Soil organic carbon and aggregation in response to thirty-nine years of tillage management in the southeastern US

Soil & Tillage Research 197 (2020) 104523

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Soil organic carbon and aggregation in response to thirty-nine years of tillage management in the southeastern US

T

Surendra Singha, Amin Nouria, Shikha Singha, Saseendran Anapallib, Jaehoon Leea, Prakash Arellic, Sindhu Jagadammaa,* a

Department of Biosystems Engineering and Soil Science, University of Tennessee, 2506 E J Chapman Drive, Knoxville, TN, 37996, USA USDA-ARS, Crop Production Systems Research Unit, 141 Experiment Station Road, Stoneville, MS 38776, USA c USDA-ARS, Crop Genetics Research Unit, 605 Airways Blvd., Jackson, TN 38301, USA b

ARTICLE INFO

ABSTRACT

Keywords: Soil organic carbon No-tillage Aggregate associated carbon Wheat-soybean double-crop Soil aggregation Permanganate oxidizable carbon No-till with wheat cover crop Resistant carbon Wet aggregate stability Water extractable carbon Continuous soybean Moldboard plow Chisel plow Disc plow Southeastern US Mean weight diameter Soil organic carbon accumulation

Agricultural management practices control soil organic carbon (SOC) content in croplands. Long-term cropping system experiments offer a great opportunity to understand the magnitude and direction of SOC change in response to management practices. Such information is very limited from the southeastern US, a region with warm and humid climatic conditions that typically favor SOC decomposition over accumulation. Therefore, this study was conducted to assess the effect of 39 years of chisel plow (CP), disc plow (DP), moldboard plow (MP), no-tillage (NT), NT with winter wheat (Triticum aestivum L.) cover crop (NTW), and NT with wheat-soybean (Glycine max L.) double crop (NTWD) on total SOC and SOC fractions including permanganate oxidizable C (POXC), water extractable C (WEC), resistant C (RC), and aggregate-associated SOC in a continuous soybean system. Additionally, aggregate size distribution, mean weight diameter (MWD), and wet aggregate stability (WAS) were determined. Results showed that NTW and NTWD significantly increased SOC and POXC compared to MP with mean SOC (g kg−1 soil) of 12.2 (NTW) ≥10.9 (NTWD) > 7.2 (MP) and mean POXC (mg kg−1 soil) of 465 (NTWD) ≥418 (NTW) > 252 (MP). The WEC and RC fractions did not differ among treatments. Across the treatments, the greatest aggregate-associated SOC concentration was found in microaggregates (0.053–0.25 mm) and the lowest in clay- and silt-size particles (< 0.053 mm). Additionally, WAS under NT systems was significantly higher (45.5–52.3 %) than under tilled treatments (21.9–29.1 %). Total SOC correlated significantly with POXC (r = 0.68, p < 0.01), RC (r = 0.46, p < 0.05), WAS (r = 0.65, p < 0.01), and aggregate-associated SOC concentrations (r > 0.6, p < 0.01). Overall, this study revealed that NT enhanced SOC and POXC accumulation and macroaggregation compared to tilled treatments. Cover cropping and double cropping further improved SOC accumulation. In conclusion, long-term adoption of different tillage intensities can strongly alter SOC dynamics in bulk soil and aggregates under continuous soybean production systems of the southeastern US.

1. Introduction Soil organic carbon (SOC) pool constitutes the largest terrestrial reservoir of C (2300 Pg C at 3 m depth) and a major sink for the atmospheric CO2 (Jobbágy and Jackson, 2000). Thus, improving C sequestration is considered as a potential option for mitigating global warming (Lal, 2004; Minasny et al., 2017). Among the terrestrial ecosystems, croplands have the greatest C sequestration potential in the top layer (Chen et al., 2018). Adoption of conservation management practices including no-tillage (NT), cover crops, crop rotations, and organic soil amendments have shown the potential to increase C sequestration in agricultural soils (Lal, 2004; West and Post, 2002),

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although the magnitude of SOC pool varies widely across soils (Bohn et al., 2002) and climates (Campbell et al., 1998; Janssen, 1984). Multiple studies showed that SOC pool is positively related to soil structural formation and stability (Lal, 2016; Six et al., 2000), soil water infiltration and retention (Causarano et al., 2008; Lal, 2006), soil thermal properties (Lakshmi et al., 2003; Perez-Brandán et al., 2012), soil fertility (Aggarwal and Power, 1997; Gavriliev, 2003; Sainju et al., 2007; Vallis et al., 1996), and overall soil health (Al-Kaisi et al., 2014; Franzluebbers, 2002; Lal, 2011; Sainju et al., 2005). Management-induced changes in SOC accumulation are generally quantified as change in total SOC, which typically takes a long time to respond to different management practices. The total SOC is composed

Corresponding author at: Tel.:865-974-2690. E-mail address: [email protected] (S. Jagadamma).

https://doi.org/10.1016/j.still.2019.104523 Received 26 July 2019; Received in revised form 30 October 2019; Accepted 26 November 2019 0167-1987/ © 2019 Elsevier B.V. All rights reserved.

