Effects of Organic Amendments Combined with Mineral Fertilizer on Soil Properties and Crop Yield in a Maize–Soybean Rotation System on Meadow Albic Soil

Meadow albic soils in the Sanjiang Plain of Northeast China are characterized by a compact plow layer, weak structural stability, low organic matter content, and limited nutrient availability, which restrict crop productivity in maize–soybean rotation systems. A two-year field experiment (2023–2024) was conducted to compare the effects of mineral fertilizer alone (CF) and CF combined with carbon-based organic fertilizer (CF+COF), humic acid organic fertilizer (CF+HA), or biochar-based fertilizer (CF+BC) on soil properties and crop yield. Soil aggregate composition, pH, organic carbon, total nitrogen, alkali-hydrolyzable nitrogen, available phosphorus, available potassium, and enzyme activities were measured together with yield and 100-grain weight. Compared with CF alone, the combined application of organic amendments generally improved soil properties and increased crop yield, although the magnitude and pattern of response differed among materials. CF+COF was more effective in increasing the proportion of medium-sized aggregates, enhancing alkali-hydrolyzable nitrogen and some enzyme activities, and achieving relatively high yields in both maize and soybean seasons. CF+HA showed comparatively balanced effects on aggregate composition and nutrient availability, whereas CF+BC was more effective in maintaining relatively high soil pH, increasing available phosphorus, and promoting larger aggregates at later growth stages. Overall, all three organic amendments combined with mineral fertilizer were beneficial for improving meadow albic soil and increasing crop yield, with CF+COF showing the best overall performance under the conditions of this study.