Reviewers: Hansen K4,5, Lee H6
Measurement unit: mass loss %, nutrient loss %; Measurement scale: root segments of individual plants; Equipment costs: €; Running costs: €; Installation effort: medium to high; Maintenance effort: low; Knowledge need: low; Measurement mode: manual (and data loggers)
In many ecosystems, roots are one of the dominant carbon and nutrient inputs into the soil. It is estimated that roots can comprise between one and three quarters of total plant input to the soil (Robinson, 2007; Freschet et al., 2013). Roots also directly interact with the soil and can start to decompose within the protection of soil aggregates and fine pores, serving to stablise soil organic matter and sequester C (Jones & Donnelly, 2004; Schmidt et al., 2011): an important contribution to offsetting greenhouse gas emissions (Marian et al., 2017). Generally, roots are more recalcitrant than leaf litter due to higher concentrations of lignin and suberin and decompose on average 1.5 to 2.8 times slower (Silver & Miya, 2001; Rasse et al., 2005; Freschet et al., 2013). Rates of root decomposition parallel those of leaf litter decomposition, and are affected by, amongst others, climate, litter quality, edaphic conditions, and the soil microbial community (See et al., 2019). Nevertheless, due to possible differences between aboveground plant parts and roots, it is necessary to study roots as a separate component to gain a more complete understanding of processes influencing plant litter decomposition. Moreover, unlike leaves, roots are buffered from changes in surface temperatures and other disturbances (e.g. erosion, herbivory, fire). Decompsosing roots belowground may therefore respond differently from surface leaf litter to global drivers such as climate and land-use change.
Studying root decomposition presents several challenges. Most notably, any extraction of roots from the soil to determine decomposition radically alters the rhizosphere interaction that could have otherwise influenced the decomposition process. This differs from studying leaf litter decomposition where material can be collected and returned to the soil surface with minimal disturbance. The challenge of disturbing roots to understand decomposition and other belowground processes has driven the development of techniques to study roots in situ. These techniques include rhizotrons, isotope labelling, x-ray tomography, and DNA-based probing (see Bardgett et al., 2014). However, many of these techniques are expensive, so the traditional approach of collecting root litter remains the predominant form through which root litter decomposition is studied. Root litter decomposition studies have been used in several climate-change experiments from in situ warming experiments (Liu et al., 2017) to space-for-time subsitutions across climatic latitudinal and elevational gradients (Marian et al., 2017). Nevertheless, root litter approaches outlined in this chapter can equally be used to investigate other major drivers of root decomposition such as land-use change (Smith et al., 2014).
22.214.171.124 What and how to measure?
Similar to aboveground litter outlined in the protocol 126.96.36.199 in the Foliar litter decomposition, the litterbag approach is quick and cheap and facilitates either a large spatial or temporal (e.g. many harvests) scale study. However, root decomposition studies usually use site-specific or pot/common garden grown litter rather than standardised litter types that can be used in leaf litter studies. This is because root-litter quality can vary by several orders of magnitude more than leaf litter (Freschet et al., 2017). Growing in the soil, roots have high plasticity at a micro spatial and temporal scale that can lead to large variations in physiochemistry and morphology across the root network of an individual plant and across species within a plant community. For example, many species have roots colonised by mycorrhiza and the extent of this colonisation has been shown to accelerate root litter decomposition (Hodge et al., 2001; Langley et al., 2006) or to slow down the decomposition of colonised root tips (Goebel et al., 2011). Therefore, any root decomposition must provide as much information as possible regarding the source and type of root litter used.
Roots for litterbags can either be collected from (semi-)natural communities, common gardens, or pots. Ideally, the environmental conditions for rearing roots in pots or common gardens should aim to emulate as far as possible the conditions experienced by the natural community (e.g. soil type and texture, water regime, temperature, mycorrhiza, soil herbivory). Nevertheless, reared roots are typically “spoilt”. Roots in pots are grown in soil with low compaction, minimal herbivory, and typically have younger root systems, higher specific root areas, and are enriched in nutrients compared to roots from natural communities (Freschet et al., 2017). Results from decomposition studies using pot-grown roots therefore should be interpreted with caution.
