An isotope study on nitrogen and phosphorus use efficiency and movement in soil in a mimicked vermicompost-based organo-mineral fertilizer

Vermicompost (VC), a stabilized organic material with high organic and humic carbon, and favorable aggregation properties, was tested as a fraction of organo-mineral fertilizers (OMFs), where organic and mineral fractions interact in hotspot areas with surrounding soil. Solutions containing ³³P radioisotope and ¹⁵N-labeled mineral fertilizers were combined with VC at two ratios of organic carbon (C_org) to mineral nitrogen (N) and phosphorus (P) (OMF_7.5C and OMF_15C) to simulate OMF granules. Control treatments included unfertilized soil (N₀P₀), mineral fertilizer (MF_NP), and sole VC at two rates (OF_7.5C and OF_15C). Nitrogen and P uptake by Italian ryegrass (Lolium multiflorum) were measured over in 8 weeks. Furthermore, MF_NP, OMF_7.5C, and OMF_15C treatments were incubated for 10 days without plant to measure atom% ¹⁵N excess and ³³P radioactivity, as indicators of N and P movement from two soil layers (surrounding fertilizer hotspot and below it). In the pot study, OMF_15C caused 24% lower biomass and less nutrient recovery derived from fertilizer (N, 11% and P, 8.5%), compared to MF_NP. In the incubation study, OMF_15C exhibited +19% atom% ¹⁵N excess in the combined two soil layers, relative to MF_NP, and +28% ³³P radioactivity in the soil surrounding the hotspot, and −89% in the soil below it. We interpreted this as a reduction in nutrient availability of the combined VC + mineral fertilizers, due to lower P mobility in soil. The combination of VC with mineral fertilizers can reduce P movement in soil. A higher C_org:N:P ratio resulted in lower nutrient use efficiency in 2 months.