Soil Science Society of America Journal 63:912-917 (1999)
© 1999 Soil Science Society of America
DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION
Nitrogen Fertilization Effects on Soil Carbon and Nitrogen in a Dryland Cropping System
Ardell D. Halvorsona,
Curtis A. Reulea and
Ronald F. Folletta
a USDA-ARS, P.O. Box E, Fort Collins, CO 80522 USA
adhalvor{at}lamar.colostate.edu
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ABSTRACT
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No-till (NT) increases the potential to crop more frequently in the Great Plains than with the conventional-till (CT) crop–fallow farming system. More frequent cropping requires N input to maintain economical yields. We evaluated the effects of N fertilization on crop residue production and its subsequent effects on soil organic C (SOC) and total soil N (TSN) in a dryland NT annual cropping system. Six N rates (0, 22, 45, 67, 90, and 134 kg N ha-1) were applied to the same plots from 1984 through 1994, except 1988 when rates were reduced 50%, on a Weld silt loam (fine, smectitic, mesic Aridic Argiustoll). Spring barley (Hordeum vulgare L.), corn (Zea mays L.), winter wheat (Triticum aestivum L.), and oat (Avena sativa L.)–pea (Lathyrus tingitanus L.) hay were grown in rotation. Crop residue production varied with crop and year. Estimated average annual aboveground residue returned to the soil (excluding hay years) was 2925, 3845, 4354, 4365, 4371, and 4615 kg ha-1, while estimated annual contributions to belowground (root) residue C were 1060, 1397, 1729, 1992, 1952, and 2031 kg C ha-1 for the above N rates, respectively. The increased amount of crop residue returned to the soil with increasing N rate resulted in increased SOC and TSN levels in the 0- to 7.5-cm soil depth after 11 crops. The fraction of applied N fertilizer in the crop residue decreased with increasing N rate. Soil bulk density (Db) in the 0- to 7.5-cm soil depth decreased as SOC increased. The increase in SOC with N fertilization contributes to improved soil quality and productivity, and increased efficiency of C sequestration into the soil. Carbon sequestration can be enhanced by increasing crop residue production through adequate N fertility.
Abbreviations: CT, conventional-till • NT, no-till • res, residue • resN, residue nitrogen • Db, bulk density • SOC, soil organic C • TSN, total soil N
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INTRODUCTION
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USE OF NO-TILL SYSTEMS to conserve more water for crop production makes it feasible to crop more frequently than is done with the conventional crop–fallow system in the central Great Plains area (Anderson et al., 1986; Shanahan et al., 1988; Halvorson, 1990; Peterson et al., 1993; Halvorson and Reule, 1994a, 1994b). An increase in cropping frequency and the low N mineralization capacity of the soils in this region require additional N to maintain sustainable yield levels (Halvorson and Reule, 1994a, 1994b; Kolberg et al., 1996).
Changing the cropping system from winter wheat–fallow with CT mechanical tillage for weed control and seedbed preparation to a more intensive NT cropping system with no mechanical tillage has potential to increase SOC (Havlin et al., 1990; Black and Tanaka, 1997; Campbell et al., 1997; Campbell and Zentner, 1997; Havlin and Kissel, 1997; Jones et al., 1997; Paustian et al., 1992; Rasmussen and Smiley, 1997). As yearly residue production is increased within a cropping system and tillage frequency is decreased, SOC levels will probably remain constant or increase with time, depending on the quantity and types of residue input into the soil (Larson et al., 1972; Rasmussen et al., 1980; Rasmussen and Rohde, 1988; Havlin et al., 1990; Parton et al., 1996). Nitrogen fertilization has been shown to increase the level of SOC within cropping systems (Rasmussen and Rohde, 1988; Paustian et al., 1992; Varvel, 1994; Campbell et al., 1997; Bowman and Halvorson, 1998); however, few studies have reported the effects of N rate on SOC accumulation.
The potential to sequester more C in soils by increasing dryland cropping intensity and N fertilization in semiarid areas could contribute positively to mitigating agriculture's effect on atmospheric CO2 levels and its effect on global climate change (Lal et al., 1998a, 1998b). Lal et al. (1998b) point out that the value of SOC is more than improving water-holding capacity and nutrient availability of the soil, its hidden value comes in its ability to help mitigate the greenhouse effect on the environment. They point out that adoption of best-management practices by farmers on cropland can help reverse the atmospheric enrichment of CO2 resulting from U.S. emissions outside of agriculture by sequestering C in soil. Thus it becomes important to understand how management practices, such as N fertilization, affect SOC. Converting to a NT system and cropping more intensively in the central Great Plains can potentially contribute to an improved environment. In addition, increasing the level of SOC will contribute to improving soil quality (Herrick and Wander, 1998; Seybold et al., 1998). Information on the long-term effects of N fertilization rates on crop residue production and its subsequent effects on SOC and TSN in NT dryland cropping systems is limited in the central Great Plains (Bowman and Halvorson, 1998). The objectives of this study were to evaluate the long-term effects of N fertilization rates on crop residue production in a NT annual cropping system and to determine the subsequent effects of returning this residue to the soil on SOC and TSN.
