Soil Temperature and Maize Nitrogen Uptake Improvement Under Partial Root-Zone Drying Irrigation

Pedosphere 26(6): 872–886, 2016 doi:10.1016/S1002-0160(15)60092-3 ISSN 1002-0160/CN 32-1315/P c 2016 Soil Science Society of China ⃝ Published by Elsevier B.V. and Science Press

Soil Temperature and Maize Nitrogen Uptake Improvement Under Partial Root-Zone Drying Irrigation Fatemeh KARANDISH1,∗ and Ali SHAHNAZARI2 1 Assistant 2 Associate

Professor, Water Engineering Department, University of Zabol, Zabol 35856-98613 (Iran) Professor, Water Engineering Department, Sari Agricultural Sciences and Natural Resources University, Sari 378 (Iran)

(Received March 23, 2015; revised September 16, 2016)

ABSTRACT Soil temperature is a major effective factor on the soil and plant biological properties. Irrigation can affect soil temperature and thereby induces a temperature effect on plant growth, which may result in an economic increase due to higher yield and plant nutrition. A field experiment was carried out to investigate the effects of three irrigation strategies including full irrigation (FI), partial root-zone drying (PRD) and deficit irrigation (DI) on soil temperature and the consequent results on the grain yield and N uptake of maize (Zea May L.). Soil temperature was measured by time domain reflectometry (TDR) sensors during the 2010 growing season. Irrigation treatments were applied from 55 to 107 d after planting. The PRD treatment caused soil temperature to be in a favorable domain for a longer period (for over 60% of the measuring dates) as a consequent result of water movement to deeper soil layers compared with the other treatments; the PRD treatment also reduced soil temperature at deeper soil depths to below the maximum favorable soil temperature for maize root growth, which resulted in deeper root penetration due to both water availability and favorable soil temperature. Compared to the FI treatment, the PRD treatment increased root water uptake by 50% and caused no significant reduction in total N uptake, while this was not observed in the DI treatment partially due to the negative temperature effect of DI on plant growth, which consequently affected the water and nutrient uptake. A longer vegetation period in the PRD treatment was observed due to higher leaf N concentrations and no significant reduction in maize grain yield occurred in the PRD treatment, compared with those in the FI treatment. Based on the results, having 15.2% water saving during the whole growing season, the PRD irrigation would positively affect soil temperature and the water and nutrient uptake as a consequent, which thereby would prevent significant reduction in maize grain yield. Key Words: uptake

full irrigation, deficit irrigation, grain yield, irrigation strategy, leaf N concentration, root growth, water saving, water

Citation: Karandish F, Shahnazari A. 2016. Soil temperature and maize nitrogen uptake improvement under partial root-zone drying irrigation. Pedosphere. 26(6): 872–886.

INTRODUCTION Soil temperature affects lots of soil physical, chemical and biological properties, which consequently can affect plant growth in different ways. Root growth has been reported to be a sensitive parameter to soil temperature (Psarras et al., 2000; Lahti et al., 2005; Callejas et al., 2009). There is a species-specific soil temperature threshold, below or over which root growth would decrease (Callejas et al., 2009). A favorable soil temperature relative to an unfavorable soil temperature would favor root growth (Callejas et al., 2009). Soil temperature could significantly affect the amount of water and nutrient uptake due to its major effect on root growth (Pregitzer et al., 2000; Dong et al., 2001; Puhe, 2003). Lower nutrient and water uptake under unfavorable soil temperature conditions ∗ Corresponding

would consequently lead to a significant reduction in yield (Schwarz et al., 1997; Aphalo et al., 2006). Despite the major role of soil temperature on plant growth, there are yet only a few investigations, in which the soil temperature variation has directly or indirectly been assessed (Kasubuchi, 1982; Tanaka and Ishii, 2000; Duna et al., 2010). Kasubuchi (1982) has illustrated a two-dimensional distribution of soil temperature in the soil surface layer using the relation between soil temperature and heat conductivity. Duna et al. (2010) have investigated the effect of the plant growth stage on the domain of soil temperature fluctuation and concluded that the difference between the minimum and maximum values of soil temperature was higher at the early season stage compared with the other growth stages. Some researchers also considered the extraction of soil temperature variations using nu-

author. E-mail: [email protected], Karandish [email protected].

SOIL TEMPERATURE, N UPTAKE AND IRRIGATION

merical models (Tanaka and Ishii, 2000). Soil water content has been reported to be a driving factor in controlling soil temperature (Hlavinka et al., 2009; Nainanayake et al., 2009; Roxy et al., 2010). Therefore, introducing a suitable irrigation strategy may help with both maintaining the soil temperature in a favorable domain for root growth and subsequently increasing nutrient and water uptake. As fresh water resources become scarce, it is difficult to irrigate crops to meet their full demand. To reduce the irrigation volume, some irrigation strategies such as deficit irrigation (DI) have been developed to aim at meeting the minimum crop water requirement. In the DI strategy, crops are exposed to water stress during the whole growing season or at some growth stages. Despite water saving, DI usually causes a significant reduction in crop yield and quality (Shahnazari et al., 2007). During the last years, a novel irrigation strategy, partial root-zone drying (PRD), has been developed (Kang and Zhang, 2004). The PRD approach is to use irrigation to alternately wet and dry two spatially distinct parts of the plant root system. In fact, in PRD one half of the root zone is irrigated, while the other half kept dried. Irrigated and dry sides are periodically switched (Dry and Loveys, 1998). Regularly alternating the wet and dry root compartments causes some plant physiological responses, which makes the PRD hypothesis to be different from the other water saving strategies (Stoll et al., 2000). The PRD irrigation has been tested for field crops and fruit trees (Kang and Zhang, 2004). Most recently, it has also been tested in vegetables (Shahnazari et al., 2007). In most cases, PRD has shown a great potential to increase irrigation water use efficiency and to maintain yield (Davies et al., 2002). Partial root-zone drying has shown a great potential to improve nutrient and root water uptake (Shahnazari et al., 2008; Hu et al., 2009; Wang et al., 2012). Since the soil water dynamics under PRD differ from those under other water saving strategies, soil temperature may be consequently different under PRD due to the major driving role of soil water content on soil temperature (Nainanayake et al., 2009; Roxy et al., 2010). Nevertheless, in spite of numerous researches on the physiological response of plants under PRD (Shahnazari et al., 2007, 2008; Hu et al., 2009; Wang et al., 2012), there is still a lack of information on the soil temperature condition under this novel irrigation strategy. Thus, the major objective of this study was to investigate the soil temperature variations under full irrigation (FI), DI and PRD and to determine subsequently their effects on the water and nutrient uptake

