The management of crop residues and soil organic matter is of primary importance in maintaining soil fertility and productivity in direct seeding systems and for minimizing agricultural impact on environmental change. The possibility of global greenhouse warming due to a rapid increase of carbon dioxide (CO2), is receiving increased attention. This concern is warranted because potential climatic changes could result in increased temperature and drought over present agricultural production areas. Thus, agriculture's role in the overall global carbon balance must be understood. We need direct measurements of CO2 loss as impacted by agricultural management practices. Management practices, such as direct seeding, need to be developed to optimize CO2 utilization from soil and plants in photosynthesis to increase crop yields. There is a definite need for information on the impact of tillage on CO2 flow from soil and how farming practices impact the carbon cycle and global climate change.
Information is needed on the short-term impacts of various tillage methods on soil quality and the carbon cycle within an agricultural production system. This work reviews several experiments dealing with short-term tillage-induced CO2 release and covers multiple objectives evaluating the effect of different management practices on CO2 loss from soil. These studies evaluated the effect of fall tillage methods, soil variability, conservation tillage tools, seasonal differences (fall versus spring tillage), and different cropping systems on short-term tillage-induced CO2 emissions. The reader is encouraged to see the original articles for additional details and data for interpreting these results. The results show when soil organic matter loss occurs and confirms that intensive tillage increases soil organic matter decomposition rate and carbon loss as CO2. Soil organic matter losses are substantially reduced if tillage intensity is reduced or eliminated as in direct seeding. The large gaseous losses of soil carbon following moldboard plowing compared to the relatively small losses with no-till have shown why crop production systems using plowing have decreased soil organic matter and why no-till or direct seeding crop production systems are stopping and reversing that trend.
Fall Tillage Methods
The first experiment to demonstrate tillage-induced CO2 losses, described in detail by Reicosky and Lindstrom (1993), was conducted on a Hamerly clay loam (fine, loamy, frigid Aeric
Calciaquoll) at the West Central Experiment Station of the University of Minnesota, Morris, Minnesota, U.S.A. in September 1991. Tillage treatments covered a range of tillage depths, soil disturbance, and residue incorporation with no-tillage (NT) as a check treatment. The four tillage treatments included moldboard plow (MP), moldboard plow followed by a disk harrow twice (MP+D2X), disk harrow (DH), and chisel plow (CP). The control treatment was no-tillage (NT) with soil and residue as left by harvest equipment from the preceding wheat crop.
The CO2 flux or loss from the tilled soil surfaces was measured using a large portable chamber described by Reicosky, (1990), Reicosky, et al. (1990), and Reicosky and Lindstrom (1993). The term gas flux refers to the flow of gas from the soil surface into the air expressed in mass of gas per unit area per unit time, in this case pounds of CO2/acre/hour. The term gas flux and gas loss have the same meaning and are interchangeable in this context. Measurements for CO2 flux were initiated within 5 min of the last tillage pass. Briefly, the chamber (volume of 114.7 ft3 covering a horizontal land area of 28.7 ft2) with mixing fans running was moved over the tilled surface until the chamber reference points aligned with plot reference stakes, lowered and data rapidly collected at 2-s intervals for a period of 80 s to determine the rate of CO2 and water vapor increase. After the appropriate lag times, data for a 30-s period was used to convert the volume concentration of water vapor and CO2 to a mass basis then linearly regressed as a function of time to reflect the rate of CO2 and water vapor increase within the chamber (corresponding to loss from the soil surface) expressed on a unit horizontal land area basis.
Relatively large initial fluxes (as large as 260 lb CO2 /acre/h) from the moldboard plow surface were observed as shown in figure 1. The CO2 losses on the MP and the MP+D2X were largest
Figure. 1. Short-term effect of fall tillage method on CO2 flux vs. time: (a) moldboard plow, moldboard plow plus disk harrow twice, and no-till; (b) disk harrow, chisel plow, and no-till. Tillage done on 4 Sept. 1991 (Day 247).
immediately after tillage probably reflecting a "flush" of CO2 released from large voids generated by the tillage event. The rapid decrease in the CO2 flux on MP from 260 lb CO2 /acre/h to 18 lb CO2 /acre/h 55 hours after tillage was noteworthy. Immediately after tillage, the MP+D2X treatment had a flux as large as 62 lb CO2 /acre/h that decreased to 18 lb CO2 /acre/h within three h. The CP treatment showed a similar trend, but with lower initial flux. These tillage methods are compared with NT where there was little change in the CO2 flux from 6 to 2 lb CO2 /acre/hr during the same 55-hour period. Cumulative CO2 evolved after tillage was calculated using numerical integration (trapezoid rule). This method assumes linear interpolation between the measured fluxes over the time interval. The areas for successive time intervals were summed to give the total amount of CO2 evolved. The cumulative CO2 flux for the 55 h period was estimated by calculating the area under the curves and resulted in 2203, 696, 330, 785, and 196 lb CO2/acre for MP, MP+D2X, DH, CP and NT respectively.
