Journal of Horticultural Science & Biotechnology (2005) 80 (2) 233–239
High irrigation frequency and transient NH4 concentration: effects on soilless-grown bell pepper By A. SILBER1,*, M. BRUNER2, E. KENIG2, G. RESHEF2, H. ZOHAR2, I. POSALSKI2, H. YEHEZKEL3, S. COHEN3, M. DINAR3 and S. ASSOULINE1 1 Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization, the Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel 2 Shaham, Extension Service, Ministry of Agriculture, P.O. Box 28, Bet Dagan 50250, Israel 3 Besor Experimental Station, P.O. Box 4, Negev 85400, Israel (e-mail:
[email protected]) (Accepted 2 November 2004) SUMMARY The effects of irrigation frequency, N concentration, and NH4-N/NO3-N ratio on rhizosphere NH4 concentrations and plant yield were investigated for bell pepper (Capsicum annuum L., cv. ‘Mazurka’) grown in perlite. The experiment comprised eight treatments, with two concentrations of N (120 or 60 mg l–1), two NH4-N/NO3-N ratios (3:7 and 1:9), and two irrigation frequencies (3 d–1, or for 1.5 min every 30 min throughout the day). The combination of high NH4 concentration in the irrigation water and high irrigation frequency reduced yield significantly. Decreasing the NH4 concentration by decreasing the NH4-N/NO3-N ratio, or by decreasing the total N concentration, improved yield. The impaired growth of plants exposed to high irrigation frequency and high NH4 concentrations was a result of high transient NH4 concentrations in the rhizosphere. Time-dependent processes, such as nitrification, reduced transient NH4 concentrations in the rhizosphere and, therefore, actual NH4 concentrations were lower under low-frequency irrigation. Modification of the NH4/NO3 ratio under high frequencies of irrigation is recommended to diminish the risks of NH4 toxicity in sensitive crops.
I
n modern agricultural systems, especially under arid or semi-arid conditions or in greenhouses using soilless culture techniques, water and nutrients are supplied simultaneously (fertigation), mainly by drip irrigation devices (Bar-Yosef, 1999; Hagin et al., 2002). Frequent application of water and nutrients ensures that the root surface and its vicinity are well-supplied with fresh nutrient solution during the irrigation events and the subsequent redistributions. Frequent replenishments prevent the formation of a depletion zone at the root surface by uptake of nutrients between successive irrigation events, decrease the concentration gradient between the medium solution and the root interface, and diminish the role of diffusion in transporting nutrient toward the roots (Silber et al., 2003). Earlier studies demonstrated that increased fertigation frequency significantly increased plant yield, especially at low nutrients concentrations (Silber et al., 2003; Xu et al., 2004). Yield improvement was primarily related to enhanced nutrient uptake, especially of P. It was suggested that the yield reduction at low fertigation frequency resulted from nutrient ion deficiency rather than water shortage, and that high irrigation frequency could compensate for nutrient deficiency. Frequent fertigation improved the uptake of nutrients through two main mechanisms: continuous replenishment of nutrients in the depletion zone near the root interface; and enhanced transport of dissolved nutrients by mass flow, because of the higher temporal water content in the medium during daytime. *Author for correspondence.
