Nutrient enrichment
N addition stimulated N sequestration in plant and soil inorganic N pools, whereas there was no significant response in the soil total N pool. Early global synthesis found that soil inorganic N concentration increased by 114.8% after N addition, with a 47.2% increase in the soil NH4+ pool and a 428.6% increase in the soil NO3- pool (Lu et al. 2011). The N content in belowground (53%) and aboveground (44%) plant pools also increased in response to N addition. However, total N content slightly increased by 6.1% in response to N addition in the present study (Lu et al. 2011). Niu et al. (2016) showed that N addition generally stimulated plant and soil inorganic N concentrations. Notably, the sizes of these N pools were relatively small and mobile (Gruber & Galloway, 2008; Schlesinger, 2009), increases in N storage in the ecosystem are limited in comparison to N input. Moreover, the stimulation on plant N uptake and soil retention generally diminish as N loading increases. This imbalance between N input and retained N emphasizes the importance of N loss by nitrification, denitrification, N2O emission, and inorganic N leaching. This was supported by Shcherbak et al. (2014), who indicated that N losses increase with N addition, even at very low levels of N loading. Similarly, Niu et al. (2016) showed that dissolved and gaseous losses of N increase exponentially with N loading rates.
In this study, nutrient addition strongly stimulated AOB abundance rather than AOA, suggesting that AOB is a more important target group for N management to reduce N loss. The differences were due to the physiological characteristic of AOA and AOB (Ouyang et al. 2016). Our findings agree with other studies, where bacteria rather than archaea drive soils nitrification in nitrogen-rich soils (Shen et al. 2018; Di et al. 2009; Habteselassie et al. 2013). Kinetic analysis showed that AOA has a high affinity for NH4+ but a lower half-saturation constant activity than AOB (Ouyang et al. 2017). This observation is consistent with culture-based studies, which showed that AOA isolates reach their maximum levels of ammonium oxidation under very low nutrient concentrations (Martens-Habbena et al. 2009). Consequently, AOB contributes to nitrification under high ammonium concentrations, while AOA dominates nitrification at low ammonium concentrations (Ouyang et al. 2016).