Understanding and quantifying whole soil-profile organic carbon transfer using an environmental tracer

Illogan

At the Illogan property, two sites were examined. First, a site with red basaltic clays (Euchrozems, upon which the ‘Dog Paddock’ site is located) and, second, a site with black earths (Vertosols, upon which the ‘Shed Paddock’ site is located) (Fig. 2). The slope (1%), aspect and elevation of each site were similar. The Dog Paddock pit was excavated 10 m to the east of a permanently mounted weather and soil moisture monitoring station (‘K1’, see Rüdiger et al. 2007). The Shed Paddock site was established ~1.5 km to the west of the Dog Paddock site. There was no evidence at either site of erosion/deposition or any disturbance other than that produced by cultivation practices associated with minimum tillage cereal and grain production. Cracking of the soil has been observed on many occasions at the Shed Paddock site during dry periods. No cracking has been observed in Dog Paddock. Further site details can be found in Wells et al. (2013).

Roscommon

The Roscommon site (K2) was located on sandy loam (Solodic) soil and comprised improved pasture and has been used for grazing (Fig. 2). There has been no cropping at the site. No soil cracking has been observed at this site. The property has been owned by the same family for over 50 years with a consistent grazing management programme.

Pembroke South

Pembroke South (K3) had Vertosol soil and had areas of both cropping and grazing (Fig. 2). Two sites were examined: one that had been cropped (Pembroke South Cropping) and an adjacent site in the same paddock (100 m to the north) adjacent to the K3 soil moisture monitoring site (Rüdiger et al. 2007) that had not been cropped and managed as pasture and grazing (Pembroke South Grazing). Large cracks have been observed at both sites here.

Pembroke North

Pembroke North (K4) had Vertosol soil and had been managed for pasture and never cropped (Fig. 2). Cracks have been observed at this site.

Site summary

All sites had low slope of 1–3% and approximately the same rainfall. All sites were well away from trees and any farm infrastructure. In summary, the sites were a mix of both pasture (grazing) and cropped sites. All sites were considered to have deep soils (depths over 1 m) and good fertility and agricultural productivity (Table 1). The Illogan site allowed an assessment of two different soils on the same property that were both managed for cropping in the same way. While K3 (Pembroke South), allowed an assessment of the same soil under both cropping and pasture.

A summary of the sites is provided in Table 1.

Soil sampling

At all sites a backhoe was used to cut trenches down to bedrock. The pits were 3–4 m long and approximately 1 m wide to allow access. Soil samples were collected by inserting steel cores into the site wall of each trench in a continual sequence down the soil profile. Each vertical profile was sampled using 200 mm long by 65 mm or 200 mm by 100 mm internal diameter cores, which were hammered into the side wall of the trench at regular intervals in a vertical line from the soil surface down to bedrock. Less deep cores (i.e. 50 and 150 mm) were collected at the surface. For insertion a steel cap or dolly was placed on top of the core. No lubricants were used on the cores when inserted.

Laboratory analysis

After collection, soil samples were immediately bagged and transported to the University of Newcastle soils laboratory, where they were weighed and then dried at 40°C for 7–10 days. The dried samples were gently disaggregated with a mortar and pestle and passed through a 2 mm sieve and the coarse (>2 mm) fraction separated. The <2 mm fraction was further disaggregated with a mortar and pestle and weighed. The bulk density of each sample was calculated using the dried soil mass and volume of each core.

The <2 mm soil fraction was ground in a mill to a fine powder and the total carbon and nitrogen content measured using a LECO 2000 analyser (LECO method, www.leco.com). The SOC is reported here in units of mass (t ha⁻¹) using the bulk density of the soil determined from the sample core and depth of the sample core. The hydrometer method outlined by Smith and Atkinson (1975) was employed to establish the particle size distribution of the <2 mm soil fraction. Bulk density of each sample was determined based on dry mass of each sample and core volume. The pH was determined using a 1:5 solution with distilled water using a Eutech pH instrument (Rayment and Higginson 1992).

