The influence of no-till coupled with straw return on soil phosphorus speciation in a two-year rice-fallow practice

Conservation tillage is well known to reduce the loss of soil phosphorus (P). However, the effects of different conservation practices on soil P speciation are still unclear. To delineate these effects, two field experiments were conducted during 2-year rice-fallow seasons with four treatments: conventional tillage/straw harvest (CTH), conventional tillage/straw return (CTR), no-till/straw harvest (NTH), and no-till/straw return (NTR). A combination of Liquid-state 31P-NMR and P K-edge XANES was used to investigate the variations in P species present in two paddy soils. Liquid-state 31P-NMR results showed that P monoesters significantly increased under NTR, and after the 2-year rice-fallow practice, the proportion of P monoesters relative to total P in the different treatments ranked as: NTR (23.25%) > NTH (22.18%) > CTR (20.29%) > CTH (19.79%) and NTR (8.61%) > NTH (8.49%) > CTR (8.31%) > CTH (8.03%) in a loam and a clay loam soil, respectively. Orthophosphate (ortho-P) also increased over time, but with no differences among treatments for the two soil types at the end of the experiment. P K-edge XANES spectra further revealed that the relative proportions of iron oxide-bound P (Fe-Pi) and calcium dihydrogen phosphate (Ca-Pi) were highest in the NTR treatment, higher by 27.6% and 11.1% in a loam soil and by 10.4% and 12.7% in a clay loam soil than that in the first rice season. However, the relative proportion of aluminum oxide-bound P (Al-Pi) was lower by 38.6% and 40.7% in the loam and the clay loam soil, respectively. Our results suggested that no-till coupled with straw return can significantly increase the soil pool of organic P, and that this treatment was beneficial to the conversion of inorganic P species to labile forms in rice paddy soils. This study highlights the transformational relationship between different P species in conservation tillage systems.