Effect of long-term fertilization on soil phosphorus characteristics and loss risk of red soil

In order to determine the effects of long-term fertilization on soil phosphorus loss risk in red soil, a study on soil phosphorus adsorption and retention and the related driving factors in red soils was conducted. A long-term fertilization experiment was started in 1986 under double corn cropping system in Jingxian County, Jiangxi Province. The treatments included no-fertilizer control (CK), sole chemical nitrogen fertilizer (N), sole chemical phosphorus fertilizer (P), sole chemical potassium fertilizer (K), chemical N, P and K fertilizers (NPK₁), double doses of chemical N, P and K fertilizers (NPK₂), sole organic manure (OM), and organic manure plus chemical N, P and K fertilizers (MNPK). Soil total phosphorus (TP), available phosphorus (Olsen-P), double acid-extractable phosphorus (Mehlich 1-P), water-soluble phosphorus (CaCl₂-P), phosphate sorption index (PSI), and phosphorus saturation degree (DPS) were measured. Isothermal adsorption and desorption characteristics of soil phosphorus were determined and the relationship between soil phosphorus parameters and soil organic carbon, pH and CEC analyzed using simple and canonical correlation analyses. The results showed that long-term application of chemical phosphorus fertilizer supplemented soil phosphorus (especially TP) and increased Olsen-P and Mehlich 1-P, but had no significant effect on CaCl₂-P. The effect of chemical fertilizer application on DPS was not significant. Phosphorus fertilizer reduced PSI, NPK₁ treatment increased PSI, but NPK₂ had no significant difference with CK. Under long-term application of organic manure (pig manure, OM and MNPK), soil TP and DPS increased, then Olsen-P, Mehlich 1-P and CaCl₂-P accumulated significantly, but PSI decreased. Results from soil P sorption isotherms simulated using the Langmuir equation produced a coefficient in the range of 0.862-0.989. CK and chemical fertilizer treatments had high maximal phosphorus adsorption (Q_m) and phosphorus adsorption affinity constant (k), while under long-term organic manure application (OM and MNPK treatments) Q_m and k reduced. The isotherms for phosphorus desorption showed that CK and chemical fertilizer treatments increased phosphorus desorption rate with increasing phosphorus concentration. Organic fertilizer treatments (OM and MNPK) had high phosphorus desorption rate under low phosphorus concentration, but low phosphorus desorption rate under high phosphorus concentration. Under long-term application of chemical fertilizers, soils phosphorus adsorption and fixation increased with new additions of phosphorus, but organic fertilizers reduced phosphorus adsorption in the soil. Soil TP, Olsen-P, Mehlich 1-P, CaCl₂-P, PSI, DPS and Q_m had significant correlation with pH, CEC, soil total organic carbon (TSOC), cold water soluble organic carbon (CWSOC) and hot water soluble organic carbon (HWSOC). There was a canonical correlation between soil phosphorus indexes and soil organic carbon, pH and CEC, with significantly canonical correlation coefficients of 0.997 and 0.951 for the first and second pairs of typical variables. The correlation coefficient between water soluble organic carbon (HWSOC and CWSOC) and the first pair of typical variables (U₁ and V₁) was highest, followed by soil pH. The study showed that the application of organic fertilizer positively affected soil phosphorus supply and retention. Soil water soluble organic carbon and pH were key indexs of phosphorus supply and risk of phosphorus loss in red soils.