Effects of dry-wet cycle aged biochar on soil fertility and rice growth in controlled irrigation paddy fields

Controlled irrigation can significantly alter the soil micro-environment after dry-wet alternation. Yet the alternating dry-wet soil conditions can often destabilize the soil microenvironment. Fortunately, the biochar application can be expected to avoid soil degradation in recent years. But the biochar itself can suffer from the aging process in the field, leading to the varying physicochemical properties over time. This study aimed to explore the effects of aged biochar during dry-wet cycles on the soil fertility and rice productivity. The physicochemical evolution of the biochar was also determined under simulated dry-wet aging, in order to quantify the dynamic regulation of the soil nutrients and rice yield in the water-saving irrigated paddy fields. Rice straw biochar was pyrolyzed at 400-600°C, and then subjected to an accelerated aging process in the laboratory. The 3, 6, and 10 dry-wet cycles (labeled as C3, C6, and C10) were also set, with the fresh biochar (CS) serving as the reference. A two-year pot experiment (2022–2023) was subsequently conducted using the japonica rice cultivar Nanjing 9108. The treatments consisted of the soil mixed with the fresh biochar (CK) and the three types of aged biochar (CC3, CC6, and CC10) at a 2% mass ratio under controlled irrigation. Scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis, and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the biochar after the experiment. Some parameters were monitored, including the soil organic carbon (SOC), total nitrogen (TN), available phosphorus (AP), available potassium (AK), and rice physiological indicators. The results indicated that the aging process induced the non-linear variations in the biochar structure. Moderate aging (6 cycles) significantly promoted the micropore evolution, thus increasing the specific surface area to a peak of 302.18 m²/g, and micropore volume by 6.86%. Conversely, excessive aging (10 cycles) led to the pore wall collapse, thus reducing the surface area. It was significantly enriched in the surface oxygen-containing functional groups (increasing the O/C ratio from 0.08 to 0.14). In the paddy soil system, the CC6 treatment shared the most effective nutrient conservation. Compared with the fresh biochar (CK), the CC6 treatment increased the SOC and TN content by 26.03%–45.40% and 14.46%–29.01%, respectively, over the two years. The highly oxidized CC10 treatment maximized the AK release. The stability was reduced in the carbon sequestration, leading to a decline in the stomatal conductance of the rice leaves. The 2-year average rice yield increased by 23.11% under the CC6 treatment, compared with the CK. A quadratic regression revealed that there was a significant correlation (R²=0.89) between the aging cycles and yield, indicating an optimal aging duration of 7.7 cycles. Moderate dry-wet cycling was enhanced to optimize the pore structure and nutrient retention, indicating the high agronomic performance of the biochar. Whereas the excessive aging degraded the physical structure to limit the yield benefits. Therefore, the biochar shared the aging threshold in the water-saving irrigation. These findings can provide a scientific basis to optimize the long-term biochar management in sustainable agriculture.