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of different fractions associated with both bulk soil and aggregates, which respond differently to different management systems (Jagadamma and Lal, 2010a; von Lützow and Kögel-Knabner, 2009). Depending on the protection mechanisms and chemical characteristics, SOC fractions can be broadly classified into active or passive categories with varying turnover rates and stability (McLauchlan et al., 2006). Management changes are reflected faster in the active than passive SOC fractions (Kaiser et al., 2014; Pandey et al., 2014; Six et al., 1999; Wander and Bidart, 2000). Thus, partitioning total SOC into different fractions can illustrate management-induced changes in C accrual in a more realistic manner (Gabarrón-Galeote et al., 2015; Madhavan et al., 2017; Skjemstad et al., 2004). In general, active SOC fraction is closely associated with soil health and agronomic benefits (Lal, 2006; Quiroga et al., 2006) while passive fraction contributes to long-term C sequestration (Kumar et al., 2014). Laboratory techniques are available to isolate multiple SOC fractions with varying stability including water extractable C (WEC), permanganate oxidizable C (POXC), hydrogen peroxide resistant C (RC), and aggregate-associated C. The WEC pool is smaller in size, but is very active and sensitive to cropland management changes (Chantigny, 2003; Scaglia and Adani, 2009). Another relatively active SOC pool, POXC, is also responsive to cropping system management (Calderón et al., 2017; Culman et al., 2012; Hurisso et al., 2016). Contrastingly, RC represents a passive SOC pool with turnover rates extending over thousands of years (Jagadamma et al., 2010). Agronomic management changes also affect soil aggregation (AlKaisi et al., 2005; Nouri et al., 2018) and C protection in aggregates (Six et al., 1999, 2000). For example, conventional tillage practices disrupt soil aggregates and expose C to microbial decomposition whereas NT not only protects aggregates from breaking down but also stimulates further sequestration of C in aggregates by retaining more plant residues (Álvarez and Álvarez, 2000; Six et al., 1999; Wright and Hons, 2004). Depending on the degree of soil disturbance and the rate of biomass incorporation, different conventional tillage practices differently influence aggregate size distribution and stability (Al-Kaisi et al., 2005). For example, chisel plow (CP) causes less soil disturbance than moldboard plow (MP), leading to the accumulation of more total as well as active SOC and enzymatic activities under CP than MP (Panettieri et al., 2013). Similarly, a study conducted on long-term corn (Zea mays L.)-soybean and continuous corn systems in Illinois showed decreased SOC pools with increased tillage intensities (Jagadamma and Lal, 2010a). As the most popular conservation management practice, NT is widely practiced in diverse cropping systems across the US. Adoption of NT and other conservation management practices are particularly relevant in southeastern US, a region that experiences hot and humid climatic conditions that stimulates SOC losses over gain and decreases agricultural productivity (Bruce et al., 1990; Franzluebbers, 2002, 2010; Sanchez et al., 1989). Thus, integration of NT with other conservation management options such as cover cropping and double cropping may accelerate SOC accumulation and improve soil health in the agroecosystems of southeastern US. The increase in SOC because of the long-term adoption of conservation tillage and crop rotations has been well documented from different regions of the US (Chalise et al., 2019; Franzluebbers, 2010; Halvorson et al., 2002; Havlin et al., 1990; Ibrahim et al., 2015; Jagadamma and Lal, 2010a; Jagadamma et al., 2007; Maiga et al., 2019). Continuous soybean under NT is a common cropping system in Tennessee, a southeastern state in which 79 % of total cropland acreage is under NT (USDA-NAAS, 2018). In addition, growing wheat as a double grain crop with soybean or as a winter cover crop in soybean systems is becoming popular in the region. Currently, > 30 % soybean acreage in Tennessee is double cropped after wheat (UTcrops, 2018). However, tillage practices such as MP, CP and disc plow (DP) are still practiced by some conventional and most organic producers in the state. Therefore, a comparative evaluation of the long-term impacts of

