Effect of nitrogen fertilization under plastic mulched and non-plastic mulched conditions on water use by maize plants in dryland areas of China

Agricultural Water Management 162 (2015) 15–32

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Effect of nitrogen fertilization under plastic mulched and non-plastic mulched conditions on water use by maize plants in dryland areas of China S.X. Li a,∗ , Z.H. Wang a , S.Q. Li b , Y.J. Gao a a Department of Plant Nutrition, College of Natural Resources and Environment Sciences, Northwest Science and Technology, University of Agriculture and Forestry, Yangling, Shaanxi 712100, PR China b State Key Laboratory of Soil Erosion and Dryland Faring on the Loess Plateau, Northwest Science and Technology, University of Agriculture and Forestry, Yangling, Shaanxi 712100, PR China

a r t i c l e

i n f o

Article history: Received 2 October 2014 Received in revised form 2 August 2015 Accepted 11 August 2015 Keywords: Maize N fertilization Plastic mulch Transpiration Water use efficiency

a b s t r a c t A field experiment was conducted in a dry sub-humid area in China to study the effect of addition of five nitrogen (N) rates on evaporation (E), transpiration (T) and water use efficiency (WUE) of spring maize (Zea mays L.) with and without plastic mulch. During plant growing period, leaf area (LA), aboveground biomass and soil water stored in 200 cm layer were measured about every 10 days. Results showed that without plastic mulch, N fertilization increased LA by 48% on average, aboveground biomass by 73% at harvest, and grain by 122% and the estimated T by 28% and with plastic mulch, LA, aboveground biomass and grain were increased by 58%, 47% and 143% respectively. Due to canopy shading and plant uptake, N fertilization remarkably increased T and decreased E. With plastic mulch, the transpiration use efficiency (WUET ) of grain was 11.4 kg ha−1 mm−1 without N while 19.6 kg ha−1 mm−1 with N addition. Without plastic mulch, the evapotranspiration use efficiency (WUEET ) was increased from 6.3 to 9.4, to 12.3, to 14.2, and to 14.9 kg ha−1 mm−1 for grain, and from 18.4 to 24.3, to 29.4, to 30.5 and to 32.2 kg ha−1 mm−1 for aboveground biomass, and WUET for grain increased from 13.3 to 15.0, to 16.4, to 18.1 and to 18.0 kg ha−1 mm−1 when N rate increased from 0, to 30, to 60, to 90 and to 120 kg N ha−1 , respectively. Although WUET for dry biomass was constant, about 39 kg ha−1 mm−1 , both WUET and WUEET for grain were increased with N rate until 90 kg N ha−1 . In conclusion, N fertilization to the N deficient soil had no influence on WUET for aboveground biomass, but significantly reduced E and increased T, LA, aboveground biomass, grain yield and WUET for grain production. © 2015 Elsevier B.V. All rights reserved.

1. Introduction As an essential substance, water determines the existence of agriculture and crop productivity. Although plant growth is subjected to a variety of environmental stresses, water deficit is ubiquitous and reduces plant growth and crop yield more than all the other stresses combined in the long run (Kramer, 1983). This is particularly true in dryland areas. Crop production depending on precipitation is called rainfed agriculture, and crop production in areas where shortage of rainfall water supply constitutes the major constraint is generally referred to as dryland agriculture (Li and Wang, 2006). Dryland agriculture is widely practiced in semiarid and sub-humid regions of the world (Stewart, 2005). Due to water scarcity being the major constraint, increasing water use by

∗ Corresponding author. Fax: +86 28 87092559. E-mail address: [email protected] (S.X. Li). http://dx.doi.org/10.1016/j.agwat.2015.08.004 0378-3774/© 2015 Elsevier B.V. All rights reserved.

plants and decreasing water loss in different ways are the major components for crop production. In flat drylands without runoff, water loss is caused mainly by crop consumption through the process of transpiration (T) and soil evaporation (E) (Howell, 1990). The two processes are defined collectively as evapotranspiration (ET) (Thornthwaite, 1948). Of the two components of ET, the E is a main process of vaporization of water occurring in soil surface, and adding E has no direct effect on crop production (Basch et al., 2012), particularly on areas where low-intensity rainfall events prevail (Lampurlanés and Cantero-Martínes, 2006; Passoura, 2006) because it is a lost water that amend the canopy temperature. On the contrary, the T portion directly influences crop production (de Wit, 1958) since the T consumption is through crop leaves and canopies, and is inseparably linked with the process of carbon dioxide assimilation from the atmosphere, and therefore it closely relates to biomass and crop production. To obtain as much as possible water use by plants for improvement of crop production in water-scarce environments, it

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is crucial to increase the total amount of water transpired (Bouman, 2007) and to reduce that evaporated. Agricultural practice and numerous experimental results have shown the serious loss of water by E on dryland areas (Li and Xiao, 1992; Peterson et al., 1993, 1996; Li et al., 2009b). The E amount is determined by many factors. Apart from the climatic conditions external to the E body (evaporative demand), soil water content, texture, and surface cover degree by growing vegetation or any form of surface mulches, are the most important ones affecting E from soil. Three stages of E are distinguished in the soil in relation with water contents: the first is when the water supply at or to the surface is sufficient to allow a more or less constant E rate as a function of the evaporative demand; the second or the rate-falling stage depends on soil properties responsible for the delivery of moisture to the evaporative zone; and the third mainly on the rate of vapor diffusion (Basch et al., 2012). Soil texture and structure affect E due to their influence on the soil hydraulic characteristics (Jalota and Arora, 2002; Ndiaye et al., 2007). Under similar soil structure conditions, finer-textured soils show higher E losses than coarse-textured soils (Prihar et al., 1996; Jalota and Prihar, 1986). At low moisture levels when water is held in coarse-textured soils between soil particles instead of in continuous pores, E occurs mainly in a slower process of diffusion rather than the conductance process in water-filled pores to reach the E zone (Ward et al., 2009). In dryland areas, precipitation is erratic and sparse; solar radiation is strong due to seldom clouds, powerful and frequent winds, and high soil temperature at daytime. These factors create a favorable condition for E that largely exceeds precipitation. In these areas, fallow practice is widely adopted to store and accumulate additional soil water from precipitation for next crop use. However, the fallow efficiency (the ratio of water stored in the soil profile to the precipitation received during the fallowing period) or water storage efficiency is very low. It is reported that the water storage efficiency in China was only in the range of 10–15% (Li and Xiao, 1992), and E losses in central Aragon (northern Spain) in the range of 55–91% based on field measurements and model simulation (Moret et al., 2007). In comparison with continuous barley, a bare-fallow crop rotation only increased an additional 20 mm of soil water storage through the fallow period (Moret et al., 2006). Fields with crop plants also suffer greatly from such a loss. For increase of water storage efficiency, reduction of E losses and conservation of more water for plant use, mulching tillage by covering soil surface with vegetation or any other materials becomes the most notable and the most easily attained management practices (Baumhardt and Jones, 2002; Mulumba and Lal, 2008) and is widely adopted. Of the mulch materials, crop residues (Jones et al., 1994) and plastic sheet (Wang et al., 2001a; Wang et al., 2001b) have been extensively used because such materials are easily obtainable. Mulching soil with those materials interferes with E process by lowering the evaporative demand through mechanical barriers or by increasing resistance to the removal of moisture over the soil and reducing the energy supply (heat flux) to the evaporative zone. Decreasing the conductivity or diffusion of water in the topsoil layer is superficially incorporated with the mulch. Numerous studies have shown the effectiveness of mulch practices (Sauer et al., 1996; Burt et al., 2005; Monzon et al., 2006; Yuan et al., 2009; Ward et al., 2009), and generally agreed that such soil mulch is substantially effective in reducing E losses at the first stage. The E reduction from a bare soil surface during initial E stage can be attained through mulch overlying the wet subsoil, thereby contributing to a more favorable soil water status. In semiarid environments with rainfall above the minimum threshold, straw mulching generally increases yields by enhancing the soil water storage (Bescansa et al., 2006; Monzon et al., 2006). However, straw or crop residue mulch has some defects: in poorly drained soils or in temperate climates with suboptimal springtime temperatures, residue retention may some-

times reduce yield below optimal levels due to decrease of soil temperature (Fabrizzi et al., 2004; Anken et al., 2004; Lal, 2008) and soil N availability (Gao and Li, 2005). The high solar reflectivity and low thermal conductivity of the crop residues prevent directly an increase in soil temperature (Hillet, 1980; Shinners et al., 1994). Trevisan et al. (2002) showed that compared with no such a cover, the soil temperature reduction amplitude could drop down to 20 cm with an oat straw cover throughout the year. Use of plastic sheets for mulch is simple and can address this problem, although it may have no such merits as crop residues. In China, plastic mulch has been extensively adopted in dryland areas for water retention (Li et al., 2013). However, in many cases, even mulch tillage is very well performed, the water use ratio, i.e., the proportion of water used by plants, is still low. It has been realized that the low water use ratio was due to low nutrient supplies, especially N (Li et al., 2000, 2009a). Although the effect of N fertilization have been extensively studied in China as well as in the world, most of the work in the past was concentrated on crop response to N fertilizers (Tong et al., 1999; Song and Li, 2003; Liu et al., 2010), N effects on crop yield and quality (Moll et al., 1982; Fischer et al., 1993; Cai et al., 1994; Fan et al., 1998; Zhu et al., 2003; Zhao et al., 2004; Dai and Sun, 2005; Li et al., 2005; Guo et al., 2008; Meng et al., 2012), N accumulation in soil, N pollution to groundwater and N influence on the environment (Ju et al., 2004, 2009 ; Zhao and Yu, 2006; Li and Wu, 2008; Huai et al., 2009), but not on its effect on water use by plants. In the previous paper (Li et al., 2013), we reported the effect of plastic mulch, and wheat straw mulch on water loss by E under fallow and cropped conditions and water use by T under cropped conditions. In this work, we found that for producing 1 g aboveground biomass or shoot dry matter of maize, about 256 g water was transpired in our experimental area no matter whether maize was cultivated under plastic mulched or non-plastic mulched conditions. Such results have made us hypothesize that the water use by T for production of a unit of the aboveground biomass is a climatic performance for a given crop, depending on climatic traits, and it may not be influenced by N fertilization. For testing such a hypothesis, and understanding the effects of N fertilization on reduction of water loss by E and increase of T, five N rates were applied to maize under plastic mulched and non-plastic mulched conditions in the experiment designed. The current paper presents results for answering those questions proposed.

