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a Dep. of Agronomy and Range Science, Univ. of California, Davis, CA 95616 USA
b Land, Air and Water Resources, Univ. of California, Davis, CA 95616 USA
c Int. Rice Research Inst., Lao-IRRI Project, Vientiane, Laos
d Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-1150 USA
cvankessel{at}ucdavis.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Abbreviations: NUE, nitrogen use efficiency • PNUE, physiological nitrogen use efficiency • SOC, soil organic carbon
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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While there is concern about increased greenhouse gas production (e.g., CH4) when straw is returned to the soil (Bronson et al., 1997; Bossio et al., 1999), burning rice straw releases amounts of both CH4 and NO2 comparable to that from decomposing straw (Miura and Kanno, 1997). By increasing soil organic carbon (SOC) levels, incorporation of rice straw may reduce the release of greenhouse gases, including CO2, until SOC reaches a maximum level. Intentional winter (fallow season) flooding is utilized in California to aid decomposition of rice straw in the field and in the process restore historical winter wetland habitat for migrating waterfowl (Elphick and Oring, 1998).
Nitrogen fertility in the rice cropping system is likely to be affected by alternative management practices that change straw retention practices and aerobic/anaerobic characteristics. Nitrogen is the most yield-limiting nutrient in rice cropping systems worldwide (Mikkelsen, 1987; Cassman et al., 1996a) and because of many opportunities for losses, especially in the alternating wet/dry cycles of rice systems, it is also the most difficult nutrient to manage (Mikkelsen, 1987; Buresh et al., 1989).
Incorporation of wheat (Triticum aestivum L.) and rice residue initially had negative yield effects on rice in a number of studies (Rao and Mikkelsen, 1976; Azam et al., 1991; Verma and Bhagat, 1992), with N immobilization one of the main causes (Rao and Mikkelsen, 1976). Yield depression following straw incorporation has been mitigated by adding mineral N (Azam et al., 1991) and the effects of N immobilization have been minimized when straw was allowed to decompose before seeding took place (Rao and Mikkelsen, 1976; Adachi et al., 1997). Plant-available N, yield, and N uptake have all been positively affected by straw incorporation in the long term (Verma and Bhagat, 1992; Cassman et al., 1996a; Kundu and Ladha, 1999).
Since one-third of total rice plant N is in the straw, some N fertilizer requirements may be replaced by returning straw to the field (Ponnamperuma, 1984; Cassman et al., 1998). Wheat–rice rotations in India have successfully reduced fertilizer N application by 29 to 40 kg N ha-1 when using straw as a replacement for fertilizer N (Mahapatra et al., 1991; Singh, 1995). Incorporation of straw has been followed by increases in microbial biomass and N mineralization (Bacon, 1990; Singh, 1995) and greater SOC and total N levels (Cassman et al., 1996a). Therefore, straw incorporation has potential for significant residual effects on soil nutrient supply.
The effects of winter (fallow season) flooding on N dynamics in a temperate rice system have not been studied extensively. Nitrogen requirements of microorganisms that decompose organic matter in flooded soils are lower than for decomposers in aerated soils (Broadbent, 1979). This results in lower net N immobilization in flooded soils than in aerobic, well-drained soils (Williams et al., 1968; Mikkelsen, 1987). Increased levels of soil organic C and N were found in a tropical rice system kept flooded most of the year compared to a rice–maize rotation that contained a long aerobic phase (Witt et al., 1998). However, this did not contribute to increased soil N supply for the first two rice crops, and long-term effects are generally unknown.
Because of the high potential for losses, N use efficiency in rice tends to be low in comparison with other major crops (Keeney and Sahrawat, 1986). Reduction of N losses would increase both soil and fertilizer N use efficiency and reduce environmental costs associated with denitrification and leaching of NO3 (George et al., 1993). Cessation of straw burning would also reduce N losses, since most of the straw N is lost in the burning process (Ponnamperuma, 1984). Additionally, straw incorporation may immobilize mineral N, which would otherwise be volatilized or denitrified (Broadbent and Tusneem, 1971).
