The response of soil organic nitrogen fractions and nitrogen availability to salinity in saline soils of the Yellow River Delta

The soil organic nitrogen composition and nitrogen availability play important roles in the soil fertility and agricultural production of saline soils. This study investigated the effects of soil salinity on soil organic nitrogen fractionation and nitrogen availability in saline soils of the Yellow River Delta (YRD). Soil samples were taken from three wheat-maize rotation fields with low (2.28 g·kg^-1, S1), moderate (3.73 g·kg^-1, S2), and high (6.69 g·kg^-1, S3) salinities on the Huibang Bohai Farm in the YRD. The crop yields were recorded, and the soil organic nitrogen fractions, including ammonia nitrogen (AN), amino acid nitrogen (AAN), amino sugar nitrogen (ASN), hydrolyzable unknown nitrogen (HUN), non-hydrolyzable nitrogen (NHN), soil inorganic nitrogen, and microbial biomass nitrogen were quantified. The activities of the nitrogen transformation-related enzymes (i.e., urease, protease, and nitrate reductase) were determined, and the relationships between the soil organic nitrogen fractions, inorganic nitrogen, crop yield, and soil salinity were analyzed. The results showed that total acid hydrolyzable nitrogen (TAHN), which is the sum of AN, AAN, ASN, and HUN, was the main fractions of soil organic nitrogen, taking up 68.79%, 61.60%, and 52.30% of the total organic nitrogen in S1, S2, and S3 soils, respectively. The contents of the four TAHN fractions (AN, AAN, ASN, and HUN) were all significantly higher in S1 than in S2 and S3 (P < 0.05), and the contents of AN, AAN, and HUN were all significantly higher in S2 than in S3 (P < 0.05). The contents of these fractions were AN > HUN > AAN > ASN in S1, and AN > AAN > HUN > ASN in S2 and S3. Conversely, the NHN content was in the order of S1 > S2 > S3, but the differences were not significant (P>0.05). For the same soil, the NHN content was lower than the TAHN content. The highest soil nitrate nitrogen content (22.08 mg·kg^-1) and microbial biomass nitrogen (20.71 mg·kg^-1) were found in S1, which was significantly higher than those in S2 and S3 (P < 0.05). The ammonium nitrogen content did not differ among the three soils. The activities of urease and nitrate reductase were in the order of S1 > S2 > S3, and the differences were significant (P < 0.05). Protease activity was significantly higher in S1 than in S2 and S3 (P < 0.05). The total yield of wheat and maize in S1 was 1.74 times of that in S2 and 5.85 times of that in S3. Correlation analyses showed that the inorganic nitrogen, microbial biomass nitrogen, AN, and HUN contents had negative exponential relationships with the soil total soluble salt content, whereas the total yield of wheat and maize and the AAN content had significant negative linear relationships with the soil total soluble salt content. The soil inorganic nitrogen content was significantly and positively correlated with the soil TAHN content. The high total soluble salt content in the soils inhibited the formation of acid hydrolyzable organic nitrogen and improved the soil nitrogen availability. These results provide theoretical support for the regulation of soil nitrogen availability in saline soils in the YRD.