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.

This study explored the effects of the application of organic materials on soil pH, microbial biomass and microbial entropy, so as to provide a scientific basis for the improvement of arable land quality.

Based on the meta-analysis, 1296 sets of data from 224 literatures around the world were integrated to comprehensively analyze the differences in soil pH, microbial biomass carbon (MBC), nitrogen (MBN), microbial entropy carbon (qMBC), and nitrogen (qMBN) under the conditions of organic material application measures (type, application amount, and application years), climate factors (annual average temperature, precipitation), and initial soil properties (pH value, organic carbon content).

After the application of organic materials, the soil pH value increased significantly by 0.23 units, increased by 4.0%, while MBC, MBN, qMBC and qMBN increased by 46.8%, 54.0%, 16.4% and 14.9%, respectively. Compared with chemical fertilizer, soil pH, MBC, MBN, qMBC and qMBN increased by 5.9%, 53.4%, 13.2%, 79.9% and 38.2%, respectively. The effect of increasing soil pH was the best when the amount of manure application was more than 10 t·hm^-2·a^-1 and the application period exceeded 10 years; however, the soil microbial biomass index could be significantly improved when the amount of fertilizer was more than 40 t·hm^-2·a^-1 and the application period was 3-10 years. Under the climate conditions of annual average temperature >16 ℃ and annual precipitation >1200 mm, the effect of applying organic materials on improving soil pH was better, especially in soils with initial soil pH value of 4.5-5.5 and organic carbon content (SOC) >12 g·kg^-1; however, under the climate conditions of 8-16 ℃ annual average temperature and annual precipitation <600 mm, the application of organic materials had better effect on the improvement of MBC, MBN, qMBC and qMBN in weakly alkaline soil (pH>7.5) with SOC≤12 g·kg^-1. The random forest model showed that the effects of applying organic materials on soil pH, MBC, MBN and microbial entropy were affected by soil initial pH and SOC, application measures of organic materials and climate conditions, respectively. The partial least squares path model further proved that the application measures of organic materials, climate conditions and initial soil properties affect microbial biomass and microbial entropy by adjusting pH value.

Organic materials most effectively elevated soil pH in acidic with high-SOC soils (pH≤5.5, SOC>12 g·kg^-1), which enhanced microbial biomass and microbial quotients in alkaline with low-SOC soils (pH>7.5, SOC≤12 g·kg^-1). The regulation of soil pH, microbial carbon and nitrogen, and microbial quotients was primarily controlled by initial soil properties, organic material application measures, and climatic conditions.

This study aimed to clarify the response and driving factors of soil bacterial diversity and dominant microbial community to fertilization measures in farmland ecosystems in northern China under different natural factors, soil properties and farmland management measures, so as to provide a scientific basis for revealing the regulatory mechanism of fertilizer application on the structure of soil microbial communities, enhancing soil biological fertility, and maintaining the stability of farmland ecosystems.

In this study, 50 papers published between 2014 and 2024 were synthesized, 207 groups of relatively independent data on bacterial diversity and dominant community were obtained, and the effects of fertilizer application on soil bacterial diversity and dominant microbial community in agricultural fields in northern China from fertilizer application measures, region, climate, soil pH level, soil SOC level, crop species, and nitrogen application were quantitatively analyzed by using the meta-analysis method and the randomized forest model.

In general, fertilization significantly enhanced the number of soil bacterial OTUs (5.6%), Chao1 index (4.3%), and the relative abundance of Ascomycota (5.3%), and significantly decreased the relative abundance of Acidobacteriaceae (-5.6%) and Greenbenders (-10.4%) (P<0.05), but there was no significant effect on the bacterial Shannon's index. In those areas with mean annual temperature <10 ℃ and annual rainfall >400 mm, fertilization significantly enhanced the number of soil bacterial OTUs, Chao1 index and Ascomycota, but significantly decreased the Acidobacteria phylum and Green Benders phylum (P<0.05). In alkaline soil and soil poor in starting organic carbon (SOC) (<12 g·kg^-1), fertilization significantly increased the number of bacterial OTUs and Chao1 index; in neutral soil and different initial SOC, fertilization significantly increased the relative abundance of Aspergillus phylum, but decreased the relative abundance of Acidobacterium phylum and Greenscope phylum. Compared with wheat and other crops, fertilizer application increased the number of bacterial OTUs (9.4%), Chao1 index (6.4%), and relative abundance of Ascomycota (4.8%), and significantly decreased the relative abundance of Acidobacteria (7.4%) and Greencurvata (11.7%) in maize (P<0.05); fertilization significantly increased the number of bacterial OTUs and Chao1 index (6.4%), and relative abundance of Aspergillus species (4.8%) in Nitrogen application >200 kg·hm^-2. Nitrogen application at >200 kg·hm^-2 significantly increased the number of bacterial OTUs, Chao1 index and relative abundance of Ascomycetes, while significantly decreased the relative abundance of Acidobacteria and Green Benders. Soil organic carbon was the main controlling factor affecting the number of soil bacterial OTU and bacterial Shannon index, annual rainfall was the main controlling factor affecting bacterial Chao1 index, annual mean temperature was the main controlling factor affecting soil bacterial Aspergillus and Acidobacteria phylum, and pH was the main controlling factor affecting soil bacterial Green Benders phylum.

It was recommended that in different regions of northern China, different fertilization measures should be selected according to local conditions, taking into full consideration of local natural factors, soil properties and farmland management measures, so as to achieve the scientific fertilization to maintain soil bacterial diversity and promote soil health.