Majid Roozeh , Majid Rajaie

a Agricultural Engineering Research Department, Darab Agricultural Research Station, Fars Agricultural and Natural Resources Research and Education Center, AREEO, Iran

b Soil and Water Research Department, Darab Agricultural Research Station, Fars Agricultural and Natural Resources Research and Education Center,AREEO, Iran

Keywords:No-tillage Corn residue Total nitrogen Nitrate accumulation Organic carbon

ABSTRACT Conservation tillage, residue management and optimizing nitrogen (N) fertilization are important strategies for soil and water conservation and sustainability of agricultural systems. In order to evaluate the effects of various residue management methods and N fertilizer rates on some soil quality indicators,accumulation of residual soil nitrate, and wheat yield under no till system, a split-plot experiment was arranged in a randomized complete block design with three replications. Main plots were three residue management methods, including retaining (R.M1), removing (R.M2) and shredding (R.M3) of corn residues. The subplots consisted of four N fertilizer rates,100 (N1),150 (N2), 200 (N3), and 250 (N4) kg N ha-1 from urea source. The results showed that soil organic carbon and total N in R.M1 and R.M3 treatments increased at the depth of 0-15 cm as compared to R.M2.The maximum accumulation of soil nitrate was observed in R.M1×N4 and R.M3×N4 interactions at the soil depth of 60-cm.The results of this study showed that R.M1×N3 and R.M3×N3 interactions had a greater impact on wheat yield than the sole effect of R.M1 or R.M3. According to our findings, application of 200 kg N ha-1 in combination with R.M1 which had once less tractor passing for mechanical shredding than R.M3, can contribute effectively to improve soil quality indicators,control soil residual nitrate and subsequently produce more wheat grain yield in the south-west of Iran.

1. Introduction

Over the past few decades, conservation tillage methods, especially no tillage, have received more attention due to reducing soil erosion, improving soil structure and organic carbon, reducing fuel consumption and decreasing production costs(Li et al.,2020;L′opez-Fando et al., 2007; Shaver et al., 2002; Zhao et al., 2009). In Iran,although conservation tillage systems have been introduced to farmers since 1999,lack of appropriate amount of soil organic matter along with improper management of fertilizers and crop residues(burning or use of residues as animal feed)has led to decreased soil fertility and consequently low productivity of agricultural systems in hot and dry regions of the country(Bahrani,Raufat,&Ghadiri,2007;Hajabbasi & Hemmat, 2000; Hemmat & Eskandari, 2004; Kabiri,Raiesi, & Ghazavi, 2015). About 10% of the regions in Iran have more than 500 mm of rainfall per year.Rest of the regions including the central and southern parts of the country with an average annual rainfall of 250 mm have arid and semi-arid climate and should be irrigated for crop production(Malekian et al.,2012). In hot and dry areas, maintaining or increasing of soil organic matter requires a series of special management techniques due to excessive heat and high rate of organic matter decomposition(Grisi et al.,1998;Haynes& Naidu, 1998; Zibilske, Bradford, & Smart, 2002). Management operations such as conservation tillage systems, especially nontillage and residue management, have been recommended as suitable measures to increase soil organic matter, water holding capacity, water use efficiency, efficient microbial activity and production sustainability (Ghimire et al., 2012; Iqbal et al., 2005;Machado et al.,2015).

