Variation of dissolved nutrient exports by surface runoff from sugarcane watershed is controlled by fertilizer application and ground cover

Abstract Overuse of chemical fertilizer due to intensive agriculture results in excess dissolved nutrient exports by runoff from watershed, causing eutrophication of water bodies. But how the time of fertilization and ground cover influence exports of dissolved nutrients by runoff from watershed have not been studied yet. This study, therefore, investigated the dynamics of dissolved nutrients (total dissolved nitrogen (TDN), NH4+-N, NO3−-N and total dissolved phosphorus (TDP)) exports, and their relationships with mode and time of fertilizer application and sugarcane cover in Nala watershed, subtropical southern China. Runoff from the outlets of three sub-watersheds was monitored and collected during sugarcane growth stages (establishment (ES), vegetative (VG), grand growth (GG) and ripening (RP)). The magnitudes of TDN, NH4+-N, NO3−-N and TDP exports from the entire watershed were 111.72, 10.97, 89.89 and 0.99 kg ha-1, respectively. The exports of TDN were 32.51, 30.74, 47.60 and 0.87 kg ha-1 during ES, VG, GG and RP, respectively, whereas the respective TDP exports during the four growth stages were 0.43, 0.16, 0.39 and 0.01 kg ha-1. The highest TDN and NO3−-N (43-47% of the total) exports during grand growth stage coincided with fertilizer topdressing and peak runoff. In contrast, the highest NH4+-N (43%) and TDP (49%) exports occurred during ES following basal fertilization with minimum sugarcane cover. The TDN exported from the watershed was equivalent to 38% of applied N fertilizer (296.4 kg ha-1) while TDP exports was equivalent to 4% of applied P fertilizer (22.90 kg ha-1). Runoff had positive relationships with the dissolved nutrient exports (P