Long-term Organic and Inorganic Fertilization Affect Soil pH, Humus Carbon Fractions, and Crop Yield in Three Soil Types

Context: Soil acidification and humus carbon depletion pose significant challenges to agricultural sustainability. Organic and inorganic fertilization influence soil pH, humus carbon, and crop yield, yet the effects in different soil types remain inadequately understood. Objective: This study aimed to (i) measure soil pH changes under different long-term organic and inorganic fertilization in granite, Quaternary red, and purple sandy shale soils, (ii) quantify humus carbon content and humification degree, and (iii) explore the implications for crop yield in these soils. Methods: A field experiment was conducted in 2023 based on a long-term study established in 1982 at the National Observation and Research Station of Farmland Ecosystem in Qiyang, south China. Six treatments were applied: CK-T (control with straw take away), CK-R (control with straw return), NPK-T (NPK with straw take away), NPK-R (NPK with straw return), OM-T (straw take away), and OM-R (straw return). Results: Soil pH was highest under the OM-R treatment, while NPK-T and NPK-R significantly reduced pH in Quaternary red and granite soils compared to purple sandy shale soil. NPK-R and OM-R treatments significantly increased humus carbon fractions, humic acid carbon (HA-C), fulvic acid carbon (FA-C), and humin carbon (HU-C) in all three soils. The NPK-R treatment increased the soil organic carbon (SOC) and soil microbial biomass carbon (SMBC) in granite soil than in Quaternary red and purple sandy shale soils. The average humic acid to fulvic acid ratio (HA:FA) across all treatments was 1.46, with the purple sandy shale soil exhibiting the highest humification degree (HA:FA 1.51), surpassing Quaternary red (HA:FA 1.49) and granite soils (HA:FA 1.38). NPK-R treatment produced the highest sweet potato (Ipomoea batatas 'Beauregard') yields in granite (25,000 kg ha⁻¹) and Quaternary red soils (24,012 kg ha⁻¹) and the highest broad bean (Vicia faba 'Aquadulce Claudia') yield (632.8 kg ha⁻¹) in purple sandy shale soil. Both crop yields were strongly correlated with soil pH, humus carbon fractions, SOC, TN, AP, AK, BD, and SWC. Conclusion: Straw return stabilizes soil pH, whereas NPK fertilizer reduces soil pH. Straw application with NPK fertilizer increases humus carbon content, nutrient concentrations, and crop yield. These findings provide valuable insights into the synergistic effects of straw and mineral fertilizers on soil properties, contributing to crop yield improvement in different soil types.