However,the use of no-till systems and residue retaining on the soil surface require greater use of herbicides and fertilizers which may lead to environmental pollution (Daryanto et al., 2017; Zhao et al., 2007; Ardell et al., 2000). Accordingly, conservation tillage systems can have a significant impact on nitrate transport among the soil profile (Alkissi & Licht, 2004). In general, retaining crop residues at the soil surface can determine the rate of chemical and biological reactions in the soil by affecting soil temperature and humidity (Brooks et al., 2004; Jackson et al., 2008) and thereby affect the nitrogen (N) cycle and increase the likelihood of denitrification and immobilization phenomena (Golabi et al., 1995;Kamkar et al., 2014). Since no-till system depending on the soil texture and climate conditions have different effects on water infiltration rate and water movement pattern in the soil,the effect of no-till on nitrate leaching can be considerably variable (Angas,Lampurlanes, & Cantero-Martinez, 2006; Lampurlanes, 2000). In some studies stated that under no tillage system, greater leaching due to higher soil moisture and the presence of preferential flows may contribute to the reduction of available N (Brye et al., 2001;Daryanto et al., 2017; Fuentes, Flury, Huggins, & Bbezdicek, 2003).In another experiment it was documented that conventional and minimum tillage methods resulted in greater nitrate accumulation in the depth of 150 cm compared with no tillage system in wheat cropping(Celik et al.,2017).In addition,the reduction of crop yield has been reported as a major constraint for a no-till system adoption (Lundy et al., 2015; Vanlauwe et al., 2014). In cases where residue is retained on the soil surface, losses due to N immobilization may lead to yield decrease which such reaction is higher when N is applied by surface broadcasting (Alvarez & Steinbach,2009; Carefoot et al.,1990; Malhi & Nyborg,1990).

To increase the wheat production, farmers usually apply the high rate of nitrogenous fertilizers.Many studies have reported that excessive N input resulted in higher nitrate leaching in soil (Dai et al., 2015; Wang et al., 2015). Finally it can be stated that retention of crop residues on soil surface in minimum/no-tillage systems,along with N fertilization play an important role in improving soil physical conditions,fertilizer use efficiency,N leaching and the crop productivity(Dalal&Chan,2001;Mohammad,Shah,Shah,&Iqbal,2003;Sainju et al.,2008).However,there are no such studies in arid and semi-arid regions of Iran. It is therefore necessary to conduct such studies under different climate,soil and crop system conditions.Thus,the objectives of this study were to(i)investigate the effect of different residue management methods and N fertilizer rates under no-tillage system on soil quality indicators,residual soil nitrate accumulation and wheat yield in hot and dry condition (ii)determine an optimum rate of N fertilization in different residue management methods in Fars province of Iran which is one of the main wheat-corn growing areas in the southwestern region of the country.

2. Material and methods

2.1. Site description and experimental design

This research was conducted from 2014 to 2016 at Darab Agricultural Research Station in Fars Province, located in the southwestern region of Iran(28°47′N,57°17׳E;1120 m above sea level)(Fig.1).The experiment site has a semi-arid climate with an average annual rainfall of 265 mm which often occurs in the fall and winter.During the two years of experiment, the maximum and minimum air temperatures were 41.5 and 12.7°C and the maximum and minimum relative humidity were 58.1 and 16.7%,respectively.Prior to the experiment initiation,the site was under continuous wheatcorn rotation system using conventional tillage. Five soil samples were collected from the depth of 0-60 cm and analyzed for soil physical and chemical properties of the experimental field before wheat planting on December 2014 (Table 1). This study was conducted using split plot experiment in a randomized complete block design with three replications. Three residue management methods were as the main plots, including retaining (R.M1),removing(R.M2) and shredding(R.M3) for both crops residue. For R.M1 treatment, all of residues from the previous crop (corn residues) left on the soil surface. In the second treatment (R.M2) residues (both flat and standing) were removed by bailing (Fig. 2). A tractor-mounted flail shredder with a 1.8 m wide was used in R.M3 treatment.The crop residues chopped into short pieces and spread on the plot area by the shredder. During operation, the deflector plate on the shredder was adjusted to the down position and angled so that the residue was directed immediately on the ground surface. The subplots consisted of four N fertilizer rates,100 (N1),150(N2),200(N3),and 250(N4)kg N ha-1from urea source which were applied only to wheat every year.The highest N rate is usually applied in more than 80% of farmers’ fields in Fars province. For corn crop,the constant amount of 250 kg N ha-1(as urea)and 50 kg phosphorus (P) ha-1(as triple superphosphate) were applied consistent with the normal farming practices and soil test. Dimensions of experimental plots were 3 × 20 m to accommodate one pass of the no-till drill.

