The current fertilizer regimes cause phosphorus deficit in paddy soils and decreased rice phosphorus uptake: a study in Shanghai, China

Excessive phosphorus (P) fertilization in intensive rice-cultivation areas in China has caused serious environmental problems. However, relatively little research has been done on investigating the soil P balance in rice production systems based on different fertilizer regimes. Our study investigated the dynamic variation of soil P balance and rice P use efficiency and productivity from 2014 to 2018 in the paddy fields under synthetic fertilizer (CF) and manure application (OF) regimes based on the practices of local farmers. Flooding water on the paddy fields tends to cause P loss via runoff and leaching. A lysimeter system was used to monitor P loss from paddy fields, which overcame the limitations of real-time monitoring discharge volume and sample-collection. The results with relatively higher rainfall intensity in 2014 and 2015, in total, 4.69 kg P ha(-1) under CF and 8.39 kg P ha(-1) under OF were lost via runoff in 2014, with 13.6 kg P ha(-1) under CF and 17.8 kg P ha(-1) under OF in 2015, whereas from 2015 to 2018, no more than 5 kg P ha(-1) was lost in CF or OF via runoff. Leaching showed a similar, varying trend for both CF and OF. Furthermore, during the 5 years, a continuous soil P deficit was observed, except in 2017 and 2018 for CF and 2018 for OF. The net P retention ranged from -15.31 to 6.2 kg ha(-1) for CF and from -23.65 to 2.64 kg ha(-1) for OF. The continuous P deficit in paddy soils might have reduced soil P storage. These huge P losses undermined rice yield production. In parallel, after 2014, the grain yield decreased from 10,885.7 in 2014 to 6266.7 kg ha(-1) in 2017 under CF and from 10,607.6 to 6649.6 kg ha(-1) under OF. Overall, these results reveal a risk of declining P storage in paddy soils and a potential menace on food safety under the current fertilizer management, suggesting that novel fertilizer strategies need to be developed for reserving P storage in paddy fields.