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a Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616
b Dep. of Agron. and Range Sci
* Corresponding author (cvankessel{at}ucdavis.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: ANI, added nitrogen interaction • FNUE, fertilizer nitrogen use efficiency • FUE–15N, fertilizer use efficiency by nitrogen-15 dilution • FUE-ND, fertilizer use efficiency by nitrogen difference
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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While burning is the traditional disposal method of straw and stubble in temperate and tropical rice-growing areas (Becker et al., 1994; Williams et al., 1972), this practice is problematic because of its contributions to air pollution (Bossio and Scow, 1995; Ponnamperuma, 1984). California legislation now restricts burning to <25% of the rice acreage, and allowable burning will likely decrease to zero. Alternatives to burning include soil incorporation or baling of the straw. Shallow flooding of fallow rice fields is commonly used in California because of its potential to enhance straw decomposition (Hill et al., 1999) and provide winter wetland habitat for migrating waterfowl (Elphick and Oring, 1998).
Addition of organic matter to the soil in the form of straw can increase soil organic matter content in rice systems (Verma and Bhagat, 1992). In the long-term, straw incorporation has resulted in increased N mineralization potential in non-rice and rice systems (Bacon, 1990). Sustained increase in microbial biomass has been observed following many seasons of straw incorporation compared with burning (Bird et al., 2001; Powlson et al., 1987). Incorporation of straw with a high C/N ratio may initially immobilize inorganic N because of the nutrients required to sustain microbial growth (Verma and Bhagat, 1992). After an initial equilibration period that may last up to 3 yr following annual rice straw incorporation (Verma and Bhagat, 1992), plant available N supply in the soil tends to increase (Bacon, 1990).
Studies on the impact of winter flooding on N dynamics in temperate rice systems have been lacking. The main effects of winter flooding, however, may be due to increased straw decomposition resulting from waterfowl activity (Bird et al., 2000) and to the effects of increased anaerobic conditions. The extended anaerobic time period during winter flooding has increased extractable inorganic N (Bird et al., 2001) after 4 yr compared with a non-winter-flooded fallow system. The increase in inorganic N is possibly due to the lower N immobilization after incorporating straw in anaerobic systems compared with aerobic systems (Acharya, 1935). It is also possible that the extended anaerobic time period during winter flooding may increase N availability to plants.
Although other studies have found that incorporation of straw can increase soil N supply to rice, the impact in temperate rice systems is not as well known. Additionally, the effect of these changes in soil N supply on N use efficiency and seasonal uptake has not been examined. The main objective of this study was to determine the impact of winter flooding and straw incorporation or burning on total N accumulation and seasonal 15N uptake and fertilizer N use efficiency (FNUE; 15N and N difference methods). The fate of 15N-labeled fertilizer in the second year and the use efficiency of 15N-labeled straw by the crop were also examined. Finally, the contribution of 15N from below- and aboveground 15N sources to rice in the second year following the application of 15N-fertilizer was assessed.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Treatments were arranged in a split-plot design, with winter flooding and no winter flooding as main-plot treatments and straw management practices as subplot treatments. Treatments were replicated four times. The straw management treatments were baling and removal, rolling, incorporation, or burning. Two of the straw management practices, burning and soil incorporation of straw, were addressed in this portion of the study. Rice variety M202 was aerially seeded each spring onto fields that were flooded approximately 1 d before seeding. The fields remained flooded throughout the growing season until crop maturity and then were drained to allow drying for harvest. Straw treatments were applied in the fall, and the water level in winter-flooded plots was maintained at 5- to 15-cm depth from November until drainage in March.
In the spring of the fourth year, 1997 at Maxwell and 1998 at Biggs, 15N-fertilizer microplots were established at each site. The 15N microplots were rectangular, 3 by 4 m, at Maxwell and circular, 2-m diam., at Biggs. The 15N-labeled urea [(NH2)2CO] was applied at a rate of 20 kg N ha-1 at 10 atom% 15N just before preplant flooding. Additional unlabeled fertilizer N was applied as aqueous NH3 and monoammonium phosphate (NH4H2PO4) to obtain a total rate of 188 kg N ha-1 with an 15N content of 1.07 atom% 15N at Maxwell. To reduce denitrification, the nitrification inhibitor nitrapyrin {N-serve 24E [2-chloro-6-(tricholoromethyl) pyridine], Dow Elanco, Indianapolis, IN) was applied at a rate of 0.4 L ha-1. At Biggs, ammonium sulfate [(NH4)2SO4] was applied to result in a total of 161 kg N ha-1, with an enrichment of 1.24 atom% 15N. To prevent lateral movement of the labeled 15N, metal barriers surrounding the microplots were inserted into the soil to a depth of approximately 15 cm. During each year of the study, additional microplots within each main plot received no fertilizer N. Phosphorus was applied as triple superphosphate at the same rate as in the N fertilized plots, 32 and 25 kg P ha-1 at Maxwell (1997) and Biggs (1998), respectively. These plots with zero N fertilizer were placed in a different location within the larger plot each year.