However, in addition to its beneficial effects on the uptake of nutrients by plants, frequent fertigation may also increase root exposure to toxic substances, such as ammonium. Although N is an essential nutrient for plant growth, high NH4 concentrations are toxic to most plants, especially at high root temperatures and under high salinity (Adams, 2002; Forde and Clarkson, 1999; Ganmore-Neumann and Kafkafi, 1980a,b; 1983; Kafkafi, 1990; Sonneveld, 2002), and may be involved in the incidence of blossom-end rot (BER) in tomato and bell pepper (Adams, 2002; Marti and Mills, 1991). BER is often associated with local Ca-deficiency in fruits (Adams, 2002; Marschner, 1995; Saure, 2001), but the direct cause of this physiological disorder remains obscure (Saure, 2001). It is generally accepted that the incidence of BER increases with high NH4 activity. Therefore, it was suggested that in susceptible crops such as tomato and sweet pepper, NH4-N should be restricted to 15%, or even to 10% of the total N supplied by irrigation (Adams, 2002; Sonneveld, 2002). Since NH4 concentration is affected by both the total N concentration and the NH4-N/NO3-N ratio, it is more practicable to express NH4 activity in terms of the solution NH4-N concentration. In Israel, the critical NH4N concentration in the irrigation water of greenhousegrown sweet pepper was found to be 14 mg l–1. Above this threshold concentration physiological disorders of the fruit, such as BER and flat fruits increased (Bar-Tal et al., 2001). However, the threshold values of NH4-N concentration in the irrigation solution, as determined in field experiments, are not definitive. Between successive irrigations, the NH4 that is added to the rhizosphere
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Irrigation frequency and NH4 effects on bell pepper 35
Temperature (°C)
A
(31°16'N, 34°24'E) from May 20 to October 23, 2001. Variations in irradiance, relative humidity, air and substrate temperatures inside the screen-house during two representative days at the beginning of the harvest period are presented in Figure 1. All climatic data inside the screen-house were recorded every 10 min by two phytomonitor stations (LPS-03MA Phytomonitor Station, PhyTech Ltd., Israel). Daily pan evaporation data at the Besor Experimental Station were supplied by The Israeli Meteorological Service.
Air Perlite
30
25
Irradiance (W m–2)
600
B
Humidity
100 400 80 200
60
Irradiance
0
0 30 DAT
12
24
36
48
Relative humidity (%)
20
40
Time (h)
FIG. 1. Climatic data inside the screen-house for two representative days (48 h) before harvest (starting at 80 DAT). A: Air temperature and perlite temperature at 10 cm depth. B: Irradiance and relative humidity.
through fertilisation is rapidly converted to nitrate, but the rates of nitrification processes and therefore the pertinent rhizosphere NH4 concentrations, are affected by environmental factors that affect the micro-organism population such as temperature, pH, electrical conductivity of the solution, and the chemical/physical properties of the substrate used. Furthermore, nitrification is a time-dependent process, therefore the transient rhizosphere NH4 concentration is also affected by the interval between successive irrigations. It is important, therefore, to assess the critical transient NH4 concentration in the rhizosphere, as well as the concentration in the irrigation solution. The aims of the present study were to explore the integrated effect of irrigation frequency, N concentration, and NH4-N/NO3-N ratio on periodic NH4 concentrations in the root area, and to assess their effects on bell pepper grown in soilless culture.
MATERIALS AND METHODS General The experiment was conducted in a screen-house (30% shade) at the Besor Experimental Station, Israel
Plant growth and experimental design Bell pepper plants (C. annuum L., cv. ‘Mazurka’) were transplanted in the centre of polypropylene containers (5.2 m long, 0.4 m wide and 0.2 m deep) filled with perlite (2 mm grain size). The density of the perlite and the water-holding capacity in the pot were 72 g l–1 and 0.55 l l–1, respectively. Each plot contained 26 plants, at a density of 3.3 plants m–2. The experiment comprised eight treatments (Table I), with two concentrations of N (N1 = 60 mg l–1 and N2 = 120 mg l–1), two NH4-N/NO3-N ratios (1:9 and 3:7), and two irrigation frequencies: “normal” (three irrigation events d–1) and “high frequency” (1.5 min every 30 min throughout