Zinc deficiency in winter oilseed rape (Brassica napus L.) is counteracted by N fertilization

Abstract

A two-year experiment was conducted on oilseed rape in 2004–2005 and 2005–2006 in north Iran. Treatments were 0, 50, 100, and 150 kg N ha⁻¹ as urea (F₀ to F₁₅₀, respectively), 100 kg N ha⁻¹ as urea +50 kg N ha⁻¹ as manure (F₁₀₀M₅₀), 50 kg N ha⁻¹ as urea +100 kg N ha⁻¹ as manure (F₅₀M₁₀₀), and 150 kg N ha⁻¹ as manure (M₁₅₀). Results indicated that seed yield in M₁₅₀ was significantly lower than in F₁₅₀ in both years. Seed protein content in the inorganic fertilization system (F₁₅₀) was significantly higher than in the organic (M₁₅₀) and integrated systems (F₁₀₀M₅₀ and F₅₀M₁₀₀). Seed oil content, however, was higher in the organic treatment than in the inorganic treatment. Less N uptake in F₁₀₀M₅₀ compared with F₁₅₀ in the first year and in F₅₀M₁₀₀ compared with F₁₅₀ in the second year did not result in similar differences in grain yield. Owing to the low content of soil-available Zn, the association between Zn and N concentration in shoot in the inorganic treatments was low. In spite of a significant increase in soil-available Zn under the organic and integrated treatments, no significant increase was observed in Zn concentration of shoot in these treatments compared with F₁₅₀. It appears that the excessive availability of P in M₁₅₀, F₅₀M₁₀₀, and F₁₀₀M₅₀ has reduced either Zn uptake or its translocation from root to shoot. Overall, it could be concluded that in order to increase Zn uptake under manure application in a Zn-deficient soil, N availability should increase.

Keywords:

• Manure
• nitrogen
• oil
• protein
• Zn

Introduction

There is a concerted effort worldwide to use green manure, legumes, and animal manure to produce the same amount of food with better quality and usage of less fossil-fuel-based inorganic fertilizers. Manures are of great interest to most farmers since they are simply available as a source of multiple nutrients and can improve soil characteristics to a great extent.

Information on the effects of organic fertilizer on seed yield and seed quality of winter oilseed rape is sparse (Rathke et al., 2006). Based on data given by Rathke et al. (2005), slurry application reduced the yield of winter oilseed rape between 7.8 and 16.6% compared with the use of inorganic fertilizers. The results on comparing the effects of manure and inorganic fertilizers on oil and protein content of oilseed rape are inconsistent. Rathke et al. (2005) found that application of mineral N fertilizer compared with slurry application resulted in lower oil content but higher crude protein content in oilseed rape. Hao et al. (2004) found that oil content of oilseed rape was significantly decreased by long-term cattle manure application. Many researchers reported the negative and positive effects of inorganic N fertilization on oil and protein content of the crop, respectively (Rathke et al., 2005). Hocking et al. (1997) showed that high N rates did not necessarily lead to lower oil content of oilseed rape.

Among micronutrients, Zn deficiency is the most important constraint to crop production in Mediterranean-type soils as in Iran (Rashid & Ryan, 2004). Zn has a unique place in the biology of the Earth. Across all phyla, from bacteria to humans, more proteins bind or require Zn for their function than do those binding all other biologically essential cations combined (Gladeshev et al., 2004). Grewal et al. (1997) found that application of 3.5 kg Zn ha⁻¹ increased seed yield of a Zn-inefficient genotype of canola by 65%. Grewal and Graham (1997) reported that oilseed rape plants grown from the high-Zn seed had better seedling vigour, increased root and shoot growth, greater leaf area, more chlorophyll concentration, and higher Zn uptake in shoot compared with those grown from the low-Zn seed in Zn-deficient soils. The influence of high-Zn seed, however, was dissipated in Zn-rich soils.

