Integrating bio char and deficit irrigation to improve sorghum productivity, soil moisture retention, and water-use efficiency under semi-arid conditions

Introduction

Semi-arid environments, which collectively span approximately 15% of the Earth's land surface, are defined by chronic moisture deficits, high potential evapotranspiration rates, and structurally degraded soils with intrinsically poor water-holding capacity [1] , posing severe and escalating challenges to sustainable food production for rapidly expanding rural populations [2] . Sorghum (Sorghum bicolor L. Moench), ranked globally among the five most important cereal crops by area harvested, is predominantly cultivated across these marginal environments by smallholder farmers owing to its inherent drought tolerance and adaptability to infertile soils [3] ; however, increasing climatic variability associated with global warming has substantially intensified in-season water stress episodes, depressing realized yields to levels well below the crop's genetic ceiling [4] . Annual sorghum yield losses attributable to combined drought stress and soil moisture deficit in sub-Saharan Africa are estimated to exceed 30-40% in years of below-normal rainfall [4] , directly threatening household food security for over 300 million smallholder farming families [2] . Climate projections further indicate that semi-arid regions of Eastern Africa will experience 15-25% reductions in seasonal precipitation by 2050, compounding the urgency of developing adaptive water and soil management technologies [5] . Deficit irrigation the intentional application of water below full crop evapotranspiration demand to optimize water productivity rather than absolute yield per unit area has emerged as a widely endorsed strategy in water-scarce dryland farming systems [6,7] ; however, its sustained agronomic effectiveness depends critically on the soil's capacity to retain and supply plant-available moisture, which is severely compromised in the low-organic-matter, coarse-to-mediumtextured soils prevalent across semi-arid Ethiopia [8] . Biochar the carbon-enriched porous solid produced by pyrolysis of biomass under limited oxygen conditions has been extensively documented to improve soil physical architecture, augment plant-available water retention, reduce bulk density, enhance cation exchange capacity, and stimulate beneficial soil microbial communities [9,10,11] ; yet the quantitative magnitude of these hydrological benefits varies substantially with biochar application rate, feedstock source, pyrolysis temperature, and soil texture class [12,13] . Despite growing independent evidence on the agronomic benefits of deficit irrigation and biochar for various field crops [14,15,16] , studies explicitly integrating both practices for dryland sorghum production in Ethiopian semi-arid conditions remain critically insufficient, constituting a significant knowledge gap with direct implications for national adaptation policy under climate change [17] . The present study was therefore designed to quantify the combined and interactive effects of graded biochar application rates and deficit irrigation levels on sorghum productivity, soil moisture dynamics, and water-use efficiency in Tigray, Ethiopia, with the central hypothesis that the co-application of optimal biochar rates and moderate deficit irrigation (75% ETc) would synergistically enhance soil moisture buffering capacity and crop water productivity beyond the effects achievable with either intervention applied independently [18] .

Materials and Methods

Materials

The experiment was conducted during the 2022 and 2023 main cropping seasons at the research farm of Mekelle University, Tigray Region, Ethiopia (14°03′N, 39°26′E; altitude 1,870 m above sea level). The site is classified within the semi-arid agro-ecological zone, receiving a mean annual rainfall of 512 mm concentrated between June and September, with a mean annual temperature of 19.4 °C. The experimental soil was a Vertisol with baseline properties: pH 7.8 (1:2.5 H₂O), organic matter 1.12% (Walkley-Black), available phosphorus 6.4 mg kg⁻¹ (Olsen), and total nitrogen 0.09% (Kjeldahl). The test crop was sorghum (Sorghum bicolor L. Moench) cultivar 'Macia', a drought-adapted variety recommended by the Ethiopian Institute of Agricultural Research for semi-arid production systems. Biochar was manufactured by slow pyrolysis of Acacia (Acacia saligna) wood chips at 450 °C under restricted oxygen for 4 h in a drum retort kiln, then ground to <5 mm particle size and characterized for: pH 9.4 (CaCl₂ suspension), electrical conductivity 1.82 dS m⁻¹, cation exchange capacity (CEC) 42.3 cmol kg⁻¹, total carbon 61.4%, total nitrogen 0.84%, and BET surface area 187 m² g⁻¹. Drip irrigation infrastructure was installed with pressure-compensating emitters calibrated at 2.0 L h⁻¹ per emitter, and an on-site automatic weather station (Campbell Scientific CR1000X) provided continuous meteorological data for evapotranspiration estimation. Reference evapotranspiration (ET₀) was calculated daily by the FAO-56

