Abstract: Fertilizers are among major contributors to the production of aboveground and belowground biomass. Soil organic carbon (SOC) concentration is an important factor affecting crop productivity in semiarid clayey soils of low fertility. A long-term experiment was established in 1998 on a Vertisol in the Mediterranean coast of Turkey, to assess the effects of four rates of application of inorganic phosphorus (P) fertilizers (0, 50, 100, and 200 kg P2O5 ha−1) on soil bulk density (ρb), carbon (C) and nitrogen (N) concentrations, SOC and N pools, C and N sequestration rate, aggregate fractions, water-stable aggregates, and the mean weight diameter (MWD). Thus, disturbed and undisturbed soil samples were collected from the 0- to 15-cm depth after wheat (Triticum aestivum L.) harvest in June 2010 to analyze soil properties.
Increase in the rate of application of P fertilizers significantly increased ρb and reduced porosity (%). The SOC concentration was significantly more in the treatment receiving 200 kg P2O5 ha−1 than in the control. The carbon-nitrogen ratio was less than 10 in control and greater than 10 in high P fertilizer treatments. Total amount and rate of C and N sequestration increased with increase in the rate of application of P fertilizers. The mean rate (kg C ha−1 y−1) of C sequestration was −110.9 for control and 556.9 for the highest rate of P fertilizer. Increase in fertilizer rate also significantly increased the rate (kg N ha−1 y−1) of N sequestration, which was −0.21 for control and 28.9 for the highest rate of P fertilizer treatment. The SOC concentrations differed among aggregate-size fractions, which were greater in 0.5- to 2.0-mm aggregate size than those in the fraction of less than 0.25 mm. Greater SOC concentrations were observed in 1- to 2-mm and 0.5- to 1-mm size fractions than in fractions of less than 0.25 mm. Concentrations of C and N decreased significantly with decrease in aggregate size of less than 0.25 mm. There was a general trend of increase in water-stable aggregates and MWD with increase in application of P fertilizer. The highest MWD (1.81 mm) was observed in the high P rate and lowest (1.36 mm) in the control. Macroaggregates were more enriched in C and N than microaggregates. The mean rate of SOC sequestration with 200 kg P2O5 ha−1 treatment is ∼560 kg C ha−1 y−1. The effects of long-term P fertilizer application on C and N sequestration finding(s) of from this study have not been previously observed.