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a University of Nebraska-Lincoln, Dep. of Agronomy and Horticulture, Lincoln, NE 68583-0915
b USDA-ARS Soil and Water Conservation Research Unit, Lincoln, NE 68583-0934
* Corresponding author (bamos2{at}unlnotes.unl.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: EC1:1, soil electrical conductivity measured in a one to one soil distilled water suspension • GWP, global warming potential • IPCC, Intergovernmental Panel on Climate Change • M1, recommended best management fertility treatment • M2, intensive fertility treatment • P1, low plant population • P2, medium plant population • P3, high plant population • TP, total porosity • WFPS, water-filled pore space
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In addition to CO2, increasing levels of atmospheric N2O and CH4 are of particular concern due to their considerably higher global warming potentials (GWP) relative to CO2. For example, over a 20-yr time period, 1 kg of N2O will have 275 times the influence on global warming as 1kg of CO2 (Intergovernmental Panel on Climate Change [IPCC], 2001). Nitrous oxide is produced when plant-available N forms are subjected to the bacterial processes of denitrification and nitrification (Firestone and Davidson, 1989), and various studies have shown that N2O emission from agricultural soil is significantly increased by application of synthetic N fertilizers (Linn and Doran, 1984; Bronson and Mosier, 1993). Global N2O emissions from row-crop agriculture are assumed to be the greatest contributor to global N2O flux (Robertson, 1993), with cultivated soils comprising 27% of the total N2O-N added from all known sources (Beauchamp, 1997). Soils comprise between 3 and 9% of the total sink for atmospheric CH4 due to consumption by methanotrophs in aerobic soils (Sylvia et al., 1998). Few studies have examined the effect of fertilizer application on CH4 uptake by cultivated soils. While Bronson and Mosier (1993) found that urea fertilization of irrigated wheat and corn did not affect CH4 uptake, Powlson et al. (1997) determined that 150 yr of N application to wheat plots maintained at a neutral pH reduced CH4 uptake by 50%.
The objectives of this study were (i) to determine the effect of different fertility management regimes on annual patterns of soil surface CO2 flux in a continuous maize production system, (ii) to develop an empirical model for prediction of soil surface CO2 flux based on relevant controlling factors (i.e., soil temperature, soil water content, LAI), which would then be used to estimate total annual soil CO2 flux under different fertility management regimes in continuous maize by means of integrating predicted values over the course of a year, and (iii) to determine the effect of different fertility management regimes on soil surface fluxes of N2O and CH4.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Management Treatments
The continuous maize systems compared in this study included three maize plant populations (P1 = 69200–76600 plants ha–1, P2 = 86400–98800 plants ha–1, and P3 = 108700–116100 plants ha–1), and two nutrient management regimes: recommended best management (M1) and intensive management (M2). The M1 N treatment was based on the current University of Nebraska-Lincoln, Department of Agronomy and Horticulture N algorithm. The input for this algorithm included a yield goal of approximately 12500 kg grain ha–1, NO–3–N concentration, and organic matter content, and was intended to follow best management practices for maize. Fertility management in the M2 treatment was designed to be nonlimiting and to supply N, P, and K to meet the requirements of a higher plant population and yield goal (approximately 16300–16900 kg grain ha–1 in the first year of production with an ultimate goal of 18800 kg grain ha–1 in subsequent years). This higher yield goal was based on the best estimate of maximum yield potential for maize under the climatic conditions in southeast Nebraska. The overall experiment is a split-split plot randomized complete block design with four replicates. The main plots are two crop rotations (continuous maize and maize–soybean), the subplots are the plant populations, and the sub-subplots are the two fertility treatments. Measurements of greenhouse gas fluxes were made in the individual sub-subplots. These sub-subplots covered eight rows and were 6.1 by 12.2 m in size. Although no permanent control plots were established for this study, when feasible, measurements were also made in the unfertilized borders at the edges of the field and between plots. These control areas covered eight rows and were 6.1 by 24.4 m in size, and measurements were made in the inner four rows of these areas. The plant population in these control areas was P3 (108700–116100 plants ha–1).
