Effects of reduced nitrogen rate on cotton yield and nitrogen use efficiency as mediated by application mode or plant density

Field Crops Research 218 (2018) 150–157

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Effects of reduced nitrogen rate on cotton yield and nitrogen use efficiency as mediated by application mode or plant density ⁎

Zhen Luoa,b, Hua Liuc, Weiping Lid, Qiang Zhaoe, Jianlong Daib, , Liwen Tianf, Hezhong Donga,b,

T

⁎

a

School of Life Sciences, Shandong University, Jinan, 250100, Shandong, China Cotton Research Center, Shandong Academy of Agricultural Sciences, Jinan, 250100, Shandong, China c Institute of Soil Fertilizer and Agriculture Water Conservation, Xinjiang Academy of Agricultural Sciences, Urumqi, 830091, Xinjiang, China d Institute of Agricultural Sciences, Bayinguoleng Autonomous Prefecture, Korla, 841000, Xinjiang, China e College of Agronomy, Xinjiang Agricultural University, Urumqi, 830052, Xinjiang, China f Institute of Cash Crops, Xinjiang Academy of Agricultural Sciences, Wulumuqi, 830091, Xinjiang, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Cotton Drip fertigation Nitrogen rate Plant density Leaf senescence Nitrogen use efficiency

Nitrogen (N) fertilization plays an important role in yield formation of field-grown cotton (Gossypium hirsutum L.), but little is known of its interaction with mode of application or plant density under irrigated production. Our objective was to determine the effects of N application rate on cotton yield, leaf senescence and N use efficiency as mediated by mode of application and plant density. To achieve this goal, two field experiments which were conducted from 2015 to 2016 using a split-plot design in randomized complete blocks. In the first experiment, the main plots were assigned to N application modes (conventional application and drip fertigation) and the subplots to N rates (375, 319, 264 and 0 kg N/ha). In the second experiment, the main plots were assigned to plant density (12 plants/m2-low density and 19.5 plants/m2-high density) and the subplots to N rates (330, 264 and 0 kg N/ha). The N rate of 264 kg/ha under drip fertigation or high plant density did not reduce cotton yield. Agronomic nitrogen use efficiency (aNUE) and nitrogen recovery efficiency (NRE) were the highest at 264 kg N/ha under drip fertigation and high plant density. Although a reduced N rate increased boll load, fertigation or high plant density relatively reduced boll load and delayed late-season leaf senescence as indicated by the increased photosynthetic rate and chlorophyll content as well as the reduced malondialdehyde concentration compared to conventional application or low plant density. The yield stability across N rates (264–375 kg N/ha) was probably due to the delayed leaf senescence and improved N use efficiency. The results suggest that the N rate could be reduced to 264 kg/ha, or 20–30% from the traditionally recommended rate, without sacrificing yield under high plant density or drip fertigation. These results are beneficial to the formulation of a scientific and rational use of N fertilizer for sustainable cotton production and environmental health.

1. Introduction The northwest inland is currently the largest cotton-growing area in China (Dai and Dong, 2014). Holding abundant sunshine and large temperature difference between day and night, this inland is an arid area with irrigated agriculture, where drip irrigation under plastic mulch has been widely adopted for cultivation of cotton (Cao et al., 2012). Despite being one of the most dominant cotton growing areas with relatively high yield and fine quality in China, low N use efficiency resulting from irrational N application have imposed a great challenge to sustainable cotton production in the area (Ju et al., 2009; Li and Zhang, 2013; Zhang et al., 2014). Therefore, it is necessary to explore the agronomic factors affecting fertilizer use efficiency in the area, so as ⁎

to reduce fertilizer inputs without yield reduction. Nitrogen is an essential macronutrient, required most consistently and in larger amounts than other nutrients for cotton production (Hou et al., 2007). Its application can enhance canopy area, photosynthesis, lint yield, fiber quality and resistance to abiotic stresses such as salinity and drought (Bondada et al., 1996; Chen et al., 2010). Thus, N nutrition is one of the most pivotal facets of cotton production (Bondada and Oosterhuis, 2001). Deficient N levels could lead to decreased boll production due to poor plant development and premature senescence (Dong et al., 2012). Consequently, N fertilizer is often applied excessively in the northwestern inland cotton regions (Mao, 2013). However, an over-dose of N will promote excessive vegetative development and delay maturity (Hodges, 2002). Studies in Australia have

Corresponding authors at: Cotton Research Center, Shandong Academy of Agricultural Sciences, Jinan, 250100, Shandong, China. E-mail addresses: [email protected] (J. Dai), [email protected] (H. Dong).

https://doi.org/10.1016/j.fcr.2018.01.003 Received 20 December 2017; Received in revised form 5 January 2018; Accepted 5 January 2018 Available online 30 January 2018 0378-4290/ © 2018 Elsevier B.V. All rights reserved.

