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a Est. Exp. Agropecuaria INTA, Balcarce, Buenos Aires, Argentina
b Est. Exp. Agropecuaria INTA, Balcarce, Buenos Aires, Argentina, and Fac. de Ciencias Agrarias (UNMP) Unidad Integrada Balcarce, C.C. 276, (7620) Balcarce, Buenos Aires, Argentina
c Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, C.C. 276, (7620) Balcarce, Buenos Aires, Argentina
* Corresponding author (hecheverr{at}balcarce.inta.gov ar)
Received for publication December 12, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CET, crop evapotranspiration • CL N2O-N, cumulative nitrous oxide nitrogen losses • CT, conventional tillage • FPL, fertilization at planting time • FV6, fertilization at six-leaf stage • MBN, microbial biomass nitrogen • NT, no-tillage • NUE, nitrogen use efficiency • UN, unaccounted nitrogen
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Compared with CT, NT systems in temperate-humid areas can change the soil environment and consequently can decrease N mineralization and nitrification while increasing N immobilization, denitrification, and/or leaching (Fox and Bandel, 1986). A greater immobilization and decrease in N mineralization in soils under NT leads to an increase in the soil organic N (Meisinger, 1984). During a period of 7 yr, NT increased soil organic C (3.8%), and consequently organic N, in soil surface (0–20 cm), with respect to CT at Balcarce (Studdert and Echeverría, 2002). Therefore, fertilizer N requirement of maize under NT is greater than CT (Meisinger et al., 1985). Consequently, it is very important to increase the recovery of applied N and maize production in NT systems if we are to increase the economic return to the farmer and reduce potential for NO–3 leaching or other adverse environmental impacts.
Nitrogen balances have been used to estimate the UN in a given agricultural production system (Legg and Meisinger, 1982). A number of 15N field balances, in maize crops under CT fertilized at planting time, have shown N deficits ranging from 15 to 27% of the applied N, and it is presumed that the deficit reflects denitrification losses (Olson, 1980; Reddy and Reddy, 1993; Bigeriego et al., 1979). Application at V6 results in losses of applied N in the range of only 6.7% (Bigeriego et al., 1979).
In the southeastern portion of Buenos Aires province, fertilizer N recovery in irrigated maize under NT is higher for FV6 than FPL (Sainz Rozas et al., 1997a, 1999). Nitrogen recovery by the maize crop was 71 and 58% (average of N rates) for FV6 and for FPL, respectively (Sainz Rozas et al., 1997a). For FPL, these authors reported denitrification losses ranging from 2.6 to 5.5% of applied N, for N rates of 210 and 70 kg ha–1, respectively. For FV6, theses losses ranged from 0.4 to 1% of applied N for the same N rates. Mean volatilization loss was 4.3 and 9.2% (average of N rates) of applied N for FPL and FV6, respectively (Sainz Rozas et al., 1999, 2001). Therefore, gaseous N losses (volatilization plus denitrification losses) were similar for both fertilization times, and it indicates that another mechanism of N loss, other than denitrification, decreased N recovery for FPL with respect to FV6.
Jokela and Randall (1997) reported greater N immobilization in organic forms when N fertilizer was applied at planting compared with V6. Nevertheless, the relative importance of this pool when calculating a whole-crop N balance has not been reported in soils under NT in Argentina. On the other hand, at Balcarce, in a maize crop under CT fertilized at planting with 200 kg N ha–1, Costa et al. (2003) reported leaching losses of 66.5 kg N ha–1. However, the magnitude of these losses has not been reported for irrigated maize crop under NT in the Argentina.
There are reports in the literature where partial aspects of N cycle (N uptake, mineral N, volatilization, and denitrification losses) as affected by N rate and application time have been studied (Sainz Rozas et al., 1999, 2001), but up to this point in time, this information has not been incorporated into an N balance equation. An N balance using part of a data set (Sainz Rozas et al., 1999, 2001) would assist in the identification of the main N loss mechanism and would thus be of use when considering approaches to decrease N loss from these NT maize systems. The objective of this research was to estimate an N balance for continuous NT irrigated maize as a function of N rate and application time.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The experimental field contains a soil complex consisting of 90% fine, mixed, thermic Typic Argiudoll and 10% fine, illitic, thermic Petrocalcic Paleudoll (petrocalcic horizon was below 70 cm). The Typic Argiudoll has a loam texture at the surface layer (0- to 25-cm depth), loam to clay loam at subsurface layers (25- to 110-cm depth), and sandy loam below 110-cm depth (C horizon). Some soil characteristics determined at the time of planting are presented in Table 1.
