Evolution characteristics of soil phosphorus and its availability under long-term fertilization in black soil region, China

Based on the long-term fertilizer experiment in Harbin, Heilongjiang Province, China(1979~2021), to study the evolution characteristics and availability changes of phosphorus (P). Contents of soil total phosphorus (TP) and Olsen phosphorus (Olsen-P) and their linear regressions were analyzed and calculated. The results showed that contents of TP and Olsen-P under treatments of phosphorus fertilizers (P, NP, PK, and NPK) increased significantly after 42 years of fertilization, with an average increase of 0.73g/kg and 77.6 mg/kg, respectively, compared to the initial content. While, contents of TP and Olsen-P decreased with the extension of fertilization years under treatments of no P fertilizer (CK, N, K, and NK)(with reductions of 18.3% and 53.6%, respectively). P availability coefficient (PAC) showed yearly declining trends under treatments of no P fertilizer, but had increasing trends with P fertilizer. The PAC of P fertilizer treatments were higher than that under treatments of no fertilizer and no P fertilizer. Soil Olsen-P increment after P application was positively correlated with the accumulated of P. For every P of 100 kg/ha surplus in the soil，Olsen-P increased by 2.34 mg/kg, 2.96 mg/kg, 1.82 mg/kg and 2.89 mg/kg in treatments of P, NP, PK and NPK. When the soil P balance was in deficient, the Olsen-P content under treatments of CK, N, K and NK would be decreased by 1.17 mg/kg, 0.87 mg/kg, 0.83 mg/kg and 1.16 mg/kg for every deficiency of P 100 kg/ha, respectively. In general, the application of P fertilizer could increase contents of soil TP and Olsen-P totally, with PAC being overall higher than treatments of no fertilizer or P fertilizer. Increment of Olsen-P varied closely with the change of soil P balance. The rational application of P fertilizer is of great significance for maintaining soil P surplus and increasing its availability in black soil regions.