2. Materials and methods 2.1. Location for field experiment A field experiment was conducted in Chengcheng County, a typical dry sub-humid area in Shaanxi Province, China. Annual precipitation averages 539 mm, of which 58% occurs in July, August, and September while other months are relatively dry. The highest annual precipitation recorded is 855.3 while the lowest is 316.2. Under natural vegetation, potential ET averages 1192.1 mm with a maximum of 1367.3 mm in a dry year and a minimum of 929.2 mm in a wet year. Average annual temperature is 11.9 ◦ C. The highest daily average temperature occurs in June (24 ◦ C), July (25.1 ◦ C) and August (24.5 ◦ C) and the lowest in January (–2.6 ◦ C). Major cereal crops are winter wheat (Triticum aestivum) and spring maize. Important cash crops include cotton (Gossypium hirsutum L.) canola (Brassica campestries L. and Brassica napus L.), peppers (Piper L.) as well as some fruit trees. Non-irrigated areas typically have one harvest a year, while irrigated areas generally have two. Because water resources are very limited, dryland farming is dominant in the county.

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Fig. 1. Changes of leaf area (cm2 plant−1 ) of maize with time at five N rates under non-plastic mulched (left) and plastic mulched (right) conditions.

2.2. Field experiment Developed on the loess parent material, the soil at the experiment site is termed Eum-Orthic Anthrosol according to the new Chinese soil taxonomy based on the diagnosis system (Soil Taxonomy Group, Nanjing Soil Institute, Chinese Academy and Cooperative Research Group on Chinese Soil taxonomy, 1995; Cooperative Research Group on Chinese Soil Taxonomy, 2001). The soil is as deep as 40–50 m in the areas with water table depth below 30 m under the ground. It is a highly calcareous (10% CaCO3 ), silty loam soil with 17% clay (<2 ␮m), 71% silt (2–63 ␮m) and 12% sand (63–2000 ␮m). Measured at 10 ◦ C, the infiltration rate of the soil was 1.39 mm min−1 on average in the range of 60 min (Tian, 1989). Spring maize (cv. Shaanxi 8410) was used as a test crop, with a density of 60,000 plants ha−1 . Two factors were involved in the experiment: cultivation and N fertilization. Maize was planted under the plastic mulched and non-plastic mulched conditions, and 0, 30, 60, 90, and 120 kg N ha−1 were applied respectively to both the plastic mulched and non-plastic mulched plots by incorporating with top 20 cm soil before maize being sown, using urea as N fertilizer. Phosphate fertilizer (calcium superphosphate, containing 12% of P2 O5 ) was applied to all treatments in the same way as N. There were 10 treatments totally in the experiment with three replications. Treatments in the field were arranged by a split design, with mulching treatments as blocks and N rates as plots. Each plot had an area of 24 m2 with 144 plants. Both blocks and plots were arranged randomly. Plastic sheets were mulched on May 4, immediately after maize emergence for the mulched treatments. The soil sample was taken at April 22 before maize was sown for determination of the initial values and at May 25, the soil and plant samples were taken as the first time during plant growth period, and then soil and plant samples were taken regularly at an interval of almost 10 days from each treatment throughout the growing season. At each sampling, maize shoots were cut off as close as possible to the soil surface and then dried and weighed for recording the aboveground biomass. At each time, soil samples were taken in 20 cm increments from 0 to 200 cm for determination of water content and total water storage in the depth and for calculation of water consumption until August 28 after maize was harvest. The 200 cm depth was chosen based on water cycle depth and maize root distribution. According to our measurement, maize rooting depth can reach 150 cm or so as the maximum depth with the major distributive layer in 0–60 cm; beyond 150 cm no root could be found. The water cycle depth amounts to about 160 cm in arable land (Li, 1989), and below 200 cm, the water content was stable without participating in water cycle in crop growing period. For this reason, nearly all the research workers take 200 cm soil samples for studying water regime in the area (Han, 1993). During maize plant growth period, soil temperature, ambient field air temperature and humidity were measured every day with the soil thermometer inserted to different depth and the aspirated

psychrometer installed at the elevation of maize canopy. Three replications were taken for each measurement a day. Leaf area (LA) was measured at each sampling time with leaf area meter. At maturity, ten maize plants were harvested from each treatment for estimation of crop yield, and soil samples were also collected at the end of the season as described above. The experiments were carried out from April 22 to August 28 in the experimental year as already reported (Li et al., 2013). A relatively large amount (38 mm) of rain fell on May 4. Additional rainfall occurred in mid June, the last part of July, and throughout August. Due to serious scarcity of rainfall at early July, 40 mm water was irrigated on July 11 artificially to ensure water uniform distribution in each plot. Total rainfall plus irrigation during the growing season was 305.1 mm. 2.3. Statistical analysis Data of leaf area, leaf area index, and shoot dry matter among mulch and N fertilization treatments were subjected to significant test using SAS software (SAS Institute, 1996), and significance was calculated based on the results of F-tests and the least significant differences (LSD) at the 0.05 probability level. Linear regression equations were used to fit the data between maize dry matter (kg ha−1 ) and T amount (mm ha−1 ) at five N rates under plastic mulched conditions, and the significance was calculated by T-test (Mo, 1992). Details of soil properties performance of the experiment, and laboratory analyses have been reported in the previous paper (Li et al., 2013). The current work aimed at demonstrating the individual effects of five N rates to soil on water use by T from maize plants under plastic mulched conditions and water loss by E from soil under plastic mulched and non-plastic mulched conditions. 3. Results 3.1. Effects of N fertilization on LA and LAI Acting as shelter, the LA characterized by leaf area index (LAI) can shade the soil, reduce the solar radiation to soil surface, lower the soil temperature, and therefore reduce the proportion of the water that would be otherwise evaporated (Cooper et al., 1987; Shepherd et al., 1987). The E can be affected by many factors, and any improved management practice that affects plant canopy and LA will result in the increase of radiation interception and decrease of E. For this reason, in order to evaluate N fertilization on water use efficiency, it is necessary to investigate its effect on LA and LAI. As shown in Table 1 and Fig. 1, the LA, together with LAI, was substantially different at different plant growing stages and varied from time to time. It was small at the early stage of plant growth, increased rapidly at the plant vigorous growth stage, and became declined at the late stage because some old leaves died and fell off.

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Table 1 Leaf area (LA) (cm2 plant−1 ) and leaf area index (LAI) of maize with five N rates under plastic and non-plastic mulched conditions. Date of determination

Leaf area of different N rate (kg ha−1 ) 0

Mean of LA of N rate

LSD0.05 for N rate

87 237 928 1431 2349 2909 3291 3179 3009 2914 106.7

80 246 847 1258 2022 2361 2655 2562 2338 2210

16.5 24.3 83.2 115.6 135.8 187.2 212.3 214.9 245.1 319.4

1890 1890

2033 2033

1656 1771

205 848 1988 3439 3553 3848 3878 3757 3241 3098 79.5

176 852 2139 3567 3867 4063 4124 3877 3840 3789 84.8

185 870 2691 3655 4047 4167 4318 4598 4232 4112 115.6

178 767 2017 3090 3498 3698 3592 3457 3186 3094

2786 2786

3029 3029

3288 3288

2658 2867

30

60

90

120

Leaf area (LA) Non-plastic mulched treatment 73 May 25 200 June 6 June 17 535 June 27 787 1543 July 6 1944 July 16 2027 July 26 1893 August 6 August 17 1547 August 28 1421 LSD0.05 for date 36.4

78 251 889 1275 1812 2145 2356 2300 2001 1840 58.1

76 271 916 1391 2061 2298 2591 2525 2332 2199 69.3

82 272 969 1405 2345 2509 3009 2914 2800 2674 81.6

Mean All treatments N addition

1197

1495 1495

1666 1666

174 550 1425 1780 2657 2876 2431 2231 2024 1998 53.3

149 714 1842 3009 3364 3536 3207 2822 2592 2472 67.3

1815

2371 2371

Plastic mulched treatment May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 LSD0.05 for date Mean All treatments N addition

32.4 123.5 141.7 154.2 178.6 209.4 241.1 334.9 353.3 299.1

Leaf area index (LAI) Non-plastic mulched treatment May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 LSD0.05 for date

0.04 0.12 0.32 0.47 0.93 1.17 1.21 1.14 0.92 0.85 0.022

0.05 0.15 0.53 0.76 1.09 1.29 1.41 1.34 1.20 1.10 0.035

0.05 0.16 0.55 0.83 1.24 1.38 1.55 1.51 1.40 1.32 0.042

0.05 0.16 0.58 0.84 1.41 1.51 1.81 1.75 1.68 1.60 0.049

0.05 0.14 0.56 0.86 1.41 1.75 1.97 1.91 1.81 1.75 0.064

0.05 0.15 0.51 0.75 1.22 1.42 1.59 1.53 1.40 1.32

0.010 0.015 0.050 0.069 0.081 0.112 0.127 0.129 0.147 0.192

Plastic mulched treatment May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 LSD0.05 for date