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The experimental design was a factorial treatment arrangement with a control (0 N) in a randomized complete block. In the first year, the factorial arrangement was a control plus a 2 x 2: two N rates [70 and 140 kg N ha–1] and two N fertilization times (planting and V6). In the second year and last year, the factorial arrangement was a control plus a 3 x 2: three N rates (70, 140, and 210 kg N ha–1) and two N fertilization times (planting and V6). In the third year, the factorial arrangement was again a control plus a 2 x 2: two N rates (70 and 210 kg N ha–1) and two N fertilization times (planting and V6). In all cases, urea was surface-broadcast.
Ammonia losses were evaluated during the 1994–1995 and 1995–1996 growing seasons for both fertilization times for the 0, 70, 140, and 210 kg N ha–1 rates, these results being part of a previous work (for more details about method of determination, see Sainz Rozas et al., 1999). A semiopen-static system (Nommik, 1973) was used to monitor NH3 volatilization losses from the plots. It consisted of a polyvinyl chloride (PVC) 30-cm-diam. and 50-cm-height cylinder per experimental unit, containing two polyurethane sponges saturated with H2SO4 (0.5 M) to capture the NH3. The sponges were changed every 24 h and washed with 1.5 L of deionized water. An aliquot of 25 mL was alkalinized with NaOH (40%), and NH3–N was determined by microdistillation (Bremner and Keeney, 1966). In 1996–1997 and 1998–1999 growing seasons, denitrification losses were determined from plots that received 0, 70, and 210 kg N ha–1 at both fertilization times. Denitrification losses were generally estimated weekly or biweekly during the growing season. Rates of N2O-N were accumulated from planting to flowering (R1) and from planting to physiological maturity (R6) in the 1996–1997 and 1998–1999 growing seasons, respectively. Denitrification rates (N2O) under field conditions were estimated by the C2H2 inhibition method (Yoshinari et al., 1977). Intact soil cores (4.2 cm i.d. x 13 cm long) were randomly obtained from between-row soil in each plot using PVC cylinders. The cylinders were sealed with two rubber stoppers, the upper stopper having a septum. Ten percent (v/v) of the gas enclosed in the cylinder was replaced with an equivalent volume of acetylene. Then, cylinders were incubated outside the laboratory in an open, but shaded environment for a 24-h period, after which 10 mL of a gas sample was removed for analyses of N2O-N concentration using a 5890 Series-II Hewlett-Packard gas chromatograph equipped with a 63Ni electron-capture detector (for more details about method of determination, see Sainz Rozas et al., 2001). Denitrification losses for 140 kg N ha–1 were not determined in any growing season. Therefore, denitrification losses for this N rate were estimated from a relationships between denitrification losses and N rates (0, 70, and 210 kg N ha–1) when this relationship was significant. If this relationship was not significant, denitrification losses from 140 kg N ha–1 resulted from an average between 70 and 210 kg N ha–1.
In each growing season, at planting time, soil samples were collected from the control plots to estimate the soil mineral N level. All treatment combinations were sampled at R6. Samples were collected at 0- to 5-, 5- to 20-, 20- to 40-, 40- to 60-, 60- to 80-, and 80- to 100-cm depths. Inorganic N was extracted from fresh samples with K2SO4 (0.5 M) and NO–3–N, and NH+4–N contents were determined by microdistillation (Bremner and Keeney, 1966).
In each experimental unit, soil N content in the MBN was determined at planting and at R6 stage by the chloroform fumigation–extraction technique (Brookes et al., 1985) at 0- to 5- and 5- to 20-cm soil depths. The results obtained in each depth were corrected by soil bulk density to transform them to kg N ha–1 to 20-cm soil depth. The MBN was calculated as:
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Each growing season, 10 maize plants were collected for determination of aboveground dry matter accumulation at the R6 growth stage. Plants were cut at ground level; separated into leaf blades, stalk plus sheaths plus tassel plus husks, and grain; and oven-dried, weighed, and milled to pass a 1-mm mesh. Reduced N was determined in each fraction by Method A (without salicylic acid modification) as reported by Nelson and Sommers (1973). Total N accumulated in each fraction was calculated as the product of its N concentration (dry weight basis) and dry weight. At maturity, 7.15 m of the two center rows of each experimental unit was hand-harvested to determine grain yield. All reported yields were corrected to 140 g kg–1 grain moisture content.
