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DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION

Recycling of Residual Soil Nitrogen in a Lowland Rice–Sweet Pepper Cropping System

^a Soil Microbiol., Soil and Water Sci. Div., IRRI, P.O. Box 3127, Makati Central Post Office, 1271 Makati City, Philippines
^b Div. of Soil Sci., Nepal Agric. Res. Council, Khumaltar, Lalitpur, Nepal

j.k.ladha{at}cgiar.org

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Residual mineral N after the dry season (DS) crop in an intensive rainfed lowland is lost upon flooding for rice (Oryza sativa L.) planting. The conservation and recycling of this N are essential for maintaining groundwater quality and system sustainability. Experiments conducted in rice–sweet pepper (Capsicum annuum L.) cropping systems in farmers' fields aimed (i) to quantify the levels of soil mineral N after the incorporation of residues of different dry-to-wet (DTW) transition crops in combination with two formulations of fertilizer N, as well as their effects on rice yields and N use efficiencies, and (ii) to estimate the soil N balance. Significant amounts of NH₄–N accumulated in soil at 15 d after incorporation of residues of indigo (Indigofera tinctoria L.) alone (12 kg ha^-1) and indigo mixed with mungbean (Vigna radiata L.) residue (24 kg ha^-1), and at 60 d after incorporation of maize (Zea mays L.) residue (8 kg ha^-1). Soil NH₄–N in treatments with maize residue was lower than that from indigo and mungbean, but it was improved when maize residue was mixed with fertilizer N. Nitrate N peaked in the upper soil layer before flooding occurred, followed by its leaching and disappearance later. Crop residues incorporated in the plot maintained low NO₃ throughout the soil profile. The crops during DTW transition reduced N losses by 33 to 72%, and residue incorporation supplied N equivalent to 87 kg ha^-1 to rice. The results suggest that a transition crop alone cannot completely reduce the N losses; therefore, strategies for reducing N fertilizer rates to better match N demand of the DS crop are needed.

Abbreviations: DAT, days after transplanting • DS, dry season • DTW, dry-to-wet • PU, prilled urea • TU, tablet urea • WFPS, water-filled pore space • WS, wet season

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
RAINFED LOWLAND RICE encounters a complex and unpredictable environment. Rice is grown in the wet season (WS) with rainwater in rotation with diverse upland crops in the DS with partial or full irrigation. Therefore, rainfed lowlands experience hydrologic conditions that fluctuate from complete flooding to drying within a season and between seasons. The alternate drying and wetting results in a reduction in soil organic matter content, large gaseous and leaching losses of N, and immobilization of nutrients such as P (George et al., 1992; Wade and Ladha, 1994; Alam and Ladha, 1997; Kundu and Ladha, 1999). On the other hand, irrigated lowland rice encounters a more stable environment where control of water minimizes changes in soil organic matter and nutrient availability (Wade and Ladha, 1994; Kundu and Ladha, 1995).

In most of the rainfed lowlands of tropical Asia, where a single crop of rice is grown, the soil can be aerobic for up to 8 mo during the year (George et al., 1992). In intensive rainfed lowlands, WS crops are rotated with diverse DS crops that are grown for 5 to 6 mo. The transition from DS aerobic to WS anaerobic soil conditions occurs for approximately 2 mo, depending on the onset of rain. Even with sufficient water from early rains, the DTW transition is too short for the production of most upland crops. Soil can also be intermittently flooded from heavy rains. Hence, weedy fallowing is the dominant practice during the DTW transition, which ends when the soil is fully flooded for rice cultivation.

The lowlands in Ilocos Norte, Philippines, have a system of WS rice followed by one or two non-rice, high-value crops in the DS, such as sweet pepper, tomato (Lycopersicum esculentum L.), garlic (Allium sativum L.), mungbean, maize, and tobacco (Nicotiana tabacum L.). Farmers usually apply N fertilizer rates that exceed crop demands to the DS crops (Lucas et al., 1999), resulting in large N losses that range from 34 to 549 kg per cropping sequence (Tripathi et al., 1997). Although the exact reasons for the excessive use of N fertilizer by the farmers and its poor utilization by DS crops are not known, high economic returns discourage farmers from being concerned about sustainability. Excessive applied N, not used by the crop, accumulates as NO₃ in the soil profile during the DS. Soil and crop management during both the DS and DTW transition substantially influences soil NO₃ (Tripathi et al., 1997). With the onset of rain and soil flooding, NO₃ either leaches down to groundwater or is lost through denitrification. This results in a net reduction of the mineral N pool at the beginning of the rice crop. A fallow period during the DTW transition provides an opportunity to integrate a crop for the purpose of capturing NO₃–N and recycling the captured N through residue incorporation in the rice crop.