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et al., 2012). Other studies in these regions have also indicated that fertigation can reduce the N input without sacrificing yield (Yan et al., 2015; Ma et al., 2016). Reports have indicated that drip fertigation can reduce N application rate and increase N use efficiency in cotton (BarYosef, 1999; Jayakumar et al., 2014). Therefore, we hyphotesized that a moderate reduction in N rate under high plant density or drip fertigation would not affect the yield of irrigated cotton in the northwest inland of China. The objectives of the present study were to determine: a) The effects of N rate on cotton growth, yield, yield components and nitrogen use efficiency as mediated by mode of application or plant density and b) If the rate of N application can be reduced without yield reduction under irrigated agriculture.

shown that about 20% of the N fertilizer inputs can be reduced without a reduction in yield (Rochester et al., 2009). Our study also suggested that N fertilizer can be used at a moderately lower rate and more efficiently than has been traditionally used in China (Dong et al., 2010). Reports from other countries further suggested that N inputs can be reduced and N use efficiency (NUE) increased, although the optimum N rate and use efficiency are affected by a number of factors like soil fertility, field management and yield potential (Boquet, 2005; Clawson et al., 2008; Janat, 2008; Kumbhar et al., 2008). There are several measures of NUE. Agronomic nitrogen use efficiency (aNUE) and nitrogen recovery efficiency (NRE) are the two most common measures. The NRE is the proportion of the applied N fertilizer that is taken up by the crop, expressed as a percentage of that applied. It indicates how well a crop uses the N fertilizer that has been applied (Rochester et al., 2007). The aNUE is defined as the increase in (seedcotton) yield per unit of fertilizer N applied (Novoa and Loomis, 1981). Maximal aNUE leads to a maximal value: cost ratio, an important economic indicator evaluating the investment benefits since both parameters are linearly related for specific input and output prices (Vanlauwe et al., 2011). It has been widely recognized that drip fertigation can increase water and nutrient use efficiency through improvement in crop yield per unit volume of water and nutrients (Bar-Yosef, 1999; Patel and Rajput, 2011). Drip fertigation not only results in good crop growth and yield advantage due to stable water content maintained near the root zone but also has the additional advantage as the water-soluble fertilizers can be injected in precise amounts (Ayyadurai and Manickasundaram, 2014). In the northwest inland of China, drip irrigation under plastic mulching, which saves water and increases water use efficiency as compared to furrow irrigation, is widely used (Hou et al., 2009). Compared with furrow irrigation, N use efficiency increased by 20–30% and cotton yield significantly increased due to increases in biological yield and leaf area index under drip fertigation (Li et al., 2004). The highest N recovery efficiency occurred at a lower N rate and cotton yield decreased significantly compared with high N rate under drip fertigation (Li et al., 2015). Consequently, an optimum N application rate under drip fertigation is important for achieving high cotton yield in the northwest inland of China. Cotton has perennial and indeterminate growth habit, consequently, it is exceedingly sensitive to environmental conditions and agronomic practices such as plant density (Darawsheh et al., 2009). The effects of plant density on cotton yield have been well documented (Bednarz et al., 2005; Feinerman, 1983; Keren et al., 1983). Although final lint yields in cotton were relatively stable across a wide range of plant densities through manipulation of boll occurrence and boll weight (Bednarz et al., 2000), the maximum lint yield can be achieved only at an optimum plant density (Feinerman, 1983), which depends on N application rate and other management practices (Zhang et al., 2011). Reports have indicated a significant interaction between plant density and N rate in the Yellow River Valley of China. Increased plant density is beneficial to cotton yields under low N rate and the yield increase due to plant density, N rate or their combinations was attributed to increases in boll number or boll weight (Dong et al., 2010). In the northwest inland of China, a new cultivation system of “short-denseearly” combined with drip fertigation has been well adopted (Tian, 2016), however, the effects of N rate, plant density and their interactions have been rarely studied under the new pattern. Thus it is necessary to clarify the effects of plant density and N fertilization rate on yield and N use efficiency, and most importantly, determine the optimum N rate for northwest inland of China. It has been widely believed that plant density, N rate and drip irrigation are three important agronomic factors for keeping high yield and sustainable development of cotton production in the northwest inland of China. A number of studies on rain-fed cotton in the Yellow River and Yangtze River valley regions have proved that the N rate can be reduced without reducing economic yield by appropriately increasing plant density in cotton (Dong et al., 2010; Mao, 2013; Wang

2. Materials and methods 2.1. Experimental sites and cultivars Two field experiments were carried out at different sites in the Northwest inland of China during the growing seasons of 2015 and 2016. The first experiment was located in southern Bayingolin city (39°51′N, 79°3′E), Xinjiang, China. The average annual sunshine duration is 4400 h with 225 d of frost-free crop growth season. Daily average temperature steadily above 10 °C starts in late March and ends in late October with a period of 210 d. Relative humidity during summer months is 40–50%, with an annual precipitation of 100.3 mm. The soil is sandy loam (the typical Xinjiang gray desert soil) with pH 8.0, organic matter 10.01 g/kg, total N 0.93 g/kg, available P 10.8 mg/ kg and available K 295 mg/kg. CRI 49, a dominant cotton cultivar in the local area, was used in the experiment. The second experiment was located in Dafeng Town (44°68′N, 87°12′E), Hutubi County, Xinjiang, China. The experimental area is in a warm-temperate arid zone with a continental climate, an average annual precipitation of 150–200 mm and an evaporation of 1600–2200 mm. Annual effective accumulated temperature is 3400–3584 °C. The soil is sandy loam (similar to the soil in the first experiment) with pH 8.0, organic matter 9.87 g/kg, total N 0.87 g/kg, available P 11.5 mg/kg and available K 308 mg/kg. Xinluzao 66, a dominant cotton cultivar in the local area was used in the experiment. 2.2. Experimental design and field management A split-plot design in randomized complete blocks with three replications was used for the first experiment. The main plots were assigned two N application modes (conventional application-30% N fertilizer plus all P and K fertilizers basally applied, and the balance 70% N applied at early flowering and drip fertigation −30% N, and all P and K used as basal fertilizer and the balance 70% N applied through drip fertigation with 15% at squaring, 27.5% at flowering and 27.5% at bollsetting). Subplots were assigned four N fertilizer (in the form of urea) rates: 375, 319, 264 and 0 kg N/ha, hereafter referred to N375, N319, N264, and N0. The N375 is the recommended rate in local high-yielding cotton fields (Tian, 2016), while the N319 and N264 are rates reduced by 15 and 30% relative to N375. Each subplot was 60 m2 (13.3 m × 4.51 m) and contained 6 rows with a plant population density of 18 plants/m2. The experiment was performed in 2015 and repeated in 2016. The second experiment was arranged into the same design as the first but the main plot was plant density (12 and 19.5 plants/m2), while N rates (0, 264 and 330 kg/ha) constituted the subplots, hereafter referred to N0, N264, and N330. As in the first experiment, 30% N fertilizer, and all P and K fertilizer were used as basal fertilizer and the balance 70% N fertilizer was applied through drip fertigation with 15% at squaring, 27.5% at flowering and 27.5% at boll-setting. Each subplot was 60 m2 (13.3 m × 4.51 m) and contained 6 rows at a plant population density of 12 or 19.5 plants/m2. The cotton cultivar, CRI 49 was used for experiment 1 while Xinluzao 66 was used for experiment 2; both were sown on 14–17th 151

