Nitrogen use efficiency, water saving and yield of rice ...

Download PDF
3 downloads 0 Views 946KB Size Report
Comment
Oct 24, 2016 - omy Special Publication. Ishaq, M., M. Ibrahim, A. Hassan, M. Saeed, and R. Lal. 2001. Subsoil compaction effects on crops in Punjab, Pakistan ...
Journal of Plant Nutrition

ISSN: 0190-4167 (Print) 1532-4087 (Online) Journal homepage: http://www.tandfonline.com/loi/lpla20

Nitrogen use efficiency, water saving and yield of rice transplanting on raised bed over traditional flat method Abdul Majeed, Abid Niaz, Atif Muhmood, Zahid Ashfaq Ahmad, Muhammad Ilyas & Abdul Wakeel To cite this article: Abdul Majeed, Abid Niaz, Atif Muhmood, Zahid Ashfaq Ahmad, Muhammad Ilyas & Abdul Wakeel (2017) Nitrogen use efficiency, water saving and yield of rice transplanting on raised bed over traditional flat method, Journal of Plant Nutrition, 40:3, 307-314, DOI: 10.1080/01904167.2016.1240190 To link to this article: http://dx.doi.org/10.1080/01904167.2016.1240190

Accepted author version posted online: 24 Oct 2016. Published online: 24 Oct 2016. Submit your article to this journal

Article views: 40

View related articles

View Crossmark data

Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=lpla20 Download by: [National Research Centre for Citrus ]

Date: 25 February 2017, At: 00:48

JOURNAL OF PLANT NUTRITION 2017, VOL. 40, NO. 3, 307–314 http://dx.doi.org/10.1080/01904167.2016.1240190

Nitrogen use efficiency, water saving and yield of rice transplanting on raised bed over traditional flat method Abdul Majeed, Abid Niaz, Atif Muhmood, Zahid Ashfaq Ahmad, Muhammad Ilyas, and Abdul Wakeel Soil Chemistry Section, Institute of Soil Chemistry and Environmental Sciences, Ayub Agricultural Research Institute, Faisalabad, Pakistan

ABSTRACT

Flat transplanting with flooded irrigation is commonly used for growing rice, but it results in ineffective use of applied fertilizer and water. The objective of this study was to compare nitrogen use efficiency and water saving of rice transplanted on beds and on flat ground. This field experiment was performed in randomized complete design for three continuous years and all treatments were repeated three times. Results showed that transplanting of four lines of rice nursery on bed and one line in furrow and nitrogen application at 150 kg ha¡1 recorded 16.06, 21.81, 16.0 and 20.21% higher paddy yield, nitrogen (N) uptake in paddy, N use efficiency and N agronomic efficiency than traditional flat method at same N level. Without loss in yield, about 25 kg ha ¡1 of N fertilizer and 24% of water can be saved with bed transplanting of rice as compared to flat method.

ARTICLE HISTORY

Received 25 June 2014 Accepted 1 April 2016 KEYWORDS

Bed and flat transplanting; N use efficiency; water saving; paddy yield

Introduction The cost of crop production, from fertilizer prices to fuel and water, has increased over the years. Need for food production around the world has increased due to increasing population growth (Barrett 2010). In Pakistan, rice ranks as second amongst the main food grain crops and it has been an important source of foreign exchange incomes in current years. Nitrogen (N) is important to rice growth and grain yield and compared to other nutrients, it is applied in higher quantity in all rice growing methods (Bond et al. 2008). Efficient use of applied nitrogen helps in improving rice grain yield and reducing the cost of production. Best nitrogen management is essential for enhancing rice yield and minimum pollution to environment. Using less nitrogen may result in lower rice yields, poorer grain quality and hence, reduced profits. When higher doses of nitrogen are applied, they can result in lower nitrogen use efficiency and higher fertilizer losses in form of leaching, volatilization, de-nitrification and nitrous oxide emissions. These nitrogen fertilizer losses results in various harmful environmental and health problems (Ju et al. 2009; Erisman et al. 2008; Xing and Zhu 2000). Generally rice is transplanted in puddled fields with continuously flooded water and is therefore a major user of fresh water resources in the world (Tuong and Bouman 2003). This conventional method induces higher bulk density, soil strength and lower permeability in subsurface layers (Naresh et al. 2014; Kukal and Aggarwal 2003). These aspects limit root growth, and water and nutrient use from the soil profile by wheat sown after rice (Ishaq et al. 2001). Flat transplanting of rice with flood irrigation can result in low water use