In a study conducted at an irrigated lowland experimental farm with a rice–fallow cropping system, George et al. (1998) examined the recycling of residue N through a DTW transition legume in the succeeding crop. But similar studies in farmers' fields of the consequences to rainfed lowlands of intensified and diversified DS crops with high inputs are lacking. Studies on N cycling in farmers' fields are important for observing the effects of farmers' management practices. In a previous study involving a rice–sweet pepper cropping system in the rainfed lowlands of Ilocos Norte, we reported mineral N accumulation ranging from 101 to 627 kg ha^-1 (top 100-cm soil profile), with 91% as NO₃–N, after a DS crop and subsequent loss of the bulk of accumulated N through leaching from WS flooding (Shrestha and Ladha, 1998). We also explored the effectiveness of catch crops in reducing NO₃ leaching losses. In continuation of this work, we now document the N nutrition and N balance of a subsequent rainfed rice crop in four farmers' fields. The study had the following specific objectives: (i) to quantify the amounts of soil mineral N as affected by the incorporation of residues of different DTW transition crops in combination with two formulations of fertilizer N, as well as their effects on rice yields and N use efficiency, and (ii) to estimate the soil mineral N balance.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Experimental Site
Of the seven farmer's fields (sites) used in the comprehensive study with the sweet pepper (DS)–rice (WS) cropping system in Ilocos Norte, Philippines (Shrestha and Ladha, 1998), four (1, 3, 5, and 7) were selected for the present study. The sites were selected based on varying NO₃ levels in groundwater and differences in soil characteristics. The sites were distributed in a watershed with a total area of 265 ha located in Magnuang, Ilocos Norte, Philippines (18°04' N latitude, 120°32' E longitude, elevation ranging from 54 to 65 m above sea level). Ilocos Norte has a humid tropical climate characterized by distinct hot and dry weather, with a water deficit for 7 to 8 mo from November to June and a monsoon season from July to October. Total annual rainfall during the experimental period (December 1994 to November 1995) was 1444 mm, 86% of which occurred from July to October. The mean annual maximum and minimum temperatures were 32.6°C and 22.8°C, respectively. Physiographically, all the sites were located on a flat plain of fluvial sediment. The soils are Inceptisol and are well to moderately well drained with a 5- to 7-m-deep groundwater table during DS and one of 0.1 to 3.7 m during WS. Soil organic C ranges from 5.4 to 8.5 g kg^-1 and Kjeldahl N from 0.7 to 1.2 g kg^-1 in the upper 25-cm depth, and both decrease with depth. Soil pH ranges from 7 to 8. Sites 1 and 3 have an exceptionally high clay content in the middle (25- to 75-cm) and lower (75- to 100-cm) soil depths, respectively (Table 1) .

Cultivation of Dry-to-Wet Transition and Wet Season Rice Crops
Indigo, indigo plus mungbean (Pagasa-7), and maize (Los Baños Lagkitan) at Site 1 and indigo at the other sites were planted as N-catch crops during the DTW transition. Additionally, a fallow plot was included at all sites. A randomized complete block design with nine replications at Site 1 (plot size 6 x 3 m) and three replications at the other sites (plot size 6 x 5 m) was used. During the DTW transition at Site 1, each treatment had nine replicated plots, which were assigned to three treatments during WS in rice, with three replications each. Rice received either no fertilizer N (control treatment), fertilizer N applied as prilled urea (PU), or tablet urea (TU) alone at the rate of 87 kg N ha^-1, or in combination with residue (one-third fertilizer N + two-thirds residue N), or an equal amount of residue N alone (see Table 2 for treatment details). At other sites, the DTW transition treatments had indigo as a transition crop and fallow, and WS treatments included no fertilizer N, prilled urea, or residue N equivalent to 87 kg N ha^-1.

Plant and Soil Sampling and Analysis
Sweet pepper, DTW transition crops, and rice from 10-, 8-, and 5.3- m² areas at the center of each plot were harvested at ground level for biomass and grain yield determination. Plant dry weight was determined from subsamples dried at 65°C for 48 h. Dried plant samples were ground for N determination by a dry combustion method with a Perkin-Elmer 2400 CHN analyzer (Perkin Elmer Corp., Norwalk, CT) (Jimenez and Ladha, 1993).