2.2. Crop management

This study focused on the response of wheat to the management of residues and N fertilizer application in a winter wheat-summer corn rotation system. Corn and wheat were respectively grown from July to December and December to June during the two years of the study. The corn single-cross hybrid 704 with 75 cm row spacing and 85,000 plant density ha-1was planted with a no-till seed cum ferti-drill in the second week of July 2014 and 2015 and harvested by a combine on December 4 and 10 in these two consecutive years.After corn harvest,the‘Chamran’wheat cultivar was sown by a no-till seed cum ferti-drill of Pierobon, TD 17.5-17 with the row spacing of 17.5 cm and at the rate of 250 Kg seed ha-1on 18 December 2014 and 12 December 2015.The soil was fertilized with urea and triple super phosphate. One-third of the N fertilizer rates were applied at planting and two-thirds at the tillering and beginning of booting stages.The amount of 90 kg phosphorus ha-1(as triple superphosphate)was also applied at planting to all plots based on the soil test. No potassium (K) fertilizer was applied to either corn or wheat crops because the soil test results showed it to be adequate(Table 1).Surface irrigation method(border irrigation)was applied during the growing period of both crops.The irrigation was performed at the 50% depletion of soil available water, which was determined gravimetrically in the top 30 cm layer of soil.During the wheat growing season, weeds were controlled using Axial (1.5 L ha-1) and Bromicide MA (1L ha-1) herbicides. Wheat grain yield was determined by harvesting an area of 1.5×18 m with a self-propelled combine on Jun 11, 2015 and Jun 4,2016.

2.3. Measurements and data analysis

2.3.1. Crop residue condition, residue cover and soil moisture

During each year of the experiment, in order to determine the amount of crop residue and its size on the soil surface,standing and flat (on-ground) residues confined in the 1 m2quadrant were collected separately at five random locations in each plot just four days after corn harvest in December 2014 and 2015.The dry matter mass of surface residue was determined by placing them in an oven for 72 h at 60°C and weighting(Chen,Tessier,&Irvine,2004).The residue cover was determined using the line-transect method(Wysocki, 1989, chap. 3). On each plot, three replications of line transect measurements were completed to estimate residue cover percent after planting. Soil moisture content, based on the dry weight, was determined before applying the N fertilizer by taking samples from the depth of 0-60 cm using an auger and at the 20 cm intervals in different stages of wheat growth (emergence,tillering,booting).The soil temperature in different treatments was measured at the depth of 10 cm before applying the N rates in accordance with the above mentioned stages of wheat growth using a calibrated electronic thermometer.

Fig.1. Location of the experimental site, Fars province, Darab, in the map of Iran.

Table 1 Physico-chemical properties of the experiment site.

2.3.2. Soil sample analysis

For each year, studied soil parameters were soil total nitrogen(STN), soil organic carbon (SOC), residual soil nitrate-N (R.N) and bulk density(B.d).STN and SOC were determined by soil sampling(0-15 cm depth) after the harvesting of wheat crop during the second and the first week of June 2015 and 2016,respectively.The soil samples were taken from each plot to a depth of 0-15 cm.Soil sample for each plot consisted of 10 soil cores diagonally across the rows, which includes samples from both inside and between the rows. Samples were pooled to have unique sample for each plot.Soil samples were air-dried and passed through a 2 mm sieve and then analyzed for STN and SOC. The SOC content was determined after pretreating the soil with 5%H2SO4to remove inorganic carbon(Walkley&Black,1934).The STN in soil samples was measured by wet combustion using micro-kjeldahl method(Bremner,1996).The R.N was determined immediately after wheat harvest on June 13,2015 and June 6,2016.Soil samples were collected at the depths of 0-30 and 30-60 cm using an auger. Soil samples for each depth were diagonally taken across the rows at five points in each plot.Soil samples from each plot contained a mixture of two cores per plot.Samples were stored in an ice-cold container and immediately transferred to the laboratory. Samples were dried at 35°C after mixing (Al-Kaisi & Licht, 2004). Ten grams of the sieved soil were shaken with 20 ml of 2M KCL solution for 30 min to extract soil nitrate.The nitrate content in the soil solution was then determined with a spectrophotometer (DR/2500, Hach Co.) calibrated according to the standard procedure using cadmium reduction method(EPA 1974). The soil C:N ratio computed as the quotient of organic carbon and total nitrogen concentration. Bulk density (B.d) was measured in each treatment using the core method (Blake &Harteg,1986). Soil samples for bulk density were collected about three weeks after wheat sowing.Intact soil cores were taken at the 2-10 and 10-20 cm depth with six samples per plot.Measurement of B.d was made on cores dried at 105°C.

