Productivity, Profitability and Greenhouse Gas Emission from Rice-Wheat Cropping System under Different Tillage and Nitrogen Management Practices

Greenhouse gas emission has become a very crucial environmental issue, which pose a potential threat to global economy and food security (Ahlawat and Kaur, 2015). Rice based cultivation is always blaming for methane (CH₄) emission Agriculture sector is a contributor of climate change by emitting anthropogenic greenhouse gases (GHGs) with a contribution of around 24% of total global anthropogenic GHGs emission (IPCC 2014). The net emission of GHGs from India was 1727.7 MT of CO₂ equivalent (Singh et al., 2015). Besides the cultivation practices, application of nitrogenous fertilizers are the major source of about 77% of total nitrous oxide emission of our country. The indiscriminant use of nitrogenous fertilizer and wetter and denser soil condition with less porosity are mainly responsible for nitrous oxide (N₂O) by escalating the process of denitrification. Therefore, with the arising challenges of global warming and climate change, forcing us to think of alternative management practices. So, to ameliorate the ill impact of climate change by reducing the level of greenhouse gases efforts should be made to develop management strategies like crop establishment methods and nitrogenous fertilizer application methods to modify the traditional rice-wheat cropping system.

Site description
 
A field experiment was conducted in 2016-19 at the Research Farm of Bihar Agricultural University (BAU), Sabour, Bihar. The research farm is under subtropical climatic condition with hot desiccating summer, cold winter and moderate rainfall. The average maximum temperature is 35-39°C whereas; minimum temperature is 5-10°C. The mean annual rainfall is around 1250 mm and precipitated during mid-June to mid-October. Daily values of weather parameters of three consecutive years (2016-17 to 2018-19) were recorded at the BAU meteorological observatory, presented in Fig 1(a), (b), (c) and (d). The initial status of soil was clay loam with pH- 7.4, Electrical Conductivity -0.26 dSm^-1, organic carbon content -4.60 g kg^-1 and available nitrogen - 228.5 Kg N ha^-1, available phosphorus-19.22 Kg P₂O₅ ha^-1 and available potassium-210.4 Kg K₂O ha^-1.
 

 
Experimental details
 
The experiment was laid out in split plot design, each plot having dimension of 4.0×5.0 m² with 8 treatment combinations, having two tillage operations i.e. zero tillage (M₁) and conventional tillage (M₂) in main plot and four N management treatments in sub plots (viz. 100% recommended dose of N (RDN) (i.e. 100 kg N ha^-1 for rice and 120 kg N ha^-1 for wheat) where ½ applied as basal with two top dressing in equal split at active tillering and panicle initiation stage (S₁). SPAD based N management practice was done (S₂). In S₂, SPAD threshold was 36 for rice and 42 for wheat whenever SPAD index goes below the critical index, the N fertilizer was applied (20 kg ha^-1) in the form of urea. The SPAD index was measured at 10 days interval, starting from 40 days after sowing of rice and 25 days after sowing of wheat to first flowering. S₃ treatment was the combination of organic and inorganic fertilizer i.e. 75% N through urea and remaining 25% N through Vermicompost which was applied 15 days before sowing. Another treatment was fixed time N management i.e. 100% RDN in four equal split i.e. at basal 20, 40  and 60 days after sowing (S₄) and replicated thrice.
 
Crop management
 
Direct seeded rice (var Rajendra Sweta) was sown in mid June with a seed rate was 50 kg ha^-1 at a row spacing of 20 cm. Manual sowing of DSR was carried out in the plot. A recommended dose of fertilizers in rice was 100 kg N + 60 kg P₂O₅ + 40 kg K₂O ha^-1 in which full P and K was applied in form of diammonium phosphate (DAP) and muriate of potash (MoP) respectively as basal and nitrogen was applied as per the treatment. Similarly for wheat, ‘HD-2967’ variety was sown manually in the mid of November through hand plough with row to row distance 22 cm using seed rate of 100 kg ha^-1. A common recommended dose of 120 kg N + 80 kg P₂O₅ + 60 kg K₂O ha^-1 were applied.
 
Crop harvest and yield
 
At crop maturity, both rice and wheat crop was harvested manually. Rice was harvested by cutting and threshed manually. Wheat grains were threshed using a plot thresher. Yields of rice and wheat were estimated by harvesting the entire plot and converted it to q ha^-1. The grain yield of rice and wheat is reported at 14% and 12%, grain moisture, respectively. For comparing the productivity of rice and wheat crops and total system productivity of the different treatments, was converted into rice equivalent yield (q ha^-1) using the following equation:

 

