The importance of soil sampling depth for accurate account of soil organic carbon sequestration, storage, retention and loss

The importance of soil sampling depth for accurate account of soil organic carbon sequestration, storage, retention and loss

Catena 125 (2015) 33–37 Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena The importance of soil sa...

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Catena 125 (2015) 33–37

Contents lists available at ScienceDirect

Catena journal homepage: www.elsevier.com/locate/catena

The importance of soil sampling depth for accurate account of soil organic carbon sequestration, storage, retention and loss K.R. Olson a,⁎, M.M. Al-Kaisi b a b

Department of Natural Resources and Environmental Sciences, S-224 Turner Hall, University of Illinois at Urbana Champaign, 1102 S. Goodwin Ave, Urbana, IL 61801, USA Department of Agronomy, 2104 Agronomy Hall, Iowa State University, Ames, IA 50011-1010, USA

a r t i c l e

i n f o

Article history: Received 28 April 2014 Received in revised form 7 October 2014 Accepted 9 October 2014 Available online xxxx Keywords: Soil organic carbon sequestration Rooting depth Soil organic carbon stocks Soil sampling depth

a b s t r a c t Soil organic carbon distribution within soil profile is highly influenced by management practices, especially tillage systems where soil environment is altered. Such changes in soil environment will affect soil carbon retention or accumulation in different layers of the soil profile. However, much published research in the area of soil organic carbon (SOC) sequestration focuses on shallow sampling depths within the 0–30 cm tillage zone when determining SOC stocks and sequestration. The objectives of this study are to quantify the SOC stock differences with depth between tillage treatments after 20 years and to determine the appropriate sampling depth when assessing SOC stocks as influenced by management practices. A 20-year moldboard plow (MP), chisel plow (CP) and no-tillage (NT) study was established with a maize–soybean rotation. The 75-cm root zone was sampled in 5-cm intervals to measure SOC stocks. The SOC sequestration, storage, retention and loss were determined for the 0–5 cm, 0–15 cm, 15–75 cm and 0–75 cm layers. The NT treatment did retain more SOC stock than the MP treatment to a 20 cm depth but the SOC stock of the 20–35 cm layer NT system was lower than the MP system. It is recommended that the depth of soil sampling has to include the entire root zone to accurately report SOC stock and the effect of tillage system on change in SOC. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The traditional method of evaluating soil C dynamics under different tillage and cropping systems is collecting soil samples to determine changes in SOC stocks. One principle which needs to be kept in mind is depth of soil sampling, which can be affected by landscape position, cropping systems, tillage systems, drainage class, and other soil forming factors that dictate the change in SOC stocks in any particular field (Olson, 2013a). The interaction between root system and soil profile has profound impact on soil C accumulation, where root system can contribute to SOC stocks. Olson (2013a) defined SOC sequestration for a land unit and suggested the SOC sequestration process should result in a net depletion of CO2 levels in the atmosphere. It is imperative that the SOC stock be measured beyond the tillage zone (0–30 cm) for the entire root zone depth to understand management practices such as tillage effects on SOC distribution when determining change in SOC stocks or sequestration rate. Generally, the interaction between, atmosphere, biosphere, and lithosphere affects nutrient vertical distributions in soil resulting in great chemical and physical gradients from surface to bedrock (Jobbagy and Jackson, 2001). Therefore, soil stratification is evident in soils and nutrient assessment including SOC are essential to have accurate account of management effects, such as tillage, on such ⁎ Corresponding author. Tel.:+1 217 333 9639; fax: +1 217 244 3219. E-mail address: [email protected] (K.R. Olson).

http://dx.doi.org/10.1016/j.catena.2014.10.004 0341-8162/© 2014 Elsevier B.V. All rights reserved.

