Effects of chemical and clay mineral additives on phosphorus transformation during cow manure and corn stover composting

Although livestock manure is an excellent fertilizer and its composting can become into more stable and nutritional organic fertilizer, it typically has a low N/P ratio compared with crop requirements. The overuse of manures leads to serious phosphorus (P) accumulation in soil, especially in vegetable and fruit production process that increased the potential of P loss in runoff making it became one of the major P sources to eutrophic water bodies. In order to decrease the risk of P runoff from fields with organic fertilizer input, several kinds of additives in organic fertilizer were studied in recent years. In current study, two kinds of additives, chemical substances (calcium oxide (CaO), magnesium oxide (MgO), ferrous sulfate (FeSO4·7H2O), alum (KAl(SO4)2·12H2O),) and clay mineral (vermiculite, zeolite, medical stone, bentonite), were chosen to study their potential effects on P stabilization, forms and transformation during cow manures and corn stalk composting. The addition ratio of additives was 2.5% of the dry weight of the compost material in different treatments. The evolution of total phosphorus (TP) and phosphorus speciation of compost were analyzed during the 35 days' composting process, while the solid samples were taken at various stages (the 0, 3rd, 7th, 10th, 14th, 21st, 28th, 35th day) of composting. Furthermore, the P contents and fractionation in composted products on 35th day were analyzed using Hedley P fractionation method, and were sequentially extracted by deionized water (H2O-P), NaHCO3 (NaHCO3-P), NaOH (NaOH-P), HCl (HCl-P) and H2SO4-H2O2 (Residual-P). The results showed that compared with the control treatment, the addition of CaO, MgO, FeSO4 and KAl(SO4)2 significantly decreased the percentage of water extracted phosphorus (WEP) in TP by 38.0%, 60.2%, 58.8% and 28.9% after 35 days composting, respectively. Adding the vermiculite and zeolite decreased the percentage of WEP in TP by 11.7%, 17.3%, respectively. The results of Hedley P fractionation showed that the addition of CaO and MgO mainly decreased the proportion of H2O-Pi, whereas increased the proportion of NaHCO3-Pi, HCl-Pi and HCl-Po, residual-P. The addition of FeSO4 and KAl(SO4)2 mainly decreased the proportion of H2O-Pi, whereas increased the proportion of NaOH-Pi and NaOH-Po, residual-P. For the addition of clay minerals, we all observed a slight decrease in H2O-Pi and NaHCO3-Pi, and a slight rise in HCl-Pi. The addition of MgO greatly increased the pH value, which may delay the composting process and increase ammonia volatilization, while the other additives had little effect on pH value changing. Therefore, FeSO4, KAl(SO4)2, zeolite and vermiculite had relative better performance for stabilizing P during cow manure composting.