Soil samples from 0- to 25-, 25- to 50-, 50- to 75-, and 75- to 100-cm depths from all plots at all sites were collected at five different times: before rice transplanting and at 15, 29, 60, and 90 d after transplanting (DAT). Samples were collected with a 5-cm-diam. auger, and each sample represented a mixed composite from three cores taken in each plot. Concurrently, soil samples were also collected by core sampler, and bulk density was determined after oven drying at 105°C. Samples were transported to the laboratory in an icebox and stored in a freezer to inhibit N transformations. After 12 h, distilled water was added to moisten the soil. After 1 h of moistening, samples were mixed thoroughly by hand and a paste was prepared for extraction. A 40-g subsample was weighed in a plastic bottle and extracted with 200 mL of 2 M KCl for 1 h. The soil suspension was then filtered through Whatman no. 1 filter paper and the filtrate stored in a refrigerator at 0°C for later analysis. Simultaneously, another subsample was weighed in a preweighed aluminum can for determination of soil water content by oven drying at 105°C.

Soil particle density was determined by the pycnometer method and water-filled pore spaces (WFPS) were calculated from the gravimetric water content for individual soil samples as described by Tripathi et al. (1997). Nitrate–N in KCl extracts was determined by Cd reduction (Dorich and Nelson, 1984), and absorbance measurements were done at a wavelength of 540 nm (Jackson et al., 1975). Exchangeable NH₄–N was determined by steam distillation with MgO (Bremner, 1965). Both NO₃– and NH₄–N concentrations (mg kg^-1) were converted to kg ha^-1 by multiplying with the appropriate bulk density and volume for that depth of soil (Rowell, 1994).

Biological Nitrogen Fixation by Legume Transition Crops and Rice
The contributions of biological N₂ fixation by indigo and mungbean were estimated by the ¹⁵N natural abundance technique using rice (cv. IR58) as the non-legume reference plant (Shearer and Kohl, 1986; Peoples and Herridge, 1990). Details are given in Shrestha and Ladha (1998). The contribution of biological N₂ fixation in lowland rice was estimated from Shrestha and Ladha (1996).

Statistical Analysis
Soil NO₃–N and NH₄–N for each depth and plant N uptake, biomass, and yield were analyzed with the GLM and correlation procedures of the SAS system (SAS Inst., 1995). The analysis of variance consisted of a repeated-measures model (using soil sampling for each depth as the repeated factor), and Duncan's multiple range test was used for mean separations. In addition, the least significant difference test was used to compare NO₃–N trends at different soil depths.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Soil Mineral Nitrogen Dynamics
Ammonium–Nitrogen
The majority of the extractable NH₄–N in the upper 100-cm soil during WS rice was in the 0- to 25-cm layer, ranging from 60 (maize residue) to 87% (indigo or mungbean residue N in combination with PU) in the 100-cm layer (data not shown). Accordingly, data on NH₄–N from lower soil depths is not presented. The incorporation of crop residues affected soil NH₄–N levels most at 15 DAT, though some differences were also observed at 60 DAT. At 15 DAT, NH₄–N from indigo or mungbean residue (24 kg ha^-1) (Treatment 7) was comparable with indigo (19 kg ha^-1) and indigo or mungbean residues in combination with PU (19 kg ha^-1) (Treatments 5 and 8, respectively), and it was higher than in other treatments (Table 2). Soil NH₄–N at 15 DAT in indigo or mungbean residue treatment was about three times higher than that of maize residue and two times that of indigo alone (Treatments 10 and 4, respectively; Table 2). Plant material that is high in N and soluble carbohydrates but low in lignin and other polyphenols, such as in the leguminous crop, generally decomposes relatively quickly (Tian et al., 1995). A lower NH₄–N in maize residue treatment was most likely due to its high C:N ratio of 36 compared with that of 14 to 15 for indigo and mungbean. The remaining N from maize residue is likely to be mineralized slowly over the succeeding years (Jensen, 1991, 1992). The NH₄–N from catch crop residue incorporation was significantly different compared with an equivalent amount of fertilizer N (PU or TU) (Table 3) and was greater with residue incorporation (Fig. 1) . Ammonium–N in treatments with residue incorporation alone (14.5 kg ha^-1) or in combination with PU (17 kg ha^-1) was comparable but higher than with residue in combination with TU. The treatments with residue plus TU (6.5 kg ha^-1) and fertilizer alone (7 kg ha^-1) had NH₄–N similar to that of the control (4 kg ha^-1) (Fig. 1). Residue quality had a significant effect on the level of NH₄–N, but the residue effect tended to disappear when the residue was combined with fertilizer N (Table 1).

