Effects of companion crops and tillage on soil phosphorus in a Brazilian oxisol: a chemical and 31P NMR spectroscopy study

Phosphorus (P) is often the main limiting factor for plant growth in highly weathered tropical soils. Phosphate use efficiency and crop yields could be increased in low-input agroecosystems through cropping systems that favor organic matter accumulation or regular additions of composts and manure. Our objective was to determine the amounts and forms of P according to companion crops (Brachiaria ruziziensis–a grass species–or Cajanus cajan–a leguminous species) and tillage systems (conventional or no-tillage). Soil P pools were determined (Presin, POlsen, PNaOH-EDTA, and Ptotal) and P species were characterized by 31P NMR spectroscopy. The concentrations of available and labile inorganic P (Presin, PiOlsen, and PiNaOH-EDTA) were greater in soil samples where companion crops (Brachiaria ruziziensis or Cajanus cajan) and maize were planted on the same row, with a more significant effect with the legume species. According to the 31P NMR, it is mostly the proportion and amount of POrthophosphate that was increased, with a slight increase of the content of PMonoester. Overall, there was a decrease of the proportion of organic P (PoNMR/PNMR) from 32 to 16% when the legume companion crop was associated with maize. The tillage systems did not lead to any change in the amounts and forms of P. Companion crops are thought to increase available P through mineralization of Po from plant residues and soil organic matter, leading to an increase of fluxes between active P pools.