Wong et al. (1999) and Adediran et al. (2004) showed that manure-compost application increased availability of Zn in soil. According to Wong et al. (1999), Zn content of cabbage (Brassica chinensis) and maize (Zea mays) was increased significantly by addition of composted manure compared with the control. Adediran et al. (2004) found that Zn content in the ear leaf of maize was similar under inorganic and organic fertilization systems but was higher under both systems compared with the control. Rupa et al. (2003) found that application of 10 t of manure ha⁻¹ increased Zn availability and uptake by wheat. In contrast, Zn deficiency has been observed where organic matter of different forms has been used (Greenland & Dedatta, 1985).

Application of manure to meet crop N requirement can greatly increase soil P content as compared with P-based fertilizer application. This is due to the lower N:P ratio of manure compared with the N:P uptake ratio of most crops (Eghball & Power, 1999b). Numerous studies with conflicting results have been conducted on P and Zn interaction. It has been reported that high P content in soil reduces Zn concentration in shoot of plants (Marschner, 1995). Conversely, Loneragan et al. (1982) showed that Zn uptake is not affected by increasing P content in the soil solution. In the case of the relation between N and Zn uptake under manure application, Gary et al. (2002) reported that to improve Zn concentration and uptake in poultry-litter-fertilized soil, it may be beneficial to apply limited amounts of N fertilizer to annual ryegrass when N is not in excess, so that there is a positive correlation between Zn uptake and N availability in soil.

Very little information exists in the literature about the effect of manure and manure + inorganic fertilizer as sources of N on Zn–P–N interactions in a Mediterranean soil and on qualitative and quantitative yield of oilseed rape. Therefore, the objective of this experiment was to assess the effects of the inorganic fertilizer, beef cattle feedlot manure, and manure + inorganic fertilizer on soil availability of Zn, P, and N and their uptake by the crop in a Mediterranean soil and therefore on the quality and quantity of oilseed rape yield.

Materials and methods

Experimental site

A two-year experiment was conducted in 2004–2005 and 2005–2006 at the Research Station of Shahid Beheshti University in Savadkooh (36° 40′ N, 53° 10′ E, 1200 m above sea level), a mountainous area in the southern zone of Mazandaran province in north Iran. Total annual rainfall of the region was 738 and 712 mm in 2004–2005 and 2005–2006, respectively, of which 540 and 578 mm occurred during the crop-growing period (October–June) of 2004–2005 and 2005–2006, respectively. Mean seasonal air temperature during fall (autumn), winter, spring, and summer of 2004–2005 was 14, 8.4, 17.9, and 22.3 °C, respectively. These values were 16.2, 8.1, 19.8, and 25.2 °C, respectively, for 2005–2006. Soil texture of the experimental plots was clay loam. Diethylenetriaminepentaacetic Acid-extractable Zn (DTPA-extractable Zn) of the soil was 0.68 mg kg⁻¹ soil. A soil with less than 0.8 mg DTPA-extractable Zn kg⁻¹ is considered to be Zn deficient (Dobermann & Fairhurt, 2000). Soil available P (18 mg kg⁻¹) and exchangeable K (479 mg kg⁻¹) appeared to be adequate for production of 3 t ha⁻¹ of oilseed rape grain yield. Information on some other soil characteristics including organic carbon (OC), total N, P, electrical conductivity (EC), and pH is presented in Table I. The experimental site had been under rice cultivation for several years until 1999. Since then it had been left fallow.

Table I. Chemical analysis of soil (0–15 cm layer) and manure applied in 2004–2005 and 2005–2006.

Source	OC (g kg^−1)	Inorganic N (mg kg^−1)	Total N (g kg^−1)	P (ppm)	EC (dS m^−1)	pH	Moisture (%)
Soil	23.3	20	2.43	18^a	1.07	7.8
Manure (2004–2005)	220	1028	13.7	3500^b	7	6.4	40
Manure (2005–2006)	200	1293	15.6	4200^b	6	7.0	45
^a,b Available and total phosphorus, respectively.

