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Minimum soil disturbance planting for rice-based rotations in northwest Bangladesh: Effects on plough pan and water balance

Mahmud, Mir Nurul Hasan (2021) Minimum soil disturbance planting for rice-based rotations in northwest Bangladesh: Effects on plough pan and water balance. PhD thesis, Murdoch University.

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Soil degradation in the rice-based cropping system of Bangladesh has prompted research to switch from conventional tillage (CT) to minimum soil disturbance crop establishment, featuring strip planting (SP) and increased crop residue retention. However, the new residue retention levels and crop establishment methods need to be tested for their water use efficiency. Therefore, two field trials were initiated to evaluate the effects of SP and bed planting (BP) with increased crop residue retention on soil physical properties, components of the water balance and water productivity in two rice-based crop rotations. Field trials were conducted during 2015-2017 in two long-term conservation agriculture (CA) experimental fields established since 2010 in two regions of northwest Bangladesh, namely 1) Alipur, the alluvial soil region, and 2) Digram, the High Barind Tract (HBT) region. The trials consisted of three tillage treatments in the main plots - SP, BP and CT. The subplots comprised of two levels of residue retention - high residue (HR) and low residue (LR). High residue and LR treatment involved the retention of respectively 50 % and 20 % by the height of the previous crop, either anchored or loose. Strip planting and BP were done with a Versatile Multi-crop Planter mounted on a two-wheel tractor (2-WT).

Seven years of continuous CA practices have provided evidence that minimum soil disturbance and increased residue retention have altered the soil physical properties in both silty loam soil at Alipur and silty clay loam soil at Digram. The physical changes were reflected in the reduction of soil BD, enhancement of total porosity (TP) and reduction of penetration resistance (PR) in the 0-20 cm soil depth. High residue treatment reduced BD from 1.37 to 1.33 g cm-3 at Alipur and 1.27 to 1.24 g cm-3 at Digram soil in the 0-10 cm soil depth compared to the LR treatment. High residue retention increased macroporosity by an average of 55 % over LR treatment. Irrespective of residue retention, the average (two soils) decrease in BD was 4.5 % and 2.6 % in 0-10 cm depth for SP and BP treatment, respectively, compared to CT. The highest BD of 1.65 g cm-3 was achieved at 10-20 cm soil depth in the CT plot, which clearly indicates a massive plough pan at this depth. However, BD of the plough pan was reduced by 3.8 % in the SP and 4.6 % in the BP treatment indicating the amelioration of subsoil compaction due to the absence of puddling over seven years. Penetration resistance in the plough pan was also decreased from 2.15 MPa (CT) to 1.93 MPa (SP) at Alipur and 2.55 MPa (CT) to 2.32 MPa (SP) at Digram. In the silty loam soil, saturated hydraulic conductivity (Ksat) at 0-10 cm under CT was 1.00 cm hr-1 which was increased to 1.39 cm hr-1 by SP and to 1.52 cm hr-1 by BP. In the silty clay loam soil, Ksat at 0-10 cm was increased from 0.32 cm hr-1 under CT to 0.66 cm hr-1 by SP and to 0.81 cm hr-1 by BP. In 10-20 cm soil depth, Ksat increased from 0.22 cm hr-1 under CT to 0.48 cm hr-1 by SP and to 0.43 cm hr-1 by BP.

Soil compaction by a 2-WT with a single wheel-pass, two wheel-passes, and four wheel-passes with and without extra loading was also tested in non-CA fields adjacent to the two long-term trials. At 0-5 cm depth, soil BD with a single wheel pass was 1.37 g cm-3, which increased to 1.40 g cm-3 after two passes, and further increased to 1.47 g cm-3 with four passes. The BD of 0-5 cm depth with no extra loading was 1.37 g cm-3 which was increased to 1.39 g cm-3 with 100 kg extra loading and further increased to 1.43 g cm-3 with 200 kg extra loading. At 5-10 cm depth, compaction by CT involving four passes indicated that a 2-WT, when frequently trafficked at this depth for many years, creates a dense soil layer that is reasonably related to the formation of the plough pan. The least limiting water range (LLWR) range could be a good indicator of soil quality in soil compaction studies since the LLWR concept includes the effects of several growth-limiting factors such as matric potential, aeration and penetration resistance that are integrated into a single parameter. Conventional tillage had a larger LLWR which is also comparable to the LLWR of strip tillage single wheel pass treatment. Conservation agriculture practice facilitates tillage, fertilizer and seeding operation in a single pass. Thus, single wheel pass traffic by a low weight 2-WT may not create measurable compaction in the surface soil and the subsurface soil.

High rice residue retention treatment increased wheat yield by 7-18 % in the whole study period (2015-2017) compared to low residue retention. Strip planting increased wheat yield by 18-25 % compared to CT in the three years. By contrast, BP increased wheat yield by 16 % compared to CT in 2015 but not in 2016 or 2017. Strip planting saved 15-36 % irrigation water for wheat growth compared to CT in three years. In contrast to SP, BP saved only 8-25 % irrigation water than CT. Irrigation water productivity of wheat was higher under SP (2.2 kg m-3) than that under BP (1.7 kg m-3) and CT (1.3 kg m-3). The results suggest that SP performed better than BP in terms of crop productivity and irrigation water productivity.

Total water losses under SP continuous flooding irrigation were 80.0-125.0 cm, while the values were 82.0-123.0 cm for BP and 66.0-86.0 cm for CT. Deep drainage during the rice crop for SP, BP and CT accounted for about 41 %, 44 %, and 39 % of the total loss, respectively. Alternate wetting and drying irrigation reduced the drainage losses by 35 %, 26 % and 48 % for SP, BP and CT, respectively. The yield of rice ranged from 6.1-6.9 t ha-1, 6.1-6.6 t ha-1 and 6.5-6.7 t ha-1 for SP, BP and CT, respectively. Irrigation water productivity for rice was higher under CT (0.88 kg m-3) compared to SP (0.66 kg m-3) and BP (0.60 kg m-3).

Improved crop yield under SP with residue retention should encourage smallholder farmers to adopt minimum soil disturbance planting in the rice-based rotation. However, altered water balance in the non-puddled minimum soil disturbance plot may require more irrigation for rice while allowing greater infiltration to groundwater. In contrast, for wheat, SP and HR had positive effects on water use and water productivity. Since water lost by deep percolation returns to the groundwater and is potentially available for reuse, non-puddled rice can beneficially increase groundwater recharge when practised in a large command area. Hence, CA practices appear to decrease the requirement for groundwater for irrigation of dry season wheat while increasing the potential for groundwater recharge, but this needs further investigation.

Keywords: Barind area (Bangladesh); bed planting; conservation agriculture; conventional tillage; deep drainage; least limiting water range; minimum soil disturbance; number of wheel passes; soil compaction; strip planting; water balance.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Agricultural Sciences
United Nations SDGs: Goal 12: Responsible Consumption and Production
Supervisor(s): Bell, Richard and Vance, Wendy
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