The rice–wheat system is one of the important food production systems in South Asia contributing to food security of the region [1
]. This system is prevalent in the fertile, alluvial Indo-Gangetic Plains of India [2
]. In addition to feeding the region with plenty of rice and wheat (~100 million tons each annually) [5
] the rice–wheat system generates an estimated 23 million tons of rice residue which is being burnt in about 2.5 million farms annually [6
]. Rice residue burning is mainly due to the compelling situation faced by farmers in the region where they have to prepare the field for subsequent wheat crop in just 20–25 days or its removal from fields alternatively adds burden of an additional operation and extra labor which will increase the total cost of cultivation.
Burning of rice residue emits an enormous amount of poisonous gases and particulate matter [7
] which hastens the already existing environmental pollution and especially the deterioration of air quality [8
]. The involvement of rice residue burning in enhancing the air pollution in Northern India has been reported by several researchers [7
]. Along with contributing to air pollution, residue burning also leads to loss of approximately 80%–90% N, 25% of P, 20% of K and 50% of S present in crop residues in the form of various gaseous and particulate matter [7
]. Further, burning also leads to loss of billions of agriculturally important microbial species present in soil which in turn affects the soil health severely [13
In addition to challenges posed by residue burning the continuous practice of rice–wheat cropping system has also caused to declining soil fertility and water table, therefore becoming a threat to sustainability [14
]. The declining ground water resources on which 98% of wheat in India is dependent for irrigation is also a major cause of concern as moisture stress conditions affects crop growth and productivity [18
Potassium is a key element involved in plant water balance mechanisms and therefore plays an important role in moisture stress tolerance of plants [19
]. In spite of having substantial potassium reserves [21
], a large quantity of K is removed by intensive cropping system. Therefore, supplementing this K through soil or foliar application becomes essential.
Urgent action is necessary for making agriculture profitable and sustainable to attain the sustainable development goal (SDG) of zero hunger [22
]. The residue burning is indirectly affecting as many as seven SGD’s of United Nations viz., no poverty, zero hunger, good health and well-being, clean water and sanitation, climate action, life on land, and life below water [23
]. The challenges posed by rice–wheat cropping system can be addressed by adopting numerous approaches such as crop diversification, intensification, precision nutrient management such as foliar application of nutrients, residue incorporation, incorporation of farmyard manure, and conservation agriculture [9
Continuous practice of rice–wheat cropping system has posed many challenges such as soil health deterioration, declining water table and air pollution resulting from rice residue burning. The hypothesis that whether RRR combined with foliar application of K can enhance wheat GY, profitability and WUE under different irrigation regimes was tested in this study. The study was planned with the objective to find out the effect of RRR, irrigation levels and foliar application of K on wheat GY parameters, WUE and profitability.
Rice residue burning creates various problems including loss of nutrients, reduction in soil carbon and destroying the beneficial soil microbial biota which results in yield penalty. Farmers supply additional nutrients to sustain the yield levels by external application of costly fertilizer inputs [35
]. The problem posed by residue burning can be minimized by RRR on the soil surface in the subsequent wheat crop. Our results proved that, when only the effect of residue retention was analyzed wheat GY and AGBM increased significantly compared to residue removal treatments. Similar results of favorable effect of residue retention on GY and AGBM was reported by [44
]. Increased yield of wheat might be due to addition of nutrients to the soil after decomposition of rice residue by microbes leading to enhancement in soil organic carbon [48
]. Additionally, residue retention imparts many beneficial effects like erosion control, decreased evaporative water losses and improved weed control. Similar results where no-tillage practices consistent of residue retention improved wheat yield were reported in other dryland wheat-growing regions [49
]. RRR led to significant increase in crop physiological parameters such as RWC, SPAD, and WUE in our experiments. Beneficial effects of residue retention in subsequent crop leading to improved physiological efficiency in terms of higher RWC and SPAD was reported in maize and sorghum by retention of barley residue [50
]. Earlier researchers also reported that RRR enhanced water productivity by conserving soil moisture from evaporation losses [51
In this study the RRR also enhanced the economic returns earned along with enhanced GY and AGBM. RRR significantly enhanced net returns and B:C ratio compared to control. RRR on soil surface as biological mulch enhancing system profitability has been reported in wheat [9
] and other crops [51
Effect of foliar application of K when analyzed independently, wheat GY and AGBM was significantly enhanced compared to control. Spraying of K2
on crop canopy increasing GY and biomass was reported in wheat and cotton [53
]. Enhanced GY under foliar application of K might be due to improved plant physiological processes involved in growth and development as K is found to be playing essential role in improving enzyme activity, photosynthetic rate, osmotic regulation, stomata movement, and water balance, cation–anion balance, and stress tolerance [56