the day), allocated to five randomised blocks. N2 is the concentration recommended by the Israeli Extension Service for greenhouse-grown sweet pepper. All the treatments received the same daily amount of water, which was sufficient to ensure that the leaching fraction (drainage versus irrigation water) would be above 0.3, and the electrical conductivity (EC) of the drainage would not exceed 3 dS m–1. Irrigation was scheduled by a computer and was applied at 0600h, 1000h and 1400h for the “normal” irrigation (I1); and from 0500h to 1800h as pulses of 1.5 min duration at 30 min intervals for the “high frequency” (I2). An extra 2 min pulse was added to both irrigation treatments at 0000h. Irrigation, N treatments, and fertilisers The emitters were located adjacent to the plants, spaced 15 cm apart, and had a discharge rate of 2.3 ± 0.2 l h–1. Samples from the irrigation solutions were collected each week, and their pH, EC and nutrient concentrations were monitored. The pH of the irrigation solution was 7.0 ± 0.2, and the P and K concentrations in the irrigation solution were 18 and 140 mg l–1, respectively. The nutrient solutions were prepared using commercial fertilisers ((NH4)2SO4, NH4NO3, KNO3, KCl and H3PO4), and tap water containing 55 mg l–1 Ca2+, 32 mg l–1 Mg2+, 120 mg l–1 Na+, 150 mg l–1 CO32–, and 170 mg l–1 Cl–. Micronutrient concentrations applied were: 0.3 mg l–1 Zn, 0.6 mg l–1 Mn, 1.2 mg l–1 Fe, 0.04 mg l–1 Cu, 0.4 mg l–1 B, and 0.03 mg l–1
TABLE I Treatments applied to bell peppers Treatment Code I1N1R1 I1N1R2 I1N2R1 I1N2R2 I2N1R1 I2N1R2 I2N2R1 I2N2R2 1
Irrigation 1
Normal Normal Normal Normal Frequent2 Frequent Frequent Frequent
Total N in irrigation solution 3
Reduced Reduced Normal4 Normal Reduced Reduced Normal Normal
NH4-N/NO3-N ratio 5
Low High6 Low High Low High Low High
NH4-N conc. in irrigation solution (mg l–1) 6 18 12 36 6 18 12 36
“Normal” 3 irrigations d–1. 2“High frequency” (1.5 min every 30 min from 0500–1800 h). 360 mg N l–1. 4120 mg N l–1. 5NH4-N/NO3-N ratio of 1:9. NH4-N/NO3-N ratio of 3:7.
6
A. SILBER, M. BRUNER, E. KENIG, G. RESHEF, H. ZOHAR, I. POSALSKI, H. YEHEZKEL, S. COHEN, M. DINAR and S. ASSOULINE Mo, all EDTA-based. Tensiometers were installed horizontally, 5 and 10 cm from the bottom of one container for each treatment. Water tension was measured every 10 s, and the average values over every 5 min were stored in a data logger (CR10X Campbell Scientific, UT, USA). As controls, four containers for each treatment, filled with perlite, two with and two without plants, were installed in the centre of the experimental screen house and were fertilised identically to the experimental treatment. They were used to evaluate net nutrient and water uptakes by the plants. Leachates from all the control containers were collected daily, and their pH, EC and volumes were monitored. Transpiration was calculated from the difference between leachate volumes of the containers with and without plants. Chemical analyses of the leachates (for NH4-N, NO3-N, P, K, Ca, Mg, Fe, Zn, and Mn) were performed each week. Concentrations of NH4-N, NO3-N, and P were determined with a Lachat Autoanalyzer K with a flame photometer, and Ca, Mg, Fe, Zn and Mn with an atomic absorption spectrophotometer. Harvest and plant analyses Red fruits (80% colour) were harvested selectively each week, from 83 d after transplanting (DAT) until the end of the experiment (154 DAT). Total fruit number, weight, physiological disorders (BER, flat or damaged fruits) and fruit quality were determined. Fruit quality was classified according to a three-grade scale: (a) export (fruit with perfect shape and weighing over 150 g); (b) local market (fruit with perfect shape and weighing under 150 g); and (c) unmarketable low quality (damaged fruit, or those with physiological disorders). At the end of the experiment, two plants from each replicate were collected and divided into leaves, stems, and fruits, then weighed immediately, washed with distilled water, dried in a ventilated oven at 60°C for 1 week, and stored for chemical analysis. The dry plant
Normal (I1) 9
No plant N2R1
8
pH
Frequent (I2) 9
N1R2
7
No plant
8
N2R1
7
N1R2
6
6 N2R2 5
20
50
9
80
110
With plants N2R1
N2R2 20
50
9
80
N1R2
7
With plants N2R1
N1R2
7
N2R2
N2R2
6
6 5
110
8
8
pH
5
20
50
DAT
80
110
5
20
50
80
110
DAT
FIG. 2. Variations of pH with time (20–110 DAT) in leachates from pots with or without plants given “normal” (I1) or “high frequency” (I2) irrigation.