Experiment description

The experiment was conducted as a randomized complete block design with four replications. Treatments included 0 (F₀), 50 (F₅₀), 100 (F₁₀₀), 150 (F₁₅₀) kg N ha⁻¹ as urea, 100 kg N ha⁻¹ as urea +50 kg N ha⁻¹ as manure (F₁₀₀M₅₀), 50 kg N ha⁻¹ as urea +100 kg N ha⁻¹ as manure (F₅₀M₁₀₀), and 150 kg N ha⁻¹ as manure (M₁₅₀). Treatments were located on the same plots during both years. Half of the urea was applied at planting and the remainder was manually side-dressed at the beginning of stem elongation. Beef cattle feedlot manure (collected during October 2004 and 2005) was applied in both years. It was incorporated into the 15 cm of topsoil by disking 2 weeks prior to planting. Manure application was based on the assumption that 35 and 20% of total N in manure would become available during the first and second year after its application, respectively. The value for the available N in manure was adapted from Eghball and Power (1999a). Based on this assumption, oilseed rape received 43 (31.3 t ha⁻¹ in the first year and 11.7 t ha⁻¹ in the second year), 28.6 (20.9 t ha⁻¹ in the first year and 7.7 t ha⁻¹ in the second year), and 14.6 t ha⁻¹ (10.4 t ha⁻¹ in the first year and 4.2 t ha⁻¹ in the second year) of noncomposted manure in treatments M₁₅₀, F₅₀M₁₀₀, and F₁₀₀M₅₀, respectively. Manure application was on a dry-weight basis. Information on OC, total and inorganic N, P, EC, pH, and moisture of manure is presented in Table I. Plots were 2.1 m wide (7 rows with 0.30 m row spacing) by 5 m long. Oilseed rape cv. Hyola was overseeded on 6 and 16 November 2004 and 2005, respectively, and thinned to 66 plants m⁻² at the three-leaf stage. Oilseed rape was cultivated under rainfed condition, so no irrigation was applied. Plots were kept weed-free during the growing season by hand weeding. No fungicide or insecticide was applied since there was no serious problem due to diseases or insects.

Sampling and analysis

Initial soil sampling was conducted at the time of plot establishment from each plot in October 2004. Subsequent soil sampling was conducted in June 2006, after completion of the second year of experiment. At each sampling, five soil cores (2.5 cm diameter, 15 cm depth) were taken from each plot. The soil was mixed thoroughly in a bucket, sieved through a 2-mm-mesh screen, and air-dried prior to analysis. Soil samples were sent to Tarbiat Modares University's Faculty of Agriculture Analytical Laboratory for the following analysis: available P and Zn. Available P was determined with the Olsen extraction method followed by spectrophotometry (Olsen & Sommers, 1994). Available Zn was determined using the DTPA method described in Page et al. (1994).

Oilseed rape was harvested in May 2005 and 2006 from a 4-m length of the middle five rows. Grain yield was adjusted to 9% moisture content. Samples were then taken from grain and shoot. Dried shoot was ground, and then digested using concentrated HNO₃ acid digestion (Page et al., 1994) for the determination of Zn using Atomic Absorption Spectrometry (AAS). Total N and P were extracted by digesting shoot tissue with 3 ml of concentrated H₂SO₄ and 1 ml of H₂O₂ at 360 °C and were determined by Berthelot reaction and molybdenum blue method, respectively. The crude protein content of seed was then calculated by multiplying N content by 6.25. Seed oil content was determined by the Soxhlet apparatus. The comparison of seed quality characteristics was done only in the second year of the experiment.

Statistical analysis

All data were analysed statistically using the General Linear Model (GLM) procedure in SAS (SAS Institute, 2000). The Duncan multiple range test (DMRT) set at 0.05 was used to determine the significance of the difference between treatment means.

Results and discussion

Grain yield and N uptake

There was a significant difference between treatments for grain yield (Table II). Increasing N rate improved the grain yield. The highest grain yield was achieved in plots fertilized with 150 kg N ha⁻¹ as urea (F₁₅₀). Grain yield in the organic treatment (M₁₅₀) was 35 and 28% less than in the inorganic treatment (F₁₅₀) in the first and second year, respectively (Table II). In the first year, grain yield in F₅₀M₁₀₀ (2493 kg ha⁻¹) was significantly lower than in F₁₅₀ (2914 kg ha⁻¹). In the second year, the difference between the integrated treatments F₅₀M₁₀₀ and F₁₀₀M₅₀ (2644 and 2359 kg ha⁻¹, respectively), and F₁₅₀ (2726 kg ha⁻¹) was not significant_. The better performance of the organic and integrated treatments in the second year compared with the first year could be related to higher temperature in fall and spring and more precipitation during growing season which causes an increase in N mineralization in manure and thus increasing N availability. Linear regression was fitted to the relation between N uptake and N applied in the inorganic treatments in order to achieve an equation to obtain actual available N in the organic and integrated treatments. According to this method, the regression equations for the first and second year were Equations (1) and (2), respectively:

