Strip Tillage and No Tillage with Integrated Agronomic Practices Improve Maize Yield and Modulate Humus Fractions and Humic Acid Molecular Properties in Sloping Farmlands of Northeast China

Conventional ridge tillage has triggered severe soil organic carbon depletion and soil erosion in sloping maize fields of humid northeast China, posing a persistent threat to black soil health and regional grain security. As representative conservation tillage systems integrated with full straw residue retention, no tillage and strip tillage exhibit prominent potential in soil protection, and their soil-improving benefits are inseparable from continuous straw carbon input; however, their regulatory effects on humus fractions and humic acid molecular properties in erosion-prone sloping farmlands remain largely unclarified. This study aimed to screen the optimal integrated tillage–cultivation mode for sloping farmlands in the northeast China black soil region and to reveal how tillage systems coupled with incremental agronomic practices affect maize yield, humus composition, and humic acid molecular characteristics in Albic soil, a representative degraded soil type of the regional black soil system. A 2-year field experiment was conducted in a typical sloping farmland of Jilin Province, with conventional ridge tillage set as the control. Five incremental integrated management practices (from baseline practice to fertilizer reduction, straw decomposition promotion, and 5–10% higher planting density) were arranged under both under no-tillage and strip-tillage systems. We analyzed dissolved organic matter fluorescence properties, carbon content of humus fractions, and humic acid molecular structural features, and performed principal component analysis for comprehensive performance evaluation of all treatments. This study demonstrates that optimized strip tillage, supported by full straw C input as an indispensable prerequisite, combined with straw decomposition promotion and a 10% planting density increase can synchronously boost soil fertility and maize yield, providing a scientific and practical tillage strategy for sustainable black soil conservation of sloping Albic farmlands in humid northeast China. Strip tillage achieved a 6.78% higher average maize yield than NT, and the maximum yield was recorded with ST5 (strip tillage combined with straw decomposition promotion and 10% planting density increase). Both no tillage and strip tillage significantly increased CDOM content, humification index and autochthonous contribution, optimized humus component distribution with elevated humic acid carbon content, humic acid carbon-to-fulvic acid carbon ratio and humic acid carbon-to-total organic carbon ratio, and enhanced humic acid aromaticity, thermal stability, and hydrophobicity. The principal component analysis results indicated that ST5 ranked first in comprehensive performance, while conventional ridge tillage ranked the lowest among all treatments. Strip tillage integrated with straw decomposition promotion and 10% increased planting density effectively modulated humus fractions, improved humic acid molecular stability, and synchronously increased maize yield. This integrated management regime provides a scientific and practical tillage strategy for sustainable black soil conservation and high-efficiency maize production in sloping Albic farmlands of humid northeast China.