Effects of reducing phosphorus fertilizer rate on root growth and phosphorus fractions in rhizosphere soils of summer maize

Objectives Winter wheat-summer maize rotation is the most common agricultural production system in the North China Plain. Excessive phosphorus (P) fertilization in wheat is concerned in this area, which might result in low fertilizer use efficiency and potentially pressure on the environment. The objective of this paper was to investigate the possibility and potential of reducing application rate of P fertilizer in winter wheat-summer maize crop rotation systems.

Methods A field experiment was conducted on fluvo aquic soil in Hengshui City, Hebei Province. Four rates of P₂O₅ application rate of 0, 112.5, 150.0 and 187.5 kg/hm² were set in winter wheat. After harvest of wheat, summer maize was planted without tillage. The grain yields, biomass and P content in above ground parts of maize were investigated; Root analysis system WinRHIZO was used to determine the root length and diameter; and the contents of different fraction of soil P were analyzed at harvest.

Results Compared to current farm's P₂O₅ input rate of 187.5 kg/hm², the maize grain yield, total root length, average root diameter and soil Olsen-P contents were not significantly affected in treatment of P₂O₅112.5 and 150.0 kg/hm² for three years' period. Under no P fertilizer application treatment, the maize yield and soil available P contents in the third year started to decline, the organic P contents in the rhizosphere soils were lower than in the non-rhizosphere soils in 2009 and 2010 year; the amounts of moderately labile organic P in soils were higher at maize seedling and mature stage than at other growth stages in 2008. When less P fertilizer was applied, moderately labile organic P contents in rhizosphere soils were remarkably lower than in non-rhizosphere at maize seedling stage. The concentrations of Ca₂-P, one of inorganic P forms in soils, declined during maize growth season. The contents of Ca₁₀-P, Ca₈-P, O-P (Occluded phosphorus), Al-P and Fe-P in the rhizosphere and non-rhizosphere soils presented no significant change among various P fertilizer rates. The concentrations of microbial biomass P in the rhizosphere soils were higher than in the non-rhizosphere soils at maize seedling stage in 2008. Compared with farmers conventional P application (187.5 kg/hm² P₂O₅), reducing P application rates did not significantly affect on the contents of microbial biomass P in rhzosphere soils at maize seedling stage.

Conclusions Under the intensive rotation system of winter wheat and summer maize in North China Plain with high soil fertility, the yield and root development of maize, and the contents of Olsen-P, organic P and microbial biomass P in the rhizosphere soils do not show significant variation when the P fertilizer input was reduced by 20% or 40% in wheat season, but those happened when no P fertilizer were applied. It is suggested that it would be available to reduce the P input by 20%-40% in winter wheat-summer maize crop rotation systems under the tested conditions.