0.10 0.33 0.85 1.07 1.59 1.73 1.46 1.34 1.21 1.20 0.032

0.09 0.43 1.10 1.81 2.02 2.12 1.92 1.69 1.55 1.48 0.040

0.12 0.51 1.19 2.06 2.13 2.31 2.33 2.25 1.94 1.86 0.048

0.11 0.51 1.28 2.14 2.32 2.44 2.47 2.33 2.30 2.27 0.051

0.11 0.52 1.61 2.19 2.43 2.50 2.59 2.76 2.54 2.47 0.069

0.11 0.46 1.21 1.85 2.10 2.22 2.15 2.07 1.91 1.86

0.019 0.074 0.085 0.093 0.107 0.126 0.145 0.201 0.212 0.179

Before June 17 for the non-plastic mulched plots and before June 6 for the plastic mulched plots, the LA was too small to be significant in shading the soil surface and reducing soil water loss by E. Since then, the LA became larger and at the middle and late stages of plant growth, it covered the soil surface more or less completely. For the non-plastic mulched plots, the averaged LAI of the five N rates was around 0.05 by the end of May, around 0.75 by the end of June, increased to more than 1.6 by the end of July, and then declined. For the plastic mulched plots, the corresponding LAI was similar to, but larger than that without mulching: it was around 0.11 by the

end of May, around 1.85 by the end of June, increased to 2.22 by the end of July and then declined. The plastic mulching had conserved more water for plant use. The actual soil water content at a given time was the net result of water conserved in soil profile and that consumed by plants. Under plastic mulched conditions, higher temperature and higher water content made plant growth more vigorous, formed larger canopies that could shade the soil surface more completely than plants in the non-plastic mulched plots. In addition, more water uptake by plants in the plastic mulched plots would, in turn, have the potential

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Table 2 Aboveground biomass (kg ha−1 ) of maize with five N rates under plastic mulched and non-plastic mulched conditions. Date of determination

Non-plastic mulched treatment May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 LSD0.05 for first 3 determinations LSD0.05 for rest determinations Mean All treatments N addition Plastic mulched treatment May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 LSD0.05 for first 3 determinations LSD0.05 for rest determinations Mean All treatments N addition

Nitrogen rate (kg ha−1 )

Mean

0

30

60

30 150 310 466 685 1814 2893 4254 5184 5368 20.4 258.6

30 150 470 793 1135 3725 5035 6271 6936 7300 25.2 318.0

30 150 510 902 1962 5115 5953 7105 8250 9112 18.6 499.8

2115

3185 3185

54 600 1515 2048 2775 4780 6513 7636 8819 8989 27.0 325.2 4373

90

LSD0.05 for N rate (kg ha−1 )

120

30 150 540 955 2184 5186 6346 7610 8984 10,006 32.4 520.2

30a 150 570 964 3455 6015 7191 8510 9991 10,798 37.2 624.0

30 150 480 816 1884 4371 5484 6750 7869 8517

3909 3909

4199 4199

4767 4767

3635 4015

54 600 1815 2456 3322 5723 7798 9143 10,332 11,362 31.8 367.2

54 600 2006 2694 3649 6286 8565 10,042 11,250 12,525 39.6 487.2

54 600 2318 3125 4232 7289 9932 11,644 13,163 14,451 43.8 608.4

54 600 2334 3147 4261 7340 10,000 11,724 13,254 14,593 52.2 799.2

5261 5261

5767 5767

6681 6681

6731 6731

54 600 1998 2694 3648 6284 8562 10,038 11,364 12,384

7.8 38.4 45.6 67.2 205.2 338.4 608.4 679.2 757.2 754.2

10.8 92.4 113.4 147.0 210.6 236.4 754.8 853.8 882.6 1082.4

5763 6110

a Aboveground biomass measured at May 25 and June 6 was no significant difference among the five N treatments for each of the plastic mulched and non-mulched treatments, and therefore the averaged values were adopted.

to reduce water loss by E since E intensity depended on soil water content. The higher values of the LAI and the steeper slope of the LA curve (Fig. 1) with time in the plastic mulched plots evidenced such a fact. Although the plant growth for reduction of the evaporative water loss was not significant in plastic mulched treatments compared to plastic mulching itself that almost completely avoided water loss, it is greatly important for the non-plastic mulched treatments that depended solely on plant shading and water uptake for reduction of water loss by E. Application of N fertilizer promoted plant vigorous growth, and therefore LA and LAI were increased significantly with N rate increase in both non-plastic mulched and mulched plots. In comparison with no N fertilization, the LA and LAI values at 120 kg N ha−1 were almost doubled. The time for their peak’ appearance and values’ decline showed another trait of N fertilization. Without N fertilization, the peak values of LA and LAI appeared much earlier, and so did the decline time. However, with application of N fertilizer, both the peak and decline time appeared later. The higher the N rate applied, the later the time for peaks of LA and LAI appearing and declining. With no doubt, N fertilization extended the duration of plant growth and shading time, further leading to more water use by plants and reduction of water loss by E. The LA and LAI differences resulted in changes of temperature and humidity in the plot microenvironment. Measured data showed that plastic mulching definitely raised soil temperature. During the entire plant growth period, the averaged soil temperature of five N rates in the 0–30 cm layer under the plastic mulched condition was 24.5 ◦ C while that under the non-plastic mulched condition was 23.6 ◦ C, the former being about 1 ◦ C higher than the

latter. However, difference in plant growth statuses had led to the changes of the averaged ambient air temperature in a reverse way during the entire period. As measured at the elevation of maize canopy each day with three replications, the averaged ambient air temperature at 8, 14 and 20 o’clock was 24.0, 32.1 and 22.5 ◦ C with a total average of 26.2 ◦ C for the non-plastic mulched plots and 23.7, 31.3 and 22.1 with a total average of 25.7 ◦ C for the plastic mulched plots, respectively, the former being somewhat higher than the latter although there was no significant difference between them. The relative humidity, on the other hand, was slightly lower in the plastic mulched plots due to lower E than that in the nonplastic mulched plots at early stages whereas there was almost no difference at late plant growth stages. Application of N fertilizer showed some influence on the ambient air temperature: with N rates of 0, 30, 60, 90, and 120 kg ha−1 , the averaged ambient temperature was 27.1, 26.4, 26.5, 25.7, and 25.6 ◦ C for the non-plastic mulched fields and 25.8, 24.9, 24.3, 23.5 and 23.9 ◦ C for the plastic mulched fields, respectively, evidencing that with N rate increases, the ambient air temperature significantly decreased probably due to abundant growth of plants that reduced solar radiation reaching to the microenvironment. In addition, at high N rate, the soil temperature was also somewhat lower compared to that at low N rate for both the non-plastic mulched and plastic mulched treatments. There was no difference in the ambient humidity among the different N rates at the early stage of plant growth probably due to little difference in T and E while at the middle or late stage, the humidity was slightly increased with N rate, probably due to high T caused by large leaf areas. It has been reported that when LAI was more than 4, there would be no water loss by E. However, in dryland areas, the LAI is generally

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Fig. 2. Changes of biomass weight (kg ha−1 ) of maize with five N rates at different determination times under non-plastic mulched (left) and plastic mulched (right) conditions.

lower than 3 even when fertilizer is applied (Gregory et al., 1984; Hamblin et al., 1987). Since maize, a sparse crop, was used in our experiment, the LAI was lower than 2 for the non-plastic mulched plots and higher than 2 for the plastic mulched plots at late stages with high N rates. 3.2. Effects of N fertilization on aboveground biomass production Since plastic sheets were overlapped between the joined edges and each maize plant was surrounded by two crescents of the joined plastic sheets in an overlapped shape, water loss by E was almost impossible. For this reason, the soil conserved more water and plants grown in the plastic mulched plots were more vigorous and plant canopies much larger than those in the non-plastic mulched plots. As a result (Table 2; Fig. 2), the plastic mulching

had substantially increased aboveground biomass. At any time of determinations, the aboveground biomass in the plastic mulched plots was significantly larger than that in the non-plastic mulched plots. At maturity, the averaged aboveground biomass of five N rates was 12,384 kg ha−1 for the plastic mulched treatment, while 8517 kg ha−1 for the non-plastic mulched treatment, the former being 45% higher than the latter. Application of N fertilizer played even more significant role in increasing aboveground biomass production (Table 2) than that without N addition. In the experimental field deficient in N supply, at the first two determinations, plants had no significant response to N fertilizer under either plastic mulched or non-plastic mulched conditions, suggesting that the N supplied by the soil was still adequate for plant requirement. However, at the third determination, plants under the plastic mulched conditions began to show

Fig. 3. Relationship between aboveground biomass (kg ha−1 ) of maize and transpired water (mm ha−1 ) under plastic mulched conditions with five N rates.

