摘要：
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 0-20 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.%以7年氮肥定位试验地玉米根茬为研究对象,通过把玉米根茬按2%比例与15 cm 和45 cm 土层深度的土壤混合后田间埋袋的方法,研究长期不同施氮量处理[分别为0 kg(N)•hm-2、120 kg(N)•hm-2和240 kg(N)•hm-2]的玉米根茬(分别用 R0、R120、R240表示),在陕西省长武黑垆土中埋藏分解1 a 后对土壤碳、氮组分的影响及根茬有机碳的分解特性。与未添加玉米根茬的对照土壤相比,玉米根茬加入能够显著增加各层土壤的微生物量碳、可溶性有机碳和矿质态氮含量,3种施氮量处理间差异不显著。随着分解时间延长,土壤可溶性有机物中结构相对复杂的芳香类化合物比例逐渐增加。分解1 a 后, R0、R120和 R240根茬的有机碳残留率在15 cm 土层中分别为44.4%、35.3%和34.9%,在45 cm 土层中分别为53.3%、44.3%和42.5%。R0根茬的碳残留率显著高于 R120和 R240；玉米根茬在15 cm 土层的碳分解率和分解速率常数显著高于45 cm 土层。采用一级动力学方程拟合玉米根茬碳残留率变化结果显示, R0、R120和 R240根茬有机碳分解95%所需要的时间在45 cm 土层比15 cm 土层分别长3.2 a、2.3 a 和1.9 a。氮肥施用量影响玉米根茬在土壤中的分解特性,在评价农田氮肥施用与土壤固碳时,应考虑不同氮肥用量下残茬养分组成及其在土壤中分解的差异。
