% in soil, 12% in coarse roots, 4.6% in coarse woody debris, and 0.8% in fine roots. In the secondary forest, located on lower slopes and valley (50–85m elevation), the total carbon stock was 274MgCha−1 and the relative importance of aboveground biomass and soil were reversed, with 38% in aboveground biomass, 52% in soil, 6.9% in coarse roots, 1.5% in coarse woody debris, and 1.3% in fine roots. Including carbon in deep subsoil (i.e. to 3m) increased soil carbon stocks by ∼40% compared to 1m depth. Overall, the 60-year-old secondary forest contained 60% as much biomass as the primary forest, while the primary forest had lower carbon stocks than other primary forests in the region.

CARBON AND NUTRIENT CYCLING THROUGH FINE ROOTS IN RUBBER (HEVEA BRASILIENSIS) PLANTATIONS IN INDIA

SUMMARYUnderstanding the growth dynamics of fine roots and their contribution to soil organic carbon and nutrient pools is crucial for estimating ecosystem carbon and nutrient cycling and how these are influenced by climate change. Rubber is cultivated in more than 10 million hectare globally and the area under rubber cultivation is fast expanding due to socio-economic reasons, apart from the importance given to this species for eco-restoration of degraded lands. An experiment was conducted to quantify fine root production, fine root turnover and carbon and nutrient cycling through fine roots in rubber plantations with different soil nutrient status and rainfall pattern. Fine root production was estimated by sequential coring and ingrowth core methods. Fine root decomposition was determined by the litter bag technique. Carbon and nutrient contents in fine roots were determined and their turnover was computed. Fine root biomass in the top 0–7.5-cm soil layer showed significant seasonal fluctuation and the fluctuations were particularly wide during the transition period from the dry season to the rainy season. Fine root production estimated by the different methods was significantly higher at the lower fertility site and during the higher soil moisture stress year. Fine root turnover ranged from 1.04 to 2.29 year−1. Root carbon and nutrient status showed seasonal variation and lower status was observed during the rainy season. The annual recycling of C, N, P, K, Ca and Mg through fine roots ranged from 590 to 1758, 30 to 85, 3 to 12, 13 to 31, 11 to 35 and 6 to 13 kg ha−1, respectively. Substantial quantities of carbon and nutrients were recycled annually in rubber plantations through fine roots. When soil moisture and nutrient stress were more severe, fine root production, turnover and carbon and nutrient recycling through fine roots were higher.

Lethal drought leads to reduction in nonstructural carbohydrates in Norway spruce tree roots but not in the canopy

Summary Heat waves and droughts are expected to increase in frequency and severity in many regions with future climate change, threatening the survival of a number of forest ecosystems. However, our understanding of the physiological processes and mechanisms underlying drought‐induced tree mortality is incomplete. Here, we present results on the physiological response of young Norway spruce trees exposed to lethal drought stress. We applied three levels of drought treatment (control, drying–rewetting, complete drought) and monitored relevant physiological functions and processes of carbon and water relations at high temporal resolution until tree death occurred. Only trees subjected to continuous drought died in our experiment. Trees subjected to drying–rewetting cycles consistently recovered in their ability to transport water, indicating that these trees do not suffer permanent damage to the hydraulic system. In all cases, drought reduced carbon assimilation, caused changes in carbon allocation and appeared to have severely reduced phloem functioning and carbon translocation. Structural growth was sacrificed for carbon investment in maintenance respiration and osmoprotection. Severe drought caused trees to rely on stored carbon reserves but, in contrast to above‐ground tissues, only root carbon pools were strongly reduced when trees died. Our results indicate that drought‐induced changes in carbon allocation, use and transport differ between above‐ and below‐ground tissues in trees. While root death may have been caused by carbon depletion, this was definitely not the case in above‐ground tissues. Our findings indicate that mortality mechanisms are not defined at the organism level but rather within tree compartments.

