Biologically Defined Soil Organic Matter Pools as ... - Springer Link

DOI: 10.1007/s00267-003-9160-z

Biologically Defined Soil Organic Matter Pools as Affected by Rotation and Tillage GEOFFREY L. DOYLE Graziglands Research Laboratory USDA-ARS 7207 El Reno Oklahoma 73036, USA CHARLES W. RICE DALLAS E. PETERSON Department of Agronomy Throckmorton Plant Sciences Center Kansas State University Manhattan, Kansas 66506-5501, USA JAMES STEICHEN Department of Biological and Agricultural Engineering Seaton Hall, Kansas Stato University Manhattan, Kansas 66506-5501, USA ABSTRACT / The importance of soil organic matter is well recognized; however, changes in C and N fractions are inadequately quantified. The objective of this study was to determine tillage and crop rotation effects on soil organic C and N fractions from a long-term (27-year) study in eastern Kansas. Cropping systems included continuous and rotation se-

Soil organic matter (SOM) has been recognized as an important constituent of soil nearly as long as human society has practiced organized agriculture (Paustian 1997a; Lal and others 1999; Loveland and Webb 2003). Recent concerns about changes in atmospheric chemistry and potential climate change have led to an increased interest in the understanding of land management effects on terrestrial C and N fluxes (Dolman and others 2003). The cultivation of soil in agriculture generally leads to an overall reduction in SOM (Buyanovsky and others 1987; Grant 1997; Paustian and others 1997a; Lal and others 1999; Loveland and Webb 2003), because of accelerated oxidation of organic C by increasing soil aeration and increasing the surface area of SOM accessible to microbial mineralization. Agricultural management practices have not been implemented with SOM management as their primary KEY WORDS: Microbial biomass C and N; Potentially mineralizable C and N; Carbon sequestration Published online May 12, 2004.

quences of wheat (Triticum aestivum L.), grain sorghum (Sorghum bicolor (L.) Moench), and soybean (Glycine max (L.) Merrill) on a Muir silt loam (fine-silty, mixed, mesic Cumulic Haplustolls). Tillage included conventional (CT), reduced (RT), and no-till (NT). Total C and N (CT and NT) were determined on all treatments. Mineralizable C and N (Co and No) and microbial biomass C and N were determined for the NT and CT soybean and sorghum rotations. Cropping systems that included wheat contained the greatest amount of CT and NT. Continuous wheat contained 2910 g C m⫺2 and 287 g N m⫺2, compared to 2225 g C m⫺2 and 222 g N m⫺2 (0 –15 cm) for continuous soybean. No-tillage contained 1128 g C m⫺2 and 109 g N m⫺2 at 0 –5 cm compared to 918 g C m⫺2 and 87 g N m⫺2 for CT. Sorghum contained 51% more Co than soybean, and NT accounted for 59% more Co than CT. More crop residue was produced and retained in rotations that included sorghum. No-tillage increased C 2440 kg ha⫺1, while CT increased C 340 kg ha⫺1 across all soybean/sorghum rotations. The highest sequestration rate (122 kg C ha⫺1 y⫺1) was observed with NT sorghum and was equivalent to ⬃3.2% of the plant material (root and shoot, less gain harvest) remaining in the soil annually.

focus. Management practices such as conservation tillage, fertilization, cropping, and manuring have instead focused on conserving soil loss and increasing crop yields, and SOM levels were of secondary importance. Tillage was historically used to control weeds, loosen and homogenize the soil and to prepare the soil for planting equipment (Peterson 1983). Conservation tillage helps to prevent the reduction of SOM, and perhaps reverse the process by decreasing residue decomposition (Grant 1997; Havlin and Kissel 1997; Loveland and Webb 2003). Surface residue decomposes more slowly than the residue incorporated into the soil in conventional tillage systems (Reicosky and Lindstrom 1993; Franzluebbers and others 1995), because there is a less favorable temperature and moisture regimen as well as less surface area accessible to microbial decomposition (Loveland and Webb 2003). Several factors make conservation tillage more desirable than conventional management practices. These include increased soil aggregation (Tisdall and Oades 1982; Wright and others 1998; Loveland and Webb

Environmental Management Vol. 33, Supplement 1, pp. S528 –S538

©

2004 Springer-Verlag New York, LLC

Soil Organic Matter and Rotation/Tillage

2003), increased infiltration capacity and reduced evaporation (Langdale and others 1992; Paustian and others 1997a; Loveland and Webb 2003), and increased SOM accumulation (Paustian and others 1997a; Lal 2002; Loveland and Webb 2003). Soil is also more susceptible to wind and water erosion after cultivation or fallow periods. Fallow decreases the quantity of plant residue returned to the soil, thus failing to offset the increase in C oxidation. Recent development of herbicides has reduced the necessity for frequent, aggressive tillage of soil as a form of weed control (Peterson 1983; Loveland and Webb 2003). As energy costs increase, management practices that reduce the number of mechanical tillage events should reduce equipment and labor costs (Peterson 1983). Improved SOM quality in conservation tillage has been attributed to increased microbial products (Grant 1997), and studies have demonstrated that fungi (predominately mycorrhizae and other filamentous fungi) are predominant in conservation tillage, while bacteria are predominant in conventional tillage (Guggenberger and others 1999; Frey and others 1999). Measurement of total soil C alone does not always identify the subtle changes in nutrient status occurring in soils converted to conservation management practices (Franzluebbers and others 1995), while the measurement of more biologically active fractions of SOM (microbial biomass C and N and potentially mineralizable C and N) have provided valuable information with respect to nutrient dynamics (Stanford and Smith 1972; Deans and others 1986; Drury and others 1991; Bremer and van Kessel 1992; Franzluebbers and others 1994; Wardle and Ghani 1995). Because most techniques for measuring soil C and N are accurate to ⫾2% of the amount present, most land management effects on SOM dynamics cannot be measured accurately until at least 10 years have elapsed since the start of the management change (Paul and others 2001). Although SOM is a continuum of an amorphous mixture of interrelated humic compounds (Paul and Clark 1996), no single fractionation technique can adequately characterize turnover rates for the whole soil (Paul and others 2001). However, Paustian and others (1992) found that first-order kinetics and a three-pool (three fraction) technique could describe SOM dynamics adequately. Collins and others (1992) found that crop rotation and residue management greatly influenced C and N dynamics, with the elimination of fallow and the addition of manure leading to an increased in both microbial biomass C and N and easily mineralizable SOM. The use of crop rotations has been shown to increase yields, and thus increases plant residue return (Peter-

