TILLAGE AND RESIDUE EFFECTS ON RUNOFF AND EROSION DYNAMICS G. V. Wilson, S. M. Dabney, K. C. McGregor, B. D. Barkoll
ABSTRACT. The carry-over effects from one year to the next of surface residue and tillage management decisions on runoff and erosion are not clear. The dynamics of runoff and erosion processes during rainfall events are likely dependent on the tillage and residue management system. The objective of this study was to elucidate the effects of tillage practices and residue management, by removal of residue cover, on the properties that describe the dynamics of the runoff and erosion processes. Six-row, 12.2 m long y 5.5 m wide, plots under conventional tillage (CT) or no tillage (NT) corn (Zea mays L.) for nine years were used in this study. Plots had an average slope of 5.7% on a Grenada silt loam (Glossic Fragiudalf) soil. Rainfall simulations were conducted on a 10.7 m y 3.7 m area within each plot at a rate of 65 mm h -1 for 1 h under natural antecedent soil-water conditions (dry run), followed by a 0.5 h simulation 4 h later (wet run), and another 0.5 h application 30 min later (very wet run). The ten treatments consisted of an incomplete 2 y 2 y 3 factorial arrangement of two tillage histories (CTh and NTh), two tillage levels (tilled and not tilled), and three residue management levels (residue left, residue removed just prior to simulated rainfall, and residue removed one year prior to simulated rainfall). The missing treatments were the NTh-tilled and CTh-not tilled with residue left. The time of runoff initiation, maximum runoff rate, flow velocity, and maximum sediment concentration were used to describe differences in runoff and erosion dynamics. Residue removal resulted in significantly sooner runoff with the NT system. There was a significant carry-over effect of residue removal with runoff initiated 35% sooner the subsequent year of removal and sediment concentrations increasing by >100%. Maximum sediment concentrations were lower for the CTh land that was not tilled than for the tilled despite the untilled land experiencing sooner runoff and higher runoff rates. Tilling NTh land resulted in significantly lower sediment concentrations than tilling CTh land, suggesting that the soil quality of NT was not immediately lost when tilled, but these beneficial properties were fully lost within one year of residue removal. Keywords. Erosion, Residue cover, Runoff, Tillage, Water pressure.
S
oil losses by erosion from U.S. cropland have been estimated to be between 2 to 6.8 billion tons per year (Trimble and Crosson, 2000). Soil loss rates in the loess hills of the Mississippi River Valley (MRV) were historically among the highest in the U.S. with erosion rates of 28.9 t ha -1 yr -1 (Langdale et al., 1995), and even the relatively flat watersheds in the Mississippi Delta have shown losses as high as 11.7 t ha -1 yr -1 under conventional tilled (CT) soybean (Murphree and McGregor, 1991). No-till (NT) cropping systems have been shown to reduce soil loss to less than 3 t ha -1 yr -1 (McGregor and Mutchler, 1992). For this reason, NT has replaced CT as the most common tillage system in these loess soils. Much of the reduction in erosion with NT has been attributed to crop residue left on the surface (Foster and Meyer, 1977; McGregor et al., 1990a, 1990b). Residue cover
Article was submitted for review in June 2003; approved for publication by the Soil & Water Division of ASAE in October 2003. The authors are Glenn V. Wilson, ASAE Member, Physical Hydrologist, Seth M. Dabney, ASAE Member, Agronomist, and Keith C. McGregor, ASAE Member Engineer, Agricultural Engineer, USDA-ARS National Sedimentation Laboratory, Oxford Mississippi; and Brian D. Barkoll, Associate Professor, Department of Civil Engineering, Michigan Technological University, Houghton, Michigan. Corresponding author: Glenn Wilson, USDA-ARS National Sedimentation Laboratory, 598 McElroy Dr., Oxford, MS 38655; phone: 662-232 -2927; fax: 662 -581 -5706; e-mail:
[email protected].
