Soil aggregates are an important controlling factor for the physico-chemical and biological processes such as ammonium (NH₄⁺) retention.  Straw return to the field is increasingly recommended to promote soil carbon (C) sequestration and improve crop yields.  However, the effects of straw return on NH₄⁺ retention at soil aggregate level in agricultural soils have seldom been investigated.  This study aimed to evaluate the influences of long-term straw return on NH₄⁺ adsorption and fixation in microaggregates (<0.25 mm) with or without soil organic carbon (SOC) oxidization.  Soil samples were collected from plots of three treatments, i.e., no fertilizer (CK), inorganic NPK fertilizers (NPK), and inorganic NPK fertilizers with rice straw return (NPKS), from a 20-year-old field trial with rice-wheat rotations in Taihu Lake Region, China.  Soil aggregates were separated using wet-sieving method.  The SOC of microaggregates was oxidized by H₂O₂.  The results showed that long-term straw return significantly increased SOC and NH₄⁺ adsorption, but inhibited NH₄⁺ fixation in microaggregates.  NH₄⁺ adsorption potential and strength - obtained from adsorption isotherms - increased, but NH₄⁺ fixation decreased along with increasing SOC in microaggregates, indicating the important role of SOC in NH₄⁺ adsorption and fixation.  This was verified by the SOC oxidization test that showed a relative decrease in NH₄⁺ adsorption potential for the NPKS treatment and an increase in NH₄⁺ fixation in all three treatments.  Therefore, long-term straw return influences NH₄⁺ adsorption and fixation by enhancing SOC content and could improve N availability for crop uptake and minimize applied N fertilizer losses in rice-wheat cropping systems.

Nitrification inhibitors, such as dicyandiamide (DCD), have been shown to decrease leaching from urea- and ammonium- based fertilizers in agricultural soils. The effect of nitrification inhibitors on nitrifier and denitrifier in short- and long-term intensive vegetable cultivation soils was poorly understood. In this study, the pot trial was conducted to investigate the differential responses of nitrifier (amoA-containing bacteria) and denitrifier (nirK-containing bacteria) to DCD in short-(soil S) and long-term (soil L) intensive vegetable cultivation soils. Quantitative polymerase chain reaction (qPCR) and terminal restriction fragment length polymorphism (T-RFLP) were employed to detect the abundance and composition of amoA- and nirK-containing communities. The results indicated that application of DCD led to a consistently higher NH4 +-N concentration during the whole incubation in soil L, while it was quickly decreased in soil S after 21 days. Furthermore, DCD induced more severe decrease of the abundance of amoA-containing bacteria in soil L than in soil S. However, the abundance of the nirKcontaining community was not significantly affected by DCD in both soils. Long-term vegetable cultivation resulted in a super-dominant amoA-containing bacteria group and less divergence in soil L compared with soil S, and DCD did not cause obvious shifts of the composition of ammonia-oxidising bacteria (AOB). On the contrary, both amoA- and nirK-containing bacterial compositions were influenced by DCD in soil S. The results suggested that long-term intensive vegetable cultivation with heavy nitrogen fertilization resulted in significant shifts of AOB community, and this community was sensitive to DCD, but denitrifiers were not clearly affected by DCD.