View larger version (28K): [in this window] [in a new window]   Fig. 1 Effect of fertilizer and residue alone or in combination with prilled (PU) or tablet urea (TU) on levels of NH4–N at 15 days after transplanting, rice N uptake, and rice grain yield at Site 1. Treatments with the same letter are not significantly different by DMRT at . Fertilizer indicates the average of two fertilizer treatments: prilled and tablet urea; Residue indicates the average of three residue treatments: indigo, indigo plus mungbean, and maize residues; §Residue + PU is the average of three residue plus prilled urea treatments: indigo residue plus PU, indigo and mungbean residue plus PU, and maize residue plus PU; ¶Residue + TU is the average of three residue plus TU treatments: indigo residue plus TU, indigo and mungbean residue plus TU, and maize residue plus TU

Nitrate–Nitrogen
Across sites and treatments, soil NO₃ during the rice crop declined at upper soil depths and increased at lower depths (Fig. 2 and 3) with an increase in WFPS after rain or flooding (Fig. 4) . The decline in soil NO₃–N concentration with an increase in WFPS may be because of denitrification and/or leaching (Linn and Doran, 1984). The decline in or loss of NO₃–N in the no-residue-incorporated plot (Treatments 1 to 3) was higher than that in the residue-incorporated plots (Treatments 4 to 12) (Fig. 2). For example, across four sites in the no-residue-incorporated plot, average initial soil NO₃–N (before flooding) was 450 kg ha^-1, but the final amount (after harvesting of rice) was 86 kg ha^-1, whereas, in the residue-incorporated plot, average initial soil NO₃–N was 256 kg ha^-1 and the final amount was 67 kg ha^-1.

View larger version (25K): [in this window] [in a new window]   Fig. 2 Soil NO3–N during the rice crop as affected by different catch crop residues and fertilizer combinations (selected treatments), and time at 7 d before transplanting (DBT), and 15, 29, 60, and 90 d after transplanting (DAT) in Site 1. (Horizontal bars represent LSD at 0.05% for treatment comparison within each depth and sampling date.) T, Treatment number; 0N, zero nitrogen; PU, prilled urea; TU, tablet urea

View larger version (42K): [in this window] [in a new window]   Fig. 3 Nitrate–N in soil layers of 25 cm in treatments with no N (0 N), prilled urea–N and indigo residue–N at five sampling dates at Sites 3, 5, and 7. (Horizontal bars represent LSD at P < 0.05 for treatment comparison within depth, site, and sampling.)

View larger version (27K): [in this window] [in a new window]   Fig. 4 Daily rainfall and water-filled pore space at Magnuang, Ilocos Norte, Philippines, in 1995

At Sites 5 and 7, the low amount of NO₃ in the catch crops similar to Site 1 was maintained throughout the soil profile, thus limiting leaching in the deeper layer (Fig. 3). Contrary to the zero N and PU treatments, which were fallow during the DTW transition period, the higher amount of NO₃ observed in the upper soil profile gradually moved down and disappeared, indicating a large loss of NO₃–N in the fallow DTW transition period. This was in agreement with the observation made by Muller et al. (1987). Yet at Site 3, the high levels of NO₃ were still maintained at the 75- to 100-cm depth, even at harvest (90 DAT). The NO₃–N was 72 kg ha^-1 in the residue-incorporated plot and 98 kg ha^-1 in the no-residue plot (Fig. 3). The high amount of NO₃–N in the soil profile at Site 3 may be because of the high clay content at the 75- to 100-cm depth compared with other sites. These findings show the positive effect of integrating a catch crop in the DTW transition and incorporating residues in the WS prior to rice planting.