distribution. It is well documented that type, thickness, and position of soil horizon can reveal the formation factors as well as management practices effects on SOC characteristics and distribution (Honeycutt et al., 1990; Marion and Schlesinger, 1985). Schlesinger (2000) suggested soils might be a sink for atmospheric carbon with the application of conservation tillage and the establishment of native vegetation on abandoned agricultural lands. Luo et al. (2010) found that adopting no-tillage in agro-ecosystem has been widely recommended as means of enhancing carbon (C) sequestration in soils. However, results are inconsistent and vary from significant increase to a significant decrease. Yang and Wander (1999) suggested that reduced tillage and no-tillage (NT) practices generally concentrate SOC in surface few centimeters; however, the use of conservation tillage does not always result in increased SOC storage. Wander et al. (1998) found NT practices increased SOC and POM-C contents by 25 and 70%, respectively compared with conventional tillage at the surface (0– 5 cm). This gain was at the expense of SOC at 5–17.5 cm depth, where SOC and POM-C decreased by 4 and 18%, respectively. It is widely believed that soil disturbance by tillage is a primary cause of the historical loss of SOC in North America and that substantial SOC sequestration can be accomplished by changing from conventional plowing to less intensive tillage such as NT and conservation tillage. Different sampling protocol can lead to different estimates of SOC stocks. Sampling and SOC analysis of the plow layer, tillage zone or management zone have often lead to different findings for the 0–20 cm layer

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than would have been determined if the root zone or a 1 or 2 m depth had been sampled and tested (Olson, 2010). This is especially true when depth of tillage is sufficient to mix the surface layer with part of the subsoil layer and to a depth below the sampling zone. In these cases the SOC rich surface layer can be buried below the shallow sampling zone. When measuring SOC sequestration, storage or retention and loss it is important to include all the SOC in the root zone, which is commonly to a depth of 1 or 2 m unless there is a root restrictive layer present, such as a very dense horizon, fragipan or bedrock. Tillage systems can influence SOC distribution, storage or retention and loss in the surface and subsurface layers (Olson, 2013a; Olson et al., 2014). Deep tillage, such as moldboard or chisel plow, can significantly alter SOC distribution in the root zone. Soil inversion by moldboard plowing can translocate surface soil SOC to lower depths. Most soil plots in SOC sequestration research studies have commonly been sampled to a 20-cm depth (ranges between 6 and 30 cm) including the North American regional SOC sequestration rate studies (Franzluebbers, 2010; Franzluebbers and Follett, 2005; Gregorich et al., 2005; Johnson et al., 2005; Liebig et al., 2005). West and Post (2002) reviewed 137 paired studies and showed more SOC stock was stored in NT than MP, but only considered, SOC measured in the top 15 or 30 cm. Kumar et al. (2012) sampled soils to 40 cm depth at two Ohio plot areas but only reported total gain in SOC for the 0–20 cm surface layer. Olson (2013b) calculated the gain or loss in SOC stock at these two Ohio plot areas for the 20–40 cm layer and found SOC stock gain for the combined 0–40 cm layer was only half as much as the reported SOC stock for the 0–20 cm layer at one site and slightly less at the other site. Clearly, the depth of soil sampling and testing did affect the SOC gain findings. Most soil sampling techniques use distance from the soil surface as a primary metric. The soil surface, however, is a reliable datum only for measurement of C concentration characteristics directly related to distance from the soil surface at the time of sampling (Wuest, 2009). Deep SOC profiles differ between the tillage treatments of interest. Deeper sampling will not completely overcome effects caused by bulk density variations and resultant change in soil surface elevation except when the SOC constituent is sampled deep enough to be approaching zero in the lower layer (Lee et al., 2009). Equivalent soil mass (massdepth) instead of linear depth can be used to correct for tillage treatment differences in soil bulk density, allowing more precise and accurate quantitative comparison of SOC constituents (Doetterl et al., 2012; Ellert and Beltany, 1995; Lee et al., 2009; Wuest, 2009). Sampling soils to the bottom of the root zone where the SOC concentration is nearly zero is recommended. Soil layers with only trace amounts of SOC present do not significantly change the total SOC stock in the soil profile (Soil Survey Staff, 1968). Kreznor et al. (1989; 1990; 1992) measured the thickness of the A horizon, the root zone and depth to parent material on a hillslope landscape prior to accelerated erosion. Fig. 1 shows how the A horizon, root zone thicknesses and depth to parent material vary with landscape position. The A horizon and root zone were thickest on the interfluve and toeslope. If one had sampled only the 20-cm layer the SOC located below that depth on the interfluve, shoulder, footslope and toeslope would not have been included. In addition, the root zone below the 20-cm layer also contained significant SOC for all landscape positions (Kreznor et al., 1989, 1990, 1992). In long-term studies, 20 to 50 years, one tillage practice can increase the SOC stock in plow layer while at the same time decreasing it in the subsoil when compared to other tillage treatments and pre-treatment SOC stocks (Olson, 2010; Zinn et al., 2005; Sa et al., 2001a, 2001b). Deeper sampling of root zone or to a 1 or 2 m can change the SOC stock and sequestration rate findings for the same soil profile if the soil had only been sampled and tested to a 20 cm depth. In a longterm tillage study in Illinois (Olson, 2010), the NT system showed SOC stock increase in the upper 0–5 cm layer, but there was a SOC loss within the 5 to 75 cm subsurface layer. The SOC stocks need to be accounted

