Nitrogen release from a 15N-labeled compost in a sorghum growth experiment

In the near future, composted bio-solids are expected to play a major role in agriculture. In order to evaluate their contribution to plant growth and nutrition, a mixed sorghum–poultry manure compost was prepared using 15N-labeled materials. Four treatments were compared in a pot trial: fertilized with compost vs. unfertilized, both of them combined with (cultivated) and without (bare) plants of fibre sorghum (Sorghum bicolor [L.] Moench.). Soil mineral nitrogen (N-min), plant growth, and N uptake were monitored over a whole growing season (167 d after fertilizer treatment; DAT). Apparent soil mineralization (ASM) and apparent recovery fraction of nitrogen by the plant (ARF) were assessed, as well as the 15N recovery fraction by the plant (15NRF). Compost enhanced sorghum biomass at mid growth (+ 200% of dry weight compared to the unfertilized). However, the difference between the control and the fertilized plants progressively decreased towards the end of the season (+ 70%). Fertilized and unfertilized plants followed different growth patterns over time, although of the same sigmoid type. Conversely, N concentration in plant tissues followed a common dilution curve, indicating that fertilized sorghum efficiently used the supplied N, avoiding luxury consumption. Apparent soil mineralization approximately reached 45% of compost total N in pots without plants. Apparent recovery fraction attained 100% at about two third of the growing season (DAT 111), then declined to about 50% because of root and leaf decline. Compared to it, 15NRF only reached ≈ 20% at mid growth (DAT 83), then declined to 12%. Despite the large difference in absolute values, ARF and 15NRF exhibited a significant correlation, indicating a common trend in time. In contrast to 15NRF, the amount of nutrient derived from fertilizer (Ndff) taken up by the plant decreased over the growth season, proving that compost contributed more to plant nutrition in the early (Ndff ≈ 50%) than in the late growing season (Ndff ≈ 25%). The large difference between ARF and 15NRF suggests that sorghum exerted a strong nutrient demand on the soil and on the fertilizer. Both 15NRF and ARF are considered valuable traits: the former better describes fertilizer behavior and actual supply of N, while the latter outlines the overall effect of fertilizer application on crop nutrition.