protects the surface from raindrop impact that would otherwise facilitate the formation of a surface crust (Alberts and Neibling, 1994). In many developing countries, crop residue is a valuable resource (e.g., fuel, feed, paper) that is completely removed at harvest. Moldenhauer and Langdale (1995) concluded from a summary of research on residue management that erosion rates approach zero as surface cover approaches 100%, 83% reduction with 50% cover, and 30% reduction with only 10% surface residue cover. Due to the time required for decomposition of residue to improve soil tilth and general soil quality, there should be a temporal effect of residue incorporation by tillage. Brown et al. (1987) conducted a rainfall simulation on silt loam soil with four levels of corn residue that was incorporated the previous year. The freshly tilled soil had significantly greater erosion rates than soil left “undisturbed” for a year. The residue rate did not impact the “undisturbed” soil, but soil loss decreased as residue incorporation increased for the freshly tilled soil. Therefore, it appears that the lack of disturbance, and not incorporation of residue, served to reduce erosion. This is consistent with observations by Van Doren et al. (1984), who found that no more than one year of lack of tillage resulted in significantly reduced soil losses, presumably due to reconsolidation of soil. The impact of no-till on runoff is not as clear as the impact on erosion. While erosion can be reduced by as much as 80% to 90% with no-till, runoff may be reduced only slightly, with 10% reduction reported by Meyer et al. (1999) for a Grenada
Transactions of the ASAE Vol. 47(1): 119 -128
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silt loam. Rhoton et al. (2002) found lower runoff, for both a Grenada silt loam and a Rayne silt loam, from long-term (>9 years) NT plots than from CT plots due to improvement in soil properties, e.g., increased organic matter, greater porosity, enhanced aggregate stability, and lower bulk density. However, many runoff and erosion studies on NT systems are conducted shortly after their establishment (Alberts and Neibling, 1994). This time delay may be one reason why some studies (e.g., Chichester and Richardson, 1992) have not found differences in runoff between no-till and conventional tillage. In contrasts to the previous citations on silt loam soils, the Chichester and Richardson (1992) study was on a Houston Black clay. However, soil texture may not be the cause of the difference, as Lal et al. (1989) observed lower runoff from NT on a poorly drained silty clay loam. Lower runoff with CT has been attributed to the breaking up of surface crust and increases in surface roughness by tillage (Steiner, 1994). How quickly the initial infiltration increase under CT is lost or how long the benefits developed under NT last when residue cover is not maintained are uncertain. Much work has been done on the impact of various tillage and residue management systems on total runoff and soil losses, but considerably less work has been conducted on the dynamic properties of these processes or their relationships during rainfall events. Surface residue can effect many of the dynamic characteristics of the runoff and erosion processes during events such as: (1) delaying the initiation of runoff, (2) slowing the runoff rate, (3) increasing the flow depth, (4) decreasing particle detachment, and (5) creating small reservoirs for sediment deposition, thereby decreasing the sediment concentration (Cogo et al., 1984; Steiner, 1994). Alberts and Neibling (1994) considered time to runoff initiation and runoff rate as the most important dynamic properties affected by surface residue. A recent report from the Soil and Water Conservation Society (SWCS, 2003), through a review of the literature and consultation with an expert panel of scientists, concluded that changes in precipitation patterns due to global warming are already being experienced. Observations indicate a clear bias towards more frequent, and greater magnitude, extreme events. Furthermore, some regions will experience an increased occurrence of drought, which can result in a lack of biomass production for residue cover. As a result, the dynamics of the runoff and erosion processes (e.g., runoff hydrograph peaks, timing, and sediment concentrations) will become dramatically more important, and quantification of the carry-over benefits from year to year of BMPs such as residue management and conservation tillage systems will become more important to flood prediction and erosion control. The objective of this study was to use long-term CT and NT plots on loessial uplands in north Mississippi to elucidate residue management and tillage effects on runoff and erosion dynamics. The carry-over benefits of residue management were determined along with the impact of tilling NT-history land and not tilling CT-history land on selected characteristics of the runoff and sediment concentration hydrographs.
