African Crop Science Journal, Vol. 11. No. 1, 2003, pp. 27-34
RECOVERY OF WHEAT RESIDUE NITROGEN15 AND RESIDUAL EFFECTS OF N FERTILISATION IN A WHEAT - WHEAT CROPPING SYSTEM UNDER MEDITERRANEAN CONDITIONS
L.L. ICHIR and M. ISMAILI1
Moulay Ismail University, Faculty of Science and techniques,
B.P. 509, 52000, Boutalamine Errachidia, Morocco
Code Number: cs03004
For cropping systems, quantification of the fate of fertiliser N and residual N is essential, to develop management practices that optimise N use efficiency. A field study was conducted in Saharan Morocco to assess the fate of fertiliser N in a wheat (Triticum durum var. Karim) - wheat cropping sequence. Therefore 85 kg ha-1 N as labelled ammonium sulfate (9.764% atomic excess) was applied in a three-split application. Fertiliser N recovery by wheat in the first year was 33.1%. At harvest, 64.8% of fertiliser N was found in the 0 - 80 cm profile as residual fertiliser-derived N; 2.1% of the applied N could not be accounted for year 1996/1997. The N unaccounted for was mainly ascribed to denitrification. The recovery of the residual labelled fertiliser N by the subsequent wheat crop was 6.4% for the treatment without residue incorporation, and 7.4% in the case of residue incorporation. The un recovered N after the second cropping season was 15.6 and 11.8% without and with residue incorporation, respectively. Loss of labelled N by the soil-plant system was not due to leaching but to denitrification. In the treatment (N+*R) with labelled residue incorporation, the %N recovery by plants was 16.2%, indicating the mineralisation of the residue applied.
Key Words: Arid-Saharan, denitrification, immobilisation, leaching, mineralisation, Triticum durum
Pour les sytèmes des cultures la quantification du sort des engrais N et les résidus de N, il est essentiel de développer les pratiques de gestion qui maximisent l'usage efficace de N. Une étude de terrain était conduite dans le sahara maroccain pour évaluer le sort des engrais N dans le blé (Triticum durum var. Karim) et la séquence de système de culture basé sur le blé. Ainsi donc 85 kg ha-1 de N comme sulfate d'ammonium étiqueté (9,764% d'exces atomique) était appliqué dans trois phases. Le recouvrement des engrais N par le blé en première année était de 33,1%. A la recolte 64,8% des engrais N était trouvé dans le profile 0-80 cm comme engrais dérivé N résiduel ; 2,1% de N appliqué ne pourra pas être compté pour l'année 1996/1997. Le N non compté était principalement attribué à la denitrification. Le recouvrement des engrais N résiduel étiqueté par la plante de blé subséquent était 6,4% pour le traitement sans incorporation de résidus, et 7,4% dans le cas d'incorporation de résidus. Le N non-recouvert après la seconde saison de culture était respectivement de 15,6 et 11,8% sans et avec incorporation de résidus. La perte de N étiqueté par le système de plante-sol n'était pas due à l'infiltration mais à la dénitrification. Dans le traitement (N+R) avec incorporation de résidus étiquetés, le pourcentage de recouvrement de N par les plantes était 16,2%, indiquant la minéralisation des résidus appliqués.
Mots Clés: Sahara aride, dénitrification, immobilisation, infiltration, minéralisation, Triticum durum
Many grain producers in the arid and Saharan Mediterranean region of Morocco apply N at high rates to meet crop needs in years of good rainfall. Nitrogen fertiliser is expensive and can amount to 15% of wheat production cost in Morocco (Corbeels, 1997). A fairly simple fertilisation strategy would be more acceptable if the losses are minimum during the intercrop season and if the residual effect on the subsequent crop was noticeable. In the studied region, deep leaching of nitrogen below the root zone during the intercropping seasons is unlikely to occur because of the low rainfall (Corbeels et al., 1998). Furthermore, anaerobic conditions which promote denitrification are rather infrequent, due to the predominantly low moisture status of the profile during the period. Hence, the residual N after harvest should be conserved during the intercropping period for the subsequent crop. As a result, recovery of the fertiliser N is expected to be high, either in the same season, or when drought has been limiting the N uptake by plants, in a subsequent season.