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Materials and methods
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The study was conducted on a Weld silt loam soil at the Central Great Plains Research Station, Akron, CO, with a soil pH of 7.2 and SOC concentration of 6.9 g kg-1 (0–15 cm depth) in 1984. The NaHCO3-extractable soil P level (0–15 cm depth) was 22 mg P kg-1 soil in 1984, which is considered very high in Colorado. The plot layout was a randomized, complete block design with four replications. At planting of each crop, N, as NH4NO3, was broadcast with no mechanical incorporation at rates of 0, 22, 45, 67, 90, and 134 kg N ha-1. The plots receiving the 134 kg N ha-1 rate actually received 179 kg N ha-1 the first two crop years of the study (1984 and 1985). This N rate was reduced in 1986 because of a significant increase in measured residual soil NO3–N in fall 1985. Nitrogen rates were reduced 50% in 1988 because of crop failure in 1987 caused by hail, and no measurable N removal occurred. Plot size was 6.1 by 12.2 m. A NT system of farming was used, with no mechanical tillage operations for seedbed preparation or weed control. Minimal soil disturbance occurred in the plots during seeding operations with NT drills. Additional plot management, grain yield, and soil NO3–N details were reported by Halvorson and Reule (1994a, 1994b).
A spring barley–corn rotation was generally followed, except when an August 1987 hail storm destroyed the corn crop allowing us to plant winter wheat in the fall of 1987 rather than spring barley in 1988 (Table 1)
. Spring barley was replaced in the rotation in 1992 with oat hay and in 1994 with oat–pea hay because of poor performance of barley in the rotation. Crop residue production was estimated by obtaining a total biomass sample from no less than a 1-m2 area of each plot. Total aboveground biomass yield (grain + residue or forage) per unit area was calculated and the grain yield subtracted to get an estimate of the aboveground crop residue returned to the soil surface. All grain yields were determined by harvesting a minimum of a 1.5 by 12 m area from each plot with a plot combine, except for hand harvesting in 1984. Total corn biomass was not determined in 1985 due to an oversight or in 1987 due to hail. An estimate of corn stover returned to the soil in 1985 was calculated by using the average stover/grain ratio from 1989, 1991, and 1993 and grain yields from 1985. In 1987, neither grain or total corn biomass was obtained due to hail on 5 Aug. An estimate of biomass production for 1987 was made by using the average estimate of corn stover returned to the soil for the other four corn years as a function of N rate. For the oat hay crop in 1992 and oat–pea hay crop in 1994, no aboveground crop residue was returned to the soil surface except the 5 to 7 cm of stubble left after cutting the hay crop. Estimates of belowground (root) residue C in the soil were made by assuming that root C equaled grain yield times 0.57 (Follett et al., 1997). Hay crop contributions to root C were estimated from a linear relationship of aboveground residue C to root C for the wheat, barley, and corn crops.
Soil samples, a composite of six random 2-cm-diameter cores per plot, were collected after hay harvest in 1994 to assess TSN and SOC in the 0- to 7.5-cm and 7.5- to 15-cm soil depths. Loose surface crop residue was brushed aside before taking the soil sample. Soil bulk density was determined in each plot for each sampling depth by collecting four random 3.2-cm-diameter cores per plot. The soil cores were composited, oven dried, and then weighed before calculating Db. Soil N and organic C were determined by dry combustion (Nelson and Sommers, 1996) using a Leco CHN-1000 autoanalyzer (Leco Corporation, St. Joseph, MI).1
Each sample was checked for free lime using the HCl fizz test. No free lime was detected. Soil bulk density values for each plot were used in calculating SOC and TSN contents. Plant residue and grain samples were analyzed for total N content using a micro-Kjeldahl N digestion (Isaac and Johnson, 1976) and Technicon autoanalyzer procedure (Technicon, 1973). Fertilizer N uptake efficiency was calculated as the sum of total aboveground biomass (or crop residue N) minus the plant N in the no N treatment divided by the total N applied in the 11-yr period. Carbon sequestration efficiency for each N rate was calculated by dividing the estimated total (11 crops) residue C returned to the soil above that without N fertilization by the change in SOC above that without N fertilization.
All statistical comparisons are at the 95% probability level unless otherwise stated. Analyses of variance were performed using SAS computer program (SAS Institute, 1991). If the analysis of variance indicated a significant F-value, a linear or quadratic function was fit to the N response data using regression functions present in the graphics program (SigmaPlot version 4.0, SPSS Inc., Chicago, IL).