Comparisons between different straw management systems in rice and their impacts on N fertility have been largely limited to tropical climates. Incorporation and burning of straw have had mixed impacts on both yield and N uptake in previous California studies (Williams et al., 1957, 1968, 1972). These studies utilized cultivation practices different from those in current use, or used rice cultivars no longer in commercial use. Therefore, further information is needed on the effects of alternative straw management using current rice cultivation practices. Winter flooding has only recently become common practice in California, so the impact of long-term winter flooding on nutrient cycling and rice production is also unknown. A multidisciplinary, long-term study on two sites was initiated to examine the effects of straw management practices and winter flooding. The objective of this portion of the study was to look at the impacts of these practices on rice yield, N uptake, and N use efficiency.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Fields were flooded during the summer growing season, then drained before harvest. Following harvest the straw treatments imposed were: (i) straw burned; (ii) straw chopped, then incorporated using a chisel plow and/or disc; (iii) straw rolled, crushed, and flattened into the soil surface using a heavy roller; and (iv) straw windrowed, then baled and removed. All treatments were accomplished after harvest in the fall and all received spring tillage. With the exception of the rolled treatment that was flooded before rolling, winter flooded treatments were flooded 10 to 15 cm following the fall straw operations.
The winter flooded treatments were drained in late March to allow time for drying before spring tillage. Spring operations included tillage, seedbed preparation, and fertilizer application, all using field-scale equipment and methods utilized by local producers. All fertilizer application occurred before seeding. Fertilizer N and P application rates are summarized in Table 2 . Nitrogen rates depended on preseason available soil N, resulting in the changes over the years. Potassium fertilizer was not applied at either site. Following fertilizer application the fields were flooded within a few days and rice variety M202 was seeded aerially at both sites. All operations utilized commercial equipment and conventional weed and pest control strategies.
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Yield
At maturity, plants in quadrats of 0.5 to 1 m2 area in the zero N and the main plots were cut at 1 to 2 cm above the soil surface, separated into grain and straw components, dried to constant mass at 60°C, and weighed. Grain yield was corrected to 140 g kg-1 water content. For comparison, yield measurements were also obtained with a small plot combine harvester on a strip (7.6 by 30.5 m) in the middle of the main plots.
Nitrogen Uptake and Nitrogen Use Efficiency
Dried straw samples were coarse ground using a Wiley mill, and then both grain and straw were ground into a fine powder with a rolling ball mill and analyzed for total N by combustion on a CNS analyzer. Nitrogen uptake was calculated from the yield measurements and total N in the plant parts.
Soil mineral N content (NO3 and NH4) was measured from cores taken at the 0- to 15-cm depth increment both before seeding and at harvest. Nitrogen available to the crop is equal to the sum of the soil mineral N at the end of the season and the N accumulated in the zero N plants, minus mineral N from the beginning of the season. Since most of the mineral N in the spring was in the form of NO3, and thus lost by denitrification following flooding (Buresh et al., 1989), and soil mineral N measured in the fall was minimal (data not shown), the available soil N over the season was based on the N uptake in the zero N plants.
Physiological N use efficiency (PNUE) (Singh et al., 1998), also called N utilization efficiency (Sowers et al., 1994; Fiez et al., 1995) or N use efficiency for grain production (Borrell et al., 1998), is equal to grain yield per unit total N uptake. PNUE was calculated as follows:
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On the other hand, N use efficiency (NUE) (Sowers et al., 1994; Fiez et al., 1995) is equal to the grain yield per unit available N and includes both a physiological and soil N supply component. NUE was calculated using the following equation:
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Statistical Analysis
Analysis of variance (ANOVA) was performed using the PROC GLM procedure in SAS (SAS Inst., 1989). The flood by block error was used as the error term in the ANOVA for the winter flooding vs. no winter flooding treatments. Contrast statements were used to compare treatment means and sets of treatment means when the ANOVA indicated treatment effects. When there was a significant straw x flood interaction the effect of winter flooding within the straw management treatments was analyzed using a contrast statement. To account for differences between years, a repeated measures model was used with time as the repeated variable. A Duncan's multiple range test was used to contrast means when winter-flooded treatments were pooled together for the NUE data. To assess compatibility of large plot harvest data with quadrat harvest data, the ANOVAs for the data sets were compared.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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). However, there was no effect on zero N fertilizer grain yield when straw was removed. The interaction between straw treatment and winter flooding in zero N fertilizer straw yield at Maxwell was more pronounced than for grain yield in 1996 through 1998 (data not shown). Winter flooding increased average zero N straw yield over no winter flooding by 26 and 22% in the incorporated and rolled treatments, respectively.
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Yield response to N fertilizer application was stronger at Maxwell than at Biggs. Fertilizer N application increased yields at Biggs on average by 30% more than zero N fertilizer yields. At Maxwell, however, application of fertilizer N resulted in an average yield increase of 105%. After 5 yr of straw management at Maxwell, increased zero fertilizer N yield due to straw retention resulted in lower yield responses to added fertilizer N (Fig. 1) .