1

2

Table II. Seed yield and shoot N uptake of oilseed rape in different fertilization treatments in 2004–2005 and 2005–2006.

	Seed yield (kg ha^−1)		Total nitrogen uptake (kg ha^−1)
Treatment*	2004–2005	2005–2006	2004–2005	2005–2006
F0	1105e**	987d	49d	42e
F50	1511d	1457c	61d	62d
F100	2129bc	2153b	104c	101c
F150	2914a	2726a	135a	131a
F100M50	2612ab	2644a	120b	126ab
F50M100	2493b	2359ab	110b	121b
M150	1896c	1974b	93c	100c
* F0, F50, F100, and F150 are 0, 50, 100, and 150 kg N ha^−1 yr^−1 as urea, respectively. F100M50 is 100 kg N ha^−1 urea +50 kg N ha^−1 as manure, F50M100 is 50 kg N ha^−1 as urea +100 kg N ha^−1 as manure, and M150 is 150 kg N ha^−1 as manure. ** Means data within each column followed by the same letter are not significantly different at the 0.05 probability level according to the Duncan multiple range test (DMRT).

Regarding these equations, actual applied N in the treatments M₁₅₀, F₅₀M₁₀₀, and F₁₀₀M₅₀ was 84.5, 112.5, and 129 kg ha⁻¹ for the first year, respectively. These values were 101, 135.5, and 143 kg ha⁻¹ for the same treatments in the second year, respectively. Based on the total N content in manure, the amount of manure application, and the above equations, N availability in manure was 19.7% in the first year. Because the C:N ratio of manure applied in the present study was similar to that of Eghball and Power (1999a, b), we considered available N of manure 35 and 20% for the first and second year to supply 150 kg N ha⁻¹ in treatments F₅₀M₁₀₀, F₁₀₀M₅₀, and M₁₅₀. However, these values showed that manure N availability in the present study was lower than in Eghball and Power's experiments (1999a,b). The C:N ratio of manure in Eghball and Power's experiments (1999a,b) was similar to that in the present study. Therefore, it appears that the differences in climatic conditions and soil characteristics were the main reasons for the differences observed in N mineralization (N availability) in manure. Beauchamp (1986) reported that less than 10% of the total N in the solid farmyard manure became available during the year of application. Change and Janzen (1996) found that after 20 years of cattle-manure application, only 56% of the N content of manure was mineralized. Consequently, more information is needed about the N mineralization of manure. Moreover, this type of plant response to cattle manure could be attributed to the slow release of the organic N from manure and the low inorganic N content (Mooleki et al., 2004).

Information on the effects of organic fertilizers on yield and quality of winter oilseed rape is sparse (Rathke et al., 2006). Rathke et al. (2005) concluded that the application of slurry alone had a small but significant influence on yield and quality of oilseed rape compared with mineral fertilization. They considered N availability of organic fertilizer to be much higher (60%) than the actual amount (11 to 69%). Stevenson et al. (1998) also found that application of cattle manure under certain environmental conditions may not entirely meet the N demand of winter oilseed rape and, therefore, that supplemental chemical N fertilizer may be needed. Mooleki et al. (2004) stated that high grain yield of oilseed rape could be achieved by application of more than 400 kg N ha⁻¹ yr⁻¹ in a land with no background of manure application. In contrast to the above results, Eghball and Power (1999a,s b) found that cattle-manure application in maize grown under conventional land preparation resulted in similar grain yield to that of chemical-fertilization treatment. Disagreement between the results of these researchers and that of the present study could be related to higher N availability of manure in Eghball and Power's experiments (1999a,b). In these two experiments, apparent N-use efficiency [(total treatment N uptake in 2 yr – total control N uptake in 2 yr)/N applied in 2 yr] * 100, was 22% for manure and 50% for the chemical N treatment. In the present study, these values were 17 and 58% for manure and chemical treatments, respectively (data not shown). The pattern of N uptake by a plant can also affect its response to manure application. Winter oilseed rape uptakes 25–30% of total N (40–80 kg N ha⁻¹) during autumn (Rathke et al., 2006), when N mineralization in manure is expected to be low. In spring, when N mineralization in manure increases due to favourable climatic conditions, winter oilseed rape starts flowering and demand for N decreases (Rathke et al., 2006).

