Long-term effects of crop rotation and nitrogen fertilization on phosphorus cycling and balances in loess-derived Mollisols

Developing meaningful agroecosystem soil P inventories necessitates moving beyond single measures of readily extractable inorganic P (Pi) limited to surface depths. We drew on a long-term (36 year) experimental field trial in the US Maize Belt (northwestern Illinois) to evaluate how crop rotation [maize-maize (Zea mays L.) vs maizesoybean (Glycine max L. Merr.)] and N fertilization (0 vs 269 kg N ha-1) impact P dynamics throughout the soil profile by using sequential fractionation and phosphatase activity assays, contextualized by soil P stocks and agronomic P balances. Distribution of P fractions by depth (0-15, 15-30, 30-60, 60-90 cm) indicate that management effects were limited to the surface soil layers (0-30 cm). Soil P fractions differed more by depth than by experimental treatments. Long-term N fertilization significantly decreased pH concurrently with labile organic P (Po) and phosphodiesterase activity. Soil labile inorganic P (Pi) was two-fold lower under N fertilization compared to zero N fertilization, reflecting greater yield and thus P export via grain harvest. Under N fertilization, integration of soybean elevated soil phosphodiesterase activity and decreased water-extractable Po. Higher stocks of soil Po than labile Pi at surface depths (0-30 cm) corroborated a hypothesized appreciable pool size of soil Po relative to the labile Pi pool to which most agronomic assessments are limited. Large negative agronomic balances over the 36-year period (-426 to -945 kg P ha-1) are suggestive of legacy P from preexperiment manure application and high native P stocks, with net P export equivalent to 11-35% of soil P stocks at 0-90 cm depth at the initiation of the experiment. These results contribute to a better understanding how N fertilization and rotation practices influence soil P cycling and stocks, thereby informing P budgets for comprehensive agroecosystem P management.

Automated summary

This study evaluated the effects of crop rotation and nitrogen fertilization on soil phosphorus dynamics using a long-term experimental field trial. The results showed that the management effects were mainly limited to the surface soil layer, with nitrogen fertilization leading to a decrease in soil pH and organic phosphorus content, while the introduction of soybean crop increased soil phosphodiesterase activity and decreased water-extractable organic phosphorus. Furthermore, the negative agronomic balances over the 36-year period indicated the presence of legacy phosphorus and suggested a significant output of phosphorus equivalent to 11-35% of the soil P stocks at the initiation of the experiment.