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2.3.3. Total N uptake and N use efficiency For determination of total N uptake, samples of each plant part were milled with a Wiley mill and screened through a 0.5-mm sieve. Total N concentration was determined by the micro-Kjeldahl method (Bremner and Mulvaney, 1982). Two most common measures including agronomic nitrogen use efficiency (aNUE) and Nitrogen recovery efficiency (NRE) were used in this study. Agronomic nitrogen use efficiency (aNUE) [kg (kg N)−1] is defined as the increase in seedcotton yield per unit of fertilizer N applied (Novoa and Loomis, 1981):

April and harvested in early October of 2015 and 2016. About 80% of the soil surface was mulched with plastic film during the entire growing season. The experimental plots were managed uniformly in accordance with local agronomic practices to ensure full seedling establishment and normal plant growth. Each plot was fertilized with 600 kg/ha of P2O5 and 90 kg/ha of K2O as basal fertilizer. An independent drip fertigation system consisting of a water tank, a fertilizing tank and drip tubes was used for the drip-fertigated plots in the first experiment and for all the plots in the second experiment. The water tank filled with irrigation water was placed 1 m above the ground to maintain enough water pressure. Three drip taps were placed in each sub plot with each tap responsible for two rows of cotton. The balance 70% N fertilizer was applied through the fertilizing tank according to the experimental design. Drip fertigation started in mid June and ended in late August or early September for a total of 10 times in both experiments.

aNUE = (Yf − Y0)/Fappl. where Yf and Y0 refer to seedcotton yields [kg ha−1] in the treatment where fertilizer N was applied and not-applied; Fappl is the amount of fertilizer N applied [kg N ha−1]. NRE = (TNUf − TNU0)/N fertilizer rate.

2.3. Data collection

where TNUf is total N uptake of N-fertilized plots and TNU0 is total N uptake of zero-N plots (Dilz, 1988).

Data were collected for biological yield, boll load, seedcotton yield, harvest index, yield components, earliness, total N uptake, net photosynthetic (Pn) rate, chlorophyll (Chl) content and malondialdehyde (MDA) concentration.

2.4. Statistical analysis Analysis of variance was performed using DPS Data Processing System (Tang and Feng, 2002). The initial combined data showed no interactions with years. Therefore, the data were pooled and presented across the two years (Steel and Torrie, 1980). Means were separated using Duncan’s multiple range tests at the 5% probability level.

2.3.1. Yield, yield components, harvest index and earliness Plants from the central four rows of each plot were manually harvested two times in October. During each harvest, 50 bolls were randomly sampled per plot to determine boll weight. After sun-drying for ten days, seed cotton was weighed. Seedcotton yield (kg ha−1) as well as the average boll weight was determined for each plot. Earliness as indicated by the percentage of the first harvest yield to total yield of seedcotton was also determined. After complete harvest, 20 randomly selected plants were removed from each plot and weighed after sundrying for 30 d. Plant biomass and harvest index (seed cotton yield/ biological yield) were then determined.

3. Results 3.1. N fertilizer effects as mediated by N application rate and mode in the first experiment 3.1.1. Yield, yield components, earliness and harvest index The mode and rate of nitrogen application as well as their interaction significantly affected seedcotton yield (Table 1). Under conventional N application, the seedcotton yield was the highest at N375, and reduced by 3.8% under N319, 16.2% under N264 and 29.2% under N0. Under drip fertigation, the yields for N319 and N264 were comparable to that for N375, but 30.6% higher than that for N0. The seedcotton yields for N375 and N319 under drip fertigation were slightly higher (2.8 and 2.7%), and that for N264 under drip fertigation was comparable, to that for N375 under conventional application. There were no differences in the boll density among N319, N264 and N375, but that under N0 decreased by 24.9% compared with that under N375 under drip fertigation (Table 1). Boll density under N264 and N0 decreased by 12.2 and 23.9% relative to N375, while that under N319 was comparable to that under N375 treated to conventional N application. The boll weight decreased with decreases in N fertilizer rate regardless of application mode; however, the boll weight under N264 was 4.5% lower than that under N375 fertilized conventionally, and was similar to that under N375 supplied by drip fertigation. Reduced N rate was beneficial to earliness, especially under drip fertigation. For conventional application, earliness under N264 was 5.9% higher than that under N375 and 4.0% lower than that under N0; however, under drip fertigation, it was 8% higher than that under N375 and comparable to that under N0. The biological yield increased with increasing N fertilizer rate regardless of application mode (Table 1), but there was no difference between modes under N375 or N319; the biological yield of N264 increased by 17.6% under drip fertigation compared with conventional application. In contrast to biological yield, the harvest index (HI) decreased with increasing N rate (Table 1). The HI of N264 increased by 7.1 and 7.7% compared with those of N375 under conventional application and drip fertigation. The HI of N264 under drip fertigation was

