Wheat (Triticum aestivum L.) is the major cereal crop grown in world. World wheat production is 695 million tons (FAO, 2011). In Pakistan it contributes 9.1 % to the value added products in agriculture and 1.7 % to gross domestic production. Currently, the area under wheat is 8.73 million hectares and total production is 25.4 million tons with an average yield of 2.9 metric ton ha-1 (Govt. of Pakistan, 2017-18). Despite a higher yield potential, average grain yield of wheat in Pakistan is well below than in most of the wheat producing countries of the world.
In Pakistan, wheat occupies a central position in regulating agricultural policies and dominates all agronomic crops in the form of total acreage and yield. Wheat is generally grown after rice and cotton in rice-wheat and cotton-wheat cropping system. Rice-wheat cropping system plays an important role not only in Pakistan but also in world food security. In Indian-subcontinent, the area under rice-wheat cropping system is 13.5 million hectares.
In Pakistan, rice is grown in Kharif season under puddle condition while irrigated spring wheat in Rabi season. The total area is about 2.4 million hectares under rice out of which 50% comprised of fine and long quality speciality rice varieties (Basmati). Fine and long quality rice varieties are late maturing which often delay and / or affect spring wheat planting. However, farmers prefer to grow spring wheat due to its high gross margins. Rice is generally harvested mechanically by combine harvester, which leaves rice stubbles in the field. To overcome the problem of rice stubbles most of the farmers burn the residues because rice stubbles incorporation required several tillage operations, which also delays wheat planting. Irregular tillage operations especially conventional tillage during the seedbed preparation and at late maturing stage of basmati rice delayed the wheat planting.
Conventional management practices including frequent plowing, chemical fertilization, and pesticide application increases crop yields but exerts negative effects on soil productivity and farm economics. Plowing improves soil tilth for crop growth and yield, alleviates soil compaction and nutrient stratification, and suppresses weeds and soil-borne diseases. However, frequent plowing fragments and mixes crop residues, increases soil aeration and temperature, disperses soil structure, accelerates decomposition of crop residues and native soil organic matter (SOM), and causes an increase in CO2 emissions into the atmosphere. Moreover, plowing led to the formation of hardpan at the plow depths, decreases water infiltration with accelerated soil erosion.
In a rice-wheat cropping system, rice is mostly grown under puddling which leads to the destruction of wet soil aggregates by plowing, sealing of pore spaces, and formation of a subsurface hardpan. This subsoil compaction reduced both the water and nutrient-use efficiencies of subsequent wheat crop owing to decreased root growth. Wheat yield reduction and degradation of soil physical properties depend on the intensity and duration of puddling operations.
Deep tillage of puddle soil reduces the compaction, increases rooting depth, and improves the yield of the following wheat crop. Deep tillage not only alleviates soil compaction but also control weeds through deep burial of weed seeds. In areas where continuous cropping is practiced, deep tillage increases the surface area of soil exposed to sunlight to control certain diseases, insects and weeds. Two passes of subsoiling were more effective than subsoiling with one-pass for not only overcoming the soil compaction but also improving the soil tilth. The soil moisture at 50 to 100 cm depth under deep tillage was more, while the water consumption reduced in the 0 to 50 cm depth. However, deep tillage is costly in terms of fuel and time.
Wheat yield can be increased by managing resources through conservation management practices. The most important technology is the conservation tillage that has made to overcome soil erosion, maximize vegetative cover on the land, increase soil organic matter, improve carbon, energy and water footprints, and sustain farm economics is zero tillage. Zero-tillage is well known as zero-till, no-till, direct seeding and direct drilling. No-till techniques have been successfully applied on more than 111 million hectare worldwide. Continuous use of reduced or no-till practices substantially improves the net profitability of crop production. The yield of zero-till wheat is equal to or even higher than the yields produced by conventional tillage. However, no-till wheat yields are often affected by weeds pressures and poor crop stand due to soil compaction, anoxic conditions, and immobilization of nitrogen.
No till conversion of plowing is one of the strategies to decrease farming costs, reduce soil erosion, and improve ecosystem services. With NT, surface accumulation of crop residues as mulch influences air, water, and energy exchange between the soil ecosystem and the atmosphere. These processes reduce soil temperature and evaporation during summer months, retain soil moisture longer-especially under dry conditions, and thereby improve crop productivity. Long-term continuous NT has been reported to produce wheat yields equal to or even higher than that of plowed fields.
In rice-wheat cropping systems, rice is harvested by combines, which leave large amount of crop residues in the fields. However, the newly introduced Happy Seeder (HS) cuts and manages the standing stubble and loose straw in front of the furrow openers, retaining it as surface mulch and sows wheat in a single operational pass of the field. Moreover, operational costs for sowing wheat are 50 to 60 % lower with HS than with conventional sowing. The HS technology provides an alternative to burning for managing rice residues and allows direct drilling of wheat in standing and loose residues. However, most constraints in transitional NT or HS are high weed pressure, poor crop stands, soil compaction and stratification of nutrients, and N immobilization. The problem of N immobilization is more acute in alternate year rice-wheat production systems due to high C:N crop residues.
Sustainable crop production depends on the efficient use of N fertilizers. In wheat production, N plays an important role in crop growth and yield. Most of the wheat varieties grown in Pakistan require substantial quantities of N because soil organic matter content is very low. High price and excessive use of N fertilizers as an insurance against crop failures have caused widespread environmental and public health problems that emphasized the need for efficient use of the N fertilizers. There are needs to evaluate the potential effects of surface residues on N transformations and crop development. Tillage operations influence the soil N dynamics because the crop residues which are incorporated in the soil by plowing decompose faster than the residues which remain on the surface under NT and HS. In NT, N release from the crop residues is slow due to partial anaerobesis and/or due to N immobilization. than in tilled systems. However, when applied in excess of crop requirements NT system has a greater loss of N fertilizer by leaching and volatilization than in CT. Current recommendations of N fertilization developed for continuously plowed systems which may not be adequate for optimum production of wheat under NT because extra nitrogen is required for residues decomposition. Therefore, the information on the effects of tillage and N fertilization on wheat production in post-harvest puddle rice fields in presence of stubbles is critical to evaluate the sustainability of the rice-wheat production systems.