Effects of nitrogen mineralization on paddy rice yield under low nitrogen input conditions in irrigated rice-based multiple cropping with intensive cropping of vegetables in southwest China

The farm household responsibility system (FHRS) was adopted in Chinese rural areas during the economic reform in the early 1980s. Since then, many farm households have increased cropping intensity by using large quantities of nitrogen (N) fertilizers in their responsible fields to increase agricultural income. However, intensive cropping systems with low N input are still common in remote places of the southwestern region of China. Maintenance and improvement of soil quality in intensive cropping systems is critical for sustaining agricultural productivity and environmental quality for future generations. The effects of intensive cropping of vegetables on paddy rice (Oryza sativa L.) yield using small quantities of N fertilizers through N mineralization of paddy soil in irrigated rice-based multiple cropping systems were studied in 15 paddy fields in Sichuan Province, China for 3 years. Intensification of vegetable cropping with negative N balance and removal of vegetable crop residues has greatly decreased total N (TN) contents in paddy soil leading to low levels of effective cumulated soil temperature and thickness of plow layer. As a result, the N mineralization in paddy field during paddy rice growing period was decreased. In addition to the low levels of chemical fertilizer N input and residual mineral N input, the lower level of N mineralization in paddy fields and low N recovery efficiency decreased the amount of N accumulated in aboveground biomass of paddy rice at maturity, resulting in limited rice yields. The amount of mineralized N only correlated with TN (partial correlation analysis). Therefore, in paddy fields with very low N input, the N mineralization in paddy soil during the paddy rice-growing period was the major limiting factor affecting the total yield increases. In addition, a decline in soil fertility can be determined using TN as an indicator. To improve paddy rice yield and avoid soil deterioration, the development and adoption of rational soil management programs are needed. These include input of plant residues, conscientious soil tillage for the maintenance of soil temperature and thickness of the plow layer, and the split application of fertilizer for the improvement of N recovery efficiency.