2.3.2. Physiological measurement Physiological parameters including boll load, Pn, Chl content, and MDA concentration were determined at the start of boll opening, from three randomly selected plants in the central four rows of each plot. Leaf area was measured by passing the leaves through a LI-3100 leaf area meter (Li-Cor, Lincoln, NE, USA), and then leaf area index (LAI) was determined on a ground area basis. Dry weights of reproductive organs (squares, flowers, green and mature bolls) were weighed after drying for 48 h at 80 °C. Boll load was expressed as dry weight of reproductive organs per unit leaf area. The youngest fully expanded leaf on the main stem (3rd or 4th leaf from the main-stem terminal) was sampled to measure net Pn rate, Chl and MDA concentration. Leaf Pn was taken between 09:00 and 11:00 h on cloudless days when ambient photosynthetic photon flux density exceeded 1500 μM/m2/s1, using a LI-6400 portable photosynthesis system (Li-Cor, Lincolin, NE, USA). Three plants per replicate were examined and the mean values were calculated. Chl contents were determined according to He et al. (2002). Briefly 0.25 g of fresh leaves was placed in a 100 ml test tube. The tissues were homogenized with a polytron after 10–15 ml pure methanol was added. The homogenate was then filtered and diluted up to 100 ml with pure methanol. The Chl concentration in the supernatant was spectrophotometrically determined by measuring the absorbances at 652 nm and 665 nm for Chl a and Chl b, respectively. Three leaves per treatment were measured separately and the measurement was repeated twice. The MDA concentration used as an indicator of lipid peroxidation was measured following procedures described in the manufacturer’s guidelines of MDA assay kit (TBA method) (Nanjing Jiancheng Bioengineering Institute, China). 152

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Table 1 Effects of modes and rates of nitrogen application on cotton yield and yield components averaged for 2015 and 2016. Treatment combination Mode of application

N rate (kg/ha)

Traditional application

375 319 264 0 375 319 264 0

Through drip fertigation

Source of variance Year (Y) Application mode (M) N Rate (R) Y×M Y×R M×R Y×M×R

Biological yield (kg/ha)

Seedcotton yield (kg/ha)

Harvest index

Boll density (no./m2)

Boll weight (g)

Earliness* (%)

15135a** 14031b 11830c 9960d 15250a 14455ab 13910b 9785d

6155b 5921c 5155d 4360e 6330a 6320a 6220ab 4395e

0.407d 0.422bc 0.436b 0.438a 0.415c 0.434b 0.447a 0.449a

1022b 1009b 897c 777d 1050a 1057a 1051a 789d

6.02a 5.87b 5.75c 5.61d 6.03a 5.98a 5.92ab 5.57d

45.7d 47.2c 48.4b 50.4a 46.1d 48.2b 49.8a 50.1a

0.0451 0.0119 < 0.0001 ns ns 0.0171 ns

0.0022 0.0017 < 0.0001 ns ns < 0.001 ns

0.0156 0.0408 < 0.0001 ns ns ns ns

0.0078 0.0086 < 0.0001 ns ns < 0.0001 ns

0.0367 0.0741 < 0.0001 ns ns < 0.0001 ns

0.0472 ns < 0.0001 ns ns 0.0209 ns

“ns” is non-significant. * Earliness is expressed as percentage cotton of the first two harvests. ** Means within a column followed by same letters are not significantly different at p < 0.05.

fertigation. Unlike the conventional application, drip fertigation increased the NRE and aNUE across N rates. The NRE in drip-fertigated plots increased by 56.5, 28.7 and 17.0% at N264, N319 and N375 compared with that in conventional application. The aNUE under drip fertigation increased by 142.4, 29.9 and 13.4% under N264, N319 and N375 compared with conventional application. The boll load, net Pn (photosynthetic) rate and malondialdehyde (MDA) concentration were significantly affected by the mode and rate of N application and their interaction (Table 2). As expected, reduced N rate decreased the net Pn and Chl content, but significantly increased boll load. The Pn under N264 decreased by 12.6 and 7.2% compared with N375 under conventional application and drip fertigation. Consistent with Pn, the Chl content decreased with decreasing N rate and that under N264 decreased by 11.1 and 10.9% compared with that under N375 applied conventionally and by drip fertigation (Table 2). In contrast to Pn and Chl content, the MDA concentration increased with decreasing N rate and for drip fertigation, the MDA under N264

slightly higher (2.5%) than that under conventional application. 3.1.2. N use efficiency and leaf senescence Total N uptake, agronomic nitrogen use efficiency (aNUE) and N recovery efficiency (NRE) were all significantly affected by the rate and mode of application and their interaction (Table 2). Total N uptake decreased with decreasing nitrogen fertilizer rate. For conventional application, the N uptake increased by 68.3% under N375, 57.2% under N319 and 45.2% under N264; for drip fertigation, the increments were 79.7, 73.9 and 71.0%, respectively. The uptake under N264, N319 and N375 in drip-irrigated plots exceeded that in conventional application plots by 17.2 and 10.1 and 6.3%. There were no differences in NRE and aNUE between N375 and N319; for conventional application, both parameters decreased by 5.1 and 38.2% under N264 compared with N319. Both the NRE and aNUE increased with decreasing N rate under drip fertigation. The NRE under N319 and N264 increased by 9.1 and 25.9% and the aNUE by 16.9 and 34.8% compared with N375 under drip

