Integrated effect of conservation agriculture and biochar on soil carbon sequestration and wheat productivity in semi-arid Alfisols

Introduction

When Chile's central valleys were first ploughed for wheat in the early 1900s, topsoil organic carbon typically ranged from 18-22 g/kg. A century of mouldboard tillage, residue burning, and continuous cropping has halved those values in many Alfisol landscapes [1] . That decline matters because soil organic carbon underpins cation-exchange capacity, water retention, and aggregate stability-all of which are already marginal in semi-arid environments receiving less than 450 mm of annual rainfall [2] . Conservation agriculture-defined by minimal soil disturbance, permanent residue cover, and diversified rotations-has shown potential to arrest carbon losses and even rebuild stocks over 5-10 years [3] . Biochar, the carbon-rich solid produced by pyrolysis of biomass, can accelerate the rebuilding process because 70-90% of its carbon resists microbial decomposition for decades to centuries [4,5] . Combining the two has been tested in tropical Oxisols and temperate Mollisols, but data from semi-arid Alfisols-common across Chile, southern Australia, and the Iberian Peninsula-remain scarce [6] . This research aimed to: (a) track soil organic carbon changes over three seasons under CA, BC, and their combination, (b) measure wheat grain yield responses, and (c) assess whether the integrated treatment offers advantages beyond what either practice delivers alone.

Baseline Soil Characterisation

Composite soil samples (0-20 cm) collected before treatment imposition in March 2021 showed the following properties: pH 6.1 (1:2.5 water), organic carbon 8.4 g/kg, total N 0.83 g/kg, available P (Olsen) 11.2 mg/kg, exchangeable K 0.34 cmol/kg, bulk density 1.41 g/cm³, and clay content 24.7%. Particle-size distribution classified the soil as sandy clay loam. Cation-exchange capacity was 12.8 cmol/kg, indicating moderate nutrient-holding ability typical of Alfisols in the region [7] .

Materials and Methods

Materials

The trial was established at the Antumapu Experimental Station of the University of Chile (33°34′ S, 70°38′ W), Santiago, on a Typic Haploxeralf at 480 m elevation. Wheat cultivar Pantera-INIA (spring bread wheat) was sown at 180 kg/ha seed rate. Biochar was produced from Pinus radiata pruning residues at 500°C in a slow-pyrolysis kiln; the product contained 68.3% C, pH 8.7, and a specific surface area of 214 m²/g. A basal fertiliser rate of 120-60-40 kg N-P₂O₅-K₂O/ha was applied across all treatments [8,9] .

Methods

A factorial randomised complete block design with four replications assigned four treatments to 16 plots (6 m × 5 m): CT (mouldboard plough + disc harrow, residue removed), CA (notill, full residue retention), BC (conventional tillage + biochar at 5 t/ha incorporated to 15 cm), and CA + BC (no-till, residue retention + surface-applied biochar). Biochar was applied once before the first sowing (May 2021). Wheat was sown annually in May and harvested in December. Soil samples were collected at 0-20 cm depth at the end of each season for organic carbon determination by wet oxidation (Walkley-Black). Grain yield was recorded from 12 m² net plots at 13.5% moisture. Data were analysed by two-way ANOVA in R; treatment means were compared with Tukey's HSD at p < 0.05 [10,11] .

Seasonal Rainfall Patterns

Rainfall during the three wheat seasons (May-December) totalled 312 mm (2021), 284 mm (2022), and 341 mm (2023).

The 2022 season experienced a 23-day dry spell in September that coincided with flag-leaf emergence. Mean growing-season temperatures were 13.8°C, 14.1°C, and 13.6°C across the three years. The 2023 season was the wettest of the three, which may partly explain the higher absolute yields recorded that year [12] .

Field Experimental Design

Plots were arranged in four blocks oriented east-west along a 2% slope. A 2 m buffer strip separated adjacent plots to prevent runoff cross-contamination. In CA and CA + BC plots, wheat stubble from the previous harvest was retained on the surface at an estimated 4.2-4.8 t/ha dry matter. Sowing in no-till plots used a Semeato SHM 11/13 direct-seeding drill. Soil organic carbon under CA + BC rose steadily to 15.7 g/kg by Year 3-an 87.0% gain over baseline-while CT declined to 7.6 g/kg (Table 1). The difference between CA + BC and the next best treatment (BC alone) was significant in all three years. The widening gap between CA + BC and CT over the three seasons (Figure 2) suggests that the carbon-building treatments deliver increasing returns as soil structure and microbial activity improve with time.

Results

Discussion

The 87% rise in soil organic carbon under CA + BC over three years exceeds most published rates for semi-arid environments, where net carbon gains of 10-30% over similar timeframes are more common [3,6] . The difference is likely driven by the combination of biochar's recalcitrant carbon pool and the reduced decomposition of surface residues in no-till, which together create a positive carbon balance [4] . The CT treatment lost carbon at a rate consistent with long-term trials on Chilean Alfisols that documented annual SOC declines of 0.3-0.5 g/kg under mouldboard tillage [1] . Wheat yield gains under CA + BC strengthened each season, which parallels the progressive improvement in aggregate stability and plant-available water reported in similar biocharno-till combinations in southern Australia [13] . The dry 2022 season actually amplified treatment differences: CA + BC outperformed CT by 50.6% that year, versus 23.6% in the wetter 2021, suggesting that carbon-enriched soils buffer yield better under moisture stress [14] . A longer monitoring period (7-10 years) would clarify whether the SOC trajectory continues to rise or plateaus as the biochar ages and surface residue inputs stabilise.

Conclusion

Combining conservation agriculture with a single biochar application of 5 t/ha raised soil organic carbon by 87.0% and wheat grain yield by 68.3% over three seasons in semi-arid Alfisols near Santiago. Neither practice alone matched the combined treatment. The integrated approach restored soil carbon toward historical levels while simultaneously lifting productivity, offering a practical management package for degraded dryland wheat systems. Verification over longer timeframes and across multiple Alfisol sites is recommended before scaling.

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