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Results and discussion
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Crop yields for each crop year and N treatment, 1984 through 1994, are reported in Table 1 and were published by Halvorson and Reule (1994a, 1994b). When averaged over all years, optimum grain yield was obtained with the application of 67 kg N ha-1. Total (11 crops) aboveground biomass (grain + residue or forage) production increased significantly with increasing N rate to near maximum with the application of 67 kg N ha-1 and then tended to level off with increasing N rate (Fig. 1a)
. Total crop residue production (barley + wheat + corn) followed similar trends to that of grain yield, increasing with increasing N rate up to the 67 kg N ha-1 rate and then tending to level off at higher N rates (Fig. 1b). Total barley and wheat residue returned to the soil surface in four crops followed the same pattern. Estimated total corn residue (stover) returned to the soil with five crops increased with the application of 22 and 45 kg N ha-1 and then tended to level off at higher N rates. No residue was considered returned by the oat hay in 1992 and the oat–pea hay in 1994 because the aboveground biomass (except for a 5- to 7-cm stubble) was removed from the field as a hay crop.
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Fig. 1 (a) Total aboveground crop biomass (grain + residue) produced in 11 crop years and (b) total aboveground crop residue, corn stover, and small grain residue returned to the soil surface from 1984 through 1994 as a function of N rate within a NT dryland annual cropping system at Akron, CO
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Changes in Db with N rate after 11 crop years are shown in Fig. 2 . Soil bulk densities in the 0- to 7.5-cm soil depth decreased significantly as the N rate and amount of biomass (Fig. 1b) returned to the soil increased. However, soil Db in the 7.5- to 15-cm soil depth was not significantly affected by N treatment. There was a significant linear inverse relationship
between soil Db (Mg m-3) and increasing amounts of residue (res, kg ha-1) returned to the soil
. The lower Db in the surface 0- to 7.5-cm soil depth will enhance the performance of disk-type drills in seed placement and enhance infiltration of water. These data point out the positive influence of returning increasing amounts of crop residue to the soil on soil Db in a NT system.
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Fig. 2 Soil bulk density in the 0- to 7.5- and 7.5- to 15-cm soil depths after 11 crop years as a function of N fertilizer rate
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Total N uptake in the total aboveground biomass of the 11 crops and the total crop residue N returned to the soil increased with increasing N rate (Fig. 3)
. The total amount of N removed in the harvested grain or forage is the difference between these two curves. An average of about 28% of the total N uptake was returned to the soil in the residue for the N fertilized plots. The increase in crop residue N returned to the soil with added N was reflected in a significant increase in TSN in the 0- to 7.5-cm depth with increasing N rate (Fig. 4)
. The change in TSN in the 0- to 15-cm soil depth was significant with increasing N rate. Changes in TSN in the 7.5- to 15-cm depth were not significant. A significant linear relationship existed between the change in TSN (kg ha-1) above that of the zero N rate and the increase in residue N (resN, kg ha-1) returned to the soil above that of the zero N rate
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Fig. 3 Total N uptake in biomass of 11 crops and total N returned to the soil surface by crop residue from nine crops as a function of N rate added each crop year
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Fig. 4 Total soil N in the 0- to 7.5-, 7.5- to 15-, and 0- to 15-cm depths after 11 crop years as a function of N rate
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The fraction of the N fertilizer applied to the 11 crops that was taken up in the total biomass and the fraction returned to the soil in the residue is shown in Fig. 5
. As the rate of N fertilization increased, the fraction of fertilizer N uptake decreased. At rates >67 kg N ha-1, the fraction dropped below 90%. This would explain the large increase in residual soil NO3–N in the 0- to 180-cm profile reported by Halvorson and Reule (1994a, 1994b) at N rates >67 kg N ha-1. As they reported, the spring 1993 residual soil NO3–N in the 0- to 180-cm profile increased from 66 kg N ha-1 for the 67 kg N ha-1 treatment to 254 kg N ha-1 for the 134 kg N ha-1 treatment. The fraction of fertilizer N returned to the soil in the residue decreased as the N rate increased (Fig. 5). These data point out the need to avoid overfertilization with N in order to reduce the quantity of potentially leachable soil N and to efficiently utilize any N applied. This agrees with the work of Raun and Johnson (1985), who reported an increase in residual soil N when N rates exceeded those needed for optimum yield.
