Long-term changes in organic and inorganic phosphorus compounds as affected by long-term synthetic fertilisers and pig manure in arable soils

Background Manures and synthetic fertilisers can affect soil pH and plant-microbe processes and thus influence P forms and composition. Aims The objective of this study was to examine the effects of the 40-year application of synthetic fertilisers or pig manure combined with synthetic fertilisers on the P forms and composition and the response of corn growth and yield to changes in P availability. Methods A long-term field experiment with unfertilised, urea, urea+superphosphate, pig manure+urea and pig manure+urea+superphosphate fertilised soils was studied. P-31 nuclear magnetic resonance spectroscopy was used to quantify P compounds. Results The maximum corn yield was found when adding pig manure+urea, despite Olsen-P was lower than on adding manure+superphosphate. Other monoester P compounds, typically related to microbial biomass and processes were highest on adding pig manure+urea. When adding superphosphate+urea+pig manure a reduction of these P compounds was observed as well as the highest accumulation of inositol hexakisphosphate (IHP). Synthetic fertilisers increased Olsen-P, orthophosphate and other monoester and total IHP and largely decreased soil pH. The addition of pig manure greatly increased the stereoisomers of IHP (myo-IHP, scyllo-IHP, neo-IHP, and D-chiro-IHP) and the concentration of orthophosphate diesters. D-chiro-IHP was not found in unfertilised soils while neo-IHP was not detected in synthetically fertilised soils. Conclusions P compounds related to microbial processes, such as other monoester, increased when adding pig manure. Additions of superphosphate on top of manures reduced other monoester, while they increased on adding synthetic fertilisers alone. Unfertilised soils were virtually free of P compounds related to microbial processes.

Automated summary

The study found that long-term application of pig manure+urea can increase corn yield, but compared to adding pig manure+superphosphate, the Olsen-P content is lower. Adding pig manure can increase P compounds related to microbial processes, while adding synthetic fertilisers can promote the accumulation of IHP.