Measurement of the CO2 flux from the tillage surfaces was continued after a major rainfall event (3 day total of 1.9 in) for up to 19 days after the initial tillage. During this period the fluxes followed the earlier trends based on the tillage method. The results in figure 2 indicate that, for at least 19 days after tillage, MP caused more CO2 to reenter the atmosphere compared to other treatments. The cumulative CO2 loss, expressed as carbon only, from each tillage surface for the 19 days after tillage was 2221, 1155, 951, 890, and 445 lb/acre for MP, MP+D2X, DH, CP, and NT, respectively. The C released as CO2 following moldboard plow, moldboard plow plus disk harrow, disk harrow, chisel plow and not tilled treatments would account for 134, 70, 58, 54, and 27%, respectively, of the C in the current years crop residue (Reicosky and Lindstrom, 1995; Reicosky et al. 1995). Considerably more C was lost as CO2 from the plowed plots than from the area not tilled.
Figure 2. Total organic carbon lost in 19 days after different tillage types relative to the amount of organic carbon in residue from the previous wheat crop.
Results suggest that depth of soil disturbance was more important than residue incorporation in determining the magnitude of short-term CO2 flux. These results differ from those of Roberts and Chan (1990) who used simulated tillage techniques to examine the importance of tillage induced increases in soil respiration as a mechanism for organic matter loss. They measured the CO2 evolution from soil cores after applying a simulated tillage and found the carbon losses that could be directly attributed to tillage ranged from .0005% to .0037% of the total which is small compared with the total carbon content of the soil. They concluded that the increase in microbial respiration due to tillage was probably not a major factor that caused losses of soil organic matter in the soils under intensive cultivation.
Using a large chamber to measure tillage-induced gas exchange, fall tillage methods affected short and intermediate-term CO2 fluxes from a harvested wheat field. Immediately after tillage, moldboard plow had the largest rate > moldboard plow plus disk twice >chisel plow >disk >no-tillage. The cumulative CO2 evolved for the 19 days of this study was in the order moldboard plow >moldboard plow plus disk twice >disk >chisel plow >no-tillage. Results suggest that high initial flux of CO2 was more related to surface soil roughness and depth of soil disturbance than to residue incorporation. The CO2 flux from all tillage plots showed small but consistent differences 19 days after tillage and 2.5 in of rain. The rates were substantially lower following the rain due to soil re-consolidation. The data suggest lower short-term soil CO2 fluxes are associated with tillage methods that limit soil disturbance. Differences in CO2 flux among tillage methods suggest potential for improved soil management to minimize agriculture's contribution to global CO2 increase. Tillage methods with low intensities that minimize depth and extent of soil disturbances will have the least impact.
The significant flush of CO2 immediately after tillage confirms the role of tillage affecting C flow within agricultural production systems. There is a need for more information on the variation and magnitude of this CO2 flush for soils around the world based on the interaction of tillage and different soil types within the landscape. De Jong, (1981) showed soil gas composition was affected by slope position in the landscape and vegetative cover in Canada. In glaciated areas, there is significant soil variability related to landscape that results in a very complex interaction of several factors that can have an effect on the CO2 released following tillage. Aspects of soil variability and tillage-induced CO2 losses were addressed in detail by Reicosky, (1995). The specific objective of that work was to quantify the variation in CO2 loss immediately after fall plowing on four different soil types previously cropped to wheat.
The experiment was conducted in the fall of 1993 at the USDA-ARS Swan Lake Research Farm located in west central Minnesota, U.S.A. The soils selected within the field are listed in Table 1 and range from moderately well to poorly drained and were formed on glacial till under tall prairie grass vegetation. The surface horizons are generally very dark with relatively high organic matter and likely developed over subsoils with high calcium carbonate.
The study area was selected by establishing two-640 ft-long transects across an area with significant variation in soil type as shown on the soils map. Two parallel transects, about 100 ft apart, were established in an east-west straight line in anticipation of moldboard plowing along the transects. The measurement locations or plots were centered on the soil-map units on the respective transect.
Commercially available moldboard plows were used for the tillage along the transects. The necessary plow width for gas exchange measurements was accomplished using two sets of plows. One was a four bottom plow 18 in wide to a depth of 9 in was pulled by the first tractor. A second set of two bottom plows (same width) pulled by another tractor immediately behind the first was required to get the necessary width for the chamber measurements. Both moldboard plows were each pulled by a medium-sized farm tractor. Plowing resulted in complete inversion of the surface layer and nearly 100% incorporation of the residue.
The CO2 loss from the plowed soil surfaces was measured using the portable closed chamber described previously, except data was collected at one s intervals for a period of 60 s to determine the rate of CO2 and water vapor increase. These sampling and calculation modifications are described in detail by Wagner and Reicosky, (1996) and Wagner et al., (1997). After the appropriate lag and mixing times, data from a thirty-second-calculation window was selected to convert volume concentration of water vapor and CO2 to a mass basis and then regressed as a function of time using a quadratic function to estimate gas losses (Wagner et al. 1997). The rough plowed surface caused occasional large holes around the perimeter of the chamber that did not allow the base of the chamber to form a good seal. These were quickly filled with soil by hand to minimize leakage. No effects of significant leakage were observed once the holes were plugged.
