The role of undersown cover crops for improving P cycling in high latitudes arable soils

Background and Aims Efficient phosphorus (P) and management is essential for sustainable arable systems. Cover crops (CCs) are promising, but their performance is uncertain in high-latitudes. This three-year study evaluated CCs’ effects on P dynamics in a P-rich soil undersown in barley in Mid-Norway (63.9°N)—one of the northernmost trials of its kind. Methods A randomized complete block design included three CC treatments: ryegrass (CC1), a ryegrass–clover mix (CC2), and a four species mix including grass, legumes and herbs (CC3), and controls without CC (with/without NPK fertilizer). Soil and plant analyses included total and available P, total N, potentially mineralizable N (PMN), pH, permanganate-oxidizable carbon, root biomass, plant P concentrations, and microbial abundance via qPCR. Statistical analysis was based on Linear Mixed Models (LMMs). Results Cover crops successfully established (average biomass: 1525 kg ha⁻ 1 ), accumulated ~ 7 kg P ha⁻ 1 , and did not reduce barley yields. LMMs showed significant effects of CC treatment on root biomass, total P, and bacteria. Pairwise comparisons also revealed that fungal abundances in CC1 and CC3 were significantly higher than in the unfertilized control. Pairwise regression revealed that soil total P was strongly predicted by root biomass (β = 1.37, P < 0.001). Available P was negatively controlled by microbial pools (Bacteria: β = -9.22, P < 0.001) and residue quality (C:P ratio: β = -0.36, P < 0.001). Conclusions CCs can be used at 63°N without yield penalty. The primary P mechanism is mass-driven sequestration (root biomass) into the stable total P pool. However, P availability is temporally constrained by residue quality and microbial competition.