When harvesting root material from natural and (semi-)natural communities, it is necessary to document plant species, timing of the year (e.g. period of peak root death in winter or dry seasons), soil depth, and management practices. Root material should be collected from multiple stands of vegetation to capture variation in root litter quality. Unless working with monocultures or underneath a single tree species without ground flora, it is necessary to extract a soil turf where the roots can be traced back to the intended plant species. In grassland and shrubland ecosystems, root production is usually concentrated in the upper 10–50 cm of the soil profile, therefore a soil turf of the correct depth should be extracted from which to harvest root material. Given limited evidence of nutrient resorption following root senescence, it is generally viewed as acceptable to use a mixture of living and dead roots to investigate root decomposition (Aerts, 1990; Silver & Miya, 2001; Freschet et al., 2013).
After harvesting a plant–soil turf or soil, roots can be washed free of any adhering soil. This is best achieved using a series of fine sieves with the finest sieve around 500 mm (0.5 mm) to collect the majority of visible roots (Livesley et al., 1999).
The collected root material can be sorted into desired functionality for the aims of the decomposition study. The majority of studies primarily focus on decomposition of fine-roots as these are believed to exhibit the fastest C turnover as well as being responsible for plant nutrient acquisition. Fine-roots are defined either by following an arbitrary diameter threshold (most commonly fine roots are defined as < 2 mm in diameter), or according to their branching position (i.e. the first-three root orders), following their function, absorptive, and transport roots (Goebel et al., 2011; McCormack et al., 2015; Freschet & Roumet, 2017). Selection of fine-roots will exclude rhizomes and taproots. Equally, selection of roots of the first-three orders will exclude the main anchoring root system. It is worth noting that when studying herbaceous roots with < 2 mm diameter cut-off, the majority of the root system is likely to be < 1mm in diameter. Therefore, providing information on root litter diameter is useful for interpretation of the results, especially with studies based on functional criteria still in their infancy.
Once sorted, root material can be pooled and oven-dried between 60 and 70 °C for 48 h to a constant weight. After drying, material can be coarsely chopped (i.e. into 2 to 3 cm sections) for thorough homogenisation. Homogeneity is important when collecting roots from a whole plant and/or from multiple stands across a community or landscape. Unless specified as part of the study aims, this pooling is important to ensure there are no biases in root properties, for example, in relation to collection depth or edaphic conditions.
Root decomposition studies typically use finer mesh sizes than leaf litter studies to exclude the ingrowth of new roots into litterbags. The smallest root diameter is around 50 µm (0.05 mm); however, mesh sizes used in root decomposition studies can range from 0.05 mm to 5 mm (Silver & Miya, 2001; Freschet et al., 2013). To exclude the ingrowth of new roots it unfortunately often requires the exclusion of most soil micro- and meso-fauna. Given our experience for short term studies (i.e. months to a year) fresh root ingrowth may be minimal, sporadic or visible (i.e. white roots) allowing the use of larger mesh sizes that permit soil fauna access (Smith et al. 2019). An alternative approach, typically used in tropical grasslands, is to place root litter into buried pots that are covered with mesh and inverted to prevent root ingrowth from plants on the soil surface but can facilitate entry of micro- and meso-fauna. However, Silver & Miya (2001) found that the buried pot approach yielded significantly lower root decay rates, presumably from the greater exclusion of living roots and rhizo-depositon that stimulates the microbial decomposition process. Other approaches of studying root decomposition in situ such as root trenching and tethering roots are also discussed by Silver & Miya (2001).
Root tissue densities are often one to two orders of magnitude lower than leaf and stem material (Kramer-Walter et al., 2016). Therefore, more root material is often required to fill a litterbag. At the same time, root litter decomposes more slowly than leaf and stem litter; hence, depending on the length of study, more root litter is likely to remain in the bag at the end of any incubation period of similar duration to most leaf litter studies (e.g. one to two years). For example, in the majority of temperate grassland ecosystems with litterbags containing 0.5 g root litter, over half of the material is still likely to be present after a year of incubation. The amount of litter used in root litter decomposition studies is therefore typically lower than quantities used in leaf litter studies: anywhere between 0.2 g and 2 g of root litter (Silver & Miya, 2001; Freschet et al., 2013). Given a smaller quantity of root litter, weighing needs to be more precise, at a mimumum ± 0.001 g.