CONTACT Abdul Majeed [email protected] Soil Chemistry Section, Institute of Soil Chemistry and Environmental Sciences, Ayub Agricultural Research Institute, Faisalabad 38000, Pakistan. Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/lpla. © 2017 Taylor & Francis Group, LLC

308

A. MAJEED ET AL.

efficiency and higher levels of crop lodging and enhanced the incidence of some crop diseases (Mollah, Bhuiya, and Kabir 2009). Different techniques for increasing paddy yield, nitrogen use efficiency and water saving in rice, such as alternate wetting and drying, direct seeding and ground cover system are being experienced. The objectives of the present study were to compare the N use efficiency, water saving and yield of rice transplanted on beds with traditional puddled flat method.

Materials and methods Site description and method of transplanting The present field experiment was conducted at the farm area of Soil Chemistry Section, Institute of Soil Chemistry and Environmental Sciences, Ayub Agricultural Research Institute Faisalabad, Pakistan for three continuous years (2011, 2012 and 2013). Ninety-centimeter-wide raised beds (60 cm top and 30 cm furrow) were made with bed planting machine following the conventional land preparation. The height of beds was 15 cm. Rice nursery was transplanted in rows in both bed and traditional (flat transplanting) methods. For beds, nursery was transplanted in four rows bed¡1, (Figure 1) while in treatments 7, 8 and 9 there were five lines bed¡1 (four lines on bed and one line in furrow). Irrigation water was applied in furrows in bed planting and by traditional flood irrigation in flat method. Time required to irrigate the field with same stream size was recorded in bed furrow system and traditional flat method. Water saving was calculated on the basis of time required to irrigate the field, rather than depth of water in both bed furrow and flat method. Description of treatments The treatments used in this field experiment included, T1, flat transplanting (N at 0 kg ha¡1) T2, bed transplanting (N at 0 kg ha¡1) T3, bed transplanting four lines (N at 75 kg ha¡1) T4, bed transplanting four lines (N at100 kg ha¡1) T5, bed transplanting four lines (N at125 kg ha¡1) T6, bed transplanting four lines (N at150 kg ha¡1) T7, flat transplanting (N at150 kg ha¡1) T8, bed transplanting five lines (N at100 kg ha¡1) T9, bed transplanting five lines (N at125 kg ha¡1) T10, bed transplanting five lines (N at150 kg ha¡1). Recommended dose of fertilizers application were done at the rate of 150 kg ha¡1 nitrogen, 90 kg ha¡1 phosphorus and 60 kg ha¡1 potash. All treatments were repeated three times in randomized complete design. The crop data of each year during the three years of the experiment were collected and statistical analysis was performed using Statistics 8.1(statistix.software.informer.com). Least significant differences (LSDs) were used for comparing treatment means (Steel, Torrie, and Dickey 1997). Plant and soil analysis Before rice transplanting, a composite soil sample was collected from the field and was analyzed for physicochemical properties (Table 1). The pH of soil paste and electrical conductivity of the soil extract was measured by method of (Mclean 1982), while phosphorus was measured on spectrophotometer by using sodium bicarbonate extraction and textural class by using hydrometer Table 1. Basic soil analysis. Soil depth (cm)

pHs

ECe (dS m¡1)

O.M (%)

Available phosphorus (mg kg¡1)

0–15 15–30

7.94 7.87

1.61 1.53

0.67 0.59

9.66 8.84

Available potassium (mg kg¡1) 208 196

Texture Sandy clay loam

JOURNAL OF PLANT NUTRITION

309

Figure 1. A) Four lines of rice transplanted on beds. B) Four lines of rice transplanted on beds and one line in furrow. C) Rice crop at harvesting stage on beds.

method (Bouyoucos 1962). Soil organic carbon (SOC) content was estimated following the method described by Ryan, Estefan, and Rashid (2001). While for potassium, soil extraction was done with ammonium acetate (1 N of pH 7.0) and potassium was determined by using PFP-7 Janway Flame photometer (Rowell 1994). Area of nine meters square was harvested randomly from the center of each plot. The harvest of each plot was collected, labeled, sun-dried and threshed individually. Grain samples were taken and dried in an oven at 70 C. For nitrogen determination, the dried ground material (0.5 g) was digested in sulfuric acid using digestion mixture copper sulfate, selenium and iron sulfate (CuSO4, Se and FeSO4), distilled and titrated against 0.1N sulfuric acid (H2SO4) (Jackson 1962).

