Catena 125 (2015) Contents lists available at ScienceDirect. Catena. journal homepage:

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1 Catena 1 (21) Contents lists available at ScienceDirect Catena journal homepage: 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 Department of Natural Resources and Environmental Sciences, S-224 Turner Hall, University of Illinois at Urbana Champaign, 112 S. Goodwin Ave, Urbana, IL 6181, USA b Department of Agronomy, 214 Agronomy Hall, Iowa State University, Ames, IA 11-11, USA article info abstract Article history: Received 28 April 214 Received in revised form 7 October 214 Accepted 9 October 214 Available online xxxx Keywords: Soil organic carbon sequestration Rooting depth Soil organic carbon stocks Soil sampling depth 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 3 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 2 years and to determine the appropriate sampling depth when assessing SOC stocks as influenced by management practices. A 2-year moldboard plow (), chisel plow (CP) and no-tillage () study was established with a maize soybean rotation. The 7-cm root zone was sampled in -cm intervals to measure SOC stocks. The SOC sequestration, storage, retention and loss were determined for the cm, 1 cm, 1 7 cm and 7 cm layers. The treatment did retain more SOC stock than the treatment to a 2 cm depth but the SOC stock of the 2 3 cm layer system was lower than the 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. 214 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, 213a). The interaction between root system and soil profile has profound impact on soil C accumulation, where root system can contribute to SOC stocks. Olson (213a) defined SOC sequestration for a land unit and suggested the SOC sequestration process should result in a net depletion of CO 2 levels in the atmosphere. It is imperative that the SOC stock be measured beyond the tillage zone ( 3 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, 21). 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.: ; fax: address: krolson@illinois.edu (K.R. Olson). 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., 199; Marion and Schlesinger, 198). Schlesinger (2) 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. (21) 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 () 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 practices increased SOC and POM-C contents by and 7%, respectively compared with conventional tillage at the surface ( cm). This gain was at the expense of SOC at 17. 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 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 2 cm layer / 214 Elsevier B.V. All rights reserved.

2 34 K.R. Olson, M.M. Al-Kaisi / Catena 1 (21) than would have been determined if the root zone or a 1 or 2 m depth had been sampled and tested (Olson, 21). 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, 213a; Olson et al., 214). 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 2-cm depth (ranges between 6 and 3 cm) including the North American regional SOC sequestration rate studies (Franzluebbers, 21; Franzluebbers and Follett, ; Gregorich et al., ; Johnson et al., ; Liebig et al., ). West and Post (22) reviewed 137 paired studies and showed more SOC stock was stored in than, but only considered, SOC measured in the top 1 or 3 cm. Kumar et al. (212) sampled soils to 4 cm depth at two Ohio plot areas but only reported total gain in SOC for the 2 cm surface layer. Olson (213b) calculated the gain or loss in SOC stock at these two Ohio plot areas for the 2 4 cm layer and found SOC stock gain for the combined 4 cm layer was only half as much as the reported SOC stock for the 2 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, 29). 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., 29). 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., 212; Ellert and Beltany, 199; Lee et al., 29; Wuest, 29). 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; 199; 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 2-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 2-cm layer also contained significant SOC for all landscape positions (Kreznor et al., 1989, 199, 1992). In long-term studies, 2 to 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, 21; Zinn et al., ; Sa et al., 21a, 21b). 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 2 cm depth. In a longterm tillage study in Illinois (Olson, 21), the system showed SOC stock increase in the upper cm layer, but there was a SOC loss within the to 7 cm subsurface layer. The SOC stocks need to be accounted 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, 213a; Olson et al., 214) 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 2 years and to determine the appropriate sampling depth when assessing SOC sequestration, storage, retention and loss. 2. Methods Interfluve A horizon (top soil) B horizon (not zone) Shoulder 2.1. Experiment site and field treatments Backslope C horizon (parent material) Footslope 2 cm Toeslope Bottom of management zone (2 cm) Fig. 1. A horizon thickness, root zone thickness and depth to parent material is shown for a hillslope landscape. 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 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, 197) which allowed for randomization of the tillage treatments no-tillage (), chisel-plow (CP), and moldboard-plow () 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 maize and soybeans to reduce deer damage. An electric fence was later used to protect the crops in the plot area. There was a 6 m wide filter strip between the plot area and the waterway.