In both years of experiment, treatment F₁₀₀M₅₀ and, in the second year only, treatment F₅₀M₁₀₀ produced similar grain yield to F₁₅₀ (Table II). Patra et al. (2000) showed that herb and essential oil yields of mint (Mentha arvensis cv. Hy 77) were significantly higher under combined application of farmyard manure and inorganic fertilizer than when using the inorganic fertilizer alone. However, Khaliq et al. (2006) observed that combination of 10 t ha⁻¹ organic fertilizer (farmyard manure + poultry manure + sugarcane filter cake) + EM (effective microorganism) + 1/2 mineral NPK in cotton yielded very similar results to those obtained from full recommended mineral NPK. Ghosh et al. (2004) also found that application of 75% NPK + 5 t ha⁻¹ farmyard manure resulted in equal and higher grain yield in sorghum and soybean, respectively, compared with 100% NPK. It could be concluded that owing to the slow rate of N release from manure, crops with high N demand such as cotton, sorghum, and oilseed rape cannot produce higher grain yield under integrated fertilization systems than under inorganic fertilization systems on a short-term basis, while this could be achieved in crops with low N demand such as soybean and mint.

In spite of less N uptake by oilseed rape in the integrated treatments (Table II), the similar yield of F₁₀₀M₅₀ and F₅₀M₁₀₀ compared with F₁₅₀ might be attributed to better synchronization of N release and crop-uptake pattern (Qian & Schoenau, 2002), and also to positive effects of manure on physicochemical and biological characteristics of soil.

Oil and protein content

Oil and protein content of seed were used to illustrate the quality of winter oilseed rape. The effects of different treatments on seed oil and protein percentage and yield are illustrated in Figure 1 and Figure 2. The seed oil content varied between 41.5 and 46.3%. The protein content ranged from 21.9 to 25.7%. The lowest oil content (41.5%) and the highest protein content (25.6%) occurred in F₁₅₀ treatment. Nevertheless, this treatment produced the highest oil yield (1203 kg ha⁻¹, Figure 2) due to high grain yield. Additionally, this treatment produced the highest protein yield (742 kg N ha⁻¹, Figure 2). The positive effect of N application on protein content and the negative correlation between oil content and protein content are well established (Hocking et al., 1997; Rathke et al., 2005).

Figure 1.  The effect of different fertilizer treatments on the protein and oil content of seed in oilseed rape in 2006. (Definition of treatments as in Table II. Data within each column followed by the same letter are not significantly different at the 0.05 probability level according to DMRT).

Figure 2.  The effect of different fertilizer treatments on the protein and oil yield in oilseed rape in 2006. (Definition of treatments as in Table II. Data within each column followed by the same letter are not significantly different at the 0.05 probability level according to DMRT).

