Long-term fertilization of P coupled with N greatly improved microbial activities in a paddy soil ecosystem derived from infertile land

• Rate of heat output is a good/novel indicator to assess soil microbial activity and soil quality.
• PCA of thermal dynamics is an excellent method to analyze integrated microbial activity.
• Long-term P–N-fertilization greatly improved microbial activities in a paddy soil ecosystem derived from infertile land.

Microcalorimetry was used to study the effects of long-term (20 years) fertilization regimes on microbial activities in a paddy soil in southern China derived from infertile land. Managements of phosphorus fertilizer coupled with nitrogen fertilizer significantly promoted the contents of total and available P, mineral N and microbial biomass C (MBC) (P < 0.05). Both principal component analysis (PCA) of calorimetric indices and metabolic quotient of heat (QT/MBC) showed that fertilization of P coupled with N, P-deficient fertilization and non-fertilized control significantly separated from each other. Redundancy analysis plot showed that rate of heat output (QT/t), peak power (Pmax) and constant of growth rate (k) were significantly correlated with soil total and available P, total and mineral N, which were greatly increased by the P fertilizer coupled with N fertilizer. In contrast, QT/MBC and peak time (tmax) were greatly increased by the P-deficient treatments. In addition, QT/t as a new introduced parameter was negatively correlated well with QT/MBC (R2 = 0.93, P < 0.01). Accordingly, integrating microcalorimetric result analyzed by PCA as well as sensitive indicators of QT/MBC, QT/t and tmax are useful to assess soil microbial activity. The higher QT/t, lower QT/MBC and tmax indicate higher microbial activity and soil quality. In conclusion, long-term fertilization of P coupled with N, especially combined organic fertilizer greatly improved soil fertility and microbial activity; in contrast, deficiency of soil P had lower microbial activity in the paddy soil derived from infertile land.