After burying the root bag in soil over a given incubation period (see next section), root litter can be extracted from litterbags, dried, and mass loss calculated as the difference between the initial dry mass and the final dry mass after incubation, similar to aboveground litter (see protocol 2.2.6 Litter decomposition). Because root litter is buried in the soil, it is more likely to be contaminated with soil, resulting in the remaining weight being higher than the initial weight. Thus, ash free corrected weight loss (i.e. the mineral residue after removal of organic matter by ignition) needs to be calculated (see protocol 2.2.6 Litter decomposition). In brief, heating undecomposed or decomposed litter in a muffle oven over 500 °C for 4 to 16 h to obtain the ash-free weight (Smith et al. 2019). Root material can also be analysed for loss of carbon and nutrients, namely total nitrogen and phosphorus, using an elemental analyser or digestion methods followed by spectrophotometry.
Installation, field operation, maintenance, interpretation
Similar to leaf litter decomposition studies, root litter should be buried underneath the same species and vegetation type as litter collection. Variation in other biological and environmental factors should be minimised in the placement of root litter. Root litter is typically buried in the soil at the main zone of root production, for example, between 5 and 20 cm below the surface in both grassland and forest sites. Samples should preferably be installed in straight lines in order to capture the same topography and facilitate the planned sequential retrieval of litterbags if carrying out multiple harvests. Once the bags have been installed, they should be marked. As they are being buried, it is ideal to tether the bag with nylon string to a metal pipe or other metallic object, for example, bottle tops. Installer’s name, burial date, and GPS location of the site of litterbag placement should be recorded: a picture of the area or sketch can also be used to aid retrieval of the bags. The precise burial location can be found using a metal detector if they have been tethered to a metal object. Belowground decomposition processes may be buffered by the soil from changes and fluctuations in surface and air temperatures that potentially influence aboveground decomposition. Therefore, root-scale micro-climate should be accounted for when conducting climate change experiments. It is thus useful to bury micro-climatic sensors (temperature, soil moisture) with litterbags where possible. Alternatively, spot-measurements can be made with hand-held probes at regular intervals during the incubation period to provide information on the micro-climate.
At each sampling point, it is advised to collect a minimum of five litterbags per litter type and per site. Care should be taken when extracting buried litterbags to avoid snagging on stones, rocks, or new root growth surrounding the bag. Any holes, tears, root, and fungal growth surrounding, or in, the litterbag should be noted in the field if possible. As much soil as possible should be removed from the surface of the litterbag. Each litterbag should be placed in a separate bag and labelled. Litter should be dried at either 60 °C or 70 °C for 48 h, then the root litter removed from the mesh and root litter weight noted. Collected litter should be stored in labelled paper bags for further chemical analyses.
Where to start
Hobbie et al. (2010), Iversen et al. (2017), Löhmus & Ivask (1995), Silver & Miya (2001), Smith et al. (2014), Smith et al. (2019)
188.8.131.52 Special cases, emerging issues, and challenges
Root decomposition in anaerobic soils
Under anaerobic conditions, root litter-mass loss may be negatively correlated with increasing soil moisture. There is evidence in wetland ecosystems with organic soils that root litter may gain mass during incubation due to deposition of metallic ions, for example, the formation of iron plaques (see Weiss et al., 2005). Geochemical or Fourier-Transform Infrared (FTIR) spectroscopy can be used to correct for metal content of litter and adjust mass loss accordingly.
Global need for more root trait data
Root traits are increasingly being recognised as an important predictor of rates of decomposition (Freshcet et al., 2013; also see protocol 4.16 Functional traits). However, root trait data is lacking in comparison to leaf trait data. Currently, root trait and fine-root trait data represent only 7% and 0.03% of data, respectively, in the global trait database TRY (Kattge et al., 2011). Root data are particularly lacking from Siberia, the Middle East, north and central Africa, and Central America (Freschet et al., 2017). More root data are necessary to understand better the key traits that govern root litter decomposition. There are several morphological, chemical, and physiological root traits that can be measured as part of a root decomposition study (see Bardgett et al., 2014; Freschet et al., 2017; See et al., 2019), although the number of traits considered will add to the cost of the study.
