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Journal of soil science and plant nutrition

versión On-line ISSN 0718-9516

J. Soil Sci. Plant Nutr. vol.17 no.3 Temuco set. 2017

http://dx.doi.org/10.4067/S0718-95162017000300011 

 

Yield and nitrogen use efficiency of rice-wheat cropping system in gypsum amended saline-sodic soil

 

B. Murtaza1*, G. Murtaza2, M. Imran1, M. Amjad1, A. Naeem, G. M. Shah1, A. Wakeel2*

 

1Department of Environmental Sciences, COMSATS Institute of Information Technology, Vehari, Pakistan.*Corresponding author: behzadmurtaza@ciitvehari.edu.pk

 2Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad-38040, Pakistan.


Abstract

It is critical to determine nitrogen use efficiency (NUE) to find to which extent higher rates of nitrogen can improve crop yield with effective management practices. Two-year field experiments were conducted to investigate yield and NUE of rice-wheat cropping system on saline-sodic soil. Treatments included were two nitrogen (N) application rates, i.e. 15% (N115)and 30% (N130) higher than the recommended rates for normal soil, along with gypsum at the rates of 50% (SGR50) and 100% (SGR100) of soil gypsum requirement. Results revealed relatively highest NO3- leaching for rice (161 and 145 mg L-1) and for wheat (97 and 93 mg L-1) during 2011-12 and 2012-13, respectively in N130 + SGR100. In this treatment, crop yield and NUE were the highest as compared to the other combinations. This resulted in reduction of yield gap by two-fold (53% to 26%) between saline-sodic and normal soils for rice-wheat. Interestingly, N130 + SGR100 proved most effective during the first year, however, N100 + SGR100 became more profitable in the subsequent year. Pearson correlation coefficients predicted significant positive correlation (p < 0.01) of yield and NUE with soil organic matter, cation exchange capacity and infiltration rate while inverse relationship was observed with electrical conductivity, pH, CaCO3, and bulk density. Based on data, it is concluded that the recommended N application together with SGR100 would be environmental-friendly and economically viable option for rice-wheat cropping system in saline-sodic soils.

Keywords: Amelioration, nitrate leaching, saline-sodic soil, nitrogen use efficiency, gypsum


1. Introduction

The commencement of 21st century is manifested by scarcity of global water resources, environmental pollution, and increased soil and water salinity (Shrivastava and Kumar, 2015). Globally, more than 8 × 108 ha of land is salt-affected either by salinity (3.97× 108 ha) or sodicity (4.34 × 108 ha) (Rozema and Flowers, 2008). In Pakistan, 29% of total geographical area (7.961 × 107 ha) is arable land, and a significant part (6.67 x 106 ha, i.e. 8.4%) of this total area is salt-affected which is rapidly expanding (Khan, 1998). It is estimated that 1.6 × 106 ha of arable land is going out of cultivation every year due to low precipitation, high surface evaporation and irrigation with brackish water (Ghassemi et al., 1995).

Salt-affected soils are characterized by high levels of water-soluble salts, high pH and high sodium (Na) contents which inhibit plant growth due to salt-induced water stress, specific ion toxicity, ion imbalance or nutritional disorders, oxidative stress, and hormonal imbalances (Munns et al., 2006). In saline-sodic soils, availability and absorption of plant nutrients is severely limited to sustain high crop production due to ion interactions, especially low nitrogen (N) because of its leaching as NO3 at high soil pH, volatilization and de-nitrification losses (Marschner, 2011). Overall, all these factors individually or in combination with each other limit N-use-efficiency (NUE) and the extent of this limitation depends on the salinity/sodicity levels, crop types and species and soil physicochemical properties (Grattan and Grieve, 1999).

There are a number of approaches to improve salt-affected soils, i.e. leaching to flush salts from root zone, application of chemical amendments and phytoremediation (Feizi et al., 2010). The reclamation of saline-sodic soils using gypsum (CaSO4. 2H2O), CaCl2, with or without organic manures is cost-effective and easy to implement (Makoi and Verplancke, 2010). Gypsum application improves the physical, chemical and biological properties of saline-sodic soils ultimately enhancing crop production on sustainable basis (Walia and Dick, 2016). Calcium (Ca) from gypsum in saline-sodic soil decreases volatilization loss of ammonium (NH4+)from the applications of NH4NO3, (NH4)2CO3, (NH4)2SO4 or ammonium phosphates fertilizers (Arshad, 1999). Moreover, Ca decreases soil pH by reacting with (NH4)2SO4 produced during the urea hydrolysis to form CaCO3 and (NH4)2SO4. Since (NH4)2SO4 is stable and slightly acidic product, therefore, it reduces NH3 loss to the atmosphere (Arshad, 1999).

