Soil carbon sequestration to depth in response to long-term phosphorus fertilization of grazed pasture

The impact of phosphorus (P) fertilizer application on the sequestration of soil organic carbon (C) was investigated in an acidic permanent pasture soil grazed by sheep. Data were collected after 20 years of P fertilization (average 19 kg P ha⁻¹ year⁻¹) compared to a non-P fertilized control, on a P deficient soil. The high P treatment had higher productivity and stocking rate (hereafter termed fertilization treatment). Soils to a depth of 110 cm were analysed for soil bulk density, whole soil C and nitrogen (N), and for C, N, P and sulphur (S) that was associated with a soil pool of fine fraction C (FFC). Fertilization and higher stocking rate had no effect on soil pH but led to a decrease in soil bulk density to depth, and an increase in total soil C that was associated with an increase in both the whole soil C (WSC) and the FFC throughout much of the soil profile. The increase in soil C was evident in both the C concentration (mg C kg⁻¹ soil) and C stocks (Mg C ha⁻¹) to a depth of at least 60 cm, with the largest difference between fertilizer treatments occurring in the top 10 cm soil layer. In the 0–10 cm layer, P fertilization increased soil organic matter (SOM) by ~1% from 3% SOM in the unfertilized treatment to 4% SOM in the fertilized treatment. Over the top 60 cm of soil profile, P fertilization increased total C stock by 12 Mg C ha⁻¹, with 92% of the C stock associated with the FFC. The higher soil C in the fertilized treatment was also correlated with a concomitant increase in N, P and S content in the FFC pool (R² = 0.97, 0.89 to 0.82 for N, P and S, respectively), whereas in the unfertilized treatment nutrient correlations with C in the FFS were observed for N and S only. Although C:N, C:P and C:S nutrient mass ratios in the FFC pool differed markedly with soil depth, no significant difference in nutrient ratios were observed due to fertilizer treatment for any of the nutrients. Collectively our data indicate that sustained P input to a texture contrast pasture soil (Yellow Chromosol) and its associated increase in stocking rate and productivity in a temperate climate had significant long-term benefit for C sequestration and accumulation of soil organic matter (SOM) throughout the soil profile.