Crop residue is a major factor in the development of management practices for optimising nutrient use efficiencies by field crops, especially in places where mineral fertilisers are scarce or expensive. Residues with nitrogen content higher than 1.5% increase the availability of N in the soil, and consequently, improve the yield of the crops (Falisse and Lambert, 1994; Davet, 1996). On the other hand incorporation of wheat straw reduces the loss of 15N applied in autumn from 60 to 47% through immobilising some of the excess nitrate (Powlson et al., 1985). Immobilised N and the N originally in straw, slowly mineralises and gradually becomes available to subsequent crops.
Several studies have been done on the recovery of residual N in alley cropping systems (Vander Meersch et al., 1993; Xu et al., 1993; Vanlauwe et al., 1996b; Ngoran et al., 1998; Vanlauwe et al., 1998). A common finding has been the low recovery of added residues N in the subsquent wheat crop. Vanlauwe et al. (1996b) estimated that 19% of Leucaena N was recovered by the aboveground biomass of the Leucaena hedgerow in two successive prunings. Ngoran et al. (1998) have found that the total recovery by maize of residue N and fertiliser N averaged 11 and 24%, respectively. The total fertiliser N recovery decreased with increasing N application at both locations (Raun et al., 1999).
Several field experiments have revealed that the total N recovery in the first crop derived from organic residues is very variable but less than 20% (Haggar et al., 1993; Vanlauwe et al., 1996a; Ngoran et al., 1998). Numerous factors are reported to affect the decomposition rates of organic residues. Inherent N content, C/N ratio, lignin and polyphenol concentrations in plant residues, as well as climatic conditions play an important role in release of available N (Tian et al., 1992). It has also been shown that soil properties can affect release of residue N (Azam et al., 1993; Awonaike et al., 1996). However, despite such similar studies described above, the residual effect of wheat fertilisation and wheat residue incorporation on wheat in wheat (Triticum durum) cropping system is unknown.
The objective of this study was to evaluate the residual effect of the common wheat fertilisation practice and the effect of wheat residue incorporation on the wheat production in a wheat - wheat cropping sequence in the Saharan Mediterranean region of Morocco.
MATERIALS AND METHODS
Study site. The study was conducted for two successive growing seasons (1996 - 1997 and 1997 - 1998) in the agricultural experimental station of Tafilalet (SMVAT), Southern Morocco. Rainfall was variable during the two growing seasons and reached only 97.8 mm in 1996-1997 and 123 mm in1997-1998. The soil was clay-loam with the following physical-chemical characteristics: total C, 0.68%; total N, 0.065%; and pH in water, 8.1; from the 0 to 20 cm soil.
Cropping season 1996 - 1997. All plots were ploughed, fertilised with 125 kg P205 ha-1 and 56 K20 ha-1. All plots were seeded with wheat (Triticum durum, var. karim) on December 22, 1996. Seeding depth was 3 cm. The plants were thinned to ensure 20 cm between one plant to another. The growing plants were irrigated following farmers local techniques of irrigation.
Labelled nitrogen was added in three applications in 9m2 subplots: (i): at the rate of 28.2 kg 15N ha-1 as (NH4)2S04 enriched with 9.764% at 15N excess at seeding. The remaining plots received the same quantity of no labelled ammonium sulfate, (ii): at the rate of 28.2 kg 15N ha-1 as (NH4)2S04 enriched with 9.764% at 15N excess +24.8 kg N ha-1 as ammonium sulfate not enriched at tillering. The remaining plots received the same quantity with no enrichment with 15N, (iii): at the rate of 28.2 kg 15N ha-1 as (NH4)2S04 enriched with 9.764% at 15N excess +24.8 kg N ha-1 as ammonium sulfate with no enrichment at flowering. The remaining plots received the same quantity with no enrichment with 15N.