different tillage intensities on SOC dynamics in the agroecosystems of Tennessee is valuable. Therefore, the present study was conducted to evaluate the long-term impacts of different intensities of tillage as well as combination of NT with other conservation practices on bulk soiland aggregate-associated SOC fractions in the soybean production systems of Tennessee. 2. Materials and methods 2.1. Site descriptions This study was conducted on a long-term experiment established in 1979 at the University of Tennessee’s West Tennessee Research and Education Center in Jackson, TN (35°37′22″ N, 88°50′47″ W; elevation 125 m). Thirty-year mean annual temperature of the region was 15.6 ℃ and the mean annual rainfall was 1375 mm. Soils of the experimental site belong to Lexington series (fine-silty, mixed, thermic, Ultic Hapludalfs) with 0–2 percent slope. Basic soil properties under each treatment are shown in Table S1. 2.2. Treatments and cultural practices Treatments included (i) chisel plowing to 20 cm soil depth followed by roller harrowing (CP), (ii) disc plowing to 10 cm depth followed by roller harrowing (DP), (iii) moldboard plowing to 25 cm depth followed by disking and roller harrowing (MP), (iv) no tillage (NT), (v) NT with winter wheat cover crop (NTW), and (vi) NT with wheat-soybean double crop (NTWD). Treatments were arranged based on completely randomized design with four replications. Tillage operations were typically applied on tilled systems in late May. In NTW system, wheat cover crop was planted in October-November and chemically terminated almost three weeks prior to sowing soybean by applying 0.71 kg ha−1 paraquat (1-methyl-4-(1-methylpyridin-1-ium-4-yl) pyridin-1ium). In NTWD system, wheat was planted in mid-October each year. In all the plots, soybeans were planted in late May of each year with four rows of soybeans per plot and 1.5 m of row spacing (plots size 18 m × 6 m). All plots were treated with 3.36 kg ha−1 alachlor (2Chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide) and 0.42 kg ha−1 metribuzin (4-Amino-6-tert-butyl-3-methylsulfanyl-1,2,4triazin-5-one) for pre-emergence weed suppression and 0.13 kg ha−1 clethodim (2-[1-[[(E)-3-chloroprop-2-enoxy] amino] propylidene]-5(2-ethylsulfanylpropyl) cyclohexane-1,3-dione) for post-emergence weed control. No nitrogen was applied for soybean crop. However, nitrogen was applied at 33.6 kg ha−1 for wheat under NTW and NTWD treatments. Details of tillage and residue management under each treatment are provided in Table S2. 2.3. Soil sampling In July 2017, a composite soil sample from each plot was collected by combining six randomly collected cores from soybean inter-row from 0−15 cm depth using a bucket type auger of 5 cm diameter. A part of soil sample was air-dried and passed through an 8 mm sieve to collect samples for aggregate size distribution and stability analyses, and another part was passed through a 2 mm sieve for bulk-soil analysis. 2.4. Bulk soil analysis Total soil C was determined by dry combustion of pulverized soil samples at 950℃ using a CN analyzer (Elementar vario TOC cube in solid mode, Hanau, Germany). Soil organic C was assumed to be equal to total soil C as soil pH was < 7.1 (Al-Kaisi et al., 2005). The SOC stocks were calculated as product of bulk soil SOC concentration, bulk density (BD), and sampling depth (Lal et al., 1998). The BD values were obtained from Nouri et al., 2018, which were measured by the 2

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undisturbed core method (Grossman and Reinsch, 2002). Permanganate oxidizable C (POXC) was determined following the methods of Weil et al. (2003) and Culman et al. (2012). Briefly, 2.5 g soil sample was mixed with 20 mL of 0.02 M potassium permanganate (KMnO4) solution. The solutions were shaken at 120 rpm for 2 min and allowed to settle for 8 min. Then, supernatant of the solution was collected and diluted 100 times. The absorbance of the diluted solution was measured at 550 nm using Biotek plate reader (Biotek Instruments, Inc., Vermont, USA). The solution absorbance was converted to POXC using calibrated KMnO4 standard curves and converted to mg kg−1 soil basis using the equation of Weil et al. (2003). The WEC was measured based on the method of Jones and Willett (2006). Briefly, 5 g of air-dried soil was mixed with 25 mL MilliQ water followed by shaking at 120 rpm for 10 min. After filtration using a filter paper (Whatman No. 42), the extracts were analyzed for total C using the CN analyzer (Elementar vario TOC cube in liquid mode, Hanau, Germany). Resistant C pool (RC) was determined by hydrogen peroxide (H2O2) oxidation (Jagadamma et al., 2010). Briefly, 1 g of air-dried soil was wetted with 10 mL deionized water and then, 30 mL of 10 % H2O2 was added and reacted at 50℃ in a waterbath. After 2–3 days when reaction was completed, the solution was centrifuged at 2500×g for 15 min and the supernatant was discarded. The oxidation process was repeated two more times and then, the samples were washed three times with 40 mL deionized water, centrifuged, and oven dried at 50℃. The SOC of the oven-dried oxidized samples were measured using a CN analyzer as described before.