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, respectively in the 3-yr repeated time analysis). In Years 3 through 5, the N content in the grain averaged 12.0 and 11.4 g kg-1 when straw was retained and removed, respectively. During the same 3 yr, N content in the straw was 7.0 and 6.4 g kg-1 when straw was retained and removed, respectively.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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At Biggs, the significant yield difference due to straw management practices was mainly because of the difference between bale/remove and the other three straw treatments. Both straw removal (burn and bale/remove) treatments resulted in losses from the system. Most of the N and C, 25% of the P, and 20% of the K in the straw are lost during burning of rice straw (Ponnamperuma, 1984). Since most of the K is in the straw, baling/removing resulted in greater losses of K from the system than did burning. The initial extractable K levels at Biggs soil at 72 mg kg-1 were considered to be almost deficient, and these levels decreased significantly after baling/removing of the straw compared with the other treatments (Hill et al., 1999). Additionally, more K deficiency symptoms appeared in the plants after baling/removing than in any other straw treatment and addition of K fertilizer significantly increased yields where straw was removed (Hill et al., 1999). Therefore, the lower yields in the bale/remove treatment can be attributed to K deficiency after removal of the straw.
The lack of yield differences between burning and incorporating of straw was also noted at Biggs in studies from the 1940s and 1960s (Williams et al., 1957, 1972). In another study yield was decreased after incorporation of straw where N concentration was <0.54% (Williams et al., 1968). This study resulted in the assumption that when the concentration of N in the straw was higher than 0.54% net N mineralization would occur. Different varieties and production practices as well as the much higher N content of the straw in our study, 0.70 and 0.79% of N in the straw returned to the soil at Maxwell and Biggs, respectively, may make the comparison between these earlier studies and ours difficult. These earlier studies also did not look into the temporal changes in N availability that occur following the incorporation of high C/N ratio straw. These temporal changes are important as in this study we found significant differences between removal and retention of straw only by the third year.
Yield of Unfertilized Rice
An initial negative effect on grain yield following straw incorporation in zero-N fertilizer treatments is common (Azam et al., 1991; Verma and Bhagat, 1992; Kludze and Delaune, 1995). However, such a decline in grain yield was not observed in our study at Maxwell (Table 4) or at Biggs (data not shown). This may be because of the smaller amount of straw added in this study (8 Mg ha-1 compared with 11 or 22 Mg ha-1 in Kludze and Delaune, 1995) or a higher indigenous soil N supply in the soils of our study due to high clay and organic matter content and lower mean soil temperature. Another potential reason could be differences between the cropping systems; in our study the straw had opportunity to decompose over the fallow winter season, while Verma and Bhagat (1992) and Azam et al. (1991) incorporated wheat straw shortly before planting rice.
The impact of straw management on grain yield where no N fertilizer was applied did not manifest itself until the third year of the study, when straw retention resulted in increased zero-N fertilizer grain yields at Maxwell. All the zero N fertilizer plots had insufficient levels of N, and yield responded to straw retention under these circumstances. Therefore, the main contributor to increased yield is most likely additional soil N following straw retention. By the third year, rice in a long-term rice–wheat rotation study in India and a 3-yr study in Japan also experienced yield increases following straw incorporation (Verma and Bhagat, 1992). A straw-applied treatment surpassed an unfertilized control in both yield and N uptake by the second season in a study from the Philippines (Becker et al., 1994). The magnitude of the beneficial yield effects tends to depend both on timing of straw incorporation due to nutrient release dynamics (Tripathi et al., 1997), and on the amount of straw incorporated (Mahapatra et al., 1991).
As seen in other experiments where N was limiting (Cassman et al., 1996b), yield response to N fertilizer application in 1998 at Maxwell decreased as zero N fertilizer yield increased (Fig. 1). After 5 yr of alternative straw management a separation between straw removal and retention appeared in this relationship, as the increase in soil N supply due to straw retention reduced the yield response to fertilizer N. While straw retention increased zero N fertilizer grain yields, the additional increase in grain yield after fertilizer N application was 6.5 and 5.2 Mg ha-1 where straw was removed and retained, respectively. Most of the decrease in fertilizer N response was likely due to the increase in N supply power of the soil following straw retention.
Yield and Winter Flooding
By the third year at Maxwell, winter flooding further increased grain yield when no fertilizer N was applied and the straw was returned to the soil. Again, since yield effects were only observed when N was limiting, winter flooding is surmised to affect the soil N supply during the growing season. Since N uptake is tied to its availability in the soil, winter flooding may improve synchrony of N uptake and N release from incorporated straw.