The experimental design was a randomised complete block with various treatments and four replicates, each plot measures 5x5 m.
Samples were taken from the grain, the straw and roots for total N and 15N analysis at crop physiological maturity. Soil samples were also taken at depths: 0 - 20; 20 - 40; 40 - 60 and 60 - 80 cm for total N and 15N analysis.
Growing season 1997 - 1998. Wheat (Triticum durum, var. Karim) was cultivated again, on all the plots, receiving 42 kg N ha-1 at sowing and 42 kg N ha-1 at tillering as (NH4)2S04. Three treatments in four replicates were considered. (i) treatment 1 (*N-R): plants residues of plots fertilised with 15N were removed. (ii) treatment (*N-R): plant residues (4.8 t ha-1) chopped into small pieces of 2 to 4 cm not enriched with 15N were transferred to plots which received 15N in the growing season 1996-1997. (iii) treatment 3 (N+*R): plots with 14N application in 1996-1997 were enriched with crop residues (4.8 t ha-1) labelled with 15N. The straw contains 1.921% 15N excess. Crop and soil sampling were performed as in the season 1996/97. Data were statistically analysed using the statistical package for social scientists (SPSS) computer package. Data were subject to analysis of variance (ANOVA), and post-anova analysis done using Fishers LSD and T-test to compare treatment means.
RESULTS AND DISCUSSION
Grain and straw yield. In 1996 -1997, the mean grain yield was 3,256 kg ha-1. The head number per m2 was 321 and 1000 seed weight was 37 g. Straw production was 4500 kg ha-1, while total N uptake was 92.7, 54.15 and 2.047kg ha-1 for grain, straw and root, respectively (Table 1).
In the 1997 -1998, grain and straw yields were higher than in the first year (Table 1). This corresponded with better climatic conditions in this year. All parameters of yield and N uptake, were not significantly different (P = 0.05) in all treatments. Our results thus, agree with Kalburtzi et al. (1990); Bremner and Kessel (1992); Myers et al. (1994) who showed that wheat yield is not affected by straw application. Almost 30-70% of dry matter and total nitrogen, respectively, were located in the grain for the different treatments.
Plant recovery of fertiliser N. Data on the recovery of fertiliser N and its distribution in the different plant parts are given in Table 2. In 1996 -1997, fertiliser N recovery was 33% for whole plant. This value was about equal to the values found by Corbeels et al. (1999) for winter wheat. It is apparent that the labelled fertiliser was mainly recovered by grain (20.8%). Christianson et al. (1990), in the study on pearl millet in Niger, also found in a year of adequate rainfall, grain uptake of fertiliser N averaged 17% and presented approximately half of the labelled 15N taken up by the whole plant. A minor part of fertiliser N was recovered in the harvested roots (0.4%). Low 15N recoveries in roots were also reported by others (Recous et al., 1988; Corbeels et al., 1999).
In 1997-1998, only 6.4% of the labelled fertiliser N applied to wheat for treatment *N-R in the previous year, was found in the following wheat crop (Table 2). These small residual N effect agreed well with reported recoveries of 1 to 7% of fertiliser N by the residual crop (Seligman et al., 1985; Ladd and Amato, 1986; Zapata and Van Cleemput, 1986; Strong et al., 1996; Corbeels et al., 1998). In our study, the low recovery by the second crop, can be explained by the fact that the rooting system developed essentially in the upper soil layer. This was probably the result of the irrigation practice. This shallow root development may have resulted in a preferential N uptake from the upper soil layers, where the unlabelled fertiliser N applied at sowing time was abundant.
For the treatment with unlabelled residues incorporation: (*N+R), nitrogen recovery in whole plant was 7.4% comparable with the treatment without the incorporation of crop residues (6.4%).