Increased N application compensates the yield reduction due to increasing levels of exchangeable Na percentage when soil physical properties are not a limiting factor in plant growth (Havlin et al., 2005). Increased uptake of Ca2+ and Mg2+, and decreased uptake of Na+ by plants resulting from the application of additional N were responsible for the greater response to N at high exchangeable Na+ levels. Thus, it appears that the beneficial effects of additional N application on crop growth in saline-sodic soils can be attributed to more than one mechanisms.Abdelgadir et al. (2010) reported that N applied to saline-sodic soil partially alleviates the adverse effects of salinity on photosynthesis, respiration, N fixation and carbohydrate metabolism through enhanced leaf surface area help in enhancing the salt tolerance of plants mainly through growth dilution effect (it distributes the salt concentration in increased surface area thereby reducing salt concentration over a large area).

A number of studies have demonstrated that soil salinity/sodicity reduced N absorption by plants especially by decreasing NUE. It is hypothesized that increased N fertilizer application may improve net N uptake and ultimately the crop yield. Hardly any information exist that adding N fertilizer at levels higher than recommended rates in saline-sodic soil can improve crop production, environmental health and economic benefit. Therefore, this work aims to determine the effect of gypsum application on NUE in saline-sodic soil in rice-wheat cropping system.

2. Materials and Methods

2.1. Experimental set-up and treatments

Two-year field experiments were conducted at village 132/GB, Dijkot (73. 0653° E, 31.1903° N), District Faisalabad, Pakistan on saline-sodic soil belonging to Typic Calciargids. Treatments were arranged in randomized complete block design (RCBD) with three replicates having a plot size of 7.06 m × 20.45 m. To minimize the boundary effects, treatments were separated with ridges of 0.60 m width and 0.30 m height, and 0.5 m around each plot was left uncropped. The recommended application rates of N in the region for wheat and rice are 120 (N100) and 125 kg ha-1 (N100), respectively. The five treatments of current study include: 1) no fertilizer and no gypsum (control), 2) recommended N + gypsum at 50% soil gypsum requirement (N100 + SGR50), 3) 30 % higher N than recommended N fertilizer + gypsum at 50 % soil SGR (N130 + SGR50), and 4) recommended N + gypsum at 100 % SGR (N100 + SGR100), and 5) 30 % higher N than recommended N fertilizer + gypsum at 100% SGR (N130 + SGR100).

2.2. Physico-chemical analysis of soil

Composite soil samples from 0-15 cm and 15-30 cm soil depths were drawn from each plot before and after harvest of the subsequent crops. Before analysis, soil samples were air-dried and ground to pass through a 2 mm sieve for homogenization. Thereafter, samples were analyzed for pHs, ECe, soluble cations (Na+, K+, Ca2+, Mg2+) and anions (CO32-, HCO3-,Cl-, SO4-, NO3-) following the methods described by Estefan et al. (2013). The ECe and pHs were determined in saturated paste extracts and saturated soil paste, respectively. Particle-size analysis was carried out by hydrometer method (Rowell, 2014). The gypsum requirement of the saline-sodic soil was determined following the protocol described by Ghafoor et al. (2004), and was applied according to the treatment plan. The amount of gypsum of saline-sodic soil was 5.18 Mg ha-1 and was well mixed with cultivator and wooden roller driven by tractor in the top layer of soil before planting the first rice crop (July 2011).