The significance of below- vs. aboveground residue as a source of 15N for the following crop was determined in the second year at Maxwell. Following harvest of the 15N-labeled crop at Maxwell in 1997, the straw from the 15N-microplots in the burned treatments was transferred onto new 15N-residue microplots in the straw-incorporated treatments. The 15N- labeled straw from the burned treatment was replaced with surrounding unlabeled straw, which was subsequently burned. From the new 15N-residue microplots established in the fall of 1997, the contribution of 15N-labeled surface straw, designated aboveground, to the subsequent crop could be measured compared with the soil plus root N and unburned stubble N, designated belowground. The 15N-residue microplot in the incorporated treatment was used to determine the entire contribution of below- plus aboveground residue pools to the subsequent rice crop. None of the straw treatments received 15N fertilizer in 1998. Characteristics of the 15N-labeled straw applied to the new 15N microplots are summarized in Table 1. The rate of 15N straw applied was equal to the amount of straw produced under straw-incorporated conditions. In the fifth year of the experiment, 15N uptake in the 15N microplots established in the spring of the fourth year represented the uptake of the previous year's fertilizer N application from both soil N pools and above- and belowground residue. Fertilizer 15N uptake in the fifth year through belowground N sources was calculated from the difference between 15N uptake in the fertilizer 15N microplots and 15N uptake where only 15N-labeled straw was applied.
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At final harvest, shoots were collected by cutting plants just above ground level. Total biomass and grain yield (1 m2) were determined from both inside and outside of the 15N microplot. There were no significant differences between yield estimates from the 15N microplots and the main plot at Maxwell. However, yield was significantly affected by microplots and deemed unreliable at Biggs, likely due to the small microplot size. Yields from the 15N microplot were used for all 15N-microplot-related calculations at Maxwell in 1997. In 1998, the main-plot yields were used because no 15N yield samples were taken due to the lack of significant difference between main plot and 15N yield in 1997. At Biggs, yields from the main plot were used in calculations of total N uptake and fertilizer use efficiency by the 15N dilution method (FUE–15N). Yields from the main plot and zero-N-fertilizer microplot were used for calculating fertilizer use efficiency by N difference (FUE-ND).
Plants from the final harvest were separated into grain and straw components, dried at 60°C, and weighed. The 15N-labeled samples were first ground in a Wiley mill, ball-milled for analysis, and analyzed for N and atom percent of 15N as described above.
Fertilizer Nitrogen Use Efficiency
Fertilizer use efficiency by the 15N dilution method (%) was calculated as follows:
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Straw N use efficiency (%), the proportion of straw N that ended up in the crop the following year, was calculated as follows:
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Fertilizer N use efficiency by the N difference method (%) was calculated as follows:
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Interactions between added fertilizer N and native soil N that change the N content in a given pool are called added N interactions (ANI) (Jenkinson et al., 1985). These interactions may result in different estimates for FUE–15N and FUE-ND.
Statistical Analysis
Data were analyzed using the PROC GLM procedure of SAS (SAS Inst., 1989). Analysis of variance (ANOVA) was used to determine treatment effects, and the flood x block mean squared error was used as the error term for winter flooding. Repeated-measures ANOVA was used to determine treatment effects over time during the growing season.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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In the fifth year of the straw management experiment, the year following the application of the 15N fertilizer at Maxwell, an average of 2.3% of the 15N fertilizer applied the previous year was accumulated by rice at plant maturity (Fig. 3) . One month later, at final harvest, the average FUE–15N of the previous year's applied fertilizer was 3.0% (Table 5). The majority of the labeled 15N uptake was from belowground sources rather than aboveground straw (Fig. 3A vs. Fig. 3B). Slightly greater uptake of 15N fertilizer in grain was seen in incorporated compared with burned plots (P = 0.078) in the second year.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Total plant N and fertilizer N uptake reached a maximum at approximately 60 to 80 d after seeding at Maxwell in both years. This is the time of maximum tillering and panicle initiation. Patrick and Reddy (1976) also found a large portion of fertilizer N uptake occurred early in the season. Other studies found N uptake to continue until much later in the growing season (Bufogle et al., 1997; Guindo et al., 1994a). This discrepancy in timing of maximum N uptake may be due to differences in soil N availability over the growing season, use of different rice varieties, climatic differences, or length of growing season. In the current study, both total N and fertilizer N recovery dropped slightly toward the end of the season. Guindo et al. (1994a) also found a similar drop in total fertilizer N recovery and total plant N content using a preflood fertilizer N application. Splitting fertilizer N application has resulted in increasing total N and static fertilizer N content (Bufogle et al., 1997) or total plant N content (Guindo et al., 1994b) toward the end of the season.
