Long-term influences on nitrogen dynamics and pH in an acidic sandy soil after single and multi-year applications of alkaline treated biosolids

• Raising soil pH depends on annual applications of alkaline treated biosolids.
• One-time use of biosolids at high rates moderately alters long term soil pH.
• Nitrogen mineralization of biosolids were greatest at the two highest rates.
• Annual applications of biosolids increased long term soil nitrogen concentrations.

We present results of a four year field study examining the changes in seasonal soil mineral nitrogen (SMN) and soil pH from the application of an alkaline treated biosolid (ATB) in an acidic sandy loam soil. Results of two management practices, annual ATB applications and a single application, and different rates of ATB (0, 7, 14, 28 and 42 Mg ha−1) were also compared over the four year study period. Corn (Zea mays L.) was used as the test crop throughout the study. Soil pH was effectively modified in all treatments receiving ATB rates, compared to the control, under both management practices but best results were achieved under annual ATB application. Soil cation exchange capacity was increased under annual ATB applications, by 3× at the highest ATB. Compared to the control, soil nitrogen and pH displayed significant changes under frequent additions with increasing rates of ATB. Our results indicate that a single application at the highest ATB rate had a residual effect on soil pH but little impact on subsequent SMN dynamics. Average soil mineral nitrogen (SMN) concentrations in the single application management ranged from 8.3 to 9.3 mg kg−1 in the final three years of the study but ranged from 8.5 to 12.1 mg kg−1 under the annual application management. In contrast, annual applications of ATB at rates ≥14 Mg ha−1 increased seasonal SMN by 15–42% and soil pH by 1–1.5 pH units. Seasonal SMN dynamics under different ATB rates and management practices were also examined using a soil nitrogen ratio (SNR) and as an area under the seasonal SMN curve.