Reviewer: Almagro M4
Measurement unit: Mg C ha-1 or kg C m-2; Measurement scale: plot; Equipment costs: €; Running costs: €; Installation effort: medium; Maintenance effort: -; Knowledge need: medium; Measurement mode: manual
The soil carbon (C) stock is the amount of C stored in the soil (Davidson & Janssens, 2006) and is the product of the long-term balance between C inputs from aboveground and belowground net productivity and C outputs from decomposition and erosion (Kirschbaum, 1995; Melillo et al., 2002). Soil C stocks are sensitive to climate change, because virtually all processes that regulate C inputs and outputs are driven by climate-related factors (such as temperature, CO2 concentration, rainfall, moisture and nutrient availability) (Tian et al., 2015). Factors other than climate that strongly influence soil C stocks are land-use (Guo & Gifford, 2002) and disturbances, such as erosion (Panagos et al., 2015), volcanic eruptions (Gísladóttir et al., 2010), fire, and human and animal activities (Laurel & Wohl, 2019). The sensitivity of soil C stocks to climatic changes implies that the C sink or source capacity of soils can drastically shift under altering climatic conditions (Kirschbaum, 1995; Melillo et al., 2002; Davidson & Janssens, 2006). Since soils store globally three times more C than the atmosphere (Chapin et al., 2011), these shifts could, in turn, induce powerful feedbacks to the climate system (Kirschbaum, 1995; Melillo et al., 2002; Davidson & Janssens, 2006; Scharlemann et al., 2014; Tian et al., 2015). This is why a detailed quantification of the current global soil C stocks and potential shifts under land-use and climate change are important.
The soil nutrient stock is the amount of nutrients stored in the soil (Chapin et al., 2011), with a focus on nitrogen (N), phosphorus (P), and potassium (K), as these elements are most frequently limiting for plant growth (Sterner & Elser, 2002). Soil nutrient stocks have strong associations with soil C stocks, as nutrients and C are tightly coupled in soil organic matter and are affected by the same processes (Hobbie et al., 2002). Furthermore, plant nutrient availability, which is often linked to soil nutrient stocks (Saynes et al., 2005), has a strong impact on plant growth and litter quality and thus indirectly on the soil C balance (Li et al., 2011).
22.214.171.124 What and how to measure?
Soil C and nutrient stocks are commonly assessed through destructive soil sampling and subsequent determination of the C and nutrient concentration in the soil (excluding particles > 2 mm) (Jobbágy & Jackson, 2000). The sampling is generally performed using manual soil augers. These can differ in type, depending on the soil texture, but have in common that the exact core volume must be known. Augers that strongly disturb the soil volume are thus not suitable for this measurement.
Measurements of soil C are mostly done by dry combustion (Matejovic, 1997; Senesi & Senesi, 2016). Other techniques have also been used in the past (most commonly the Walkley‐Black method (wet acidified dichromate oxidation) or loss on ignition), and conversion factors for comparison can be found in Soon & Abboud (1991). A new, less laborious method for laboratory and field determination of soil C, i.e. laser-induced breakdown spectroscopy (LIBS), is currently being developed (Senesi & Senesi, 2016). Methods used for soil nutrient determination depend on the nutrient under investigation. Nitrogen is generally measured together with C using dry combustion (Matejovic, 1997). The other most commonly measured nutrients (P and K) are measured by acid destruction with H2SO4, salicylic acid, H2O2, and Se (Wallinga et al., 1989). Other important elements (Ca, K, Mg, Na, Mn) can be determined with the same method.
Installation, field operation, maintenance, interpretation
At the sampling spot, the vegetation and the superficial litter layer are removed down to the topsoil layer (Vadeboncoeur et al., 2012). Typically, one core is taken per plot, but in heterogeneous environments it is more desirable to collect a number of smaller cores that are subsequently pooled before analyses and provide a more accurate characterisation of the plot. The latter technique (with more but smaller volumes) yields larger uncertainty in the core volume, and thus in the transformed C and nutrient content per unit area measurements.
To determine the soil carbon stock per unit area, the entire soil depth needs to be sampled. A core is taken down to the bedrock and is split up into different depth layers to obtain information about the depth profile of the stocks (Maillard et al., 2017). In general, a subdivision is made between topsoil (0–10 cm depth) and subsoil (> 10 cm depth), but in some cases, it might be important to use a finer scale. Alternatively, soil horizons are used as subdivisions instead of fixed soil depth (Maaroufi et al., 2015). For more information on the importance of sampling design, the spatial scale of the sampling, and sampling depth, one can consult Maillard et al. (2017) and Allen et al. (2010).
Soil samples are sieved at 2 mm to exclude stones and roots and dry weight is determined (optimally the samples are dried at a maximum of 40 °C to avoid volatilisation of N). Subsequently, the C and nutrient content of finely ground aliquots is determined (Matejovic, 1997). Finally, the C and nutrient stocks per unit area can then be calculated by applying the following equation:
where S is the C or nutrient stock over the entire soil depth, which is split into n layers. Ci is the C or nutrient concentration for layer i (expressed in%), DWi is the dry weight of the core taken from layer i, and A is the surface area of the core.
