Long-term agricultural management maximizing hay production can significantly reduce belowground C storage

Liming and fertilization of grasslands have been used for centuries to sustain hay production. Besides improving hay yields, these practices induce compositional shifts in plant and soil microbial communities, including symbiotic arbuscular mycorrhizal (AM) fungi. However, in spite of increasing interest in soil carbon (C) sequestration to offset anthropogenic CO₂ emissions, little is known about the long-term effects of these agronomic interventions on soil C stocks. We examined how plants, AM fungi, and soil C respond to more than seven decades of annual applications of lime, mineral nitrogen (N), and mineral phosphorus (P) to test the hypotheses that (1) management practices increasing aboveground plant production decrease C allocation to roots, AM fungi and the soil; and (2) the relative availability of N and P predicts belowground C allocation in a consistent manner. Our study was conducted at the Rengen Grassland Experiment, established in 1941. Lime combined with N increased hay yields and promoted development of AM fungal hyphae in soil, while reducing relative C allocation to roots. Simultaneous enrichment of soil with lime, N, and P further boosted hay production, promoted grasses and suppressed other plant functional groups. This treatment also decreased soil organic C and strongly suppressed AM fungi in the soil, although the response to P varied among different AM fungal taxa. Our results indicate that agricultural management practices aimed at maximization of hay production may, in the long run, significantly (-20%) reduce belowground C storage. This is a great concern with respect to the intended use of grasslands as anthropogenic CO₂ sinks because the fertilization-induced decrease in soil C stocks can partly or fully negate the C sequestration potential of the grassland ecosystems as a whole.