Table 1 shows the N application schedule for the two fertility management regimes in continuous maize during the three growing seasons. Granular preplant fertilizer was broadcast and disked, and for the M2 treatment, included supplemental nutrients in addition to NH4NO3. In 1999, the M2 preplant fertilizer included P, K, S, and Fe applied at the rates of 44, 84, 20, and 10 kg ha–1, respectively. In 2000, the M2 preplant fertilizer included P, K, S, Fe, and Zn applied at 45, 85, 21, 12, and 6 kg ha–1, respectively. In 2001, the preplant fertilizer applied to the M2 plots included only 45 kg ha–1 P and 85 kg ha–1 K as supplemental nutrients. Subsequent N was applied as NH4NO3 and was surface broadcast between the rows. The plots were kept well watered during the growing season through a drip tape system. During the 1999 and 2000 growing seasons, the tape ran along the base of the plants in every row. In 2001, the drip tape was buried at a depth of 30 to 38 cm at a 60-cm spacing beneath the between row area to conserve water as well as avoid rodent damage.
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v) by total soil porosity (TP) (Linn and Doran, 1984). Water-filled pore space is reported here as a percentage but is used as a fraction in empirical equations. Bulk densities from the soil cores were used in calculations of v and TP when gravimetric samples were taken. During use of the HydroSense, additional soil cores were taken periodically to determine average within row and between row bulk density for WFPS calculations. At various times during the growing season it was necessary to substitute WFPS readings from an adjacent M1 plot for WFPS of a particular M2 plot when the HydroSense readings were unusually high due to higher electrical conductivity of the M2 treatment (presumably due to the higher soil NO–3 content of these plots).
Nitrous Oxide and Methane Flux
Nitrous oxide and CH4 fluxes were measured on 3 d during the 2000 growing season (23 May, 12 July, and 24 August) and on 3 d in 2001 (17 May, 24 July, and 22 August). These samples were taken in the P2 plant population of continuous maize for the M1 and M2 treatments, as well as in the control areas adjacent to each continuous maize block. Two static chambers per plot were installed in the between row location. These chambers had a diameter of 15 cm and covered an area of 176.7 cm2, and could be closed with a vented lid. They were inserted (without lids) into the soil to a depth of 7.5 cm at least 24 h before sampling, leaving a head space of 1325 cm3. Using a syringe, 20-mL gas samples were extracted through a septum in the lid of each chamber at 0, 15, and 30 min after closing. These samples were then injected into 10-mL evacuated vials sealed with septa and aluminum collars. Nitrous oxide and CH4 were analyzed by means of an automated gas sampling system attached to a gas chromatograph (Varian 3700) as described by Arnold et al. (2001). Fluxes were calculated using an equation published by Hutchinson and Mosier (1981), which assumes that flux decreases over time due to a decrease in the concentration difference between the soil and the headspace. For data that did not fit this assumption, flux was calculated from the slope of concentration versus time curve. Soil moisture and temperature was measured at each chamber using the techniques described above.
Electrical Conductivity, pH, and NO–3–N
In 2001, measurements of EC1:1, pH, and NO–3–N were made in the laboratory on subsamples taken from soil cores (top 7.5 cm) collected at the time of N2O and CH4 sampling. Electrical conductivity was determined for a 1:1 soil-water suspension (by mass) with a conductivity meter (Markson model 1062). A pH meter (Oaktron 510 series) was then used to determine the pH of the 1:1 soil-water suspensions. A separate subsample of dry soil was analyzed for NO–3–N content by means of water extraction and Cd reduction (Gelderman and Beegle, 1998). Electrical conductivity was also measured in the field in the top 7.5 cm of soil with a conductivity meter (Hanna Instruments Dist WP, Woonsocket, RI) adapted for in situ measurements by John Doran and Spencer Arnold of the USDA-ARS Soil and Water Conservation Research Unit, Lincoln, NE. The instrument was mounted on a pole and wired to metal probes that were pushed directly into the soil. In 2000, the first version utilized copper probes. Electrical conductivity was measured in 2000 near each static chamber in the between row area whenever N2O and CH4 fluxes were measured. A sturdier version with steel probes was assembled in 2001. This version also allowed for a temperature correction of EC readings. In 2001, in situ EC measurements were made concurrent with the 24 July and 22 to 23 August N2O and CH4 samples. On 24 July 2001, 500 mL of distilled water was poured into each chamber after gas samples were extracted, and EC was measured later in the evening within the chamber. In situ EC measurements were made on 23 Aug. 2001, approximately 24 h after addition of the 500 mL of distilled water. Distilled water was used in 2001 to reduce the variability of field EC measurements attributable to variability in soil water content. Analysis of variance was performed for all sampling days.