Interfluve

Shoulder

20 cm Backslope

Footslope Toeslope

A horizon (top soil)

B horizon (not zone)

C horizon (parent material) Bottom of management zone (20 cm)

Legend Fig. 1. A horizon thickness, root zone thickness and depth to parent material is shown for a hillslope landscape.

for in the root zone in order to assess tillage system effects and plant contributions to SOC stock change. Much of the contradiction in SOC stock and SOC sequestration findings (Olson, 2013a; Olson et al., 2014) is partially a result of differing soil sampling depths/protocols. The objectives of this study are to quantify the SOC stock differences as affected by depth between tillage treatments after 20 years and to determine the appropriate sampling depth when assessing SOC sequestration, storage, retention and loss.

2. Methods 2.1. Experiment site and field treatments A long-term tillage experiment was started in 1989 at the Dixon Springs Agricultural Research Center in southern Illinois. The soil at the study site was a moderately eroded phase of Grantsburg silt loam (fine-silty, mixed, mesic Typic Fragiudalf) (Soil Survey Staff, 1999) with an average depth of 64 + 6 cm to a root-restricting fragipan. The area had an average slope gradient of 6%. Starting with maize (Zea mays L.) in 1989, maize and soybean [Glycine max (L.) Merr] were grown in alternate years. The experimental design was two Complete Latin Squares and each square having three rows and three columns (Cochran and Cox, 1957) which allowed for randomization of the tillage treatments no-tillage (NT), chisel-plow (CP), and moldboard-plow (MP) both by row (block) and by column. This replication was used to control random variability in both directions. Each tillage treatment was randomized six times in 18 plots with a size of 9 m × 12 m. The columns were initially separated by 6 m buffer strips of sod. Later the buffer strips were planted to NT maize and soybeans to reduce deer damage. An electric fence was later used to protect the crops in the plot area. There was a 60 m wide filter strip between the plot area and the waterway.

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30

100

25

88

20 62 15 10 5 0 88 1990

1994

2000 Years

MP retained in storage

The results and discussion will focus on NT and MP SOC stocks present in Figs. 2–5. The SOC stock baseline was determined at 5 cm

109 100

Mg C/ha 0–5 cm layer

12 10 8

54

6 4 2 2000 Years

Percent of SOC retained

14

0 2009

2005

MP SOC loss below steady state (lost SOC) Steady state Fig. 3. SOC stocks of the pre-treatment baseline and after 20-years on the MP and NT plots in the 0–15 cm surface layer.

intervals for both the NT and MP treatments prior to tillage treatment implementation and at the end of a 20-year study. The SOC stock means of the 6 pre-treatment MP and 6 pre-treatment NT plots were not statistically different, therefore they were combined as the pretreatment SOC baseline for both systems (NT and MP). The root zone was sampled and SOC reported periodically over time (Olson, 2010; Olson et al., 2005). In order to determine tillage system effect on SOC stock and sequestration after 20-years the NT and MP SOC data points for the following soil layers, 0–5, 0–15, 15–75, and 0–75 cm, were compared to pre-treatment SOC stock means for the same layers (Figs. 2, 3, 4, and 5). Fig. 2 shows the pre-treatment baseline and the NT and MP SOC stocks after 20-years for the top 5 cm depth. Clearly, after 20 years the NT 0–5 cm layer had more SOC stock than the pre-treatment baseline and the NT surface layer did sequester SOC on this sloping and eroding site. After 20-years the MP 0–5 cm layer had lost SOC stock when compared to the pre-treatment SOC baseline. Comparison of the NT and MP