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Table 3 Water stored, total water loss, water loss by evaporation (E) and transpiration (T) with five N rates under plastic mulched conditions. Date for determination

Water stored in 0–200 cm layer

Rainfall + irrigation (mm ha−1 )

Between adjoining 2 periods

Cumulative amount

Water loss (mm ha−1 )

Between adjoining 2 periods

Cumulative water loss (mm ha−1 ) estimated Cumulative amount

E

T

N rate at 0 kg ha−1 April 22 May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 Total

320.6 353.9 349.3 348.2 349.3 299.2 370.0 344.2 373.4 373.4 380.6 380.6

48.8 4.6 13.2 24.2 1.8 121.4a 1.8 53.6 12.2 23.5 305.1

48.8 53.4 66.6 90.8 92.6 214.0a 215.8 269.4 281.6 305.1

15.5 9.2 14.3 23.1 51.9 50.6 27.6 24.4 12.2 16.3 245.1

15.5 24.7 39.0 62.1 114.0 164.6 192.2 216.6 228.8 245.1

9.8 13.0 16.5 20.0 20.0 20.0 20.0 20.0 20.0 20.0

5.7 11.5 22.5 42.1 94.0 144.6 172.2 196.6 208.8 225.1

N rate at 30 kg ha−1 April 22 May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 Total

320.6 353.9 349.3 343.9 341.0 280.6 330.3 310.0 334.7 332.7 335.7 335.7

48.8 4.6 13.2 24.2 1.8 121.4a 1.8 53.6 12.2 23.5 305.1

48.8 53.4 66.6 90.8 92.6 214.0a 215.8 269.4 281.6 305.1

15.5 9.2 18.6 27.1 62.2 71.7 22.1 28.9 14.2 20.5 290.0

15.5 24.8 43.3 70.4 132.6 204.3 226.4 255.3 269.5 290.0

9.8 13.0 16.5 20.0 20.0 20.0 20.0 20.0 20.0 20.0

5.7 11.5 26.8 50.4 112.6 184.3 206.4 235.3 249.5 270.0

N rate at 60 kg ha−1 April 22 May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 Total

320.6 353.9 349.3 341.5 335.9 259.5 319.5 289.8 311.6 308.0 309.2 309.2

48.8 4.6 13.2 24.2 1.8 121.4a 1.8 53.6 12.2 23.5 305.1

48.8 53.4 66.6 90.8 92.6 214.0a 215.8 269.4 281.6 305.1

15.5 9.2 21.0 29.8 68.2 71.4 31.5 31.8 15.8 22.3 316.5

15.5 24.7 45.7 75.5 143.7 215.1 246.6 278.4 294.2 316.5

9.8 13.0 16.5 20.0 20.0 20.0 20.0 20.0 20.0 20.0

5.7 11.5 29.2 55.5 123.7 195.1 226.6 258.4 274.2 296.5

N rate at 90 kg ha−1 April 22 May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 Total

320.6 353.9 349.3 337.2 327.0 249.7 284.9 253.8 270.4 264.1 262.0 262.0

48.8 4.6 13.2 24.2 1.8 121.4a 1.8 53.6 12.2 23.5 305.1

48.8 53.4 66.6 90.8 92.6 214.0a 215.8 269.4 281.6 305.1

15.5 9.2 25.3 34.4 79.1 86.2 32.9 37.0 18.5 25.6 363.7

15.5 24.7 50.0 84.4 163.5 249.7 282.6 319.6 338.1 363.7

9.8 13.0 16.5 20.0 20.0 20.0 20.0 20.0 20.0 20.0

5.7 11.5 33.5 64.4 143.5 229.7 262.6 299.6 318.1 343.7

N rate at 120 kg ha−1 April 22 May 25 June 6 June 17 June 27 July 6 July 16 July 26 August 6 August 17 August 28 Total

320.6 353.9 349.3 336.9 326.6 248.7 283.3 252.0 268.3 261.8 259.7 259.7

48.8 4.6 13.2 24.2 1.8 121.4a 1.8 53.6 12.2 23.5 305.1

48.8 53.4 66.6 90.8 92.6 214.0a 215.8 269.4 281.6 305.1

15.5 9.2 25.6 34.5 79.7 86.8 33.1 37.3 18.7 25.6 366.0

15.5 24.7 50.3 84.8 164.5 251.3 284.4 321.7 340.4 366.0

9.8 13.0 16.5 20.0 20.0 20.0 20.0 20.0 20.0 20.0

5.7 11.5 33.8 64.8 144.5 231.3 264.4 301.7 320.4 346.0

a

Total water received during the period, including 40 mm irrigated water.

22

S.X. Li et al. / Agricultural Water Management 162 (2015) 15–32

Table 4 Water stored, total water loss, water loss by evaporation (E) and transpiration (T) with five N rates under non-plastic mulched conditions. Date for determination

Water stored in 0–200 cm layer

Rainfall + irrigation (mm ha−1 )

Cumulative water loss (mm ha−1 ) estimated

Between adjoining 2 periods

Cumulative amount

T

E

N rate at 0 kg ha−1 (T estimated with equation y = 38.854x, where y is biomass and x is T) April 22 320.6 336.8 48.8 48.8 May 25 332.9 4.6 53.4 June 6 330.1 13.2 66.6 June 17 332.5 24.2 90.8 June 27 279.5 1.8 92.6 July 6 July 16 339.9 121.4a 214.0a July 26 309.9 1.8 215.8 327.1 53.6 269.4 August 6 315.3 12.2 281.6 August 17 333.7 23.5 305.1 August 28 333.7 305.1 Total

32.6 8.5 16.0 21.8 54.8 61.0 31.8 36.4 24.0 5.1 292.0

32.6 41.1 57.1 78.9 133.7 194.7 226.5 262.9 286.9 292.0

0.8 3.9 8.0 12.0 17.6 46.7 74.5 109.5 133.4 138.2

31.8 37.2 49.1 66.9 116.1 148.0 152.0 153.4 153.5 153.8

N rate at 30 kg ha−1 (T estimated with equation y = 38.896x, where y is biomass and x is T) April 22 320.6 May 25 336.8 48.8 48.8 332.0 4.6 53.4 June 6 328.3 13.2 66.6 June 17 June 27 332.6 24.2 90.8 291.1 1.8 92.6 July 6 344.2 121.4a 214.0a July 16 309.3 1.8 215.8 July 26 319.0 53.6 269.4 August 6 August 17 314.1 12.2 281.6 324.8 23.5 305.1 August 28 Total 324.8 305.1

32.6 9.4 16.9 19.9 43.3 68.3 36.7 43.9 17.1 12.8 300.9

32.6 42.0 58.9 78.8 122.1 190.4 227.1 271.0 288.1 300.9

0.8 3.9 12.1 20.4 29.2 95.8 129.4 161.2 178.3 187.7

31.8 38.1 46.8 58.4 92.9 94.6 97.7 109.8 109.8 113.2

N rate at 60 kg ha−1 (T estimated with equation y = 39.055x, where y is biomass and x is T) 320.6 April 22 336.8 48.8 48.8 May 25 June 6 330.9 4.6 53.4 327.7 13.2 66.6 June 17 June 27 331.9 24.2 90.8 292.1 1.8 92.6 July 6 * 332.3 121.4 214.0* July 16 308.6 1.8 215.8 July 26 332.6 53.6 269.4 August 6 August 17 307.0 12.2 281.6 315.3 23.5 305.1 August 28 Total 315.3 305.1

32.6 10.5 16.4 20.0 41.6 81.2 25.5 29.6 37.8 15.2 310.4

32.6 43.1 59.5 79.5 121.1 202.3 227.8 257.4 295.2 310.4

0.8 3.8 13.1 23.1 50.2 131.0 152.4 181.9 218.2 233.3

31.8 39.3 46.4 56.4 70.9 71.3 75.4 75.5 77.0 77.1

N rate at 90 kg ha−1 (T estimated with equation y = 39.005x, where y is biomass and x is T) 320.6 April 22 336.8 48.8 48.8 May 25 June 6 331.0 4.6 53.4 326.1 13.2 66.6 June 17 333.0 24.2 90.8 June 27 288.8 1.8 92.6 July 6 a 331.7 121.4 214.0a July 16 302.9 1.8 215.8 July 26 323.7 53.6 269.4 August 6 300.7 12.2 281.6 August 17 297.9 23.5 305.1 August 28 297.9 305.1 Total

32.6 10.4 18.1 17.3 46.0 78.5 30.6 32.8 35.2 26.3 327.8

32.6. 43.0 61.1 78.4 124.4 202.9 233.5 266.3 301.5 327.8

0.8 3.8 13.8 24.1 56.0 133.0 162.7 195.1 230.3 256.5

31.8 39.2 47.3 54.3 68.4 69.9 70.8 71.2 71.2 71.3

N rate at 120 kg ha−1 (T estimated with equation y = 39.035x, where y is biomass and x is T) 320.6 April 22 336.8 48.8 48.8 May 25 330.8 4.6 53.4 June 6 323.8 13.2 66.6 June 17 335.0 24.2 90.8 June 27 270.0 1.8 92.6 July 6 a 324.2 121.4 214.0a July 16 294.4 1.8 215.8 July 26 324.2 53.6 269.4 August 6 288.3 12.2 281.6 August 17 290.7 23.5 305.1 August 28 290.7 305.1 Total

32.6 10.6 20.2 13.0 66.8 67.2 31.6 23.8 48.1 21.1 335.0

32.6 43.2 63.4 76.4 143.2 210.4 242.0 265.8 313.9 335.0

0.8 3.8 14.6 24.7 88.5 154.1 184.2 207.8 255.9 276.9

31.8 39.4 48.8 51.7 54.7 56.3 57.8 58.0 58.0 58.1

Between adjoining 2 periods

a

Cumulative amount

Water loss (mm ha−1 )

Total water received during the period, including 40 mm irrigated water.