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of maize. MATERIALS AND METHODS Site and climatic conditions The field experiment was carried out in 2010 at the research farm of Sari Agricultural Sciences and Natural Resources University (SANRU) (36.3◦ N, 53.04◦ E) in Sari, Iran. The mean elevation of the site is 15 m above sea level. Based on the DeMarten method (Oliver, 2005), the climate of the area is humid. The mean annual rainfall at the site is 616 mm and the class “A” pan evaporation is 2 500 mm. About 70% of annual rainfall occurs over the October–March period. The long-term annual average, minimal, and maximal air temperatures are 17.3, −6 and 38.9 ◦ C, respectively (Darzi-Naftchali et al., 2013). Weather data were collected at the SANRU weather station, less than 1 km distance from the site of the experiment. The climate condition during the 2010 growing season (May 26–September 9) is shown in Fig. 1. Daily minimal temperature ranged from 15.5 to 28.6 ◦ C during the growing season with a mean of 23.3 ◦ C. The lowest (26.4 ◦ C) and highest (38.1 ◦ C) values of daily maximal temperature occurred on the 95th and 78th d after planting, respectively. The lowest (51%) and highest (83.5%) values of relative humidity occurred on the 13th and 95th d after planting, respectively. Daily reference evapotranspiration (ETo ) varied between 2.2 to 9.2 mm with a mean of 6.7 mm totaling 543.4 mm in the 2010 growing season. Totally, 8 mm precipitation was recorded for the whole growing season of 2010, which occurred from 1 to 54 d after planting. No rainfall occurred during the stress period (55–107 d after planting). The average wind speed during the whole growing period was 4.37 m s−1 . The surface soil (0–20 cm) of the study site is classified as Typic Haplxereptd based on Soil Survey Staff (2014). Chemical properties of the surface soil are as follows: electrical conductivity (EC), 1.64 dS m−1 ; organic matter, 12.2 g kg−1 ; organic carbon, 7.1 g kg−1 ; nitrogen (N), 0.6 g kg−1 ; phosphorus (P), 3.6 mg L−1 and potassium (K), 75 mg L−1 . The soil physical properties in 0–100 cm profile are summarized in Table I. Prior to planting, field capacity (FC) and permanent wilting point (PWP) of different soil samples were determined at suction of about 30 and 1500 kPa, respectively, using a pressure plate apparatus. The average water table in the experimental field was about 122 cm below the soil surface at the beginning of the growing season, and then it continued to fall to 211 cm below the soil surface at the end of the experiment.

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Fig. 2 Horizontal section of drip lines, drippers and plant positions in the experimental field.

Maize single-cross hybrid 704 was planted 5 cm deep on May 26, 2010, parallelling to the drip lines, with 60-cm row and 20-cm within-row plant spacings (Fig. 2). The agricultural activities and fertilization during the maize growing season are summarized in Table II. Crops were irrigated by the surface drip irrigation system during the whole growing season. The volume of water applied to each plot was measured with flow meters. The FI treatment was fully irrigated and soil water content was kept nearly close to the field capacity during the whole growing season. Also, all plots (including FI, PRD and DI treatments) received the same irrigation water depth from 1 to 55 d after planting. The water saving treatments (PRD and DI treatments) were applied in the period from the beginning of tassel emergence (July 19, 2010, 55 d after planting) until maturing stage (September 9, 2010, 107 d after planting), which is called “stress period” in this research. The PRD and DI treatments applied in the period from the beginning of tassel emergence until maturity was to avoid plant stress (Edmeads et al., 1992; Bola˜ nos et al., 1993). During the stress period (from 55 to 107 d after planting), the PRD and DI treatments were scheduled to receive 75% of the irrigation amount of the FI treatment at each irrigation event. In the FI and DI treatments, both drip lines of

Fig. 1 Temporal variations of climate variables during the 2010 growing season. Tmin = daily minimal temperature; Tmax = daily maximal temperature; ETo = daily reference evapotranspiration.

Experimental design and crop management The experimental field area was 15 m × 33 m, which was divided into 9 plots (each 5 m × 11 m). The field experiment was a complete block design with three replicates of three surface drip irrigation treatments: FI, PRD and DI. The surface drip irrigation system was set prior to the planting and implementation of the field experiment. The horizontal section of drip lines, drippers and plant positions in the experimental field were as shown in Fig. 2. Two drip lines were used for each crop row, one at each side. Each drip line had drippers every 40 cm and the dripper discharge was 2 L h−1 . TABLE I Soil physical properties in 0–100 cm profile in the experimental field Depth cm 0–20 20–40 40–60 60–80 80–100 a) Permanent

Soil texture Sandy clay loam Clay loam Clay loam Clay loam Clay loam wilting point.