¹³⁷Cs analysis

Dried soil samples of approximately 1000 g (<2 mm fraction) were placed in Marinelli containers and gamma emissions were counted on a hyper-pure germanium spectrometer (GEM Series HPGe Coaxial Detector System) over a period of 24 h with counting errors of approximately ±10% or less achieved. The Loughran (1994) method was utilised to determine the concentration of ¹³⁷Cs in the soil samples, which were then converted to areal activity (Loughran et al. 2002).

Statistical analysis

Assessments were conducted using linear regression and Student t-tests with KaleidaGraph software (www.synergy.com) and GraphPad Prism (https://www.graphpad.com). Significance was P < 0.05 unless stated otherwise, where P ≤ 0.05 means that the test hypothesis is false or should be rejected. All figures were created using the KaleidaGraph program.

Soil properties

Observation of the trench wall of the Vertosol sites showed a small number of fine roots from the present and past cereal crops at depths greater than 1 m. However, the majority of roots were concentrated in the near surface. There were also well-defined vertical lines of darker material, suggestive of either decayed plant roots or organic rich surface material that had been transported down cracks. No texture difference (by hand) was found despite the darker colour.

The sites had different clay content based on soil type (Figs 3 and 4). For Illogan (Dog Paddock) (Fig. 3) and Roscommon sites (Fig. 4), the clay content was lower than that of the Vertosol sites. Roscommon had clay concentration of less than 10% for the top 60 cm, which increased after this depth. Illogan (Dog Paddock) had a clay content of less than 40% for the top 25 cm, which increased after this depth. The Vertosols of Illogan (Shed Paddock) and Pembroke South all had clay concentrations >50% down the profile. Pembroke North had clay content >50% for the top 1 m, which declined after this depth. All soil cores had pH less than 7 (range 5–7). There was a generally increasing trend in bulk density with depth for all sites.

Fig. 3.

Clay content depth concentration at Illogan: (top) Dog Paddock and (bottom) Shed Paddock. Dog Paddock had four soil profiles and Shed Paddock had three individual soil profiles samples in each trench, with each profile approximately 1 m apart to provide replicates.

Fig. 4.

Clay content depth concentration for (top) Roscommon, (middle) Pembroke South cropped and pasture and (bottom) Pembroke North.

Total SOC varied considerably for each site (Table 2). Here SOC is expressed in terms of carbon stock. Roscommon had the lowest stock (~65 t ha⁻¹) and Pembroke North the highest (402 t ha⁻¹). For the Illogan sites, there was considerable variability at each trench. Stocks had range of 77–84 t ha⁻¹ for Dog Paddock and 94–166 t ha⁻¹ for Shed Paddock. In general, the lower clay sites (Roscommon and Dog Paddock) had the lowest SOC stocks, with the higher clay Vertosols having the largest stock.

All sites had unique SOC depth distribution profiles. For Illogan (Dog Paddock), SOC was concentrated at the 5–10 cm depth and for Shed Paddock, SOC was concentrated at greater depths (Figs 5 and 6). The lower clay Roscommon site had SOC concentrated in the top 20 cm, with a rapid decline at depth. For Pembroke North and South, SOC was highest at the surface and declined down the profile.

Fig. 5.

SOC depth distribution for (top) Dog Paddock and (bottom) Shed Paddock.

Fig. 6.

SOC stocks for (top) Roscommon, (middle) Pembroke cropped and pasture and (bottom) Pembroke North.