Equation (2). Aggregate-associated SOC content = weight of each aggregate size x aggregate-associated SOC concentration (2) To measure the wet aggregate stability (WAS), 4 g of air-dried aggregates of size 1−2 mm were placed on a 0.26 mm sieve and allowed for capillary wetting from the bottom. Then, the sieves with the aggregates were lowered and raised in distilled water for 3 min at a rate of 35 times per minute using a wet sieving apparatus (Eijkelkamp Agrisearch Equipment, Giesbeek, the Netherlands). Thereafter, unstable and stable fractions were collected, oven-dried at 105℃ and weighed. Sand correction of the stable aggregate fraction was done by placing the fraction on 0.26 mm sieve, washing with deionized water, collecting the sand fraction retained on top of the sieve and oven drying (Kemper and Rosenau (1986). Finally, WAS was calculated using Equation (3): WAS (%) = (Fraction remained on sieve - sand) / (Total sample - sand) x 100 (3) 2.6. Statistical analysis Analysis of variance (ANOVA) was conducted using the PROC GLIMMIX procedure in SAS v. 9.4 statistical package (SAS, 2012). Tillage treatments were considered as fixed effect and replicates were considered as random effect. Least square means were separated using the Least Significant Difference (LSD) at p < 0.05. Canonical correlation analysis was conducted using PROC CORR procedure in SAS and the strength of covariance between SOC fractions was expressed as the Pearson’s correlation coefficients.

2.5. Aggregate analysis Aggregate size distribution was determined by dry sieving method which involved placing 100 g of air-dried and 8 mm sieved soil sample on top of a stack of sieves of sizes 4.75, 2, 0.25 and 0.053 mm, and shaken using a vertical sieve shaker apparatus (CSC sieve shaker, Fairfax, VA) for 5 min at an amplitude of 0.1 mm. Aggregates were fractioned into five size classes: 4.75−8 mm (very large macroaggregates, VLMA), 2–4.75 mm (large macroaggregates, LMA), 0.25−2 mm (small macroaggregates, SMA), 0.053-0.25 mm (microaggregates, MiA) and < 0.053 mm (clay- and silt-size particles, CSP). Aggregate size distribution was calculated by weighing aggregates remained on top of each sieve and expressing as a fraction of the initial mass of sample used. The mean weight diameter (MWD) was calculated using Eq. (1) (Youker and McGuinness, 1957).

MWD =

n i=1

x¯i wi

3. Results and discussion 3.1. Total soil organic C in bulk soil Bulk soil SOC concentration and SOC stocks varied significantly across the treatments but both showed almost identical response to treatments (Table 1). The SOC under the three no-tilled treatments NT, NTW, and NTWD were 39 %, 69 %, and 51 % greater than MP (7.2 g kg−1 soil), but there were no significant differences among the no-tilled treatments. Similarly, SOC among the three tilled treatments (CP, DP, and MP) were not significantly different. Furthermore, SOC under NT and NTWD was not significantly different from CP and DP treatments but that under NTW was 36 % greater than CP (9 g kg−1 soil) and 33 % greater than DP (9.2 g kg−1soil) Similar results were also reported by Duiker and Beegle (2006) from a study conducted on a silt loam soils in Pennsylvania, which showed significantly greater SOC concentration within the top 15 cm in NT system compared to MP. They also found that SOC in NT did not differ from that in CP and DP systems. In general, tillage practices that involve mechanical soil inversion, such as MP, accumulate less SOC in the surface soil layer than reduced tillage

(1)

where, xi is the mean diameter of aggregates size fraction on each sieve (mm), wi is the mass of aggregates remained on each sieve, and n is the number of aggregate size fractions. The aggregate-associated SOC concentration (g kg−1 aggregates) in each aggregate size class was determined using the Elementar CN analyzer in solid mode, as described in Section 2.4, and then, aggregateassociated SOC content in soil (g kg−1 soil) was calculated using

Table 1 Bulk soil carbon fractions, bulk densities and SOC stocks under different tillage systems. Treatments

SOC g kg−1

POXC mg kg−1

WEC mg kg−1

RC g kg−1

BD Mg m−3

SOC stocks kg m−2

MP CP DP NT NTWD NTW

7.22 8.95 9.16 10.0 10.9 12.3

252 302 306 301 465 418

59.3 52.2 54.3 54.8 60.8 60.0

0.483 0.470 0.467 0.433 0.537 0.510

1.45 (0.009)ab 1.47 (0.005)a 1.43 (0.01)b 1.46 (0.01)a NA 1.48(0.006)a

1.57 1.97 1.97 2.21 NA 2.72

(0.36)c (0.61)bc (0.60)bc (1.57)ab (0.25)ab (0.81)a

(48.4)c (47.5)bc (66.2)bc (83.4)bc (12.5)a (22.5)ab

(2.09)a (6.39)a (7.10)a (11.9)a (8.76)a (7.02)a

(29.1)a (40.4)a (23.3)a (59.0)a (64.4)a (5.77)a

(0.07)c (0.14)bc (0.13)bc (0.36)ab (0.17)a

Means (standard error) followed by different lowercase letters within a column are significantly different across tillage treatments at p ≤ 0.05. MP: Moldboard plow; CP: Chisel plow; DP: Disk plow, NT: No-tillage; NTWD: No-tillage with wheat- soybean double crop; NTW: No-tillage with wheat cover crop. SOC: soil organic carbon, POXC: permanganate oxidizable carbon, WEC: water extractable carbon, RC: resistant carbon, BD: bulk density (adopted from Nouri et al., 2018). NA; BD data was not available. 3