Sand 49 40 30 37 36

Silt % 22 25 36 30 28

Clay

Field capacity cm3

27 35 34 33 34

0.30 0.32 0.32 0.32 0.32

PWPa)

Bulk density

0.15 0.14 0.14 0.14 0.14

g cm−3 1.40 1.38 1.35 1.37 1.37

cm−3

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TABLE II Summary of agricultural activities and fertilization during the 2010 maize growing season Date

Time after planting

Crop intervention

Fertilizer(s) used

May 26 June 12 July 14 July 19 September 9

d – 17 50 55 107

Planting and fertilization Fertilization Fertilization Onset PRDa) Harvest

150 kg ha−1 triple superphosphate 65 kg ha−1 urea and 50 kg ha−1 potassium sulphate 135 kg ha−1 urea and 100 kg ha−1 potassium sulphate

a) Partial

root-zone drying.

each crop row worked simultaneously, but in the PRD treatment and during the stress period, just one drip line worked at each irrigation event while the other line was kept off to ensure PRD. Since previous studies reported 7–10 d as the best period for the PRD cycling for maize (Yazar et al., 2009), the irrigation was shifted between the two sides of the plants weekly in the PRD treatment of this research. Fertilizer rates were determined according to the conventional practices. Nitrogen, K and P fertilizers were applied as urea, potassium sulphate and triple superphosphate, respectively (Table II). Fertilizers were applied via irrigation using the drip irrigation system (fertigation) except triple superphosphate, which was distributed prior to planting by hand. All other agricultural operations such as plowing, spraying and weeding were done in accordance with the conventional method used in the region. All plants in a plot (apart from those in the first and end rows of each plot) were harvested on September 9, 2010 (107 d after planting) to determine the grain yield. Grains were removed from the cobs, dried at 70 ◦ C for 24 h, and then adjusted to 14% moisture.

Decagon time domain reflectometry (TDR) sensors (Decagon, USA). For irrigation scheduling, 6 Decagon TDR sensors were installed in the root zone of the FI treatment (Fig. 3). Sensors of Decagon TDR were installed at depths of 0–5 cm (Sensor a), 10–30 cm (Sensor b), 35–40 cm (Sensor c), 45–65 cm (Sensor d), 70– 75 cm (Sensor e) and 80–100 cm (Sensor f) as shown in Fig. 3. Sensors a, c and e (5TE, Decagon, USA) were used to measure soil temperature, soil electrical conductivity and volumetric soil water content. Sensors b, d and f (10HS, Decagon, USA) were employed to measure volumetric soil water content. Measured volumetric soil water contents by Sensors a, b, c, d, e and f were used in Eq. 1. All treatments received the same amount of irrigation water from 1 to 55 d after planting. During the stress period (i.e., from 55 to 107 d after planting), the PRD and DI treatments received 75% of the irrigation volume of the FI treatment.

Irrigation requirement Daily irrigation requirement of the FI treatment was calculated by Eq. 1: InFI =

6 ∑ [(θFCj − θBIj ) × zj ]

(1)

j=1

where InFI is the net irrigation depth for the FI treatment (mm), θFCj is the volumetric soil water content at field capacity for the jth soil layer (cm3 cm−3 ) (j = 1, 2, 3, 4, 5 and 6 for 0–7.5, 7.5–32.5, 32.5–42.5, 42.5–67.5, 67.5–77.5 and 77.5–100 cm soil depths, respectively), θBIj is the volumetric soil water content before irrigation events for the jth soil layer (cm3 cm−3 ), and zj is the jth soil layer depth (mm). The θBIj was measured daily one hour before each irrigation event during the whole growing season with

Fig. 3 Sketch of Decagon time domain reflectometry (TDR) sensor (5HS and 10TS, Decagon, USA) placement in the plots at depths of 0–5 cm (Sensor a), 10–30 cm (Sensor b), 35–40 cm (Sensor c), 45–65 cm (Sensor d), 70–75 cm (Sensor e) and 80– 100 cm (Sensor f) in vertical position under soil surface with full irrigation. Sensors 5HS were able to measure soil electrical conductivity, soil temperature and volumetric soil water content, and Sensors 10TS were able to measure just volumetric soil water content.

Soil temperature monitoring As shown in Fig. 4, 25 IDRG-SMS T2 sensors (Tehran, Iran) were installed for one replicate in each

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treatment to simultaneously measure volumetric soil water content and soil temperature; i.e., totally 75 IDRG-SMS T2 sensors were installed for three treatments. It was assumed that each probe was measuring a volume of 0.002 m3 , called a “calculating pixel” in the current study; therefore, there were 25 calculating pixels in the root zone of one plant in each treatment (Fig. 4). As shown in Fig. 4, two emitters (one on each drip line) allocated to each plant called the “first dripper” and the “second dripper”, respectively. As mentioned above, in the PRD treatment, just one drip line was operating at each irrigation event during the stress period (i.e., 55–107 d after planting). Thus, just one of the two allocated drippers of each plant was emitting at each irrigation event. It was assumed that Position A was the place of the off dripper in the PRD treatment and Positions B, C, D and E were 5, 10, 15 and 20 cm apart from Position A, respectively (Fig. 4). Also, Position E was assumed to be the place of emitting dripper in the PRD treatment. Since the PRD irrigation was weekly shifted between two sides of the plant during the stress period, Position A (or E) could thus be the place of “first dripper” or “second dripper” in the PRD treatment (Fig. 4), which depended on the number of the irrigation events. In the FI and DI treatments, both drip lines and consequently both emitters were operating simultaneously for all irrigation events. Thus, both Positions A and E were the places of the emitting drippers in the FI and DI treatments.

F. KARANDISH AND A. SHAHNAZARI

Daily reading IDRG-SMS T2 sensors, the absolute value of soil temperature variations due to irrigation (|∆T |) was calculated as the difference between the measured soil temperature at one hour before and one hour after each irrigation event. To determine vertical |∆T | changes between treatments, the average of |∆T | in Positions A and B for each soil layer was considered as the soil temperature variation for the dried side of this layer, and the average of |∆T | in Positions C, D and E was considered as the soil temperature variation at the wetted side, for the PRD treatment. To determine the lateral |∆T | changes, the average of |∆T | for the 0–100 cm soil depth in a particular position (i.e., Position A) was calculated. Calculating root water uptake Root water uptake (WU) from the 0–20, 20–40, 40–60, 60–80 and 80–100 cm soil depths at the wetted (irrigated) side in the PRD treatment was calculated based on a soil water balance equation (Allen et al., 1998), and the same procedure was applied to calculate WU in half of the root zone in the FI and DI treatments. The soil water balance equation was summarized as follows (Allen et al., 1998): In + ∆Sm = DWm + ETm