Due to the anticipated rapid decline in the CO2 loss as a function of time after plowing, gas exchange measurements were made with the portable chamber within 30 to 40 s after the pre-marked area was plowed. The sequence of events was as follows: the tractors pulling plows moved through the experimental plot area to a pre-determined reference point, then stopped and waited while the chamber was quickly moved over the plot area and three successive measurements taken. Upon completion of the three measurements, the plows were moved through the next experimental plot area to the next pre-marked location and the chamber moved over the pre-marked area to repeat another series of three measurements. The sequence was repeated across different soils on the transect until all seven pre-marked plots were measured and the cycle repeated starting on the first plot measured that day. The south transect was plowed from west to east and the CO2 loss evaluated on 23 September (Day 266). The north transect was plowed on 24 September 1993 (Day 267) from east to west. Chamber measurements were made facing into the wind. Within any single day, four measurement cycles were completed on each transect. On Day 267, about 24 h after tillage on the south transect only, another cycle of measurements was completed. On both days, at the start of each cycle, triplicate measurements were made on a no-till plot with surface residue as left by the field combine. Based on the location on the soils map, the soil was a Parnell, about 20 m north of the north transect.
The change in CO2 flux as a function of time after plowing is illustrated in figure 3 for three different soils in the north transect. The three different soils, Barnes1, Vallers1 and Parnell2, represent the lowest, middle and highest initial fluxes on Day 267, respectively. The trends were similar and represent the change in the CO2 flux from one min to 3.5 h after plowing and reflect the highest and lowest rates in the study. The CO2 flux for Parnell2 soil in figure 3 decreased from a high of 1017 to 428 lb CO2 /acre/h within 8 min to about 71 lb CO2 /acre/h after 3.5 h.
Figure 3. The CO2 flux as a function of time after plowing for three different soil series on the north transect that represent low, median, and high fluxes measured on D 267.
All of these fluxes were larger than the no-till plot average of 3 lb CO2 /acre/h. The decrease in the CO2 flux as a function of time after plowing was fitted to a reciprocal linear function and a modified exponential function. The coefficients for the equations summarized in figure 3 have no physical meaning and were extremely variable across all soils and within the same soil series. The magnitude of the initial fluxes and the rapid decline in CO2 fluxes immediately following plowing for as long as 3.5 h suggest a significant amount of gaseous C can be lost depending on soil type and location within the landscape.
The cumulative CO2 loss from one min to about 3.5 h after plowing on the south transect is summarized in figure 4. The cumulative flux ranged from 537 lb CO2 /acre for Vallers3 to a low of 176 lb CO2 /acre for Barnes2. However, the initial measurements on Barnes2 were not made until eight minutes after actual plowing due to operator error and may not be a representative. More realistic is Barnes3 that had 316 lb CO2 /acre evolve during the 3.5-h period after plowing. These cumulative fluxes illustrate differences due to soils and their location within the landscape. All fluxes were significantly larger than no-till at 5 lb CO2 /acre on the same day illustrating significant loss of CO2 in the initial flush immediately after plowing. Results show substantial spatial variation in the total amount of CO2 evolved after plowing along the transect and within the same soil series.
Significant differences in the CO2 fluxes between south transect plowed areas and the no-till after 24 h indicated significant CO2 was lost overnight. The change in CO2 flux from 3.5 h to 24 h after plowing ranged from 10 lb CO2 /acre/h on the Hamerly5 soil to 39 lb CO2 /acre/h on the Vallers3 soil. The fluxes after 24 h were still 6- to 10-times larger than that from the no-till plot.
Figure 4. The cumulative CO2 loss for the first 3.5 hours after plowing day for soils on south transect (Day 266).
The decreasing trend would likely have continued until the next perturbation, i.e. another tillage event or rainstorm that could cause surface sealing or cold temperatures or drier soils that could decrease the CO2 flux.
Figure 5. The cumulative CO2 loss for the first 24 hours after plowing for soils on south transect (Day 266) using the trapezoid rule for numerical integration.
Cumulative CO2 losses for plots on the south transect 24 h after plowing on Day 266 and can be used to approximate the cumulative CO2 loss for 1 day are shown in figure 5. The values for 24 h may be subject to error due to the long time between the last two measurements, however they represent first approximations.
The cumulative CO2 flux values ranged from a high of 1279 lb CO2 /acre to a low of 682 lb CO2 /acre. The cumulative CO2 fluxes along the south transect were 961, 903, 1279, 964, 1114, 802, and 682 lb CO2 /acre , respectively for the Hamerly5, Vallers4, Vallers3, Vallers2, Hamerly4, Barnes3 and Barnes2 soils. These values can be compared with the no-till which lost 59 lb CO2 /acre during the same period. Cumulative CO2 losses for the 24-hour period after plowing were significantly larger than no-till and there were substantial differences between soils. The results show lingering effects of the plow perturbation for at least one day.