Table 2 Effects of modes and rates of nitrogen application on cotton N use efficiency and leaf senescence averaged across 2015 and 2016. Treatment combination Mode of application

N rate (kg/ha)

Traditional application

375 319 264 0 375 319 264 0

Through drip fertigation

Source of variance Year (Y) Application mode (M) N Rate (R) Y×M Y×R M×R Y×M×R

Total N uptake (kg/ha)

N recovery efficiency(%)

N agronomic efficiency (kg/kg)

Pn (μmolCO2/ m2/s)

Chl (mg/g FW)

MDA (mmol/g FW)

Boll load* (g/ m2)

350b** 327c 302d 208e 372a 360ab 354b 207e

37.6d 37.3d 35.4e – 44.0c 48.0b 55.4a –

4.79d 4.89d 3.02e – 5.43c 6.35b 7.32a –

24.6c 23.2d 21.5e 18.2f 26.5a 25.5b 24.6c 17.6f

2.08b 1.92cd 1.85de 1.76e 2.28a 2.12b 2.03bc 1.78e

0.127d 0.132cd 0.148ab 0.157a 0.110e 0.121d 0.139bc 0.155a

195d 212c 229b 242a 158f 174e 189d 248a

0.0275 0.0109 < 0.0001 ns ns 0.0003 ns

0.0013 0.0002 < 0.0001 ns ns < 0.0001 ns

0.0215 0.0018 < 0.0001 ns ns < 0.0001 ns

0.0001 0.0103 < 0.0001 ns ns 0.0001 ns

0.0256 0.0104 < 0.0001 ns ns ns ns

0.0372 0.1196 < 0.0001 ns ns ns ns

0.0025 0.0202 < 0.0001 ns ns 0.0006 ns

“ns” is non-significant. * Boll load was expressed as dry weights of reproductive organs (g) with respect to the corresponding leaf area (m2) at peak boll-setting. Net photosynthetic rate (Pn), chlorophyll (Chl) and malondialdehyde (MDA) concentration of the main-stem leaves were measured at the onset of boll opening. ** Means within a column followed by same letters are not significantly different at p < 0.05.

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Table 3 Effects of plant density and N fertilizer rate on cotton yield and yield components averaged for 2015 and 2016. Treatment combination Plant density (plants/m2)

N rate (kg/ha)

12

330 264 0 330 264 0

19.5

Source of variance Year (Y) Plant density (D) N rate (R) Y×D Y×R D×R Y×D×R

Biological yield (kg ha−1)

Seedcotton yield (kg ha−1)

Harvest index

Boll density (no./m2)

Boll weight (g)

Earliness* (%)

13746b** 12488c 10551e 14386a 13399b 11804d

5636a 5345b 4653d 5625a 5601a 5005c

0.410c 0.428b 0.441a 0.391d 0.418b 0.424a

952b 933c 829e 970a 974a 891d

5.92a 5.73b 5.61b 5.80a 5.75ab 5.62b

46.5e 48.1d 49.7c 50.6c 52.2b 55.4a

0.0056 0.0461 < 0.0001 ns ns ns ns

0.0046 0.0014 < 0.0001 ns ns 0.0059 ns

0.0135 0.0205 < 0.0001 ns ns ns ns

0.00217 ns < 0.0001 ns ns < 0.0001 ns

0.00346 ns 0.0002 ns ns ns ns

0.0001 0.0082 0.0001 ns ns ns ns

“ns” is non-significant. * Earliness is expressed as percentage cotton of the first two harvests. ** Means within a column followed by same letters are not significantly different at p < 0.05.

plant density in total N uptake, but had 10.6% more uptake than the combination of N264 and low plant density. There was no difference in NRE between N330 and N264 under low plant density, while N264 increased NRE by 16.6% relative to N330 at high plant density. N264 decreased aNUE by 11.8% relative to N330 at low Plant density, but increased it by 20.6% relative to N330 at high plant density. Boll load, Pn, MDA concentration and Chl content in late season main-stem leaves were all significantly affected by N rate and plant density, but not by their interaction. The Pn increased with increasing plant density or N rate. It increased by 13.5% under high plant density under N264. Consistent with Pn, the Chl content increased with increasing plant density or N rate. It increased by 14.3% at high plant density under N264. However, the MDA concentration and boll load decreased with increasing N rate or plant density. The MDA was the lowest at high plant density and N330. High plant density decreased MDA by 9.0, 8.4 and 8.9% under N330, N264 and N0. It also decreased the boll load by 5.6, 6.8 and 10%, respectively.

increased by 26.4% compared with that under N375. The boll load increased with decreasing N rate. Under N319, N264 and N0, it increased by 8.7, 17.4 and 24.1% compared with that under N375 in conventionally applied plots; for the drip-fertigated plots, it increased by 10.1, 19.6 and 57.0%, respectively. Drip fertigation also decreased boll load compared with conventional application. 3.2. N fertilizer effects as mediated by N rate and plant density in the second experiment 3.2.1. Yield, yield components, earliness and harvest index Biological yield, seedcotton yield and harvest index were significantly affected by plant density and N fertilizer rate; seedcotton yield was also affected by their interaction (Table 3). The seedcotton yield was highest under N330 and lowest under N0 regardless of plant density. However, the reduced N rate (N264) decreased the seedcotton yield by 5.2% compared with N330 under low plant density, but not under high plant density. The N330 produced the most biological yield under high plant density and second highest yield under low plant density. Treatment with N264 and N0 decreased biological yield by 9.2 and 13.2% under low plant density and by 6.9 and 17.9% under high plant density compared with N330. However, the harvest index increased as N rate decreased. For N264 and N0 it increased by 4.4 and 7.6% under low plant density and by 6.9 and 8.4% under high plant density compared with N330. The boll density was significantly affected by plant density and N rate interaction (Table 3). For N330 and N264 the greatest boll density occurred under high plant density; low plant density slightly reduced the boll density. The 20% reduction in N rate (N264) did not decrease boll weight under high density, but decreased it by 3.2% under low density. Earliness was significantly affected by N fertilizer rate and plant density, but not by their interaction. Earliness increased with decreasing N rate or increasing plant density. On average, high plant density increased earliness by 5% compared with the low plant density, while N330 and N264 increased earliness by 3.8 and 9.7%, respectively.