2.3.3. Seeding depth, seed scatter index, speed of emergence and stand establishment

The seeding depth of wheat as affected by residue management methods was determined by measuring the CFL (chlorophyll-free length, from the seed remnants to the onset of the green stem) of the seedlings (Tessier, Saxton, Papendick, & Hyde, 1991). When final plant count was completed, five wheat seedlings were uprooted from each row and their CFL were measured. The uniformity of seeding depth was characterized by the seed scatter index (SSI) represented by the standard deviation of measured seeding depths (Tessier, Saxton, Papendick, & Hyde, 1991). The speed of emergence was calculated by: SE = Σ (Ni/di)/L (Tessier,Saxton, Papendick, & Hyde,1991), Where SE is the speed of emergence per unit row length (plants m-1per day), Nithe number of newly emerged seedlings counted per day (di) and L the length of the row counted(m)in each plot.Plant counts were made in a 2 m long staked row at 4 random locations on each plot at 2,4,6,8,10,and 12 days after the first seedling emergence and weekly thereafter until a stable count was obtained.

2.3.4. Data analysis

Data were analyzed using SAS statistical software and the differences between the means of the soil properties and crop yield were determined with Duncan’s multiple range test at P ≤0.05.

3. Results and discussion

3.1. Crop residue condition, soil temperature and moisture content

The condition of corn residue density and size obtained from the different residue management methods are given in Table 2. As shown by the results mechanical shredding of standing residue(RM3)reduced the height of stubble and consequently its weight by 65.5 and 59.5%compared to R.M1,respectively.R.M2 treatment had the lowest residue density per unit area and the highest residue length than the other treatments. The findings also revealed that the average residue cover was lowest for R.M2 treatment and the use of R.M1 and R.M3 methods covered more than 30% of the soil surface (Table 2). In addition, the use of mechanical shredding(R.M3)among these methods resulted in greater uniformity in the distribution of residue and its cover over the soil surface (86.8%).Nearly 84.7%of the 13.3 t ha-1of flat(on-ground)residue in R.M3 was less than 15.7 cm in length and fairly evenly spread (Table 2).The effect of different residue management methods on soil temperature and moisture content in accordance with different growth stages are shown in Table 3. Generally in R.M1 and R.M3 methods due to more amounts of residue, higher soil surface cover and smaller evaporation rate,the heating effect of radiation was minor,resulting in the reduction of soil temperature and enhancement of soil moisture content (Tables 2 and 3).Mean comparisons showed that the mean soil surface temperature at different stages of fertilization in R.M3 and R.M1 treatments was 18.9% and 10.1%lower than R.M2, respectively (Table 3). In R.M1 and R.M3 treatments, topsoil cover and higher moisture content led to the reduction of soil surface temperature and evaporation.In fact, this coating acts as an insulator, preventing the absorption of solar energy and heat transfer from the atmosphere to the soil. (Chen et al., 2014; Shen et al., 2018). On the other hand, the R.M2 management method with lower volume of residue caused an increase in soil surface temperature and decreased soil moisture content compared to R.M1 and R.M3 methods. These results are in agreement with the findings of Romero et al.(2015),Li et al.(2013),Chen et al. (2005) and Licht and Al- Kaisi (2005) that examined the impact of tillage systems and residue management and showed that in no-till system with straw removed, soil temperature was higher than no-tillage and straw retained system.However,studies by Steward et al.(2018),Verma et al.(2011),Karhu et al.(2010)and Ambebe et al. (2009) revealed that soil temperature controls biological and biochemical processes in the soil that in turn affect soil organic matter formation, fertilizer efficiency, root development,nutrient uptake and disease occurrence.