Measurements within one minute after plowing as opposed to five minutes after plowing increased the maximum initial CO2 flux nearly three-fold over observations of Reicosky and Lindstrom (1993) for another Hamerly soil. Enhanced CO2 loss and the subsequent entry of oxygen into the soil should shift the gaseous equilibrium and result in enhanced organic matter decomposition. There was significant spatial variation along the transects within the landscape that was partly related to soil type, and probably related to soil organic C and water content at the time of tillage currently being evaluated in more detail. The spatial variation in the CO2 flux from different soil types was complex and highly dependent on landscape position that could have been related to the water content of the surface layer at the time of tillage.
The spatial distribution of soil properties is an important source of variability in a field experiments. In this work, the rate of CO2 loss was partially dependent on the position of the soil within the landscape. The ability to characterize this spatial variability can be critical in field studies where the properties under investigation are not uniformly distributed. Temporal variation in associated processes are complex and difficult to quantify. Critically important is the dynamic temporal variation from the perturbation of the soil system when it is moldboard plowed. The temporal trends superimposed on the spatial variation in landscapes only further complicate the analysis. Quantifying these positional trends of spatially distributed phenomena will require further work to provide policy makers with sufficiently accurate quantitative data for environmental quality decisions.
Conservation Tillage Tools
Conservation tillage tools are primarily designed for residue management and to meet conservation compliance. Emphasis is placed on leaving surface residue primarily for erosion control. There is little information on how the degree and extent of soil mixing by these tools may affect soil carbon and CO2 losses. A study was designed to evaluate tillage-induced CO2 losses caused by different conservation tillage tools. The experiment was conducted at the USDA-ARS Swan Lake Research Farm 24 August, 1994 on a Barnes loam (Udic Haploborolls, fine loamy, mixed). This study was conducted on spring wheat residue (variety Marshall) and weed mix that had been killed with Roundup two weeks before tillage.
Conservation tillage tools, also referred to as combination implements, consist of a wide variety of basic tillage components, commonly found as part of other tillage implements mounted on toolbars, that are adjustable to vary residue cover. Because of the wide variety of these combination implements commercially available, the specific equipment used in this study will be briefly described as follows: a John Deere*2 Model 2800 moldboard plow, the Howard Paraplow*2 (model 410B), the White*2 model 445 Conser-Till chisel plow, the DMI*2 530 Ecolo-Tiger chisel plow, the Glencoe*2 SS 7400 Soil Saver, and the John Deere*2-510 Disk Ripper. Gas exchange was measured as described before, only measurements were initiated within 60 s after tillage.
The CO2 flux as a function of time after tillage in the second experiment showed the moldboard plow and Paraplow had the highest initial fluxes with the other conservation tillage tools intermediate between moldboard plow and the area not tilled. The average initial flux for the moldboard plow was 437 lb CO2/acre/h that decreased to about 62 lb CO2/acre/h within 5 hours after tillage. The four conservation tillage tools had initial fluxes that ranged from 125 to 187 lb CO2/acre/h and eventually decreased to about 31 lb CO2 /acre/h at the end of 5 hours. The flux decreased rapidly with time after tillage primarily due to soil drying and continued gas loss.
The cumulative carbon dioxide fluxes were determined by calculating the area under the curves of CO2 flux versus time using numerical integration and are summarized in figure. 6. The cumulative flux represents the total amount of CO2 lost during the first 5 hours after tillage, which was the longest period common to all tillage equipment. The cumulative CO2 loss for 5 hours after tillage was 724 lb CO2/acre for the moldboard plow and the smallest on treatment area not tilled at 52 lb CO2/acre. The Paraplow was second at 702 lb CO2/acre with the other remaining conservation tillage tools intermediate ranging from 200 to 248 lb CO2/acre. All conservation tillage tools produced more CO2 than the NT treatment, but significantly less than the moldboard plow.
The average cumulative short-term CO2 loss (5 hours) for four conservation tillages was only 31% of the moldboard plow. The moldboard plow treatment lost 13.8 times as much CO2 as the soil area not tilled, compared to the average of four different conservation tillage tools that lost only 4.3 times. The smaller CO2 loss following conservation tillage tools is significant. While primarily designed to leave crop residue on the surface for erosion control, these conservation tillage tools can have a second beneficial effect that results in less CO2 loss. These preliminary results suggest that progress is being made in developing conservation tillage tools that can further enhance soil C management.
Fall versus Spring: Tillage methods
Information is needed on the short-term impacts of timing of various tillage methods on C dynamics within agricultural production systems that have seasonally dependant operations. In colder climates, different methods for both fall and spring tillage are commonly used as described by Reicosky, (1997). The objective was to measure the effect of different tillage methods on the CO2 flux from soil in the fall and spring. Canopy gas exchange techniques allow measurements of short-term fluxes that can contribute to a better understanding of the underlying processes that decrease soil C. A large portable field chamber was used to measure gas exchange following both fall and spring tillage using different implements. Various combinations of soil disturbance and residue incorporation were established using conventional tillage equipment commonly used in the northern Corn Belt of the U.S.A.
