Impact of fertilizer source on the dynamics of carbon and nutrients in a podzol designated for land-use conversion

Boreal lands have long been considered unsuitable for agriculture. Climate change projections support a shift towards increased agricultural production in boreal ecosystems via land use conversion. Depending on the conversion protocol, the new plough layer may variably reflect the properties of different soil depths. This study assessed how adding mineral or organic fertilizer sources to distinct soil layers within ploughing depth (0-15 cm [topsoil] or 15-30 cm [subsoil]) affects nutrient uptake and crop productivity. Locally relevant organic waste fertilizers (dairy manure [DMN], recirculating aquaculture system supernatant [SUP] and slurry [RAS]), were compared against mineral fertilizer. Tall fescue was used to verify agronomic responses. Starting available nutrient concentrations were balanced as necessary with mineral N and P fertilizers. Except for NH4-nitrogen, both soils were of apparent similar fertility (total C < 1%); unfertilized, neither could support significant plant growth. Subsoil-based growth was fast but limited in time reflecting immediately available nutrients. Topsoil led to delayed, extended growth. This might be due to dichotomous priming effects: during the duration of the experiment up to 36% of subsoil carbon was lost, while the topsoil gained up to 49% carbon, most apparent for DMN. While RAS led to the greatest N and P acquisition efficiencies for topsoil, it had no such effect for subsoil, suggesting distinct fixation and mineralization processes. Dissimilar C cycling and nutrient acquisition for the two soil depths indicates that site-specific considerations that include soil health parameters with commonly tested available nutrients are needed where podzols are farmed after land-use conversion.