]. Foliar application of K positively increased RWC, SPAD and WUE significantly in this study. The results are in consistency with earlier reports by [56
], but differ from those when an otherwise well-fertilized wheat crop received additional K regardless of residue retention [59
]. Nonetheless, [60
] suggested that wheat response to K fertilization in semi-arid regions was more consistent in high-yielding environments, similar to those included in our study. The profitability of K application treatment was significantly higher in terms of net returns earned and BC ratio. Foliar spray of K increasing net returns and BC ratio was in agreement with earlier reports in wheat [61
], cotton [62
], toria [63
], and ground nut [64
Wheat GY, RWC, WUE, NR, and BC ratio were significantly affected by interaction of residue retention treatment and irrigation levels. GY was significantly increased under IAS and ICS treatments when residue was retained. The increased GY under IAS is similar to individual effect of residue retention while in case of ICS, significantly higher GY might be due to decreased soil water evaporation due to residue retention [44
]. In this study residue retention under IAS treatment did not have any significant effect on RWC, this might be because under IAS there was no dearth of soil moisture. In case of ICS and ICS + IFS, as there was moisture stress, the RRR enhanced RWC significantly compared to residue removal due to residue acting as soil mulch in protecting moisture loss. Increase in WUE values was highest under ICS treatment with residue retained, again might be due to the obvious fact that residue conserved soil moisture, suppressed weed growth, and created a favorable microenvironment in the root zone. Residue retention under ICS condition led to enhanced net returns and B:C ratio compared to residue removal because of the higher GY and WUE under this interaction.
There was no interaction found between residue retention and K application in this study. This might be because rice residue is known to release K along with many other nutrients upon its decomposition [48
Foliar application of K and RRR were together analyzed to study their interaction effect indicated that, except RWC under ICS all other parameters were statistically similar. This might be due to involvement of K in stress mitigation mechanisms of plants subjected to water stress. In case of all other parameters and treatments being non-significant the crop was not subjected to water stress under IAS and ICS + IFS treatments. Upon this if there is some partial stress is there under ICS + IFS treatment, the effect might have been nullified by residue retention treatments being included in the analysis.
Relationship of traits studied with GY was assessed through Pearson correlation analysis, which showed that except WUE (r = −0.70), all other traits were positively correlated to GY. The correlation observations in this study are in close conformity with earlier reports in wheat and barley [65
], except that for WUE and GY, which is usually reported as positive [68
]. The discrepancy between our findings and those by other authors relating GY and WUE positively are likely because when water resources were the same, increases in grain yield increased WUE; but when the total amount of water available increased, WUE decreased vastly and thus they negative correlation across the whole dataset. Simple regression of GY with water available clearly showed a strong positive trend where GY increased with additional irrigation water provided to the crop. While the scatter plot of residuals of regression between GY Vs water and water available showed absence of any clear trend. This might be due to influence of combined effect of rice residue, irrigation levels and K application.
Economic and Environmental Impact
RRR treatment generated an additional income of 151 $
in this study. Earlier researchers [9
] in 2019 reported that rice residue burning makes a substantial contribution to air pollution in Indo–Gangetic plains of India. Air pollution is the second highest health risk factor in northern India is a cause of concern. Further the atmospheric smog resulting from a mix of factors including residue burning is costing heavily in terms of closure of thousands of schools in New Delhi alone. Disruptions in road transportation due to low visibility caused by smog many a times lead to road accidents [7
]. An estimated 23 million ton of rice residue is being burnt in about 2.5 million farms annually [6
]. Burning of rice residue emit an estimated 8.57 Mt of CO, 141.15 Mt of CO2
, 0.037 Mt of SOx, 0.23 Mt of NOx, 0.12 Mt of NH3
and 1.46 Mt NMVOC, 0.65 Mt of NMHC, 1.21 Mt of particulate matter [7
] which could be effectively avoided and managed by residue incorporation into wheat.
In this paper we investigated the effect of rice residue retention, irrigation levels and foliar application of K on wheat GY, AGBM, HI, TPM, GrPMS, TGW, GPS, RWC, SPAD, WUE, Net returns, and BC ratio. There was a clear advantage of residue retention, which was reflected by significant higher GY, AGBM, TPM, and GrPMS compared to residue removal. Residue retention also enhanced physiological performance of crop by favorable effects on RWC, SPAD, and WUE. RRR enhanced net returns by 624.4 $ ha−1 with BC ratio of 1.62. Residue retention increased WUE especially under limited water available conditions and led to higher returns of US$151 which would be of great help to the farmers who face shortage of water under changing climatic scenario. Adoption of RRR over rice–wheat area might lead to substantial reduction in environmental degradation resulting from its burning.