2.3. Rice and wheat crop husbandry

Each year, only one crop of rice and wheat was grown. During the first year, rice (Oryza sativa L., super basmati) seedlings (2-3 seedlings per hill) were transplanted on July 07, 2011 and harvested on November 04, 2011. Row to row and hill to hill distance was maintained at 20 cm. The seedlings were transplanted without puddling the soil. The experimental plots were irrigated with canal water using a cut-throat flume to estimate the amount of water applied. Fertilizers N, phosphorus (P) and potassium (K) were applied at 125, 75 and 60 kg ha-1 as urea, triple super phosphate (TSP) and sulfate of potash (SOP), respectively. Full dose of P and K, and 1/3rd of N dose were applied at the time of transplantation while the remaining N was applied in two equal splits at 34 and 46 days after transplantation (DAT) of rice. At physiological maturity, straw and paddy yield were recorded. Soil samples were taken from each experimental plot and were analyzed for physicochemical soil properties after harvest of the rice crops.

Soon after harvest of the first rice crop, wheat (Triticum aestivum L. cv. Faisalabad 2008) was sown using 100 kg ha-1 seed rate on November 12, 2011 and harvested on April 26, 2012. Sowing was done using a drill machine while maintaining a row to row distance of 20 cm and a depth of seed in soil at 5 cm. The fertilizers N, P and K were applied at 120-100-60 kg ha-1 as urea, TSP and SOP, respectively. All the P and K were applied at the time of sowing while half of N was applied at the time of sowing. The remaining N was applied in two equal splits at tillering i.e. 28 days after germination (DAG) and booting stages (54 DAG). The wheat crop was irrigated five times with known volume of canal water; the first irrigation was applied at day-30 after sowing and subsequent four irrigations were given at an interval of 20 days. The weedicide (Isoproturon 50 WP) was applied using a Knapsack sprayer 55 days after sowing (i.e. after the second irrigation). After harvesting at maturity, grain and straw yield was recorded. The NUE was calculated using the formula: kg DM production per kg N uptake while agronomic use efficiency was calculated with (YN-Y0)/FN.

Where YN and Y0 are yield with and without N application, respectively and FN is amount of N fertilizer applied.

Soil samples were collected from each treatment plot after the harvest of both the wheat crops. This samples were analyzed for pHs, ECe, SAR and soluble cations and anions by following the methods described by Estefan et al. (2013).

2.4. Leachate collection

Ceramic cups were installed at a 60 cm depth for leachate collection before the transplantation of first rice crop. These cups were installed randomly in the plots (2 m away from the borders). All holes were initially augured at a 65 cm depth, a slurry of fine sand was then poured into the auger hole and ceramic cups were installed vertically. A plug of bentonite was inserted into the hole during back-filling to prevent water movement along the shaft. Solutions were withdrawn from the cups using a syringe connected to a long tube, and transferred to a storage bottle. Leachate was collected until it had been calculated that drainage from the field had stopped which corresponded to the collection of five leachates during rice cropping and three leachates during wheat cropping. All the leachate samplers were treated with 1 M HCl. Leachates were collected by applying 0.06-0.07 MPa suction with the help of suction pump and analyzed within 24 h of leachate collection, or otherwise stored at temperature 4 °C until analysis.

The experiments were continued under the same management practices for further one year. For rice, during the second year, seedlings were transplanted on July 15, 2012 and harvested on November 17, 2012. While during the second year, wheat crop was sown on November 28, 2012 and harvested on April 28, 2013. Average range of temperature, relative humidity, evapotranspiration and total rainfall received during growing seasons are given in our previous study (Murtaza et al., 2017).

2.5. Statistical analysis

Before performing analysis of variance (ANOVA), the data were tested for normality of distribution and homogeneity of variance. Significant differences in physicochemical parameters of rice and wheat were examined using ANOVA. All the data were statistically analyzed using computer software ,Statisix 8.1,. The significant differences between the treatment means were compared using least significant difference (LSD) test at 5% probability level (Steel et al., 1997). For physicochemical parameters, the value of the Pearson correlation coefficient (r) was calculated at p < 0.05.