Penman-Monteith method [6] , and crop evapotranspiration (ETc) was derived using published stagespecific crop coefficients for sorghum (Kc initial = 0.4, Kc mid = 1.1, Kc end = 0.55).

Methods

A randomized complete block design (RCBD) with a 4×3 factorial arrangement was employed, comprising four biochar application rates (B₀: 0, B₁: 5, B₂: 10, B₃: 20 t ha⁻¹) and three deficit irrigation levels (I₁: 100% ETc full irrigation; I₂: 75% ETc moderate deficit; I₃: 50% ETc severe deficit), yielding 12 treatment combinations replicated three times across 36 plots (3.5 m × 4.0 m each) separated by 1.0 m buffer strips. Biochar was broadcast and manually incorporated into the top 15 cm of soil two weeks before sowing. Seeds were planted at 75 cm × 25 cm spacing (~53,333 plants ha⁻¹) and thinned to one plant per stand at 14 days after emergence (DAE). Uniform fertilization (60 kg N ha⁻¹ as urea in two split applications and 46 kg P₂O₅ ha⁻¹ as triple superphosphate at sowing) was applied across all plots. Data recorded included: plant height (cm) at 30, 60, and 90 DAE; leaf area index (LAI) at 60 DAE using a LI-COR LI-3100C area meter; days to 50% heading; panicle length (cm); 1,000-grain weight (g); grain yield (t ha⁻¹, adjusted to 12.5% moisture); harvest index (%); volumetric soil moisture content (SMC, %) at 0-20 cm and 20-40 cm depths measured weekly with a Diviner 2000 capacitance probe (Sentek Technologies, Australia); and water-use efficiency (WUE, kg m⁻³) calculated as grain yield divided by total seasonal irrigation water applied [6] . Data from both seasons were subjected to combined two-way ANOVA with season as a random effect using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA) [17] . Treatment means were compared using Tukey's Honest Significant Difference (HSD) test at α = 0.05. Pearson correlation coefficients and simple linear regression were computed to quantify relationships between soil moisture, biochar rate, and grain yield across all treatments [18] . [3,10,14] .

Results

Sorghum growth was significantly (0.01) influenced by biochar rate, irrigation level, and their interaction (Table 1). Plant height at 60 DAE ranged from 103.2 cm (B₀I₃) to 141.5 cm (B₃I₁), with biochar-amended plots consistently taller than unamended controls across all irrigation levels. Importantly, B₃I₂ achieved a plant height (140.3 cm) statistically equivalent to the fully irrigated maximumbiochar treatment (B₃I₁: 141.5 cm), demonstrating that 20 t ha⁻¹ biochar fully compensated for a 25% irrigation reduction in terms of vegetative growth [11,14] . Leaf area index at 60 DAE followed an identical trend, with B₃I₂ (2.79) and B₃I₁ (2.82) significantly superior to all other treatments, while B₀I₃ recorded the minimum LAI (1.54) alongside the most delayed heading (69.2 days), indicating physiological drought-stress-mediated phenological acceleration under the absence of soil moisture amendment [3, 4] . Panicle length was maximized under B₃I₂ (29.1 cm), exceeding even B₃I₁ (28.6 cm), likely reflecting improved assimilate partitioning under biochar-mediated moisture supply during the reproductive phase [9] . [6,9,15] .

