Table 1_Mechanistic insights into the role of biochar, alum, and iron sulfate in nutrient retention and DOC-associated leaching from poultry litter-amended soils.docx

IntroductionIndiscriminate application of poultry litter (PL) often exceeds crop nutrient demand, resulting in nitrate (NO3-N) and phosphate (PO4-P) leaching to groundwater. Conventional amendments, i.e., alum and iron sulfate, effectively immobilize nutrients but may induce soil acidification. Biochar (BC), a carbon-rich byproduct, has emerged as a potentially sustainable alternative. However, a systematic comparison with established amendments is required, along with the role of manure-derived dissolved organic carbon (DOC) on nutrient leaching dynamics, before broader adoption.
Materials and methodsAn in-situ pot experiment was conducted using coarse-textured soil in 24 leaching columns under open-field conditions with corn (Zea mays) as the test crop. Six treatments were evaluated: (i) soil without PL, (ii) soil with 20 t ha−1 PL (iii) soil with 20 t ha−1 PL + 10% BC-1 (w/w relative to PL), (iv) soil with 20 t ha−1 PL + 10% BC-2, (v) soil with 20 t ha−1 PL + 10% alum, and (vi) soil with 20 t ha−1 PL + 10% iron sulfate. Leachate samples across five natural rainfall events were analyzed for NO3-N, PO4-P, and DOC.
ResultsIn unamended PL-treated soil, NO3-N and PO4-P concentrations ranged from 0.53–51.5 and 3.88–20.5 mg L−1, respectively. With amendments, NO3-N concentrations ranged from 1.94–42.0 (BC-1), 1.72–39.2 (BC-2), 1.07–22.1 (alum), and 1.55–46.7 mg L−1 (iron sulfate), while PO4-P concentrations ranged from 3.20–13.9, 3.18–13.6, 1.51–8.25, and 1.99–11.6 mg L−1, respectively. Across, five rainfall events, amendments reduced cumulative average PO4-P loading by 14.5% (BC-1), 33% (BC-2), 48% (alum), and 52% (iron sulfate) and NO3-N loading by 37% (BC-2), 41% (alum), and 33% (iron sulfate), whereas BC-1 increased cumulative average NO3-N loading by 10% compared to unamended manure soil. DOC exhibited significant positive correlations with NO3-N (r = 0.54, p < 0.05) and PO4-P (r = 0.83, p < 0.001). Grain yield was similar in unamended and BC-amended soils, significantly higher than alum- and iron sulfate-amended soils.
ConclusionThe role of amendments (BC-2, alum, and iron sulfate) was significant for PO4-P retention. The NO3-N retention was modest and influenced by crop growth stages, while PO4-P loss was governed by rainfall amount.