Table 2 Residue size and density as affected by different residue management methods.

3.2. Soil organic carbon (SOC) and soil total nitrogen (STN)

Table 4 shows that the various residue management methods significantly influenced the SOC content in the upper soil layer(0-10 cm). The highest SOC content was observed in R.M1 and R.M3 treatments at the depth of 0-10 cm (Table 5). In fact an average amount of 13.7 Mg ha-1of crop residue was leaved on the soil surface in the R.M1 and R.M3 treatment (Table 2). In other words,retaining residues on the soil surface(R.M1)or chopping the residue(R.M3)into various length pieces increased SOC compared to R.M2 (Table 5). It is accepted that the high amount of crop residue would have impact on soil organic carbon(Casado-Murillo& Abril, 2013). The higher SOC in R.M1 and R.M3 can be probably attributed to the huge quantities of crop residues left on the soil surface, effects on soil temperature and consequently greater biological activity(Tables 2 and 3).Surface placement of crop residues in R.M1 and R.M3 affects near-surface moisture and temperature of the soil during the growing season (Table 3). The efficient decomposition of plant residue by soil microorganisms requires optimum soil temperature and moisture(Han et al.,2012;Bauer et al.,2008).In other hand, deposition of plant residues on the soil surface in R.M1 and R.M3 can probably provide an energy source for soil microorganisms and lead to more decomposition of crop residues with time (Nachimuthu & Hulugalle, 2016; Mangalassery et al.,2015; Helgason et al., 2010; Dou et al., 2008) and subsequently create a more favorable condition for SOC accumulation in the surface soil (Table 5). These results match with several previous studies (Chen et al., 2009; Gupta Choudhury et al., 2014; Malhi,Lemke, & Wang, 2006; Mohammmad et al., 2012; Noe & Abril,2008) that higher SOC might be due to the role of crop residues in conserving soil moisture, stabilizing organic carbon and protecting carbon from mineralization in the combination of no-tillage and residue retained system compared to straw removed system.

The amount of SOC was affected by different rates of N fertilizer in the order of OC.N1＞OC.N2＞OC.N3＞OC.N4 in the wheat crop(Table 5).Generally,the plots that received more N fertilizer had the lowest amount of SOC after harvest.The lowest amount of SOC was observed when N was applied at the rate of 250 kg ha-1(Table 5).This can be attributed to the increased microbial activity resulted in higher rate of biodegradation and oxidation of crop residues(Haynes & Naidu,1998; Reicosky et al.,1995). In fact in our study,application of high N fertilizer (N3, N4), probably lead to faster decomposition of the residues on the soil surface due to the growth of microbial population that would obtain energy from SOC and as a consequence would reduce the humification process (Jia et al.,2006; Sall et al., 2003). The results also showed that the amount of carbon released from residues decreased with the reduction of N consumption and consequently the SOC in the surface layer was significantly increased in lowest rate of N consumption (Table 5).Similar results can be found in research done by Malhi, Lemke, &Wang, 2006, though Al-Kaisi & Guzman, 2013 and Al-Kaisi et al.(2017) showed that the rate of decomposition of corn residue treated with low N rates (34, 67 kg N ha-1) was not significantly different.However,the increase in decomposing efficiencies is due to changes in decomposing communities that have a greater N requirement (Ågren et al.,2001).

Table 3 Soil temperature and soil moisture content under different residue management methods.

Table 4 P-values for soil organic carbon, soil total nitrogen, C:N ratio, bulk density, residual nitrate-N, and grain yield.

Table 5 Soil organic carbon(SOC),soil total nitrogen(STN),C:N ratio and bulk density(B.d)as affected by residue management methods and N fertilizer rates.