S529

son 1983; Mallarino 2001; Lo´ pez-Bellido and others 2001; Legere 2002). The inclusion of soybeans, oats, or alfalfa in corn production in lowa increases corn yield 0.1 to 16% (Mallarino 2001). In addition to increasing yield, crop rotation affects the “fresh” or active pool of SOM. (Franzlubbers and others 1995; Loveland and Webb 2003). A sorghum–wheat–soybean rotation increased the active fraction of SOM by 30% in Texas (Franzlubbers and others 1995). The objective of this study was to quantify changes to soil organic C and N pools, to include total C and N, potentially mineralizable C and N, and microbial biomass C and N, during a long-term (27 years) rotation and tillage study in eastern Kansas.

Materials and Methods Site Description The experimental site was located on the Kansas State University Agronomy Farm, located in Ashland Bottoms (Riley County; 39°07'N, 96°37'W), approximately 14 km south of Manhattan, Kansas. The plots have been used in a long-term rotation and tillage study initiated in 1974 with the objective of evaluating the effects and feasibility of conservation tillage systems within eastern Kansas (Peterson 1983). The soil was a Muir silt loam (fine-silty, mixed, mesic Cumulic Haplustoll). The Riley County soil survey reports the eastern edge of the field where the study was conducted as a Reading silt loam (fine, mixed, mesic Typic Arguidoll); however, pH, cation exchange capacity, and particle size analysis found no significant differences between treatments indicating a consistent, single soil (Muir silt loam) across the entire study area. The treatments consisted of three crops and three tillage methods. The three crops were soybean (B) (Glycine max (L.) Merrill), grain sorghum (S) (Sorghum bicolor (L.) Moench) and winter wheat (W) (Triticum aestivum L.). These three crops were arranged into five cropping systems in the following combinations: W/B, B/S, S/S, B/B, and W/W, with no use of cover crop (summer fallow for the wheat rotations and winter fallow for the sorghum and soybean rotations). The three tillage treatments were as follows. 1.

No-tillage (NT) plots did not undergo disk or chisel operations prior to planting, or following harvest. Crops were planted directly into plant residue, and fertilizer was broadcast prior to planting, but not incorporated into the soil. Weed control consisted of herbicide use only. 2. Conventional tillage (CT) plots received tillage op-

S530

G. L. Doyle and others

erations as close to local farming practices as possible. Plots were disked (15 cm) and chiseled (25 cm) once each, after harvest in the fall and left unplanted until the spring, unless wheat was in the cropping sequence. Wheat was normally planted using a drill in October. In the spring, sorghum and soybean seedbed preparation was conducted using a disk or rotary tiller (5–10 cm) as needed to incorporate broadcasted fertilizer, and was followed by crop planting. A combination of mechanical tillage and chemical herbicides were used for weed control, usually not exceeding two operations per growing season, but varied according to residue amounts, precipitation, weed emergence, and seedbed condition. 3. Reduced tillage (RT) plots received similar seedbed preparation as the CT plots. However, RT plots were not tilled in the fall, and weed control consisted of herbicide use only; herbicide was broadcast onto the soil and plant surface but not incorporated with tillage. Dimensions of the study plots were 6.1 ⫻ 18.3 m, whereas row spacing for the soybean and grain sorghum was 76.2 cm, and 20 cm for the wheat. All plots received yearly applications of 65 kg N ha⫺1 prior to planting. Herbicide application rate and type varied from year to year based on weed production prior to and after planting. Soil Sampling Plot composite soil samples (300 – 600 g, field moist) were obtained from all plots in late April 2001 at depths of 0 –5 and 5–15 cm, prior to planting of soybean and grain sorghum. Samples were placed in polyurethane whirl-pak bags (NASCO Inc., Modesto, CA) and stored in a cooler with ice for approximately 3 hours before transport to the laboratory, where they were stored at 5°C until they were prepared for analysis. After passage through a 4-mm sieve, field-moist samples were then prepared for the various analyses. Oven-dry weight was determined from drying 10-g samples at 105°C until constant weight was achieved. Additionally, bulk density was determined using a thin, stainless steel core (5-cm id, 5-cm increments) that was forced into the soil to a depth of 15 cm, removed, and the 5-cm increments were placed in individual whirl-pak bags (NASCO Inc.) and brought back to the laboratory for drying (105°C, 48 hours). A 50-g subsample from each composite plot sample was air dried and particle size distribution was determined using a modification of the pipette method of Kilmer and Alexander (1949) and the Soil Survey Lab-

oratory Methods Manual (1996). An additional 100-g subsample was given to the Kansas State University Soil Testing Laboratory for determination of pH and cation exchange capacity. Total C and N Soil C and N was determined from air-dried subsamples (two per plot), which were meticulously cleaned of plant material and ground to a fine powder with either a mortar and pestle, or a stainless-steel ball mill. Samples were then analyzed through direct combustion, chromatographic separation, and analysis on an automated CN-analyzer (Carlo Erba Elemental Analyzer, Model 1500 CNS Analyzer; Carlo Erba Strumentazione, Milan, Italy). Because soil pH was less than 7, it was assumed that all measured C was organic C (Havlin and Kissel 1997). Microbial Biomass C and N Microbial biomass C and N were determined from subsamples taken from the soybean and sorghum, NT, and CT plots at the 0 –5-cm-depth increment only, using the chloroform fumigation–incubation method (Jenkinson and Powlson 1976; Voroney and Paul 1984; Garcia and Rice 1994) and described here briefly. Field-moist soil (25 g) was added to two separate 125-mL Erlenmeyer flasks and preincubated for 5 days at 25°C (Voroney and Paul 1984). Enough distilled H2O was added to bring the samples to field capacity (⬃25 % vol/vol). After the preincubation period, one flask of each sample was placed inside a glass desiccator along with approximately 25- to 30-mL alcohol/pentene-free chloroform (CHCl3). The desiccator was evacuated with a mechanical vacuum pump (⬃0.033 MPa), and the chloroform was allowed to boil for 60 seconds vented, and evacuation was repeated two more times. After the evacuation, the valve to the desiccator was closed and the samples and CHCl3 remained in the desiccator for 18 to 24 hours, after which the CHCl3 beaker was removed and the desiccator was evacuated for 3 minutes, then vented repeatedly for at least eight cycles, to ensure all chloroform vapors were removed. After the final evacuation, samples in fumigated and nonfumigated flasks were placed in 940-mL mason jars fitted with a rubber septa and incubated for 10 days at 25°C. Using a 1-mL syringe, a gas sample was taken from the headspace, and the evolved CO2 concentration was measured using a Shimadzu Gas Chromatography-8A gas chromatograph (Shimadzu Scientific Instruments Inc., Columbia, MD) equipped with a 2-m Porapak Q column (0.318 id). Helium (14-mL min⫺1) was the mobile phase, and a thermal conductivity detector was used (70°C).