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systems such as CP and DP (Al-Kaisi et al., 2005; Angers and EriksenHamel, 2008; Duiker and Beegle, 2006). We found no differences in SOC among the three NT systems, despite higher residue C input in NTW and NTWD compared to NT. This indicates that long-term NT is a better strategy to increase the SOC build-up than more residue addition through cover cropping or double cropping in the warm and humid climate of the study region. These findings are in accordance with the study conducted in the southeastern US by Jagadamma et al. (2019), which reported that NT systems integrated with winter wheat cover crop did not accumulate more SOC than NT with no cover crop in the top 15 cm of soil profile. Wegner et al. (2018) also reported that higher organic matter inputs from winter cover cropping integrated with NT did not increase SOC than that of NT alone on silt clay loam soils. We also found that NT, regardless of cropping systems, accumulated more SOC than tilled systems. On the other hand, some past studies reported that tillage practices redistributed SOC from the surface to the subsurface soil layers (Dıaz-Zorita and Grove, 2002; Franzluebbers, 2010; Jagadamma et al., 2019; Luo et al., 2010; Olson, 2010; Stetson et al., 2012; Wander et al., 1998; Singh et al., 2019). Since we only considered the top 15 cm of the soil profile, the effect of tillage on redistribution of SOC from surface to sub-surface soil cannot be evaluated. Other longterm studies conducted on silt-loam soils in humid climates reported similar results with higher SOC under NT than under tillage in the upper 15 cm soil profile (Al-Kaisi et al., 2005; Duiker and Beegle, 2006). Although combined effect of NT and cover cropping on SOC accumulation was similar to NT alone in our study, results from other agroecological regions across the US showed increased SOC accumulation when cover crops were integrated into no-till systems (Blanco-Canqui et al., 2011; Chalise et al., 2019; Kibet et al., 2016; Maiga et al., 2019; Olson et al., 2014).

2016; Tirol-Padre and Ladha, 2004). It is possible that the slight increase in POXC due to no-tillage could have been masked by the more prominent effect of higher residue inputs under NTW and NTWD treatments. Plaza-Bonilla et al. (2014) also reported that POXC is more sensitive to management changes when organic matter inputs are abundant. 3.2.2. Water extractable carbon Water extractable carbon did not vary significantly among the treatments, and the mean values ranged from 52.2–60.8 mg kg−1 soil (Table 1). The lack of treatment response to WEC, one of the most active pools of SOC (Chantigny, 2003; Scaglia and Adani, 2009), was unexpected. However, in soils with higher clay content, as in this study, WEC can be sorbed onto the clay minerals irrespective of the treatments (Bolan et al., 2011; Nie et al., 2018; Ozdes et al., 2011; Singh et al., 2017, 2016; Singh et al., 2018). A study by Si et al. (2018) showed significant WEC variations among tillage treatments when the sampling depth was 0−10 cm, indicating that perhaps shallow sampling depth is a key factor to observe significant treatment response to WEC concentration. In NT systems, easily decomposable C inputs are typically concentrated in the top few centimeters of soil. By collecting a single sample to represent the top 15 cm layer in this study, the higher WEC concentrations in the surface layers of NT systems may have been diluted (Chu et al., 2017, 2019; Jagadamma et al., 2019; Singh et al., 2019). 3.2.3. Resistant carbon Resistant carbon (RC) did not vary significantly among the treatments, and the mean values across the treatments ranged from 0.43 to 0.51 g kg−1 soil (Table 1). This fraction represents only 4.2–6.5% of bulk soil total SOC and relatively lower proportions were associated with no-tilled treatments (4.2–4.9 % of bulk soil SOC) than tilled treatments (5.1–6.5 % of bulk soil SOC). Jagadamma and Lal (2010b) also reported no significant differences in RC among long-term NT, CP, and MP treatments under continuous corn cropping system in Ohio. Lack of response of RC with long-term tillage practices indicate that long-lived C pools in soil are not affected by management changes unlike the relatively active pools such as POXC (Kumar et al., 2012; Lal, 2006; Quiroga et al., 2006; Scaglia and Adani, 2009). Regardless of the treatments, the mean RC values in this study, as percentage of bulk soil SOC, were consistently lower than similar results reported by several past studies (Grasset et al., 2009; Helfrich et al., 2007). However, the trend of increased RC with increasing tillage intensity, though statistically not significant, was consistent with the findings of Jagadamma and Lal (2010b). Higher proportion of passive SOC fractions in tilled than no-tilled soils were reported by several past studies (e.g., Grasset et al., 2009; Helfrich et al., 2007; Kaiser and Guggenberger, 2003).