3. Results

3.1. Chemical properties of salt-affected soil during crop growth

Before the cultivation of wheat 2011-12, pHs, ECe and SAR of experimental plots ranged from 8.53 to 8.71, 7.55 to 10.53 dS m-1 and 32.61 to 44.51 (mmol L-1)1/2 at 0-15 cm soil depth, respectively. At 15-30 cm soil depth, the respective values ranged from 8.50 to 8.69, 7.40 to 8.99 dS m-1 and 34.56 to 36.57 (mmol L-1)1/2 (Table 1). After wheat harvest 2012-13, the highest decrease in pHs was observed at N100 + SGR100 and the lowest in the control treatment. The treatment N100 + SGR100 remained non-significant (p ≤ 0.05) with N130 + SGR100 at 0-15 cm soil depth. At 15-30 cm soil depth, the highest decrease in pHs was observed with N100 + SGR100 (3.9 %) followed by N130 + SGR100 (3.7 %), N100 + SGR50 (3.2 %), N130 + SGR50 (2.8 %) and control treatment (0.5 %). In saline-sodic soil, the highest decrease in ECe was observed for N100 + SGR100 (66.51%) and the lowest was for the control treatment after wheat harvest 2012-13 at 0-15 cm soil depth. At 15-30 cm soil depth, the maximum observed reduction in ECe at N130 + SGR100 differed significantly (p≤0.05) with the control treatment. Post-wheat crop (2012-13) soil analysis showed a significant decrease in SAR with the varying rates of N and gypsum (Table 1). The reduction in SAR was the highest for treatment N100 + SGR100 and the lowest for the control treatment. Treatment N100 + SGR100 significantly differed from other treatments except N130 + SGR100 at both the soil depths.

Table 1. Effect of N and gypsum treatments on pH, EC and SAR of salt-affected soil under experiments

Values in a column sharing same letter (s) are statistically similar at p ≤ 0.05; NS, Treatments differed non-significantly at p ≤ 0.05; * Treatments differed significantly p ≤ 0.05.a Chemical properties before the start of experiment; b Chemical properties after the harvest of experiment.

3.2. Crop growth and yield

3.2.1. Rice

During 2011, paddy yield was significantly (p < 0.05) affected by different rates of N fertilizer (Figure 1). The highest paddy yield of 1.21 t ha-1 was obtained for treatment N130 + SGR100 and the lowest of 0.98 t ha-1 in case of the control treatment. Further, the highest straw yield was obtained with N130 + SGR100 (1.58 t ha-1) and the lowest (1.38 t ha-1) was with the control treatment. Straw yield was significantly affected due to the different rates of N fertilizer application along with gypsum i.e. N100 + SGR50, N130 + SGR50, N100 + SGR100 and N130 + SGR100 treatments. It seemed that SGR100 was necessary to achieve reclamation and obtain rationally high paddy and straw yields on saline-sodic soils. Similarly,during 2012, the maximum paddy yield of 1.92 t ha-1 was obtained in treatment N130 + SGR100 which remained the highest as compared to the other treatments. Straw yield was also affected significantly at all N and gypsum application levels (Figure 1) and maximum straw yield of 2.46 t ha-1 was observed with N130 + SGR100 which significantly differed with the control treatment, whereas it remained non-significant with other treatments i.e. at N100 + SGR50, N130 + SGR50 and N100 + SGR100 treatments.

Figure 1. Effect of N and gypsum application rates on paddy yields in saline-sodic soil, where (C) no fertilizer and no gypsum, (N100 + SGR50) recommended N + gypsum at 50% SGR, (N130 + SGR50) 30 % higher N than recommended N fertilizer + gypsum at 50 % soil SGR, and (N100 + SGR100) recommended N + gypsum at the rate of 100 % SGR, and (N130 + SGR100) 30 % higher N than recommended N fertilizer + gypsum at 100% SGR. Fisher's LSD test (p ≤ 0.05) was performed. Treatments which were not significantly different from one another are illustrated with same letter on the columns. SGR=Soil Gypsum Requirement

3.2.2. Wheat

Significant increase in wheat yield was observed in both the years (2011-12 and 2012-13). Among the treatments, the highest wheat grain yield of (3.12 t ha-1) during 2011-12 and 3.53 t ha-1 during 2012-13 was obtained for N130 + SGR100 but it did not differ from treatment N100 + SGR100 (Figure 2). Overall, it was observed that treatment N130 + SGR100 performed better for grain as well as straw yield.