 
Greenhouse gas (GHG) collection and analysis
 
The greenhouse gases i.e. CH₄, N₂O and CO₂ were collected from both rice and wheat field through Pyrex glass gas chamber with the help of 50 mL disposable injection syringe with three way leur lock. At each sampling date, GHG samples were collected at 0, 30 and 120 minutes interval from each plot. The Gas samples were analyzed for CH₄, N₂O and CO₂ concentrations by a gas chromatograph (Trace GC 1100, Thermo Fischer) equipped with two detectors. N₂O was detected by an electron capture detector (ECD) and CH₄ was detected by flame ionization detector (FID). CO₂ was reduced with hydrogen to CH₄ in a nickel catalytic converter at 350°C and then detected by the FID. The carrier gas was nitrogen at a flow rate of 35 mL min^-1. The temperatures for the column and ECD detector were maintained at 60°C and 300°C, respectively. The oven and FID were operated at 60°C and 300°C, respectively. The gas emission flux was calculated from the difference in gas concentration according to the equation of Zheng et al., (1998) :

 

 
Where,
F is the gas emission flux (mg m^-2 hr^-1), ρ is the gas density at the standard state, h is the height of chamber above the soil (m), C is the gas mixing ratio concentration (mg m^-3), t is the time intervals of each time (h) and T is the mean air temperature inside the chamber during sampling.
 
Global warming potential (GWP) and GHGs intensity (GHGI)
 
GWP of different treatments was measured as CO₂ equivalent using the following equation (IPCC, 2007).
 
GWP (CO₂ equivalent kg ha^-1) =  (CO₂)+(CH₄ x 21)+(N₂O x298)
       
Based on a 100-year time frame, the GWP coefficients for CH₄ and N₂O are 21 and 298, respectively, when the GWP value for CO₂ is taken as 1 (IPCC, 2007).
 
GHGI was estimated on the basis of grain yield produced (Shang et al., 2011) :

 
 

 

Statistical analysis
 
Analysis of variance (ANOVA) was done to determine treatment effects (Gomez and Gomez, 1984) using microsoft excel 2007.

Grain yield and system productivity
 
Zero tillage recorded highest rice and wheat grain yield 45.1 q ha^-1 and 41.3 q ha^-1, respectively (Table 1). Zero tillage attributed 7.54% and 8.0% higher yield than conventional tillage practices. Split application of nitrogen recorded highest grain yield in both the crop; 45.3 q ha^-1 and 40.8 q ha^-1 in rice and wheat, respectively. Similarly, system productivity was superior in zero tillage and split application of nitrogenous fertilizer and lowest in conventional practices. Our result was also concord with the result of Singh and Kumar (2014). This was due to the fact that zero tillage facilitates a favorable soil environment through altering soil properties and soil organic matter content and thus nitrogen availability enhanced. This will promote better crop growth and root mass and ultimately enhance plant growth attributes and dry matter content. Zhang et al., (2015) was found that, 30% residue retention under zero tillage system that creates a permanent or semi-permanent organic soil cover and reduces the evaporation losses from the soil and allows soil borne microorganism to accelerate the microbial degradation of soil organic matters and balancing the soil nutrient content which enhance the vegetative growth of plant. Moreover, there is an increase in infiltration of water into the soil and also increase organic matter retention under zero tilled condition that eliminate soil erosion, improving soil biological fertility and building up the more resilient soil for crop growth (Sharma, 2007). Zero tillage improves soil physical, chemical and biological parameters and providing good mount of nutrient to crops that attribute to better yield determining parameter and good yield. While in conventional tillage system more tillage causes more erosion and soil degradation and severe soil loss from topsoil layer especially warmer areas where thin top soil layer are generally found. Split application nitrogenous fertilizer increases nitrogen availability to plant that enhances cell elongation and cell division in meristematic tissue of the plant which aids good growth of the crop. Under S₄ treatment there was a quick and adequate nutrient supply that improves the nitrogen use efficiency of crop. Moreover, split application of nitrogenous fertilizers supply nutrient as per the need of crop and reduce the chance of nitrogen losses through weeds, leaching, denitrification and volatilization, expressing higher growth, yield attributes and yield of crop. This result is also in close agreement with the findings of Singh et al., (2018) and Mauriya et al., (2015).
 


Economics and profitability
 
Zero tillage ensures lower cost of production as compared to the conventional tillage (Table 2). The result showed that zero tillage reduced the cost of production by 3.0% over conventional tillage; however an additional 16% (15536 INR ha^-1) net profit was also obtained. Similarly, split application of nitrogenous fertilizer gave an additional net return of 10890 INR ha^-1 despite its higher cost of cultivation (Rs 68328 ha^-1) over normal fertilization schedule (Rs. 67928 ha^-1). Exclusion of tillage practices and beneficial aspect of zero tillage could help not only reduce the cost of cultivation but also harness higher yield, hence higher net income and B:C ratio (Jat et al., 2014). Split application of nitrogen would help to augment the crop nitrogen demand at required time. Thus resulted higher yield and ultimately more income and higher B:C ratio.
 

