Opportunity costs for maize associated with localised application of sewage sludge derived fertilisers, as indicated by early root and phosphorus uptake responses

Phosphorus recycling from waste and localised placement of fertilisers can potentially improve sustainable P management in agriculture. However, knowledge about root and plant P uptake responses to placement of complex waste-derived fertilisers is lacking. Sewage sludge (SS) and sewage sludge ash (ASH) were tested against triple superphosphate (TSP) in a rhizobox setup where maize shoot and root growth and architecture were followed for 30 days. The three P sources were either mixed homogenously into the soil (slightly acidic, low in available P and moderate P fixing capacity; labelled with P-33) or localised in a patch close to the seed. Localisation of TSP and SS both induced increased root length density in and around the fertiliser patch. For TSP this was followed by enhanced dry matter yield and fertiliser P uptake compared to the mixed source. In contrast, P uptake from SS was not enhanced by the localisation, and while the uptake from the seed was similar, the uptake from soil was lower probably due to weaker root development in the remaining soil. No root response was found for localised ASH, whereas mixed ASH more than doubled dry matter yield and P uptake in comparison. Young maize plants' responses to fertiliser localisation imply opportunity costs and under the given soil conditions, localisation of SS and ASH (contrary to TSP) did not entail an overall benefit for the plant.