Biochar Impact On The Water Relations Of Nerica Rice Grown On A Compacted Rhodic Kandiustalf

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2021-09

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University of Ghana

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ABSTRACT Soil compaction has become a major challenge in tropical agriculture due to the increasing use of heavy agricultural machinery for tillage. Soil compaction sets in a spiral of soil degradation processes through reduced infiltration increased runoff and soil erosion. Ultimately, soil and crop productivity are impaired. Several remedial measures have been proposed to minimize soil compaction-induced degradation. Among these is biochar application to soils. Though the impact of biochar on enhancing soil physical properties is increasingly reported in the literature, data in Ghana continue to be scant and research also continues to lag on this front. The focus of this was three-fold. First, the impact of biochar application on compacted soils with regard to their physical and hydraulic properties such as the bulk density, moisture characteristic, runoff and infiltration, was investigated using laboratory studies. Second, the growth, yield and water use efficiency of upland rice (Nerica 14) under a range of biochar-amended compacted soils were studied under greenhouse conditions. For laboratory and greenhouse studies, the soil used was Toje series (Rhodic Kandiustalf). In a final study, two (2) biochar-modified runoff models were assessed for their suitability in predicting runoff from biochar-amended compacted Toje series. The laboratory study was a Completely Randomized Design (CRD) in factorial arrangement with three (3) compaction levels (Field D1= 1.3 Mg/m3; medium D2 = 1.5 Mg/m3 and high bulk density D3 = 1.7 Mg/m3), rice husk biochar at 3 rates (B0, B10 and B20 corresponding to 0, 10 and 20 ton/ha, respectively) was used. The treatment units which were in PVC pipes of 20 cm height and 16 cm diameter were irrigated from a rainfall simulator. Data were collected on the infiltration and saturated hydraulic conductivity. The variation of bulk density with biochar application was also determined. The soil moisture characteristic (SMC) was determined on samples of the various treatments using the Haines equipment and the dominant soil pore size was derived from the SMC. In the case of the greenhouse study, a similar experimental arrangement was used but with the PVC column heights of 40 cm. In addition, Nerica rice was planted and intermittently irrigated until maturity. There were 3 levels of irrigation (low: seasonal irrigation = 338.5 mm; medium = 419 mm and high = 569.5 mm). Both biochar application rate and bulk density and their interactions showed significant effects on the pore size and saturated hydraulic conductivity (Ksat). The dominant pore size increased from 0.0075 cm for the very high compacted soil (D3) at 0 ton/ha biochar application to 0.015 cm for low compacted soil (D1) at 20 ton/ha biochar application. Similarly, the Ksat was lowest (0.78 cm/h) for D3 with no biochar application and highest for D1 with 20 ton/ha biochar application. The increased pore size for the low density and high biochar application may explain the high Ksat values since according to Poiseuille’s Law, the water flow rate is proportional to the 4th power of the pore radius. It was observed that biochar application significantly reduced the bulk density. Soil compaction also significantly impacted infiltration parameters. Data analysis based on Horton’s (1948) infiltration model showed that the highest value for the initial infiltration rate, io = (134.7 cm/s) was observed for D1 with 20 ton/ha biochar application rate and least (7.7 cm/s) for D3 at 0 ton/ha biochar application rate. With respect to Philip’s (1957) infiltration equation, the sorptivity, S, was highest (12.1 cm/min 0.5) forD1 at 20 ton/ha biochar and least for D3 (2.06 cm/min 0.5) at 0 ton/ha biochar. The biochar-induced parameters used to predict the runoff using the modified Horton and Philip’s equations predicted the laboratory determined infiltration into compacted soils (R2 = 0.75). The greenhouse results showed that biochar application reduced the bulk density, offsetting the soil compaction effect on plant growth as well as the runoff, drainage and evapotranspiration Components of the water balance. The actual seasonal evapotranspiration (ETa) was reduced with increasing soil bulk density for each water regime. The highest yield was recorded for the treatment Combination of the high-water regime, 10 ton/ha biochar application and bulk density, D1. In general, the grain yield response to soil compaction was in the D1 > D2 > D3. The Water Use Efficiency (WUE) decreased with increasing density levels except for D2. Biochar had only a small significant effect on the WUE of the compacted soils. The input of the biochar and soil compaction modified ETa in the Doorembos and Kassam (1979) yield production function showed that rice yields under varying soil compaction and biochar application could be satisfactorily predicted (R2 = 0.67; Willmott d-index (0.89). Runoff was well predicted using the models of Ive et al. (1976) and the USDA Natural Resource Conservation Service (NRCS, 1972) Curve Number (CN) that were modified to respond to soil compaction and biochar application, for the laboratory studies. Overall, the performance of the two modified models was acceptable with R2 > 74% and Willmott index d > 0.67. In general, it can be concluded that the soil compaction problems induced by tillage can be effectively addressed by biochar application.

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MPhil Soil Science

Keywords

Ghana, The Water Relations, Rice, soil

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