(2)

where In is the net irrigation depth for the considered treatment (mm), ∆Sm is the initial soil water depletion (mm) up to a specific soil depth (m) (i.e., m = 1, 2, 3, 4 and 5 for 0–20, 0–40, 0–60, 0–80 and 0–100 cm

Fig. 4 Diagram of IDRG-SMST2 sensor (Tehran, Iran) grid placement in the soil. It was assumed that Position A was the place of the off dripper, Positions B, C, D and E were 5, 10, 15 and 20 cm apart from the off dripper in the PRD treatment, respectively. Both emitters in Positions A and E were the emitting drippers in FI and DI treatments and just the emitter in Position E was the emitting dripper in the PRD treatment. As the PRD irrigation was weekly shifted between two sides of the plant, the emitting dripper in Position E in the PRD treatment would thus be the first or second dripper depended on the number of the irrigation events during the stress period. If “first dripper” was emitting, thus “first set” was adopted; if “second dripper” was emitting, thus “second set” was adopted.

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soil depths, respectively) and over a specific time period (i.e., between two consecutive irrigation events), DWm is the drainage water (mm) from a specific soil depth (m), and ETm is the daily evapotranspiration (ET, mm) up to a specific soil depth (m). As irrigation was performed daily, Eq. 2 was thus used for a daily period (i.e., from before the irrigation event on the nth d after planting till before the irrigation event on the (n + 1)th d after planting). The net irrigation depth of the FI treatment (InFI ) was calculated by Eq. 1. The net irrigation depth (In ) at dried side of the PRD treatment was equal to zero in Eq. 2. The ∆Sm was calculated as follows: ∆Sm =

m ∑ [ 2(θei − θ¯ei ) + 2(θdi − θ¯di ) + i=1

] (θci − θ¯ci ) × zi ÷ 5

(3)

for the wetted side of the PDR treatment, ∆Sm =

m ∑ [ 2(θai − θ¯ai ) + 2(θbi − θ¯bi ) + i=1

] (θci − θ¯ci ) × zi ÷ 5

(4)

for the dried side of the PDR treatment and ∆Sm =

m ∑ [ (θai − θ¯ai ) + (θbi − θ¯bi ) + (θci − θ¯ci ) + i=1

] (θdi − θ¯di ) + (θei − θ¯ei ) × zi ÷ 5

(5)

for the FI and DI treatments, where θai , θbi , θci , θdi and θei are the volumetric soil water contents (cm3 cm−3 ) in Positions A, B, C, D and E, respectively, in the ith soil layer (i = 1, 2, 3, 4 and 5 for 0–20, 20–40, 40–60, 60–80 and 80–100 cm soil depths, respectively) measured before irrigation event on the nth d after planting and θ¯ai , θ¯bi , θ¯ci , θ¯di and θ¯ei are the volumetric soil water contents in the mentioned positions measured on the (n + 1)th d after planting (the next day), and zi is the depth of the ith soil layer (i.e., zi = 200 mm). It was assumed that half of Position C and both Positions D and E were at the wetted side of the PRD treatment. In the PRD treatment, irrigation water was just added to the half of the root zone, therefore DWm for the dried side of the PRD treatment was assumed to be zero (i.e., both In and DWm are equal to zero at the dried side of the PRD treatment). The DWm for the FI and DI treatments and for the wetted side of the PRD treatment was calculated as in Eq. 6: DWm = In −

m ∑ (θFCi − θi ) × zi i=1

(6)

where θFCi is the volumetric soil water content at field capacity in the ith soil layer (cm3 cm−3 ) and θi is the average of measured volumetric soil water content (cm3 cm−3 ) in the ith soil layer before the irrigation event on the nth d after planting. Assuming that half of Position C and both Positions D and E were at the wetted side of the PRD treatment, θi was calculated as follows: θi = (θci + 2θdi + 2θei ) ÷ 5

(7)

for the wetted side of the PRD treatment and θi = (θai + θbi + θci + θdi + θei ) ÷ 5

(8)

for the FI and DI treatments. All parameters are previously introduced. Having daily ETm , root WU for both sides of the plant was calculated as follows: { ET1 for i = m = 1 WUi=m = (9) ETm − ETm−1 for 2 ≤ m ≤ 5 where WUi is the root water uptake (both evaporation and transpiration) from the ith soil layer (i.e., WU1 , WU2 , WU3 , WU4 and WU5 are the root water uptake from 0–20, 20–40, 40–60, 60–80 and 80–100 cm soil depths, respectively). Cumulative values of soil water balance components could be calculated as a summation of daily values over the considered period (i.e., during the stress period or the whole growing season). Measuring N uptake Three plants per plot were sampled 66, 74, 81 and 89 d after planting to determine the N concentration of leaves by the Kjeldahl method (Keeney and Nelson, 1982). Also, at the final harvest, the total N concentration in the aboveground plants (including leaves, stem and yield) was determined as well; root N was not measured in this study. Total N uptake (TNU) in the aboveground plants was calculated by multiplying the total N concentration in the aboveground plants by the biological yield. Mosier et al. (2004) described four agronomic indices commonly used to describe nutrient use efficiency: partial factor productivity (PFP, kg crop yield per kg applied nutrient); agronomic efficiency (AE, kg crop yield increase per kg applied nutrient); apparent recovery efficiency (ARE, kg nutrient taken up per kg applied nutrient); and physiological efficiency (PE, kg yield increase per kg nutrient taken up). Apparent recovery efficiency (ARE, the ratio of N uptake to total N fertilizer applied (200 kg N ha−1 )) was used in this study to determine N fertilizer use efficiency.