Figure 2. Effect of N and gypsum application rates on wheat yield during amelioration of saline-sodic soil. (C) no fertilizer and no gypsum, (N100 + SGR50) recommended N + gypsum at 50% SGR, (N130 + SGR50) 30 % higher N than recommended N fertilizer + gypsum at 50 % soil SGR, and (N100 + SGR100) recommended N + gypsum at 100 % SGR, and (N130 + SGR100) 30 % higher N than recommended N fertilizer + gypsum at 100% SGR. Fisher's LSD tests (p ≤ 0.05) were performed. Any treatment which were not significantly different from one another are illustrated as same letter on the columns. SGR=Soil Gypsum Requirement

3.3. Agronomic use efficiency and nitrogen use efficiency

The agronomic use efficiency (AUE) was the highest with N130 + SGR100 treatment while it was the lowest with N100 + SGR50 during both years of rice (2011-12) (Table 2). During wheat crop (2012-13), the highest AUE was associated with N100 + SGR100 which was significantly different from control treatment.

Table 2. Effect of applied N rates on NUE and AUE of wheat and rice on salt-affected soil, where(C) no fertilizer and no gypsum, (N100 + SGR50) recommended N + gypsum at 50% SGR, (N130 + SGR50) 30 % higher N than recommended N fertilizer + gypsum at 50 % SGR, and (N100 + SGR100) recommended N + gypsum @ 100 % SGR, and (N130 + SGR100) 30 % higher N than recommended N fertilizer + gypsum at 100% SGR.

*Treatments differed significantly at p < 0.05.

The highest NUE was recorded with control treatment and the lowest with N130 + SGR100 for rice-wheat during both years. In rice 2011, maximum and minimum recorded NUE was 76.95 and 57.19 kg kg-1, respectively. While during second year, the highest NUE was observed with control (73.93 kg kg-1) treatment and the lowest (50.30 kg kg-1) was being with N130 + SGR100. Minimum NUE in rice for both years was observed with N130 + SGR100 and the highest was with control treatment.

Similarly, in wheat 2011-12, the highest NUE with control treatment was 86.19 kg kg-1 and minimum (63.55 kg kg-1) was observed with N130 + SGR100. While during second year, the highest and lowest recorded NUE was 88.69 and 59.73 kg kg-1 with control and N130 + SGR100 treatm

The effect of gypsum and N application rates on NO3- leaching in saline-sodic soil during rice cropping in 2011 and 2012 has been presented in Figure 3. The highest NO3- concentration (42.07 mg L-1) was recorded in the first leachate at N130 + SGR100 which did not differ significantly from the treatment N100 + SGR50. During the second leaching, the lowest amount of NO3- (1.75 mg L-1) was recorded at control treatment and the highest amount in N130 + SGR100. There was a gradual increase in NO3- concentration in the third, fourth and fifth leachates. The maximum NO3- was recorded in the fifth leachate at N130 + SGR100. During rice cropping (2012), the highest amount of NO3- leaching (27.35 mg L-1) was recorded at N130 + SGR100 and the lowest amount of NO3- leaching (12.05 mg L-1) was observed at control treatment (Figure 3). In second leachate, a significant decrease in leached NO3- contents was observed with all applied rates of N compared to the first leachate. An increase in leached NO3- contents observed during third and fourth leaching seems due to the improvements in infiltration rate (IR) and hydraulic conductivity (HC) of soil with the use of gypsum. In fifth leachate, the lowest NO3- concentration was observed with control treatment and the greatest was with N130 + SGR100 treatment.

Figure 3. Effect of N application rates on nitrate leaching during wheat and rice crops on saline-sodic soil. (C) no fertilizer and no gypsum, (N100 + SGR50) recommended N + gypsum at 50% SGR, (N130 + SGR50) 30 % higher N than recommended N fertilizer + gypsum at 50 % soil SGR, and (N100 + SGR100) recommended N + gypsum at 100 % SGR, and (N130 + SGR100) 30 % higher N than recommended N fertilizer + gypsum at 100% SGR. Columns show the mean while bars on columns show standard error. SGR=Soil Gypsum Requirement