MATERIALS AND METHODS Field plots at the Nelson Farm experimental site in the loessial uplands of north Mississippi (89.957° W, 34.565° N)
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were used for this study. Plots with long-term history of conventional tillage (CTh) and no tillage (NTh) corn (some plots had grain sorghum grown the first few years) were selected for study, and the management was changed when necessary to obtain the ten treatments listed in table 1. The ten treatments consisted of an incomplete 2 × 2 × 3 factorial arrangement of two tillage histories (CTh and NTh), two tillage levels (tilled and not tilled), and three residue management levels (RL, RR0, and RR1). The RL was residue left on the surface and corn planted but not emerged prior to simulated rainfall, RR0 was residue removed just prior to simulated rainfall, and RR1 was residue removed one year earlier, maintained fallow with chemicals, and then removed again prior to simulated rainfall. The two treatments not included were NTh-tilled and CTh-not tilled with residue left. Soils were predominately Grenada silt loam (fine silty, mixed, thermic, Glossic Fragiudalf) with some areas consisting of Memphis and Loring silt loams soils, which all have similar surface properties. Description of soil physical properties for these treatments can be found in Rhoton et al. (2002) and Dabney et al. (2003). Conventional tillage consisted of chisel/disk preplant operations and one to two post-emergence cultivations. The only soil disturbance under NT was one pass with a no-till planter. Tillage and other management practices were up and down the slope. Residue removal was by hand with corn stalks cutoff at the soil surface and the stalk base left in place. Residue left consisted predominantly of corn stalks that were not shredded. Residue cover was measured on each plot prior to rainfall simulations but after residue removal and tillage using the line method (Laflen et al., 1981). The rainfall simulator (Meyer, 1960) consisted of a series of oscillating Veejet nozzles (80100) located approximately 3 m above the soil surface. Nozzles traversed the area horizontally in two dimensions in order to apply a uniform rainfall application with an impact energy of 211 kJ ha -1 mm -1. Rainfall was applied to a 3.7 m × 10.7 m area (fig. 1) within plots at a rate of 65 mm h -1 for 1 h under antecedent soil-water conditions (dry run), followed by a 0.5 h simulation 4 h later (wet run), and another 0.5 h application 0.5 h later (very wet run). According to McGregor and Mutchler (1992), rainfalls of 65 mm h -1 with 0.5 h and 1 h durations are expected in this region about once every year and ten years, respectively. A pit was dug adjacent to each plot at the upslope end for hydrometeric monitoring (fig. 1). Tensiometers were installed horizontally from the pit face with the 2.5 cm long cup positioned 30 cm into the plot area. TDR rods were also Table 1. Treatment designations with their corresponding replications (Reps), tillage history, current tillage practice, and residue management. Reps History Tillage Residue Designation 2 3 2 2 1 2 5 2 3 2 [a]
[a]Conv.
tillage Conv. tillage Conv. tillage Conv. tillage Conv. tillage No-tillage No-tillage No-tillage No-tillage No-tillage
Tilled Tilled Tilled Not tilled Not tilled Not tilled Not tilled Not tilled Tilled Tilled
Residue left Residue removed Residue removed Residue removed Residue removed Residue left Residue removed Residue removed Residue removed Residue removed
Cth-tilled, RL Cth-tilled, RR0 Cth-tilled, RR1 Cth-not tilled, RR0 Cth-not tilled, RR1 Nth-not tilled, RL Nth-not tilled, RR0 Nth-not tilled, RR1 Nth-tilled, RR0 Nth-tilled, RR1
Conv. = conventional
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installed horizontally from the pit face and extended 10 to 30 cm into the plot area. A total of 11 tensiometers and 12 TDR probes were installed at the 5, 10, and 15 cm depths (fig. 1). Soil water pressures were monitored on a 1 min cycle and volumetric water contents on a 5 min cycle starting well before rainfall was applied and continuing until steady-state conditions were established after the very wet rainfall simulation was terminated. The time of runoff initiation was recorded and runoff rate measured by collecting the runoff for 15 s intervals every 3 min until rainfall was terminated, at which point runoff was collected for 15 s every minute until runoff ceased. Runoff volume was recorded and sediment content analyzed by decanting excess water and then evaporating to dryness. Dye tracer was released on the surface at the upper end of plots during each simulation. The time for tracer to move 6.1 m downslope was recorded to determine the linear flow velocity. The SAS Proc Mixed procedure (SAS, 1999) was used to determine differences in least square means between treatments, by run (dry, wet, very wet). Significance in this article is set at a P level of 0.05. Soil samples were taken within the plot area immediately prior to rainfall application of each run to determine soil water contents. These data were used to account for variations between runs due to antecedent moisture using a covariant analysis. The moisture effect was not significant; therefore, moisture was not included in the final analysis of variance.