Winter flooding increased the period in which the soil remains under anaerobic conditions. Continuous rice rotations in southeast Asia have resulted in increased levels of soil organic matter, although an apparent N deficiency in the system led to a decline in rice yield (Cassman et al., 1998). Unlike the situation in southeast Asia, the additional anaerobic period associated with winter flooding in California (up to 5 mo) led to an increase in N mineralization as measured by N uptake in unfertilized rice (Table 6). One main difference between the systems is the significantly greater aerobic period during spring field preparation and autumn harvest in California rice production.
Significant increases in respiration and changes in metabolic diversity due to C inputs and winter flooding were noted after the first year of the straw management treatments in this experiment at Maxwell (Bossio and Scow, 1995). This continued into the second year of treatments, as winter flooding affected relative abundance of fungal versus bacterial populations, characteristic of the differences between aerobic and anaerobic communities (Bossio and Scow, 1998). Adaptations within the microbial community during winter flooding may affect the behavior of that community during the growing season, resulting in potential for different residue decomposition rates and/or timing of that decomposition.
Nitrogen Uptake and Nitrogen Use Efficiency
Retention of straw resulted in increased N uptake in both N fertilizer and zero N fertilizer plots at Maxwell. Similar increases in plant N uptake after straw incorporation have been noted elsewhere (Becker et al., 1994). Where N fertilizer was supplied, higher N uptake as a result of straw retention did not correspond to higher yield. This suggests that the additional N supplied from straw retention was in excess of N needs at current N fertilization rates. Reduction of fertilizer N rates by the difference in N uptake between treatments where straw was removed or retained (16 kg N ha-1 in winter flooded treatments) would be unlikely to result in a yield decline.
The increased soil N supply following winter flooding when residue was retained, as evidenced by increased N uptake may be related to microbial community adaptation or changes in decomposition timing. Since decomposition rates decline in anaerobic conditions (Broadbent, 1979), winter flooding may result in lower N mineralization rates during the winter. Losses of N during the fallow season would then be limited since the majority of N mineralization would tend to occur during the aerobic periods and in the warmer growing season.
The additional soil N supply following straw retention could be either a direct result of the N added in the straw or from reduced N losses (Mikkelsen, 1987) that reflect changes in microbial dynamics (Bossio and Scow, 1995). Addition of high C residue ties up mineral N within microbial biomass, preventing loss via denitrification or volatilization (Bacon, 1990). Long-term experiments have found greater soil organic matter (Verma and Bhagat, 1992) and microbial biomass C and N (Powlson et al., 1987) following years of residue incorporation, which may result in greater available N pools.
The increased soil N availability that resulted from straw retention translated to lower PNUE, as seen at Maxwell in Table 7. Since PNUE will be maximized at the optimum N supply rate from soil and fertilizer N, the N supply was sufficient or in excess at current fertilizer N application rates.
Nitrogen use efficiency, which has both a physiological and soil N supply component, decreased with the increase in soil N supply (Tables 5 and 8), suggesting that some of the decrease in NUE may have been due to the increased soil N supply. This decrease in NUE after straw retention suggests that N fertilizer rates could be adjusted by the third year of straw incorporation because of the increase in soil N supply. Nitrogen losses due to denitrification and volatilization would then be reduced since these processes tend to occur at greater rates under high mineral N concentrations (Focht, 1979; da Silva and Stutte, 1981).
Rice production at Maxwell was more efficient in N use than at Biggs (Table 8). As detailed in the above discussion on yield, possible explanations for the differences between the sites may be the K deficiency at Biggs, and its lower soil organic matter, clay content, and pH. Lower soil organic matter content may result in reduced N cycling in the system due to decreased microbial activity. The Biggs soil, with lower clay and soil organic matter contents, may also be subjected to greater N losses due to less adsorption of N onto clay particles and organic matter.
When available N was in excess, most of the additional N uptake due to residue retention was partitioned within the straw, resulting in a lower ratio of grain N/straw N (data not shown) and a lower ratio of grain production/total plant N (Table 7). When N was limiting, such as in the zero N fertilized rice plots, the increased N uptake due to straw retention was partitioned within the grain, resulting in higher ratios. Similar effects on harvest index (grain/straw) have been found, with harvest index increasing in N limited conditions after more N was supplied (Adachi et al., 1997). Also, although dependent on cultivar, harvest index generally decreases when additional N is added to rice already at maximum yield (Borrell et al., 1998). Therefore, an optimum level of available N would maximize the harvest index and the utilization of N in grain production.
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Conclusions |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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ACKNOWLEDGMENTS |
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Received for publication January 10, 2000.
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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