235
material (DM) was ground to pass a 20-mesh sieve. Samples (100 mg) were wet-ashed with H2SO4-H2O2 and analysed for Na, K, organic-N and P. HClO4-HNO3 ashing was used to analyse for Ca, Mg and micronutrients. Element concentrations were determined as mentioned above. Free nitrate was extracted from the dry leaf powder by vigorous shaking in deionised water (with a water:leaves weight ratio of 3:1) for 1 h at room temperature. The extract was filtered and diluted 10-fold with deionised water before measurement with a nitrate/nitrite meter (Merck RQflex). Statistics Statistical analyses were carried out with JMP software (SAS Institute, 2002). Data for dry weight, leafN, –P and –K during the vegetative stage, yield parameters, and nutrient concentrations in leaves and fruit during the productive stage, were analysed for treatment effects using the general linear model procedure of SAS. Differences among means were tested with the standard least squares mode of ANOVA, followed by a Tukey pair-wise comparison by means. Differences with a probability larger than 95% were taken as significant. Quadratic regression and the NLIN procedure of JMP software was used to obtain the relation between leaf NO3, –N and leaf Cl concentrations. RESULTS Leachate analysis and the integrated effect of irrigation frequency, N concentration, and source of pH Increasing the NH4 supply, either by increasing total N concentrations or by raising the NH4-N/NO3-N ratio, resulted in a decreased pH of the leachates from the control containers (Figure 2). The pHs of leachates (with or without plants) of treatment N1R1 at both irrigation frequencies were similar to those of N2R1 (Figure 3) and, therefore, are not shown. The pHs of leachates from unplanted pots, exposed to high irrigation NH4-N concentrations under frequent irrigation, decreased continuously during the growth period and reached a value of pH 5.4 compared with pH 5.8 under normalfrequency irrigation (I2N2R2 and I1N2R2 treatments, respectively; Figure 2). Throughout the growth period, the average NH4-N concentration in the leachate from unplanted I1N2R2 and I2N2R2 pots was 10–15 mg l–1 compared with low values of 0.1–0.4 mg l–1 in all other leachates. As the NH4-N concentration decreased, the NO3-N concentration simultaneously increased, so that the total N concentration in leachate from unplanted pots remained almost equal to that in the irrigation water. Acquisition of NH4 by plants in all planted pots further reduced the NH4-N concentrations of leachates below the analytical detection limit. EC and element concentrations (except for N) in leachates from unplanted and planted control pots were not affected by any of the treatments applied. The EC of leachates from all unplanted pots during the growth period remained almost equal to that of irrigation water (1.6–2.0 dS m–1), while it increased to 3.0 ± 0.3 dS m–1 in the presence of plants. Accumulation of elements in the substrate following water loss due to transpiration was the main cause of the EC increase. Na, Cl, Ca, and Mg2+ concentrations in leachates from planted pots rose to
Irrigation frequency and NH4 effects on bell pepper
236 Top
Water tension (kPa)
0
1
2 93 DAT 3
0
8
Normal: I1 16
Bottom
0
Water tension (kPa)
Frequent: I2
24
32
40
48
40
48
Frequent: I2
1
2 Normal: I1 93 DAT 3
0
8
16
24
32
Time (h) FIG. 3. Variation with time (h) of tensiometer readings located 10 and 5 cm from the bottom of a container (Top and Bottom, respectively) with plants under I1N2R2 and I2N2R2 treatments during two representative days (48 h, starting at 93 DAT). Treatment acronyms are detailed in Table I.