Microbial biomass C (MBC) was then calculated: MBC ⫽

关共CO 2 ⫺ C兲 fumigated soil ⫺ 共CO 2 ⫺ C兲 nonfumigated soil兴 kc (1)

where kc ⫽ 0.41, which is the fraction of lysed biomass respired as CO2 (mineralized C) over the 10-day incubation (Jenkinson and Powlson 1976; Voroney and Paul 1984). After measuring the CO2 ⫺ C, 100 mL of 1M KCl were added to the flasks and shaken for at least 1 hour at 300 RPM on an orbital shaker. The suspension was then allowed to settle before being filtered gravimetrically using number 2 Whatman filter paper (Whatman International LTD., Maidstone, England). The filtrate was analyzed colorimetrically to determine inorganic N (NH4⫹-N and NO3⫺-N) using an Alpkem Autoanalyzer (Alpkem Corp., Clackamus, OR, Bulletins A303-So21 and A303-S170). Microbial biomass N (MBN) was calculated from (Voroney and Paul 1984) the following.

冋

共NH 4⫹ ⫹ NO 3⫺ 兲 fumigated soil ⫺ 共NH 4⫹ ⫹ NO 3⫺ 兲 nonfumigated soil MBN ⫽ kn where kn is calculated as k n ⫽ ⫺0.014

冉

冊

册

共CO 2 ⫺ C兲 fumigated soil ⫹ 0.39 共NH 4⫹ ⫹ NO 3⫺ 兲 fumigated soil

(2)

(3)

Potentially Mineralizable C and N The pools of potentially mineralizable C and N and their rate constants were determined from laboratory incubations as first described by Stanford and Smith (1972) and modified by Deans and others (1986) and Garcia (1992). Four cores from each replicate of the soybean and grain sorghum whole-plots, and the CT and NT subplots were prepared following the 4-mm sieving procedure described previously. Polyvinyl chloride (PVC) cylinders (5.08-cm id, 10-cm height) were filled with ⬃125 g of field-moist soil at a bulk density of 1.0 g cm⫺3 and stored at 5°C until initiation of incubations. Plastic mesh was glued to the base of the PVC cylinders to retain the soil. For leaching of mineralized NH4⫹-N and NO3⫺-N during the incubation, the cylinders were set on plastic Buchner funnels (7-cm diameter) that were attached to side-arm Erlenmeyer flasks connected to a vacuum pump. A cellulose filter with a bubbling pressure of 0.0685 MPa (Millipore Corp., Bedford, MA) was attached to the bottom of the Buch-

S531

ner funnel to allow the soil to equilibrate to a water potential of ⫺0.003 MPa. To maximize contact between the base of the core and the filter, a layer of 29-␮mmean-diameter glass beads was placed on the filter prior to the cores being placed in the funnel. To minimize soil disturbance during leaching, glass wool was placed on the soil surface prior to the leaching with 100 mL of 0.01 M CaCl2 solution. This volume removed 75% of the inorganic N (data not shown). Garcia (1992) used ⬃400-mL of 0.01 M CaCl2, in order to remove 85–95% of the inorganic N in prairie soils, but this study modified the volume in order to minimize soil disturbance. The leachate was brought up to a known volume with 0.01 M CaCl2 solution, and an aliquot was saved for analysis of NH4⫹-N and NO2⫺-N ⫹ NO3⫺-N. Inorganic N in the leachate was determined colorimetrically using an Alpkem Autoanalyzer (Alpkem Corp., Bulletins A303-S021 and A303-S170). The samples were leached five times during the incubation on days 28, 56, 84, 175, and 256. Fifty milliliters of a N-free solution was added to the cores to replenish nutrients lost by the leaching. The N-free solution contained 100, 24, 113, 0.5, and 4 mg L⫺1 of Ca, Mg, S, P, and K, respectively; the pH was approximately 7 (Garcia 1992). Applying a vacuum of ⫺0.033 MPa for 6 hours was used in order to equilibrate the soil water potential before returning the samples to 940-mL mason jars and incubated at 35°C. Every 2–7 days, the CO2-C evolved from the soil cores was determined using 1-mL gas samples from the headspace of the Mason jar and analyzed as described previously. After measuring the CO2-C, and to allow the soil samples to remain aerobic, the mason jars were opened for approximately 15 minutes allowing the headspace to equilibrate with atmospheric O2 levels. The amount of potentially mineralizable C was estimated by fitting the cumulative respired CO2 for each core to a first-order, one-pool model using a non-linear curve fitting procedure (PROC NLIN; SAS Institute Inc., 1995) in the form: C min ⫽ C o关1 ⫺ exp共 ⫺ kt兲兴

(4) ⫺1

where Cmin ⫽ mineralized C in ␮g CO2 ⫺ Cg soil at time t; Co ⫽ potentially mineralizable C in ␮g CO2 ⫺ Cg⫺1 soil; kCo ⫽ rate contant of mineralization in day⫺1; and t ⫽ time in days. At the end of the incubation (256 days), the cores were disassembled. Moisture content was determined (105°C, until constant weight was achieved) and the final CaCl2 extraction was used to determine inorganic N content. Total C and N were also determined using direct combustion as described previously.