The experiment was conducted in the fall of 1993 and the spring of 1994 at the USDA-Agricultural Research Service Swan Lake Research Farm on a relatively uniform Barnes loam (fine loamy, mixed, Udic Haploborolls) formed on glacial till under tall prairie grass vegetation. The surface horizon is generally very dark with relatively high organic matter developed over subsoil with high calcium carbonate. The previous cropping history of the experimental area was corn, soybean and spring wheat using conventional tillage (moldboard plow, disk harrow and cultivation) for 80 years.
Commercially available tillage implements were used to establish different tillage treatments (unreplicated) as described by Reicosky and Lindstrom (1993). Typical plot size was one or two implement widths by 65 ft long to insure proper tillage action. The first treatment was moldboard plow (MP) tillage, using a three-bottom plow with bottoms 18 in wide, to a depth of 10 in, which resulted in complete inversion of the surface layer and nearly 100% incorporation of the residue. The second treatment was the same moldboard plow tillage to 10 in, followed by disk harrowing (MP + DH). This resulted in the same depth and degree of soil disturbance, but with smaller aggregates and a less porous surface. The third treatment was disk harrowing (DH) once, which resulted in shallow soil disruption (3 in) and partial incorporation of residue. The last treatment, chisel plowing (CP) once (6 in deep), used a standard chisel plow with 11 shanks on 12-in centers and 3 in-wide twisted shovels staggered on three bars, for complete soil disruption. The primary difference between DH and CP was the depth of soil disturbance (6 in for CP, 3 in for DH). Disk and chisel tillage are commonly used for overwinter wind and water erosion control. The check treatment was not tilled (NT), with soil and wheat residue left by harvest equipment.
Initial fall tillage in 1993 was on 4 Oct. 1993 (DY 277) in the order MP, MP+DH, DH, and CP. Tillage in spring 1994 was on 19 April (DY 109) in the same order. The portable chamber measurements for CO2 flux were initiated within 1 min of the last tillage pass. The CO2 fluxes from the tilled soil surfaces were measured using the large portable chamber described previously with modifications described by Wagner and Reicosky, (1996) and Wagner et al., (1997). After the appropriate lag and mixing times, data from a thirty-second-calculation window was selected to convert volume concentration of water vapor and CO2 to a mass basis and then regressed as a function of time using a quadratic function to estimate gas fluxes (Wagner et al. 1997). Tillage was done both in fall 1993 and spring 1994 when the surface soil water potential was marginally high for tillage of this soil.
The not-tilled (NT) plots were selected for gas exchange measurements over undisturbed soil with crop residues as left by the combine after harvest. The NT designation implies no soil disturbance after harvest of the 1993 wheat crop, which was established using conventional tillage and planting equipment.
Generally, fall tillage in western Minnesota,U.S.A., is done in late September, October, and early November with a span of about 60 days for primary tillage. Similarly in the spring, much of the spring tillage takes place late April and May. The long-term temperature record shows a wide range in air temperatures during both fall and spring tillage periods. However, the absolute temperature values are reasonably close for both fall and spring tillage periods.
The CO2 fluxes from both fall and spring NT treatments were nearly the same and only slightly above zero throughout the study period. The large CO2 flux immediately after fall moldboard plowing (MP) and the subsequent decline as a function of time agrees with the previous work using the same equipment on a different soil. The consistent differences between the fall and spring MP are apparently related to some factor other than tillage type. The initial CO2 flux from the fall MP was 1088 lb CO2/acre/hr and then decreased to 54 lb CO2/acre/h within 2 h after tillage. The corresponding values for the spring MP were 161 and 18 lb CO2/acre/h.
These CO2 flux differences between fall and spring probably reflect differences in soil CO2 concentration from limited spring microbial activity since the surface soil thawed about 3 weeks previously. Campbell et al. ( 1970, 1971 and 1973) showed the effect of spring and fall incubation conditions on nitrogen transformations and that diurnally fluctuating low temperatures caused a reduction in microbial population. In this work, the fluctuating low temperatures in the spring were around a mean surface soil temperature that was gradually increasing after soil thaw
from < 32 F to about 45 F at the time of tillage. In the fall, the soil temperatures were fluctuating around a decreasing mean temperature that reflects higher microbial activity from higher soil temperatures prior to tillage. The interaction of water content, temperature and the type of tillage may result in slightly lower fluxes during the spring. The CO2 fluxes from the not-tilled (NT) treatments in the fall and spring were similar and are within measurement error of the chamber technique. Soil variability also needs to be considered along with a multitude of temporal biological factors that could affect the CO2 flux.
The cumulative CO2 losses for 80 h after tillage are summarized in figure. 7. These values represent cumulative loss from time of tillage to the end of the fall or spring test periods. The fall MP treatment shows the largest cumulative CO2 loss followed by CP and MP+DH that were all larger than DH or NT. The trends with tillage intensity are qualitatively the same for both fall and spring, with consistent quantitative differences showing CO2 loss in the fall greater than
in the spring. The differences between the fall and spring may be due to small differences in water content and temperature or to differences in recent microbial activity (Campbell et al. 1971, 1973).
