New methodology for soil aggregate fractionation to investigate phosphorus transformations in iron oxide‐rich tropical agricultural soil

Summary Understanding phosphorus (P) dynamics at the soil aggregate level is necessary to increase P‐use efficiency and for reducing reliance on external P inputs. Traditional methods of soil fractionation are often unsuitable for the study of P dynamics within soil aggregates of tropical soil because of the large amounts of strongly P‐sorbing iron oxide concretions. Here, we present an alternative method of soil fractionation to assess carbon (C), nitrogen (N) and P cycling in a very weathered tropical soil (total P, 883 ± 19 mg P kg −1 ) rich in iron concretions (total P, 1136 ± 28 mg P kg −1 ). This wet‐sieving method removes large iron concretions (> 2000 µm) without any marked effect on smaller soil aggregates, which reduces the error associated with overestimation of aggregation. It reduces variation in nutrient content within larger soil aggregates, and also enables nutrient cycling within water‐stable soil aggregates to be studied. In addition, because it uses field‐moist soil it is possible to measure microbial biomass, community structure, or both, within aggregate fractions. Although this method is important for measuring and understanding C, N and P dynamics within soil aggregate fractions, both the nutrient content and biochemical characteristics of the iron concretions need to be considered to obtain a deeper understanding of nutrient dynamics within the whole soil. Highlights An aggregate fractionation method for tropical soil rich in iron concretions. This method can separate large iron concretions from soil aggregates. Error and variation in aggregation measurements and associated nutrient concentrations are reduced. Measurement of P in concretions and aggregates separately provides a better understanding of P dynamics.