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Statistical analysis Plant data collected in this study were subjected to the analysis of variance (ANOVA) procedure (SAS Institute Inc., 1988). Appropriate standard errors of means were calculated. Duncan’s test was applied to compare measured parameters from plants that had experienced different irrigation treatments (P = 0.05). RESULTS AND DISCUSSION Irrigation depth The daily irrigation requirements for all treatments (Fig. 5) included the cumulative irrigation water requirement. During the stress period, the FI, PRD and DI treatments received 372, 279 and 279 mm, respectively. During the whole growing season, the FI, PRD and DI treatments received 613, 520 and 520 mm, respectively. Compared with the FI treatment, the PRD and DI treatments saved 15.2% of irrigation water during the whole growing season (from 1 to 107 d after planting); i.e., all treatments were fully irrigated from 1 to 55 d after planting and irrigation treatments were applied from 55 to 107 d after planting (the stress period). During the stress period, the PRD and DI treatments received 75% of irrigation water of the FI treatment. Thus, despite 25% water saving during the stress period, the amount of water saved in the PRD and DI treatments during the whole growing season would be 15.2% compared to the FI treatment. Soil temperature The absolute values of the soil temperature varia-

tions (|∆T |) due to irrigation in all calculating pixels are summarized in Table III for all irrigation treatments. Regardless of the treatments, the highest |∆T | in each soil layer was observed in the place of emitting dripper, accounting for about 50% of the whole |∆T | in each soil layer; the longer the distance from the emitter, the less the soil temperature variation. For the FI and DI treatments, the lowest |∆T | occurred at 10 cm apart from the emitting dripper, while for the PRD treatment, the least |∆T | occurred in the place of the off drippers for all soil layers. In addition, regardless of the lateral positions, surface soil layers had more contribution of |∆T | compared with the deeper ones. Over 90% of the whole |∆T | occurred at the 0–60 cm soil depth after each irrigation event. The highest and lowest |∆T | occurred at the 0–20 and 80–100 cm soil depths, respectively, for all treatments. Higher |∆T | near the emitting drippers and in the surface soil layer could be attributed to the higher soil water content variations at the considered places after each irrigation event. Many studies have reported a significant linear relation between soil temperature variations with respect to soil water content variations (Nainanayake et al., 2009; Roxy et al., 2010). Duna et al. (2010) have reported higher soil temperature fluctuations at the early maize growth stages due to more soil water content variations at the considered growth stage. Dourado-Neto et al. (1999) and Hlavinka et al. (2009) have also reported higher soil temperature fluctuations under higher soil water content variations due to irrigation, precipitation or root water uptake. In agreement with the literature, a strong linear regression was observed between |∆T | and the absolute soil water

Fig. 5 Daily and cumulative irrigation requirements for all irrigation treatments, full irrigation (FI), partial root-zone drying (PRD) and deficit irrigation (DI), during the whole growing season.

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TABLE III Soil temperature variations after irrigation events with different soil depths and lateral positions for all irrigation treatments during the stress period Treatmenta)

Depth

|∆T |b) in different lateral positionsc) Position A

FI

PRD

DI

cm 0–20 20–40 40–60 60–80 80–100 Average 0–20 20–40 40–60 60–80 80–100 Average 0–20 20–40 40–60 60–80 80–100 Average

Position B

Position C

Position D

Position E

Average

3.360 1.256 0.567 0.031 0.000 1.043 3.896 1.715 1.447 0.069 0.015 1.429 2.509 0.930 0.414 0.012 0.000 0.773

4.317 1.791 0.605 0.031 0.000 1.349 5.006 2.633 2.021 0.184 0.031 1.975 3.226 1.332 0.442 0.012 0.000 1.003

3.475 1.447 0.368 0.025 0.000

◦C

4.241 1.791 0.567 0.031 0.000 1.326 0.015 0.000 0.000 0.000 0.000 0.003 3.169 1.332 0.414 0.012 0.000 0.985

3.514 1.332 0.031 0.015 0.000 0.978 0.031 0.015 0.015 0.000 0.000 0.012 2.624 0.988 0.012 0.006 0.000 0.726

1.944 1.064 0.069 0.015 0.000 0.619 1.370 0.949 0.375 0.031 0.000 0.545 1.447 0.787 0.041 0.006 0.000 0.456

2.064 1.063 0.772 0.057 0.009 2.595 1.074 0.264 0.009 0.000

a) FI

= full irrigation; PRD = partial root-zone drying; DI = deficit irrigation. absolute value of the soil temperature variations (|∆T |) due to irrigation was calculated using the difference of measured soil temperature at one hour before and after each irrigation event during the stress period (from 55 to 107 d after planting). c) Position A was the place of the off dripper, Positons B, C, D and E were 5, 10, 15 and 20 cm apart from the off dripper in the PRD treatment, respectively. Both emitters in Positions A and E were the emitting drippers in FI and DI treatments; just the emitter in Position E was the emitting dripper in the PRD treatment.

b) The

variations (|∆θ|) in this study (R2 = 0.915, root mean square error (RMSE) = 1.6 ◦ C) (Fig. 6).

dripper (Position E) and 5 cm apart from it (Position D) in the PRD treatment was significantly higher than those in the FI and DI treatments. The average |∆T | in the place of emitting dripper (Position E) and in Position D in the PRD treatment was 1.46 and 1.37 times higher than that in the corresponding side of the FI treatment, and was 1.97 and 1.85 times higher than that in the corresponding side of the DI treatment, respectively (Table IV). No significant difference was observed between the FI and PRD treatments in term TABLE IV Statistical comparison of soil temperature variations at 0–100 cm soil depth in Positions C, D and E for different irrigation treatmentsa) Position

Sum of |∆T |b) at 0–100 cm depth FI

Fig. 6 Relationship between absolute soil temperature variations (|∆T |) and absolute soil water content variations (|∆θ|).

Table III well reflected the considerable differences in the lateral and vertical |∆T | between the PRD and FI (or DI) treatments. Table IV shows the statistical comparison of the average |∆T | in different lateral positions among all treatments. For the entire 0–100 cm soil profile, the average |∆T | in the place of emitting

PRD

DI

◦C

C D E a) FI

0.62ac) 1.04b 1.35b

0.55a 1.43a 1.98a

0.46b 0.77c 1.00c

= full irrigation; PRD = partial root-zone drying; DI = deficit irrigation. b) See Table III for the description of |∆T |. c) For each position, the same letter indicates no significance difference between treatments at P = 0.05.