RESULTS HYDRODYNAMIC RESPONSES Even though rainfall simulations were carried out over a 50 d period each year, with few exceptions, the soil water pressures prior to dry runs suggested that antecedent conditions were close to field capacity for all treatments. The dynamics of the soil water pressure response was highly dependent on the tillage and residue treatments, as seen in figures 2 and 3. However, there were some common features worth discussing. The 5 cm tensiometers exhibited the quickest response, followed by the 10 and 15 cm depths, as the wetting front advanced down the profile. With few exceptions, runoff was initiated prior to the 5 cm tensiometer response. Once the wetting front reached the tensiometers, soil water pressures increased rapidly. Steady-state soil water pressures were obtained, on average, within 46, 57, and 82 min at the 5, 10, and 15 cm depths, respectively, following rainfall initiation for the dry run. Due to the wetter antecedent conditions, steady-state was reached more quickly during the wet and very wet runs, with respective times following rainfall initiation of 26 and 17 min at the 5 cm depth, 42 and 24 min for the 10 cm depth, and 49 and 29 min for the 15 cm depth. Soil water pressures were observed to decrease slightly (few centimeters) between the dry and wet runs, but negligible changes occurred between the wet and very wet runs. Soil surfaces generally exhibited dramatic increases in water content, as indicated by the 5 cm depth in figures 2 and 3, with time shortly after initiation of the dry rainfall event. These increases ranged from 3.4% to 18.4%, with an average across all treatments of 10.5%. Despite changes in soil water pressures at the 5 cm depth being similar to those at the 10 and 15 cm depths, the changes in water content at these deeper
ÓÓÓÓÓÓ ÓÓÓÓÓÓ Dye release area
Tensiometer and TDR location Soil Surface 5 cm 10.7 m 5 cm
5 cm
Tensiometer 3.7 m
TDR Probe
Flume
Figure 1. Diagram of plot layout with circular cutout illustrating the positions on the pit face where tensiometers and TDR probes were installed horizontally underneath the plot area. The area where the dye tracer was released is indicated at the upper end of the plot.
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Figure 2. Runoff (flow), sediment concentration, soil water pressure, and soil water content by volume over time for the CTh-tilled for RL (left) and RR1 (right) treatments. Solid arrows indicate time to runoff initiation (upper graphs) and to tensiometer response (lower graphs); dashed lines indicate maximum runoff rates and maximum sediment concentrations.
depths were considerably smaller, averaging 5.3% and 2.8%, respectively. Changes in water content were minimal for the wet and very wet runs, with respective increases averaging 2.3% and 1.0% at the 5 cm depth, 1.6% and 1.0% at the 10 cm depth, and 1.0% and 0.6% at the 15 cm depth. A couple of plots exhibited no increase at the 15 cm depth during these later two runs. The hydrodynamic responses illustrated in figures 2 and 3 suggest that there are differences between the tillage and residue treatments. To determine if these differences were significant, the hydrodynamic responses were characterized by the following descriptive properties, as indicated by the arrows in figures 2 and 3: time to runoff initiation (min), time for tensiometer response (min), end-of-run surface pressure (cm H2O), maximum runoff rate (mm h -1), and linear flow velocity (m s -1). Due to mechanical difficulties, the TDR data were not sufficient for statistical analysis. Since tensiometers cannot be installed immediately at the surface, the end-of-run surface pressure was estimated by a linear extrapolation to the surface from the water pressures at the 5, 10, and 15 cm depths at the time rainfall was terminated for each run.
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Continuity in Tillage System and Residue Impacts The long-term NT system that is maintained as NT with corn residue left (NTh-not tilled, RL) provided 70.0% surface cover. The surface cover decreased dramatically to 10.2% immediately following residue removal (RR0) and to 4.0% the subsequent year (RR1). The value for NTh-not tilled with residue removed (NTh-not tilled, RR1) is close to the surface cover under CTh-tilled with residue left (CThtilled, RL), which was only 5.0% due to incorporation of residue by tillage. Residue removal decreased the surface cover under CTh-tilled even further to just 1.7% for RR0 and 1.5% for RR1. These differences in residue cover along with the tillage system itself impact the runoff hydrodynamics. The increased permeability of the freshly tilled soil is clearly evident by the longer times to runoff initiation for tilled than observed for not tilled (table 2) under dry antecedent conditions. The differences are significant when residue is tilled into land that has a CT history as compared to not tilling NTh land regardless of its residue management. Thus, such watersheds will exhibit flashier streamflow responses than predominately tilled watersheds during the first storm following tillage. However, the opposite is likely true for subsequent events due to changes in the hydraulic properties of tilled soil.
