Pathways of dissolved unreactive phosphorus loss under long-term crop straw and manure application

Dissolved unreactive phosphorus (DUP) loss can increase the risk of water organic and inorganic compound pollution. Because DUP loss under different fertilizers has not been evaluated, selecting a fertilizer that decreases losses but maintains yields remains challenging. Therefore, we compared DUP losses via overland flow, leaching, and sediment in lysimeter plots (8 m × 4 m × 0.6 m) with mineral and organic fertilizers for two years. The six treatments, with three replicates, included no fertilizer (CK), mineral fertilizer (NPK), pig manure only (OM), pig manure combined with NPK (OMNPK), crop straw only (RSD), and crop straw combined with NPK (RSDNPK). The results showed that leaching accounted for 54–89% and was the major route for DUP loss. Compared with NPK (0.074 kg ha−1 year−1), in the OM and OMNPK, the annual DUP losses via leaching were substantially increased by 11.1% and 8.6%, respectively, whereas they were decreased by 13.2% and 6.2% in the RSD and RSDNPK treatments, respectively. There was a significant positive linear relationship between the soil P surplus and the annual total DUP loss loading (r = 0.80, p = 0.05). Compared with the NPK (43.44 kg ha−1 year−1), the values of the soil P surplus under the RSD and RSDNPK treatments were greatly reduced by 122.6% and 14.5%, thereby decreasing the annual total DUP loss by 34.5% and 26.4%, respectively. In conclusion, pig manure application can increase DUP leaching risk, whereas the combination of mineral fertilizers and crop straw residues can decrease the DUP loss without compromising the crop yield.