Environmental tracer

All sites had a unique ¹³⁷Cs depth distribution (Figs 7 and 8). All sites displayed a decrease in ¹³⁷Cs with depth, except for Roscommon where it was present in the top 20 cm only. In the non-Vertosols (Illogan Dog Paddock and Roscommon), the migration of ¹³⁷Cs appeared limited to approximately 20–25 cm maximum depth. For Vertosols, ¹³⁷Cs was detected to depths of 60–80 cm (Pembroke North). For the Vertosol sites, the ¹³⁷Cs depth distribution inconsistently declined in concentration down the profile. However, for Pembroke North, there was an absence of ¹³⁷Cs at 40–60 cm, with detectable concentrations at 60–80 cm. Similarly for Pembroke South (cropped), there was an absence of ¹³⁷Cs at 20–40 cm, with detectable concentrations at 40–60 cm. While we do not have data on cracking depths, this increased concentration suggests that this was a maximum potential crack depth and was the maximum depth that material labelled with ¹³⁷Cs was translocated.

Fig. 7.

¹³⁷Cs depth concentration at Illogan: (top) Dog Paddock and (bottom) Shed Paddock.

Fig. 8.

¹³⁷Cs depth distribution for (top) Roscommon, (middle) Pembroke cropped and pasture and (bottom) Pembroke North.

Down-profile SOC movement and SOC stock

There are two methods of delivery of labelled soil to depth: local and non-local supply (Murphy 2015). The Vertosols examined here have shrink–swell properties, with cracks developing as soil dries and allowing surface soil materials to fall into the cracks (Wilding and Tessier 1988). This material is then incorporated into the subsoil (i.e. pedoturbation) (Yaalon and Kalmar 1978). This process is responsible for the peaks in ¹³⁷Cs activity found at depth, as labelled soil is transported down into the subsoil via cracks. Southard and Graham (1992) found ¹³⁷Cs present in Vertosols at depths up to 72 cm. Al-Masri (2006) found the vertical distribution of ¹³⁷Cs to be related to soil type, allowing movement of material to depth. Curtis (1996) also suggested that surface cracks enable soil fines labelled with ¹³⁷Cs to move down the soil profile. Several other studies have examined the depth distribution of ¹³⁷Cs and SOC. The oldest sites sampled to depth in the local area were sampled to bedrock in 2006 (Stanley catchment, Martinez et al. 2010) with ¹³⁷Cs being present to depth. This suggests that transport of material down the soil profile predates the time of later sampling here.

For untilled non-cracking soils, the ¹³⁷Cs depth distribution profile is that of an exponential decline within the top ~20 cm, with little ¹³⁷Cs below this. The authors have demonstrated this to be the case for sites within the Krui catchment and surrounds (Martinez et al. 2009; Hancock et al. 2015). The relatively shallow presence of ¹³⁷Cs at the non-Vertosol sites (Dog Paddock and Roscommon) suggests that the movement of SOC and other soil components from the surface is via diffusive processes (Wells et al. 2012, 2013; Wells and Hancock 2014).

There were contrasting relationships with SOC and clay for the Vertosol and non-Vertosol sites. For the Dog Paddock, there was a significant result with SOC and clay (Fig. 9 top); however, there was no relationship at Shed Paddock (Fig. 9 bottom). This finding is a result of Dog Paddock having an increase in clay with depth (Fig. 3 top), with Shed Paddock having a constant clay content down the profile and no evident pattern (Fig. 3 bottom). Similar relationships were found for Pembroke South and North, with no significant relationship found with clay (not displayed for brevity). No relationship was found at Roscommon, likely due to the low SOC content and low clay (not shown for brevity).

Fig. 9.

Relationship between SOC and clay for Illogan: (top) Dog Paddock and (bottom) Shed Paddock.

There was a consistent and significant relationship with SOC and ¹³⁷Cs for Dog and Shed Paddocks (r = 0.92, P < 0.001) (Fig. 10). Similar relationships were found for Pembroke South and North, but there was no relationship between SOC and clay or SOC and ¹³⁷Cs for Roscommon. This suggests that clay is not the sole control on SOC movement at these sites (and likely other sites), and a portion of SOC associated with clay will move along pathways traced by ¹³⁷Cs also fixed to clay.

