Effects of quantitative fertilization and straw mulch on runoff nitrate- and ammonium-nitrogen losses under natural rainfall events in purple soil sloping croplands, Southwest China

Quantitative fertilization and straw mulch play a pivotal role in regulating runoff nitrate- (NO3-N) and ammonium -nitrogen (NH4-N) losses from sloping croplands. However, the directions and magnitudes of these effects remain largely elusive at the plot scale. This study investigated the effects of decreased and/or increased fertilizer combined with straw mulch on NO3-N and NH4-N runoff losses under different rainfall categories. Conventional fertilizer (CK), decreased fertilizer (DFT) and its combination with straw mulch (DFT + SMT), increased fertilizer (IFT) and its combination with straw mulch (IFT + SMT) were assigned at four sites with runoff plots in the purple soil sloping croplands, southwest China. Runoff depth, the concentrations and losses of total nitrogen (TN), NO3-N and NH4-N as well as NO3-N/TN and NO3-N/NH4-N molar ratios in runoff were determined in the wet season in 2019. Results showed that runoff depth significantly increased as elevated rainfall amount in the plots with straw mulch. TN concentration was significantly different among different rainfall categories for the treatments of IFT and IFT + SMT, showing 9.12-132.91% and 2.56-102.33% higher in the heavy rain than that in other rainfall events, respectively. Higher NO3-N and NH4-N concentrations were found in the treatment of IFT in the high rainfall amount. The changes in TN, NO3-N and NH4-N losses had trends similar to runoff depth, indicating that the N runoff loss was dominantly affected by the runoff rather than the concentration. NO3-N/TN and NO3-N/NH4-N were markedly higher in the light rain than that in other rainfall events, but were not differed by the varied applications of fertilizers and their combinations with straw mulch. The results suggest that NO3-N and NH4-N runoff losses are mostly affected by the rainfall rather than the quantitative fertilization and straw mulch in a short-term experimental period. Our results have great significance for identifying the effective measures to control the nitrogen runoff loss in sloping croplands.