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).