Maize (Zea mays L.) is the 2nd important crop after wheat in the Khyber Pakhtunkhwa Province of Pakistan. The yield of maize is very low (1868 kg ha⁻¹) in Khyber Pakhtunkhwa (semiarid climate) than the average yield (3990 kg ha⁻¹) in the country (Amanullah et al., 2009a,b; MINFAL, 2012). The reasons for low crop productivity in semiarid climate are: (1) low soil moisture availability (2) low soil fertility (Amanullah et al., 2012), and (3) indiscriminate use of chemical fertilizers by smallholders (Amanullah et al., 2015a). According to Oad et al. (2004) and Amanullah and Khan (2015), the major problems in the way of increasing yield at farmer's fields are the inappropriate nutrients supply. Among the plant nutrients, nitrogen (N) management is one of the most important factor required for improving crop productivity and profitability under semiarid climates (Amanullah, 2016). According to Guo et al. (2016), N is the most yield-restraining nutrient in crop production globally. Our previous research work on maize and nitrogen management revealed that proper nitrogen rates and nitrogen timings (splits) management improve maize growth and development (Amanullah et al., 2007, 2008, 2009a,b; Amanullah and Shah, 2010), yield and yield components (Amanullah et al., 2009a), grain quality (Amanullah et al., 2010), and growers income (Amanullah et al., 2010) in the study area. In another study (Amanullah et al., 2015b), we noticed that the integrated use of nitrogen (120 or 150 kg N ha⁻¹) along with compost (2 t ha⁻¹) improved yield and yield components in maize under deep (45 cm) than shallow (15 cm) tillage system under semiarid climate.

Increasing human population in Pakistan is resulting in less land for cultivation. Therefore, increase in crop productivity depends on higher yield per unit area. Increased cropping intensity especially under cereal based cropping system removes plant nutrients (Amanullah and Inamullah, 2016) from the soil, in addition to natural nitrogen losses (Gehl et al., 2005). Therefore, it has become of foremost importance to select high yielding varieties and genotypes that can uptake and utilize nitrogen more efficiently (Hirel et al., 2001). Efficient nitrogen fertilizer management is essential for achieving economic yields and for enhancing N use efficiency (Pan et al., 2012; Yousaf et al., 2014). Selection of high yielding cultivars is important for food security and growers incomes (Geiger, 2009). Best nitrogen management practices play an important role in increasing crop productivity (Fageria et al., 2006). Best nitrogen management practices minimize N losses and increases N availability for crops which increase nitrogen use efficiency and reduce negative impacts of N on the environment (Havlin et al., 2009). According to Amanullah (2014), N use efficiency (NUE) decrease with increase in N rate and the NUE was higher with application of ammonium sulfate (AS) over calcium ammonium nitrate (CAN) and urea. The hybrid maize had higher NUE than the local cultivars (Pioneer-3025 > Jalal > Azam). Ammonium sulfate increases NUE, however, its higher cost make it less profitable N-fertilizer for the maize growers in Northwest Pakistan (Amanullah et al., 2015a). The decline in the price of AS could make it more profitable and most suitable N-fertilizer source under semiarid climates in terms of higher NUE (Amanullah, 2014), higher net returns (Amanullah et al., 2015b), and better seed quality (Khan, 2016).

In the semiarid climates, growing maize hybrids are more efficient than local cultivars in terms of higher NUE and grower's income. However, no previous research was reported to investigate the response of different maize genotypes to different N sources × N levels interaction. This experiment was conducted with an overall objective to find the best level and best source of N fertilizer for improving yield and yield components of maize genotypes. Sustainable crop production relies on the continuous renewal of soil fertility through a balance between N demand and supply in cropping systems (Yousaf et al., 2016). Better management of high yielding crops with lower N loss is desperately needed to achieve sustainable agriculture (Duan et al., 2014). The current research project was therefore conducted to investigate yield and yield components response of maize genotypes [Pioneer-3025 (hybrid) in comparison with local cultivars (Jalal and Azam)] to N rates and sources to find out best N source and their level for higher maize productivity in the study area.