The effect of gypsum and N application rates on NO3- leaching in saline-sodic soil during the wheat cropping (2011-12) is shown in Figure 3. The highest NO3- concentration (42.07 mg L-1) observed in the first leachate was at treatment N130 + SGR100 which did not differ significantly from N130 + SGR50. In the second leachate, a decrease in NO3- concentration were recorded. The lowest NO3- concentration (5.50 mg L-1) was recorded at control treatment and the highest (28.21 mg L-1) at N130 + SGR100. A decrease in NO3- leaching in the second leachate was attributed to increased uptake of NO3- by plants during the vegetative growth period. There was an increase in NO3- in the third leachate which seems due to the third N application coupled with slow growth rate of wheat crop. The highest leaching of NO3- was recorded at N130 + SGR100. During wheat cropping (2012-13), the greatest amount of NO3- leaching (31.56 mg L-1) was recorded with N130 + SGR100 which did not differ significantly from NO3- leaching at N130 + SGR50, while the smallest amount of NO3- leaching (10.63 mg L-1) was observed at control treatment (Figure 3). A significant decrease in NO3- in second leachate was observed as compared to first leachate. The minimum NO3- concentration was recorded at control treatment and the highest was at N130 + SGR100. An increase in NO3- contents was observed in third leachate with respect to second where the highest NO3- concentration (31.90 mg L-1) was observed at N130 + SGR100 and the lowest (8.98 mg L-1) at control treatment. Overall, the maximum NO3- leaching was noted at N130 + SGR100, while the minimum N leaching occurred with the control treatment during rice-wheat crop production.

3.4. Nitrogen uptake by rice and wheat crops

The effect of gypsum and N application rates on N uptake by rice (paddy and straw) and wheat (grain and straw) in both years has been presented in Table 3. In both years, the highest N uptake was recorded at treatment N130 + SGR100. In rice 2011, the highest nitrogen uptake recorded in paddy and straw was 19.51 and 29.15 kg ha-1, respectively. While during second year, the highest N contents in paddy and straw were 37.24 and 49.82 kg ha-1, respectively. The highest uptake in rice was observed with N130 + SGR100 for both years and the lowest was with control treatment.

Table 3. Effect of applied N rates on nitrogen uptake (kg ha-1) of wheat and rice on salt-affected soil, where (C) no fertilizer and no gypsum, (N100 + SGR50) recommended N + gypsum at 50% SGR, (N130 + SGR50) 30 % higher N than recommended N fertilizer + gypsum at 50 % SGR, and (N100 + SGR100) recommended N + gypsum @ 100 % SGR, and (N130 + SGR100) 30 % higher N than recommended N fertilizer + gypsum at 100% SGR.

*Treatments differed significantly at p < 0.05.

Similarly, in wheat grain and straw, maximum nitrogen uptake was recorded at treatment N130 + SGR100 in both years. In wheat 2011-12, the highest N uptake by grain and straw was 45.92 and 65.23 kg ha-1, respectively. While during second year, the highest recorded uptake of N was 56.02 and 65.83 kg ha-1, respectively which also differed significantly from the respective control treatment.

3.5. Pearson correlation between physicochemical properties of soil and yield parameters

Data showed inverse relationships between EC, pH, SAR and CaCO3 with grain and straw yield (p < 0.01) while CEC and OM correlated positively with the yield at 5% level of significance (Table 4). With respect to bulk density (BD), the highest significant values of correlation coefficient (r) were observed with grain and straw yield i.e. -0.9506 and -0.9705, respectively. The values of correlation coefficients between IR and yield were highly significant (p < 0.01), and the highest values were 0.8861 and 0.8627 with grain and straw, respectively. Furthermore, there were negative correlations between NUE and soil chemical properties (EC, pH, SAR and CaCO3), and values of correlation coefficient for NUE were significantly higher than infiltration rate (IR). However, the highest r values of NUE with OM, grain, and CEC were 0.9695, 0.8962, and 0.8253, respectively.

Table 4. Pearson correlation coefficients (r) between physicochemical properties of soil and yield parameters after 2 year of experiments

BD: bulk density, CEC: cation exchange capacity, CaCO3: calcium carbonate, EC: electrical conductivity, IR: infiltration rate, NUE: nitrogen use efficiency, OM: organic matter, SAR: sodium adsorption ratio

3.6. Economic evaluation

The practical economics of the experiments were calculated using the market prices of common and variable inputs while toll prices of rice and wheat (Table 5). The agriculture on degraded and poor soils is generally less attractive because of high initial cost for soil reclamation. The gross income was maximum with addition of N130 + SGR100. The yield gap between the salt-affected and normal soil decreased from 53% to 26% at N130 + SGR100. While, benefit cost ratio estimation revealed that rice-wheat cropping system is profitable with treatment N100 + SGR100 as compared to other treatments (Table 5). However, investment is profitable in rice-wheat production at N100 + SGR100 since an investment of 1 PKR, it gains 2.15 PKR with net return of 1.15 PKR. It is encouraging to note that the cost of treatments has been recovered from the first crop, while in subsequent year; treatment N100 + SGR100 became more profitable.