Quantifying root litter decomposition in situ
Radioactive measurements have been used to estimate root age, turnover, and root C residence time (Staddon, 2004; Lukac, 2012). In order to carry out isotopic analysis, roots need to be extracted from the soil and thus have similar caveats to root litter decomposition studies. For example, Strand et al. (2008) demonstrated that poor sampling of shorter-lived smaller roots from a CO2 enriched pine plantation resulted in an over-estimation of the longevity of root C. To date, only a few studies have combined isotope labelling and litter decomposition to address rates of root C mineralisation and stabilisation into soil organic matter. Personeni & Loiseau (2004) demonstrated the use of the technique when decomposing 13C-labelled root litter underneath pot-grown living plants, yet this approach needs investigating in situ underneath natural communities.
Bardgett et al. (2014), Freschet & Roumet (2017), Freschet et al. (2017), Goebel et al. (2011), Laughlin (2016), McCormack et al. (2015), See et al. (2019)
More on methods and existing protocols
Global reviews on root decomposition across biomes, climates, soil types, and considering various methodologies include the Fine-Root Ecology Database (FRED: http://roots.ornl.gov), Iversen et al. (2017), and Silver & Miya (2001)
Aerts, R. (1990). Nutrient use efficiency in evergreen and deciduous species from heathlands. Oecologia, 84(3), 391-397.
Bardgett, R. D., Mommer, L., & de Vries, F. T. (2014). Going underground: root traits as drivers of ecosystem processes. Trends in Ecology & Evolution, 29(12), 692-699.
Fine-Root Ecology Database (FRED), http://roots.ornl.gov), Oak Ridge National Laboratory, USA, last accessed 15th February 2018.
Freschet, G. T., & Roumet, C. (2017), Sampling roots to capture plant and soil functions. Functional Ecology, 31(8), 1506-1518.
Freschet, G. T., Cornwell, W. K., Wardle, D. A., Elumeeva, T. G., Liu, W., Jackson, B. G., … Cornelissen, J. H. C. (2013) Linking litter decomposition of above- and below-ground organs to plant–soil feedbacks worldwide. Journal of Ecology, 101(4), 943-952.
Freschet, G. T., Valverde-Barrantes, O. J., Tucker, C. M., Craine, J. M., McCormack, L. M., Violle, C., … Roumet, C. (2017). Climate, soil and plant functional types as drivers of global fine-root trait variation. Journal of Ecology, 105(5), 1182-1196.
Goebel, M., Hobbie, S. E., Bulaj, B., Zadworny, M., Archibald, D. D., Oleksyn, J., … Eissenstat, D. M. (2011). Decomposition of the finest root branching orders: linking below-ground dynamics to fine-root function and structure. Ecological Monographs, 81(1), 89-102.
Hobbie, S. E., Oleksyn, J., Eissenstat, D. M., & Reich, P. B. (2010). Fine root decomposition rates do not mirror those of leaf litter among temperate tree species. Oecologia, 162(2), 505-513.
Hodge A., Campbell, C. D., & Fitter, A. H. (2001). An arbuscular mycorrhizal fungus accelerates decomposition and acquires nitrogen directly from organic material. Nature, 413(6853), 297-299.
Iversen, C. M., McCormack, M. L., Powell, A. S., Blackwood, C. B., Freschet, G. T., Kattge, J., … Violle, C. (2017). A global Fine-Root Ecology Database to address below-ground challenges in plant ecology. New Phytologist, 215(1), 15-26.
Jones, M. B., & Donnelly, A. (2004). Carbon sequestration in temperate grassland ecosystems and the influence of management, climate and elevated CO2. New Phytologist, 164(3), 423-439.
Kattge, J., Diaz, S., Lavorel, S., Prentice, I. C., Leadley, P., Bönisch, G., … Wirth, C. (2011). TRY–a global database of plant traits. Global Change Biology, 17(9), 2905-2935.
Kramer-Walter, K. R., Bellingham, P. J., Millar, T. R., Smissen, R. D., Richardson, S. J., & Laughlin, D. C. (2016). Root traits are multidimensional: specific root length is independent from root tissue density and the plant economic spectrum. Journal of Ecology, 104(5), 1299-1310.
Langley, J. A., Chapman, S. K., & Hungate, B. A. (2006). Ectomycorrhizal colonization slows root decomposition: the post-mortem legacy effect. Ecology Letters, 9(8), 955-959.
