Medium-term effects of different types of N fertilizer on yield, apparent N recovery, and soil chemical properties of a double rice cropping system

Controlled-release, slow-release and stabilized nitrogen (N) fertilizers can potentially increase crop production and reduce N losses. However, information is limited regarding their medium- and long-term effects on rice productivity and soil chemical properties. Therefore, we investigated the effects of these three types of N fertilizer on rice grain yield, apparent N recovery efficiency (ANR), and soil chemical properties in a double cropped rice system over a 7-year period in Zhejiang Province, China. This study examined six N treatments: no N fertilizer (CK), split application of prilled urea (Urea), a single basal application of polymer-coated sulfur coated urea (PSCU, formed by coating SCU with a polymer coating), urea with a nitrification inhibitor (UNI), urea formaldehyde (UF), and a mixture of PSCU and prilled urea (Mix). The effect of N treatment varied over the 7 years, but annual grain yields of double cropped rice with UNI, UF, and Mix were significantly higher than with Urea in 6/7, 4/7 and 3/7 years, respectively, and similar to yield with Urea in the other years. Performance of PSCU was variable, with significantly higher yields than Urea in 3/7 years, but lower yields in 2/7 years. Compared to Urea, the UNI treatment increased the grain yield and ANR of early rice by averages of 9.2 and 24%, respectively, and increased the grain yield and ANR of late rice by averages of 8.6 and 22%, respectively. Total N in the topsoil (0-20 cm) of the CK declined gradually during years 2-4. At the end of each of years 4-7, total soil N in all N fertilized treatments exceeded that in the CK. After 7 years, the five N fertilized treatments had 8-17% higher topsoil total N than the initial value, with no significant difference between these treatments. Available P increased over time in all treatments, with higher values in the CK than the five N fertilized treatments after the first year. After 7 years, available P was 387-529% higher in the N fertilized treatments than the initial value, and with no significant differences between these treatments. However, available K declined considerably in the five N fertilizer treatments during the first four years, but then steadily increased during the next three years when the application rate was increased from 120 to 200 kg K ha(-1). Soil pH declined over the seven years, more so in PSCU and Mix when pH declined from 5.2 to 4.5, compared with declines to 4.7-4.9 in the other treatments. Although fertilizer for the UNI treatment cost $29 ha(-1) more than for the Urea treatment, the higher yield/revenue and lower fertilization labor cost of UNI led to increases in net profit of $ 326 and 329 ha(-1) year(-1) in early and late rice, respectively. Overall, application of UNI was the superior alternative to prilled urea in the double rice cropping system because of consistently higher yield and profitability, while maintaining soil fertility at similar levels to the other N fertilizer options.