Rice Yield, Nitrogen Uptake, and Nitrogen Use Efficiency
Across sites and treatments, rice grain yield ranged from 3.1 to 6.6 Mg ha^-1, with a corresponding N uptake of 48 to 146 kg ha^-1. Both yield and N uptake were highest in the treatment with TU at Site 1 and with indigo residue at Site 3 (Table 4) . Grain yield and N uptake were significantly different with residues vs. without residue, and with PU vs. TU. But both parameters did not differ among residue treatments (Table 3). At Site 1, yield with TU (Treatment 3) was 1.2 Mg ha^-1 higher than the equivalent amount of PU (Treatment 2), and it was similar to indigo residue alone (Treatment 4), indigo mixed with mungbean residue plus TU or PU (Treatments 8 and 9), and maize residue plus TU (Treatment 12). At Site 3, rice yield and N uptake were similar in the fallow–0 N and indigo residue treatments because of high residual soil mineral N. For an unknown reason, at Site 5 yield and N uptake with indigo residue tended to be lower than with an equivalent amount of PU. The increase in grain yield from catch crop residue N ranged from 39 to 90% of fallow during DTW transition, which was equivalent to or higher than the equivalent amount of PU (87 kg N ha^-1) (Table 4).

The agronomic N-use efficiency, which is estimated by the difference in grain yield in the treatment with and without applied N divided by the amount of applied N, tended to be highest in the treatment with TU and residue plus urea. It was lowest in the maize treatments (Table 4). These results indicate a better synchrony of N release and crop demand when a slow-release fertilizer such as TU or residue plus urea is used.

Nitrogen Balance
An apparent mineral N balance was calculated to evaluate the effect of DTW transition and WS management on reducing N loss (Table 5) . At Site 1, the N loss from the 100-cm soil profile ranged from 208 to 516 kg ha^-1. It was highest in fallow and lowest in the plot with maize grown during the DTW transition and residue incorporated in rice. The efficiency of DTW transition crops in decreasing N loss was in the order of maize > indigo + mungbean > indigo. Maize was highly efficient in its ability to capture a larger amount of soil N (177 kg ha^-1) than indigo (151 kg ha^-1) and indigo plus mungbean (118 kg ha^-1). At Sites 3 and 5, losses were similar to those of Site 1. At Site 7, the loss in the indigo plot was negligible. The reduction in N loss due to integration of a catch crop during the transition and incorporation for rice ranged from 33 to 72% of the N lost in the fallow. The inability of the transition crops to reduce N loss at Sites 1, 3, and 5 compared with Site 7 is due to the high buildup of soil mineral N (ranging from 494–908 kg ha^-1, 97% being NO₃–N), which was beyond the capacity of the transition crop to capture. At Site 7, indigo was able to reduce N loss by 72% because of lower soil mineral N (260–313 kg ha^-1). At Sites 1, 3, and 5, residual mineral N was two to three times higher than the uptake by the transition crop, resulting in a loss of residual N of up to 83% (Shrestha and Ladha, 1998). This indicates that the strategy of the N transition crop alone is not effective enough to reduce significant N loss. There is an urgent need to explore other avenues, especially reducing the N fertilizer rates to better match the demand for N to DS crops. Nitrogen loss was lower with TU than with PU. It is important to note that, despite growing a transition crop, soil mineral N after the rice harvest did not decrease and was similar to that of fallow. At all sites except Site 3, higher mineral N ranged from 260 to 604 kg ha^-1 in the early part of the DTW transition and decreased to 43 to 78 kg ha^-1 at harvest (Table 4).

	ACKNOWLEDGMENTS

 
We acknowledge the technical assistance of Mr. M. Alumaga and Mr. B. Pacia and thank Dr. T.F. Marcos, Dr. E.L. Calacal (president, Mariano Marcos State Univ.), Dr. S.R. Obien (director, PhilRice), Dr. C. Piggin (program leader, Rainfed Lowland Rice Ecosystems, IRRI), Dr. T.P. Tuong (water engineer, IRRI), and Dr. C.G. McLaren (head, Biometrics Div., IRRI) for their suggestions throughout the study and constructive comments on the manuscript. We are also indebted to Dr. Bill Hardy, Communication and Publications Services, IRRI, for his editorial assistance. On behalf of the Rainfed Lowland Rice Research Consortium, we acknowledge the Asian Development Bank (ADB) and the Directorate General for International Cooperation (DGIS), Ministry of Foreign Affairs, the Netherlands, for their partial funding of this research.

Received for publication February 19, 1999.

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Shrestha, R.K. Articles by Ladha, J.K. Search for Related Content PubMed Articles by Shrestha, R.K. Articles by Ladha, J.K. Agricola Articles by Shrestha, R.K. Articles by Ladha, J.K.