Effects of long-term nitrogen fertilization on maize root decomposition characteristics at different soil depths

Root residues in fields after harvesting crops are the basic materials for soil organic carbon (C). Root residues are critical for the maintenance of organic matter and improvement of soil fertility. The application of inorganic fertilizers not only increases crop yield, but also affects the allocation of photosynthate in aboveground and belowground systems of crops. The effect of different fertilization on returned crop root biomass into soil has widely been studied. Nitrogen (N) fertilizer accounts for the largest fertilizer use in agricultural production. However, it has still not been clear whether the chemical composition and decomposition dynamics of crop root residues were affected by N level. Meanwhile, soil nutrient cycle may also be affected by root decay. Consequently, a field experiment was conducted to evaluate soil organic C decay under different levels of N fertilizer application to maize. The experiment also studied the dynamics of soil available C and N contents affected by the addition of maize roots in black loessial soils. In October 2010, maize roots in the 020 cm soil depth were collected from three N fertilization treatments in a 7-year-long field experiment. Maize roots gathered from the plots under the 0 kg(N)·hm^-2, 120 kg(N)·hm^-2 and 240 kg(N)·hm^-2 treatments were marked as R₀, R₁₂₀ and R₂₄₀, respectively. After mixing with soil at 15 cm and 45 cm depths at 2% proportion, the decomposition characteristics of the three N-fertilized roots were determined for 368 days after buried in soil. The results showed that in contrast to the control treatment (without root addition), the contents of soil microbial biomass C, soluble organic C and mineral N increased significantly under addition of N-treated maize roots at both soil depths. However, there was no obvious difference between R₀, R₁₂₀, and R₂₄₀ treatments. The specific absorbance at 280 nm (UV280) increased with increasing decomposition time, which suggested that the portion of aromatic and complex compounds in soil organic matter increased. After one year of decomposition, residual ratios of C in roots under R₀, R₁₂₀ and R₂₄₀ treatments were respectively 44.4%, 35.3% and 34.9% at 15 cm depth, and 53.3%, 44.3% and 42.5% at 45 cm soil depth. Root decomposition ratio and decay rate constant were significantly higher at 15 cm than that at 45 cm soil depth. Simulated equations of remaining maize root C with the first order kinetics fitting indicated that the time to reach 95% root C decomposition under R₀, R₁₂₀ and R₂₄₀ prolonged, respectively, by 3.2, 2.3 and 1.9 years at 45 cm soil depth, compared with that at 15 cm soil depth. It was concluded that the effect of decomposition of crop residues on soil C and N accumulation and cycle in farmland soils need more attention in the study of soil carbon sequestration.