4. Discussion This study has clearly shown the individual and interaction effects of N rate and application mode or plant density on plant biomass, yield, yield components, harvest index and N use efficiency of irrigated cotton. The most important findings were that a 30% N reduction (N264) did not adversely affect seedcotton yield under drip fertigation, but increased yield by 5 and 20.7% compared with N319 and N264 under conventional application. “Buffering” effects of plant density on seedcotton yield were also found in drip irrigated cotton in the second experiment. The relatively low N rate (N264) did not reduce seedcotton yield compared with N330 under high plant density, but reduced yield by 5.2% compared with N 330 under low plant density. The phenomenon of reduced N rate without yield reduction (yield stability) was probably attributed to the delayed late-season leaf senescence and improved N use efficiency (aNUE and NRE). This study has also provided new insights into the relationship between leaf senescence and N fertigation or plant density in an arid area. Although reduced N rate seemed to increase late-season leaf senescence as a result of the increased boll load, drip fertigation or high plant density significantly delayed leaf senescence to some extent, compared to the conventional application or low plant density, and maintained a relatively higher photosynthetic capacity in the late season. This can be regarded as one of the important mechanisms for saving N fertilizer without yield reduction under fertigation or dense seeding.

3.2.2. N use efficiency and leaf senescence Total N uptake, aNUE and NRE were all significantly affected by Plant density and nitrogen fertilizer rate; aNUE and NRE were also significantly affected by their interactions (Table 4). As expected, high plant density or N rate favored N uptake. The uptake increased by 77.1 and 62.5% under N330 and N264 at low Plant density; it increased by 74 and 69% under N330 and N264 at high Plant density. The combination of N264 and high plant density did not differ from that of N330 and high 154

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Table 4 Effects of plant density and N fertilizer rate on cotton N use efficiency and leaf senescence averaged for 2015 and 2016. Treatment combination Plant density (plants/m2)

N rate (kg/ ha)

12

330 264 0 330 264 0

19.5

Source of variance Year (Y) Plant density (D) N Rate (R) Y×D Y×R D×R Y×D×R

Total N uptake(kg/ ha)

N recovery efficiency(%)

N agronomic efficiency (kg/kg)

Pn (μmolCO2/m2/ s)

Chl (mg/g FW)

MDA (mmol/g FW)

Boll load *(g/ m2)

340b** 312c 192d 355a 345ab 204d

44.8b 45.5b – 45.8b 53.4a –

2.98a 2.62b – 1.88d 2.26c –

21.5a 19.2b 16.5c 22.6a 21.8a 18.7b

1.95bc 1.82cd 1.65e 2.16a 2.08ab 1.78de

0.133cd 0.142bc 0.158a 0.121e 0.130de 0.144b

214cd 234b 249a 202d 218c 224bc

0.0035 0.0221 < 0.0001 ns ns ns ns

0.0057 0.0379 0.0015 ns ns 0.0063 ns

0.0468 0.0016 0.8081 ns ns 0.0049 ns

0.0021 0.0449 0.0002 ns ns ns ns

0.0328 0.0008 0.0003 ns ns ns ns

0.0058 0.0111 0.0002 ns ns ns ns

0.0001 0.0058 0.0006 ns ns ns ns

“ns” is non-significant. * Boll load was expressed as dry weights of reproductive organs (g) with respect to the corresponding leaf area (m2) at peak boll-setting. Net photosynthetic rate (Pn), chlorophyll (Chl) and malondialdehyde (MDA) concentration of the main-stem leaves were measured at the onset of boll opening. ** Means within a column followed by same letters are not significantly different at p < 0.05.

1997) due to corresponding changes in reproductive partitioning of dry matter (Ali et al., 2009; Darawsheh et al., 2009). In the present study, the decreased N fertilizer rate reduced the biological yield but increased the harvest index as reported previously (Ali et al., 2009; Dong et al., 2012). Previous studies have also indicated that high cotton yield can be achieved under adequate dry matter production and harvest index (Mao, 2013). In our first experiment, a 15–30% N reduction under drip fertigation did not decrease biological yield but increased the harvest index significantly. Although a 20% N reduction decreased biological yield, the harvest index increased significantly under high density. The yield stability with reduced N rate under drip fertigation was achieved by maintaining a moderate biological yield and relatively more partitioning of assimilates to reproductive organs as indicated by a relatively higher harvest index; the yield stability with reduced N rate under high plant density was achieved mainly through the increased harvest index. The results further reinforced the strong compensatory ability of cotton under different N application modes and plant density.

