Nutrient addition affects stability of soil organic matter and aggregate by altering chemical composition and exchangeable cations in desert steppe in northern China

Nitrogen and phosphorus can influence the stability of soil organic matter (SOM) and aggregate, which are important for ecosystem functions. However, understanding remains limited regarding the impact of nutrient addition on soil aggregate stability, aggregate-relevant organic matter and nutrients in desert steppe. Here, we studied the variation of soil aggregate stability, nutrients, and the exchangeable cations distribution characteristics, and the chemical composition and thermal stability of SOM were analyzed using thermogravimetric (TG) and Fourier transformed infrared spectroscopy. Samples of soil were collected under four conditions, phosphorus addition (P), nitrogen addition (N), combined phosphorus and nitrogen addition (NP), and no addition (CK), from 0 to 10 cm depth over 4-year experiments in desert steppe in northern China. The percentage of micro-aggregates (0.25-0.053 mm) and large macroaggregates (>2 mm) dramatically increased by 34.2% and 18.1%, respectively, under the N treatment condition compared to the CK treatment, and micro-aggregate content significantly increased under the P treatment, respectively. Nitrogen addition significantly increased the soil aggregate stability index, but the P addition reduced their value. The both macro-aggregates (>0.25 mm) and silt-clay (<0.053 mm) were preferentially enriched with soil nutrients and the exchangeable cations. The N and P addition was beneficial to nutrients accumulating in macro-aggregates, and enhanced the aliphatic-C and aromatic-C abundances respectively. The TG-T-50 value (i.e., temperature when 50% of SOM is lost) increased after nutrient addition, indicating that the SOM has higher thermal stability, especially in the micro-aggregates. The addition of N may have potentially a greater influence on aggregate stability through their influence exchangeable polyvalent cations (Ca2+ and Mg2+), and stability of SOM was influenced by chemical composition and exchangeable K+, Na+, Ca2+ contents in desert steppe. Overall, nutrient addition effects stability of SOM and aggregate by altering chemical composition and exchangeable cations in desert steppe in northern China.