Saline soil organic matter characteristics of aggregate size fractions after amelioration through straw and nitrogen addition

Abstract Saline soil organic matter (SOM) composition and distribution are largely unknown. A coastal field experiment was designed to investigate the effects of straw and nitrogen addition on SOM characteristics by diffuse reflectance Fourier‐transform mid‐infrared (FTIR) spectral and amino sugar analysis. In each growing season, maize/wheat straw was applied at rates of 5.0 × 10 3 kg ha −1 (S 5 ) and 1.0 × 10 4 kg ha −1 (S 10 ), and inorganic nitrogen was applied at rates of 75 kg ha −1 (N 75 ), 150 kg ha −1 (N 150 ), and 300 kg ha −1 (N 300 ). N 150 without straw addition was the control treatment (CK). Dry‐sieving technique was used to fractionate soils into macroaggregates (>0.25 mm, MA), microaggregates (0.053–0.25 mm, MI), and silt‐plus‐clay particles (<0.053 mm, SC). Results showed that SOM and amino sugar contents were efficiently increased in the S 5 and S 10 treatments compared with CK, which were significantly higher in MA and MI than in SC ( p < 0.05). In straw‐addition treatments, SOM and amino sugar contents were significantly higher in S 5 N 300 and S 10 N 300 ( p < 0.05). The contribution of microbial necromass C to soil organic C (SOC) was 8.9–17.1%, and the fungal residue was dominant in bulk and aggregate soils. The amount of recalcitrant form of organic C decreased with SOM content increase, and the abundance of aromatic C groups increased under higher salt stress. Therefore, we suggested that microbial necromass accumulation could not be the main pathway of SOC sequestration, and enriching recalcitrant organic C probably led to SOM stability in saline soils.