4.1. Cotton yield and harvest index Irrigation and N fertilization are essential for cotton growing in arid regions like northwestern China (Li et al., 2016). Under drip fertigation, water-soluble fertilizers can be injected through the system in precise quantities (Patel and Rajput, 2011; Ayyadurai and Manickasundaram, 2014). Thus drip fertigation under mulching has been widely adopted in the northwest inland of China in recent years (Dai and Dong, 2014). Plant density is also one of the important factors regulating cotton yield in such an area (Ma and Sun, 2013). However, there has been a controversy about the optimum amount of N fertilizer and plant density in the northwest inland of China (Zheng et al., 2000; Deng et al., 2013; Chen et al., 2014; Bai et al., 2017), mainly because the individual effects of nitrogen application mode, N fertilizer rate and plant density on cotton yield and N use efficiency have been well documented (Bondada et al., 1996; Hou et al., 2007; Ali et al., 2007; Boquet, 2005; Rinehardt et al., 2004; Clement-Bailey and Gwathmey, 2007; Sawan et al., 2008; Jayakumar et al., 2014), but the interaction between nitrogen rate and application mode or plant density in the arid area is poorly understood. In the present study, seedcotton yield was affected by N rate, application mode and their interaction, and by N rate, plant density and their interaction. We found that the traditional high N rate produced relatively higher seedcotton yield regardless of application mode or plant density, but a 15–30% N reduction under drip fertigation or 20% N reduction under high density did not cause significant yield reduction. In other words, high plant density or drip fertigation is an important measure to save N fertilizer without sacrificing yield in the arid area. Previous studies showed that agronomic measures, such as increasing planting density or organic fertilizer inputs as well as improving application modes were beneficial to the stability of cotton yield under N reduction in the Yellow River valley of China (Dong et al., 2010, 2012; Mao, 2013). This study further confirmed that N application mode or planting density is an important guarantee for reducing N fertilizer without reducing yield. Cotton yield formation is determined by the accumulation of total biomass and the proportion of reproductive tissue to total biomass (Bange and Milroy, 2004; Dai et al., 2015). As an indeterminate species with strong compensatory ability, cotton growth and biomass accumulation are sensitive to the environment and cultural practices (Sadras et al., 1997; Reddy et al., 1997). However, cotton yield is quite stable in a series of environmental changes such as water availability, nitrogen supply and plant density (Kimball and Mauney, 1993; Sadras et al.,

4.2. Leaf senescence Leaf senescence is a degenerative process involving the orderly and sequential changes in physiology and biochemistry (Thomas and Stoddart, 1980), which are largely due to the accumulation of activated oxygen species in chloroplasts and mitochondria (Wright, 1999; Mittler, 2002). If the activated oxygen species were not promptly eliminated, the membranes of most plant cell organelles would be destroyed, and then more malondialdehyde (MDA) would accumulate (Noctor and Foyer, 1998; Mittler, 2002). Since leaf photosynthesis is closely related to leaf senescence (Smart, 1994; Bleecker and Patterson, 1997; Dong et al., 2008), the MDA together with Pn and Chl content have been used as valid indicators of plant senescence. In the present study, drip fertigation increased the chlorophyll content and Pn rate significantly compared with conventional application, while high density increased the Pn rate and chlorophyll content, but decreased the MDA content compared with low density at 20% N reduction. Based on leaf photosynthesis and MDA concentration, drip fertigation and high plant density delayed leaf senescence. Leaf senescence can inhibit photosynthesis and assimilate supply for boll and fiber development, leading to reduced boll weight and harvest index (Bauer et al., 2000; Peng and Krieg, 1991). Since the late-season period coincides with boll development, the decreased boll weight and harvest index and the consequent reduction in seedcotton yield might be attributed to leaf 155

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well as plant density. Seedcotton yield was also affected by plant density and N rate interaction. The 30% N reduction (N264) under drip fertigation or 20% N reduction (N264) under high plant density did not reduce cotton yield. The reduced N rate without yield reduction was attributed to the delayed leaf senescence and improved N use efficiency as indicated by aNUE and NRE. The results suggest that N fertilizer rate can be moderately reduced without yield reduction under drip fertigation or high plant density in irrigated cotton.

senescence under conventional application and low density under reduced N application. Boll load is an indicator of the source-sink relationship, which is essentially a comprehensive reflection of relationships between vegetative and reproductive growth. Large boll loads, rather than plant disease or poor rooting structure, are likely to constitute the most important factor in predisposing cotton to premature senescence (Wright, 1999). Dong et al. (2012) reported a significant negative correlation between boll load and leaf senescence at the onset of boll opening, and a large boll load can easily cause early leaf senescence, resulting to yield and quality reduction in cotton (Dong et al., 2006; Wright, 1999). Our recent study showed that boll load up-regulated the expression of senescence-associated genes and induced leaf senescence (Chen et al., 2018). In the present study, a decreased N rate increased boll load but drip fertigation and high density delayed leaf senescence by decreasing boll load. A 30% N reduction (N264) increased boll load compared with N375 and N319 under drip fertilization, but considerably decreased boll load compared with N264 under conventional N application. Similarly, a 20% N reduction (N264) increased boll load compared with N330, but decreased boll load compared with N264 under low density. The results suggested that drip fertigation and high density could suppress leaf senescence under reduced N fertilizer rate. The performance of cotton at the boll-opening stage is called maturity performance (Kong and Dong, 2011). Normal senescence favors normal maturity performance. Senescence too early or too late can reduce the crop yield and quality (Wright, 1999; Dong et al., 2006). Earliness is very important for improving cotton quality and decreasing input of fertilizer, irrigation, as well as providing proper time for rotation with other crops (Basbagi et al., 2007; Yu et al., 2017). Earliness may be dependent on variety, plant density, soil fertility and other environmental conditions (Heitholt and Sassenrath-Cole, 2009). Rao and Weaver (1976) showed that an increase in plant density increased earliness but a contrary report also exists (Kerby et al., 1990). In the present study, N reduction accelerated leaf senescence, but increased earliness. The decreased seedcotton yield following N reduction was due to early maturity caused by premature senescence. However, drip fertigation or high density increased earliness, but delayed leaf senescence, suggesting a relatively appropriate maturity performance.