S532


Table 1. Surface (0 –15 cm) soil properties measured in April 2001, after 27 years using five cropping systems and tillage research Bulk density pH

CEC (cmol kg⫺1)

Sand (g kg⫺1)

Silt (g kg⫺1)

Clay (g kg⫺1)

CT (mg m⫺3)

RT (mg m⫺3)

NT3 (mg m⫺3)

5.2

20

8.6

69.7

21.7

1.36

1.36

1.40

NT ⫽ no-tillage, RT ⫽ reduced tillage, CT ⫽ conventional tillage CEC ⫽ Cation Exchange Capacity.

The nitrogen mineralization data were fitted to a one-pool model, a two-pool model, and a mixed-order model (Stanford and Smith 1972; Deans and others 1986; Garcia 1992) using a nonlinear curve-fitting procedure (PROC NLIN; SAS Institute Inc. 1995). The two-pool and mixed-models either failed to converge or resulted in higher root mean squared error; therefore, only the one-pool model was used: N min ⫽ N o 关1 ⫺ exp(⫺k Not兲]

(5)

where Nmin ⫽ mineralized N in ␮g N g⫺1 soil at time t; No ⫽ potentially mineralizable N in ␮g N g⫺1 soil; kNo ⫽ rate contant of mineralization in day⫺1; and t ⫽ time in days. Statistical Analysis For the total C and N measurements, data were analyzed as a split-split-plot design, replicated four times, with rotation (wheat, sorghum, and soybean) as whole plot, tillage (no-till, conventional till, and reduced tillage) as the subplot and depth (0 –5 cm and 5–15 cm) as the sub-sub-plot (Steel and Torrie 1982). Analysis of microbial biomass C and N, potentially mineralizable C and N, and their respective rate constants were analyzed using a split-plot design, replicated four times, with rotation (sorghum and soybean) as the whole plot and tillage (no-till and conventional till) as the subplot at the 0 –5-cm depth (Havlin and Kissel 1997). Analysis of variance and separation of means by the least significant difference test were performed using SAS procedures (SAS Institute Inc. 1995).

Results Soil Properties Soil bulk density was different between tillage treatments, but similar between rotations (Table 1). Conventional and reduced tillage both had bulk densities of 1.36 Mg m⫺3, whereas no-tillage was 1.40 Mg m⫺3. All treatments had similar pH values (5.2) for the 0 –15-cm depth. Depth, tillage, and rotation demonstrated significant treatment effects alone, with the only signifi-

Table 2. Soil total C and N content after 27 years using three tillage regimens (conventional [CT], reduced [RT], and no-tillage [NT]) Total C Tillage 0 –5 cm NT RT CT 5–15 cm NT RT CT

Total N

g kg⫺1

g m⫺2

g kg⫺1

g m⫺2

16.1ba 15.1b 13.5a

1128 1025 918

1.27c 1.42b 1.56a

109 96 87

11.8c 11.8c 12.0c

1652 1602 1626

1.18d 1.18d 1.2d

166 160 164

Tillage and tillage ⫻ depth were significantly different; values followed by the same letter are not significantly different (LSD [0.05]). a

Table 3. Soil total C and N content after 27 years of using five cropping systems Rotation

Total C 0 –15 cm ⫺1

g kg Wheat/soybean Soybean/sorghum Sorghum/sorghum Soybean/soybean Wheat/wheat

14.6ca 12.9b 12.9b 11.1a 14.1c

⫺2

gm

2886 2563 2549 2225 2910

Total N 0 –15 cm g kg⫺1

g m⫺2

1.40c 1.25b 1.25b 1.11a 1.46c

284 250 249 222 287

a

Rotation was significantly different as indicated by different letters, while rotation ⫻ tillage, rotation ⫻ depth, and rotation ⫻ tillage ⫻ depth were not (LSD [0.05]).

cant interaction being depth by tillage for both total C and N (Table 2 and 3). In general, total C and N decreased with depth and soil disturbance. Across all rotations at the 0 –5-cm depth, the largest concentration of CT (16.1 g kg⫺1) and lowest concentration of NT (1.27 g kg⫺1) was in no-till. Expressed on an area basis, these concentrations translate into 1128 g C m⫺2 and 109 g N m⫺2; therefore, because of the soil bulk density differences, NT has a greater amount of NT. The NT CT 0 –5 cm was followed by reduced tillage


Table 4. Soil microbial biomass C (MBC) and N (MBC) for three rotations (sorghum; soybean) at the 0 –5-cm depth for conventional (CT) and no-tillage (NT) regimens

Tillage NT CT Rotation Soybean/soybean Soybean/sorghum Sorghum/sorghum

MBC g m⫺2

MBN g m⫺2

22.3aa 10.1b

5.6a(C:N⫽3.98) 2.8b(C:N⫽3.61)

14.9a 18.5a 15.2a

3.8a 4.2a 4.7a

Tillage was significantly different, whereas rotation and rotation ⫻ tillage were not, as indicated by values followed by different letters (LSD [0.05]).

a

(15.1 g kg⫺1) and then conventional tillage (13.5 g kg⫺1). Expressed on an area basis, the 0 –15-cm depth CT had the lowest total N (251 g m⫺2), followed by RT (256 g m⫺2), whereas NT had the highest total N (275 g m⫺2) (Table 2). There were equal amounts of C and N among all tillage treatments at the 5–15-cm depth. Across both depths, the rotations that included wheat generally had the greatest CT and NT, with the wheatsoybean system containing 14.6 g C kg⫺1 and 1.40 g N kg⫺1, whereas continuous soybean contained only 11.1 g C kg⫺1 and 1.11 g N kg⫺1 (Table 3). Microbial Biomass C and N Microbial biomass C (MBC) and N (MBN) was estimated for the 0 –5-cm depth, B-B, B-S, S-S sequences, and no-till and conventional tillage only. Treatment means demonstrated no significant rotation ⫻ tillage effects seen in MBC or MBN (Table 4). Continuous soybean, continuous sorghum, and soybean–sorghum rotations both had similar levels of MBC and MBN; however, the CT plots contained 45% less MBC and 50% less MBN than was estimated for the no-till plots (Table 4). Mineralizable Carbon Mineralizable C (Co) was significantly different among rotation and tillage, as well as significant interaction between rotation and tillage, whereas there were no significant differences between mineralization rates, except for the NT S-S kCo (Table 5). When tillage by rotation interactions were compared there was no significant difference in Co within the CT rotations, whereas the NT continuous sorghum had 68% more Co when compared to NT continuous soybean, and 42% more than the NT soybean-sorghum rotation. The Co in NT continuous soybean was not significantly different from that in the CT continuous sorghum and soy-