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Figure 3. Runoff (flow), sediment concentration, soil water pressure, and soil water content by volume over time for the NTh-not tilled for RL (left) and RR1 (right) treatments. Solid arrows indicate time to runoff initiation (upper graphs) and tensiometer response (lower graphs); dashed lines indicate maximum runoff rates and maximum sediment concentrations. Table 2. Treatment means for time to runoff initiation, maximum runoff rate, linear flow velocity, and maximum sediment concentration.[a] Runoff Initiation Time Maximum Runoff Rate Linear Flow Velocity Maximum Sediment Conc. (min) (mm h -1) (m h -1) (mg L -1) Tillage
Residue
Dry
Wet
V. Wet
CTh-t CTh-t CTh-t
RL RR0 RR1
12.0 a 8.1 abc 8.5 abc
0.9 cd 0.8 d 0.8 d
0.6b c 0.5 c 0.5 c
53.2 abc 55.2 b 55.9 abc 60.6 ab 57.0 abc 61.6 ab
49.9 c 61.7 ab 63.6 ab
180.6 a 238.3 a 399.6 ab 248.9 a 341.5 a 337.5 ab 247.1 a 320.1 a 348.6 ab
34650 cd 44050 cd 57800 bcd 56500 bc 71000 bc 70400 bc 67250 abc 83100 ab 90050 ab
CTh-nt CTh-nt
RR0 RR1
2.3 ac 2.5 bc
1.1 cd 1.0 cd
0.6 bc 0.6 bc
60.5 ab 62.8 ab 60.3 abc 65.4 ab
63.4 ab 65.5 ab
271.8 a 235.0 a 231.7 ab 293.2 a 430.6 a 457.9 a
36950 bcd 41100 d 44850 de 50900 abcd 58200 bcd 66500 bcd
NTh-t NTh-t
RR0 RR1
7.5 abc 10.9 ab
1.3 bc 0.9 cd
0.8 bc 0.5 c
51.8 bc 49.1 c
59.1 b 61.7 ab
59.6 b 63.2 ab
256.2 a 273.0 a 280.0 ab 142.7 a 306.9 a 340.4 ab
22133 d 94000 a
33300 d 38867 de 110950 a 101250 a
NTh-nt NTh-nt NTh-nt
RL RR0 RR1
3.0 c 3.5 c 2.3 c
2.3 a 1.7 b 1.2 cd
1.7 a 1.0 b 0.6 c
56.0 abc 58.3 ab 59.1 ab 62.4 a 63.4 a 62.5 ab
61.8 ab 64.7 a 62.0 ab
193.2 a 162.0 a 131.8 b 168.4 a 237.2 a 261.3 ab 197.1 a 263.5 a 287.1 ab
8450 d 16360 d 71150 ab
20750 d 13550 f 24380 d 23840 ef 49750 cd 52050 cd
[a]
Dry
Wet
V. Wet
Dry
Wet
V. Wet
Dry
Wet
V. Wet
Different letters indicate that treatments within a column are significantly different at the 0.05 level.