 
Seasonal GHGs emission, GWP and GHGI
 
Tillage and nitrogen management had a significant influence on greenhouse gases (GHGs) emission. Lowest GHGs emission was found under zero tillage irrespective of nitrogen management (Fig 2). Significant (p=0.5) variation was found in CH₄ and CO₂ emission in different phenological stages, but no significant variation was found in N₂O emission in different growth stages. Least amount of total seasonal CH₄ (48.89 kg ha^-1 in kharif and 6.25 kg ha^-1 in rabi), CO₂ (38.26 kg ha^-1 in kharif and 157.03 kg ha^-1 in rabi) and N₂O (1.60 kg ha^-1 in kharif and 21.67 kg ha^-1 in rabi) emission was obtained from zero tilled plots and spit application of nitrogenous fertilization emitted less amount of CH₄ (55.44 kg ha^-1 in kharif and 5.52 kg ha^-1 in rabi), CO₂ (40.39 kg ha^-1 in kharif and 147.52 kg ha^-1 in rabi) and N₂O (1.61 kg ha^-1 in kharif and 19.35 kg ha^-1 in rabi) (Table 3). Interaction of tillage and nitrogen management was found non-significant. Basically, anaerobic conditions are prerequisite for activities of methanogenic bacteria that enhance methane production. Adding to this methane oxidation potential would get disturbed by tillage operation. Thus under zero tillage, no disturbance of the soil causes less exposure soil organic matter resulted in lower chance of methane emission. Moreover under zero tillage system soil has high bulk density as because of reduced porosity (total porosity and pore size) that enhances retention of methane in soil and prevents the flow of methane in soil. It may improve oxidation of methane by methanotrophs resulting in lower methane emission. Under aerobic condition non-microbial methane emission is common from wheat crop. Besides this, higher carbon dioxide release was found in response to tillage that means the ploughing operation break down the soil aggregate and exposed the soil organic matter for microbial decomposition under conventional tillage system. Furthermore, soil pore character i.e. total porosity and pore size of the soil are stronger envisages of carbon dioxide flux than soil organic matter and presence of microbial biomass carbon (Sapkota et al., 2015). Conventional tillage increases the porosity of the soil which favours the respiration of aerobic microorganism by recovering movement of water and air within the soil that augment carbon dioxide emission (Wassmann et al., 2000). Although, there is a large ambiguity regarding the higher nitrous oxide emission from zero tillage system than conventional tillage system but after long term practice of zero tillage may reduce the nitrous oxide emission (Ahmed et al., 2009). The nitrification and denitrification process both are responsible for nitrous oxide emission (Liu et al., 2015). Split application of nitrogenous fertilizer (S4) reduced the methane, carbon dioxide and nitrous oxide from rice-wheat system. That is because of splitting doses of nitrogen enhance nitrogen use efficiency. Actually, the required quantity of nitrogen will be available to the plant at right time augment NUE and lower the losses of nitrogen through denitrification, leaching and volatilization (Bhatia et al., 2012). However 100% inorganic fertilization through neem coated urea (S₁) endorsed maximum nitrous oxide emission (2.08 kg ha^-1 in kharif and 27.99 kg ha^-1 in rabi). The application of the nitrogenous fertilizer as basal to the soil increases nitrous oxide emission by providing substrate to denitrifing bacteria in the soil. Thus, nitrogen is lost through various pathways before it is utilized by the crop as the chance of mineralization is more under non-splitting fertilization (Jain et al., 2013). The GWP was significantly lower under zero tillage system 9576 kg CO_2eq ha^-1. Three top dressing of nitrogenous fertilizer (S₄) could lower down the Global Warming Potentiality by 7966 kg CO_2eq ha^-1. The minimum greenhouse gas Intensity (GHGI) of the system was recorded 0.94 kg CO_2eq kg^-1 grain yield under zero tillage practice (M₁) and it was lower at 0.71 kg CO_2eq kg^-1 grain yield in split application of nitrogen management practices (S₄) under rice-wheat cropping system. The cost of cultivation for both rice and wheat crop was lowered under zero tillage system as compared to the conventional tillage system. This may be due the fact that under zero tillage plot labour requirement for ploughing was less because of no tillage operation which curtailed down the cost of cultivation. Similarly, SPAD based nitrogen management practice (S₂) had the lower cost of cultivation because of 30% lower nitrogenous dose was applied in this treatment that acquired the lower input cost. The highest system gross return was obtained under zero tillage treatment (M₁) in rice and wheat crop even in the cropping system as compared to the conventional tillage treatment (M₂). This is because of zero tillage condition incurred higher economic and biological yield through efficient utilization of available resources and good management practices (Raju et al., 2012). Split application of nitrogenous fertilizer (S₄) gave significantly superior in gross return of both crop even the system as compared to the other management practices. Actually splitted dose of nitrogenous fertilizer increases the nitrogen use efficiency of the crop which promotes yield attributing character and yield of the crop.
 

 

       
Pearson’s correlation study also revealed that significant positive correlation between total N₂O emission with GWP (r=0.984, p<0.01) and GHGI (r=0.946, p<0.01), but significant negative correlation between GHGI and net return (r= -0.843, p<0.05) was found (Table 4). This result implies that N₂O emission was the key modulator of total GWP and GHGI (Fig 3). The modulation in the nitrogen management could curb the GWP and lowered down the yield scaled GHGs emission. Moreover, negative correlation of GHGI with net return visualized the beneficial aspects of zero tillage and split nitrogen application in enhancing profitability and combat climate change.