The global agricultural crisis of saline-alkali soil currently affects over 900 million hectares of arable land, challenging water uptake, nutrient absorption, and beneficial microbial communities. A three-year controlled pot experiment published in BMC Plant Biology by Yuting Wang, Guangli Tian, Qingqing Zhao, Dongwei Li, and Shuai He explored the long-term residual effects of a single corn straw-derived biochar application on these soils, focusing on cotton growth and soil bacterial communities. The study investigated four biochar application rates—0%, 1%, 3%, and 5% (w/w)—on both salt-sensitive and salt-tolerant cotton varieties. The results revealed a crucial optimal dose effect and a significant temporal trajectory to biochar’s benefits, highlighting the need for careful management in soil remediation.

The researchers observed that biochar’s impact on soil properties and cotton growth was not immediate or static, but exhibited significant temporal heterogeneity. The primary benefit of salinity mitigation was seen exclusively in the second year, where biochar treatments reduced total soil salt content (TSC) by 0.80–1.39g⋅kg−1 compared to the control. This effect disappeared by the third year, likely due to the biochar aging and losing its porous structure, which promotes salt leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More. Biochar consistently increased soil organic carbon (SOC), total nitrogen (TN), and available potassium (AK) each year in a dose-dependent manner (the higher the dose, the larger the increase). Compared to the control, SOC increased by 135.15%−763.19%, TN by 12.88%−241.92%, and AK by 22.00%−168.84%. The effect on AK became more pronounced with duration. In contrast, biochar had no impact on available phosphorus (AP) in any year, and AP levels were observed to decline over time across all treatments. This AP depletion, coupled with a delayed increase in soil pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More by 0.34–0.44 units in the third year, likely contributed to the eventual decline in cotton growth.

Cotton growth showed an “A-shaped” response over the three years—limited benefits in the first and third years, but maximum improvement in the second year. The most significant finding regarding dosage was the clear optimal dose effect at 1%(w/w) biochar application. At this rate, cotton growth parameters significantly surpassed the control in the second year- Aboveground biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More increased by 24.13%−80.72%; Plant height increased by 12.07%−29.65%; Stem diameter increased by 13.99%−33.22%.

However, higher application rates of 3% and 5% had no detectable effect on cotton growth, suggesting that excessive biochar may impede root development through physical pore obstruction. Biochar altered the soil’s bacterial community composition, clustering primarily by planting year. Interestingly, biochar had limited impact on bacterial alpha diversity (Shannon and Chao1 indices) within the same growing season. Null model analysis revealed a major shift in the underlying ecological processes over time. Deterministic processes, where environmental conditions filter species, predominated throughout. Years 1 and 2: Assembly was dominated by homogeneous selection (62.09% and 67.03%, respectively), meaning the environment strongly favored microbes with conserved adaptive traits; Year 3: A significant transition occurred to heterogeneous selection (84.06%), suggesting adaptive changes in the soil environment, likely driven by nutrient (AP) limitation.

Crucially, the 1% biochar treatment showed an elevated proportion of ecological drift (around 10%), which is associated with reduced environmental selection pressure. This suggests that the low, optimal dose improved the soil environment enough to allow cotton to recruit beneficial microorganisms more freely, independent of strict environmental filtering.

Source: Wang, Y., Tian, G., Zhao, Q., Li, D., & He, S. (2025). Temporal dynamics and optimal dose effects of biochar on soil properties, cotton growth, and bacterial community assembly in saline-alkali soils. BMC Plant Biology, 25(1), 1337.