Liu, Y., Liu, S., Wan, S., Wang, J., Wang, H., & Liu, K. (2017). Effects of experimental throughfall reduction and soil warming on fine root biomass and its decomposition in a warm temperate oak forest. Science of the Total Environment, 574, 1448-1455.
Livesley, S. J., Stacey, C. L., Gregory, P. J., & Buresh, R. J. (1999). Sieve size effects on root length and biomass measurements of maize (Zea mays) and Grevillea robusta. Plant and Soil, 207(2), 183–193.
Löhmus, K., & Ivask, M. (1995). Decomposition and nitrogen dynamics of fine roots of Norway spruce (Picea abies (L.) Karst.) at different sites. Plant and Soil, 168-169, 89-94.
Lukac, M. (2012). Fine root turnover. In S. Mancusco (Ed.), Measuring Roots (pp. 363-373). Berlin: Springer.
Marian, F., Sandmann, D., Krashevska, V., Maraun, M., & Scheu, S. (2017). Leaf and root litter decomposition is discontinued at high altitude tropical montane rainforests contributing to carbon sequestration. Ecology and Evolution, 7(16), 6432-6443.
McCormack, M. L., Dickie, I. A., Eissenstat, D. M., Fahey, T. J., Fernandez, C. W., Guo, D., … Zadworny, M. (2015). Redefining fine roots improves understanding of below-ground contributions to terrestrial biosphere processes. New Phytologist, 207(3), 505-518.
Personeni, E., & Loiseau, P. (2004). How does the nature of living and dead roots affect the residence time of carbon in the root litter continuum? Plant and Soil, 267(1), 129-141.
Rasse, D. P., Rumpel, C., & Dignac, M. (2005). Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation. Plant and Soil, 269(1-2), 341-356.
Robinson, D. (2007). Implications of a large global root biomass for carbon sink estimates and for soil carbon dynamics. Proceedings of the Royal Society B: Biological Sciences, 274(1626), 2753-2759.
Schmidt, M. W. I., Torn, M. S., Abiven, S., Dittmar, T., Guggenberger, G., Janssens, I. A., … Trumbore, S. E. (2011). Persistence of soil organic matter as an ecosystem property. Nature, 478(7367), 49-56.
See, C. R., McCormack, M. L., Hobbie, S. E., Flores-Moreno, H., Silver, L. W., & Kennedy, P.G. (2019). Global patterns in fine root decomposition: climate, chemistry, mycorrhizal association and woodiness. Ecology Letters, 22(6), 946-953.
Silver, W. L., & Miya, R. K. (2001). Global patterns in root decomposition: Comparisons of climate and litter quality effects. Oecologia, 129(3), 407-419.
Smith, S. W., Woodin, S. J., Pakeman, R. J., Johnson, D., & Wal, R. (2014). Root traits predict decomposition across a landscape‐scale grazing experiment. New Phytologist, 203(3), 851-862.
Smith, S. W., Speed, J. D. M., Bukombe, J. Hassan, S. N., Lyamuya, R., Mtweve, P. J., Sundsdal, A., & Graae, B. J. (2019) Litter type and termites regulate root decomposition across contrasting savannah land-uses. Oikos, 128(4), 596-607.
Staddon, P. L. (2004). Carbon isotopes in functional soil ecology. Trends in Ecology & Evolution, 19(3), 148-154.
Strand, A. E., Pritchard, S. G., McCormack, M. L., Davis, M. A., & Oren, R. (2008). Irreconcilable differences: fine-root life spans and soil carbon persistence. Science, 319(5862): 456-458.
Weiss, J. V., Emerson, D., & Megonigal, J. P. (2005). Rhizosphere iron (III) deposition and reduction in a L.-dominated wetland. Soil Science Society of America Journal, 69(6), 1861-1870.
Authors: Smith, SW1, Taghizadeh-Toosi, A2, Ostonen, I3
Reviewers: Hansen K4,5, Lee H6
1 Department of Biology, Norwegian University of Science and Technology, Trondheim, Norway
2 Department of Agroecology, Aarhus University, Tjele, Denmark
3 Institute of Ecology and Earth Sciences, University of Tartu, Estonia
4 Swedish Environmental Protection Agency, Stockholm, Sweden
5 Swedish Environmental Research Institute IVL, Stockholm, Sweden
6 NORCE Norwegian Research Centre and Bjerknes Centre for Climate Research, Bergen, Norway