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The effect and relative contributions of C and P inputs on soil microbial biomass P (MBP) accumulation were studied in three long-term soil fertility experiments with various soil and climate characteristics at Qiyang, Yangling, and Wulumuqi. The maximum of soil MBP in all three sites was 47.8 mg P kg-1. The MBP accumulated per unit in soil (mg P kg-1 soil) was correlated with a 4.91 mg kg-1 increase in Olsen P. For each unit increase in P surplus (kg P ha-1), manure C (kg C ha-1), and stubble C (kg C ha-1), MBP accumulation increased by 330, 3.7, and 13 units (μg P kg-1 soil), respectively. The soil MBP was positively correlated with crop yield and P uptake, making the soil MBP a useful soil P fertility index. The critical levels of the soil MBP pool were 140 kg ha-1, 57–62 kg ha-1, and 33–35 kg ha-1 in acidic red soil, loessial soil, and grey desert soil, respectively. This is the first report to establish a quantitative index of soil fertility based on the soil MBP pool. Our findings demonstrate that C input is a good driver of soil MBP accumulation. Integration of the soil MBP as an index of soil P fertility into agricultural P management is useful to help manage mineral P fertilizers as part of sustainable agricultural practices. [ABSTRACT FROM AUTHOR]

• Straw-induced priming effect (PE) in two arable soils subjected to different fertilization regimes were investigated. • PE in soils under different fertilization regimes was negitively linked to the MAOC/SOC ratio. • The fungal abundances and ratio of fungal/bacterial growth correlated positively with SOM-derived CO 2. • The PE intensities were significantly lower in GZL soil compared to UQ soil. • Manure application greatly decreased PE via increased mineral protection and N availability. Rational chemical fertilizer application, especially combined with manure or straw amendment, serves as a key strategy for facilitating soil organic carbon (SOC) sequestration and soil fertility improvement. However, the effects of fertilization managements on the SOC priming induced by straw return remains unclear. Here, a microcosm was conducted to clarify the mechanisms of 13C-labeled maize straw addition effects on priming effect (PE) in two 29-year fertilized soils (Urumqi, UQ; Gongzhuling, GZL) subjected to four fertilization regimes (no fertilizer, CK; chemical fertilizers, NPK; combined application of NPK with straw, NPKS; combined application of NPK with manure, NPKM). Results showed that straw addition strongly increased native SOC decomposition by 15–44%, inducing positive PEs across sites and fertilization treatments. Compared to CK treatment, the NPK and NPKS treatments significantly increased PE by 26% and 98%, respectively, in the UQ soil, but had little effect on the PE in the GZL soil. The NPKM treatment strongly decreased PE by 51–55%, mainly due to the increased proportion of mineral-associated organic carbon (MAOC) and N availability, and decreased fungal abundances, as compared to the CK treatment in UQ and GZL soils. The PE intensities were significantly lower in GZL soil (5.1–11.7 mg CO 2 -C/g SOC) relative to UQ soil (13.7–54.8 mg CO 2 -C/g SOC), primarily due to the higher MAOC/SOC ratio and N availability in the former across fertilization regimes. Overall, manure application has the potential to mitigate soil C loss via priming, mainly resulting from enhanced SOC stabilization via mineral protection and increased N availability, thus facilitating SOC sequestration in agroecosystems. [ABSTRACT FROM AUTHOR]