S533

bean-sorghum rotation. The only significant difference among mineralization rates was in the NT continuous sorghum compared to all other rotation by tillage combinations. The NT sorghum mineralization rate is equivalent to a laboratory Co turnover rate of 254 years (1/kCo), compared to a turnover rate of 143 years for both the soybean-sorghum and continuous soybean NT soil. Continuous soybean contained 51% more Co than the continuous soybean, whereas the soybean-sorghum was not significantly different from the two continuous monocultures and fell between the values for the continuous monocultures (Table 5). Conventional tillage has 60% less Co across rotations when compared to no-till; however, there was no difference between rates of mineralization between CT and NT. Mineralizable Nitrogen No significant differences were seen in No or kn across rotations or tillage (Table 5). Relationships Among CT, NT, Co, No, MBC, and MBN Tillage and rotation were significant for Co:CT, as were treatment interactions (Table 6). The Co pool represented 25% of the CT in CT and 46% in NT. The Co in continuous soybean represented 28% of the CT, the continuous sorghum was 46%, and the soybean/ sorghum rotation was 31%. Tillage was significantly different for the ratio of MBC to CT, with 1.2% MBC in CT and 2.1% in NT. For the three rotations MBC was 1.5 to 1.7% of the total C. Rotation did affect the proportion of MBC to Co, but tillage did not significantly affect this ratio.

Discussion Despite a large body of research demonstrating the effect of crop rotation, fertilization, disturbance, and tillage on plant productivity and SOM accumulation or loss (Tiessen and others 1982; Peterson 1983; Buyanovsky and others 1987; Langdale and others 1992; Collins and others 1992; Franzlubbers and others 1995; Paustian and others 1997a; Havlin and Kissel 1997; Schimel and others 2001; Loveland and Webb 2003), there remains a substantial gap in our knowledge of the size and flux of SOM pools across a regional scale because of the influence of climate, management history, soil type, and landscape processes (Carter and Rennie 1982; Paul and others 2001; VandenBygaart and others 2002; Loveland and Webb 2003). The formation of SOM is a function of the decomposition of plant material and recombination with microbial and plant

S534


Table 5. Potentially mineralizable C (Co), N (No), and their respective rate constants (kx) as determined from onepool model fitting at the 0 –5-cm depth Effect Rotation S/S B/S B/B Tillage CT NT Tillage ⫻ rotation S/S CT B/S B/B S/S NT B/S B/B

Co g C m⫺2

kCo day⫺1

No g N m⫺2

kNo day⫺1

487aa 334ab 237b

0.00517b 0.00642ab 0.00713a

5.4a 4.1a 5.5a

0.0236a 0.0281a 0.0274a

203b 502a

0.00652a 0.00595a

1.5a 8.8a

0.0327a 0.0200a

222c 232c 154cd 752a 436bc 320c

0.00640a 0.00641a 0.00677a 0.00394b 0.00642a 0.00750a

1.2a 2.2a 1.2a 9.6b 5.9b 9.8b

0.0273a 0.0324a 0.0383a 0.0199a 0.0239a 0.0165a

a Rotation, tillage, and rotation ⫻ tillage were significant for Co, as indicated by a different letter, all other effects were not, except for kCo (LSD [0.05]). S ⫽ sorghum, B ⫽ soybean, CT ⫽ conventional tillage, NT ⫽ no-tillage.

Table 6.

AVOVA of Co:No, Co: CT, MBC: CT and MBC:Co in the 0 –5-cm depth as influenced by rotation and tillage

Effect Rotation B/B B/S S/S Tillage CT NT Source of Variation Tillage (T) Rotation (R) T⫻R

Co:No

Co: CT

MBC: CT

MBC:Co

142aa 269a 160a

0.28a 0.31a 0.46b

0.0174a 0.0175a 0.0148a

0.061a 0.059a 0.034b

293a 88b

0.25a 0.46b

0.0124a 0.0207b

0.051a 0.051a

0.0131 ns ns

0.001 0.0083 0.0067

0.003 ns ns

ns 0.0239 ns

a

A different letter within a column indicates significant differences (LSD [0.05]). Co,No ⫽ potentially mineralizable C and N, CT ⫽ total C, MBC ⫽ microbial biomass C, S ⫽ sorghum, B ⫽ soybean, CT ⫽ conventional tillage, NT ⫽ no-tillage.

byproducts (Paul and Clark 1996). As plant inputs increase, and disturbance decreases, one would expect an increase in MBC and MBN, as well as Co and No. Woods (1989) determined that in soils from adjacent undisturbed and cultivated plots in Wyoming, microbial biomass C and N was more than five times greater in the first 2 cm of undisturbed soil, whereas respirable C and mineralizable N concentrations were 8 and 18 times greater in the same depth increment when compared to deeper depths. After 3 cm, concentrations remained equal in both disturbed and undisturbed soils. In an earlier paired rangeland and cropland study conducted in North Dakota, decreases in microbial biomass C and N, mineralizable C and N, and total C and N all correlated negatively with increasing soil disturbance (Schimel and others 1985).