Effects of thinning on fine root biomass and carbon storage of subalpine <I>Picea asperata</I> plantation in Western Sichuan Province, China

Effects of thinning on fine root biomass and carbon storage of subalpine <I>Picea asperata</I> plantation in Western Sichuan Province, China

Temporal variations of biomass, carbon and nitrogen of roots under different tree species

Seasonal changes of root biomass, concentrations and masses of carbon and nitrogen in roots and root diameter classes (<2mm, 2–5mm and>5mm) were evaluated under four adjacent coniferous plantation sites: Oriental spruce (Picea orientalis L.), Austrian pine (Pinus nigra Arnold), Turkish fir (Abies bornmulleriana L.), Scots pine (Pinus sylvestris L.) and natural Sessile oak (Quercus petraea L.) forest. The root biomass, carbon and nitrogen concentrations, carbon and nitrogen masses, C/N ratio of roots were evaluated with regards to species, root diameter classes and seasons. The significantly (p<0.05) lowest annual mean root biomass was 258 g/m2 in Austrian pine compared with tree species. The fine root class has significantly the lowest carbon content, with the 42% value, among other root diameter classes. However, the temporal variations of root C (%) presented no statistical significance. The annual mean nitrogen concentrations ranged from 0.56% (Turkish fir) to 0.74% (Austrian pine) among species. We also found that root nitrogen concentrations in January were significantly different from those of other sampling times. C/N ratios showed statistically significant differences in root diameter classes that the ratios were found, 49 for fine roots, 111 for small roots and 143 for coarse roots, respectively.

Root carbon inputs to the rhizosphere stimulate extracellular enzyme activity and increase nitrogen availability in temperate forest soils

The exudation of carbon (C) by tree roots stimulates microbial activity and the production of extracellular enzymes in the rhizosphere. Here, we investigated whether the strength of rhizosphere processes differed between temperate forest trees that vary in soil organic matter (SOM) chemistry and associate with either ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi. We measured rates of root exudation, microbial and extracellular enzyme activity, and nitrogen (N) availability in samples of rhizosphere and bulk soil influenced by four temperate forest tree species (i.e., to estimate a rhizosphere effect). Although not significantly different between species, root exudation ranged from 0.36 to 1.10 g C m−2 day−1, representing a small but important transfer of C to rhizosphere microbes. The magnitude of the rhizosphere effects could not be easily characterized by mycorrhizal associations or SOM chemistry. Ash had the lowest rhizosphere effects and beech had the highest rhizosphere effects, representing one AM and one ECM species, respectively. Hemlock and sugar maple had equivalent rhizosphere effects on enzyme activity. However, the form of N produced in the rhizosphere varied with mycorrhizal association. Enhanced enzyme activity primarily increased amino acid availability in ECM rhizospheres and increased inorganic N availability in AM rhizospheres. These results show that the exudation of C by roots can enhance extracellular enzyme activity and soil-N cycling. This work suggests that global changes that alter belowground C allocation have the potential to impact the form and amount of N to support primary production in ECM and AM stands.

Scenario uncertainties in estimating direct land-use change emissions in biomass-to-energy life cycle assessment

The use of biomass for energy production has increasingly been encouraged in the United States, in part motivated by the potential to reduce greenhouse gas (GHG) emissions relative to fossil fuels. However, the GHG-intensity of biomass-derived energy is highly dependent on how the biomass is obtained and used. We explore scenario uncertainty in GHG estimates in the Calculating Uncertainty in Biomass Emissions (CUBE) model and find that direct land-use change emissions that result during the biomass production often dominate the total “farm-to-hopper” GHGs. CUBE represents each land-use change decision as a conversion of land from one of four specified baseline ecosystem to produce one of seven feedstock crops, both distinct by geographic region, and then determines the implied changes in soil organic carbon, root carbon, and above-ground biomass. CUBE therefore synthesizes and organizes the existing literature to represent direct land-use change emissions in a way that can be more readily incorporated into life cycle assessment. Our approach to representing direct land-use change literature has been applied to a specific set of data and offers immediate implications for decisionmakers, but it can also be generalized and replicated in the future, making use of improved scientific data on the magnitude and rates of direct land-use change emissions as it becomes available.

Characterization of a NADH-Dependent Glutamate Dehydrogenase Mutant of Arabidopsis Demonstrates the Key Role of this Enzyme in Root Carbon and Nitrogen Metabolism

The role of NADH-dependent glutamate dehydrogenase (GDH) was investigated by studying the physiological impact of a complete lack of enzyme activity in an Arabidopsis thaliana plant deficient in three genes encoding the enzyme. This study was conducted following the discovery that a third GDH gene is expressed in the mitochondria of the root companion cells, where all three active GDH enzyme proteins were shown to be present. A gdh1-2-3 triple mutant was constructed and exhibited major differences from the wild type in gene transcription and metabolite concentrations, and these differences appeared to originate in the roots. By placing the gdh triple mutant under continuous darkness for several days and comparing it to the wild type, the evidence strongly suggested that the main physiological function of NADH-GDH is to provide 2-oxoglutarate for the tricarboxylic acid cycle. The differences in key metabolites of the tricarboxylic acid cycle in the triple mutant versus the wild type indicated that, through metabolic processes operating mainly in roots, there was a strong impact on amino acid accumulation, in particular alanine, γ-aminobutyrate, and aspartate in both roots and leaves. These results are discussed in relation to the possible signaling and physiological functions of the enzyme at the interface of carbon and nitrogen metabolism.