Table 5. Economics of applied treatments for rice and wheat (two each) crops from salt-affected soils, where (C) no fertilizer and no gypsum, (N100 + SGR50) recommended N + gypsum at 50% SGR, (N130 + SGR50) 30 % higher N than recommended N fertilizer + gypsum at 50 % SGR, and (N100 + SGR100) recommended N + gypsum @ 100 % SGR, and (N130 + SGR100) 30 % higher N than recommended N fertilizer + gypsum at 100% SGR.

Pak Rs=Pakistani rupees
Benefit Cost Ratio = TR/TC, TR mentions the total revenue, which properly analyzed as the benefit generated through rice-wheat production. TC remarked as the total cost appropriately evaluated as the total expenditures of rice-wheat cultivation.

4. Discussion

Nitrogen cycling in soil-plant system is very dynamic and complex due to climatic, soil, and plant factors and their interactions (Fageria and Baligar, 2005). The fraction of the applied N not taken up by plants is vulnerable to loses by volatilization, denitrification and leaching. The NUE can be increased by improving uptake efficiency of the N inputs, thereby minimizing the aforementioned losses and potentially reducing environmental risks (Ladha et al., 2016; Ortega et al. 2016). Adoption of new management strategies, such as cropping systems, affects C/N ratio, and organic and inorganic pools of N in the soil. In this study, during rice growth, submerged soil having high leaching fraction (LF) promoted desalination and desodication of calcareous soils due to valence dilution, in-situ mineral weathering and root action (Jenny, 2012). The decrease in pHs at N100 + SGR100 could be attributed to supply of Ca2+ followed by leaching of Na+ salts below 30 cm soil layer in saline-sodic soils (Table 1). However, at 0-15 cm soil depth during rice crop, NH4+ along with gypsum may decrease pHs, increase leaching, permeability, macro porosity and flocculation, lower bulk density and reduce surface crusting (Rengasamy, 2010). Treatment N130 + SGR100 and N100 + SGR100 remained better to lower the ECe because of improved IR of water (Table 1). Heavy irrigation water to rice crop increased leaching of soluble salts, lowering ECe more effectively at 0-15 cm soil depth compared to ECe at 15-30 cm soil depth. In gypsum added plots, the composition of soil solution had higher levels of Ca2+ that might have favored the sustainability in IR and HC. All the treatments significantly lowered soil SAR after rice and wheat harvest and gypsum application was highly effective in lowering soil SAR by decreasing dispersion through Na+-Ca2+ exchange (Qadir et al. 2014).

The crop NUE is controlled by N uptake and assimilation. Nitrate taken up by plant roots is assimilated in the roots while its major part is transported to the shoots where it is transformed to NH4+ in the cytoplasm (Lam et al., 1996). Ammonium ions in the solution are generally bound to negatively charged soil surfaces and transported to surface water bodies through runoff, whereas excessive NO3--N is leached from vadose zone to the phreatic zone, thereby contaminating the water in aquifers (Howden et al., 2013). The most relevant measure of the plant physiological NUE is the change in grain/paddy yield per unit accumulation of N in plant biomass. The higher paddy and wheat grain yield with N and gypsum application is due to induced growth that encouraged plant tolerance against the predominant EC and SAR stresses. Relatively higher yield with gypsum may be owing to its favorable effects on soil physical and chemical properties (e.g. improving soil permeability, hydraulic conductivity), particularly with favorable Ca2+: Na+ ratios in the soil solution (Murtaza et al., 2009). The broadcasting of gypsum along with N has a greater potential to decrease the soil salinity, resultantly, increased rooting depth and uptake rates of N, P, K, Ca, Cu and Mn. Decline in soil alkalinity usually leads to deeper rooting, and higher water and N uptake by plants (Fageria and Baligar, 2005). The N dose is directly related to the rice crop yield as the N requirement of rice crop depends upon the targeted yield (Prasad, 2006)