The moldboard plus disk harrow (MP+DH) and the chisel plow (CP) showed similar trends but with lower magnitudes. The differences in CO2 fluxes between the seasons were consistent in all four tillage treatments and showed a gradual decline with time as the soil dried. The freezing rain (0.2 in) in the afternoon two days after spring tillage had only a minor effect on CO2 flux the next day, however did affect soil evaporation. All tillage methods were larger than not-tilled (NT) used for reference. However, in the fall, the CO2 fluxes from DH were only slightly higher than NT.
Cumulative water losses during the same periods are summarized in figure 8. The differences between tillage methods are smaller than differences in CO2 loss. All tillage methods showed a trend related to tillage intensity and were higher than the area not-tilled in the fall. Higher cumulative evaporation in the spring was probably related to the 0.2 in rain 2 days after tillage and higher solar radiation and evaporative demand. Short-term effects suggest more evaporation immediately after MP tillage which had the roughest surface and the largest porosity that resulted in maximum CO2 and water fluxes. Evaporation immediately after tillage decreased with tillage intensity. However, as the soil dried, evaporation decreased and became more dependent on the potential evaporative demand and surface soil water content.
Soil evaporation trends are slightly different from the CO2 trends, primarily because the evaporation is more strongly related to potential evaporative demand than to soil loosening. The highest evaporation rates occurred immediately after tillage primarily because the tillage was done close to midday when the potential evaporation demand for that day was highest. The differences in evaporation immediately after tillage primarily reflects the tillage effect and secondarily the time of tillage. Temporal trends were related to potential evaporative demand and soil drying, especially in the fall. Evaporation differences between the fall and spring were erratic and decreased with time after tillage.
Cropping Systems and Tillage Methods
Information is needed on the variation and magnitude of the CO2 flux and N transformations caused by tillage of different soil types and cropping systems. The objectives of this study described in detail by Reicosky et al. (1997), were to compare measurements of the soil CO2 flux using a soil chamber and canopy chamber and to examine the interactions between short-term soil CO2 flux and soil N transformations from three cropping systems on a vertisol that had three different tillage practices imposed. Measurements were made on three cropping systems (coastal bermudagrass, continuously-cultivated sorghum, and no-till sorghum) that had three different tillage practices (moldboard plow, chisel plow, and untilled as the control) imposed on a vertisol.
Measurements of soil CO2 flux were made on 6 and 7 May, 1994, at the Blackland Research Center, Temple, Texas, U.S.A. on three fields, all within 0.5 mile of each other, each having a Houston Black clay (fine montmorilinitic thermic Udic Pellusterts) and a different cropping history. On 11 April, 1994, glyphosate was applied to the areas that were to be tilled in each field so there was no live vegetation in any field on the day of tillage. The fertility history was similar for all three fields and is considered typical for the area. The cropping systems were single fields in place for decades and as a result did not allow complete randomization and replication. Further details for the three cropping systems were as follows:
1. Coastal bermudagrass. This field was in bermudagrass and had not been tilled for > 30 years. The grass was cut for hay about two times per year and the field was fertilized annually. On 25 April and 3 May, 1994 the dead grass was cut in this field.
2. Continuously-cultivated sorghum. This field had been in continuous row crops or, occasionally, small grains for more than 30 years. Crop residues were shredded and incorporated into the soil. Annual tillage practices usually included three passes with a 8-in deep disk and one pass with a chisel plow, a typical central Texas tillage intensity. Sorghum was seeded in April 1994.
3. No-till sorghum. This field was under a no-till system since 1991, during which there had been no tillage and shredded crop residues had been left on the soil surface. Grain sorghum was planted in April 1994. Prior to initiating the no-till system, this field was managed in a manner similar to that of the continuously-cultivated sorghum. Thus, the tillage intensity for this field is intermediate between the bermudagrass and continuously-cultivated sorghum fields.
Two primary tillage treatments and a control (untilled) were used in each cropping system on 6 May, 1994. The control treatment had no soil disturbance by tillage. Tillage treatments were adjacent to each other separated by alley s for traffic and were 65 ft long. Within each tillage strip, three measurement sites were identified for replication of soil CO2 flux measurements. Primary tillage implements used sequentially were moldboard plow and chisel plow. The moldboard plow consisted of three 16 in-wide bottoms that plowed about 8 in deep. Two adjacent passes were required to achieve the necessary width for the canopy chamber measurements. The chisel plow was 10 ft-wide, with shanks spaced 12 in apart and penetrating about 8 in deep. Each primary tillage was followed by a secondary tillage with two passes of a disk harrow about five hours after the primary tillage.
In each field, soil CO2 flux measurements were made initially in the untilled treatment, followed by tillage with the moldboard plow and soil CO2 flux measurements on that treatment, and finally chisel plow tillage and flux measurements on that treatment. This sequence took about 45 min. for each field. Measurements and tillage were initiated in the no-till sorghum field at 0715 h Central Standard Time (CST) on 6 May 1994, followed by measurements in the bermudagrass and continuously-cultivated sorghum fields. These sequences of soil CO2 flux measurements were repeated in each tillage treatment and cropping system throughout the day on 6 May until about 1700 h CST. Each sequence of CO2 flux measurements took about 2.25 hours to complete measurements in all fields. The disk harrow tillage treatment was imposed in the same order in these fields. One final set of soil CO2 flux measurements was made in each field beginning at 24 hours after tillage on 7 May 1994 to give a total of eight measurements on each treatment.
