[Effect of Long-Term Fertilization on Organic Nitrogen Functional Groups in Black Soil as Revealed by...

Nitrogen (N) is a common limiting nutrient in crop production. The N content of soil has been used as an important soil fertility index. Organic N is the major form of N in soil. In most agricultural surface soils, more than 90% of total N occurs in organic forms. Therefore, understanding the compositional characteristics of soil organic N functional groups can provide the scientific basis for formulating the reasonable farmland management strategies. Synchrotron radiation soft X-ray absorption near-edge structure (N K-edge XANES) spectroscopy is the most powerful tool to characterize in situ organic N functional groups compositions in soil. However, to our most knowledge, no studies have been conducted to examine the organic N functional groups compositions of soil using N K-edge XANES spectroscopy under long-term fertilization practices. Based on a long-term field experiment (started in 1990) in a black soil (Gongzhuling, Northeast China), we investigated the differences in organic N functional groups compositions in bulk soil and clay-size soil fraction among fertilization patterns using synchrotron-based N K- edge XANES spectroscopy. Composite soil samples (0-20 cm) were collected in 2008. The present study included six treatments: farmland fallow (FALL), no-fertilization control (CK), chemical nitrogen, phosphorus, and potassium fertilization (NPK), NPK in combination with organic manure (NPKM), 1.5 times of NPKM (1.5 NPKM), and NPK in combination with maize straw (NPKS). The results showed that N K-edge XANES spectra of all the treatments under study exhibited characteristic absorption peaks in the ranges of 401.2-401.6 and 402.7-403.1 eV, which were assigned as amides/amine-N and pyrrole-N, respectively. These characteristic absorption peaks were more obvious in clay-size soil fraction than in bulk soil. The results obtained from the semi-quantitative analysis of N K-edge XANES spectra indicated that the relative proportion of amides/amine-N was the highest in both bulk soil and clay-size soil fraction, and it was the most major forms in soil organic nitrogen functional groups. Compared with the FALL treatment, the relative proportion of amide/amine-N was lower whereas that of Pyrrole-N was higher in the CK treatment. In the treatments with combined chemical fertilizers and organic manure, the relative proportion of amide/amine-N decreased with increasing application rates of organic manure, while that of Pyrrole-N had an opposite trend. In bulk soil, the relative proportion of amide/amine-N was the highest for the NPKS treatment than for the other treatments. On the other hand, the relative proportion of nitrile/aromatic-N was the highest for the Fallow treatment than for the other treatments in clay-size soil fraction. It is feasible to use N K-edge XANES spectroscopy for characterizing in situ the changes of organic N functional groups in soil under different fertilization practices.