A whole-crop N balance approach (Meisinger, 1984) was used to estimate the UN in the soil–plant system, according to the following equations:
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Solving for UN:
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Nf = fertilizer N input
Nmin = N produced from mineralization
Nsinb = NO–3–N at the beginning growing season
Nsine = NO–3–N at the end of the growing season
Ngr = N removed in grain
Nvol = volatilization N losses
Ndesn = cumulative N2O-N losses
Nstover = stover returned N
Nroot = root returned N
MBN = change in soil microbial biomass N content (0–20 cm) between physiological maturity (end) and planting (beg), i.e., MBN = MBNend – MBNbeg
In the 1996–1997 and 1998–1999 growing seasons, the occurrence of a rainfall event (18 mm) shortly after FPL and FV6 (1 d after fertilization) prevented N volatilization (Fox et al., 1986), and therefore volatilization losses did not contribute to the UN.
Nitrogen mineralized during the growing season was determined using the model reported by Echeverría et al. (1994). This model integrates the potentially mineralizable N (N0), N mineralization constant, soil temperature, and soil moisture at the 20-cm depth during the growing season. The N0 was determined by anaerobic incubations of 14 d (Echeverría et al., 2000).
Nitrogen returned in crop root to the soil was estimated as a proportion of total N accumulated in aerial biomass. These values were 6.6 and 5.3% for control and fertilized treatments, respectively (Uhart and Andrade, 1995).
Treatment effects were evaluated by analysis of variance using the Statistical Analysis System (SAS) (SAS Inst., 1985). Following the F test in ANOVA, multiple comparison of means (p < 0.05) was conducted with a Fisher's protected least significant difference (LSD).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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These results indicate that N recovery by the maize crop was greater when N was applied at the V6 stage than when N was applied at planting time. Additionally, it appears that the recovered N was predominately deposited into the grain as previously reported by Bigeriego et al. (1979). The larger rate of N recovery with the FV6, which is just before maximum plant need, may be due to lower N losses through denitrification or leaching or lower N immobilization in organic forms (Bigeriego et al., 1979; Wells and Bitzer, 1984; Jokela and Randall, 1997).
Mean fertilizer N recovery as estimated by the difference method (estimated by subtracting N uptake of the control treatment from the N uptake of the fertilized treatments) ranged from 43 to 53% when the fertilizer was applied at planting time. These values are greater than those reported by Olson (1980) but similar to those reported by other authors (Kitur et al., 1984; Reddy and Reddy, 1993; Jokela and Randall, 1997). The FV6 resulted in estimated fertilizer N recovery values between 62 and 74%, which is less than those reported by Bigeriego et al. (1979) but greater than those reported by Jokela and Randall (1997). The difference with these last authors can be explained by greater water availability in our experiment after the V6 stage. On the other hand, when N recovery was calculated by the whole-crop available N method (mineralized N plus mineral N at planting plus N fertilizer), the observed values were 46 and 53% for FPL and FV6, respectively (Tables 4 and 5).
Nitrogen in Soil Microbial Biomass
Nitrogen immobilization by microbial biomass was increased by N fertilization in all growing seasons but the 1994–1995 season (Tables 4 and 5). The lack of N immobilization determined in 1994–1995 with respect to the other growing seasons could be explained by the effect of C availability on soil biomass size (Bonde et al., 1987) because the preceding crop was soybean in 1994–1995 and maize in other growing seasons. In turn, N immobilization was not different for the fertilized treatments, and the main difference was observed between control and the fertilized treatments (Tables 4 and 5). For maize crops growing under rainfed conditions, Jokela and Randall (1997) reported more N in organic forms for FPL compared with FV6. This behavior could be due to longer time and sometimes better moisture conditions for microbial immobilization between the time of application and maximum crop N uptake. However, under irrigation, soil water content after V6 would not be a very restrictive factor of the microbial activity. In Balcarce, mean soil temperature at a 20-cm depth is 14.5 and 17.5°C for October and November (period between planting and V6), respectively (Della Magiora, personal communication, 1985). Therefore, this variable could also have limited microbial growth and N immobilization for planting with respect to V6 fertilization because optimum temperature for soil N mineralization and immobilization ranged from 25 to 30°C (Alexander, 1977).