Soil CO2 flux was measured in all cropping systems and tillage treatments using two types of instrumentation, a soil chamber and a canopy chamber described previously by Reicosky, (1990). The soil chamber CO2 flux was measured using a small, cylindrical, vented soil chamber (volume = 0.026 ft3 and diameter = 4 in) that had a Li-Cor (Li-Cor, Inc., Lincoln, Nebraska) Model 9960-035 sensor head and a Li-Cor Model 6200 Photosynthesis System attached. The soil chamber was placed on polyvinyl chloride collars randomly located in each treatment and buried to a depth of 1.5 in. The rate of CO2 increase inside the chamber was measured for about 15 to 30 s. Concurrent measurements of soil temperature at 0.2 in depth were made at the time of each soil CO2 flux measurement. Further details on this method are presented by Dugas (1993).
The first post-tillage soil chamber flux measurement was usually made within 20 s after completion of tillage at three locations within each tillage treatment at the same time as canopy chamber measurements. Collars were adjacent to the areas used for canopy chamber measurements. Once collars were placed in a tillage treatment, they were only removed briefly for the disk harrow tillage. The three measurements for each tillage treatment were averaged and individual measurements were used as replicates. Three canopy chamber measurements were sequentially made for each tillage treatment, each at a predetermined distance (ca. 10, 20, and 30 ft) from the end of the tilled area. Each canopy chamber measurement was used as a replicate. The soil CO2 flux was calculated from the rate of CO2 concentration increase inside each chamber. The CO2 fluxes (positive upward) were expressed on a land area basis, which, for tilled surfaces, is less than the exposed soil surface area due to the surface roughness (Reicosky and Lindstrom, 1993).
The soil CO2 fluxes measured by both the soil chamber and canopy chamber were greatest immediately after tillage in all cropping systems initially reported by Reicosky et al (1997). The considerably smaller flux from untilled treatments represents the flux that originates within the soil and from surface residue decomposition due to microbial activity on the moist soil surface. The much greater fluxes from the tilled treatments were due to physical CO2 release from soil pores and solution (Reicosky and Lindstrom 1993).
Fluxes from the untilled treatment were slightly > 0 for the entire period for both chambers. For the two tilled treatments, fluxes immediately after tillage were greatest in the bermudagrass for both the soil chamber and canopy chamber, likely due to higher CO2 concentrations in soil pores from increased organic C and surface residue. The next highest post-tillage flux was measured in no-till sorghum, likely due to the greater amount of surface residue in this system relative to the continuously-cultivated sorghum, which had the lowest flux immediately after tillage. Two hours after tillage, fluxes from all fields decreased rapidly to a value about 20% of the maximum. Thus, a large percentage of the CO2 loss occurred immediately after primary tillage suggesting that physical release was the primary controlling mechanism. The disk harrow tillage approximately five hours after primary tillage caused only a slight flux increase in all cropping systems. The fluxes measured with canopy chamber were not affected while the percentage increase for the soil chamber was larger. The breakdown of the large soil aggregates following secondary tillage was noticeable on the bermudagrass, but caused only minor surface mixing and breakdown of large aggregates on both sorghum plots resulting in little effect on CO2 flux.
The CO2 fluxes 24 hours after tillage measured by the canopy chamber were essentially equal in the tilled and untilled treatments for the continuously-cultivated sorghum and no-till sorghum. However, fluxes remained about 10 times greater in the bermudagrass tilled treatments, presumably due to higher soil C concentrations and root decomposition rates. The soil chamber also measured a greater flux in the bermudagrass, although the soil chamber flux from the bermudagrass was considerably smaller relative to the canopy chamber flux.
Fluxes immediately after tillage in the tilled treatments measured by the soil chamber were only about 10% of those measured by the canopy chamber. The larger flux difference between methods in this study was likely related to the following: 1) the small diameter of the soil chamber precluded a representative measurement in these tilled soils where the size of soil clods and air gaps (ca. 12 in) was about 3 times greater than chamber diameter, 2) increased turbulence and dynamic pressure differences inside the canopy chamber caused by the mixing fans that may have biased the flux measurement from this instrumentation, and 3) the surface area and porosity of soil under the canopy chamber after tillage was considerably greater than that under the soil chamber due to increased soil roughness. Reicosky and Lindstrom (1993) calculated a 50% increase in soil surface area for plots that were tilled with a moldboard plow. Breakage of large soil aggregates or clods is likely an important process affecting soil CO2 flux after tillage and the small diameter of the soil chamber likely precludes a representative measurement for very rough soil surfaces.