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Various studies have shown that the release of CO2 from decomposing soil organic matter is largely a function of soil water content and temperature (Howard and Howard, 1993). As can be seen in the data for the 2000 growing season (Fig. 2), increases and decreases in these controlling factors tended to mirror increases and decreases in soil CO2 flux. However, both soil temperature and WFPS reached maximum values earlier in the season than does soil CO2 flux, and they remained at these high values for some time after soil surface CO2 flux had dropped back down to preplant levels. Soil CO2 flux increased throughout May and June as the plant increased in biomass, reaching a maximum around anthesis. Martens (1990) reported a decrease in C translocation to the soil and declining rate of root growth as maize plants reached anthesis. While soil temperature and WFPS remained high, soil CO2 flux declined steadily throughout the rest of the growing season as more carbohydrates were allocated to grain fill and less to the roots. Soil surface CO2 flux decreased even more as the plants eventually reached physiological maturity and senescence. Qian et al. (1997) showed that root-released C decreases as maize plants age. While day to day variations in soil surface CO2 flux seem to mirror variations in soil temperature and moisture, the seasonal shape of the soil surface CO2 flux curve reflects an increase in biomass and root C allocation and then a decline in root exudates and eventual plant senescence. Singh and Gupta (1977) list phenologic stage as one of the factors governing root respiration. Soil CO2 flux measurements during the growing season represent a combination of root respiration, microbial respiration in the bulk soil, and respiration of the rhizosphere microbial community that predominantly uses root-released C as an energy source. This suggests that plant phenology exerts a great influence on soil CO2 flux through control of belowground C allocation.
Soil Surface Carbon Dioxide Flux—Effect of Plant Roots
Between row and within row flux, temperature, and WFPS are plotted for all sampling days during the 27-mo study in Fig. 3. Within row flux was on average 64% higher than between row flux. Within row flux was higher than between row flux on all but one of the 46 sampling days that rows could be distinguished and was significantly higher than between row flux on 33 d (p values ranged from 0.0408 to <0.0001). Over the course of the growing season, within row flux was 10 to 198% greater than between row flux, with maximum flux differences due to location of measurement generally occurring between V12 and dent stage. However, the effect of plant roots on soil surface CO2 flux was also evident in early seedling stages and after harvest during root decomposition. In 1999, soil surface CO2 flux at the V2 stage was 16% higher within row and was significantly higher by 42% (p = 0.0013) at the V2 stage in 2000. Soil surface CO2 flux was significantly higher in the within row area than in the between row area by 115% (p = 0.0006) 3 d after harvest in 1999 and by 89% (p < 0.0001) 11 d after harvest in 2000.
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Between Year Comparisons
Between year differences in soil surface CO2 flux, soil temperature, and WFPS were examined by selecting a set of measurements and time period that were common to all three growing seasons. The data set was therefore limited to the M1 and M2 treatments of the P2 plant population of continuous maize since these plots were sampled in all three seasons. Measurements were limited to those made from the V8-V9 leaf stage through the last sampling day before physiological maturity to obtain three comparable data sets. This comparison among the three seasons is shown in Table 3. The 2000 season had significantly higher soil surface CO2 flux values than both the 1999 season (p < 0.0001) and the 2001 season (p < 0.0001). In addition, soil surface CO2 fluxes were significantly greater in the 2001 season than in the 1999 season (p = 0.01). While continuous maize plots during the first growing season of this experiment (1999) followed a previous soybean crop, the second (2000) and third (2001) growing seasons followed a previous maize crop. Residue returned to the soil after the 1998 soybean harvest was estimated to be 2.8 Mg ha–1, while maize residue returned to the soil after the 1999 and 2000 harvests was measured at 9.6 and 11.1 Mg ha–1 respectively. It is therefore likely that the significantly greater soil surface CO2 flux observed during the 2000 and 2001 growing seasons was due to a higher level of residue input and substrate decomposition.