Cropping systems and tillage research conducted in warmer climates have reported increases in crop yield and SOM under systems that include rotation and reduced tillage. Wood and Edwards (1991) found that after 10 years of conservation tillage in Alabama, CT and NT concentrations to a depth of 10 cm were 67% and 66% higher, respectively, when compared to plow tillage. Cropping systems that included a higher frequency of corn experienced a less pronounced increase in CT and NT compared to tillage. Some researchers in cooler climates (Carter and Rennie 1982) have found no significant changes in soil C and N even after more than 16 years of no-till in western Canada. These researchers suggest that no change in SOM occurred within the soil profile, but rather a redistribution of C, with NT concentrating C


S535

Table 7. Yield summary for sorghum and soybean rotations within the conventional tillage and no-tillage treatments (1992–1998) Estimatedb residue returned (kg ha⫺1)

Estimatedc root return (kg ha⫺1)

Estimatedd plant C (kg ha⫺1)

2782 3366 6423

4173 4207 6423

2087 2104 3212

2504 2524 3854

2533 3124 7075

3800 3905 7075

1900 1953 3538

2280 2343 4245

⫺1

a

Yield (kg ha No tillage Soybean/soybean Soybean/sorghum Sorghum/sorghum Conventional tillage Soybean/soybean Soybean/sorghum Sorghum/sorghum

)

a

Unpublished data, Kansas State University, Department of Agronomy Experimental Farm, Ashland Bottoms, KS. Grain to shoot ratio; sorghum ⫽ 1.0, soybean ⫽ 1.5. c Root-to-shoot ratio ⫽ 0.5. d Estimated as 40% of plant dry weight b

near the soil surface. VandenBygaart and others (2002) found a similar lack of NT effect in a 15-year study conducted in Ontario. In contrast to Carter and Rennie (1982), SOM values reported here indicate that redistribution did not occur, because there was no significant difference in SOM between tillage systems below 5 cm. The results presented in this paper demonstrated results similar to those of Karlen and others (1998) as well as those reviewed by Loveland and Webb (2003). After 27 years, plots in this study had significantly less CT and NT under continuous soybean compared to all other rotations, and significantly less microbial biomass C and N, potentially mineralizable C and total C and N were measured in CT compared to NT. Continuous sorghum consistently produced 35% more residue with NT, and 46% more with CT when compared to the other rotations (Table 7). This is consistent with Havlin and others (1990) and Varvel (1994), who found that cropping systems that included high residue-producing crops resulted in greater soil organic C and N, whereas the inclusion of soybean in the rotation decreased SOM because of low-residue inputs. Values of Co:No in this study were much higher than reported for a tallgrass prairie site (Garcia 1992), as well as for agricultural soils (Omay 1996). In this study, as in Omay (1996), mineralizable and microbial biomass fractions of SOM were sensitive to management. Microbial biomass C and N did not show significant sensitivity to cropping; however, MBC and MBN did respond significantly to tillage, with NT soil containing greater than 50% more MBC and MBN compared to CT soil. Interestingly, the MBC:MBN ratio in NT equaled 3.98 (Table 4), whereas CT equaled 3.61,

which equals a 9% shift. Fungal biomass MBC:MBN values range from 15:1 to 4.5:1, whereas bacteria have a much smaller range of 5:1 to 3:1 (Paul and Clark 1996). The MBC:MBN values determined in this study suggest that the NT soils contained a higher percentage of fungal biomass compared to the CT soil, which supports the results of Guggenberger and others (1999) as determined through analysis of cell-wall residues extracted from samples taken from the same plots as this study. One unique difference between this study and that by Omay (1996) was that the only significant difference in MBC:CT (or so-called microbial quotient) was due to tillage, and not to rotation. Therefore, under the conditions of this study the microbial quotient may be a good indicator of the efficiency of conversion of plant inputs to MBC under different tillage practices. Efficient conversion of plant inputs to microbial biomass should influence the relative proportions of Co and No. The effect of tillage on Co were significant, as NT continuous sorghum contained 752 g C m⫺2 as Co, while CT continuous soybean contained only 154 g C m⫺2 (Table 5). Monoculture sorghum averaged 42% more Co when compared to the soybean–sorghum rotation. In contrast, Franzluebbers and others (1995) reported that monoculture sorghum averaged 30% less Co when compared to a rotation of sorghum– soybean– wheat in south central Texas. This is perhaps due to shorter fallow periods and thus greater rhizodeposition when wheat is included in the rotation, as well as soil textural, temperature, and moisture differences. The Texas soil was silty clay loam and contained 31% clay and 45% silt compared to 21% clay and 70% silt in the Ashland Bottoms soil (Table 1). Many researchers have

S536


reported a positive correlation between clay content and a soil’s capacity to experienced increases in active SOM (Bremer and van Kessel 1992; and Loveland and Webb 2003). Prior to the rotation and tillage experiment, the history of the Ashland Bottoms included ⬃60 years of continuous moldboard plow or CT wheat production. Control plots were absent from the treatment structure of this study, making C sequestration estimates uncertain. Based on the assumption that SOM has reached an equilibrium after so many years of continuous cultivation (Buyanovsky and others 1987), an initial CT and NT value can be estimated from the SOM measurements made by Peterson (1983) of the CT continuous wheat plots. In 1981, the CT continuous wheat plots contained 13 g C kg⫺1 and 0.11 g N kg⫺1 (884 g C m⫺2 and 75 g N m⫺2) in the 0 –5-cm-depth interval. Based on these estimated initial CT values, the monoculture sorghum (1004 g C m⫺2) had sequestered 120 g C m⫺2 and 20 g N m⫺2, and the soybean–sorghum rotation experienced a similar response over 20 years (1981– 2001). The monoculture soybean lost 88 g C m⫺2 and gained 6 g N m⫺2. No-till experienced an increase of 244 g C m⫺2 (2440 kg C ha⫺1) and 34 g N m⫺2, whereas CT experienced an increase of 34 g C m⫺2 and 12 g N m⫺2 across all soybean–sorghum rotations. The estimates from this study for the NT increase in C are similar to those measured by Karlen and others (1998), where they measured C storage during the first 11 years after conversion of moldboard plow to chisel plow (3625 kg C ha⫺1) and no-till (4000 kg C ha⫺1) in the 0 –5-cm-depth increment in corn plots established in a tile drained field in Iowa. Karlen and others (1998) measured an almost twofold increase in C storage during the 15th year of the study, and attributed this to unusually high plant productivity. Assuming a linear sequestration rate (two measured values, 1981 and 2001), and using the current crop yield averages (unpublished data, Kansas State University, Department of Agronomy Experimental Farm, Ashland Bottoms, KS) to estimate residue return, an estimated C sequestration rate can be determined using the following assumptions (Table 7). 1.