Incorporation of straw increased total plant N in N fertilized treatments by 13 and 23 kg N ha-1 at Maxwell in the fourth and fifth years of the study (Table 2). Although this was not significant in 1997, repeated measures ANOVA revealed a significant difference over five years (Eagle et al., 2000). The increase in N uptake due to straw incorporation in unfertilized microplots was even more substantial (Eagle et al., 2000). However, on average only 3.5% of the straw N directly entered the following year's crop (Fig. 4). Therefore, the impact of straw incorporation on N availability is much larger than would be suggested from the recovery of one year's worth of straw-N in the subsequent crop. Additional benefits following the incorporation of organic material, such as mineralization of other nutrients and improved soil quality, may lead to an increase in total N accumulation in the crop by supplying other limiting nutrients and increasing microbial activity.
Nitrogen-15 Fertilizer Use Efficiency
The FUE–15N values measured in this study are comparable to the 30 to 50% values reported in research on tropical lowland rice (Bronson et al., 2000; De Datta et al., 1968). These values tend to be lower than those reported for upland crops, which are also dependent on crop and soil types, production methods, and timing of fertilizer application (Macdonald et al., 1997). Other studies have also reported that application of fertilizer N later in the growing season increases FUE–15N (Patrick and Reddy, 1976). Bronson et al. (2000), however, did not notice any difference in FUE–15N between split fertilizer applications with different times of application. The FUE–15N values have been noted in the range of 72 to 79% when 15N fertilizer was applied 27 and/or 55 d after emergence (Norman et al., 1992).
The FUE–15N was greater when straw was burned rather than incorporated over the growing season at Maxwell (Fig. 2) and at final harvest at Biggs (Table 3). Incorporating straw compared with burning it increased the soil N availability through an increase in net N mineralization and corresponding dilution of fertilizer 15N. At final harvest, the increase in fertilizer N uptake in the total plant at Biggs and in the grain at Maxwell when straw was burned was associated with an increase in soil N uptake in the straw at both locations when straw was incorporated (Tables 2 and 4). Therefore, the rate of fertilizer N application may be reduced when straw is incorporated.
After four and five seasons of straw incorporation in situ, greater N immobilization shortly after residue incorporation and greater N mineralization during the growing season was observed compared with where straw was burned (Bird et al., 2001). Further, four seasons of residue incorporation increased the C and N contents of the active light and mobile humic fractions (Bird et al., 2002). Clearly, the incorporation of straw for a prolonged period of time changed the overall N dynamics and cycling in the soil and caused a net increase in the N supply power of the soil, reflected in higher yields and total N uptake of the unfertilized rice crop. Adjustment of fertilizer N application to better reflect soil N supply should be considered to increase FNUE in rice systems (Cassman et al., 1993).
Added Nitrogen Interactions
The recovery of fertilizer N varied widely whether based on the 15N isotope dilution or the N difference method (Table 3). Such a large difference in the recovery of fertilizer N indicates the strong presence of an ANI. The ANI observed at Maxwell and Biggs could be apparent or real. Apparent ANI is caused by mineralization–immobilization turnover, in which newly mineralized unlabeled N replaces fertilizer 15N ions in solution (Jenkinson et al., 1985). This process is microbially driven, with concomitant N immobilization of added fertilizer N and mineralization of native soil N. At the Maxwell site, a sustained, greater microbial biomass C and N pool was observed after Year 4 along with greater N fertilizer recovered as labile humic N (mobile humic acid) in incorporated plots compared with burned plots (Bird et al., 2001; Bird et al., 2002). These results suggest that the microbial stabilization of fertilizer N leads to the enhanced apparent ANI with incorporating rather than burning. Apparent ANI is also the most likely contributor when the ANI increases, with a longer contact period between fertilizer N and the soil N pools (Schnier, 1994). Because the uptake curves for fertilizer N and total N were similar in shape (Fig. 1 and 2), fertilizer N and soil inorganic N are likely in the same or similar pools, making pool substitution of labeled and unlabeled N probable (Hart et al., 1986). It has been suggested that apparent ANI likely constitutes the majority of observed ANI (Jenkinson et al., 1985).