Fig. 10.

Relationship between SOC and ¹³⁷Cs for the Illogan: (top) Dog Paddock and (bottom) Shed Paddock. SOC and ¹³⁷Cs were significantly correlated for both Dog Paddock (r = 0.92, P < 0.001) and Shed Paddock (r = 0.82, P < 0.002) when all profile data were combined.

The data here allow a means to determine an approximate age and rate of SOC movement down the profile. If we assume a maximum depth of 80 cm for the labelled material and that ¹³⁷Cs arrived on the surface during 1945–1972, then this suggests that an age range of between 65 (2010 subtract 1945) and 38 years (2010 subtract 1972) of material using 2010 as an average date of sampling. Therefore, at the earliest, material has migrated to its maximum depth 65 years prior, with material assumed to be still migrating. At the other extreme, if labelled soil was only transported at the end of the fallout period in 1972, then the labelled material has a maximum age of approximately 40 years.

The age of 65 years can be considered a maximum age because there was no fallout prior to this. However, it is more likely that the labelled material is much younger than these ages and transported in less time. Soil would be continually migrating down the soil profile. The authors have observed cracks large enough to place an entire arm down in 2005 at Pembroke South. It is also likely that modern material is continuing to move down the profile. It is also recognised that the SOC quantified here may be much older than the ¹³⁷Cs fallout period. However, dating (i.e. using carbon-14) of this SOC is beyond the scope of this project and using the approach here provides a first pass assessment of age and migration rate.

SOC contribution by erosion to depth and age

Murphy (2015) suggests that there are no definitive data to suggest a SOC content threshold for soil productivity. Smith et al. (2015) suggest that a SOC content of 2% is a threshold for soil erosion stability for Vertosols. Below 2%, soil aggregates slake, this being an inherent property of Vertosols. They found that that when SOC was greater than 3.5% in surface soil, aggregate stability was improved, suggesting an increase in SOC can reduce erosion. Here, there was a range of surface SOC concentration, with Illogan (Shed Paddock) having SOC of 1.81% while the Pembroke sites had range of 1.05–7.35% (Table 2). There was no visual evidence of soil structural decline or erosion at any of the sites.

There are no specific erosion rates for each site examined here. However, if we assume that the erosion rate is 5 t ha⁻¹ year⁻¹ (Martinez et al. 2009, 2010), and hillslope soils have an approximately 2–3% OC on the surface (assuming this is the most erosionally active or mobile layer), this provides 0.1–0.2 t ha⁻¹ of eroded or potentially mobilised OC per year for the pasture sites. If all eroded sediment was captured by soil cracks then this would be a maximum of 0.2 t ha⁻¹ year⁻¹ (on average) of OC delivered down the soil profile. This is a likely gross overestimate and would be a maximum rate of delivery, as cracks will not always be present due to wetting up of the soil and closure of the cracks with eroded material and OC being transported over the cracks and downslope. Given that the SOC stock for the Vertosol sites exceeded 30 t ha⁻¹ in the top 0.2 m (Figs 5 and 6), this suggests that only a small fraction of available SOC is provided by external delivery.

If we assume a maximum SOC age of approximately 40 years (based on the last atmospheric fallout in 1972) and that this material does not degrade once down in the profile, then this suggests a maximum of 4–8 t ha⁻¹ of OC may be a result of external delivery. Therefore the majority of SOC is likely from in-situ production.

SOC and sequestration

There are several implications for carbon sequestration. The ability and extent to which SOC can be increased in the agricultural environment by adoption of different management and landuse is not well understood (Badgery et al. 2014; Murphy 2015; Hobley et al. 2016; Conrad et al. 2017). Some authors have concluded that the potential for increased SOC accumulation is limited in agricultural settings (Robertson and Nash 2013). Hoyle et al. (2013) suggest that the capacity to store more SOC in the surface is limited and that the subsoil has additional storage capacity.