310

A. MAJEED ET AL.

Nitrogen use efficiency and its components N uptake by grain and nitrogen use efficiency (NUE) was calculated according to formulae given by Rehim et al. (2012), Dobermann (2005) and Fageria, Baliger, and Jones (1997). N uptake D

N contents .%/ in grain£ Yield .kg ha ¡ 1 / 100

NUE D

Grain yield .kg ha ¡ 1 / N dose applied .kg ha ¡ 1 /

NAE D

½Grain yield .kg ha ¡ 1 / in fertilized plot  ¡ ½Grain yield .kg ha ¡ 1 / in control plot N dose applied .kg ha ¡ 1 /

Results Soil properties, grain yield and water saving Basic soil analysis showed that the soil was low in organic matter, marginal in available phosphorus (P) but sufficient in potassium (K) (Table 1). The results showed (Figure 2) that transplanting of rice on beds (four lines on bed and one line in furrow) and application of 125 kg N ha¡1 gave statistical similar yield as compared to 150 kg N ha¡1 application in flat puddled fields with continuously flooded water. Increasing nitrogen application up to 150 kg N ha¡1 in bed transplanting (four lines on bed and one line in furrow) enhanced the yield up to 3.83 t/ha which was significantly higher than yield of rice transplanted on flat ground (3.30 t/ha) at same N rate. It showed that about 25 kg N ha¡1 can be saved with bed transplanting of rice. But transplanting of four lines of rice on beds and application of 150 kg N ha¡1 produced comparatively low paddy yield as compared to flat method with same level of N application (Figure 2). By transplanting of rice plants in the furrows between beds, increased the number of plants as compared to traditional flat planting technique (Photograph, b). Transplanting of four lines on bed and one line in furrow and application of 150 kg N ha¡1 produced 16.06% higher paddy yield as compared to flat method. Less irrigation time with same stream size was recorded to irrigate the field in bed furrow system as compared to traditional flat method. About 24% water saving in term of time was recorded in bed furrow system as compared to flat method (Figure 3).

Figure 2. Effect of nitrogen levels and sowing methods on paddy yield of rice. Note: N-0 (F) D T1, (N at 0 kg ha¡1 flat transplanting), N-0 (B) D T2, (N at 0 kg ha¡1 bed transplanting), N-75 (B) D T3, (N at 75 kg ha¡1 bed transplanting four lines), N-100 (B) D T4, (N at 100 kg ha¡1 bed transplanting four lines), N-125 (B) D T5, (N at 125 kg ha¡1 bed transplanting four lines), N-150 (B) D T6, (N at 150 kg ha¡1 bed transplanting four lines), N-150 (F) D T7, (N at 150 kg ha¡1 flat transplanting) N-100 (B) D T8, (N at 100 kg ha¡1 bed transplanting five lines), N-125 (B) D T9, (N at 125 kg ha¡1 bed transplanting five lines), N-150 (B) D T10, (N at 150 kg ha¡1 bed transplanting five lines).

JOURNAL OF PLANT NUTRITION

311

Figure 3. Percent increase in different parameters by N application at 150 kg ha¡1 on bed transplanting (four lines on bed and one line in furrow) over N application at 150 kg ha¡1 on traditional flat transplanting.

N concentration and uptake in paddy On an average three years pooled data showed (Table 2) that maximum N concentration was found in bed transplanting as compared to flat method with continuously flooded water. Similarly maximum N uptake (53.34 and 48.28 kg/ha) in grain was observed in bed transplanting (where N was applied at 150 and 125 kg/ha), while 43.79 kg/ha N uptake in flat method (where N was applied at 150 kg/ha) was recorded. It showed that higher N uptake by grain was recorded even by 125 kg/ha N application on beds (four lines on bed and one line in furrow) as compared to 150 kg/ha N application in traditional flat method. Overall higher nitrogen accumulation recorded by grain in bed transplanting system as compared to conventional flat fields (Table 2). Nitrogen application at 150 kg ha¡1 in bed transplanting (four lines on bed and one line in furrow) increased 4.51 and 21.81% higher nitrogen concentration and uptake in grain as compared to application of 150 kg N/ha in traditional flat method with continuously flooded water (Figure 3).