Consolidation of tilled soil occurs rapidly during runoff events, thereby decreasing the hydraulic conductivity of the surface. A decrease in the hydraulic gradient was evident by the soil water pressures becoming close to the same value at, or shortly after, the end of each run. Thus, the profile approached a unit gradient with time near the end of these rainfall events, in which case the infiltration rate approached
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the hydraulic conductivity. The hydraulic conductivity was not necessarily the saturated value. Estimation of the surface pressure at the end of each run (table 3) suggests that the soil was often under a negative water pressure even while runoff was occurring. It is interesting to note that negative surface pressures were extrapolated for the end of each run for the residue left
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Table 3. Treatments means for time to tensiometer response at three depths and the extrapolated surface soil water pressure for the end of each run.[a] 5 cm Depth Time 10 cm Depth Time 15 cm Depth Time (min) (min) (min)
Surface Water Pressure (cm)
Tillage
Residue
Dry
Wet
V. Wet
Dry
Wet
V. Wet
Dry
Wet
V. Wet
Dry
Wet
V. Wet
CTh-t CTh-t CTh-t
RL RR0 RR1
11.0 bcd 6.3 cd 16.0 bc
6.0 bc 6.3 bc 1.0 c
9.0 ab 6.7 b 2.0 b
18.0 ab 13.7 b 38.0 ab
11.0 ab 8.7 b 2.0 bc
11.0 ab 9.0 b 2.0 b
26.0 b 17.0 b 73.0 ab
14.0 bc 10.7 c
12.0 b 13.7 b 5.0 b
-6 abc 4 bc 6 abc
-4 a -4 a 23 a
-6 a -5 a 24 a
CTh-nt CTh-nt
RR0 RR1
9.0 cd 22.0 ab
9.0 ab
10.0 ab
18.0 ab 74.0 a
11.0 ab
20.0 a
16.0 bc
23.0 ab
32 c -30 a
7a
6a
NTh-t NTh-t
RR0 RR1
3.0 d 32.0 a
2.0 c 1.0 c
3.5 b 1.0 b
3.7 ab 49.0 ab
2.5 c 1.0 c
6.0 b 2.0 b
8.7 b 64.0 b
6.0 c 2.0 c
8.5 b 4.0 b
1 abc 74 d
3a 21 a
3a -5 a
NTh-nt NTh-nt NTh-nt
RL RR0 RR1
9.0 cd 3.0 d 6.5 cd
16.0 a 2.5 c 3.0 bc
17.0 a 3.8 b 1.0 b
16.0 b 7.8 ab 35.5 ab
18.0 a 6.8 bc 3.5 bc
17.0 a 5.8 b 4.0 b
11.3 b 49.5 b
49.0 a 10.0 c 24.0 b
46.0 a 12.3 b 12.0 b
-4 abc -9 ab 11 bc
-10 a 2a 88 a
-6 a 3a 74 a
[a]
150.0 a
Different letters indicate that treatments within a column are significantly different at the 0.05 level.
treatments with both CTh-tilled and NTh-not tilled. These surface water pressures increased to near zero for treatments that had residue removed immediately before rainfall (RR0) and increased further to high positive pressures at the surface for treatments that had residue removed and maintained fallow for one year (RR1) (table 3). These findings suggest that surface seals developed within one year following residue removal such that at the end of runs, when the profile was near steady-state, the surface was under positive pressures, while negative pressures existed just 5 cm below the surface. Both factors, surface seal development and decreased hydraulic gradient, result in shorter times to runoff initiation in subsequent rainfalls. The Ti for the tilled systems (table 2) decreased substantially from the dry to subsequent runs. However, NT that was left undisturbed was fairly stable with minor changes in Ti between the dry and subsequent runs, particularly for the RL system. The resulting Ti for NTh-not tilled land was significantly longer during the wet and very wet runs than for the CTh-tilled land. In contrast, the time to soil water pressure response at the 5 cm depth actually increased from the dry to wet and very wet runs for the NTh-not till land with RL. As a result, this treatment had significantly longer tensiometric response times than the residue -removed treatments under CTh-tilled and NTh-not tilled. The differences in runoff dynamics due to tillage interacted with the impacts of residue management. Increases in residue cover generally increase the time to runoff initiation (Alberts and Neibling, 1994). Tillage effectively incorporated residue left on the surface such that the residue cover was only 5.0% for the RL treatment on CTh land that was tilled. As a result, differences in Ti for CTh-tilled among residue treatments were not significant. The Ti decreased significantly from the dry to the subsequent runs, which tended to equalize residue removal effects even further. Thus, residue removal had a negligible effect on runoff timing under CT. Residue played a critical role in runoff dynamics for the NT systems, with residue removal resulting in significantly flashier, shorter Ti response under NT with RR0 than RL during the wet and very wet conditions. Residue creates small reservoirs that tend to slow the rate of runoff and increase the time to runoff initiation. Once these systems were wet up, differences in antecedent moisture and hydraulic gradients
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were minimal, and the differences in Ti between RL and both residue removal treatments were evident. Additionally, the carry -over effect of residue removal from the initial (RR0) to the subsequent year (RR1) was evident by a 35% decrease in Ti on average, which was significant for the wet and very wet runs. These results demonstrate the importance of residue cover with NT systems. As observed for RL, runoff occurred significantly sooner under NT than CT (table 2) during the first event (dry run) after tillage, but Ti decreased by >92.5% for CT during the subsequent events, while NT was less affected (