The results of the experiment at the end of the growing season showed that the effect of different residue management methods on the STN was significant (Table 4). Maximum STN was observed in the R.M3 treatment and minimum STN occurred in the R.M2 treatment in the 0-15 cm of soil layer(Table 5).The increase of STN in R.M3 treatment can be due to the more amounts of residues on the soil surface,higher biological processes,more N mineralization and subsequent increase in transformation of soil N (García-Gil et al., 2004). Our results agree with others who stated the impact of residue management and seasonal effect on the decomposition rate of plant residues in arid and semiarid regions (Halvorson,Wienhold, & Black, 2002; Kumar & Goh, 2000; Malhi Lemke, &Wang, 2006; Zhang et al., 2008). There was no significant difference in average STN concentration observed between different rates of N in the upper soil layer.This proves that N has penetrated into the lower layers of the soil and manifested itself in the form of nitrate N.This result is in agreement with findings stated by Grant et al. (2016).

The soil C:N ratio as affected by different residue management methods are shown in Table 4. The results revealed that R.M3 and R.M1 treatments relative to R.M2 led to a significant C:N ratio reduction in the depth of 0-15 cm of the soil surface layer(Table 5).The lowest C:N ratio value occurred in the R.M3 treatment(Table 5). The C:N ratio is usually considered as a key factor controlling organic matter decomposition, nutrients release and N mineralization and immobilization (Bossche, Bolle, Neve, &Hofman, 2009). Decrease in C:N ratio in R.M3 might be attributed to relatively even distribution of crop residues on the soil surface and faster decomposition resulted from better contact of soil microorganism with crop residues (Table 2). Feng et al. (2014),Haene et al. (2008) and Stemmer et al. (1999) reported that in reduced and no tillage systems with different crop residue, C:N ratio changes in the soil surface layer are influenced by the residue contact with soil and their exposure to microbial population and nutrients.

The C:N ratio in the soil surface layer was significantly affected by different amounts of N fertilizer (Table 4). The results showed that C:N ratio was decreased at the end of the growing season as the N fertilizer rate was increased. The lowest C:N ratio was observed when N was consumed at a rate of 250 kg ha-1(Table 5).As it was stated by Jia et al. (2006), Haynes and Naidu (1998) and Reicosky et al.(1995),N fertilization would have led to a decrease of the soil C:N ratio with subsequent increased microbial population and faster decomposition of the crop residue.

Fig.3. Residual soil nitrate-N content as affected by different residue management methods;Different letters above the histograms indicate significant differences at the 0.05 level(ANOVA and Duncan’s multiple range test).

3.3. Residual soil nitrate (R.N) accumulation

Analysis of variance showed that different residual management methods had a significant effect on the amount of R.N after wheat harvest (Table 4). According to the results, R.M1 treatment accumulated the highest amount of nitrate N at the soil depth of 0-60 cm after wheat harvest.R.M2 treatment with a 31.1%decrease had the lowest nitrate accumulation compared to R.M1(Fig.3).The mechanical shredding (R.M3) reduced the R.N content by 13.7%compared to R.M1 (Fig. 3). Differences in the amount of nitrate leaching to the lower layers of soil in different residual management methods,in addition to the more immobilization of inorganic N by microbes due to retaining or returning crop residues to the soil(Kuzyakov&Xu,2013;Gentile et al.,2009),might be attributed to changes in some soil properties such as bulk density(B.d),moisture content (Tables 3 and 5), cone index (data not shown) and infiltration rate resulted from the high traffic intensity of tractors used for residue management operations in R.M2 and R.M3 treatments.Collection and transport of corn residues from the field involve tractor traffic, which can increase soil compaction (Wilhelm et al.,2004). As the number of residue management operations was increased in R.M2 and R.M3 treatments,the higher soil bulk density(B.d) was obtained at the both depths (Table 5). The B.d values in the 2-10 cm and 10-20 cm layers had significant differences between residue management methods (Table 5). On average,13.4%increase was found in B.d at the 0-20 cm layers in R.M2 and R.M3 as compared to R.M1.According to Mtyobile et al.(2020),there was a negative correlation between the soil water content and the B.d.When soils are compacted due to the high traffic intensity of the tractor,the amount of macropores is reduced and subsequently the B.d is increased (Weninger et al., 2019; Pe~na-Sancho et al., 2017;Moret & Arrue, 2007; Pagliai & Vignnozi, 2002). The major loss of the largest pores caused by soil compaction can lead to the change of pore size distribution, soil hydraulic properties and water movement (Dairon et al., 2017; Kinoshita, Schindelbeck, & vanEs,2017; Dexter, 2004; Richard et al., 2001). According to the facts stated,the lower nitrate leaching under R.M2 and R.M3 treatments might be ascribed to the higher compaction of soil compared to R.M1. In other hand, in R.M1 treatment (without collection and transport of corn residues from the field by tractor)the built-up of the macropores allows more water to infiltrate which might explain the higher nitrate leach load (Daryanto et al., 2017).