Grain yield was the parameter most strongly governed by the biochar × irrigation interaction (0.01, Table 2). The highest yield (3.62 t ha⁻¹) was recorded under B₃I₂, statistically at par with B₃I₁ (3.54 t ha⁻¹) while consuming 25% less irrigation water, confirming a superior agronomic water productivity outcome. The yield advantage of B₃I₂ over the unamended full irrigation control (B₀I₁: 2.43 t ha⁻¹) was 49.2%, validating the central hypothesis. The fully irrigated unamended control produced lower yield than all biochar-amended treatments regardless of irrigation level, underscoring the strong independent soil-fertility contribution of biochar beyond moisture effects [9,11,16] . Harvest index ranged from 29.6% (B₀I₃) to 47.1% (B₃I₂), confirming efficient dry-matter partitioning to grain under the combined amendment regime. The 1,000-grain weight under B₃I₂ (33.8 g) exceeded B₃I₁ (33.4 g), suggesting that biochar-mediated moisture buffering during grain-filling may have prolonged the effective grain-fill period under moderate deficit [14,15] . [10,11,12] .

Soil moisture content at both 0-20 cm and 20-40 cm depths responded significantly ( 0.01) to biochar rate, irrigation level, and their interaction (Table 3). At 0-20 cm, SMC peaked at 31.2% under B₃I₁ and remained substantially elevated (27.8%) under B₃I₂, demonstrating that 20 t ha⁻¹ biochar effectively dampened the moisture loss imposed by a 25% irrigation reduction. Conversely, unamended plots under moderate deficit (B₀I₂) retained only 15.3% SMC a 31.7% reduction relative to B₀I₁ (22.4%) confirming the critical vulnerability of unamended semi-arid soils to deficit irrigation regimes [8,10] . The finding that B₃I₃ maintained SMC (22.4%) at 0-20 cm statistically equivalent to the fully irrigated unamended control (B₀I₁: 22.4%) is particularly remarkable, suggesting that 20 t ha⁻¹ biochar can compensate for the water deficit imposed by a 50% reduction in applied irrigation [11,12,13] . Pearson correlation analysis revealed a highly significant positive relationship between SMC at 0-20 cm and grain yield (r = 0.91, 0.01) and between biochar rate and WUE (r = 0.88, 0.01) across all irrigation levels [17,18] . [6,7,14,18] .

Water-use efficiency improved progressively with increasing biochar rates and was highest under B₃I₂ (1.84 kg m⁻³), representing a 58.6% increase over B₀I₁ (1.16 kg m⁻³) and a 44.9% gain over B₀I₂ (1.27 kg m⁻³) (Table 4). The WUE of B₃I₂ exceeded that of B₃I₁ (1.69 kg m⁻³), confirming that moderate deficit with maximum biochar is the agronomically and water-economically superior production system [7,17] . These results collectively validate the hypothesis that co-application of biochar and deficit irrigation achieves synergistic water productivity gains [18] .