In the treatment (N+*R), the percentage of the recovery was 10.3; 5.5 and 0.3 for the grain; straw and roots, respectively. This showed some mineralisation of the residues. The N recovery from fertilisers was twice that from the residues. Similar results have been presented by Azam et al. (1991) who found that more 15N from ammonium sulfate was translocated to the shoot whereas more 15N from Sesbania aculeata remained in the soil biomass. Ngoran et al. (1998) also found that the total recovery of residue N and fertiliser N by maize averaged 11 and 24%, respectively. Previous studies have shown that release and availability of residue N to crops depends on the chemical composition of organic material and the soil type. After 20 weeks of decomposition, Azam et al. (1991) observed that only 55% Sesbania residue were mineralised.
Residual labelled fertiliser N. Total labelled, residual fertiliser N in soil (up to 80 cm) declined from 64.8% after wheat harvest in year 1 to 42.8% (*N-R) and to 45.6% (*N+R) after harvest of the year 2 crop (Table 3). In year 1, fertiliser N recovery was 37.9% in the top 0 - 20 cm (23.7%), in the 20 - 40 cm and (3.2%) in 40-60 cm. No fertiliser N was found in the 60-80 cm layer, indicating that leaching losses were limited during the growing season.
In 1997-1998, fertiliser N recovery for treatment *N-R was 15.3% in the 20 cm layer, 20.5% in 20 - 40 cm and 7% in 40-60 cm. In the layer 60 - 80 cm there was no labelled nitrogen. For the treatment with residue (*N+R), 17.3% of the labelled N was in the 0-20 cm, 23.2% in the 20-40 cm, 5.1% in the 40-60 cm and 0% in the 60-80 cm. Thus, labelled N losses by the soil-plant system are not due to leaching losses (Table 3).
Overall, the upper soil layer (0 - 20 cm) was depleted of N in second year (Table 3). These results showed that a small quantity of labelled nitrogen migrates to deep layers.
Fertiliser N balance and losses. The recovery of fertiliser N by the 1996/97 wheat crop and the following crop (1997/98) accounted for about (39.5%) of the total N added as fertiliser (Table 4). The recovery of fertiliser N by the soil at wheat harvest in 1997-1998 was 42.8% for *N-R and 45.6% for *N+R.
Total N recovery (wheat + wheat + soil) at harvest in 1997/98 accounted for 82.3% for *N-R and 86.1% for *N-R of the applied fertiliser (Table 4). This means that about 17.7% (*N-R) and 13.9% (*N+R) of the total N added as labelled fertiliser could not be accounted for after two cropping seasons. The loss of residual labelled fertiliser N during the first harvest was 2.1% and the second harvest, 15.6% for (*N-R) and 11.8% for (*N+R). Similarly, Jedidi (1998) showed that losses of 15N varies from 0.3 to 11% of introduced 15N. These losses are less important in the case of organic amendments material incorporation in soil.
Denitrification of the residual labelled fertiliser N was presumably a major N loss mechanism by the system soil-plant. These losses were not due to leaching because 60-80 cm soil depth contained no labelled fertiliser N.
It is evident that irrigation practices, which led to flooded soil for short periods favoured N loss probably through denitrification. This process is known to be important in warm climates, when temporary water logging conditions occur (Feigenbaum et al., 1984; Atta and Van Cleemput, 1988; Strong et al., 1992; Corbeels et al., 1998).
The present investigation shows that carry-over of fertiliser N from one growing season to the next in soils under Arid Saharan Mediterranean climate is limited. Losses of residual fertiliser N during the two growing seasons are small. The recovery by wheat in the first season from applied N was 33%. The recovery by the following crop was barely 6.4% for the treatment without residue and 7.4% for the treatment with residue incorporation. This could be attributed to climatic conditions, to insufficiency of the irrigation water and of the availability of nitrogen, especially in the higher soil layer (0 -20 cm). In the treatment with labelled residue (N+*R), the N recovery was 16%.
We gratefully acknowledge the financial support of the International Atomic Energy Agency (IAEA). We thank Dr. Felipe Zapata and Dr. Gamini Keerthisinghe for their help and advice. We are grateful to the technical staff of the IAEA Seibersdorf Laboratory for 15N analysis. The staff of Tafilalet where the research was conducted is gratefully acknowledged.
©2003, African Crop Science Society
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