3.2. Soil organic C fractions in bulk soil 3.2.1. Permanganate oxidizable carbon Permanganate oxidizable carbon was significantly influenced by the treatments (Table 1). The mean POXC for NTWD was 465 mg kg−1 soil, which was significantly higher compared to all other treatments except NTW (417 mg kg−1 soil). In contrast to total SOC, POXC of NTWD was significantly higher and that of NTW was numerically higher than NT (301 mg kg−1 soil), meaning that POXC was more sensitive to the additional organic C inputs in the NTWD and NTW systems from wheat residues. However, POXC from the NT treatment was not significantly different compared to that from tilled treatments such as CP (302 mg kg−1 soil), DP (300 mg kg−1 soil) and MP (252 mg kg−1 soil). Our results are in accordance with a study conducted by Awale et al. (2013) in midwestern US, in which POXC did not differ significantly between NT and tillage treatments when soil was sampled from 0−15 cm depth. Results indicated that higher residue input to soil increased relatively active POXC fraction, not total SOC (Hurisso et al.,

Table 2 Aggregate size distribution and mean weight diameter under different tillage systems. Treatments

MP CP DP NT NTWD NTW

Aggregate size distribution (%) VLMA (4.75−8 mm)

LMA (2–4.75 mm)

SMA (0.25−2 mm)

MiA (0.053–0.25 mm)

CSP (< 0.053 mm)

10.2cC 16.1bcC 14.5bcC 15.7bcC 26.6bB 58.0aA

28.9aB 29.6 aB 26.4aB 28.4aB 35.7aA 27.6aB

46.0aA 41.4aA 40.5aA 40.5aA 26.1bB 9.3cC

8.08abC 6.30abC 9.10aC 7.85abC 3.82bcC 1.46cC

6.83bC 6.60bC 9.49aC 7.57abC 7.81abC 3.67cC

MWD (mm) 2.16c 2.5bc 2.29c 2.43c 3.2b 4.74a

Numbers followed by different lowercase letters within a column are significantly different across tillage treatments at p ≤ 0.05. Numbers followed by different uppercase letter within a row are significantly different across aggregate size fractions at p ≤ 0.05. MP: Moldboard plow; CP: Chisel plow; DP: Disk plow, NT: Notillage; NTW: No-tillage with wheat cover crop; NTWD: No-tillage with wheat- soybean double crop. VLMA: very large macroaggregates; LMA: large macroaggregates; SMA: small macroaggregates; MiA: microaggregates, CSP: clay- and silt-size particles. 4

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3.3. Aggregate fractions

et al., 2018, 2019; Yang and Wander, 1998). In general, WAS is strongly influenced by the amount and type of organic matter inputs (Barzegar et al., 2002; Lichter et al., 2008; Maiga et al., 2019; Nascente et al., 2015; Six et al., 2000), however, increased organic matter inputs from NTW and NTWD treatments in this study did not increase WAS compared to NT alone.

3.3.1. Aggregate size distribution and mean weight diameter Aggregate size distribution and MWD varied significantly among treatments (Table 2). Soils under NTW had the highest proportion of very large macroaggregates (VLMA) of 4.75−8 mm size (58 %) and lowest proportion of clay- and silt-size particles (CSP) of < 0.053 mm size (3.67 %) compared to all other treatments. The proportion of large macroaggregates (LMA) of 2–4.75 mm size did not vary significantly among tillage treatments whereas small macroaggregates, (SMA) of 0.25−2 mm size was the highest under CP, DP, MP, and NT (40.5–46%) followed by NTWD (26.1 %) and the least under NTW (9.3 %). The microaggregates (MiA), 0.053-0.25 mm size, also followed a similar trend as that of SMA with NTW showed the lowest proportion (1.46 %) which was statistically similar to NTWD (3.82 %) but lower than CP, DP, MP and NT (6.3–9.1%). Overall, NT treatments that added more crop residues (NTW and NTWD) facilitated macroaggregation with proportionately higher larger size aggregate (> 2 mm) compared to other treatments. Consequently, the MWD was the highest for NTW (4.74 mm) followed by NTWD (3.2 mm) compared to other treatments, which were statistically identical with mean MWD ranged from 2.16 to 2.5 mm. The positive effect of crop residue input and concomitantly increased SOC on macroaggregate formation was reported by several studies (Blanco-Canqui et al., 2013; De Gryze et al., 2005; Eynard et al., 2004; Nouri et al., 2018). In warm and humid regions, as in the case of this study location, formation and retention of aggregates is highly constrained by the faster decomposition of crop residues, particularly the residues of leguminous crops like soybean with lower C: N ratio. Rapidly decomposing organic matter has a more immediate and transient effect leading to aggregate breakdown (Griffiths and Burns, 1972), whereas slowly decomposing organic matter has slow but more stable effect on aggregate formation (Blanco-Canqui and Lal, 2004; Martin and Waksman, 1941). Consequently, we found more larger-sized aggregates (> 2 mm) than smaller-sized aggregates from NTW and NTWD treatments that returned both soybean and wheat residues than other treatments, which only returned soybean residues to soil.