Grain-to-shoot ratios of 1.5 for soybean, and 1.0 for sorghum (Havlin and Kissel 1997) 2. Root-to-shoot ratio of 0.5 (Hebert and others 2001) 3. 40% of plant residue is C (Doyle 2002, unpublished data) A net increase in SOM in the NT monoculture sorghum (2440 kg C ha⫺1) over 20 years equals a sequestration rate of 122 kg C ha⫺1 y⫺1 or 3.2 % of the

sorghum residue (root and shoot, less grain harvest) remains in the soil each year, and less in the other treatment combinations. This rate of 122 kg C ha⫺1 y⫺1 is comparable to the C storage rates reported by Paustian and others (1997b) for Swedish studies (⬃200 –300 kg C ha⫺1 y⫺1) receiving straw amendments every other year during a NT study. Much of the information on global terrestrial C and N cycles are interpreted on a regional or global scale; thus studies such as this one better delineate the SOM dynamics in soils under common land management practices conducted in eastern Kansas. The results of this study demonstrate that total soil C can be increased through the use of management practices that enhance the more biologically active SOM pools. A more robust understanding of the SOM dynamics within the Ashland Bottoms cropping and rotation study will be obtained through the quantification of Co and No for the cropping systems that include wheat, as well as examination of some of the physical processes effecting C sequestration in these soils.

Acknowledgments We thank Adam Grant for his endless efforts in the laboratory preparing samples for analysis, as well as for many hours of work with the GC. This research was supported by the U.S. Department of Energy’s National Institute for Global Environmental Change (NIGEC) through the NIGEC Great Plains Regional Center at the University of Nebraska-Lincoln. (DOE Cooperative Agreement No. DE-FC03-90ER610100). Financial support does not constitute an endorsement by DOE of the views expressed in this article. Contribution No. 03-262J of Kansas Agric. Exp. Stn.

References Bremer, E., and C. van Kessel. 1992. Seasonable microbial biomass dynamics after addition of lentil and wheat residues. Soil Science Society of America Journal 56:1141–1146. Buyanovsky, G. A., C. L. Kucera, and G. H. Wagner. 1987. Comparative analysis of carbon dynamics in native and cultivated ecosystems. Ecology 68:2023–2031. Carter, M. R., and D. A. Rennie. 1982. Changes in soil quality under zero tillage farming systems: Distribution of microbial biomass and mineralizable C and N potentials. Canadian Journal of Soil Science 62:587–597. Collins, H. P., P. E. Rasmussen, and C.L. Douglas Jr. 1992. Crop rotation and residue management effects on soil carbon and microbial dynamics. Soil Science Society of America Journal 56:783–788. Deans, J. R., J. A. E. Molina, and C. E. Clapp. 1986. Models for predicting potentially mineralizable nitrogen and decom-


S537

Dolman, A. J., E. D. Schulze, and R. Valentini. 2003. Analyzing carbon flux measurements. Science 301:916.

Lal, R. 2002. Why carbon sequestration in agricultural soils. Pages 21–30 in J. M. Kimble (ed.), Agricultural practices and policies for carbon sequestration in soil. CRC Press, Boca Raton, FL.

Drury, C. F., J. A. Stone, and W. I. Findlay. 1991. Microbial biomass and soil structure associated with corn, grasses, and legumes. Soil Science Society of America Journal 55:805– 811.

Langdale, G. W., L. T. West, R. R. Bruce, and W. T. Miller. 1992. Restoration of eroded soil with conservation tillage. Soil Technology 5:81–90.

Franzlubbers, A. J., F. M. Hons, and D. A. Zuberer. 1994. Seasonal changes in soil microbial biomass and mineralizable C and N in wheat management systems. Soil Biology and Biochemistry 26:1469 –1475.

Legere, A. 2002. Residual effects of crop rotation and weed management on a wheat test crop and weeds. Weed Science 50:101–11.

position rate constants. Soil Science Society of America Journal 50:323–326.

Franzlubbers, A. J., F. M. Hons, and D. A. Zuberer. 1995. Soil organic carbon, microbial biomass, and mineralizable carbon and nitrogen in sorghum. Soil Science Society of America Journal 59:460 – 466. Frey, S. D., E.T. Elliott, and K. Paustian. 1999. Bacterial and fungal abundance and biomass in congenital and no-tillage agroecosystems along two climatic gradients. Soil Biology and Biochemistry 31:573–585. Garcia, F. O. 1992. Carbon and nitrogen dynamics and microbial ecology in tallgrass prairie. Ph.D. Dissertation. Kansas State Univ., Manhattan. (Dissertation Abstract 92-35625). Garcia, F. O., and C. W. Rice. 1994. Microbial biomass dynamics in tallgrass prairie. Soil Science Society of America Journal 58:816 – 823. Grant, R. F. 1997. Changes in soil organic matter under different tillage and rotation: Mathematical modeling in ecosys. Soil Science Society of America Journal 61:1159 –1175. Guggenberger, G., S. D. Frey, J. Six, K. Paustian, and E. T. Elliott. 1999. Bacterial and fungal cell-wall residues in conventional and no-tillage agroecosystems. Soil Science Society of America Journal 63:1188 –1198. Havlin, J. L., . Kissel, L. D. Maddux, M. M. Claasen, and J. H. Long. 1990. Crop rotations and tillage effects on soil organic carbon and nitrogen. Soil Science Society of America Journal 54:448 – 452. Havlin, J. L., and D. E. Kissel. 1997. Management effects on soil organic carbon and nitrogen in the east-central Great Plains of Kansas. Pages 381–386 in E. A. Paul (ed.), Soil organic matter in temperate agroecosystems: Long- term experiments in North America. CRC Press, Boca Raton, FL. Herbert, Y., E. Guingo, and O. Loudet. 2001. The response of root/shoot partitioning and root morphology to light reduction in maize genotypes. Crop Science 41:363–371. Jenkinson, D. S., and D. S. Powlson. 1976. The effects of biocidal treatments on metabolism in soil—V. A method for measuring soil biomass. Soil Biology and Biochemistry 8:209 –213. Karlen, D. L., A. Kumar, R. S. Kanwar, C. A. Cambardella, and T. S. Colvin. 1998. Tillage system effects on 15-year carbonbased and simulated N budgets in a tile drained lowa field. Soil and Tillage Research 48:155–165.