S.X. Li et al. / Agricultural Water Management 162 (2015) 15–32

stronger responses to N application and the aboveground biomass was almost increased with N rate increase until N rate reaching 90 kg ha−1 that was no difference with N rate of 120 kg ha−1 in aboveground biomass production, whereas under non-plastic mulched conditions plants were still relatively small and needed small amount N, and thus showed irregular responses to N rates. Although each N rate significantly increased aboveground biomass compared to control, yet there was no regular pattern for the increase of aboveground biomass with N rates. Thereafter, N supply to soil became necessary for both plastic mulched and non-plastic mulched treatments, and the aboveground biomass was regularly increased with N rate increase even though the net increase was decreased at high N rate compared to that at low N rate. 3.3. Effects of N fertilization on increase of T and reduction of E In addition to shading the soil surface by large canopies, plant growth reduces evaporative water loss by taking up water, and this leads to reduction of soil water amount available for E. Additional growth during this period is then “free” in terms of water use, since water that is not transpired would otherwise evaporate directly from the moist soil surface (Cooper et al., 1987). All this will increase T and decrease E. The N fertilization remarkably promoted plant growth, increased plant aboveground biomass and thus intimately related with the transpired water. As shown in Tables 2 and 3, at the high N rate, the aboveground biomass was high, and the transpired water large, while at the low N rate, the reverse was the case. The water loss in plastic mulched plots was mainly due to maize plant T, especially when plant grew lager. However, at the early fourth determinations, maize plants were small and had little function to prevent solar radiation from reaching the soil surface, and there was almost no difference in water loss due to E that occurred from the minor openings around plants and at time of lifting out plastic sheets for receiving rainfall or irrigation. Therefore, the amount of E for prior to June 17 was estimated by simply subtracting the E values in the follow, plastic mulched plots from the total lost water in the cropped, plastic mulched plots except the E of the fourth determination (June 27) that was calibrated from plant biomass. In such a manner, water loss by E from plastic-mulched plots was estimated to be 20 mm at the first four time determinations. Thereafter, as plants grew larger and shaded the soil completely, such a loss was considered negligible, and thus the water loss was regarded as transpired water solely (Li et al., 2013). By this way, the total transpired water in entire maize growth period was estimated to be 225.1, 270.0, 296.5, 343.7 and 346.0 mm ha−1 for N rate at 0, 30, 60, 90 and 120 kg ha−1 respectively under the plastic mulched conditions. For identifying the role of N fertilization on water use by T and water loss by E, we set up five linear regression equations for each N rate using the aboveground biomass data harvested at different time as function in term of kg ha−1 (Table 2) and water consumption by T as estimated above at the same time as variable in term of mm ha−1 under plastic mulched conditions (Table 3) with the method and procedures established (Li et al., 2013). As shown in Fig. 3, the aboveground biomass versus the transpired water was fitted very well with linear equations and all determination coefficients (R2 ) for fitting the equations were higher than 0.96 (n = 10), being significant at 0.01 level. The equations were: y = 38.854x, y = 38.896x, y = 39.005x, y = 39.055x and y = 39.035x (where y is the aboveground biomass or dry shoot weight and x is T) for N rate at 0, 30, 60, 90 and 120 kg ha−1 , respectively. The straight line passed through the origin, indicating that T was the plant performance, and that it would have not occurred in the absence of biomass production. The slope of the line represented the “climate” in the growing season. Theoretically, all data points should fall exactly on the line only if the “climate” was always the same during the entire growing

23

season. Under controlled conditions, this would be feasible while under field conditions, the climate had somewhat changes during the season. Using these equations or Fig. 3 to directly calculate, we found that for producing 1 g aboveground biomass of maize, about 256 g water was transpired in the experimental area. In looking at the points in Fig. 3, it showed that in mid-season with hotter and drier climate, the points were below the line and in the early season with cooler and late season with more humid climate, the points were above the line. This says it took more than 256 g of water to produce 1 g of biomass during the middle season, and less than this amount during the early and late periods. The lower air temperature (21.3 ◦ C) in early season, high air temperature (25.4 ◦ C) in hottest season, and more rainfall data in August have provided strong evidence for the conclusion. It is interesting to note that the figure of 256 g of water for producing 1 g of biomass was totally in agreement with that calculated from the average of five N rates (Li et al., 2013), suggesting that although N fertilization greatly raised the total T consumption and total biomass production, yet the total T consumption was parallel to the biomass production, and both were increased in the same proportions with N rates, and therefore the N rates had no influence on T amount for production of one unit aboveground biomass Vegetative growth has a direct relation to water use, crop yield and WUE for a given supply of soil water (Basch et al., 2012) Using equations established from plastic mulched plots and the aboveground biomass data from non-plastic mulched plots (Table 2), we were able to calculate water loss by T at five N rates under the non-plastic mulched conditions, and then to determine the corresponding water loss by E. As shown in Table 4, in the nonplastic mulched plots, water loss pattern by T was similar to plastic mulched plots throughout the growing season: large amounts of water lost due to E were found during the early growing period, almost no water loss during the middle growing period, and a small amount of water loss due to E during the late growing period. Application of N fertilizer played a significant role in increasing water use by T in the non-plastic mulched plots as in the plastic mulched plots. Without N fertilization, the water consumed by T was only 138.2 mm ha−1 , equivalent to 47% of the total water loss whereas the E was as high as 153.8 mm ha−1 or 53% of the total water consumed. In contrast, the T amount was increased to 187.7, to 233.3, to 256.5 and to 276.9 mm ha−1 , equivalent to 62%, to 75%, to 78%, and to 83% of the total water consumed while E was correspondingly decreased to 38%, to 25%, to 22% and to 17% when N rate was added at 30, 60, 90 and 120 kg ha−1 , respectively. 3.4. Effect of N fertilization on water use efficiency (WUE) Both mulching and N fertilization substantially increased the total aboveground biomass production and the grain yield was proportionally up with the biomass (Table 5). Conservation of water by mulching and application of N fertilizer both played significant roles in increase of water use by plants and this was the basis for increase of crop yield. The quantity of water used to produce plant biomass or crop yield can be evaluated in several ways. Of these, water use efficiency (WUE) is extensively adopted. Strictly, it is not the “efficiency” because a true efficiency is a comparative term (i.e., dimensionless) requiring a theoretical maximum value. For this reason, some researchers prefer to use other terms such as water use “coefficient” or “ratio” (Gregory, 1988). Further, as Sinclair et al. (1984) pointed out, “WUE has been used interchangeably to refer to observations ranging from gas exchange by individual leaves for a few minutes, to grain yield response to irrigation treatments through an entire season”. Despite some arguments, the term is still adopted widely by agronomists in China and elsewhere in the world. For calculation of WUE, water quantity is commonly

24

S.X. Li et al. / Agricultural Water Management 162 (2015) 15–32

Table 5 Effects of plastic mulch and N rates on total water loss, transpiration and evaporation, and water use efficiency. Determination items

Plastic mulched treatment Grain (kg/ha) (all treatments) Grain (kg/ha) (N addition) Dry shoot matter (kg/ha) All treatments N addition Initial water in 2 m layer (mm) Harvest indices Rainfall plus irrigation (mm) Water in 0–2 m at harvest (mm) Total water loss (ET, mm) Evaporation (E, mm) Transpiration (T, mm) T/ET WUEET Grain Dry matter WUET Grain (all treatments) Grain (N addition) Dry matter (all treatments) Dry matter (N addition) Non-plastic mulched treatment Grain (kg/ha) All treatments N addition Dry shoot matter (kg/ha) All treatements N addition Harvest indices Initial water (mm) in 2 m layer Water in 0–2 m at harvest (mm) Total water loss (ET, mm) Transpiration (T, mm) Evaporation (E, mm) T/ET (all treatments) T/ET (N addition) WUEET Grain (all treatments) Grain (N addition) Dry matter (all treatments) Dry matter (N addition) WUET Grain (all treatments) Grain (N addition) Dry matter (all treatments) Dry matter (N addition)

N rate (kg/ha) 0.0

30.0

60.0

2565

4193 4193

5888 5888

8989 320.6 28.5 305.1 380.6 245.1 20.0 225.1 0.92

11362 11362 320.6 36.9 305.1 335.7 290.0 20.0 270.0 0.93

90.0

12525 12525 320.6 47.0 305.1 309.2 316.5 20.0 296.5 0.94

7328 7328 14451 14451 320.6 50.7 305.1 262.0 363.7 20.0 343.7 0.95

Mean

LSD0.05

5503 6237

187.4 154.2

120.0 7539 7539 14593 14593 320.6 51.7 305.1 259.7 366.0 20.0 346.0 0.95

12384 13233 320.6 43.0 305.1 309.4 316.3 20.2 296.3 0.94

10.5 36.7

14.5 39.2

18.6 39.6

20.2 39.7

20.6 39.9

16.9 39.2

11.4

15.5 15.5 42.1 42.1

19.9 19.9 42.2 42.2

21.3 21.3 42.0 42.0

21.8 21.8 42.2 42.2

18.0 19.6 41.7 42.1

39.9

1833

2814 2814

3825 3825

5368

7300 7300 38.5 320.6 324.8 300.9 187.7 113.2 0.62 0.62

9112 9112 42.0 320.6 315.3 310.4 233.3 77.1 0.75 0.75

34.1 320.6 333.7 292.0 138.2 153.8 0.47

6.3 18.4

13.3 38.8

4646 4646 10006 10006 46.4 320.6 297.9 327.8 256.5 71.3 0.78 0.78

4988 4988 10798 10798 46.2 320.6 290.7 335.0 276.6 58.4 0.83 0.83

154.1 85.3

8517 9304 41.4 320.6 312.2 313.2 218.5 94.8 0.69 0.75

754.2 653.5 0.5 3.1 3.2 4.2 5.4 7.1

12.3 12.3 29.4 29.4

14.2 14.2 30.5 30.5

14.9 14.9 32.2 32.2

11.4 12.7 27.0 29.1

15.0 15.0 38.9 38.9

16.4 16.4 39.1 39.1

18.1 18.1 39.0 39.0

18.0 18.0 39.0 39.0

16.2 16.9 39.0 39.0

WUEET = Y /ET WUET = Y/T WUEP = Y/TP where Y is crop yield, either total dry biomass or economical products; ET, evapotranspiration; T, total transpired water; TP, total precipitation in the plant growing season. The WUEET calculated by ET may be used for evaluating effects of cropping practices on

1.3 2.4

3621 4068

9.4 9.4 24.3 24.3

measured as the residual term in water balance equation and expressed as total water use, that is, E directly from the soil surface plus T, or ET during plant growing season (Cooper et al., 1983). It may also be defined as T alone or as the total water input (precipitation only on drylands) to a system in a plant growing season. Such relations can be expressed in following equations:

982.4 824.1 3.1 0.7 2.3 4.3 4.8 2.2 3.4

1.1 0.9 2.3 1.9

the difference of water loss by both E and T under different crop production systems as well as on the degree of effectiveness of water use. The WUET calculated by T may be used for providing information on water metabolism function of a plant and on the relation between plant growth and water use. The WUEP calculated by TP, including different water losses and residual water still existing in soil profile during plant growing period, may be of significance for a long-term study in evaluation of effects of cropping measures on the bioavailability of water resources (Sinclair et al., 1984). In the current study, we estimated ET as growing season precipitation and irrigation plus soil water depletion from maize seeding to harvest, and using the plastic mulch technique, we estimated T, the transpired water and by subtracting T from the total soil water loss (ET) as measured at different time, we calculated the water loss by E, all at five N rates. Based on our experimental data and parameters obtained, all the three relations could be used to calculate their corresponding WUE. However, since it was not a long-term study, we

