Dynamics of changes in soil quality properties in a semi‑arid ecosystem following poplar wood biochar application under field conditions

Introduction
The rapid expansion of arid and semi-arid regions, driven by climate change and unsustainable land management, has led to severe soil degradation, fertility loss, and increasing threats to global food security. In this challenging context, biochar, as a highly stable carbon-based soil amendment, holds significant potential for improving critical soil chemical properties. However, most existing research focuses on short-term effects, and the long-term temporal dynamics of key soil indicators following a single biochar application in semi-arid ecosystems remain poorly understood. Current evidence suggests that initial agronomic benefits may gradually decline over time due to surface aging processes and nutrient leaching. Therefore, determining the optimal application rate and effective time horizon to sustain soil health is crucial for sustainable land restoration. This study aimed to monitor the three-year effects of a single application of poplar wood biochar at 25 and 50 t/ha on soil organic matter, total nitrogen, carbon-to-nitrogen ratio, pH, and electrical conductivity under semi-arid field conditions. The main hypothesis was that soil chemical responses to biochar would follow a non-linear trajectory, with higher application rates demonstrating greater functional stability over the three-year monitoring period. This targeted approach provides a robust scientific framework for cost-effective and ecologically sound decision-making in degraded land restoration programs worldwide.
 
Materials and Methods
This field experiment was conducted at the Urmia University research farm (37°39′N, 44°58′E; 1362 m a.s.l.) under a cold semi-arid climate with ~340 mm annual rainfall and 11.5°C mean temperature. A completely randomized design with three replications evaluated single applications of poplar wood biochar at 0 (control), 25, and 50 t/ha across nine 10-m² plots. Biochar was produced via slow pyrolysis at 450°C (120 min, 10–15°C/min heating rate), sieved to 0.5–1.0 mm, and uniformly incorporated into the top 25 cm of loam-clay soil (texture verified by hydrometer) at trial initiation, with no further amendments over three years. Composite soil samples were collected annually at the end of each cropping cycle. Soil organic matter was determined via the Walkley-Black wet oxidation method (Van-Bemmelen conversion factor 1.724), total nitrogen by standard Kjeldahl digestion-distillation-titration, and the C/N ratio was subsequently calculated. Soil pH and electrical conductivity were measured in saturated paste extracts following established laboratory protocols. Data normality and variance homogeneity were confirmed using Shapiro-Wilk and Levene’s tests. Temporal and treatment effects were analyzed through repeated-measures ANOVA, with mean separations performed using the LSD test at p≤0.05 in SPSS v27. All experimental procedures strictly followed standardized agronomic and soil analytical guidelines.
 
Results and Discussion
Repeated measures ANOVA revealed significant effects of biochar application and time on all soil chemical properties (p<0.05). Organic matter increased significantly in both biochar treatments: at 25 t/ha, values were 197%, 120%, and 95% higher than control in years one, two, and three, respectively, rising from 2.88% to 1.77% (w/w); at 50 t/ha, increases reached 267%, 195%, and 151%, with values increasing from 3.56% to 2.29% (w/w). Total nitrogen showed significant increases only at 50 t/ha across all three years (0.26%, 0.22%, 0.20%; representing 108%, 97%, and 67% above control), whereas the 25 t/ha rate yielded smaller gains of 65%, 42%, and 22%. The C/N ratio increased by 75%, 53%, and 62% at 25 t/ha and by 71%, 46%, and 52% at 50 t/ha relative to control, with no significant difference between application rates. pH increased modestly by 2.3%, 1.5%, and 1.6% at 25 t/ha and by 4.3%, 2.9%, and 2.2% at 50 t/ha versus control, converging by year three. Electrical conductivity rose within the non-saline range: at 25 t/ha, values increased from 1.37 to 1.08 dS/m, representing a decline in relative enhancement from 40% to 5.2% above control; at 50 t/ha, values increased from 1.65 to 1.11 dS/m, with relative enhancement decreasing from 70% to 8.1%. The significant treatment×time interaction for organic matter and electrical conductivity indicates temporal sensitivity, whereas non-significant interactions for nitrogen, C/N ratio, and pH suggest stable, uniform effects. These quantitative patterns demonstrate that 25 t/ha suffices for structural and regulatory improvements, while 50 t/ha is necessary for sustained nitrogen retention in semi-arid soils over a three-year horizon.
 
Conclusion
The findings demonstrate that biochar’s influence on soil chemical parameters follows a dynamic, non-linear trajectory over a three-year period. Initial application yields peak improvements, gradually transitioning toward surface aging and functional equilibrium by year three. While soil organic matter maintained significant enhancements at both 25 and 50 t/ha, long-term total nitrogen stabilization exclusively required the higher rate. Conversely, the C/N ratio, pH, and electrical conductivity reached practical saturation thresholds by the final year, indicating that doubling the application from 25 to 50 t/ha offered no additional agronomic benefit for these indicators. This divergent response highlights the necessity of a targeted, objective-driven biochar strategy in semi-arid agroecosystems. Farmers prioritizing structural improvement, chemical buffering, and runoff mitigation can confidently adopt 25 t/ha as a cost-effective solution. In contrast, low-input systems requiring sustained nitrogen autonomy must apply 50 t/ha to maintain adequate nutrient reservoirs across multiple cropping cycles. Uniform application guidelines without specific restoration objectives risk significant economic inefficiencies. Given the study’s constraints to a single feedstock, pyrolysis temperature, and loam-clay texture, future research should emphasize multi-year monitoring, isotopic tracing of nutrient fluxes, and integration with complementary nature-based practices. These efforts will refine stability models and establish optimized, site-specific protocols for resilient dryland agriculture. Adaptive management frameworks remain essential for scaling implementation.