30 25

100 86 80

20 15 10 5 0 88 1990

1994

Legend MP retained in storage

NT

MP

MP

2000 Years Legend

MP retained in storage

2005

NT

NT SOC loss below steady state (lost SOC)

MP SOC loss below steady state (lost SOC)

MP SOC loss below steady state (lost SOC)

Fig. 2. SOC stocks of the pre-treatment baseline and after 20-years on the MP and NT plots in the 0–5 cm surface layer (Olson et al., 2014).

0 2009

MP

NT SOC above steady state (sequestered SOC)

Steady state

Percent of SOC retained

3. Results and discussion

NT

NT SOC loss below steady state (lost SOC)

Mg C/ha 15–75 cm layer

Soil samples were collected in September of 1988 (prior to the establishment of the tillage experiment in spring of 1989) and in June of 2009, at 5 cm increments to 75 cm. The sampling depth was limited to 75 cm due to the presence of a root restricting fragipan at a 64 + 6 cm depth. Previous soil sampling found only trace amounts of SOC concentration present below a 75 cm depth (Olson et al., 2005), probably from previous grass roots penetrating the fragipan along the prism faces. Four soil cores (3.2 cm diameter), one from near each of the four corners of the plot (1.5 m from adjacent, above or below plot, and 1.5 m from border strip), were obtained for each depth and composited by crumbling and mixing for each depth separately. The soil samples were air-dried and pulverized and passed through a 2-mm sieve prior to analysis. The SOC concentration was determined after removal of un-decomposed plant residue using the modified aciddichromate organic carbon procedure number 6A1 (Soil Survey Staff, 2004). Field moist core bulk density was determined by centering the sampler ring on each 5-cm layer (Soil Survey Staff, 2004) using a Model 200 soil core sampler (5.6 cm in diameter and 6 cm high) manufactured by Soil Moisture Equipment Corp.

1994

0 2009

Legend

2.2. Field and laboratory methods

0 88 1990

2005

Percent of SOC retained

Mg C/ha 0–15 cm layer

The implements used in each tillage system and depth of tillage were as follows: NT (John Deere No-Till planter with wavy coulters), CP (straight-shanked chisel plowed to 15 cm with disking to 5 cm), and MP (moldboard plowed to 15 cm with disking to 5 cm). In the spring of each year the MP and CP treatments were conducted followed by 2 disking operations and planting of either maize or soybeans. In May of odd years, maize was planted at the seeding rate of 64,000 seeds ha − 1. Fertilizers were applied of 218 kg ha − 1 N (liquid and injected), 55 kg P ha− 1 (dry and surface applied), and 232 kg K ha− 1 (dry and surface applied). In even years, soybeans were planted at a rate of 432,000 seeds ha− 1 and no N fertilizer was applied.

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Steady state Fig. 4. SOC stocks of the pre-treatment baseline and after 20-years on the MP and NT plots in the 15–75 cm surface layer.

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55

Grantsburg Soil Organic Carbon 100

50

30 25

Legend

20

MP retained in storage

15

NT SOC loss below steady state (lost SOC)

10

MP SOC loss below steady state (lost SOC)

NT

MP

20 25

Legend

30

MP subsoil SOC loss below baseline

35

NT subsoil SOC loss below baseline

40

Sequester SOC

45

Steady state

1994

15

Depth (cm)

Mg C/ha 0–75 cm layer

70

35

Percent of SOC retained

10

40

0 88 1990

5

87

45

5

0

2000 Years

2005

0 2009

Fig. 5. SOC stocks of the pre-treatment baseline and after 20-years on the MP and NT plots in the 0–75 cm surface layer (Olson et al., 2014).