The significant effect of different rates of N fertilizer on the R.N is shown in Table 4.The N fertilizer application rate is one of the most important factors affecting the vertical distribution of nitrate N in the soil profile. Generally, plots that received more than 150 kg N ha-1had significantly higher R.N in the lower layer of the soil after wheat harvest. Mean comparison revealed that R.N had an increasing trend with increase of N application from N1 to N4.Nitrate residue peak in the 0-60 cm soil layer was 76.8 N kg/ha by 250 N addition(N4)in our study.On average treatments of N2 and N3 could decrease nitrate N in the 0-60 cm soil layer by 51.1 and 29.4%, respectively compared with the highest N level (Fig. 4).Other researchers have also reported similar reports on the direct relationship between increased nitrate N in the soil profile and increased N fertilizer application (Al- Kaisi & Mensah, 2007; Dai et al., 2015; Yan, Zhang, Zhang, Abdelrahman, & Gao, 2016; Yang,Gao, Cao, & Chen, 2009).

Residue management methods and N fertilizer rates caused significant differences in R.N.Also,the interaction effect of residue management methods and N rates on R.N content was significant(Table 4).The combination of R.M1 and R.M3 systems with N rates higher than150 kg ha-1compared to other treatments accumulated the highest amount of nitrate N in the soil profile (Fig. 5). The highest amount of R.N was observed from the interaction of N4 × R.M1. Our findings showed that nitrate N was decreased by 22.9% in N3 × R.M1 compared to N4 × R.M1. This trend was also observed when comparing the N3 × R.M3 with N4 × R.M3 interactions (Fig. 5).

Fig. 4. Residual soil nitrate-N content as affected by different fertilizer rates; Different letters above the histograms indicate significant differences at the 0.05 level (ANOVA and Duncan’s multiple range test).

Fig. 5. Residual soil nitrate-N content as affected by interaction of residue management methods and fertilizer rates Different letters above the histograms indicate significant differences at the 0.05 level (ANOVA and Duncan’s multiple range test).

3.4. Treatment effects on grain yield

Different methods of corn residue management had a significant effect on wheat yield (Table 4). The highest wheat yield was observed in R.M2 treatment. Application of R.M1 and R.M3 treatments compared to R.M2 resulted in 11.9%and 4.8%decrease in plant yield respectively(Fig.6).The concentration of on-ground residue in the R.M1 and R.M3 might have played a major role in this incident.The increase of yield in R.M2 can probably be attributed to the lower amount of residue remaining on the soil surface, and subsequent reduction of the physical effects of residue on seed scatter, emergence speed and stand establishment (Tables 2 and 6). The finding revealed that the R.M1 and R.M3 promoted the greatest seed scatter index (SSI) from the mean seeding depth and subsequently decreased the emergence speed while R.M2 promoted the lowest SSI and the fastest emergence of wheat seed. In fact, removing crop residue in R.M2 treatment provided the highest stand establishment which was 21.7 and 13.3% higher as compared to R.M1 and R.M3 respectively (Table 6). Furthermore, as corn residues have a slow decomposition rate (Barraco et al., 2007; Reis et al., 2011), this can immobilize a relevant amount of soil mineral N reducing its availability to wheat crop sown later than corn(Morris, Miller,Orson,&Froud-Williams, 2010). In this regard, some studies have also reported that N fertilizer efficiency is reduced in the condition of plant residue retention which has been ascribed to enhancement of N immobilization(Aulakh et al.,2001;Barraco et al.,2007;Bird et al.,2001). The results also revealed that in conditions where plant residues were converted to smaller pieces by the shredder and distributed almost uniformly on the soil surface (RM3), crop yield was increased by 8.1% compared to R.M1 (Fig. 6). These results are consistent with the findings of Roozbeh and Pooskani (2003), and Bueno,Amiama,&Hernanz,2007 who reported that wheat yield in crop-beds managed by stem chopper was significantly increased due to greater planting depth uniformity and better stand establishment.

