Phosphorus balance and availability in a purple paddy soil under long-term different fertilization

Objectives Soil phosphorus (P) availability is an essential index for assessing soil fertility. This study aimed to examine responses of soil total phosphorus (TP) and Olsen phosphorus (Olsen-P) to a paddy soil P balance under a long-term fertilization to offer science-based support for P management in the southwest of China.

Methods The long-term fertilization experiment, initiated in 1982 on a purple paddy soil in Sichuan Province, consisted of eight treatments including no fertilizer (CK), three chemical fertilizer treatments (N, NP and NPK), single pig manure (M), and chemical fertilizer plus manure treatments (MN, MNP and MNPK), all the treatments repeated four times. Soil Olsen-P and TP were analyzed, the soil apparent P balance was calculated based on P input and crop removal, and the linear regressions between them were conducted thereafter.

Results The annual P removal by the crops in CK and N treatments was about 13.22 kg/hm², showing a stable or a declining pattern within experimental years, while the P removal by the crops under the M and MN treatments was 1.73 times of that in the CK and N treatments, showing a rising pattern in the M and MN treatments with time. The P removal by the crops in the P treatments (NP, NPK, MNP and MNPK) ranged from 41.71 to 45.62 kg/hm², but the quantity decreased with advance of experimental time. Soil P was always in deficit for the no P treatment, while amounts of annual soil P surplus were from 8.76 kg/hm² to 88.79 kg/hm² in the P treatments (NP, NPK, M, MN, MNP, and MNPK). The treatments with chemical P plus manure accumulated higher soil P than the treatments with either manure or chemical P fertilizer alone, and the P surplus in the chemical P plus manure treatments continued to increase with experimental time. Soil TP and Olsen-P responded highly with soil P surplus or deficit. Their increments in the treatment of chemical P or chemical P plus manure were positively correlated with soil P balance, while no such relationships were observed between TP or Olsen P and the soil P balance in the no P and manure treatments. For every mean surplus of 100 kg P/hm² in the soil, amounts of TP would be correspondingly increased by 0.14, 0.16, 0.015and 0.018 g/kg, and those of Olsen-P by 15.76, 17.19, 1.96 and 1.85 mg/kg in the treatments of NP, NPK, MNP and MNPK, respectively.

Conclusions Soil P availability varied closely following the change of soil P surplus or deficit and was also correlated with P sources (chemical source or organic source of P). The increase in soil TP and Olsen-P in the chemical P treatments was significantly higher than that in the manure treatment in the purple paddy soil.