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of the average |∆T | in Position B. However, the average |∆T | in Position B in the PRD treatment was significantly (1.2 times) higher than that in the DI treatment. These results confirm a larger wetting front advance in the PRD treatment, which partly could be due to the significant linear relation between |∆T | and absolute soil water content variation (|∆θ|) observed in this study (Fig. 6). The PRD treatment also caused a significant increase in the vertical |∆T | at the wetted side compared with the FI and DI treatments (Table V). For all soil depths, |∆T | at the wetted side of the PRD treatment was significantly higher than that at the corresponding side of the DI and FI treatments. An exception was for the 0–20 cm soil depth, at which no significant difference in term of average |∆T | was observed between the FI and PRD treatments, while this difference was significant between the PRD and DI treatments as well as between the FI and DI treatments. The significant difference between the FI and DI treatments could be justified by the favorable soil water content condition in the FI treatment, which consequently affected soil temperature condition. The values of the vertical |∆T | at soil depths of 0–20, 20–40, 40–60 and 60–80 cm at the wetted side of the PRD treatment were on average 1.07, 1.29, 3.10 and 3.67 times higher than those of the corresponding soil depths in the FI treatment, respectively, and were on average 1.43, 1.74, 4.29 and 9.63 times higher than those of the corresponding soil depths in the DI treatment, respectively (Table III). TABLE V Statistical comparison of soil temperature variations at the wetted side (Positions C, D and E) of the partial root-zone drying (PRD) treatment and those at the corresponding side of the full irrigation (FI) and deficit irrigation (DI) treatments at different soil depths Soil depth

Sum of |∆T |a) in Positions C, D and E FI

cm 0–20 20–40 40–60 60–80 80–100

PRD

DI

◦C

3.21ab) 1.37b 0.41b 0.03b 0.00b

3.42a 1.77a 1.28a 0.09a 0.02a

2.39b 1.02c 0.30b 0.01c 0.00b

Table III for the description of |∆T |. each soil depth, the same letter indicates no significant difference between treatments at P = 0.05. a) See

b) For

The average daily soil temperature at different soil depths of all treatments during the stress period is given in Fig. 7. The average daily soil temperature was affected by the different irrigation regimes. The soil

temperature in the FI treatment ranged from 27.3 to 34 ◦ C with a mean value of 29.8 ◦ C for the whole 0–80 cm soil depth during the stress period. The maximal, minimal and average soil temperatures at the considered soil depth were 34, 25.2 and 28.5 ◦ C, respectively, for the PRD treatment and were 34, 29.8 and 31.5 ◦ C, respectively, for the DI treatment during the stress period. For over 60% of the stress period, the average soil temperature in the whole root zone for the PRD treatment lay below the threshold of maximum favorable soil temperature for maize root growth (i.e., 28 ◦ C). This period was considerably longer than those for the FI and the DI treatments. The PRD also caused a better soil temperature condition for deeper soil layers. The average soil temperatures at soil depths of 0–20, 20–40, 40–60 and 60–80 cm in the PRD treatment during the stress period were 29.5, 28.2, 27.1 and 29.3 ◦ C, respectively, which were about 2% to 13% lower than those in the FI and DI treatments. The higher difference of soil temperature was observed at soil depths of 40–60, 60–80 and 80–100 cm compared with those at soil depths of 0–20 and 20–40 cm. Regardless of the treatments, Fig. 8 shows different vertical patterns of soil temperature. The soil temperature at the 0–20 and 20–40 cm soil depths seemed to have more fluctuations, as compared with deeper soil depths, probably due to more effect of air temperature and irrigation (Dourado-Neto et al., 1999; Hlavinka et al., 2009) than those in the deeper layers. The measured daily soil temperature before the irrigation event increased with increasing soil depth up to the 40–60 cm soil depth and thereafter, it decreased with increasing soil depth. A best-fitted third-degree polynomial function could be fitted on the average values of soil temperature for the different soil depths during the stress period (Fig. 8). Different conditions in terms of soil temperature under PRD could be ascribed to the fact that only half the root system was irrigated at each irrigation event. This could markedly influence the lateral and vertical advance of the wetted side and subsequently may then affect the soil temperature especially in deeper soil layers. The daily monitoring of soil water content revealed that the lateral and vertical water movement after each irrigation event was higher in the PRD treatment (data not shown), which consequently led to lower soil temperature compared with the FI and DI treatments. Some other studies also reported that a suitable irrigation management would prevent increasing soil temperature over the favorable maximum soil temperature for root growth (Nainanayake et al., 2009; Roxy et al.,

SOIL TEMPERATURE, N UPTAKE AND IRRIGATION

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Fig. 7 Daily changes of soil temperature during the stress period (from 55 to 107 d after planting) at different soil depths for all irrigation treatments, full irrigation (FI), partial root-zone drying (PRD) and deficit irrigation (DI). Soil temperature data are measured at one hour before each irrigation event.

2010) due to increasing thermal conductivity after watering the dried soil. Kasubuchi (1975) has reported that higher thermal conductivity of more wetted soils would control soil temperature in deeper soil layers. Soil temperature for maize root growth was better during the stress period (Fig. 7). Results of some studies demonstrated that maize root growth generally

starts at about 9 ◦ C and reaches a maximum growth rate at 28 ◦ C (Walker, 1969; Barber et al., 1988). Lower soil temperature than 9 ◦ C has been reported to decrease the maize root water uptake by decreasing water velocity (Kozlowski and Pallardy, 1997) and root permeability (Lambers et al., 2008) or through affecting the structure and function of cell membranes (Kozlo-

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Fig. 8 Vertical pattern of average soil temperature during the stress period (from 55 to 107 d after planting) for all irrigation treatments, full irrigation (FI), partial root-zone drying (PRD) and deficit irrigation (DI). Each point noted the daily average of measured soil temperature at the considered depth during the stress period. Vertical bars indicate the domain of the soil temperature variation at the considered soil depth for different days during the stress period. Lateral bars indicate the soil depth intervals.