350–400, 450–600, 80–120 and 50–60 mg l–1, respectively. K+ concentrations in these leachates remained at the same level as in the irrigation water (140 mg l–1), but P concentrations dropped from 18 to 7–10 mg l–1. Water tension in the substrate and transpiration Daily variations in water tension in containers during two representative days are shown in Figure 3 for the high-N and high-NH4/NO3-N ratio treatments. Water tensions in the upper part of the containers were higher than in the lower part, but the trends were almost the same. Water tension decreased sharply during irrigation events, as the water content increased, then increased significantly shortly afterwards, mainly as a result of drainage out of the pot (Figure 3). The subsequent moderate tension increase, over a longer period, was due to water uptake by the roots. As the surface of the medium was entirely covered by the pepper canopy, evaporation was negligible and transpiration was the main cause of water loss. The amplitude of the variations in water tension decreased as the irrigation frequency increased (Figure 3), indicating that the drainage volumes flowing out of the container after each individual irrigation event increased as the irrigation frequency decreased. Lower amplitudes of water tension
variation indicate higher temporal mean moisture contents and, therefore, greater water availability to the plant. Note that for both irrigation frequencies, the highest value of water tension during daylight hours was reached around 1500h (Figure 3), as the atmospheric sink for water reached its highest value because of maximum air temperature and irradiance (Figure 1). The daily water dose during this period was 3 l per plant, which was provided in the “high-frequency” treatments by 1.5 min pulses every 30 min from 0500h to 1700h, corresponding to an equivalent rate of 0.3 l h–1. Thus, the increase in water tension in the middle of the day (Figure 3) indicates that the transpiration rate exceeded water flux to the roots. As air temperature and irradiance decreased in the afternoon (Figure 1), the water tension returned to its former values. The further increase during the night was due to drainage. Treatment effects on the transpiration rates were insignificant; therefore only the transpiration rate pattern of I1N2R2 plants is presented (Figure 4). Irrigation increased as a result of continuous increases in transpiration rates elicited by plant growth, until 50 DAT (July 10, 2001). The transpiration rate then remained steady for the remainder of the growing period, at a value between the potential evaporation calculated from pan measurements and the potential transpiration calculated with the Penman-Monteith equation (P-M, Figure 4). Although the calculated VPD for water did not increase after 50 DAT, the daily irrigation doses were increased to 3.6 l plant–1 in order to meet the constraint of an EC below 3 dS m–1 in the drainage water. Thus, water application under a high VPD should be increased in order to maintain reasonable EC values in the substrate during the hot Summer. Plant growth and yield The combination of high N rate and low NH4-N/NO3N ratio (treatment N2R1) enhanced vegetative growth, whereas a high ammonium rate (treatment N2R2) depressed vegetative growth under both irrigation frequencies (Table II). The dry weight (DW) data presented in Table II shows: (1) when given a high NH4N/NO3-N ratio fertiliser, above-ground DW was greater TABLE II The effect of irrigation frequency (I), N application rate (N) and NH4-N/NO3-N ratio (R) on the total dry weight (DW) of shoots, and on leaf-N, -P and -K concentrations during the vegetative growth period Treatments I1N1R11 I1N1R2 I1N2R1 I1N2R2 I2N1R1 I2N1R2 I2N2R1 I2N2R2 Mean
DW (g plant–1)
N (g kg–1 DW)
P
K
22.5 24.0 25.6 20.1 21.8 21.8 28.3 19.5 22.9
47 50 55 56 50 49 54 55 52
5.3 5.1 5.8 6.0 5.3 5.6 5.8 6.4 5.7
70 71 81 66 69 76 78 73 73
Source of variation 0.701 I2 0.011 N2 2
