Effects of partial substitution of organic nitrogen for inorganic nitrogen in fertilization on salinity and nitrogen utilization in salinized coastal soil

Salinization reduces soil N utilization and must be controlled. The objective of this study was to evaluate the effect of application of organic and inorganic fertilizers in various combinations on soil salt content and N utilization. A series of field experiments were conducted in newly reclaimed and salinized farmland in Dongtai, Jiangsu. Maize 'Changjiangyu 8' was grown in soils receiving no fertilization (CK) or applied with one of the following combinations of fertilizers:OM1 (manure only), OM3/4 (1/4 chemical fertilizer and 3/4 manure), OM1/2 (1/2 chemical fertilizer and 1/2 manure), OM1/4 (3/4 chemical fertilizer and 1/4 manure), and OM0 (chemical fertilizer only). All combinations provided 225 kg·hm^-2 N. Soil samples were collected before sowing, during the growing stage of maize, and after maize was harvested. Samples were analyzed for salt content, pH, moisture, and inorganic N. Harvested maize plants were analyzed for N. Crop N uptake and soil N balance and use efficiency were computed. Application of manure increased soil porosity and reduced soil density and surface salt. As a result, the electric conductivity of the soil decreased, and the decrease appeared to be proportional to the rate of manure application. Soil pH was not affected. Application of manure increased soil moisture and organic matter content, and the increase was in proportion to the rate of manure application. Application of OM1/4 resulted in the highest corn N uptake, N content, and yield among all treatments. The combination was also highest in N harvest index, N recovery, N agronomy efficiency, and N partial productivity. While balance analysis showed loss of N from the soil for all treatments due to leaching, runoff, and volatilization, the loss was lowest for OM1/4. Application of combined 3/4 chemical fertilizer and 1/4 manure to provide 225 kg·hm^-2 N reduced soil salinity and resulted in the highest corn production and N efficiency. Meanwhile, it also had significant influence on the improvement of soil water holding capacity, soil structure and organic matter content.