Phosphorus and carbon status of a paddy soil under different fertilization regimes

Phosphorus (P) in soil particulate fraction (PF; > 53 mu m) is suggested to have a significant importance in soil P cycling. However, the effects of continuous fertilization on P-PF and its association with soil organic carbon (SOC) in paddy soils have not been well studied. We sampled paddy soils at 0-20 cm from a long-term field experiment (initiated in 1981) conducted under humid subtropical conditions in China, which has five fertilization treatments with equivalent P input (135 kg P2O5 ha(-1) year(-1)) except the control treatment (CK). Changes in total P (Pt), inorganic P (Pi), organic P (Po), and SOC under different fertilization managements were evaluated in the whole soil, in the PF, and in the mineral-associated fraction (MAF; < 53 mu m). Continuous fertilization increased the contents of SOC and P in all soil fractions. Both Po and organic carbon in PF were the most sensitive variables to fertilization, indicating that they constitute a useful tool to detect the effects of management practices. Among the fertilization treatments, organic amendments significantly increased Po-PF contents more than chemical fertilizer applied only (p < 0.05), although they had equivalent P input. The paddy soil without fertilization showed a more significant decrease in Pi compared with Po. The SOC/Po ratios were significantly lower in fertilization treatments (especially those with manure or straw incorporation) than in CK and decreased from PF to MAF. A significant relationship was found between Po-PF contents and rice P uptake during the growing season. These results demonstrate that Po-PF may also play a significant role in P cycling of paddy soil, and thus, it would be better to consider Po-PF in soil diagnosis to promote P management of paddy soil, especially for that under long-term organic amendments.