Microbial biomass organic N transforms progressively into more stable organic compounds as the microbial biomass dies (Jenkinson and Parry, 1989). Data from the Broadbalk experiment show that the half-life for soil microbial biomass is on the order of 1 yr (Jenkinson and Parry, 1989). Moreover, these authors suggest that a large proportion of the organic products liberated by dead microbial cells are mineralized and again used for the growth of new microbial populations (cryptic growth). These antecedents would indicate that in our experiment, N immobilization would not have been largely underestimated if we had only considered the MBN since a great part of the N immobilized from the fertilizer would be part of this N pool at the end of the growing season. For the 1995–1996, 1996–1997, and 1998–1999 growing seasons, the mean value for the immobilized N fertilizer was estimated to be 13.4 kg N ha–1 and was calculated as the difference between fertilized treatments (average N rates) and the control treatment. This value is lower than that reported by Olson (1980), but similar to that of Reddy and Reddy (1993), for fertilizations at planting time for maize grown under CT management.
In the 1995–1996 and 1996–1997 growing seasons, no significant relationship was found between crop N uptake and change in MBN (MBN at physiological maturity minus MBN at planting) in the preceding growing season (Fig. 2a). Therefore, this N pool would not have contributed to N available for these treatments. This result can be explained by the low N immobilization determined in each growing season (Tables 4 and 5) and by the progressive transformation, as the microbial biomass dies, into active humus N, which has a turnover time of 3.1 yr (Jenkinson and Parry, 1989). Therefore, the change in MBN in the preceding growing season would not have contributed to available N, and therefore the UN for the fertilized treatments would not have been largely overestimated.
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Residual Soil Nitrate
In the 1994–1995, 1995–1996, and 1998–1999 growing seasons, soil inorganic N at the end of the growing season was significantly affected by an interaction between N rate and fertilization time. Mineral N was significantly greater only for the highest N rate applied to V6 stage (Tables 4 and 5), and it indicates that high N rates applied at that stage exceeded crop requirements. Similar results have been reported by Bigeriego et al. (1979) and Jokela and Randall (1997). When N remaining in the soil profile after maize harvest is high, the potential for NO–3–N pollution of groundwater increases significantly from harvest to the next spring (Liang et al., 1991). However, for FPL, N rate did not increase soil NO–3–N content at the end of the growing season. This result would indicate that greater N losses, maybe through nitrate leaching, happened during the growing season for this fertilization time.
In the 1995–1996 and 1996–1997 growing seasons, no significant relationship was found between crop N uptake for unfertilized and fertilized treatments and residual NO–3–N in the preceding growing season (Fig. 2b), and therefore, this N pool would not have contributed to available N, and the UN for the fertilized treatments would not have been largely overestimated. This result can be explained by the low soil NO–3–N content determined at physiological maturity in the 1994–1995 and 1995–1996 growing seasons (Tables 4 and 5) and by N losses happening during fallow period.
Unaccounted Nitrogen
In 1995–1996 and 1998–1999, there was a significant interaction (P < 0.05) between N rate and the fertilization time for the UN. Increasing N rate increased UN mainly for FPL (Tables 4 and 5). Despite the interaction, the UN for FPL was significantly higher than FV6 in the 1994–1995, 1995–1996, and 1998–1999 growing seasons (Tables 4 and 5). In the 1996–1997 growing season, UN was affected only by the N rate (Table 5). As it has been mentioned, water excess in January (Fig. 1) could have caused nitrate leaching, diminishing the difference between fertilization times.
Air mean temperatures of October and December (time of planting and V6 fertilization, respectively) were similar in the 1994–1995 and 1995–1996 growing seasons (Table 1). Consequently, volatilization losses for the same N rate and application time were similar (Table 4) because volatilization rate and air mean temperature are highly associated (Sainz Rozas et al., 1997b). Therefore, volatilization losses would not have been very different in the other years in which these losses were not determined, because air mean temperature of October and December showed little variation through the years (Table 1).