3.3.3. Aggregate-associated SOC concentrations and contents Among all the treatments, MP had lower SOC concentration in all aggregate size classes except LMA (2–4.75 mm) (Fig. 2). Among all aggregate size classes, regardless of the treatment differences, MiA (0.053–0.25 mm) had relatively greater SOC concentrations despite less proportion of aggregates (Table 2), while CSP (< 0.053 mm) had relatively lower SOC concentrations. Similar results of decreased SOC concentrations in CSP fraction under no-tilled and tilled cropping systems were reported from the agricultural soils of Georgia (Bossuyt et al., 2002; Wright and Hons, 2004). When aggregate-associated SOC contents were calculated on the basis of unit mass of soil, results were significantly different among different treatments and aggregate size fractions (Table 3) and the treatment differences were mostly similar to that of aggregate size distribution (Table 2). Aggregate-associated SOC contents in VLMA were: NTW > NTWD = NT = CP = DP = MP. The SOC contents in LMA did not differ among treatments but that in SMA followed the trend: NTW < NTWD = NT = CP = DP = MP. In MiA size fraction, SOC content was the greatest for DP. Regardless of the treatments, CSP fraction stored the least amount of SOC among all aggregate size fractions. Aggregate-associated SOC content in CSP fraction was the greatest in DP followed by NTWD compared to other treatments, however the CSP in general stored an order of magnitude lower SOC than larger aggregates. Overall, no-tilled treatments with wheat residue input (NTW and NTWD) showed greater SOC storage in larger aggregate size fractions (> 2 mm) compared to tilled treatments and NT alone, which stored more SOC in SMA. Higher residue retention coupled with NT management promotes macroaggregation, which physically protects SOC within the aggregates (De Gryze et al., 2005; Gupta and Germida, 1988; Jagadamma and Lal, 2010a; Li et al., 2019; Jiao et al., 2006; Six et al., 1999, 2000). Further, regression analysis of percent change in aggregate-associated SOC content with percent change in aggregate size distribution revealed differential responses based on aggregate size fractions when transitioning from conventional tillage (MP, CP, DP) to conservation tillage (NT, NTWD, NTW) (Fig. 3). Strong positive relationship (R2 > 0.85) between percent change in aggregate-associated SOC content with percent change in aggregate size distribution were found in all cases, except when CP was transitioned to

3.3.2. Wet aggregate stability Wet aggregate stability, WAS, differed significantly between tilled and no-tilled treatments as NT, NTW and NTWD showed significantly higher WAS (45.5–52.3%) compared to CP, DP and MP treatments (21.9–29.1%) (Fig. 1). However, WAS did not vary significantly among tilled (CP, DP, and MP) and no-tilled (NT, NTW, and NTWD) treatments. The unfavorable effect of tillage on WAS has been reported by several studies (Blanco-Canqui et al., 2011; Maiga et al., 2019; Nouri

Fig. 1. Wet aggregate stability under different tilled and no-tilled treatments. MP: Moldboard plow; CP: Chisel plow; DP: Disk plow, NT: Notillage; NTWD: No-tillage with wheat – soybean double crop; NTW: No-tillage with wheat cover crop. Different letters denote statistically different means based on Least Square Difference (LSD) at p ≤ 0.05.

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Fig. 2. Effects of tillage treatments on aggregated-associated SOC concentrations (g kg−1 aggregates). Different letters denote statistically different means within an aggregate size fraction based on Least Square Difference (LSD) at p < 0.05. VLMA: very large macroaggregates; LMA: large macroaggregates; SMA: small macroaggregates; MiA: microaggregates, CSP: clay- and silt-size particles.

Table 3 Aggregate-associated SOC contents in bulk soil under different tillage regimes. Treatments

MP CP DP NT NTWD NTW

Aggregate-associated SOC contents (g kg−1 soil) VLMA (4.75−8 mm)

LMA (2–4.75 mm)

SMA (0.25−2 mm)

MiA (0.053–0.25 mm)

CSP (< 0.053 mm)

0.78bC 1.58bB 1.45bC 2.41bABC 4.13bA 9.82aA

2.42aB 2.77aA 2.36aB 3.88aAB 3.58aA 2.91aB

3.67aA 3.91aA 4.09aA 4.5aA 2.98aA 1.13bB

0.92abcC 0.9abcB 1.44aC 1.18abBC 0.73bcB 0.29cB

0.42bcC 0.46bB 0.66aC 0.53abC 0.65aB 0.28cB

Numbers followed by different lowercase letters within a column are significantly different across tillage treatments at p ≤ 0.05. Numbers followed by different uppercase letter within a row are significantly different across aggregate size fractions at p ≤ 0.05. MP: Moldboard plow; CP: Chisel plow; DP: Disk plow, NT: Notillage; NTW: No-tillage with wheat cover crop; NTWD: No-tillage with wheat-soybean double crop. VLMA: very large macroaggregates; LMA: large macroaggregates; SMA: small macroaggregates; MA: microaggregates, CSP: clay- and silt-size particles.