S.X. Li et al. / Agricultural Water Management 162 (2015) 15–32

concentrated on WUEET and WUET as keys for an overall evaluation of agricultural measures and plant water metabolism. As shown in Table 5, both plastic mulching and N fertilization greatly increased T and reduced E. On average of five N rates, the plastic mulched treatments consumed 316.3 mm water and the non-plastic mulched 313.2 mm. Although the total water amounts consumed were almost no difference between soil surface treatments, the water loss by E was greatly different. In the plastic mulched field, the water loss by E was only 20 mm as estimated above whereas that in the non-plastic mulched field was 94.8 mm or 30.3% of the total water loss during the entire plant growing period. As a result, the averaged WUEET was substantially different: it was 39.2 kg ha−1 mm−1 for aboveground biomass and 16.9 kg ha−1 mm−1 for grain in plastic mulched plots while 27.0 and 11.4 kg ha−1 mm−1 , respectively, in the non-plastic mulched field, the former being 45% and 48% higher for aboveground biomass and grain yield respectively than the latter. Application of N fertilizer played even more important role in rise of T and reduction of E, both the increase in T and decrease in E being linearly related to N rates. Using the average of total water consumption ET to make comparison, the N fertilized plots consumed 334.1 mm ha−1 water on average and the control (without N addition) 245.1 mm ha−1 water under the plastic mulched conditions while the corresponding treatments consumed 318.5 and 292.0 mm ha−1 water under the non-plastic mulched conditions. The differences in water utilization were due to different T/ET and different biomass and grain yield production with five N rates under the two cultivated conditions. However, in any case, the ratio of crop yield increase by N fertilization was much higher than that of water consumed. For the plastic mulch treatments, the highest grain yield with addition of 120 kg N ha−1 was almost 2.94 fold compared to the lowest without N addition; and for the nonplastic mulched treatments, the highest was 2.72 fold of the lowest. As a result, the magnitude of WUEET was consistently increased for both grain and biomass with N rate increase. Having been mulched, the WUEET was increased from 10.5 to 14.6, to 18.6, to 20.2 and to 20.6 kg ha−1 mm−1 for grain, and from 36.7, to 39.2, to 39.6 to 39.7 and to 39.9 kg ha−1 mm−1 for aboveground biomass when N rate was increased from 0 to 30, to 60, to 90 and to 120 kg ha−1 , respectively. Not having been mulched, the variation pattern of WUEET with N rates was similar to, but the values were lower than those under the plastic mulched conditions: the WUEET values were 6.3 kg ha−1 mm−1 for grain and 18.4 kg ha−1 mm−1 for aboveground biomass in plots without application of N fertilizer while 9.4, 12.3, 14.2, and 14.9 kg ha−1 mm−1 for grain and 24.3, 29.4, 30.5 and 32.2 kg ha−1 mm−1 for aboveground biomass in plots with 30, 60, 90 and 120 kg N ha−1 addition, respectively. Clearly, due to rise of aboveground biomass and higher crop yield than the water consumed as a basis, N fertilization raised WUEET and correspondingly resulted in the increases of T to ET ratios. The WUET differed from the WUEET . Under the plastic mulched conditions, the transpired water was only 20 mm less than ET, and therefore the WUET was not greatly deferent from the WUEET . Under the non-plastic mulched conditions, however, WUET was much higher than WUEET , approximating to the WUET in the mulched plots. On average of five N rates, the WUET was 18.0 and 16.2 kg ha−1 mm−1 for grain and 41.7 and 39.0 kg ha−1 mm−1 for aboveground biomass under the plastic and non-plastic mulched conditions, respectively. Clearly, soil surface treatments did not exhibit great impact on the WUET values for both grain and biomass. Application of N fertilizer performed another pattern. Similar to soil surface treatments, N fertilization had no significant influence on the WUET of the aboveground biomass as shown in Table 5 where the measured values of aboveground biomass were used for calculation. The same trends could also be gained from equations

25

established on Fig. 3. In those equations, y was biomass (aboveground biomass) in term of kg ha−1 and x, transpired water in term of mm ha−1 . Values for slopes in the equations were indicative of WUET and the equality of the slopes showed that the WUET values were almost no difference for different N rates. Such a result can be expected since the WUET is the reciprocal of the so-called “transpiration coefficient” (defined as water consumption amount by T for producing 1 unit of product) in plant physiology (Pan and Dong, 1982), and therefore it is another form for expressing how much water is consumed for producing 1 unit aboveground biomass. Using the measured biomass at harvest to calculate, WUET values for aboveground biomass had the same trends as shown by the equations, and varied from 39.9, to 42.1, to 42.2, to 42.0 and to 42.2 kg ha−1 mm−1 under plastic mulched conditions and from 38.8, to 38.9, to 39.1, to 39.0 and to 39.0 kg ha−1 mm−1 under non-plastic mulched conditions when N rate was increased from 0, to 30, to 60, to 90 and to 120 kg ha−1 . Changes in WUET under plastic mulched conditions indicated variations in microenvironment conditions due to N application, and the higher WUET values with N fertilization were due to less water consumption by T at the harvest period with more humid climate (Fig. 3). In contrast, the WUET values under the non-plastic mulched conditions were constant due to T calculated from the established equations that had almost no influence on WUET compared to the concretely measured results. However, in both cases, the WUET values were in a reasonable range. Viewed as a whole from either the values from slopes of equations or those calculated from the aboveground biomass measured at harvest, the WUET was fairly constant. All this further confirmed that in a given climate, maize plant metabolize almost the same amount of water to produce 1 unit aboveground biomass (Ehlers and Goss, 2003; Steduto et al., 2007). In contrast, the WUET of grain production was of great difference: it varied from 11.4, to 15.5, to 19.9, to 21.3 and to 21.8 kg ha−1 mm−1 under the plastic mulched conditions and from 13.3, to 15.0, to 16.4, to 18.1 and to 18.0 ha−1 mm−1 under the non-plastic mulched conditions with N rates from 0, to 30, to 60, to 90 and to 120 kg ha−1 , respectively (Table 5). Fig. 4 clearly illustrated the total water consumption, T and E from 0 to 200 cm depth with five N rates under plastic mulched and non-plastic mulched conditions 3.5. Relation of N fertilization to harvest index The differences in WUET of grain production were due to the harvest index that was defined as ratio of economic yield to aboveground dry matter yield (Donald and Hamblin, 1976) and that was useful in characterizing a wide range of agronomic experiments. Grain yield is determined by multiplying the harvest index value times the amount of aboveground biomass, and the aboveground biomass is directly proportional to the amount of T that is affected by the vapor pressure deficit. The harvest index is affected by many factors and it is genetically controlled to maximum only when water and nutrient supply are adequate. Under dryland conditions, water is always limited so harvest index values seldom approach the maximum values. Prihar and Stewart (1990) estimated the genetic harvest index for grain sorghum and maize to be approximately 0.53 and 0.60, respectively. The harvest index values can decline significantly when water is severely limited during the grain filling period, and under extreme conditions, the harvest index can be zero. As a rule, the aboveground biomass yield is closely correlated to grain production. Since aboveground biomass includes grain, it is evident that there should be indicative of the high degree of self correlation that must exist between the total dry matter and grain (Howell, 1990). For avoiding such a self correlation, we calculated the relations of N rates to harvest index and WUET to harvest index rather than grain yield- biomass production relationships.

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Fig. 4. Total water consumption (TWC), water transpired (T) and water evaporated (E) from 0 to 200 cm layer at N rates from 0 to 120 kg ha−1 under plastic mulched (left) and non-plastic (right) conditions.

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Fig. 5. Relation of harvest index (%) to N rate (kg ha−1 ) applied under non-plastic sheet mulched and plastic sheet mulched conditions (data from Table 5).

Results revealed that, under adequate supply of other nutrients, N fertilization significantly increased the grain production (Li et al., 1994) and thus the harvest index. Although the slopes of the lines in the two figures are significantly different under plastic and non-plastic mulched conditions due to water consumption difference, the harvest index was highly correlated with N rates (Fig. 5) and with the WUET (Fig. 6). Grain production is the purpose for crop plantation, and increasing more grain than biomass is another benefit for application of N fertilizer. From the results obtained, we can conclude that for increasing water use efficiency, the first thing we have to do should be reduction of water loss by E and increase of water use by T and then raising the T efficiency or T coefficient for grain production. At any rate, adequate supply of N fertilizer for a soil deficient in this nutrient available will play multiple roles.