stocks suggests that NT increased SOC stocks by two fold (13 Mg C ha−1 vs. 6.5 Mg C ha−1) in the 20-year study for the 0–5 cm layer. The NT SOC stock is consistently above the pre-treatment stock baseline assumed at a steady state. The NT SOC stock was 9% higher at the end of the study than that for the pre-treatment baseline. During the same period the MP plots lost 46% of the SOC stock in the upper 5 cm as a result of plowing and erosion. The SOC stock and sequestration findings for the 0–15 cm layer (Fig. 3) were different. In 20-years the NT system had lost 12% of SOC stock and the MP lost 38%. With the inclusion of the 5 to 15 cm layer the NT system was just losing SOC stock at a slower rate or retaining more SOC stock than MP. After 20-years, the 15– 75 cm layer of the NT had 14% less SOC stock than that of the pretreatment baseline and the 15–75 cm layer of the MP had 20% less SOC than that of baseline treatment (Fig. 4). Some studies used shallow soil depths (commonly 20 cm but ranges from 6 to 30 cm) to determine soil C sequestration by different tillage systems (Franzluebbers, 2010) and may not account for or explain the dynamics of SOC movement and accumulation at lower depths. The other significant factor in annual cropping systems where considerable amount of tillage is done at various intensities is the influence of tillage on SOC distribution within the soil profile due to mixing effect (Al-Kaisi et al., 2005; Baker et al., 2007). The SOC stock findings of the root zone (0–75 cm) (Fig. 6) show that the NT treatment lost 13% and the MP lost 30% of their SOC stocks when compared to pre-treatment baseline. The SOC sequestration noted with NT in Fig. 2, for the 0–5 cm layer, disappeared when the SOC for the entire root zone (0–75 cm) was calculated. The SOC stock loss in the 5– 75 cm subsurface and subsoil layers were much greater than the 0– 5 cm gains and resulted in significant root zone SOC losses from the pre-treatment baseline value. The NT system showed a net SOC storage increase or sequestration in the upper 0–5 cm layer (Fig. 6); however, there was greater SOC stock loss within the 5 to 75 cm layer. For the 15 to 35 cm layer the NT SOC stock loss is greater (13%) than that for MP during the 20-years. The lower subsurface and subsoil layers of NT treatment SOC loss was greater than the NT SOC stock gain in the 0– 5 cm layer (Fig. 6). The SOC stocks need to be accounted for the entire root zone in order to assess tillage system effect on SOC sequestration, storage, retention and loss. In many studies where SOC was measured to a depth greater than 30 cm no significant difference between the volumes of SOC in plowed vs. no-tilled system. The only difference was the

50

NT subsoil SOC loss below MP

55

NT and MP (1988) baseline NT (2009)

60

MO (2009)

65 70 75

2

4

6 8 10 12 14 16 18 Change in 20 years Mg C/ha

20

22

Fig. 6. SOC stock changes from the pre-treatment baseline and after 20-years on the MP and NT plots with depth (Olson et al., 2014).

location of the SOC within the soil profile. In NT plots the SOC was concentrated in the top 30 cm, but was dispersed to greater depths in tilled plots (Baker et al., 2007; VandenBygaart et al., 2003). The depth of soil sampling does make a difference when measuring SOC sequestration and stocks. If the sampling depth includes only the surface layer or tillage zone the SOC stock account will in some cases differ as compared to the entire root zone or upper 1 to 2 m soil profile. Therefore, to prevent sampling depth from being a co-variable, it is imperative that soils be sampled to depth of the root zone or to a 1 to 2 m depth when measuring SOC stock or trying to determine SOC sequestration, storage, retention or loss. 4. Conclusions The MP treatment lost SOC stock in the 0–20 cm layer when compared to the pre-treatment SOC and maintained the SOC stock for the 20 to 75-cm layer. The NT treatment sequestered SOC in the 0–5 cm layer (9% more) when compared to pre-treatment SOC stock but lost SOC in 5 to 35 cm layer (13%) and maintained the SOC stock for the 35 to 75-cm layer. The NT treatment did retain more SOC stock than MP to a 20 cm depth, but from 20 to 35 cm layer the SOC stock was lower than that for MP and the same for the 35 to 75 cm layer. This can be a result of the moldboard plow mixing the A horizon materials into the 20 to 35 cm layer of MP system and translocation of rich SOC surface layer to lower depth. The depth of sampling can and does affect the reported SOC sequestration and stock findings. The need to quantify SOC stocks and SOC sequestration requires sampling of the entire root zone or to a depth of 1 or 2 m. This greater depth (entire root zone) of sampling is recommended to eliminate the inconsistent findings when assessing the SOC sequestration, storage, retention and loss.