Discussion

The superior grain yield and WUE achieved under B₃I₂ relative to the unamended full irrigation control constitute the principal and most practically significant finding of this study, aligning with the foundational premise that biochar amendments fundamentally alter the hydrological behavior of semi-arid soils [9,10] . The improvement in SMC with increasing biochar rates observed across both soil depths corroborates findings by Hardie et al. [10] , who documented 18-34% increases in plant-available water in soils amended with wood-derived biochar at 20 t ha⁻¹. The mechanism is well established: biochar's macro-porous structure and elevated surface area (187 m² g⁻¹ in this study) introduce additional meso-and micropore water-storage sites within the soil matrix, reducing hydraulic conductivity at high water contents and thereby buffering the rate of soil water depletion between irrigation events [11,12] . The high CEC of the biochar used (42.3 cmol kg⁻¹) further enhanced nutrient retention in the initially low-fertility Vertisol, contributing complementary fertility benefits that would have supported root system development and improved the efficiency of applied fertilizer [13] . The statistical equivalence of B₃I₂ and B₃I₁ in grain yield extends the conclusions of Ramadhan and Hamed [14] , who reported comparable yield maintenance under biochar-assisted deficit irrigation in wheat grown in Iraq, and Abel et al. [15] , who documented up to 45% increases in water retention in sandy soils amended with biochar. Notably, our results from heavier Vertisols suggest that biochar benefits extend beyond the sandy soils where most evidence has been generated, an important finding for clay-dominant semi-arid landscapes common across the Ethiopian Highlands. The progressive increase in WUE from 1.16 to 1.84 kg m⁻³ with increasing biochar rate under moderate deficit mirrors the water productivity improvements predicted theoretically by Woolf et al. [17] and those reported empirically by Kizito et al. [18] for East African soils under biochar amendment. The drought-stressmediated phenological acceleration observed in B₀I₃ with heading delayed by up to 8.8 days relative to B₃I₂ reflects a well-documented adaptive mechanism in sorghum under soil moisture limitation [3] , and the biochar-mediated suppression of this response suggests that improved soil water availability effectively shielded reproductive development from water-deficit-induced advancement. The strong positive correlation between SMC at 0-20 cm and grain yield (r = 0.91) confirms that surface-layer soil moisture dynamics are the primary mediator through which biochar and irrigation interact to determine sorghum yield under the semi-arid conditions studied, with direct implications for irrigation scheduling strategies in smallholder dryland systems [1,6,16] .

Conclusion

This study provides comprehensive and statistically robust evidence that the strategic co-application of biochar at 20 t ha⁻¹ in combination with 75% ETc deficit irrigation represents the most agronomically efficient and watersaving production system for sorghum under semi-arid conditions in Ethiopia and comparable dryland agroecologies. The biochar at this application rate effectively buffered soil moisture depletion resulting from a 25% irrigation deficit, enabling the crop to achieve grain yields and WUE statistically indistinguishable from, and in terms of water productivity superior to, the fully irrigated unamended control. The marked improvement in volumetric soil moisture content across both 0-20 cm and 20-40 cm soil layers confirms that pyrolyzed organic matter fundamentally restructures the water retention dynamics of semi-arid Vertisols, and that these benefits persist even under the most severe irrigation deficit imposed in the study (50% ETc), where biochar-amended soils maintained moisture levels equivalent to the unamended fully irrigated control. The progressive relationship between biochar application rate and both grain yield and WUE across all irrigation levels suggests that the optimum biochar rate may exceed 20 t ha⁻¹, warranting future investigations at higher rates within cost-benefit constraints. In terms of practical agricultural recommendations, smallholder farmers in the semi-arid regions of sub-Saharan Africa should be encouraged to produce biochar from locally abundant biomass sources including crop stalks, acacia and eucalyptus prunings, and by-products of domestic charcoal production using low-cost drum or retort kilns that are readily constructible from locally available materials. A one-time biochar incorporation of 15-20 t ha⁻¹ to a depth of 15 cm prior to sorghum sowing, followed by annual maintenance applications of 3-5 t ha⁻¹, is recommended to sustain soil moisture benefits and minimize annual production costs. Drip or furrow irrigation scheduling should target 75% of ETc during the critical grain-fill growth stage, as demonstrated by the WUE gains in this study, conserving approximately 25% of seasonal water input without yield penalty. National agricultural extension services in Ethiopia, Eritrea, Somalia, and other semi-arid Eastern African countries should integrate biochar-assisted deficit irrigation as a priority climate-smart adaptation intervention for smallholder sorghum producers, particularly given projections of intensifying rainfall deficits across the region through 2050. Future research should examine residual soil moisture effects of single high-rate biochar applications over three or more seasons, the interaction of biochar with split versus full-season nitrogen management under deficit irrigation, and life-cycle cost-benefit analyses of on-farm biochar production to establish a rigorous economic basis for technology dissemination and adoption at scale.

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