Objectives Exploring the impact of partial replacement of chemical fertilizers with organic matter on soil moisture content and photosynthetic characteristics, and clarifying the pathways and effects of its promotion on maize yield increase can provide theoretical and technical support for achieving green and high-yield maize production.

Methods Field experiments were conducted at Shandong Agricultural University in 2016 and 2017, involving three treatments: two organic matter substitution models—full return of wheat straw (S) and mixed return of cattle manure and straw (SM)—and a chemical fertilizer control (CK). From the flowering to full maturity stages of maize, soil volumetric moisture content was monitored at 10 cm intervals from 0 to 120 cm. The relative chlorophyll content (SPAD) and photosynthetic fluorescence parameters of leaves were measured at five key growth stages after maize flowering. Above-ground biomass and yield of maize were investigated at maturity, and correlation analysis was conducted to assess the impact of various factors on maize yield.

Results Partial replacement of chemical fertilizers with organic matter significantly increased maize yield, biomass, and harvest index. Compared with CK, the SM treatment increased maize yield by 4.2% and 6.3% in 2016 and 2017, respectively, while the harvest indices for S and SM treatments increased by 2.5% and 3.0%, and 5.0% and 4.7%, respectively, compared to CK. Organic substitution effectively improved deep soil moisture content, with S and SM treatments showing an average increase in soil moisture content of 8.6% and 9.5% in the 30-40 cm soil layer compared to the 20−30 cm layer, and an increase of 1.8% and 3.6% compared to the same layer in CK. Compared with CK, the two-year average SPAD values of maize at various growth stages under the SM treatment increased by 2.0% to 6.3% compared to CK. Other photosynthetic fluorescence parameters, except for intercellular CO₂ concentration (Ci), were superior in SM and S treatments compared to CK. Compared with the S treatment, the SM treatment significantly increased net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr), while reducing Ci, achieving similar Fv/Fm and PI_ABS values to the Pn. The PI_ABS of the S treatment increased by 12.8% to 17.4% compared to CK, while the SM treatment showed an even greater increase of 20.5% to 26.4%. Correlation analysis indicated significant correlations between maize yield and soil moisture content, Gs, and other photosynthetic fluorescence parameters during the grain filling and maturity stages.

Conclusions The fertilization method of partially replacing chemical fertilizers with organic matter can effectively increase deep soil moisture content from the flowering to late summer stages of maize, maintain higher SPAD values and PSII functionality in leaves after maize flowering, promote photosynthesis, enhance photosynthetic efficiency, and increase photosynthetic productivity after maize flowering. The mixed return of cattle manure and straw as a partial replacement for chemical fertilizers has a more positive impact on soil moisture content and post-flowering photosynthetic performance of maize than the sole application of chemical fertilizers or the single substitution of chemical fertilizers with straw alone, thus being more conducive to high and stable maize yields.