It is clear from Table 3, that the rest (N treated plots) had produced number of rows ear⁻¹ (NOR ear⁻¹), number of seeds row⁻¹ (NOS row⁻¹), number of seeds ear⁻¹ (NOS ear⁻¹) than control plots (N not applied). In both the years, NOR ear⁻¹, NOS row⁻¹, and NOS ear⁻¹ increased significantly (P ≤ 0.05) with application of N in two higher than the two lower rates. Application of AS (ammonium sulfate) produced more NOR ear⁻¹ than urea and CAN (calcium ammonium nitrate). On the other hand, application of CAN produced more NOS row⁻¹ and NOS ear⁻¹ as compared to urea (Table 3). The hybrid maize had produced higher NOR ear⁻¹, NOS row⁻¹, and NOS ear⁻¹ than the two local cultivars in both the years (Table 3). Increase in N level produced more NOR ear⁻¹ and the increase was more in the maize hybrid than the two local cultivars that resulting in significant N-level × genotypes (N × G) interaction (Figure 1). Interaction of nitrogen source and nitrogen rates (N × S) in year one (Figure 2A) and year two (Figure 2B) indicating that the increase in N level had increased number of seeds per row while using different N sources. Interaction of N source and genotypes (S × G) indicated that in both years i.e., year one and two (Figures 3A,B, respectively), maize hybrid performed better by producing more number of seeds per row with each N source as compared with the two local maize cultivars. Interaction of N source and rates (N × S) had significant effects on NOS ear⁻¹ (Figure 4), indicating that the NOS ear⁻¹ increased to maximum with CAN applied at the rate of 150 kg N ha⁻¹. While in case of urea the NOS ear⁻¹ increased to maximum with the highest rate of 200 kg N ha⁻¹ (Figure 4). Application of AS at the rate of 150 kg N ha⁻¹ increased NOS ear⁻¹ in both years. The NOS ear⁻¹ increased to maximum in hybrid maize with application of CAN as compared to other N sources resulting in significant S × G interaction (Figure 5). The interaction between N rates and genotypes (N × G) indicated that NOS ear⁻¹ increased in hybrid maize at all four levels of N and increased to maximum at the highest rate of 200 kg N ha⁻¹ (Figure 6). On the other hand, the NOS ear⁻¹ in the local cultivars increased up to 150 kg N ha⁻¹ and further increase in N level had not significantly increased NOS ear⁻¹.

Likewise, it is clear from Table 4 that the N rest had produced more number of ears per 100 plants (NOEP 100 plants), grain yield, stover yield, and shelling percentage than the control plots. In both years NOEP 100 plants, grain and stover yields (Table 4) increased significantly with application of N in two higher than the two lower rates. The effect of nitrogen sources was not significant for NOEP 100 plants (Table 4). Ammonium sulfate treated plots produced higher grain yield, followed by CAN, while less grain yield was recorded from urea (Table 4). Application of CAN produced more stover yield, followed by AS, while less stover yield was recorded from urea treated plots. The hybrid maize had produced higher NOEP 100 plants, grain, and stover yields than the two local cultivars in the two years (Table 4). Interaction between N rates and genotypes (N × G) had significant effects on NOEP 100 plants (Figure 7). The NOEP 100 plants increased in maize hybrid while increasing N level up to 200 kg N ha⁻¹, while in the local cultivars NOEP 100 plants increased up to 150 kg N ha⁻¹ and further increase in N had no significant effect on NOEP 100 plants. Interaction between N rates and genotypes (N × G) had significant effects on grain yield in 2nd year (Figure 8). The grain yield increased in maize hybrid while increasing N rate as compared to the two local maize cultivars. The grain yield of the two cultivars increased with increase in N up to 150 kg N ha⁻¹ and further increase in N rate 200 kg N ha⁻¹ did not increased grain yield significantly. Interaction between N source and N rates (N × S) had significant effects on stover yield in the first year only (Figure 9). The stover yield increased with increase in N rates while using each N fertilizer source. Interaction between N rates and genotypes (N × G) indicated that stover yield increased in hybrid maize with increase in N level up to 200 kg N ha⁻¹, while in the local cultivars stover yield increased up to 150 kg N ha⁻¹ and further increase in N up to 200 kg N ha⁻¹ did not increased the stover yield significantly (Figure 10).

Nitrogen management is one of the most important factor for improving crop productivity under semiarid climates. The results of this study confirmed the existence of significant variability in yield and yield components of maize depending on genotypes inheritance, differences in N fertilizer source and rates as well as differences in the weather condition. Application of nitrogen at the rate of 150 and 200 kg N ha⁻¹ in the form of urea, 100 and 150 kg N ha⁻¹ in the form of CAN (calcium ammonium nitrate) and 50 and 100 kg N ha⁻¹ in the form of ammonium sulfate was more beneficial for improving maize productivity in the study area. The current higher price of fertilizers along and the less purchasing power had negative impact on smallholder's income. The superiority of AS over urea probably may be due to the presence of sulfur (24%) which is lacking in urea. Ammonium sulfate has an acidifying effect on soil, therefore, its continuous use may be advantageous on alkaline soils especially under arid and semiarid climates. The superiority of CAN over urea may be attributed to the availability of N in the two different forms (ammonium and nitrate) which make the N availability for long time than urea. Decrease in the cost of hybrid seeds and N-fertilizers especially ammonium sulfate will decrease production cost and could increase crop productivity, profitability and sustainability under semiarid climates.

Amanullah designed the research project, analyzed the data, and wrote the manuscript. AI and AA performed the experiments, collected the data, made tables and figures. SF and BP helped in the editing of the manuscript.

This research was supported by The University of Agriculture Peshawar from its endowment fund.