Table 2. Nitrogen use efficiency and its components in bed and flat transplanting of rice. Treatments N-0 (F) N-0 (B) N-75 (B) N-100 (B) N-125 (B) N-150 (B) N-150 (F) N-100 (B) 5 line N-125 (B) 5 line N-150 (B) 5 line LSD

Paddy N content (%)

Paddy N uptake (kg/ha)

NUE kg/kg N

NAE kg/kg N

1.19 G 1.22 F 1.30 E 1.33 CD 1.38 AB 1.39 A 1.33 DE 1.33 DE 1.36 BC 1.39 A 0.03

15.99 G 18.01 G 29.27 F 33.15 F 39.46 D 42.78 C 43.79 C 38.52 D 48.28 B 53.34 A 2.95

30.00 A 24.86 B 22.93 C 20.50 D 22.00 CD 29.01 A 28.47 A 25.52 B 1.54

10.36 D 10.13 D 11.14 D 10.68 D 13.06 C 14.28 BC 16.69 A 15.70 AB 1.52

Note: N-0 (F) D T1, (N at 0 kg ha¡1 flat transplanting), N-0 (B) D T2, (N at 0 kg ha¡1 bed transplanting), N-75 (B) D T3, (N at 75 kg ha¡1 bed transplanting four lines), N-100 (B) D T4, (N at 100 kg ha¡1 bed transplanting four lines), N-125 (B) D T5, (N at 125 kg ha¡1 bed transplanting four lines), N-150 (B) D T6, (N at 150 kg ha¡1 bed transplanting four lines), N-150 (F) D T7, (N at 150 kg ha¡1 flat transplanting) N-100 (B) D T8, (N at 100 kg ha¡1 bed transplanting five lines), N-125 (B) D T9, (N at 125 kg ha¡1 bed transplanting five lines), N-150 (B) D T10, (N at 150 kg ha¡1 bed transplanting five lines).

312

A. MAJEED ET AL.

N use and agronomic efficiency The maximum N use efficiency (30.0 kg grain/kg N uptake) was found by application of 75 kg N/ha (Table 2) in bed transplanting (four lines on bed) followed by 100, 125 and 150 kg N /ha application on beds (four lines on bed and one line in furrow). By application of 150 kg N/ha on beds (four lines on bed and one line in furrow) caused 25.52% higher nitrogen use efficiency (Figure 3) as compared to conventional flat puddled methods with continuously flooded water at same level of N application. Similarly results showed (Table 2) that higher N agronomic efficiency (16.69 kg/kg) recorded by 125 kg/ha N application on beds (four lines on bed and one line in furrow). The comparison of bed and flat transplanting (Figure 3) showed that 20.21% higher N agronomic efficiency were recorded where four lines of rice nursery on bed and one line in furrow were transplanted as compared to conventional flat system at same rate of N application.