Cumulative fluxes for the 24-hour period after tillage, calculated using the trapezoid rule, were about five times greater for the canopy chamber than the soil chamber (figure 9). For treatments with tillage, cumulative canopy chamber measurements ranged from a high of 1035 lb CO2 /acre/day for coastal bermudagrass to a low of 285 lb CO2 /acre/day for continuously-cultivated sorghum. Absolute differences between cropping systems were less for fluxes measured by the soil chamber. Relative differences between cropping systems were smaller than the effect of tillage for both chambers. For both chamber methods, cumulative fluxes for each tillage method were greatest for the bermudagrass, and were similar for the no-till and continuously-cultivated sorghum. Fluxes were least from the untilled treatment in all cropping systems and for both methods. The bermudagrass flux, which was largest even under the untilled treatment, was likely enhanced due to root decomposition.
Figure 9. Cumulative soil CO2 flux for 24h after tillage from three cropping systems and three tillage treatments as measured by a canopy chamber (left) and soil chamber (right). Each cumulative flux represents the mean of three replicates, and error bars represent +/- 1 SE.
For the soil chamber, cumulative fluxes from the moldboard plow treatments were consistently higher in all three cropping systems, while for the canopy chamber measurements the moldboard plow was greatest only for no-till sorghum. Differences in cumulative flux due to primary tillage method were small, but differences between cropping systems were substantial as indicated by the error bars (figure 9).
Two dynamic chamber methods, a soil chamber and canopy chamber, were compared for measuring soil CO2 flux from three cropping systems and three tillage treatments. For tilled surfaces, flux differences between chamber methods were large, while both showed similar temporal trends. Fluxes were greatest in the bermudagrass and least in the continuously-cultivated sorghum. Fluxes in the moldboard plow treatment were usually the greatest, and fluxes in the untilled treatment were considerably smaller than fluxes from either tillage treatment. Cumulative CO2 flux for 24 hours from the three cropping systems after tillage with either a moldboard plow or a chisel plow was considerably greater from an established bermudagrass pasture than from a no-till sorghum field or a continuously-cultivated sorghum field. Differences were related to previous tillage history, soil organic C concentration, and surface residue. Fluxes from the tilled treatments, either with a moldboard or chisel plow, were considerably greater than from the untilled treatment. The soil chamber and canopy chamber showed similar qualitative temporal and treatment trends. The disparity between the chamber methods suggests a critical need for a reference or standard method for quantitative comparison of CO2 fluxes, especially for tilled soils.
Summary and Conclusions
The composite results presented in this overview highlights the importance of direct seeding and helps explain the long-term decrease in soil organic matter as a result of intensive tillage on agricultural soils. The short-term large CO2 losses following moldboard plowing compared to the relatively small losses with no-till or conservation tillage illustrates why crop production systems with moldboard plowing have decreased soil C and why no-till systems are reversing this trend. The moldboard plow not only lifts, fractures, inverts and opens the soils which allow rapid CO2 and oxygen exchange, but also incorporates residue into the soil which feeds a microbial population explosion. Intensive tillage, such as moldboard plowing, that disturbs the soil to depths and leaves the soil surface in a very rough, porous condition can result in substantial CO2 loss. The spatial variation of carbon flux across the landscape was related to soil properties and water content a time of tillage and adds complexity to the interpretation of fluxes from large land areas. With conservation tillage, most crop residues are left on the soil surface, but only a small portion is in intimate contact with the soil moisture and available to the microorganisms. The average cumulative short-term CO2 loss for four conservation tillage tools was only 31% of the moldboard plow. The moldboard plow treatment lost 13.8 times as much CO2 as the soil area not tilled, compared to the average of 4 different conservation tillage tools that lost only 4.3 times. The trends with tillage intensity were qualitatively the same for both fall and spring, with consistent quantitative differences showing greater CO2 loss in fall than in spring. The differences between the fall and spring may be due to small differences in water content and temperature and limited previous microbial activity. Cumulative CO2 flux for 24 hours from the three cropping systems after tillage with either a moldboard plow or a chisel plow was considerably greater from an established bermudagrass pasture than from a no-till sorghum field or a continuously-cultivated sorghum field. Quantative differences in chamber methods were noted and qualitative trend differences were related to previous tillage history, soil organic C concentration, and surface residue. Present data shows intensive tillage decreases soil carbon and supports increased adoption of new and improved forms of conservation tillage equipment that offers significant potential to preserve or increase soil C levels. The large gaseous losses of soil carbon following moldboard plowing compared to the relatively small losses with no-till have shown why crop production systems using plowing have decreased soil organic matter and why no-till or direct seeding crop production systems are stopping and reversing that trend. Reversing the trend of decreasing soil carbon using less tillage intensity will be beneficial to agriculture as well as to the global population through better control of the global carbon balance.
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|SOIL SERIES||DRAINAGE||DEPTH of A1||BULK DENSITY||SOIL ORGANIC MATTER||CLAY CONTENT||PH|
(Aeric Calciaquolls fine-loamy, frigid)
|PARNELL silty clay loam (Typic Argiaquolls, fine, montmorillonitic, frigid)||VERY POOR||22.1||74.8-81.0||6-10||27-40||6.1-7.8|
VALLERS silty clay loam
(Typic Calciaquolls, fine; loamy, frigid)