soil available phosphorus, soil enzymes, c, n and p availability, plant nutrition, fertilization, soil microorganisms, Plant culture, SB1-1110

Pot experiments were conducted in 2017 with two cotton cultivars (CCRI 79 and LMY 28) and three phosphorus (P) levels: 3, 8 and 12 mg P2O5/kg as P0, P1 and P2, respectively. In this study, the soil water-soluble organic carbon content increased as the soil available P (AP) increased, while there were no significant variations for soil total organic matter content among the three AP levels. The activities of invertase, cellulase and urease in cotton soil decreased significantly in the P0. There were positive correlations between invertase and cellulose activities with soil organic carbon and inorganic-nitrogen (N); these correlated negatively with soil C/N ratio and AP level. In addition, high soil AP can raise soil AP and enhance alkaline phosphatase activity, which had a significant negative relationship with the soil C/P ratio. Urease activity had a significant positive relationship with soil NH4+-N, C/P and N/P, as well as a negative correlation with soil C/N. Moreover, soil NH4+-N and NO3--N in the P1 and P2 were lower than in the P0, which might be an effect of high AP on soil N availability.