During wheat cropping in 2012-13, the highest AUE with N100 + SGR100 on saline-sodic soil could have been due to improved soil properties i.e. ECe decreased from 10.53 to 7.27 dS m-1 and SAR from 41.33 to 26.48 (mmol L-1)1/2 (Table 1). The relationship between N dose and paddy yield was significant (Figure 1) and showed that yield increased with increasing N dose up to a certain level and thereafter, at constant rate with subsequent dose of N (Singh et al., 2010). The NUE decreased with increasing N dose which is consistent with the diminishing law of return. Excessive application of N in rice increased leaching losses due to heavy irrigation requirements of rice which has detrimental impacts on the diversity and functioning of non-agricultural bacterial and plant ecosystems (Galloway et al., 2013). The Ca ions decrease the soil pH by precipitating carbonates and by forming a complex Ca-soil replacing Na+ and ultimately leaching the Na+ from root zone. Calcium also improves the N uptake by plant roots especially when the plants are young, and preserves the structural and functional integrity of plant membranes, stabilizes cell wall structures, regulates ion transport and controls ion exchange behavior as well as cell enzyme activity (Läuchli and Grattan, 2007).

Primary sources of inorganic N taken up by the plant roots are NH4+-N and NO3--N. During rice production in 2011, there was a gradual increase in NO3- concentration in the third, fourth and fifth leachates (Figure 3). As observed above, the higher NO3- content in fifth leachate seemed due to the third N application coupled with a decrease in N absorption just before crop maturity. Higher than permissible NO3- especially in the first leachates was recorded at N130 + SGR100 which not only decreases fertilizer use efficiency but also contaminates groundwater reservoir (Boufekane and Saighi, 2015). During rice crop of 2012, an increase in leached NO3- contents observed during third and fourth leaching seems due to the improvements in IR and HC with the use of gypsum (Gharaibeh et al., 2009) by aggregating soil particles. Increased aggregate stability in gypsum amended soils suggests formation of strong bonding involving Ca2+ bridges (Bronick and Lal, 2005).

During wheat 2011-12, the maximum NO3- concentration observed in the first leachate was at treatment N130 + SGR100 which did not differ significantly from N130 + SGR50. A significant decrease in NO3- leaching in the second leachate was attributed to increased uptake of NO3- by plants during the vegetative growth period. The observed increase in NO3- contents in third leachate could be due to the ameliorative role of gypsum which enhanced soil flocculation, and increased IR and HC of soil. Yazdanpanah (2013) reported that during reclamation process of saline-sodic soils in consequence of leaching, N tends to be removed out of soil profile. Several studies documented adverse effects of high NO3- levels, most notably methemoglobinemia, stomach cancer and non-Hodgkin’s Lymphoma (Ward et al., 1994).

Although the highest yield was obtained when 30% extra N was applied with 100% gypsum addition and the benefit cost ratio was also better, nevertheless considering highest NO3 leaching it cannot be recommended. The treatments N100 + SGR50 and N100 + SGR100 produced promisingly higher yield with low NO3 leaching which seems better option for sustainable crop production (Figure 3).

5. Conclusions

The results of this study showed that the highest NO3- leaching for rice and wheat was observed with N130 + SGR100 but remained higher for wheat than for rice during reclamation of saline-sodic soil. In this treatment, crop yield and AUE were also the highest as compared to the other combinations. Recommended and higher levels of N fertilizer increased the growth and yield of rice and wheat when applied along with gypsum. Application of higher N fertilizer (N130) had maximum yield for rice and wheat, nevertheless the net returns were not economical for the latter case. Economic returns of wheat were observed at recommended dose of N along with gypsum as per reclamation requirements. All these observations led us to conclude that the treatment N100 + SGR100 would be environmental friendly and economically viable option for rice-wheat cropping system in saline-sodic soils.

Acknowledgements

The authors gratefully acknowledge Higher Education Commission (HEC) of Pakistan for financial assistance to carry out this work. Special thanks are due to Prof. Dr. Abdul Ghafoor and Prof. Dr. Rien Van Genuchten for their valuable feedback on the manuscript.

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