Relationships between altitudinal gradient and plant carbon isotope composition of grassland communities on the Qinghai-Tibet Plateau, China

Foliar and root carbon isotope composition (δ13C) of grassland communities on the Qinghai-Tibet Plateau, China, was obtained by the biomass weighting method and direct measurement. We investigated the characteristics and altitudinal patterns of foliar and root δ13C and determined which environmental factors influenced foliar δ13C most. Foliar δ13C of alpine steppe was significantly higher than that of alpine meadow and temperate steppe. For alpine meadow, root δ13C was significantly higher than of foliar δ13C. Foliar δ13C increased with altitude at an average rate of 0.60‰ km−1 for the whole grassland ecosystem. This rate was lower than that at species level. However, there were no significant relationships between root δ13C and altitude. Atmospheric pressure was a more important factor than temperature and precipitation in its influence on the altitudinal pattern of foliar δ13C at the community level.

Estimations of Total Ecosystem Biomass and Carbon Storage for Fir (Abies nordmanniana S. subsp. bornmülleriana (Mattf.)) Forests (Western Black Sea Region)

With increasing CO2 in the atmosphere, there is an urgent need of reliable estimates of biomass and carbon pools, especially in Turkey where there is a serious lack of data. Information on amount of carbon biomass resulting from direct field measurements is crucial in this context, to know how forest ecosystems will affect the carbon cycle and also to validate the measurements. Biomass data were collected over the different vegetation types and land uses of pure fir forests in the Western Black Sea region in Turkey. Using site-specific allometric equation, we estimated biomass and carbon pools. We used GIS technology to develop a carbon biomass map of our study area. We estimated aboveground carbon, root carbon, and soil organic carbon down to 30 cm depth. This study will provide estimates of biomass and carbon pools from pure fir forests in the Western Black Sea region as well as an appropriate methodology to estimate based on ecosystem carbon storage components.

Predicting soil respiration using carbon stock in roots, litter and soil organic matter in forests of Loess Plateau in China

Understanding the dominant variables driving soil respiration is critically important for predicting soil CO2 emission and assessing the carbon balance of forest ecosystems. In a small catchment of the semiarid Loess Plateau in China, soil respiration and soil biophysical factors were studied on sites of five forest types, comprising plots established in a pure Pinus tabulaeformis plantation, a pure Robinia pseudoacacia plantation, a mixed P. tabulaeformis and R. pseudoacacia plantation, a pure Platycladus orientalis plantation, and a natural Populus davidiana stand. Soil temperature at the 10 cm depth was found to be the most predominant factor controlling the temporal pattern of soil respiration, accounting for 11–40% seasonal variation in the rate of soil CO2 efflux across forest types. By applying an empirical model and the calculated temperature sensitivity of soil respiration (Q10) and the rate of basal soil respiration (R10), annual soil CO2 emission was estimated separately for each forest type using the automatically monitored data of soil temperature at the 10 cm depth. The annual soil CO2 emission varied greatly with forest types and ranged from 647.71 g C m−2 y−1 in the P. orientalis plantation to 1448.50 g C m−2 y−1 in the natural P. davidiana stand. Annual soil CO2 efflux is better predicted by soil organic carbon content and the amount of carbon in roots, litter and top soil than soil temperature when data are pooled for all plots of the five forest types. A first order exponential analysis indicates that about 77% of the variation in annual soil CO2 efflux is explained by root carbon stock, 63% by the combined carbon stock in roots, litter, and top soil, and 48% by the combined carbon stock in litter and top soil. We conclude that annual soil CO2 efflux can be predicted by carbon pools in roots and soils rather than by soil temperature in watersheds where spatial variation in soil temperature is relatively small in the semiarid Loess Plateau of China.

