Phosphorus pools in acid sulfate soil are influenced by soil water content and form in which P is added

Phosphorus availability in acid soils is influenced by interactions of P and Fe-oxides which are modulated by soil water content. Decomposition of organic material influences these interactions. But little is known about the effect of P added as inorganic P or manure on P pools in an acid sulfate soil under varying water content. Sandy acid sulfate soil (pH adjusted to 5.5) was amended with 400 mg P kg⁻¹ as inorganic P, manure or combinations of inorganic P and manure. Amendment treatments included inorganic P alone (100F), manure alone (100M) and P added as 50% inorganic P + 50% manure (50M/50F) as well as an unamended control. The soil was submerged for 2 weeks, then the overlying water was removed and the soil dried to maximum water-holding capacity. The soil was incubated for 4 weeks at this water content followed by submergence for 2 weeks. In the unamended control, P pools were very low and not affected by soil water content. At the end of the first submergence, labile phosphate and oxalate extractable P were two-fold higher in 100F than in 100M and 50M/50F. Oxalate extractable Fe was higher in 100M than 100F and remained unchanged until the end of the experiment. In the following moist incubation and after the second submergence, labile phosphate remained unchanged in 100M, but halved in 100F. Labile phosphate strongly increased in 50M/50F and was about two-fold higher than in the other two amended treatments. Oxalate extractable P was two-fold higher in the moist period than at the end of the first submergence in manure treatments and 30% higher in 100F. It then decreased again after re-submergence in the manure treatments and became lower than in 100F. It can be concluded that with inorganic P addition, labile phosphate was initially very high, but then strongly decreased in the moist period due to removal of phosphate with the overlying water and to binding of P to amorphous Fe where it remained after re-submergence. With manure on the other hand, labile phosphate was lower than with 100F initially, but then remained stable. Phosphate in the overlying water was low with manure which indicates that, in contrast to inorganic P addition, the threat of P transfer into the surrounding environment is low.