The highest oil content was achieved in M₁₅₀, but this treatment produced only 459 kg oil ha⁻¹. Chemical N fertilization (F₁₅₀) versus manure application (M₁₅₀) resulted in lower oil content. In the inorganic treatments, increasing N rate up to 100 kg ha⁻¹ had little effect on oil content, while another 50 kg ha⁻¹ increment in applied N reduced oil content dramatically compared with control. In the integrated and organic treatments (F₁₀₀M₅₀, F₅₀M₁₀₀, and M₁₅₀), increasing N rate had a negative impact on seed oil content. Mason and Brennan (1998) and Cheema et al. (2001) found a negative influence of N fertilization on the oil content of the seed. This could be attributed in part to competition for carbon skeleton during carbohydrate metabolism (Bhatia & Rabson, 1976). The synthesis of both fatty acids and amino acids requires carbon compounds from the decomposition of carbohydrates. Since the carbohydrate content of protein is lower than that of oil (Lambers & Poorter, 1992), increased N supply intensifies the synthesis of protein at the expense of fatty acid synthesis and thus reduces the oil content of the seed. Results of Hocking et al. (1997) and Mason and Brennan (1998), however, showed that high N rates did not always affect the oil content of oilseed rape. The present results also suggested that there has been sufficient carbohydrate for oil synthesis up to 100 kg N ha⁻¹, since the application of less than 100 kg N ha⁻¹ did not change the protein content of the seeds significantly. Information on the effects of organic fertilizers on seed quality of winter oilseed rape is sparse. In the present study, it appears that lower uptake of N in the organic treatment compared with the inorganic treatment caused a higher availability of carbohydrates for oil biosynthesis. This result is also supported by the results of Rathke et al. (2005).

Zn and N uptake

By an increase in chemical N application rate, N concentration in shoot increased significantly (Table III). N application also increased Zn concentration in shoot, although it was not significant. Pederson et al. (2002) reported that in an acidic soil containing high levels of Zn and P, N concentration was highly correlated with P, Cu, and Zn concentration in shoot, suggesting that improvement in N fertility would improve P, Cu, and Zn concentration in forage (annual grasses). A positive relation has been also demonstrated between Zn supply and protein content (Marschner, 1995). Low association between N concentration and Zn concentration in shoot in the present study could be related to the low level of available Zn in soil (0.68 mg kg⁻¹ soil) compared with the Pederson et al. (2002) experiment. These researchers had used poultry litter fertilizer, which is rich in Zn. In the integrated and organic treatments (F₁₀₀M₅₀, F₅₀M_100, and M₁₅₀), by an increase in the proportion of manure, the available Zn in soil enhanced significantly compared with F₁₅₀ but Zn concentration in shoot did not show any significant difference. One reason for such a response could be related to the reduction in N uptake and therefore N concentration in shoot in F₅₀M₁₀₀ and M₁₅₀ (Table II). The significant increase in Zn concentration of shoot in F₁₀₀M₅₀ compared with control confirms this hypothesis (Table III). It appears that N availability should increase in order to simultaneously increase Zn uptake and Zn availability under manure application in a Zn-deficient soil.

Table III. The effect of different fertilizer treatments on soil-available P and Zn and their concentration in shoot of oilseed rape in 2006.

	Soil (mg kg^−1)		Shoot
Treatment*	P	Zn	N (%)	P (%)	Zn (mg kg^−1)
F0	22.5d**	0.44c	0.99c	0.19b	26b
F50	33.7d	0.6bc	1.20b	0.22ab	33ab
F100	31.3d	0.49c	1.28b	0.21ab	29ab
F150	51.8cd	0.55bc	1.48a	0.24ab	30ab
F100M50	82.2b	0.77b	1.39ab	0.29ab	34a
F50M100	82.7b	1a	1.14b	0.33ab	27ab
M150	105.1a	1.17a	1.08c	0.35a	29ab
* Definition of treatments as in Table II. ** Means data within each column followed by the same letter are not significantly different at the 0.05 probability level according to DMRT.

Zn and P uptake

There was no difference between treatments in the case of available Zn in initial soil sampling (data not shown). There was also no significant difference between inorganic treatments with respect to the available P and Zn (Table III)_. Treatments M₁₅₀, F₅₀M₁₀₀, and F₁₀₀M₅₀ increased the available Zn in soil by 113, 82 and 40%, respectively, relative to F₁₅₀. Higher Zn availability could be related to the soluble organic acids produced through decomposition of organic matter. These organic acids can mobilize soil-bound Zn and restrict the fixation of soluble Zn by soil components by chelating with the element. Besides, mineralization of immobilized Zn takes place in a later period (Mandal et al., 1993).

