Our objectives were to investigate whether AP affects the soil bacterial community composition and diversity in high-level available phosphorus (AP) soils. The bacterial community was analysed through high-throughput sequencing using the Illumina MiSeq platform. Fifteen soils, including barren land, cropland and greenhouse soils which were sandy loam Fluvisols, were selected from different fields in Beijing, China, with AP contents ranging from 5.03 to 391.45 mg kg⁻¹. Statistical analyses revealed high AP (>100 mg kg⁻¹) decreased alpha diversity (Shannon’s index, H’) but not beta diversity of the soil bacterial community. The sequencing of 16S rRNA genes showed that Proteobacteria, Bacteroidetes and Acidobacteria were the dominant phyla in sandy loam Fluvisols. AP, soil organic carbon (SOC) and total nitrogen (TN) had synergistic influence on the shift of the bacterial community composition. Moreover, AP was the main driving factor affecting the soil bacterial community composition compared with other environmental factors. The members of the Proteobacteria, Bacteroidetes and Actinobacteria belonging to copiotrophic taxa typically increased in relative abundance in high-P soils, while oligotrophic taxa (mainly Acidobacteria) decreased in relative abundance. Our results demonstrated that the bacterial community composition would shift from oligotrophic to copiotrophic with increasing levels of AP.

In recent years, continuous efforts have been made to understand the impact of biochar on arable soil fertility. Little is known about whether the biochar derived from municipal sewage sludge has positive impacts on urban soil. In this study, we pyrolyzed spray-dried municipal sewage sludge at 200 °C, 300 °C, 500 °C, and 700 °C for 2 h in a muffle furnace and then amended it into an urban soil to grow turf grass in pots. The outcomes demonstrated that biochar incorporation caused remarkable increases in soil organic C, black C, total N, available P, and K by 3–8, 7–25, 2–9, 10–19, and 1.4–2 times, respectively. The dry matter of turf grass increased by 43–147%, probably due to the nutritional improvement after biochar addition. The turf grass grown in biochar-added soil had 4–70% lower heavy metals than that in the control, although the soils had much higher total heavy metals, which might imply that biochar amendment reduced the bioavailability of heavy metals. Considering the cost of biochar production and its impacts on both urban soil and grass, it would be alternative to convert the spray-dried municipal sewage sludge into biochar at 200 °C for 2 h and then used as an urban soil amendment.

Drying and rewetting (DRW) events are very common in arable land. However, it is not clear how the frequency of DRW stress history influences soil carbon (C) and phosphorus (P) dynamics under field conditions. In this study, an arable loam calcareous soil was treated with simulated farming practices that included wheat straw and nitrogen incorporation and three DRW cycles at intervals of 14 days during a 90-day experimental period of incubation at 25°C. The DRW events significantly increased cumulative CO2-C evolution, but the increase rate of cumulative CO2-C evolution declined with increasing DRW cycles. Microbial biomass C (MBC) and P (MBP) decreased by 9–55% and 9–29%, respectively, following each DRW event, but recovered to the level before DRW treatment within 7 days. Frequent drying and rewetting caused significant increases in both extractable organic C and NaHCO3-extractable P, by 10–112% and 10–18%, respectively. The fluctuation of the tested parameters became less with increasing frequency of DRW cycles. Changes in microbial biomass, either MBC or MBP, were poorly correlated with those of extractable organic C and NaHCO3-extractable P. Overall, frequent DRW cycles had much stronger and longer lasting impact on soil biomass P dynamics than biomass C. These findings may imply certain links among soil moisture, microbial activity and nutrient bioavailability that are important in water and nutrient management.