The findings here demonstrated that SOC was highest for high clay sites and also suggests that organic carbon can be more readily translocated to depth in Vertosols than for other soils. In several soil pits (Illogan Shed Paddock and Pembroke South), the presence of ¹³⁷Cs was not continuous down the profile, with an absence at one depth and detectable concentrations below. However, while having potential for rapid translocation, the exposure to oxygen and water provides an environment for breakdown of organic material. These feedbacks are likely to be complex, as the addition of organic matter at depth will change the soil shrink–swell properties and may either enhance or reduce OC breakdown (Murphy 2015). Long-term studies would provide insights into the capabilities of these soils to sequester OC.

There is potential for increasing SOC stocks at these sites. The similarity between the Shed Paddock and Pembroke South in terms of soil depth and clay content suggests that both sites have the potential to have similar SOC stock. However, Shed Paddock had considerably lower SOC stock than Pembroke South. The property is currently a well-run enterprise using no tillage for cropping and advanced management practices. How more biomass can be produced to increase SOC both at the surface and at depth while maintaining farm profitability is an open question.

Several authors have questioned how land management and vegetation can influence soil properties at depth (Pringle et al. 2011; Murphy 2015; Hobley et al. 2016; Conrad et al. 2017). Dalal et al. (2021) found that the introduction of a pasture phase in the cropping cycle may reduce SOC loss. Given that Roscommon and Pembroke North have never been cropped and, in recent decades, there have been no fertiliser inputs, how SOC can be increased is an important question, given that they are optimally economically managed (Badgery et al. 2014; Murphy 2015; Minasny et al. 2017).

Further issues and data needs

There are several possible explanations for the presence of ¹³⁷Cs at depths greater than that driven by diffusive processes.

It is likely that soil has been, and continues to be, delivered to the cracks and moves down the profile via cracks – with cracking observed at Shed Paddock, Pembroke North and South sites. The process is likely to be episodic with drying periods opening up soil cracks and periods of rainfall that produce runoff delivering eroded surface soil to the cracks. We hypothesise that it is only large rainfall events that produce surface runoff in this environment to generate sufficient eroded material to be delivered to the cracks (Hancock et al. 2019).

Other tracers such as ⁷Be would provide storm-scale detail on the movement of surface materials (Zapata 2002; Walling et al. 2011). A pathway forward would be to sample down a soil profile with cracks immediately after a large storm and assess whether short-term tracers such as ⁷Be were detected. Dating of SOC (i.e. radiocarbon method) would provide information on longer ages. Carbon isotopes (i.e. δ¹³C) would also provide insights (i.e. Pringle et al. 2011) into plant-based carbon inputs as well as geochemical analysis (Lawrence et al. 2015). This is currently being investigated.

All soil samples were collected in a vertical line down the soil profile. There was no attempt to seek cracks or areas of darker soil suggestive of organic enrichment. Therefore, it is possible that considerably more labelled material has made its way further down the soil profile than that captured using the sampling method here of samples collected in a vertical line. More targeted sampling would provide further insights.

At Illogan, multiple profiles were collected for each trench and were sampled approximately 1 m from each other. Both sites demonstrated considerable variability in SOC even within the same trench (Table 2, Fig. 5). For carbon accounting this presents an issue not just for quantifying SOC on the surface but also evaluating SOC whole-profile stocks. This raises the question of how many soil profiles should be sampled such that a statistically defensible value can be obtained, given the variability observed here.

The findings highlight the need for more long-term field data (i.e. Conrad et al. 2017; Jha et al. 2020) and model development to provide enhanced understandings of whole soil profile SOC distribution and change (Luo et al. 2010) across a range of soil types and climates (Murphy 2015) as well agricultural management practices (Badgery et al. 2014). Innovation in surface and subsurface methodology for SOC quantification is needed to advance spatial and temporal distribution of SOC at the point and hillslope scale (Robertson and Nash 2013).