Application of 31P NMR spectroscopy in determining phosphatase activities and P composition in soil aggregates influenced by tillage and residue management practices

Soil phosphorus (P) composition and phosphatase activities in aggregates are essential for agricultural productivity and remain poorly understood. A field experiment was conducted from 2007 to study the effect of tillage systems (conventional tillage, T and no tillage, NT) and crop residue management (0, 50% and 100% crop residue incorporation/coverage) on P composition determined by 31P nuclear magnetic resonance (NMR) and phosphatase activities in soil aggregates (>2 mm, 0.25–2 mm and 0.053–0.25 mm). The results showed that crop residue input influenced the concentrations of soil phosphate monoesters and diesters, alkaline phosphomonoesterase (AlP), acid phosphomonoesterase (AcP), phosphodiesterase (PD) activities, and soil aggregate stability significantly, and the addition of crop residue was significantly more effective than tillage. The NT had significantly higher soil phosphatase activities than tillage treatment but not more soil P content. The 0.25–2 mm aggregates showed higher total P, organic P, concentrations of monoesters and diesters, and AlP activity. The structure equation model showed that soil aggregate stability could increase concentrations of monoesters and diesters indirectly by its direct effects on soil phosphatases. Our results suggest that NT and crop residue input could increase the P store and sustainable supply in soil aggregates and that the 0.25–2 mm size aggregates may play an important role in soil organic P maintenance and transformation.