Exploring the interface of soil organic matter with phosphorus cycling: A size density fractionation approach

Soil organic matter (SOM) is a key determinant of soil fertility, as it regulates soil physical properties and nutrient cycling. Because SOM generally declines in agricultural soils, the application of organic matter-rich amendments is a common mitigation strategy. However, the processes underlying SOM accrual after the application of these materials remain insufficiently understood and are likely to intersect with both abiotic and biotic processes involving phosphorus (P) and other elements. Since SOM and soil P are often studied using separate methodological approaches, we applied a sodium polytungstate (SPT)-based size-density fractionation method adapted to combined SOM and P analyses. We hypothesized that this approach would isolate particulated and mineral-associated fractions relevant to SOM and P cycling, and that long-term application of organic amendments would produce detectable changes in the fractional distribution of C, P, and selected elements, potentially providing insight into the associations involved in SOM accrual. We fractionated Luvisols subjected to long-term fertilization with sewage sludge or household waste compost, as well as an unfertilized control soil as follows: Coarse particles (2 mm-50 µm) were separated into fractions CL (< 1.8 g cm −3 ), CDA (> 1.8 g cm −3 ), and CDB (≫ 1.8 g cm −3 ), while fine particles (< 50 µm) were separated into fractions FL (< 2.4 g cm −3), FDA (> 2.4 g cm −3 ), and FDB (≫ 2.4 g cm −3). Fraction δ 15 N and total element concentrations were measured, fractional recoveries of C, N, P, Fe, and Ca were calculated relative to whole (i.e. non-fractionated) soils, and dominant organic matter species were characterized using pyrolysis-GC-MS. Compost-amended soils reached 3.5% C and sludge-amended soils 2.2% C, compared with 1.2% in unfertilized soils, despite the sludge treatment receiving only one quarter of the cumulative C inputs of compost over two decades. Recovery of C across fractions was influenced by amendment type. For example, in compost-amended soils, C recovery in the CDA fraction increased significantly. Recoveries of C and P were positively correlated across fractions (R 2 = 0.58–0.83), and, in some fractions, were also associated with Ca or Fe recoveries, suggesting a role of P-, Ca- and Fe-mediated associations in SOM accumulation. Our results show that size-density fractionation is a possible approach for exploratory research studying the physical fractions where C and P processes co-occur, providing a basis for further investigations.