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Estimating Annual Soil Surface Carbon Dioxide Flux
Since soil surface CO2 flux measurements in this study were not continuous, estimates of total flux for the fertilized plots over the course of a full year were based on an empirical equation. This equation was fitted to all data collected in the M1 and M2 plots from 11 June 1999 through 9 Sept. 2000, a total of 1322 sets of flux, WFPS, and soil temperature measurements, along with LAI estimates for each sampling day. Reports in the literature indicate that 8 to 52% of all the carbohydrates produced per day in photosynthesis are respired by the roots during the same time period, and this percentage varies widely with age of plants, growth conditions, and species (Lambers et al., 1996). It was felt that LAI, through its relationship to photosynthetic capacity and subsequent below ground C translocation, could serve as a parameter that would reflect the contribution of root respiration to total soil surface CO2 flux from emergence to physiological maturity. While root biomass and exudates are difficult and time-consuming to measure, LAI is a relatively simple measurement that is commonly made in agronomic studies, making it more suitable for an empirical equation such as ours. Leaf area estimates were obtained by fitting curves through available LAI data for each combination of treatment, plant population, and rotation used in the model data. A curve-fitting program using the Marquardt–Levenberg algorithm was used to determine coefficients. Data obtained after harvest in 2000 was not included in the curve fit to retain an independent data set with which to test the equation.
The equation uses a simple exponential relationship involving the sum of soil temperature and LAI. In addition, it incorporates a relationship between WFPS and relative soil respiration for repacked cores of 11 medium- to fine-textured soils derived by Doran et al. (1990). The coefficients of the quadratic expression of WFPS were fixed to those published by Doran et al. (1990), while the coefficients of the exponential expression of soil temperature and LAI were estimated by the Marquardt–Levenberg algorithm. This equation is as follows:
 | [1] |
The equation was used to estimate daily average soil surface CO2 flux for the five sampling days from 26 Sept. 2000 through 17 Mar. 2001 on which measurements were made in the post-harvest fallow M1 and M2 plots and for the seven sampling days during the 2001 growing season (15 June 2001 through 18 Aug. 2001) on which measurements were made in the M1 and M2 treatments in continuous maize. The input data used to test Eq. [1] consisted of 200 sets of measured WFPS, measured soil temperature, and LAI values estimated from fitted curves specific to each fertility treatment. It should be stressed that this was an independent data set not used to parameterize the equations. These estimates are compared with measured daily average soil surface CO2 flux in Fig. 4. A line fitted through the data points had a slope of 0.815, an intercept on the y axis of 0.018, and an R2 of 0.90, showing that it performed well at predicting these soil surface CO2 flux values, considering that daily measured and predicted flux during the 2001 growing season was based on only 16 sets of measurements.
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 | [2] |
Equation [2] was used to calculate hourly or half hourly flux for the M1 and M2 treatments of P2. Soil temperature input for Eq. [2] consisted of half hourly soil temperature readings at 0.1 m from an automated weather station (AWS) located in the center of the study center (DOY 8–256) or hourly 0.1-m soil temperature readings from an AWS located in a grassy field within 250 m of the study area (DOY 1–7 and 257–366). Leaf area index values for Eq. [2] were estimated from fitted curves, and WFPS values were estimated by interpolating between measured daily values. Hourly and half hourly soil CO2 flux estimates were averaged over each 24-h period. Integration under curves of predicted flux plotted against day of year yielded an emission estimate of 11500 kg C ha–1 yr–1 for the M1 treatment and 11600 kg C ha–1 yr–1 for the M2 treatment. Therefore, based on both actual flux measurements and estimated values, it seems that the intensive fertility treatment results in little difference in soil surface CO2 flux compared with the standard recommended treatment (M1). However, intensive levels of N application may have an indirect effect on soil CO2 flux, as declining pH levels necessitate increased lime applications, which in turn, potentially increase soil surface CO2 flux as neutralization of the soil solution proceeds.