A significant difference was observed between F₀ and F₁₀₀M₅₀ only with respect to shoot Zn concentration (Table III). These results showed that manure has increased the available Zn in soil, but not Zn uptake or its translocation from root to shoot. Adediran et al. (2004) also found that organic fertilizer application increased Zn availability in soil and its content in ear leaf of maize compared with control but not compared with inorganic fertilizer. Rupa et al. (2003) reported that application 10 t ha⁻¹ manure resulted in a slight increase in the available Zn and its concentration in shoot and therefore dry matter of wheat. Katyal and Randhawa (1983) reported that regular application of farmyard manure prevented the occurrence of Zn deficiency. Wong et al. (1999) also reported that Zn content of cabbage and maize increased significantly by the addition of compost.

In the present experiment, shoot Zn concentration failed to increase under manure application in spite of higher Zn availability in soil, which could be related to the high P availability in soil and therefore shoot in the organic and integrated treatments compared to the inorganic treatment (Table III). In the present study, manure was applied on the basis of N requirement of oilseed rape (150 kg N ha⁻¹ for an expected yield of 3 t ha⁻¹). Therefore, this application added P to the soil several times more than the crop requirement (Table I; 159.5 kg P ha⁻¹ equal to 84 mg kg⁻¹ soil). In addition, initial soil sampling indicated adequate available P in soil (18 mg kg⁻¹). Therefore, treatments M₁₅₀, F₅₀M₁₀₀, and F₁₀₀M₅₀ increased the available P in soil by 103, 60, and 59% and P concentration in shoot by 45, 38, and 21%, respectively, compared with F_150. At high levels of soil P, most of the absorbed Zn remains in root due to inactivation of Zn or due to increased binding of Zn to the root cell walls, as reported by Rupa et al. (2003), resulting in less Zn translocation to the shoot. Moreover, high P content in the shoot might decrease solubility and mobility of Zn both within the cells and in long-distance transport to the shoot apex (Marschner, 1995).

It appears that where available P exceed 80 mg kg⁻¹ soil, Zn uptake and its translocation from root to shoot decrease in oilseed rape (Table III). Rupa et al. (2003) reported that addition of P and Zn as high as 40 and 7.5 mg kg⁻¹, respectively, to a soil with adequate available Zn (0.8 mg kg⁻¹), increased dry matter and Zn concentration of wheat compared with control, while at 80 mg P kg⁻¹ soil, wheat dry matter and the root Zn concentration did not change but shoot Zn concentration decreased. By application of 160 mg P kg⁻¹ soil, Zn availability in both soil and dry matter, and Zn concentration of root and shoot, significantly decreased compared with application of 40 mg P kg⁻¹ soil. However, in this experiment, Zn uptake is discussed in relation to applied P not in relation to soil-available P. Safaya (1976) also showed that Zn-deficiency symptoms became evident on maize plants when fertilized with 75 mg P kg⁻¹ soil.

We conclude that combined application of organic and chemical fertilizers could result in similar grain yield in oilseed rape compared with that of chemical fertilizer alone. Application of manure as a source of crop nutrition resulted in better synchronization of N release in soil and crop-uptake pattern. It was also revealed that integrated fertilization systems favour oilseed rape with respect to seed oil content. Manure application increased Zn availability in soil. Regarding a positive association between N and Zn concentration in shoot, the increase in soil-available Zn did not result in higher Zn concentration in shoot, which was due to less N uptake under organic fertilization systems. A negative association was also found between soil-available P and Zn concentration in shoot. Considering this finding, application of manure reduced Zn concentration in shoot of oilseed rape due to less translocation of this nutrient from root to shoot. Overall, integrated application of the organic and chemical fertilizers would be beneficial to oilseed rape, farmers, and the environment if an appropriate proportion of each component (organic and chemical) were selected so that a balance between different macro- and micro-nutrients released in soil and crop requirement could be achieved.

Acknowledgements

The authors would like to thank Tarbiat Modares University's Faculty of Agriculture Analytical Laboratory for providing facilities for soil and plant analyses, Shahid Beheshti University for providing field facilities. The authors would also like to greatly thank the two anonymous reviewers for their critical comments.