NT (R2 = 0.007). Additionally, the magnitude of changes also varied among transitions (i.e. magnitude of change was higher when relatively intensive MP was transitioned to NTW compared to other transitions). The magnitude of change was higher under higher aggregate sized fractions than that of other aggregate sized fractions, as VLMA were the most responsive to the most of the transitions. These results are in accordance with the aggregate hierarchy model concept that states that microaggregates are bound together to form macroaggregates with the help of organic binding agents resulting in SOC-enriched macroaggregates (Six et al., 2000; Tisdall and Oades, 1982). Disruption of macroaggregates by increased tillage intensity results in decreased SOC-enriched macroaggregates (Elliott, 1986; Six et al., 2000).

Lal, 2006; Lucas and Weil, 2012; Quiroga et al., 2006). However, RC did not show many significant correlations with other SOC fractions. 4. Conclusions This study focused on a comprehensive evaluation of changes in bulk soil SOC as well as several SOC fractions in response to 39 years of tillage management in a continuous soybean system in the southeastern US. Overall, the results revealed higher total SOC accumulation by NT treatments compared to tilled treatments, which was mainly achieved through integrating NT with wheat as a cover crop (NTW) or wheat as a double crop (NTWD) in the soybean systems. We also observed that WAS, aggregate-associated SOC and POXC were also favored under NTW management along with macroaggregate formation. By evaluating SOC accumulation and distribution among bulk soil and aggregate fractions under diverse long-term management practices, this study provided a broader understanding of the management-induced SOC changes under continuous soybean system in the warm and humid climatic conditions of the southeastern US.

3.4. The relationship among measured SOC fractions The correlations among bulk soil SOC, POXC, WEC, RC, WAS, and aggregate-associated SOC concentrations are shown as Pearson’s correlation coefficients in Table 4. In general, total SOC was positively correlated with all studied parameters except WEC (p < 0.05). This is also true for POXC (0.51 < r > 0.78, p < 0.05) and WAS (0.52 < r > 0.65, p < 0.05). A strong positive correlation between SOC and POXC suggests that POXC can be a reliable proxy for total SOC changes. Similar positive correlations between SOC and POXC were also reported in other studies (Calderón et al., 2017; Culman et al., 2012;

Declaration of Competing Interest The authors declare no conflict of interest.

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Fig. 3. Response of different aggregate size fractions to transition from MP, CP and DP treatments to NT, NTWD and NTW. MP: Moldboard plow; CP: Chisel plow; DP: Disk plow, NT: No-tillage; NTW: No-tillage with wheat cover crop; NTWD: No-tillage with wheat-soybean double crop. Percent change in mass of different aggregate-size fractions was regressed against percent change in aggregateassociated SOC content under each treatment transition. VLMA: very large macroaggregates; LMA: large macroaggregates; SMA: small macroaggregates; MiA: microaggregates, CSP: clay- and silt-size particles.

Table 4 Pearson’s correlation coefficients (r) among measured properties and SOC fractions. Bulk soil properties

Bulk soil properties

Aggregate-associated SOC

SOC POXC WEC RC WAS VLMA LMA SMA MA CSP

Aggregate-associated SOC

SOC

POXC

WEC

RC

WAS

VLMA

LMA

SMA

MiA

CSP

– 0.68*** 0.30 0.46** 0.65*** 0.68*** 0.60*** 0.85*** 0.84*** 0.81***

– 0.30 0.58** 0.55** 0.58*** 0.51** 0.67*** 0.78*** 0.70***

– 0.59*** 0.42* 0.48** 0.38 0.56*** 0.46** 0.44**

– 0.50* 0.54** 0.36 0.43 0.53*** 0.69***

– 0.64*** 0.52** 0.59*** 0.65*** 0.62***

0.79*** 0.56*** 0.64*** 0.70***

– 0.55*** 0.52*** 0.67***

– 0.85*** 0.79***

– 0.79***

–

*p ≤ 0.1 level, ** p ≤ 0.05, *** p ≤ 0.01. SOC: soil organic carbon, POXC: permanganate oxidizable carbon, WEC: water extractable carbon, RC: resistant carbon, WAS: wet aggregate stability, VLMA: very large macroaggregates, LMA: large macroaggregates, SMA: small macroaggregates, MiA: microaggregates, CSP: clay- and silt-size particles.

Acknowledgments

References

The authors would like to thank personnel at the University of Tennessee’s West Tennessee Research and Education Center for managing the field trails and helping with sample collection. We extend our gratitude to Dr. Liesel Schneider for statistical consulting.

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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.still.2019.104523. 7

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