Effects and Dominant Controlling Factors of the Combined Application of Organic and Chemical Fertilizers on Elevating Soil Fertility and Maize Yield in the Black Soil Region of Northeast China

This study aimed to elucidate the impacts and dominant factors of combined application of nitrogen, phosphorus, and potassium fertilizers with manure (NPKM) and manure alone (M), compared with application of nitrogen, phosphorus, and potassium fertilizers (NPK), on soil properties and maize yield across different soil types, soil textures, soil pH levels, and initial SOC contents, to provide a scientific basis for mitigating black soil degradation and promoting high and stable maize yields.

Based on 75 published studies and 477 paired datasets regarding the effects of fertilization on soil fertility and maize yield, this study employed a meta-analysis to quantitatively assess the impacts of NPKM and M treatments (relative to NPK) on soil physicochemical and biological properties and maize yield of the black soil in Northeast China, considering soil types, textures, pH levels, and initial SOC contents. Furthermore, principal component analysis and a random forest model were employed to identify the relative importance of different soil properties in determining the soil fertility index and maize yields.

Compared with NPK, NPKM significantly reduced soil bulk density (BD) by 6.2%, with the largest reduction in pH<6.5 (12.3%). The NPKM treatment increased SOC by 24.0%, with the highest increase (30.6%) when initial SOC was less than 20 g·kg^-1. It also enhanced AN, AP, and AK, with larger increments under pH 6.5-7.0. Regarding maize yield, NPKM increased it by more than 25% on average, with the largest increase (43.7%) at pH 6.5-7.0. The M treatment increased SOC by an average of 21.1%, with the greatest increase (42.5%) at low initial SOC, but its enhancing effect gradually weakened as the initial SOC content increased. Overall, the effect of the M treatment was weaker than that of NPKM, and under some conditions, it even reduced total phosphorus (TP) by 12.6% and total potassium (TK) by 3.7%. The soil fertility indices for NPKM and M treatments were 0.7 and 0.6, respectively, both indicating a high fertility level. The PCA and random forest analyses showed that SOC was the core driver of increasing maize yield. The increment of maize yield under NPKM was primarily dependent on SOC, TN, SMBC, and BD, with a cumulative explanatory rate of 60.2%. In contrast, the yield increase under M was mainly dependent on SOC, BD, pH, SMBC, and SMBN with a cumulative explanatory rate of 63.7%.

In the black soil region of Northeast China, NPKM significantly reduced BD and enhanced SOC, nutrient availability, microbial biomass, and maize yield, having a more comprehensive effect than that under M. Our study demonstrated that NPKM was more effective for improving soil fertility and securing high and stable maize yields. It also identified SOC as the key driver of soil fertility and maize yield under NPKM treatment, and concluded that this fertilization method is the optimal approach for enhancing black soil fertility and achieving high and stable maize yields. Additionally, NPKM represented an effective strategy for enhancing soil fertility, preventing degradation, ensuring food security, and promoting sustainable agricultural development.