Incorporation of calcium cyanamide and straw reduces phosphorus leaching in a flooded agricultural soil

Calcium cyanamide and straw additions coupled with one-month of flooding conditions can effectively inhibit soil diseases by generating anaerobic conditions and hydrolysing cyanamide during the summer fallow period in agricultural settings. However, the response of soil phosphorus (P) forms and mobility to the calcium cyanamide and straw additions under flooding conditions remain unknown. This study employed column leaching devices to monitor the P fractions and related movement in a flooded soil with six treatments: control, 0.5 g kg⁻¹ calcium cyanamide (CC), 2 g kg⁻¹ straw (S2), 0.5 g kg⁻¹ calcium cyanamide coupled with 2 g kg⁻¹ straw (CS2), 10 g kg⁻¹ straw (S10) and 0.5 g kg⁻¹ calcium cyanamide coupled with 10 g kg⁻¹ straw (CS10). The results showed that the CC significantly reduced the cumulative P leaching by 26.3%, compared with the control, attributed to the fixation of dissolved P by calcium hydroxide hydrolyzed from calcium cyanamide. The S2, CS2, S10 and CS10 reduced the cumulative P leaching by 10.4%, 41.0%, 70.0% and 80.5%, and enhanced soil P adsorption maximum and the proportions of NaOH-P fraction, compared with the control, respectively. The reason for low P mobility in treatments with straw addition could be attributed to the strengthened association between dissolved P and poorly crystalline Al/Fe/Mn oxides in soils due to an acidification effect induced by decomposition of straw under flooded conditions. Such acidification further coupled with calcium hydroxide was responsible for the lowering of P mobility in CS2 and CS10 treatments. Redundancy analysis indicated that topsoil organic carbon is the most important factor explaining 32.9% of the variations in soil P fractions and cumulative leached P across the treatments. This study highlights that calcium cyanamide in combination with straw additions suppressed soil P leaching under flooded conditions during the summer fallow period, which provided a net water quality benefit slowing release of P to the environment. Future studies should be carried out in agricultural regions with various soil mineral matrices.