Nitrous Oxide, Methane, Electrical Conductivity, NO–3–N, and pH
Measurements made of N2O flux, CH4 flux, and EC during the 2000 and 2001 growing seasons and NO–3–N and pH during the 2001 growing season are shown in Table 4. While no significant difference in N2O flux was seen between the fertilized treatments on 23 May 2000 after M1 and M2 plots had received identical amounts of preplant N (see Table 1.), a significant difference was seen on 12 July, 49 d after an additional 100 kg N ha–1 had been applied to the M1 treatment and 34 d after an additional 263 kg N ha–1 had been applied to the M2 treatment. The greater amount of N applied to the intensive treatment resulted in a significantly higher N2O flux in comparison with both the M1 treatment and the unfertilized control, and this effect was still evident when measurements of N2O flux were made 11 wk after the final fertilizer application (August 24). Field measured electrical conductivity was significantly higher in the M2 treatment later in the season. A good correlation (R2 = 0.89) was seen between field-measured EC for the two fertility treatments and N2O flux in 2000 (Fig. 5). In this figure, electrical conductivity values were adjusted to account for naturally occurring anions not related to fertilizer inputs by subtracting EC values measured on the same day in the control areas (Smith and Doran, 1996). This strong relationship was not seen in 2001 when distilled water was added to the chambers before EC measurement. No significant differences in CH4 flux were observed during the 2000 season.
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There are several possible reasons for the difference in the pattern of N2O flux seen in the two growing seasons. While two splits of N were applied to the growing crop in 2000, this application was spread out over three splits in 2001. This change in timing of N application in the M2 treatment may have allowed for more efficient uptake of NO–3 by the plants, therefore making it less available for denitrification. Also, lower NO–3 levels throughout the growing season would allow for a lower N2O/N2 ratio in denitrification products. Increased levels of NO–3 seem to inhibit the reduction of N2O to N2 during denitrification, thus increasing the N2O/N2 ratio of the products (Blackmer and Bremner, 1978; Smith and Doran, 1996). Since NO–3–N levels were not measured during the 2000 growing season, it is not possible to verify whether or not they were lower in the M2 treatment in 2001. However, as seen in Table 4, NO–3–N levels measured in the top 7.5 cm in 2001 proved to be significantly higher in the M2 plots than in either the control areas or the M1 plots on 24 July and 22 August (p values ranged from 0.0009 to 0.0184).
Based on the considerably higher NO–3 levels in the M2 treatment, one would expect that N2O flux would again be significantly higher in the M2 treatment than in the M1 treatment and the control area. However, the soil surface layer was generally drier in 2001 due to the change in irrigation technique. It is possible that the potential for high rates of denitrification existed in the M2 treatment in 2001, but that this potential was not met due to a generally lower WFPS. As mentioned previously, standing water in the area of one of the M2 chambers seemed to trigger an extremely high N2O flux (246 g N ha–1 d–1), which contributed to the high standard deviation on 24 July.
A third possible explanation for the lower N2O fluxes later in the 2001 season in the M2 treatment is the progressive lowering of soil solution pH due to greater nitrification of NH+4 applied at a higher rate. If the resulting NO–3 had exceeded plant requirements, excess hydronium ions produced during nitrification would not have been sufficiently neutralized during NO–3 uptake. Patriquin et al. (1993) describe how this decoupling of soil-plant N cycling also decouples the cycling of protons and can result in acidification of soil. By the time the third set of measurements was taken on August 22 and the full N rates had been applied, nitrification of NH+4 had caused the pH in both treatments to drop to levels significantly lower than the control area. In the M2 treatment, pH dropped to 4.8, a level significantly lower than that of the M1 treatment. It is possible that this low pH inhibited the microorganisms involved in the N transformations that produce N2O. While both nitrification and denitrification have an optimum pH range of 6.5 to 8 (Smith and Doran, 1996), nitrification is especially sensitive to low pH, and its rate becomes negligible below pH 5.0 (Bouwman, 1990). Since WFPS was generally below 80%, a level above which denitrification rates increase sharply (Linn and Doran, 1984), it is likely that nitrification was the major source of N2O in this system and its production could have been decreased by lowering pH levels. Electrical conductivity was significantly higher than both the M1 treatment and the control in 2001. Laboratory measured EC1:1 was highly correlated with NO–3–N concentration for individual soil samples (R2 = 0.91), suggesting that the relatively high EC values found in many of the M2 samples were a result of the greater amounts of N fertilizer applied in that treatment.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication March 17, 2004.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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