3 K.R. Olson, M.M. Al-Kaisi / Catena 1 (21) The implements used in each tillage system and depth of tillage were as follows: (John Deere No-Till planter with wavy coulters), CP (straight-shanked chisel plowed to 1 cm with disking to cm), and (moldboard plowed to 1 cm with disking to cm). In the spring of each year the 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, seeds ha 1. Fertilizers were applied of 218 kg ha 1 N (liquid and injected), 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, seeds ha 1 and no N fertilizer was applied Field and laboratory methods Soil samples were collected in September of 19 (prior to the establishment of the tillage experiment in spring of 1989) and in June of 29, at cm increments to 7 cm. The sampling depth was limited to 7 cm due to the presence of a root restricting fragipan at a cm depth. Previous soil sampling found only trace amounts of SOC concentration present below a 7 cm depth (Olson et al., ), 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. m from adjacent, above or below plot, and 1. 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, 24). Field moist core bulk density was determined by centering the sampler ring on each -cm layer (Soil Survey Staff, 24) using a Model 2 soil core sampler (.6 cm in diameter and 6 cm high) manufactured by Soil Moisture Equipment Corp. 3. Results and discussion The results and discussion will focus on and SOC stocks present in Figs. 2. The SOC stock baseline was determined at cm Mg C/ha cm layer SOC above steady state (sequestered SOC) Mg C/ha 1 cm layer SOC loss below steady state (lost SOC) 1 29 Fig. 3. SOC stocks of the pre-treatment baseline and after 2-years on the and plots in the 1 cm surface layer. intervals for both the and treatments prior to tillage treatment implementation and at the end of a 2-year study. The SOC stock means of the 6 pre-treatment and 6 pre-treatment plots were not statistically different, therefore they were combined as the pretreatment SOC baseline for both systems ( and ). The root zone was sampled and SOC reported periodically over time (Olson, 21; Olson et al., ). In order to determine tillage system effect on SOC stock and sequestration after 2-years the and SOC data points for the following soil layers,, 1, 1 7, and 7 cm, were compared to pre-treatment SOC stock means for the same layers (Figs. 2, 3, 4, and ). Fig. 2 shows the pre-treatment baseline and the and SOC stocks after 2-years for the top cm depth. Clearly, after 2 years the cm layer had more SOC stock than the pre-treatment baseline and the surface layer did sequester SOC on this sloping and eroding site. After 2-years the cm layer had lost SOC stock when compared to the pre-treatment SOC baseline. Comparison of the and Mg C/ha 1 7 cm layer SOC loss below steady state (lost SOC) Fig. 2. SOC stocks of the pre-treatment baseline and after 2-years on the and plots in the cm surface layer (Olson et al., 214). Fig. 4. SOC stocks of the pre-treatment baseline and after 2-years on the and plots in the 1 7 cm surface layer.

4 36 K.R. Olson, M.M. Al-Kaisi / Catena 1 (21) Mg C/ha 7 cm layer SOC loss below steady state (lost SOC) Fig.. SOC stocks of the pre-treatment baseline and after 2-years on the and plots in the 7 cm surface layer (Olson et al., 214). Depth (cm) Grantsburg Soil Organic Carbon subsoil SOC loss below baseline subsoil SOC loss below baseline Sequester SOC subsoil SOC loss below and (19) baseline (29) MO (29) stocks suggests that increased SOC stocks by two fold (13 Mg C ha 1 vs. 6. Mg C ha 1 ) in the 2-year study for the cm layer. The SOC stock is consistently above the pre-treatment stock baseline assumed at a steady state. The SOC stock was 9% higher at the end of the study than that for the pre-treatment baseline. During the same period the plots lost 46% of the SOC stock in the upper cm as a result of plowing and erosion. The SOC stock and sequestration findings for the 1 cm layer (Fig. 3) were different. In 2-years the system had lost 12% of SOC stock and the lost 38%. With the inclusion of the to 1 cm layer the system was just losing SOC stock at a slower rate or retaining more SOC stock than. After 2-years, the 1 7 cm layer of the had 14% less SOC stock than that of the pretreatment baseline and the 1 7 cm layer of the had 2% less SOC than that of baseline treatment (Fig. 4). Some studies used shallow soil depths (commonly 2 cm but ranges from 6 to 3 cm) to determine soil C sequestration by different tillage systems (Franzluebbers, 21) 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., ; Baker et al., 27). The SOC stock findings of the root zone ( 7 cm) (Fig. 6) show that the treatment lost 13% and the lost 3% of their SOC stocks when compared to pre-treatment baseline. The SOC sequestration noted with in Fig. 2,forthe cm layer, disappeared when the SOC for the entire root zone ( 7 cm) was calculated. The SOC stock loss in the 7 cm subsurface and subsoil layers were much greater than the cm gains and resulted in significant root zone SOC losses from the pre-treatment baseline value. The system showed a net SOC storage increase or sequestration in the upper cmlayer(fig. 6); however, there was greater SOC stock loss within the to 7 cm layer. For the 1 to 3 cm layer the SOC stock loss is greater (13%) than that for during the 2-years. The lower subsurface and subsoil layers of treatment SOC loss was greater than the SOC stock gain in the 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 3 cm no significant difference between the volumes of SOC in plowed vs. no-tilled system. The only difference was the 7 7 location of the SOC within the soil profile. In plots the SOC was concentrated in the top 3 cm, but was dispersed to greater depths in tilled plots (Baker et al., 27; VandenBygaart et al., 23). 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 Change in 2 years Mg C/ha Fig. 6. SOC stock changes from the pre-treatment baseline and after 2-years on the and plots with depth (Olson et al., 214). The treatment lost SOC stock in the 2 cm layer when compared to the pre-treatment SOC and maintained the SOC stock for the 2 to 7-cm layer. The treatment sequestered SOC in the cm layer (9% more) when compared to pre-treatment SOC stock but lost SOC in to 3 cm layer (13%) and maintained the SOC stock for the 3 to 7-cm layer. The treatment did retain more SOC stock than to a 2 cm depth, but from 2 to 3 cm layer the SOC stock was lower than that for and the same for the 3 to 7 cm layer. This can be a result of the moldboard plow mixing the A horizon materials into the 2 to 3 cm layer of 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.

5 K.R. Olson, M.M. Al-Kaisi / Catena 1 (21) 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 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.,. Soil carbon and nitrogen changes as influenced by tillage and cropping systems in some Iowa soils. Agric. Ecosyst. Environ. J. 1, Baker, J.M., Ochsner, T.E., Venterea, R.T., Griffits, T.J., 27. Tillage and soil carbon sequestration what do we really know? Agric. Ecosyst. Environ. J. 118, 1. Cochran, W.G., Cox, G.M., 197. Experimental Designs (2nd). John Wiley and Sons, Inc., New York. Doetterl, S., Six, J., Van Wesemael, B., Van Oost, K., 212. 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