Alternatively, the C and nutrient stock can be expressed per volume instead of per area. This is necessary when it is not desirable or possible to sample the entire soil depth. In that case, the C or nutrient stock of the soil layer of interest (Si) is calculated using the volume of the sampled core (Vi):
Where to start
Davidson & Janssens (2006), Jobbágy & Jackson (2000), Maillard et al. (2017), Melillo et al. (2002), Senesi & Senesi (2016)
126.96.36.199 Special cases, emerging issues, and challenges
When calculating the C and nutrient content in stony soils, the stone volume (stones > 2 mm; Novák et al., 2011) of the soil should be taken into account, as the volume occupied by stone is assumed not to contain a significant amount of C (Rytter, 2012). The correction is generally made with the ‘quantitative pit method’ (Vadeboncoeur et al., 2012), where the rock fragment density is estimated by digging a pit with dimensions large enough to obtain a representative sample of the soil (< 2 mm) and stone (> 2 mm) volume. Subsequently, the volume of the pit and stones are measured. For the pit volume, the pit is lined with plastic and the volume of water necessary to fill it up to the soil surface is tracked (Vadeboncoeur et al., 2012; Mehler et al., 2014; Beem-Miller et al., 2016). The volume of stones is estimated by the water displacement method or by using a hydrostatic scale (Mehler et al., 2014).
In cases when the quantitative pit method is unsuitable (e.g. due to its destructive, labour-intensive or costly nature), other, but less accurate methods can be used. The most common alternatives are the hammer, hydraulic push, and rotary coring methods (Beem-Miller et al., 2016).
In soils containing a substantial amount of large roots, the same technique can be used as in stony soils (see above) to exclude the root volume from the soil volume (Vadeboncoeur et al., 2012; Mehler et al., 2014). In soils with small roots (e.g. grasslands), the root volume is generally neglected.
Soils on calcareous bedrock contain inorganic C in the form of carbonates (e.g. calcite or dolomite). Methods assessing soil C, such as dry combustion or wet oxidation, provide a bulk measure of total soil C consisting of organic and inorganic C components. In most climate-manipulation experiments the primary interest is on how organic C is affected. Therefore, in order to assess effects on organic C, inorganic soil C content needs to be subtracted from total soil C. Under specific conditions, climate manipulations could not only affect organic C, but also directly affect the dissolution or formation of soil carbonates (e.g. their pedogenic formation due to precipitation manipulation in arid or semi-arid environments).
Carbonate contents (and consequently their share in organic C) can be assessed in several ways. Most approaches are based on the comparison of a soil sample containing organic + inorganic C (= total soil C) with a sample free of inorganic C. The technically easiest way of assessing the carbonate content is to treat soil samples with HCl and then conduct a volumetric determination of the released CO2 (Nelson & Sommers, 1982). This gasometric approach is called the ‘Scheibler’ or calcimeter method. The calcimeter method is straightforward, albeit time and labour-consuming, because only a limited number of samples can be treated simultaneously. To assess the carbonate content of a higher number of samples simultaneously, acid washing (e.g. Midwood & Boutton, 1998; Schnecker et al., 2016) or acid fumigation (Harris et al., 2001; Walthert et al., 2010) can be applied to remove carbonates from the samples. These techniques have the advantage that the soil samples can also be used for further analyses (e.g. for isotope measurements). While acid washing can lead to loss of acid-soluble organic C, acid fumigation has been successfully used with calcite (Harris et al., 2001) and even dolomite (Walthert et al., 2010) without solute C loss. Beside these methods, carbonate content can also be assessed non-destructively by FT-IR spectroscopy (Tatzber et al., 2007) or near-infrared analysis (NIRA) (Ben-Dor & Banin, 1990). Such methods are, however, comparatively sophisticated and may be applied only when a detailed characterisation of organic C is foreseen anyway. The thermal gradient (ThG) method has been shown to perform well, especially with regard to the assessment of dolomite contents (Vuong et al., 2016).
Theory, significance, and large datasets
Davidson & Janssens (2006), Kirschbaum (1995), Melillo et al. (2002), Scharlemann et al. (2014), Tian et al. (2015)
More on methods and existing protocols
Allen et al. (2010), Jobbágy & Jackson, (2000), Maillard et al. (2017), Senesi & Senesi (2016); Vadeboncoeur et al. (2012)
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Authors: Leblans NIW1, Stuart-Haëntjens E2, Schindlbacher A3, Vicca S1
Reviewer: Almagro M4
1 Centre of Excellence PLECO (Plants and Ecosystems), Biology Department, University of Antwerp, Wilrijk, Belgium
2 Climate Impacts Research Centre, Department of Ecology and Environmental Science, Umeå University, Abisko, Sweden
2 Department of Biology, Virginia Commonwealth University, Richmond, USA
3 Department of Forest Ecology and Soils, Federal Research and Training Centre for Forests, Natural Hazards and Landscape, Vienna, Austria
4 BC3-Basque Centre for Climate Change, University of the Basque Country, Leioa, Spain