Studies using 15N-labeled fertilizer in rice systems have often found positive ANIs (Cassman et al., 1993). Added N interaction in tropical rice production systems ranged from -7.0 to 22.6 kg N ha-1 at various times throughout the growing season (Schnier, 1994). The ANI increased where fertilizer N was in contact with the soil for a greater period of time. The degree of pool substitution, and thus the nature of the observed ANI, depends on method of fertilizer application (Schnier, 1994). At Biggs, ANI was estimated at 14 kg N ha-1 while at Maxwell, values were much higher, i.e., 51 and 47 kg N ha-1 when straw was incorporated and burned, respectively. At Maxwell, the higher ANI, if it was associated with higher N turnover rates, might have been caused by higher organic matter content. Also, the greater yield response to fertilizer N at Maxwell could have resulted in a higher real ANI due to more root penetration or root exudates and turnover.
The FUE–15N at Biggs was lower than at Maxwell and can be partially explained by the poor growing season in 1998 (El Nino) compared with the better growing season in 1997, which had a warmer and dryer spring. However, differences in soil characteristics and management practices play a large role because comparisons of FUE-ND from 1995 through 1998 are consistently lower at Biggs than at Maxwell (average of 43% at Biggs vs. 66% at Maxwell). The low soil extractable K at Biggs (Hill et al., 1999) may also contribute to lower N use efficiencies.
Residual Nitrogen-15 Fertilizer
Tracing the fate of the 15N fertilizer through the second growing season indicated that belowground pools (root and microbial derived) are more significant sources of plant available N than incorporated straw N. In the spring before the second year (May 1998), 21% of the original 15N-labeled fertilizer was measured in the top 15 cm of the soil profile (Bird et al., 2001). At final harvest of the second year, the crop had accumulated 15% of that 15N, or approximately 3% of total amount of 15N fertilizer applied in the previous year (Table 5), compared with 39 and 36% of the fertilizer 15N taken up the year before in burned and incorporated treatments, respectively. Therefore, as expected, by the second year, the N added as fertilizer was in less-available forms.
In contrast, the incorporated straw alone contributed 3.5% of its total N, and 3.5% of the fertilizer 15N within the residue, to the subsequent crop. Only 13% of the 15N-labeled fertilizer in the second-year crop came from the rice residue. Therefore, the availability of the 15N residue and 15N soil pools appeared to be different, and the belowground 15N pools were more important N sources to the crop. Unfortunately, most other field studies only followed the fate of 15N-labeled straw (Becker et al., 1994) or combined roots and shoots (Norman et al., 1990). A separation of above- vs. belowground contributions as a source of N for the subsequent crop is seldom made. In addition, belowground sources of 15N include root, crown, and microbially immobilized fertilizer N, making it difficult to assess the importance of these pools in the years following fertilizer addition.
From our study, it appears that in rice cropping systems, the aboveground contribution may not be as important a source of N as belowground sources of N such as remaining fertilizer 15N, belowground 15N-labeled residues (roots or exudates), or 15N immobilized by microbial biomass during the year of fertilizer application. Nitrogen fractions, including mobile humic substances, light fraction, and microbial biomass, are the most active soil N pools and likely contribute the majority of the 15N label to the second-year crop (Bird et al., 2001; Bird et al., 2002).
While the straw N use efficiency was low, the cumulative effect of 4 and 5 yr of straw incorporation on N nutrition was greater than direct N flow from straw to crop. Although only 1.8 kg N ha-1 straw N was directly available to the crop in the year following incorporation, total N uptake increased by 23 kg N ha-1 in 1998. Therefore, these results, combined with the evidence that yield and N uptake only began to be affected by straw management by the third year of the long-term study (Eagle et al., 2000), suggest that active N pools in incorporated plots were increasing over time, thereby enhancing available N more as incorporation practices continued.
Neither straw management nor winter flooding affected the total amount of 15N fertilizer remaining in the system at Maxwell 2 yr after application (Bird et al., 2001) even though more fertilizer N was removed from the system where straw was burned, both in the grain and the burned straw. The low straw use efficiency may have been due to losses of the residue N following spring tillage. This could contribute to the lack of difference between treatments, both in 15N plant uptake and total 15N recovery.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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A large difference in the FUE–15N and the FUE-ND was observed. The difference was likely caused by a strong ANI, whereby unlabeled N in the microbial biomass was substituted for 15N-labeled fertilizer. Hence, the recovery of fertilizer by 15N isotope underestimates the role of fertilizer N as a source of N for the rice crop. The belowground pools of residual 15N after application of 15N-labeled fertilizer were more important than incorporated straw in availability of N to the next rice crop, and the small uptake of straw N in the first year compared with a much higher effect of straw N in later years indicates significant N cycling between various soil N pools in subsequent years.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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