F. KARANDISH AND A. SHAHNAZARI

PRD treatment and from the corresponding side of the FI and DI treatments, and the results were illustrated as in Fig. 9. Applying PRD had a considerable effect on root WU. The values of root WU at the wetted side of the PRD treatment were considerably higher than those at the corresponding side of the FI and DI treatments. Totally, 300.6 mm water was taken up from the wetted side of the PRD treatment at the 0–100 cm soil depth during the stress period, which was on average 1.50 and 1.75 times higher than those from the corresponding side of the FI and DI treatments, respectively. The difference between root WU from the wetted side in the PRD treatment and from the corresponding side of the FI treatment was more noticeable at the 40–60 cm (WU3 ) and 60–80 cm (WU4 ) soil depths. The values of WU3 and WU4 at the wetted side of the PRD treatment were 51.39 and 6.18 mm, respectively, which were 3.54 and 3.1 times higher than those in the corresponding side of the FI treatment, respectively, and were 4.11 and 4.12 times higher than those in the corresponding side of the DI treatment, respectively. The values of WU3 and WU4 in the half of the root zone in the FI treatment were 14.6 and 2 mm, respectively (Fig. 9). Nevertheless, totally, 2.75 mm water was taken up from the 80–100 cm soil depth (WU5 ) in the PRD treatment, which was less than 1% of the whole root water uptake in this treatment during the stress period. However, no water was taken up from the 80–100 cm soil depth of the FI and DI treatments. The cumulative water uptake (CET) during the stress period from the whole root zone (i.e., both wetted and dried sides) were 400, 375 and 340 mm for the FI, PRD and DI treatments, respectively (Table VI). Statistical analysis revealed that there was no significant differe-

wski and Pallardy, 1997). Also, an increase in soil temperature beyond the favorable maximum threshold (28 ◦ C) can decrease the root water uptake (Nainanayake et al., 2009). Based on this knowledge, it is evident that a better condition in term of soil temperature for root growth was provided due to different soil water conditions in the PRD treatment (Fig. 7), which was reflected in the water and nutrition uptake increase as shown below. Water and N uptake and grain yield Root water uptake at each soil depth was measured during the stress period from the wetted side of the

Fig. 9 Cumulative root water uptake at different soil depths from the wetted side of partial root-zone drying (PRD) treatment and from the corresponding side of full irrigation (FI) and deficit irrigation (DI) treatments during the stress period (from 55 to 107 d after planting).

SOIL TEMPERATURE, N UPTAKE AND IRRIGATION

TABLE VI Total nitrogen uptake (TNU) in the aboveground plants, leaf nitrogen content (LN), apparent recovery efficiency (ARE), cumulative evapotranspiration (CET) during the stress period (from 55 to 107 d after planting) and grain yield (GY) for all irrigation treatmentsa) Parameter ha−1 )

TNU (kg LN (g kg−1 )

AREd) (kg kg−1 ) CET (mm) GY (Mg ha−1 )

Measuring time

FI

PRD

DI

At harvest 66 DAPc) 74 DAP 81 DAP 89 DAP At harvest For stress period At harvest

178ab)

174a 31.4a 26.7a 19.4a 16.4a 0.87a 375a 6.9a

161b 25.8c 23.1b 15.6b 14.3b 0.80b 340b 5.8b

28.8b 25.7a 18.1a 15.8a 0.89a 400a 7.0a

a) FI = full irrigation; PRD = partial root-zone drying; DI = deficit irrigation. b) Means followed by the same letters in a row are not significantly different between treatments at P = 0.05. c) Days after planting. d) ARE was calculated as N taken up per kg N applied (200 kg N ha−1 ).

nce in CET between the FI and PRD treatments. However, this difference was significant between the FI and DI treatments and also between the PRD and DI treatments. Both the FI and PRD treatments caused significantly higher root WU than the DI treatment (Table VI). Many studies have reported a significant reduction in root WU under DI (Gavloski et al., 1992; NeSmith and Ritchie, 1992; Jama and Ottman, 1993; Traore et al., 2000; Shahnazari et al., 2007). The enhanced root WU in the PRD treatment is in agreement with the findings of some other studies (Ponie et al., 1992; Fort et al., 1997; Rodriguez and Grady, 2000; Kang et al., 2002; Kang and Zhang, 2004; Liu et al., 2006; Sepaskhah and Ahmadi, 2010). A plausible cause of higher soil water extraction may lie in a considerable change in root system in the PRD treatment. Kang and Zhang (2004) reported that PRD could enhance the extension and initiation of primary and secondary roots and thus could increase root dry mass and hydraulic conductivity, enhancing the WU rate of plants at the wetted side of the PRD treatment. The result was supported by Sepaskhah and Ahmadi (2010) and Liu et al. (2006). In addition, the root penetration to deeper soil layers under PRD has a great effect on increasing root WU. Fort et al. (1997) and Ponie et al. (1992) observed that root systems extend to deeper soil layers under PRD and induce the initiation and growth of secondary roots. Although root growth was not measured in this study, higher root WU at the wetted side of the PRD treatment confirmed the root system improvement (Fig. 9). The higher contribution of deeper soil layers to the total