Fig. 6. Grain yield as affected by different residue management methods; Different letters above the histograms indicate significant differences at the 0.05 level (ANOVA and Duncan’s multiple range test).

Table 6 Sowing depth, seed scatter index, speed of emergence and stand establishment as influenced by different residue management methods.

Fig. 7. Grain yield as affected by different fertilizer rates; Different letters above the histograms indicate significant differences at the 0.05 level (ANOVA and Duncan’s multiple range test).

Analysis of variance indicated that different fertilizer rates had significant effects on grain yield (Table 4). Mean comparison revealed that crop yield was increased by enhancing N rate from N1 to N4 (Fig. 7). The increase of N application from N3 to N4 had no significant effect on grain yield improvement (Fig. 7) which, however resulted in substantial N residues in the soil (Fig. 4). The findings showed that the optimum amount of N needed to achieve the highest yield was 200 kg ha-1(N3).However,application of N2 and N1 treatments resulted in 14.3% and 26.3% decrease in yield compared to N3 respectively (Fig. 7). As it was demonstrated by Gentile et al.(2009),the combined use of crop residues and mineral fertilizers has helped to synchronize the N supply with crop demand and reduce N losses.According to the results,the interaction between residue management methods and different amounts of N fertilizer on yield was significant (Table 4). The combination of R.M2 and R.M1 systems with N values greater than 150 kg ha-1produced the highest yield compared to the other interactions.The minimum grain yield was observed in the R.M3×N1 treatment and the maximum occurred in the R.M2 × N3 interaction (Fig. 8). The results also revealed that grain yield in R.M3 × N3 decreased by 8.4% as compared to R.M1 × N3 (Fig. 8). Our findings showed that the application of 200 kg N ha-1(N3) in combination with R.M1 and R.M2 gave an optimum grain yield and reduced the residual soil nitrate N as compared to other interactions(Figs.5 and 8).The present results are supported by Feng et al. (2014), Gentile et al.(2009) and Habtegebrial, Singh, & Haile, 2007 who indicated that the rate of applied N can be reduced under suitable residue management method resulting in the reduction of N losses risk from corn-wheat cropping system and leading to improvement of wheat grain yield.

Fig.8. Grain yield as affected by interaction of residue management methods and fertilizer rates;Different letters above the histograms indicate significant differences at the 0.05 level (ANOVA and Duncan’s multiple range test).

4. Conclusion

The amount of NO3-N remaining in the soil after plant harvest was greatly affected by different residue management methods and N rates. The results of this study showed a significant increase in R.N content with increasing fertilizer rate (especially in the amounts more than 150 kg N/ha). The methods of residues remaining on the soil surface resulted in more nitrate N accumulation than the removing method.The lowest N required to achieve maximum yield was seen in N3 treatment. Based on our findings,since the presence of crop residue at no-till system can conserve soil and organic matter, thus application of 200 kg N ha-1 in combination with R.M1, which had once less tractor passing for mechanical shredding than R.M3, can contribute effectively to improve soil quality indicators, control soil residual nitrate and subsequently produce more grain yield in the south-west of Iran.These results are important findings for farmers in arid and semiarid regions of Iran where there are concerns about the management of N fertilizer use in no tillage systems affected by different residue management methods.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgement

We acknowledge the Agricultural Research, Education and Extension Organization,Iran for funding this research.