The effect of phosphogypsum on greenhouse gas emissions during cattle manure composting

Phosphogypsum (PG), a by-product of the phosphate fertilizer industry, reduces N losses when added to composting livestock manure, but its impact on greenhouse gas emissions is unclear. The objective of this research was to assess the effects of PG addition on greenhouse gas emissions during cattle feedlot manure composting. Sand was used as a filler material for comparison. The seven treatments were PG10, PG20, PG30, S10, S20, and S30, representing the rate of PG or sand addition at 10, 20, or 30% of manure dry weight and a check treatment (no PG or sand) with three replications. The manure treatments were composted in open windrows and turned five times during a 134-d period. Addition of PG significantly increased electrical conductivity (EC) and decreased pH in the final compost. Total carbon (TC), total nitrogen (TN), and mineral nitrogen contents in the final composted product were not affected by the addition of PG or sand. From 40 to 54% of initial TC was lost during composting, mostly as CO2, with CH4 accounting for < 14%. The addition of PG significantly reduced CH4 emissions, which decreased exponentially with the compost total sulfur (TS) content. The emission of N2O accounted for < 0.2% of initial TN in the manure, increasing as compost pH decreased from alkaline to near neutral. Based on the total greenhouse gas budget, PG addition reduced greenhouse gas emissions (CO2-C equivalent) during composting of livestock manure by at least 58%, primarily due to reduced CH4 emission.