Discussion Effect of transplanting method on paddy yield and water saving The efficient use of applied nitrogen supports in enhancing the paddy yield and reduced the cost of crop production. Present study indicates that higher paddy yield was recorded by transplanting of rice on raised beds as compared to flat puddled fields with continuously flooded water, due to higher nutrient uptake. From flat to bed changing layout modifies the hydrology of the method and allows better control of irrigation. Applied water moves laterally from the furrow into the bed, and is driven upwards towards the bed surface by capillarity. This changed hydrology in bed planting affects nutrient uptake, transport and increased paddy yield compared with flood irrigation in flat system (Farooq et al. 2009). Similarly grain yield improved by bed planting in transplant aman rice as compared to flat method was also reported by Hobbs and Gupta (2003) and Meisner, Talukdar, and Hossain (2005). Likewise, Tang, Zheng, and Huang (2005) described that 6.7% rice yield increased by bed planting method as compared to traditional cropping system. Rice yield planted on raised beds was superior to rice grown in flat method, due to effective nitrogen utilization by rice plants, influenced better paddy yield in raised bed method (Ockerby and Fukai 2001). Bed planting technique provides a natural opportunity to decrease compaction by keeping traffic to the furrow bottoms and therefore avoids the negative effects of puddling that increased paddy yield (Govaerts, Sayre, and Ceballos 2006). The comparison of water saving (Figure 2) in both system showed that almost 24% water can be saved in term of time required to irrigate the field in bed planting system because water is applied only in small furrows instead of large surface area in flat method. N use efficiency and its components Comparison between bed and flat transplanting of rice showed that 4.51, 21.81, 16.0 and 20.21% higher N concentration in paddy, N uptake, N use efficiency and N agronomic efficiency were recorded in bed transplanting with same rate of N application, it may be due to more root proliferation and biomass (Figure 2). Highest N use efficiency was recorded in bed planting by applying N at 75 kg ha¡1. Applying N fertilizer in higher quantities might have caused in more N losses. Lower nitrogen concentration in paddy and N use efficiency was recorded in flat planting. It can be due to ineffective use of applied nitrogen by plant roots and more leaching and volatilization losses (Mollah, Bhuiya, and Kabir 2009). It shows that raised bed caused rice plant roots to use applied nitrogen fertilizer more efficiently than flat planting even when nitrogen fertilizer were applied in lower amounts and it suggests that this soil manipulation caused reduction in nitrogen losses (Table 2). The agronomic efficiency of applied nitrogen fertilizer was significantly higher in bed transplanting treatments compared to flat system. The higher nutrient uptake in bed planting method is mainly due to less leaching loss of nutrients and availability of sufficient moisture for mineralization of native as well as applied nutrients. Bed planting improved nutrient use efficiency by providing good conditions to growing roots than flat planting (Naresh et al. 2014). Similarly, higher nitrogen and chlorophyll contents in flag leaves were recorded in

JOURNAL OF PLANT NUTRITION

313

bed sowing as compared to flat due to more effective nitrogen uptake and use efficiency (Shah et al. 2013 and Fahong, Xuqing, and Sayre 2004). In our study it is also interesting to note that nitrogen uptake in the grain, where no nitrogen fertilizer was applied, was consistently higher in bed transplanting method, it indicates that rice roots grow copiously on beds to take up nitrogen from a large volume of soil (Table 2). Rice transplanting on beds in rice-wheat system not only save the water but also reduces N loses, improves fertilizer use efficiency and grain yield.

Conclusions From the three years findings, it concludes that 16% higher paddy yield and 21.81% higher nitrogen uptake recorded by transplanting of four lines of rice on beds and one line in furrows as compared to traditional flat method with 150 kg N ha¡1 application. Without loss in yield 25 kg ha¡1 nitrogen fertilizer and 24% water can be saved in bed furrow transplanting system of rice as compared to flat planting.

Acknowledgments We are very thankful to Professor Dr. Rai Niaz Ahmad, Ex-Director, Water Management Research Center, University of Agriculture, Faisalabad Pakistan, for providing bed planter.