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Acknowledgments This study has been published with the funding support of the Director of the Office of Research at the University of Illinois at Urbana-Champaign, Urbana, IL. The NRES Research Project 65-372 was funded as part of the Regional Research Project 367 and in cooperation with the North Central Regional Project NC-1178 (Soil Carbon Sequestration) and North Central Regional Project NCERA-3 (Soil Survey). References Al-Kaisi, M., Yin, X., Licht, M., 2005. Soil carbon and nitrogen changes as influenced by tillage and cropping systems in some Iowa soils. Agric. Ecosyst. Environ. J. 105, 635–647. Baker, J.M., Ochsner, T.E., Venterea, R.T., Griffits, T.J., 2007. Tillage and soil carbon sequestration—what do we really know? Agric. Ecosyst. Environ. J. 118, 1–5. Cochran, W.G., Cox, G.M., 1957. Experimental Designs (2nd). John Wiley and Sons, Inc., New York. Doetterl, S., Six, J., Van Wesemael, B., Van Oost, K., 2012. Carbon cycling in eroding landscapes; geomorphic controls on soil organic C pool composition and C stabilization. Global Change Biology 18 (7), 2218–2232. Ellert, B.H., Beltany, J.R., 1995. Calculation of organic matter and nutrient storied in soils under contrasting management regions. Can. J. Soil Sci. 75, 529–538. Franzluebbers, A., 2010. Achieving soil organic carbon sequestration with conservation agricultural systems in southeastern United States. Soil Sci. Soc. Am. J. 74, 347–357. Franzluebbers, A.J., Follett, R.F., 2005. Greenhouse gas contributions and mitigation potential is agricultural region of North America. Soil Tillage Res. 83, 25–52. Gregorich, E.G., Rochette, P., VandenBygaart, A.J., Angers, D.A., 2005. Greenhouse gas contributions of agricultural soils and potential mitigation practices in eastern Canada. Soil Tillage Res. 83, 73–94. Honeycutt, C.W., Heil, R.D., Cole, C.V., 1990. Climatic and topographic relations of three Great Plains soils. I. Soil morphology. Soil Sci. Soc. Am. J. 54, 469–475. Jobbagy, E., Jackson, R., 2001. The distribution of soil nutrients with depth: global patterns and the imprint of plants. Biogeochemistry 53, 51–77. Johnson, J.M.F., Reicosky, D.C., Allmaras, R.R., Sauer, T.J., Venterea, R.T., Dell, C.J., 2005. Greenhouse gas contributions and mitigation potential of agriculture in the central USA. Soil Tillage Res. 83, 73–94. Kreznor, W.R., Olson, K.R., Banwart, W.L., Johnson, D.L., 1989. Soil, landscape, and erosion relationships in a northwest Illinois watershed. Soil Sci. Soc. Am. J. 53, 1763–1771. Kreznor, W.R., Olson, K.R., Johnson, D.L., Jones, R.L., 1990. Quantification of postsettlement deposition in a Northwestern Illinois sediment basin. Soil Sci. Soc. Am. J. 54, 1393–1401. Kreznor, W.R., Olson, K.R., Johnson, D.L., 1992. Field evaluation of methods to estimate soil erosion. Soil Sci. 153, 69–81. Kumar, S., Kadono, A., Lal, R., Dick, W., 2012. Long term no till impacts on organic carbon and properties of two contrasting soils and corn yields in Ohio. Soil Sci. Soc. Am. J. 76, 1798–1809. Lee, J., Hopmans, J.W., Rolston, D.E., Baer, S.G., Six, J., 2009. Determining soil carbon stock changes: simple bulk density corrections fail. Agric. Ecosyst. Environ. 134, 251–256.

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