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Fig. 5 Percentage of fertilizer N applied in the total aboveground biomass and residue returned to the soil as a function of N rate
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The mass of SOC increased significantly with increasing N rate in the 0- to 7.5-cm soil depth, but not in the 7.5- to 15-cm soil depth (Fig. 6)
. A more rapid buildup of SOC in the shallower vs. the deeper layer is consistent with the observations reported by Follett et al. (1997) and Huggins et al. (1998). The increase in SOC in the 0- to 15-cm depth (Fig. 6) with increasing N rate was significant
. This increase in SOC mass with increasing N rate reflects the response of crop biomass to added N and the quantity of residue returned to the soil. Based on the regression equation in Fig. 6, 2 Mg ha-1 more SOC had accumulated in the 0- to 15-cm soil depth after 11 crops in the 134 kg N ha-1 treatment than with the zero N treatment. This equates to an annual increase in SOC of 182 kg C ha-1 yr-1 at the 134 kg N ha-1 rate. At the 67 kg N ha-1 rate, the annual increase in SOC was estimated to be 140 kg C ha-1 yr-1. The accumulation rate for the 67 kg N ha-1 treatment is slightly higher than the accumulation rate of 100 kg C ha-1 yr-1 associated with N fertility reported by Lal et al. (1998b). The annual SOC accumulation rate for the 45 kg N ha-1 treatment was 94 kg C ha-1 yr-1, which is very close to the annual accumulation value reported by Lal et al. (1998b). The soil C/N ratio was not influenced by N fertilization in this study, remaining constant across all N rates at 9.0 in the 0- to 7.5-cm depth and 8.9 in the 7.5- to 15-cm depth.
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Fig. 6 Total soil organic C in 0- to 7.5-, 7.5- to 15-, and 0- to 15-cm depths as a function of N rate after 11 crop years
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Carbon sequestration is very important when considering the effects of farming practices on greenhouse gas emissions, such as CO2 (Lal et al., 1998b). Cropping systems and practices that enhance C sequestration of atmospheric CO2 are beneficial. Data from this study indicate that N fertilization increased the SOC in this NT annual cropping system. Based on relationships for winter wheat used by Follett et al. (1997), we estimated the influence of N fertilization on C sequestration by assuming that the aboveground crop residue returned to the soil surface contained 40% C and that root C contributions were equal to 57% of grain yield. These estimates of C content are based on 14C studies with winter wheat (Follett et al., 1997), but we currently do not know if such estimates are reasonable for crops such as corn. Estimates of root residue C from the hay crops were made on the basis of a linear relationship
that we observed between estimates of aboveground residue C and estimated root residue C based on grain yield for the other crops. Weed C contributions were disregarded since observed weed control was excellent in this study. The percentage of change in C sequestration above that of the zero N rate is shown in Fig. 7
. Carbon sequestration efficiency, when based on only mass of aboveground residue C indicates an increase of 
30% at the highest N rate above that of the zero N rate. However, when estimated root C was included with aboveground residue C in the total plant C estimate, the increase in C sequestration efficiency was 11% higher at the highest N rate than for the zero N rate. These calculations illustrate that knowledge about the mass of C in plant roots that are incorporated into the SOC pool is critical to the calculation of plant-residue C storage efficiency. However, there is a serious gap in research knowledge concerning the mass of belowground residue C produced by plant roots from various crops. This information is extremely important when addressing the effects of cropping practices on C sequestration as it is related to concerns about global climate change.
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Fig. 7 Estimates of C-sequestration efficiency in the 0- to 15-cm soil depth as a function of N rate after 11 crop years when considering surface residue C inputs only and surface residue C plus estimated root residue C inputs
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Summary
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The positive effects of N fertilization on SOC were clearly demonstrated in the 0- to 7.5-cm soil depth in this long-term dryland annual cropping study under NT conditions. Nitrogen fertilization significantly increased crop residue inputs to the soil, resulting in increases in TSN and SOC in the 0- to 7.5-cm depth after 11 crops. The increase in SOC with increasing N fertilization rate decreased soil Db and contributed to improved soil quality (Herrick and Wander, 1998). Carbon sequestration efficiency was improved by N fertilization. This study shows that managing NT cropping systems for optimum yield with adequate N fertility will have positive environmental effects and that N fertilization will enhance SOC accumulation and productivity in the central Great Plains. A good N fertility program helps sequester atmospheric CO2 into SOC by increased plant growth and subsequently, the return of organic C to the soil for storage as soil organic matter in a NT system.Raun Johnson 1995
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ACKNOWLEDGMENTS
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The authors would like to thank Dr. G.A. Peterson, Colorado State University, Fort Collins, and L. Sherrod, USDA-ARS, Fort Collins, CO for performing the C and N analyses.
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NOTES
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Contribution from USDA-ARS. The U.S. Department of Agriculture offers its programs to all eligible persons regardless of race, color, age, sex, or national origin, and is an equal opportunity employer.
1 Trade names and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment of the product by the authors or the USDA, Agricultural Research Service. 
Received for publication July 28, 1998.
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ABSTRACT
INTRODUCTION