4. Discussion 4.1. Function of T in agriculture and its improvement Water limited environments currently comprise about half of the earth’s land surface and are expected to continue to expand (Newman et al., 2006). As a result, agricultural water resource limitations have become a major issue for a corresponding increase in food demand (Béziat et al., 2013). The drastically increased world population and the overall standard of living improvement further increase the competition for the same limited water resource and will produce pressure on agriculture (Kite, 2000; Yermiyahu et al., 2007). The water scarcity is more serious in China with only about one quarter of water of the world average per capita and the conflict among agriculture, industry and municipalities demanding more water than available (Li and Peng, 2009). The global climate change will locally impact the mean and variance of temperature as well as the amount and distribution of precipitation and atmospheric CO2 concentrations (IPCC, 2007); agriculture will be strongly impacted

by these changes (Brouder and Volenec, 2008). In this context, improvement of the efficiency of precipitation and irrigation water is vital for sustainable development of water resources and environment protection. In agricultural and natural vegetation land surface, if runoff and drainage do not occur, the only way for water consumption is through ET, that is, E from the soil and T through the stomata of plants. Being the important element of the hydrological cycle and major water balance component, ET serves as a regulator of key ecosystem processes by linking stomatal activity, carbon exchange and water use. The accurate assessment of ET and quantification of its components are fundamental to help investigate if irrigation can be improved and available water be used more productively (Kite, 2000; Zhao et al., 2013). However, the function of E and T within ecosystems is distinctly different. Driven by atmospheric demand, soil water potential and hydraulic conductivity, E constitutes a large fraction of ET in sparse natural and agricultural ecosystems under row crops and orchards with crops at initial growth stages due to considerable areas of exposed soil. It has been reported that E is about 30–60% in wheat field (Cooper et al., 1983) and 30–80% in range lands (Wilcox et al., 2003). Although E may have effects in moderating vapor pressure deficit (Leuning et al., 1994), and create a more favorable microclimate in some cases for plant use of T (Kustas and Norman, 1999; Kustas and Agam, 2013), or act to reduce T (Agam et al., 2012) up to 50% as a result of E associated with sprinkler irrigation (Tolk et al., 1995), it is largely a non-beneficial, wasteful water loss and does not directly contribute to production. The T, on the other hand, is the process of water moving from soil, through roots, stem and leaves of a plant into the atmosphere (Jones and Tardieu, 1998). Regulated by opening and closing of the stomata of a plant (Jones and Tardieu, 1998), it is the only water that passes through the crop associated with plant growth and productivity (Passioura, 1976; Tanner and Sinclair, 1983) and the only important physiological indicator of the plant’s water metabolism. The degree of T rate and T coefficient (de Wit, 1958; Sinclair et al., 1984) are all related with

Fig. 6. Relation of WUET (kg mm−1 ha−1 ) of grain to harvest index (%) under plastic mulched and non-plastic mulched conditions (data from Table 5).

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the formation of aboveground biomass and crop yields. Changes in crop management have frequently been reported to increase T by a crop (Gregory, 1989). For this reason, in cropping systems, an effective use of rainfall or irrigation requires that the volume of water transpired and T coefficient by the crop should be maximized while that of E minimized. This is the basic strategy for crop production, especially in dryland areas. On drylands of China, the major constraint for crop production is the shortage of water supply, and adoption of measures that can increase water use by plants is obviously essential. The major water losses in the drylands include runoff and E. Runoff is an undesirable water flow process (Guzha, 2004) over soil surface or through soil by which soil is more or less eroded and the amount of water available for productive T is reduced. The seriousness of runoff and its induced soil erosion have long been documented in the Loess Plateau, a major part of dryland areas in China, due to short time of torrential rain, or rainstorms in summer, leading to the topsoil being depleted and soil depredated (Brown and Wolf, 1984; Gao and Bai, 1994; Wei et al., 2000). However, as a whole, water loss by runoff mainly occurs on sloping lands while that by E occurs widespread, not only on sloping lands, but on flat lands as well. Although this loss is not clearly sighted by eyes as that of runoff, its loss is really serious since it occupies a large portion of the total lost water and occurs on all lands. For reduction of water loss by such a way, great attention has been given to mulching techniques. As reported in our previous paper (Li et al., 2013), mulching, either plastic or wheat straw, plays a significant role in conservation of water and has been received as priority by famers and agricultural scientists. Due to interception of solar radiation and reduction of wind speed close to the soil surface (Jalota and Prihar, 1990), the presence of crop residues or plastic sheets on the soil surface has been shown to conserve water by suppression of E (Bond and Willis, 1969), especially early in the crop season when the crop canopy is small, The water saved by E in the early growth period could be used as T in early and late seasons for increasing water productivity (Lascano et al., 1994; Li et al., 2008). Laboratory studies showed that in a single drying cycle, cumulative E from straw-mulched soil initially lagged behind that from non-mulched soil, but with time, total water loss was similar to or even exceeded that of non-mulched soil (Jalota and Prihar, 1990; Jalota, 1993). However, in field, where rain and irrigation interrupt drying cycles, the effects of multiple drying and wetting cycles are likely to further interact with the influence of mulch on E. The effectiveness of mulch in conserving moisture is most pronounced when soils are wet and E is in evaporation stage 1 (Bond and Willis, 1970; Steiner, 1989) such as after irrigation or precipitation. Ritchie (1972) and Yunusa et al. (1993) also suggest that mulch is effective only in the early periods of plant growth when canopy cover is small, since plants can influence microclimate and the uptake of water for T. The function of the plastic mulch in restricting water movement can be attributed to physical impedance of the movement of water vapor from the soil to the atmosphere; modification of the soil surface energy balance (Horton et al., 1996) via the interception of solar radiation (Jalota and Prihar, 1990). Without mulch plants may have had higher T by regulating stomata opening and closing in response to lower water availability, while with mulch the stomata may have remained open for a longer time (Singh et al., 2011). Loomis (1983) emphasizes that where water is limiting, several basic strategies should be followed to bring crops to maturity within the available supply: (1) ensure that a large proportion of the available water goes to T; (2) achieve a high level of production per unit of T; and (3) achieve a balance between seasonal use of water and seasonal supply. The present study evidences that for reaching such purposes, N fertilization plays a great role not only making the plants to fully use water by T, but also to conserve water in soil from E loss, and

finally bring maize to maturity with high yield and limited water supply. 4.2. Importance of N fertilization in increase of water use by plants It has been found that in many cases, even mulching is properly conducted, the water use by plants may be still low enough and the effect of mulch may be not ideal if soil nutrients are deficient, particularly N. As an essential element, N is used by plants and living organisms in large amounts to produce a number of complex organic molecules such as amino acids, proteins and nucleic acids that play extremely important roles in their lives (Mengel and Kirkby, 2001). An adequate and balanced N nutrient supply from soil is a prerequisite for abundant root and aboveground plant growth, and therefore for crop yields as well as for WUE (Liu et al., 1998; Hatfield et al., 2001; Ali and Talukder, 2008). Studies have shown that the effect of different textural soils on growth and yield of potato (Solanum tuberosum L.) (Ahmadi et al., 2011a,b) and peanut (Songsri et al., 2009) was correlated with differences in N uptake since the long-term N-mineralization potential was different in different textural soils (Knight et al., 2008). However, almost all of soils worldwide are deficient in N supply, and there is a great gap between plant demand and soil supply. For much of human history, N supply limited crop production, and still does in many subsistence and low-input farming systems. The application of chemically fixed N has strongly helped promote agricultural development for supplying food to man and fodder to animals (Evans, 1998), enabling the enormous and unprecedented expansion of the global human population. For this reason, application of N fertilizer has been regarded as a priority for nutrient supply. This is especially true for three cereals, wheat, maize and rice that are major crops for consumption of chemical N fertilizers. Agriculture development in dryland areas is closely associated with the use of N fertilizer as in other regions, and N fertilizer has an indissoluble bond with agriculture (Li et al., 2009a). It has long been recognized that application of N fertilizer or improvement of soil N fertility is an essential way for crop management. Although N fertilization is to raise crop yield as the main purpose and has never been regarded as a measure for water conservation, it plays a very important role in increase of the proportion of water used by plants. The principal basis of N nutrient input on improvement of water use is to promote plant growth and allow more rapid expansion of the canopy. This will result in the following effects: First, vigorous plant growth by promotion of N nutrient input can take up more water from soil: that will in turn reduce water content in soil, and lead to reduction of soil water loss by E, as water that is not transpired by plants would otherwise be evaporated directly from the moist soil surface. Second, changes in nutrient status affect shoot growth, and thereby the extent of ground covers and the volume of water transpired (Cooper et al., 1983). In cropping lands, the ground cover is determined by the crop canopy that is generally characterized by LA or LAI. Acting as shelter, the LA and LAI, can shade the soil surface more completely, reduce and even avoid the solar radiation directly to soil surface, lower the soil temperature, and therefore reduce the proportion of water loss by E (Cooper et al., 1987; Shepherd et al., 1987). When the plant canopy is large, and its duration is long, E losses from the soil surface are often small, and T losses commensurately greater (Cooper et al., 1983). Transpiration (T) of water through plants will consume energy, and cool the plant-growing microenvironments; the decline of temperature in the microenvironment will in turn reduce water loss by E. In addition, the LA can directly increase plant T consumption, and there is a linear relationship between T and biomass, and the slope of the line depends on the nutrient availability. Crop growth under field conditions is