Linking litter production, quality and decomposition to vegetation succession following agricultural abandonment

Agricultural land abandonment has been increasing worldwide for environmental and socio-economic reasons, and knowledge of its key ecological processes (e.g., carbon (C) and nitrogen (N) input and accumulation) in relation to vegetation succession can provide important information for ecosystem management and greenhouse gas emissions mitigation. In order to better understand the above- and belowground litter dynamics following agricultural abandonment, we simultaneously studied the litter and fine root production, quality, decomposition, C and N input in ecosystems along a secondary successional gradient (i.e., grassland, shrub-grass land, young secondary forest, and mature secondary forest) following agricultural abandonment in China's Qinling Mountains. Results showed that the significant increase of aboveground woody plant litter and decrease of grass litter during vegetation succession led little changes in total litter production and annual total C and N input in different succession stages, while the fine root production, fine root biomass, C input from fine root production increased significantly with stand age. The initial litter C concentration and fine root carbon: phosphorous ratio (C:P) were the main factors in explaining the variations of decomposition rates of litter and fine root, respectively. The increasing C concentration in litter and the increasing C:P ratio in fine root during vegetation succession had potentially driven the decreases in litter and fine root decomposition rate respectively. The accumulation of litter standing crop during vegetation succession could be attributed to the decreases in litter decomposition rate partly caused by changes in litter quality, rather than the increases in litter production. Our results imply that the changes in vegetation type have a much smaller role in the annual total litter production and the total litter C and N input than previously assumed, while the changes in quality and decomposition rate may have largely influenced C accumulation in stand floor and soil during secondary succession following agricultural abandonment.

Variation of carbon age of fine roots in boreal forests determined from 14C measurements

The main objectives of this study were to determine how the carbon age of fine root cellulose varies between stands, tree species, root diameter and soil depth. In addition, we also compared the carbon age of fine roots from soil cores of this study with reported values from the roots of the same diameter classes of ingrowth cores on the same sites. We used natural abundance of 14C to estimate root carbon age in four boreal Norway spruce and Scots pine stands in Finland and Estonia. Age of fine root carbon was older in 1.5–2 mm diameter fine roots than in fine roots with <0.5 mm diameter, and tended to be older in mineral soil than in organic soil. Fine root carbon was older in the less fertile Finnish spruce stands (11–12 years) than in the more fertile Estonian stand (3 and 8 years), implying that roots may live longer in less fertile soil. We further observed that on one of our sites carbon in live fine roots with the 1.5–2 mm diameter was of similar C age (7–12 years) than in the ingrowth core roots despite the reported root age in the ingrowth cores – being not older than 2 years. From this result, we conclude that new live roots may in some cases use old carbon reserves for their cellulose formation. Future research should be oriented towards improving our understanding of possible internal redistribution and uptake of C in trees.

Alteration of pasture root carbon turnover in response to superphosphate and irrigation at Winchmore New Zealand

Grassland net primary productivity, carbon (C) allocation to roots, root production and subsequent turnover time in grazed pastures have significant implications for modelling landuse effects on global carbon dynamics. The objective of this work, using the decline of assimilated 13C, was to quantify C allocation to roots, root production and turnover time in response to long-term fertiliser and irrigation treatments. The treatments on the pastures on a long-term research site at Winchmore, Canterbury, New Zealand were 0 or 375 kg ha−1 y−1 superphosphate under irrigation and, on a nearby irrigation experiment, unirrigated or irrigated (with fertilisation of 250 kg ha−1 y−1 superphosphate) when the soil water content fell to 20% w/w (50% available moisture). The pasture treatments were pulse-labelled using 13CO2 within portable gas-tight enclosures. Separate micro-plots were 13CO2 pulse-labelled in late spring, summer and autumn. Below ground net primary production (206 g C m−2 y−1) was similar in unfertilised, unirrigated, fertilised and irrigated pastures, despite marked differences in above ground production. Unfertilised/irrigated and unirrigated/fertilised treatments had greater root biomass, root C allocation and longer root turnover time (1.9 y and 2.0 y, respectively) than fertilised and irrigated treatments (1.3 y). These root turnover times appeared to be consistent with the improved substrate quality attributed to the species found in the irrigated and fertilised treatments.