Acknowledgements This work was supported the young fund for Shandong Academy of Agricultural Science (2015YQN20), by National Key research and development program of China (2017YFD0201906), the special fund for Taishan Scholars (No. tspd 20150213) from Shandong Province, and China Agricultural Research System (CARS-15-15). References Ali, M.A., Mushtaq Ali Mueen-ud-Din, Y.K., Yamin, M., 2007. Effect of nitrogen and plant population levels on seed cotton yield of newly introduced variety CIM-497. J. Agric. Res. 45 (4), 289–298. Ali, H., Afzal, M.N., Muhammad, D., 2009. Effect of sowing dates and plant spacing on growth and dry matter partitioning in cotton (Gossypium hirsutum L.). Pak. J. Bot. 41 (5), 2145–2155. Ayyadurai, P., Manickasundaram, P., 2014. Growth, nutrient uptake and seed cotton yield as influenced by foliar nutrition and drip fertigation in cotton hybrid. Int. J. Agric. Sci. 10 (1), 276–279. Bai, Y., Mao, S.C., Tian, L.W., Li, L., Dong, H.Z., 2017. Advances and prospects of highyielding and simplified cotton cultivation technology in Xinjiang cotton-growing area. Sci. Agric. Sin. 50, 38–50 (in Chinese). Bange, M.P., Milroy, S.P., 2004. Growth and dry matter partitioning of diverse cotton genotypes. Field Crops Res. 87, 73–87. Bar-Yosef, B., 1999. Advances in fertigation. Adv. Agron. 65, 1–70. Basbagi, S., Ekinci, R., Genver, O., 2007. Combining ability and heterosis for earliness characters in line x tester population of Gossypium hirsutum L. Hereditas 144, 185–190. Bauer, P.J., Frederick, J.R., Bradow, J.M., Sadler, E.J., Evans, D.E., 2000. Canopy photosynthesis and fiber properties of normal- and late planted cotton. Agron. J. 92, 518–523. Bednarz, C.W., Bridges, D.C., Brown, S.M., 2000. Analysis of cotton yield stability across population densities. Agron. J. 92, 128–135. Bednarz, C.W., Shurley, W.D., Anthony, W.S., Nichols, R.L., 2005. Yield, quality, and profitability of cotton produced at varying plant densities. Agron. J. 97, 235–240. Bleecker, A., Patterson, S., 1997. Last exit: senescence abscission and meristem arrest in Arabidopsis. Plant Cell 9, 1169–1179. Bondada, B.R., Oosterhuis, D.M., 2001. Canopy photosynthesis, specific leaf weight, and yield components of cotton under varying nitrogen supply. J. Plant Nutr. 24 (3), 469–477. Bondada, B.R., Oosterhuis, D.M., Norman, R.J., Baker, W.H., 1996. Canopy photosynthesis, growth, yield and boll 15N accumulation under nitrogen stress in cotton. Crop Sci. 36, 127–133. Boquet, D.J., 2005. Cotton in ultra-narrow row spacing: plant density and nitrogen fertilizer rates. Agron. J. 97, 279–287. Bremner, J.M., Mulvaney, C.S., 1982. Nitrogen—total. In: Page, A.L. (Ed.), Methods of Soil Analysis: II. Chemical and Microbiological Properties. ASA and SSSA, Madison, WI, pp. 595–624. Cao, W., Ma, Y.J., Zhang, S.J., Zhuang, L.L., 2012. Study on deficit irrigation technology for cotton border irrigation in arid area-taking Yuli County as an example. Water Saving Irrig. 8, 4–8. Chen, W.P., Hou, Z.N., Wu, L.S., Liang, Y.C., Wei, C.Z., 2010. Effect of salinity and nitrogen on cotton growth in arid environment. Plant Soil 326, 61–73. Chen, G.W., Yu, Y., Li, H., 2014. On the strategic shift of high-yielding cultivation theory in cotton in Xinjiang—from exploring heat energy potential to light energy. Xinjiang Reclam. Sci. Technol. 1, 3–6 (in Chinese). Chen, Y.Z., Kong, X.Q., Dong, H.Z., 2018. Removal of early fruiting branches impacts leaf senescence and yield by altering the sink/source ratio of field-grown cotton. Field Crops Res. 216, 10–21. Clawson, E.L., Cothren, J.T., Blouin, D.C., Satterwhite, J.L., 2008. Timing of maturity in ultra-narrow and conventional row cotton as affected by nitrogen fertilizer rate. Agron. J. 100, 421–431. Clement-Bailey, J., Gwathmey, C.O., 2007. Potassium effects on partitioning, yield, and earliness of contrasting cotton cultivars. Agron. J. 99, 1130–1136. Dai, J.L., Dong, H.Z., 2014. Intensive cotton farming technologies in China achievements: challenges and countermeasures. Field Crops Res. 155, 99–110. Dai, J.L., Li, W.J., Tang, W., Zhang, D.M., Li, Z.H., Lu, H.Q., Eneji, A.E., Dong, H.Z., 2015. Manipulation of dry matter accumulation and partitioning with plant density in relation to yield stability of cotton under intensive management. Field Crop Res. 180, 207–215. Darawsheh, M.K., Khah, E.M., Aivalakis, G., Chachalis, D., Sallaku, F., 2009. Cotton row

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