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dependent on the ability of the canopy to intercept the incoming radiation, and on its conversion into biomass (Gifford et al., 1984), and all this is related to LAI and canopy architecture. Andersen et al. (1996), Ferreira and Carr (2002) and Shahnazari et al. (2007) reported an association of higher crop N-uptake with denser crop canopies and high LAI values that intercept more light and ultimately produce higher yields. The N uptake enhances crop yield by increasing LAI and the intercepted photo-synthetically active radiation (de Medeiros et al., 2001; Williams et al., 2003) and thus raises the crop T coefficient (Ritchie and Burnett, 1971; Ayars et al., 2003). Third, N fertilization promote the development of root systems. It has been reported that plant nutrients determine to a certain extent the growth of roots (Rockstrom and Barron, 2007) because the source of the substrate for root growth is from photosynthesis, which depends on the LA and per unit leaf rate or LAI, both of which are nutrient-dependant. A larger root system may not only tap a greater volume of stored water, but also reduce the volume of water otherwise lost in E (Knight et al., 2008). When the roots are not impaired by pathogens, the higher N status of the crop leads to a larger root system and to more soil water extraction (Liu et al., 1998; Angus and van Herwaarden, 2001) than the lower N status crops. As a final result, N fertilization significantly increases WUE and crop yield as reported by a variety of experiments (Liu, 2000; Li et al., 2009a) and the present study from us. Our experiment clearly demonstrated the important effects of N fertilizer input on such functions to the soil deficient in this element available. In addition to improvement of the root growth (Du et al., 1995a) and physiological properties of maize plants (Du et al., 1995b; Xu and Shan, 1995), reduction of E and increase of T were the basis of N fertilization in improvement of WUE and crop yield. 4.3. Transpiration—aboveground biomass relations Crop WUE, is usually defined as the amount of aboveground dry matter produced by a unit of water consumption in both T and E (Sinclair et al., 1984). Physiologically, WUE depends on the response of CO2 and H2 O diffusion to partial stomata closure (Jarvis et al., 1999). Factors affecting the amount of water stored in soils, such as infiltration and storage capacity, and the ability of roots to extract the water from the soil, have major effects on crop yields (Carberry et al., 2011; Chaves and Oliveira, 2004) and climate factors, such as vapor pressure deficits and fraction of diffuse radiation (Doherty et al., 2010), may limit the apparent water-limited potential yield for crops. In history and at present, WUE is widely adopted for evaluation of water use by plants (Deng et al., 2006). However, the inclusion of E from the soil surface in the denominator makes the WUE for any particular crop highly variable depending upon locations, years and crop management (Doyle and Fischer, 1979). For avoiding the instability of WUE and for assessing crop productivity in regions from which little quantitative data are available, some workers prefer to use a robust parameter with a more conservative behavior rather than WUE. In the mid-20th century, de Wit (1958) states that normalization of water productivity (WP*) for different evaporative demands of the environment can make its value conservative for application in different environments. The WP* is defined as ratio between crop biomass and the integral of normalized daily T over the growth duration of the crop (Steduto et al., 2009) and the normalized daily T is calculated as the ratio of daily actual T over daily potential reference ET. The parameter is relatively insensitive to variation in soil nutrient status (Tanner and Sinclair, 1983; Stanhill, 1986) and varies only slightly with contrasting climatic conditions. Evidence for the conservative nature of WP* has been given by Steduto et al. (2007), who also conclude that C3 and C4 species have different values of WP*. Within the C3 and C4 species groups, WP* tends to be conservative and the investigated

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C3 species, sunflower, wheat and chickpea (Cicer arietinum), could be grouped in one value of WP* (Steduto and Albrizio, 2005) and such a similar value was further confirmed by Yuan et al. (2013). The similarity in values from different authors provides further evidence for the purported consistency of WP* and demonstrates that the tested C3 crops shared a common value of water productivity in years of their study. Of course, the concepts of WP* and WUE differ fundamentally and the results have no basis for comparison. However, when WUE takes T as variable (WUET ), a stable, linear relation between T and aboveground biomass exists for at least some crops that have been studied. The linearity between crop biomass and water uptake has been observed since the early 1900s (Briggs and Shantz, 1913), and values for T coefficient have been found more conservative (Ludlow and Muchow, 1990), but are reported only rarely. Sinclair and Weise (2010) state that in a 2 kPa transpiration environment, a C4 crop growing will have a T rate of approximately 220 g water per day for each g of plant growth while a C3 crop will require 330 g for each g of plant growth. The water requirement, however, is greatly influenced by temperature, radiation, humidity and wind in the environment close to where plants are growing (Stewart and Lal, 2012). In our study, we use the transpiration-aboveground biomass relationship as WUET established from the plastic mulched plots to estimate E and T in the non-plastic mulched plots, and evidences that such a relation can be successfully used for estimation of E and T for maize, a C4 crop under a given condition. Calculated from the WUET , for producing 1 g of aboveground biomass, about 256 g of water was transpired. Certainly, there existed some environmental climate variations at plant growth period and with different N rates. When the environmental climate was hotter and drier, it took more than 256 g of water to produce 1 g of maize biomass whereas when the environmental climate was cooler or wetter, it took less than this amount water to produce 1 g of maize biomass. Similarly, different N rates show some slight difference in the T coefficient, For example, without N application, leaves were not so abundant, and E was higher, resulting in a drier microenvironment and thus higher water amount than 256 g was consumed for producing 1 g of biomass while with N fertilization, less water was evaporated, and the microenvironment was relatively wet, and therefore a lower water consumption (T) was observed for producing 1 g of biomass. However, as a whole, the water amount used for producing a unit of aboveground biomass was stable. It is clear that the result obtained from the relationship between T and aboveground biomass is the comprehensive performance of crop type and the environments. For a given type of crops under a certain macroclimate condition in a region, the water used for producing a unit of aboveground biomass is constant with minor variation modified by the plant-growing microclimate.

5. Conclusion (1) An adequate supply of N fertilizer to soil deficient in this element significantly promoted plant growth. With addition of 120 kg N ha−1 , LA and LAI in both the plastic and non-plastic mulched plots were almost doubled compared with no N fertilization. As a direct result, maize aboveground biomass production was regularly increased with N rate increase. (2) Due to abundant plant growth and large canopy formation by N fertilization, resulting in greatly shading the soil surface and higher uptake of water by plants, the transpired water as estimated was increased from 225.1, to 270.0, to 296.5, to 343.7 and to 346.0 mm ha−1 under the plastic mulched conditions and from 138.2, to 187.7, to 233.3, to 256.5 and to 276.9 mm ha−1 under the non-plastic mulched conditions for N rate of 0, 30, 60, 90, 120 kg ha−1 , respectively. Under the non-plastic mulched

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conditions, the T was equivalent to 47%, 62%, 75%, 78%, and 83% of the total water consumed while E was equivalent to 38%, 25%, 22% and 17%.with N addition at rates of 0, 30, 60, 90 and 120 kg ha−1 , respectively.. (3) Under our experimental conditions, for producing 1 g of biomass, about 256 g of water was needed. This figure was totally in agreement with that calculated from the average of five N rates in the previous study. The fairly constant WUET showed that N rates had no impact on “transpiration coefficient” of the aboveground biomass although N fertilization greatly raised the total transpired water amounts. The results further revealed that in a given climate, maize plant metabolize almost the same amount of water to produce one unit biomass. In contrast, the WUET of grain production varied from 11.4, to 15.5, to 19.9, to 21.3 and to 21.8 kg ha−1 mm−1 under plastic mulched conditions and from 13.3, to 15.0, to 16.4, to 18.1 and to 18.0 ha−1 mm−1 under non-plastic mulched conditions with N rate changes from 0, to 30, to 60, to 90, and to 120 kg ha−1 , respectively. To sum up, N fertilization to the soil deficient in this nutrient significantly increased the total water use by T and increased the harvest index, resulting in improved maize grain yield; while the aboveground biomass and T relation was stable. Acknowledgements This work was part of the projects (30230230, 49890330, 39770425 and 30070429) successively supported by the National Natural Science Foundation of China (NFSC). The authors would like to take the opportunity to express their sincere thanks to the NFSC, for its kindness of supporting such projects in succession. Sincere thanks are given to the editors and three anonymous reviewers for their careful work on the manuscript; without their critical reviews, some mistakes may be not found in the original manuscript and the paper may not appear in such a manner. References Agam, N., Evett, S.R., Tolk, J.A., Kustas, W.P., Colaizzi, P.D., Alfieri, J.G., Mckee, L.G., Copeland, K.S., Howell, T.A., Chavez, J.L., 2012. Evaporative loss from irrigated inter-rows in a highly adjective semi-arid agricultural area. Adv. Water Res. 50, 20–30. Ahmadi, S.H., Andersen, M.N., Lærke, P.E., Plauborg, F., Sepaskhah, A.R., Jensen, C.R., Hansen, S., 2011a. Interaction of different irrigation strategies and soil textures on the nitrogen uptake of field grown potatoes. Int. J. Plant Prod. 5, 263–274. Ahmadi, S.H., Plauborg, F., Andersen, M.N., Sepaskhah, A.R., Jensen, C.R., Hansen, S., 2011b. Effects of irrigation strategies and soils on field grown potatoes: root distribution. Agric. Water Manage. 98, 1280–1290. Ali, M.H., Talukder, M.S.U., 2008. Increasing water productivity in crop production—a synthesis. Agric. Water Manage. 95 (11), 1201–1213. Andersen, M.N., Heidmann, T., Plauborg, F., 1996. The effects of drought and nitrogen on light interception, growth and yield of winter oilseed rape. Acta Agric. Scand. B: Soil Plant 46, 55–67. Angus, J.F., van Herwaarden, A.F., 2001. Increasing water use and water use efficiency in dryland wheat. Agron. J. 93 (2), 290–298. Anken, T., Weisskopf, P., Zihlmann, U., Forrer, H., Jansu, J., Perhacova, K, 2004. Long-term tillage system effects under moist cool conditions in Switzerland. Soil Tillage Res. 78 (2), 171–183. Ayars, J.E., Johnson, R.S., Phene, C.J., Trout, T.J., Clark, D.A., Mead, R.M., 2003. Water use by drip irrigated late season peaches. Irrig. Sci. 22, 187–194. Basch, C., Kassam, A., Friedlich, T., Santos, F.L., Gubiani, P.I., Calegari, A., Reichert, J.M., dos Santos, D.R., 2012. Sustainable soil water management systems. In: Lal, R., Stewart, B.A. (Eds.), Soil Water and Agronomic Productivity. CRC Press, Taylor & Francs Group, Boca, Raton, London, New York, pp. 229–288. Baumhardt, R.L., Jones, O.R., 2002. Residue management and paratillage effects on some soil properties and rain infiltration. Soil Tillage Res. 65 (1), 19–27. Bescansa, P., Imaz, M.J., Virto, I., Enrique, A., Hoogmoed, W.B., 2006. Soil water retention as affected by tillage and residue management in semiarid Spain. Soil Tillage Res. 87 (1), 19–27. Béziat, P., Rivalland, V., Tallec, T., Jarosz, N., Boulet, G., Gentine, P., Ceschia, E., 2013. Agricultural and Forest Meteorology 177 46–56. Bond, J.J., Willis, W.O., 1969. Soil water evaporation—surface residue rate and placement effects. Soil Sci. Soc. Am. Prod. 33, 445–448.

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