References Barrett, C. B. 2010. Measuring food insecurity. Science 327:825–828. Bond, J. A., T. W. Walker, B. V. Ottis, and D. L. Harrell. 2008. Rice seeding and nitrogen rate effects on yield and yield components of two rice cultivars. Agronomy Journal 100:393–397. Bouyoucos. 1962. Hydrometer method for making particle analysis of soil. Journal of Agronomy 54:464–465. Dobermann, A. 2005. Nitrogen use efficiency-state of the art. IFA international workshop on enhanced efficiency fertilizers. Frankfurt, Germany. June 28–30. http://I/soilfertility.rml.edu/Materials%20to%20include!Nitro gen%20use%20efficiency%20state%20of0/o20the%20ar_Dobermann.pdf Erisman, J., M. Sutton, J. Galloway, Z. Klimont, and W. Winiwarter. 2008. How a century of ammonia synthesis changed the world. Nature Geoscience 1:636–639. Fageria, N. K., V. C. Baliger, and C. A. Jones. 1997. Growth and mineral nutrition of field crops. 2nd ed. New York, USA: Marcel Dekker, Inc. Fahong, W., W. Xuqing, and K. Sayre. 2004. Comparison of conventional, flood irrigated, flat planting with furrow irrigated, raised bed planting for winter wheat in China. Field Crops Research 87:35–42. Farooq, M. N., A. Kobayashi, O. I. Wahid, and S. M. A. Basra. 2009. Strategies for producing more rice with less water. Advanced Agronomy 101:351–388. Govaerts, B., K. D. Sayre, and J. M. Ceballos. 2006. Conventionally tilled and permanent raised beds with different crop residue management, effects on soil C and N dynamics. Plant and Soil 280:143–155. Hobbs, P. R., and R. K. Gupta. 2003. Resource-conserving technologies for wheat in the rice-wheat system. In Improving productivity and sustainability of rice-wheat systems: Issues and impact, vol. 65, 149–217, American society of Agronomy Special Publication. Ishaq, M., M. Ibrahim, A. Hassan, M. Saeed, and R. Lal. 2001. Subsoil compaction effects on crops in Punjab, Pakistan II: root growth and nutrient uptake of wheat and sorghum. Soil Tillage Research 60:153–161. Jackson, M. L. 1962. Soil chemical analysis. Prentice Hall: Inc. Englewood Cliffs, New Jersey. U.S.A. Ju, X. T., G. X. Xing, X. P. Chen, S. L. Zhang, L. J. Zhang, X. J. Liu, Z. L. Cui, B. Yin, P. Christie, and Z. L. Zhu. 2009. Reducing environmental risk by improving N management in intensive Chinese agricultural systems. Proceedings of the National Academy of Sciences of the United States of America 106:3041–3046. Kukal, S. S., and G. C. Aggarwal. 2003. Puddling depth and intensity effects in rice-wheat system on a sandy loam soil I: development of subsurface compaction. Soil Tillage Research 72:1–8. Mclean, E. O. 1982. Soil pH and lime requirement. In Methods of soil analysis part 2: Chemical and microbiological properties, ed. A. L. Page, R. H. Miller and D. R. Keeney, vol. 9, 2nd ed., 199–209. Madison, WI: American Society of Agronomy. Meisner, C. A., H. M. Talukdar, and I. Hossain. 2005. Introduction and implementing a permanent bed system in the rice-wheat cropping pattern in Bangladesh and Pakistan. In Proceedings of the ACIAR Workshop on Permanent Bed Planting Systems, Griffith, NSW, Australia. Mollah. M. I. U., M. S. U. Bhuiya, and M. H. Kabir. 2009. Bed planting, a new crop establishment method for wheat in rice-wheat cropping system. Journal of Agricultural and Rural Development 7:23–31.

314

A. MAJEED ET AL.

Naresh, R. K., S. S. Tomar, D. Kumar, S. Purushottam, S. P. Singh, A. Dwivedi, and V. Kumar. 2014. Experiences with rice grown on permanent raised beds, effect of crop establishment techniques on water use, productivity, profitability and soil physical properties. Rice Science 21:170–180. Ockerby, S. E., and S. Fukai. 2001. The management of rice grown on raised beds with continuous furrow irrigation. Field Crops Research 69:215–226. Rehim, A., M. Hussain, M. Abid, M. Z. Haq, and S. Ahmad. 2012. Phosphorus use efficiency of Triticum aestivum L. as affected by band placement of phosphorus and farmyard manure on calcareous soils. Pakistan Journal of Botany 44:1391–1398. Rowell, D. L. 1994. Soil science. Methods and application. UK: Longman Scientific & Technical. Ryan, J., G. Estefan, and A. Rashid. 2001. Soil and plant analysis laboratory manual. 2nd ed. Aleppo, Syria: International Center for Agricultural Research in Dry Areas. Shah, S. S. H., A. Hassan, A. Ghafoor, and A. Bakhsh. 2013. Soil physical characteristics and yield of wheat and maize as affected by mulching materials and sowing methods. Soil and Environment 32:14–21. Steel, R. G. D., J. H. Torrie, and D. A. Dickey. 1997. Principles and procedures of statistics: A biometrical approach, 3rd ed., New York, USA: McGraw Hill Co. Tang, Y. L., J. G. Zheng, and G. Huang. 2005. Studies on permanent bed planting with double zero tillage for rice and wheat in Sichuan basin. Southwest China Journal of Agricultural Sciences 18:25–28. Tuong, T. P., and B. A. M. Bouman. 2003. Rice production in water scare environments. In Water productivity in agriculture: Limits and opportunities for improvement, ed. J. W. Kijne, R. Barker, D. Molden, 53–67. Wallingford, UK: C.A.B. I. Publishing. Xing, G. X., and Z. L. Zhu. 2000. An assessment of N loss from agricultural fields to the environment in China. Nutrient Cycling in Agroecosystems 57:67–73.