Available phosphorus status and critical threshold for leaching in greenhouse soils influenced by different fertilizer sources

It is significant to study soil phosphorus (P) leaching resulted from excessive inputs of manure or chemical fertilizer in greenhouse soils. Soil P movement not only depends on content but also on forms of P supplied by fertilizers. Therefore, research on soil P status influenced by manure and chemical fertilizers is important for scientific fertilization and preventing non-point source pollution. In this paper, laboratory soil incubation experiment was conducted to investigate soil available P (Olsen-P) and calcium chloride extracts P (CaCl₂-P) contents influenced by manure and inorganic P sources at different rates0, 50, 100, 150, 300, 600 mg(P)·kg^-1(dry soil) in three soils with different planting years in Raoyang County, Hebei Province. The three soils included soil next to the greenhouse without planting vegetables (control), plastic greenhouse soil with 30 years of planting vegetables (plastic greenhouse soil) and solar greenhouse soil with 4 years of planting vegetables (solar greenhouse soil). The main purpose was to study the effects on available P content in greenhouse soils and determine the threshold of P leaching under manure and inorganic fertilizers application by mathematical stimulation. The results showed that both organic and inorganic P input increased soil Olsen-P and CaCl₂-P contents significantly in three soils. For the control soil with low Olsen-P content, inorganic fertilizer improved Olsen-P content more greatly compared to manure when the fertilizer rate was more than 50 mg(P)·kg^-1(dry soil). But for the plastic greenhouse soils, only under higher P input300 and 600 mg(P)·kg^-1(dry soil), inorganic fertilizer showed more significantly Olsen-P-increasing effect than manure. And there was no significant difference between inorganic and organic fertilizers at low P supply. In general, soil CaCl₂-P content was higher under inorganic fertilizer treatments than organic fertilizer treatments in three types of soil, especially under high P input rate>300 mg(P)·kg^-1(dry soil). Two segment linear regression analyses results showed that the critical soil P leaching thresholds under organic and inorganic P addition in greenhouse soils were 87.8 mg·kg^-1 and 198.7 mg·kg^-1, respectively. With the increase of soil Olsen-P, the increment rate of soil CaCl₂-P with inorganic P addition was twice as much as that with organic P addition. Therefore, in the high P greenhouse soils in Hebei Province, the input of inorganic P fertilizer should be reduced. All P input from manure and inorganic fertilizers should be prohibited in greenhouses soil with Olsen-P content exceeding 198.7 mg·kg^-1. In vegetable greenhouses with soil Olsen-P below 198.7 mg·kg^-1, manure replacement of inorganic fertilizer should be intensified.