883

water depletion in the PRD treatment also implies the higher root density and consequently higher root surface area, which has been reported to have a great effect on the water and nutrition uptake from the soil matrix (Wang et al., 2012). Considerable increases in root WU from the 60–80 and 80–100 cm soil depths for the PRD treatment also confirmed higher root extension to deeper soil layers in this treatment (Fig. 9). As noted above, improved root growth could partially be ascribed to the more favorable soil temperature in the PRD treatment. Soil temperature can strongly influence root initiation and growth (Hogue and Neilson, 1986; Tagliavini at al., 1991; McMichael and Burke, 1998; Dong et al., 2001). Many studies have also reported that the pattern of root growth could be changed due to different soil temperatures (Psarras et al., 2000; Lahti et al., 2005; Callejas et al., 2009). Maize roots tended to grow more horizontally under the more favorable soil temperature in the surface soil layers (King, 1892). Roots will penetrate to the deeper soil layers if there is a favorable soil temperature condition in the sub-soil (Callejas et al., 2009), consequently resulting in higher WU from the deeper soil layers (Pregitzer et al., 2000; Puhe, 2003). In addition to higher root WU, N uptake in the PRD treatment also improved partially due to more favorable soil temperature. Table VI shows leaf nitrogen content (LN) at different sampling dates, total nitrogen uptake (TNU) in the aboveground plants at harvest, and N fertilizer ARE. Despite lower irrigation water application, no significant reduction in LN was observed in the PRD treatment compared with the FI treatment. In addition, LN in the PRD treatment was slightly higher than that in the FI treatment 74, 81 and 89 d after planting (not significant), and was significantly higher than that for the FI treatment 66 d after planting. Water stress under the DI treatment caused a significant reduction in LN compared to that for FI or PRD treatments. Total N uptake (TNU) in the aboveground plants were 178, 174 and 161 kg ha−1 in the FI, PRD and DI treatments, respectively. Statistical analysis showed no significant reduction in TNU between the FI and PRD treatments. Total N uptake in the PRD treatment was significantly (about 8.1%) higher than that in the DI treatment. The FI treatment also led to significantly higher TNU compared with the DI treatment, which could be ascribed to favorable soil water content condition in the root zone, consequently leading to better soil temperature condition compared with the DI treatment. Consistent with TNU, no significant reduction in ARE was observed in the PRD treatment

884

(0.87 kg kg−1 ) compared with the FI treatment (0.89 kg kg−1 ) during the whole growing season. Also, the weekly analysis of nutrition uptake showed the considerably higher ARE in the PRD treatment compared with the FI treatment during the stress period (data not shown). Nevertheless, ARE in the DI treatment was significantly (9% and 10.1%) lower than that in the PRD and FI treatments, respectively. Higher LN and TNU in the PRD treatment were in consistent with the findings of Kirda et al. (2004) and Shahnazari et al. (2008), who reported that PRD had better N uptake with minimal mineral N residual left in the soil after maize harvest. Higher LN and TNU in the PRD treatment could be ascribed to an increase in available N in the soil (Kirda et al., 2004; Shahnazari et al., 2008; Hu et al., 2009; Wang et al., 2009; Wang et al., 2012). Haverkort et al. (2003) have reported that N availability increases the length of vegetation growth and postpones leaf senesce due to more N nutrition in leaves. Improved N availability and consequently higher N uptake could partially be ascribed to the more favorable soil temperature under PRD treatment. Dong et al. (2001) reported the major role of soil temperature on the N availability and subsequent N recovery from the soil. Alphalo et al. (2006) also reported increases in root water and nutrient uptake due to a better N availability under more favorable soil temperature conditions. They observed that an unfavorable soil temperature would result in a significant reduction in the leaf chlorophyll and stomatal conductance and an increase in cytoplasm resistance and water potential. Many studies have reported a significant relation between the leaf N and chlorophyll levels (Rodriguez and Grady, 2000; Gianquinto et al., 2003). Soil temperature could affect the soil N availability through two ways: i) by promoting the root growth and ii) by modifying the nitrification rate. Soil N availability has a close relation with the rate of nitrification, which is highly associated with the soil temperature. Sakai (1959) has reported that increasing soil temperature from 25 to 35 ◦ C would delay the nitrification in three different soils. They have reported 25 ◦ C as the most favorable soil temperature for nitrification. The PRD treatment had the closest values of soil temperature to 25 ◦ C as shown in Fig. 7. The stimulation of nitrification under PRD has been reported by some other researchers (Nourbakhsh and Karimian Eghbal, 1997; Shahnazari et al., 2008). Based on this knowledge, the lower LN in the DI treatment could partially be ascribed to the more unfavorable soil temperature in the root zone compared with that in the PRD treatment.

F. KARANDISH AND A. SHAHNAZARI

Therefore, it could be concluded that a more favorable soil temperature could result in improving root growth and nitrification rate and consequently higher root water uptake, better N nutrition uptake in leaves and more plant N accumulated, which all have been reported to be the major driving factors on grain yield, by increasing the vegetation growth and maintaining a greater photosynthesis rate (Varvel et al., 1997; Gianquinto et al., 2003). This is in agreement with the results of the present study, in which no significant reduction in grain yield occurred between the FI (7 t ha−1 ) and PRD (6.9 t ha−1 ) treatments, whereas a significant reduction was observed in the DI treatment (5.8 t ha−1 ). Ismail et al. (2007) found that soil temperature plays a major role in tomato yield. They found a significant reduction in tomato yield under higher irrigation frequency than that under the less one due to the higher soil temperature. CONCLUSIONS Of the three irrigation treatments applied from 55 to 107 d after planting the DI and PRD treatments resulted in 15.2% water saving. In all treatments, the places closer to the emitting dripper had higher soil temperature fluctuations, with the maximum in the surface soil layer. Nevertheless, comparatively, the highest soil temperature fluctuations were observed in the PRD treatment, which resulted in soil temperature within a favorable domain for root growth even in deeper soil layers, and consequently, the promotion of the root system enhanced the root water uptake after reirrigating the dried side. Also, a better N availability in the PRD treatment, which was due to the improved root growth (or function) and increased nitrification rate at more favorable soil temperature, caused a significant increase in plant N nutrition and consequently prevented considerable grain yield reduction in the PRD treatment compared to the FI treatment. Thus, based on the results, it could be concluded that PRD is the best water saving method in the study area. ACKNOWLEDGEMENT The authors would like to appreciate Sari Agricultural Sciences and Natural Resources University (SANRU), Iran for giving the site for field investigation and to appreciate University of Zabol for financial support of this research. REFERENCES Aphalo P J, Lahti M, Lehto T, Repo T, Rummukainen A, Man-

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