Lopez-Bellido, L., R. J. Lopez-Bellido, J. CE. Castillo, and F. J. Lopez-Bellido. 2001. Effects of long-term tillage, crop rotation and nitrogen fertilization on bread-making quality of hard red spring wheat. Field Crops Research 72:197–210. Loveland, P., and J. Webb. 2003. Is there a critical level of organic matter in the agricultural soils of temperate regions: a review. Soil and Tillage Research 70:1–18. Mallarino, A. P. 2001. Impacts of crop rotation and nitrogen fertilization on crop production. Iowa State University Northern Research and Demonstration Farm Annual Progress report: ISRF01-22. Omay, A. B. 1996. Fate and availability of nitrogen in cornsoybean rotation. Ph.D. Dissertation. Kansas State Univ., Manhattan. (Dissertation Abstract 96-37244). Paul, E. A., and F. E. Clark. 1996. Soil microbiology and biochemistry, 2nd ed. Academic Press, London, New York. Paul, E. A., S. Morris, and J. S. Bohm. 2001. The determination of soil C pool sizes and turnover rates: Biophysical fractionation and tracers. Pages 193–206 in R. Lal, J. M. Kimble, R. F. Follett, and B. A. Stewart. (eds.), Assessment methods for soil carbon. CRC Press, Boca Raton, FL. Paustian, K., W. J. Parton, and J. Persson. 1992. Modeling soil organic matter in organic-amended and nitrogen-fertilized long-term plots. Soil Science Society of America Journal 56:476 – 488. Paustian, K., H. P. Collins, and E. A. Paul. 1997a. Management control on soil carbon. Pages 15–50 in E. A. Paul (ed.), Soil organic matter in temperate agroecosystems: Long-term experiments in North America. CRC Press, Boca Raton, FL. Paustian, K., G. I. Agren, and E. Bosatta. 1997b. Modeling litter quality effects on decomposition and soil organic matter dynamics. Pages 313–335 in G. Cadisch, and K. E. Giller. (ed.), Driven by nature: Plant litter quality and decomposition. CAB International, Oxford, UK. Peterson, D. E. 1983. Crop production as affected by cropping sequence and method of seedbed preparation in conservation tillage. M. S. Thesis, Kansas State Univ., Manhattan. Reicosky, D. C., and M. J. Lindstrom. 1993. Effect of fall tillage method on short term carbon dioxide flux from soil. Agronomy Journal 85:1237–1243. SAS Institute 1995. Statistical analysis user’s guide: Statistics. Version 6.2. SAS Institute, Cary, NC.

Kilmer, V. J., and L. T. Alexander. 1949. Methods of making mechanical analyses of soils. Soil Science 68:15–24.

Schimel, D. S., D. C. Coleman, and K. A. Horton. 1985. Soil organic matter dynamics in paired rangeland and cropland topsequences in North Dakota. Geoderma 36:201–214.

Lal, R., J. M. Kimble, R. F. Follett, and C. V. Cole. 1999. The potential of US cropland to sequester carbon and mitigate the greenhouse effect. CRC Press, Boca Raton, FL.

Schimel, D. S., J. I. House, K. A. Hibbard, P. Bousquet, P. Ciais, P. Peylin, B. H. Braswell, M. J. Apps, D. Baker, A. Bondeau, J. Canadell, G. Churkina, W. Cramer, A. S. Den-

S538


ning, C. B. Field, P. Friedlingsten, C. Goodale, M. Heimann, R. A. Houghton, J. M. Melillo, B. Moore III, D. Murdiyarso, I. Noble, S. W. Pacala, I. C. Prentice, M. R. Raupach, P. J. Rayner, R. J. Scholes, W. L. Steffen, and C. Wirth. 2001. Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems. Nature 414:169 –172. Soil Survey Laboratory Staff. 1996. Soil survey laboratory manual. Soil survey investigation report No. 42. USDANRCS. U.S. Government Printing Office, Washington, D.C. Stanford, G., and S. J. Smith. 1972. Nitrogen mineralization potentials of soils. Soil Science Society of America Proceedings 36:465– 472. Steel, R. D. G., and J. H. Torrie. 1982. Principles and procedures of statistics. McGraw-Hill Inc, New York. Tiessen, H., J. W. B. Stewart, and J. R. Bettany. 1982. Cultivation effects on the amounts and concentrations of carbon, nitrogen, and phosphorus in grassland soils. Agronomy Journal 74:831– 835. Tisdall, J. M., and J. M. Oades. 1982. Organic matter and water stable aggregates in soils. Soil Science 33:141–163. VandenBygaart, A. J., X. M. Tang, B. D. Kay, and J. D. Aspinall. 2002. Variability in carbon sequestration potential in no-till

soil landscapes of southern Ontario. Soil and Tillage Research 65:231–241. Varvel, G. E. 1994. Rotation and nitrogen fertilization effects on changes in soil carbon and nitrogen. Agronomy Journal 86:319 –325. Voroney, R. P., and E. A. Paul. 1984. Determination of Kc and Kn in situ for calibration of the chloroform fumigationincubation method. Soil Biology and Biochemistry 16:9 –14. Wardle, D. A., and A. Ghani. 1995. A critique of the microbial metabolic quotient (qCO2) as a bioindicator of disturbance and ecosystem development. Soil Biology and Biochemistry 27:1601–1610. Wood, C. W., and J. H. Edwards. 1992. Agroecosystem management effects on soil carbon and nitrogen. Agriculture, Ecosystems and Environment 39:123–138. Woods, L. E. 1989. Active organic matter distribution in the surface 15 cm of undisturbed and cultivated soil. Biology and Fertility of Soils 8:271–278. Wright, S. F., A. Upadhyaya, and J. S. Buyer. 1998. Comparison of N-linked oligosaccharides of glomalin from arbuscular mycorrhizal fungi and soils by capillary electrophoresis. Soil Biology and Biochemistry 30:1853–1857.