Root carbon input in organic and inorganic fertilizer-based systems

In agroecosystems, carbon (C) inputs come from plant roots, retained shoot residues and in some cases from applied manures. Manure and shoot derived C inputs are relatively easy to determine. Conversely, high costs associated with root measurements have caused knowledge on root C input to remain scant. This study aimed at determining macro-root C input and topsoil root related respiration in response to nutrient management and soil fertility building measures. We sampled roots and shoots of cereals and catch crops in inorganic and organic fertilizer-based arable cropping systems in a long-term experiment in 2 years, 2008 and 2010. Sampled shoots and macro-roots of catch crop mixtures and cereals were characterized for dry matter (DM) biomass (C was estimated as 45 % of DM biomass). We also measured topsoil root-related soil respiration throughout the growing season of winter wheat by subtracting soil respiration from soil with and without exclusion of roots. Catch crop roots accounted for more than 40 % of total plant C. For spring barley in 2008 and spring wheat in 2010, root C was higher in the organic than in the inorganic fertilizer-based systems. However, for winter wheat in 2008 and spring barley in 2010, there were similar amounts of root C across systems. The measurements of topsoil root-derived respiration also showed no difference across systems, despite large differences in harvested cereal yields. Cereal biomass shoot-to-root (S/R) ratio was higher (31–131 %) in inorganic than in organic fertilizer-based systems. Our findings show that macro-roots of both cereal crops and catch crops play a relatively larger role in organically managed systems than in mineral fertilizer based systems; and that the use of fixed biomass S/R ratios to estimate root biomass leads to erroneous estimates of root C input.

Root carbon flow from an invasive plant to belowground foodwebs

Aims Soil foodwebs are based on plant production. This production enters belowground foodwebs via numerous pathways, with root pathways likely dominating supply. Indeed, root exudation may fuel 30–50 % of belowground activity with photosynthate fixed only hours earlier. Yet we have limited knowledge of root fluxes of recent-photosynthate from invasive plants to belowground foodwebs.

Root carbon and protein metabolism associated with heat tolerance

Extensive past efforts have been taken toward understanding heat tolerance mechanisms of the aboveground organs. Root systems play critical roles in whole-plant adaptation to heat stress, but are less studied. This review discusses recent research results revealing some critical physiological and metabolic factors underlying root thermotolerance, with a focus on temperate perennial grass species. Comparative analysis of differential root responses to supraoptimal temperatures by a heat-adapted temperate C3 species, Agrostis scabra, which can survive high soil temperatures up to 45 °C in geothermal areas in Yellow Stone National Park, and a heat-sensitive cogeneric species, Agrostis stolonifera, suggested that efficient carbon and protein metabolism is critical for root thermotolerance. Superior root thermotolerance in a perennial grass was associated with a greater capacity to control respiratory costs through respiratory acclimation, lowering carbon investment in maintenance for protein turnover, and efficiently partitioning carbon into different metabolic pools and alternative respiration pathways. Proteomic analysis demonstrated that root thermotolerance was associated with an increased maintenance of stability and less degradation of proteins, particularly those important for metabolism and energy production. In addition, thermotolerant roots are better able to maintain growth and activity during heat stress by activating stress defence proteins such as those participating in antioxidant defence (i.e. superoxide dismutase, peroxidase, glutathione S-transferase) and chaperoning protection (i.e. heat shock protein).

Carbon inputs by wheat and vetch roots to an irrigated Vertosol

Research on the amounts of carbon that can be added to Vertosols of New South Wales and Queensland by crop roots in irrigated cotton farming systems is sparse. The objective of this study was to determine the amounts of carbon added to soil by roots of wheat (Triticum aestivum L.) and purple vetch (Vicia benghalensis L.) sown in rotation with irrigated cotton (Gossypium hirsutum L.). Measurements were made from 2008 to 2010 in an ongoing experiment near Narrabri, northern NSW, using a combination of soil cores and minirhizotron observations. The experimental treatments were: cotton monoculture; cotton–vetch (CV); cotton–wheat, in which wheat stubble was incorporated into the beds with a disc-hiller (CW); and cotton–wheat–vetch, in which wheat stubble was retained as in-situ mulch (CWV). Vetch was killed by a combination of mowing and contact herbicides, and the residues were retained as in situ mulch. Root length per unit area of vetch in CWV and wheat in both CW and CWV was comparable, although wheat had a higher concentration of roots in surface 0.10 m. Root growth of the CV treatment was sparse. Root carbon available for addition to soil was greater with vetch than with wheat and was in the order: vetch in CWV (5.1 t C/ha.year) &gt; vetch in CV (1.9 t C/ha.year) &gt; wheat in CW (1.6 t C/ha.year) = wheat in CWV (1.7 t C/ha.year). Intra-seasonal root mortality accounted for 12% of total root carbon in vetch and 36% in wheat. The remaining fraction consisted of carbon in the root mass at the end of the growing season. Carbon sequestered by root inputs of the rotation crops was estimated to be ~0.34 t C/ha.year for the vetch and wheat crops in the CWV rotation, 0.10 t C/ha.year for vetch in CV, and 0.08 t C/ha.year for wheat in CW. Rotation CWV was, therefore, the most effective in sequestering carbon from roots.