For both fertilization times, almost all denitrification losses were measured between planting and flowering (period elapsed between October and January) because after flowering, denitrification rate is limited by N availability (Sainz Rozas et al., 2001). Accumulated rainfall plus irrigation between October and January ranged from 523 to 403 mm for 1996–1997 and 1998–1999 growing seasons, respectively (Fig. 1). Nevertheless, CL N2O-N, as a percentage of applied N, were something higher in 1998–1999 than 1996–1997 growing season (Table 5). Therefore, these losses would not have been very different in the others years because accumulated rainfall plus irrigation between October and January ranged from 474 to 521 mm for 1994–1995 and 1995–1996 growing season, respectively (Fig. 1).
Unaccounted N in 1994–1995 and 1995–1996 for both application times, and for FV6 and FPL in 1996–1997 and 1998–1999 growing seasons, respectively, cannot be attributed solely to NO–3–N leaching losses. This is because denitrification losses were not determined in the first two growing seasons and volatilization losses were not determined in the last two growing seasons. For FPL, when denitrification or volatilization losses (average of years) were subtracted from UN in the years in which these losses were not determined, the mean UN (through the years) was 55, 69, 86, and 103 kg N ha–1 for 0, 70, 140, and 210 kg N ha–1, respectively. These values represent N losses of 20.0, 22.1, and 22.8% of applied N. However, for FV6, these values were 56, 46, 49, and 34 kg N ha–1 for the same N rates. The lower N losses for the fertilized treatments at V6 with respect to the control treatment are explained by an overestimation of different N pools determined. Greater crop N uptake from soil organic matter and increased root growth in fertilized plots (Rao et al., 1991) could partially explain the overestimation and the negative values observed for FV6. For FPL, this effect would have been counteracted by the existence of greater N losses from soil. However, the overestimation for the treatments fertilized at V6 was not very important.
Unaccounted N for FPL could be attributed to NO–3 leaching because rainfall was higher than CET at the beginning of the growing season while for FV6, early December, rainfall plus irrigation generally were less or slightly higher than CET after the FV6 (Fig. 1). From experiments with continuous maize crops fertilized at planting time with 200 kg N ha–1, Toth and Fox (1998) reported that rainfall events greater than 15 mm immediately after surface fertilization increased the leachate NO–3–N concentration early in the growing season and increased NO–3 leaching. These results are in agreement with those observed in our experiment due to rainfall events that happened from 1 to 8 d after fertilization.
Nitrate leaching losses determined indirectly in this experiment for the two higher N rates applied at planting time are lower than nitrate leaching loss reported by Walters and Malzer (1990). These authors worked with maize under CT in sandy soils and reported NO–3 leaching loss of 30% of N applied (180 kg N ha–1). However, the NO–3–N leaching loss in our study is higher than losses reported by Jeminson et al. (1994). These authors worked on silt-loam texture soils and reported NO–3 leaching losses of 14% of N applied (200 kg N ha–1) for the same application time under CT. These losses happened mainly at the beginning of the growing season. This difference would be attributed to the larger infiltration capacity in soils under NT than CT because of the presence of surface mulch and the larger number of continuous macropores that are open at the soil surface (Unger and McCalla, 1980). At Balcarce, in a maize crop under CT fertilized at planting time with 200 kg N ha–1, Costa et al. (2003) reported leaching losses of 66.5 kg N ha–1. These losses could be higher than those reported in our experiment because these authors evaluated leaching losses for a longer period.
Nitrogen Fate as Affected by Rate and Application Time
When N fertilizer was applied at the V6 stage, the crop accumulated a greater proportion of the applied fertilizer N than when the fertilizer was applied at planting time (Fig. 3). Total gaseous losses (denitrification plus volatilization) represented a fraction ranging from 7.6 to 13.8% of applied N (Fig. 3). Denitrification losses were more important for FPL and accounted for almost 80% of total gaseous losses for the lowest N rate (70 kg N ha–1) while volatilization was a more important mechanism loss for FV6 and accounted for up to 82% of total gaseous losses for the highest N rate (210 kg N ha–1) (Fig. 3).
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Residual NO–3–N increased with increasing N rate for FV6, but this N pool did not increase for FPL (Fig. 3). Therefore, it is very important that producers do not apply N in excess of crop requirements because N remaining in the soil profile after harvest is susceptible to leaching and thus increases the risk of groundwater pollution.
The MBN was similar for both fertilization times (Fig. 3), and therefore, the greater N uptake and grain yield for the V6 fertilization were attributed to the existence of greater N leaching for the planting fertilization because total gaseous losses were similar for both fertilization times.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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