Effects of Continuous Application of Composted Sewage Sludge on Soil Organic Nitrogen Fractions and δ¹⁵N in Sandy Fluvo-aquic Soil

In order to provide a theoretical basis for fertilizing and improving sandy fluvo-aquic soil quality, soil total nitrogen, content of different fractions of active organic nitrogen and soil δ¹⁵N values were determined after continuous application of nonhazardous composted sewage sludge (SW) in this study. Four-year field experiment was conducted in a wheat-maize rotation crop field during 2013-2016 in Henan province, China. Based on different fractions of soil active organic nitrogen and stable nitrogen isotope technology, and the response of soil physical and chemical properties, we studied the response of soil active organic nitrogen and soil δ¹⁵N values to composted sewage sludge. The results were that 1) Continuous application of SW significantly raised the content of soil carbon and nitrogen, with 265.83% and 284.31% increase, respectively, by the 45 t•hm^-2 SW treatment in comparison with the control group (CK). The SW treatments significantly enhanced the active organic nitrogen component content and the particulate organic nitrogen (PON) by 104.57%-280.91%. The content of active nitrogen components in the treatment were presented as particulate organic nitrogen (PON)>light fraction organic matter nitrogen (LFON)>dissolved organic nitrogen (DON)>soil microbial biomass nitrogen (SMBN). The allocation of PON/TN and SMBN/TN increased, and the allocation of LFON/TN and DON/TN significantly decreased with the increasing application of SW, which resulted in decrease of the total distribution proportion of active nitrogen, indicating that the 45 t•hm^-2 SW treatment contained enough stable organic nitrogen components for the accumulation of organic nitrogen. 2) Soil amended with SW significantly reduced soil δ¹⁵N values. The pathway analysis and the principal components analysis revealed that the rate of SW had a significantly effect on the soil δ¹⁵N values and soil nitrogen content, while correlation analysis showed that soil δ¹⁵N values had the strong relationship with the soil TN, PON and LFON contents, which indicated that the δ¹⁵N could reflected the effect of soil improvement with the SW. 3) Redundancy analysis further revealed that soil pH, soil moisture and soil fertility had significant effects on the content of soil active organic nitrogen fractions and their δ¹⁵N values. Soil pH and soil integrated fertility index (IFI) were the most significant factors influencing soil active organic nitrogen fractions and their δ¹⁵N values (P<0.05), and they could explain 48.2% and 17.5% of the variation respectively, through the redundancy analysis. The results showed that four years of SW application, particularly the 45 t•hm^-2 SW treatment, could significantly increase the soil nitrogen content and nitrogen quality, and thus improved the soil fertility. δ¹⁵N could be used as a good indicator for the dynamic changes of soil nitrogen and soil quality after SW application.