AGRICULTURE IS the mainstay of economic backbone in Pakistan. This sector contributes by 21 per cent to gross domestic product (GDP). However, in the recent past, the agricultural productivity has been endangered by many of the challenges that were never foreseen earlier. Rice and wheat crops are grown in sequence, and also with other crops throughout the country. These crops are of significance due to the food security as well a commodity for foreign exchange earnings. The rice-wheat cropping system is the most intensive production system in Asian countries, and in Pakistan it covers an area of about 2.2 m ha.
Evidence is now appearing that the productivity of rice-wheat system is stagnant and total factor productivity is declining because of a fatigued natural resource base and, therefore, sustainability of rice-wheat cropping system is at risk. A huge amount of crop residues (7-10 t ha-1) is produced annually in this cropping system. Recent mechanization in harvesting paddy through combine harvesters leaves a sizeable amount of rice straw in the field. About 40 per cent nitrogen (N), 30-35 per cent phosphorus (P), 80-85 per cent potassium (K), and 40-50 per cent sulfur (S) acquired by rice during its growth remains in vegetative plant parts at crop maturity. Rice and wheat exhaustively feeds the nutrients and the rice-wheat system is heavily depleting the soil of its nutrients. A rice-wheat system yielding 7 t ha-1 of rice and 4 t ha-1 of wheat removes more than N 300, P 30 and K 300 kg ha-1 from the soil. A 10 t ha-1 crop removes more than 730 kg NPK ha-1 that is rarely returned under the current cultural practices.
The malformed practice of straw removal in case of wheat, and burning for rice crop, has posed serious edaphic and ecological problems. A farming system that does not take in to account of proper residue management may result in mining the soil of major nutrients leading to net negative balance and multi-nutrient deficiencies in such a system. Declining soil fertility due to poor crop residue management has been acknowledged as a major agricultural problem in many developing countries. This is also one of the obvious reasons for the yield decline in the rice-wheat system.
Farmers commonly remove wheat straw for feeding the animals. However, rice straw due to high lignin and silica and low protein content demerits for this purpose. Its management remains a key issue as it interferes with tillage and seeding operations. Burning is a common farming practice to dispose crop residues/stubbles in RW system. Wheat plantation after paddy harvest is energy and time consuming process and is also becoming expensive due to sky rocketing fuel prices. Preparation of field vacated by rice involves removal or utilization of rice stubbles/straw. Disposal and utilization of these in the short time is difficult, compelling farmer to burn the residues to have a clean field. Nonetheless, burning helps clear soil surface and control soil borne pest and pathogens, it otherwise contributes to air pollution, killing of beneficial soil insects, microorganisms and massive loss of nutrients that can be recycled into the system.
Burning caused almost complete loss of N, the most limiting nutrient in crop production. Burning of crop residues has been identified as the potent source of emission of greenhouse gases. Burning of one tone straw can produce as much as 3 kg particulate matter, 60 kg Co2, 1460 kg Co, 199 kg ash and 2 kg So2. Such emissions bring modifications in atmospheric chemistry of regional environments that has linkages with global climate change scenario. Long term burning could have a negative impact on microbial population as well as their diversity which may take 5 years or longer to recover. Bacterial population involved in nitrification is particularly vulnerable to burning. Contrarily, residue retention and incorporation is believed to increase the same.
In order to maintain and restore soil health and subsequent productivity within rice-wheat cropping system, proper handling and management of crop residues is of immense important, but for various reasons, the amount of residue being returned to the soil is not adequate. Management of rice residues offers a great challenge to agriculturists for enhancing sequestration of carbon and maintaining the sustainability of production. Crop residue is a valuable natural resource and needs to be managed as an important component of the system for improving soil health, managing weeds and sustaining the natural ecosystem, as well as enhancing productivity. Several management options like incorporation, surface retention, mulching and direct seeding in zero tillage can be opted as alternatives to current detrimental practice of residue burning. Every option has its own merits and demerits and needs to be evaluated for specificity of location, soil and situation. Assessing the impact of different residue management practices on soil characteristics can serve as an important tool in developing fertilizer recommendation practices.
Among a wide range of soil management practices aiming at restoring soil fertility, use of organic amendments particularly the use of crop residues seems striking one as it is abundantly and readily available. Incorporation of plant residues, coupled with appropriate tillage, can increase soil organic carbon, or if used as mulch, the residue can modify soil temperature, conserve moisture and help control weeds by smothering and allelopathic effects. Proper management of residues in rice-wheat system can be used as a natural tool for weed management because many of the cultivars of both rice and wheat have phytotoxic effects through release of secondary metabolites into rhizosphere during their decomposition. Manipulation of crop residue allelopathy can give way to non-chemical weed management. With the innovation of resource conservation technologies like zero tillage, burning of rice straw can be overcome by directly drilling wheat in rice stubbles. Zero tillage remains the most successful and widely adopted resource conservation technology in rice-wheat cropping system till date.
Effects of different options of residue management on decomposition and nutrient release rates, soil microbial activity, as well as the release of phytotoxic compounds (allelochemicals) from rice and wheat residues need to be studied. In modern and highly productive systems, rational efforts have been made to identify and understand the aspects identified above. In Pakistan, no collaborative effort has so far been made to address these problems. This requires an integrated approach towards addressing different issues, and coming up with practical solutions that are not only viable but also economically feasible and ecologically sound. Such information will also be crucial for sustainability of rice-wheat cropping system through improvement in soil physical, chemical and biological properties.
The writers are associated with Weed Science and Allelopathy Lab, Department of Agronomy, University of Agriculture, Faisalabad. They can be reached at <firstname.lastname@example.org>