Coastal saline soil aggregate formation and salt distribution are affected by straw and nitrogen application: A 4-year field study

Little is known about the effects of straw incorporation on saline soil aggregate formation or salt distribution in coastal zones. In this study, a 4-year coastal wheat/maize rotation field experiment was employed. In each growing season, maize/wheat straw was applied at the rates of 5.0 × 10³ kg ha⁻¹ (S) and 1.0 × 10⁴ kg ha⁻¹ (2S), and inorganic N was applied at the rates of 75 kg ha⁻¹ (N1/2), 150 kg ha⁻¹ (N), and 300 kg ha⁻¹ (N2). Treatment without straw addition and applied with 150 kg ha⁻¹ inorganic N was used as the control (CK). Dry- and wet-sieving techniques were used to fractionate the soils into large macroaggregates (> 2 mm, LM), small macroaggregates (0.25–2.0 mm, SM), microaggregates (0.053–0.25 mm, MI), and silt-plus-clay particles ( 0.25 mm, LA + SA) ranged from 74.2 %–88.3 % and 28.4 %–37.6 % in straw applied treatments, which significantly increased by 14.7 %–19.0 % and 21.1 %–32.4 % compared to CK, respectively (p < 0.05). The mean weight diameter (MWD) and aggregate stability rate (AR) enhanced as straw and N application rate increased. After straw application, soil organic carbon (SOC) preferentially accumulated in MI fractions, which was significantly higher than other aggregate fractions, except SN1/2 and CK (p < 0.05). MWD and AR were significantly negatively correlated with soil salinity (p < 0.05), and soil salinity decreased by 20.5 %–26.9 % in straw treatments compared to the initial soil salinity. In the CK and SN1/2 treatments, soil salinity of LA + SA fractions was significantly higher than < 0.25 mm fractions (MI + CS) (p < 0.05). The same difference in soluble Ca²⁺ was also observed in CK, SN1/2, or SN. In conclusion, saline soil aggregate formation and stability improved after straw and N application, which caused the salinity to decrease and affected salt distribution in aggregates. The findings of this study suggest that adequate N should be applied in order to maximize saline soil reclamation efficiency with straw.