Effects of nitrogen fertilizer on the composition of maize roots and their decomposition at different soil depths

The objective of this study was to determine the effect of nitrogen (N) application on the carbon (C) and N composition of maize roots and their decomposition dynamics at the depths of 15 cm and 45 cm. Maize roots were collected from 0, 120, and 240 kg N ha−1 fertilized plots (R0, R120, and R240, respectively) of a 7-year long-term field experiment. Maize roots were mixed (2% w/w) with soil samples taken from depths of 15 and 45 cm. The mixtures were added to litter bags and buried at 15 and 45 cm depths in the field for 386 days. The root N content was 90-104% greater, the C to N ratio was 43-50% less, and the lignin to N ratio was 51-57% less in the roots of N fertilized (R120 and R240) compared with no N fertilized (R0). Compared to the R0 addition soil samples, the contents of mineral N, microbial biomass C, and soluble organic C in soils mixed with R120 and R240 were greater by 23-37%, 143-297%, and 20-118%, respectively. After 386 days, the remaining C content in roots ranged from 25 to 31% in the R120 and R240 samples compared to 36-38% in R0 samples. Therefore, the increased N content and decreased C to N ratio with fertilization resulted in slightly faster root decomposition. Nevertheless, maize roots from N fertilized plots left more organic C in the soil due to their much greater biomass; therefore, N fertilization led to a greater C input. We conclude that N fertilization affects not only the composition of maize roots, but also their decomposition in soil.