Phosphorus leaching in high-P soils under maize silage and interseeding cover crop system

Managing soil phosphorus (P) in agroecosystems with high P levels is crucial to mitigate P loss in water sources without compromising yields. The objective of this study was to assess maize silage (Zea mays L.) + interseeding cover crop of annual ryegrass (Lolium multiforum) or using a relay cover crop and starter P (P0, P10, and P20) on (i) P leaching, (ii) changes in soil P status, and (iii) maize yield, cover crop biomass and P uptake. A six-year study was conducted between 2018 and 2023 in two sites. Leachate samples were collected every two weeks between 2021and 2023. The concentration of P in leachate exceeded 0.5 mg L^–1 indicating a high risk of P leaching at the current levels of soil P. High concentration of P in leachate with a peak at 0.9 mg L^–1 in August and 1.45 mg L^–1 in October during 2021 and 2022 was pronounced under interseeded compared to relay cover crop system due to improve water infiltration contributing to the downward movement of P. In contrast, the concentration of P in leachate was high under relay cover crops in February and March particularly in 2023, which could be attributed to the delay in cover crops establishment and growth in early spring. The concentration of P extracted using anion exchange membranes (PO_4AEMs) increased with increasing starter P rate during the first weeks after planting maize silage. There were spikes of PO_4AEMs, reaching 1.18 and 1.73 µg cm^–2 d^–1 during the 4th, 7th, and 8th weeks after planting maize which coincided with high soil moisture content occurring after irrigation or rainfall events. The lack of dry matter yield response to starter P at harvest was due to high initial soil P. The soil P status remained significantly high averaging 196.6 mg kg^–1 as Mehlich-3 P, despite negative cumulative P budgets. A close observation of the relationships among soil P indicators shows a trend of rapidly decreasing water extractable P under interseeding compared with relay cover crops. It is possible that the production of high biomass under interseeding cover crops increased the amount of P taken up from the soil. We conclude that interseeding cover crops within cash crops in high P soils could be effective at decreasing soil P, but could enhance P leached in the first years of cropping due to high water infiltration along the soil profile.