Changes in nitrogen pools in the maize-soil system after urea or straw application to a typical intensive agricultural soil: A 15N tracer study

A 15N maize pot experiment was conducted to compare the N value of fertilizer alone and fertilizer combined with straw at an equivalent N rate. The four treatments were control (CK), 15N-urea, 15N-urea plus straw, and 15N-straw plus urea. Soil N pools, maize N and their 15N abundance were determined during maize growth. At maturity 26.0% of straw N was assimilated by maize in the urea plus straw treatment. From the eighth leaf stage to maturity, urea plus straw had a significantly (P < 0.05) higher concentration and percentage of exogenous substrate N present as soil total N (TN), particulate organic N (PON), and mineral associated total N (MTN) in bulk and rhizosphere soils than the urea-only treatment. From silking to maturity in the urea plus straw treatment, rhizosphere soil significantly (P < 0.05) increased the percentage of exogenous substrate N present as inorganic N (Inorg-N) and MTN, and significantly (P < 0.05) decreased that present as PON and microbial biomass N (MBN) compared with the bulk soil. From the eighth leaf stage to maturity, rhizosphere soil significantly (P < 0.05) increased the percentage of straw N present as Inorg-N and MTN except for MTN at the silking stage, and significantly decreased (P < 0.05) that present as PON compared with the bulk soil. Overall, straw was an available N source to the crop, and the increase in straw N availability needs to be considered from the interaction of fertilization practices and the crop rhizosphere.