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

Abstract

Purpose: 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). Materials and methods: Soil P pools were determined (Presin, POlsen, PNaOH-EDTA, and Ptotal) and P species were characterized by 31P NMR spectroscopy. Results and discussion: 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. Conclusions: 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.