Soil aggregate size influences the impact of inorganic nitrogen deposition on soil nitrification in an alpine meadow of the Qinghai–Tibet Plateau

Abstract

Background Ammonium (NH4+) and nitrate (NO3−) are two inorganic forms of nitrogen (N) that are deposited from the atmosphere into soil systems. As the substrate and product of soil nitrification, these two forms of inorganic nitrogen will affect or be affected by the soil net nitrification rate (Nr). Our knowledge regarding soil nitrification is mainly derived from studies with bulk soil. However, soil is composed of different aggregate fractions, which may have an important impact on Nr. Methods In 2017, we collected soil samples from an alpine meadow of the Qinghai–Tibet Plateau and separated them into four soil aggregates (2–4, 1–2, 0.25–1, and <0.25 mm) using the dry sieving method. The four soil aggregate sizes amended with the 2 N deposition forms (NH4+-N and NO3−-N) were then incubated at 25 °C for 28 days, and the soil aggregates for each treatment were collected on day 0, 7, 14, 21, and 28 to determine the NO3−-N concentration. The soil Nr and contribution of soil aggregates to the nitrification rate in the bulk soil were calculated. Results There were differences in the physicochemical properties of the soil aggregates. The addition of N and aggregate size had strong effects on soil Nr, which were significantly increased under high levels of NH4+ addition across all soil aggregates. The Nr during the 4 week incubation period differed among aggregate sizes. Nr in the 2–4 mm aggregates was higher than in the other aggregates, which was correlated with the maximum values of the soil porosity observed in the 2–4 mm aggregates. Furthermore, almost half of the soil was composed of aggregates of <0.25 mm, indicating that the <0.25 mm aggregates made a higher contribution to the nitrification rate in the bulk soil than the other aggregates, even though these aggregates had a lower nitrification ability. Overall, our study revealed that the soil nitrification rate was influenced by both the N addition and soil aggregates, and that the 2–4 mm aggregates had a dominant effect on the response of soil N transformation processes to future nitrogen deposition in the alpine meadow.

Soil properties rather than plant diversity mediate the response of soil bacterial community to N and P additions in an alpine meadow

“…The average annual temperature is −0.45°C, with extremes of −29°C and 27°C distributed between January and July per year. The soil is clay-loam and classified as Mat-Gryic-Cambisol (Li et al, 2020 ). The main species of plants in this study area include Leymus secalinus, Stipa purpurea, Poa pratensis, Kobresia humilis, Melissitus ruthenica, Potentilla chinensis , and Artemisia frigida .…”

Nitrogen addition enhances nitrogen but not carbon mineralization in aggregate size fractions of soils in a Pinus massonia plantation

“…Third, in the present study higher levels of microbial biomass were evident in smaller aggregates. Higher microbial load in microaggregate has been associated with increased levels of activities for hydrolases including urease, nitrate reductase, L-glutaminase, and beta-glucosidase, thus contributing to elevated rates of N mineralization such that following N input (Khorsandi and Nourbakhsh, 2007;Li et al, 2021), such N mineralization increased. The results indicated the highest net N conversion and nitrification rates in microaggregates under N90 treatment conditions (Figure 5B), indicating that high N input can raise effective soil N concentrations and intensify mineral N loss, reducing the soil's N fixation capacity (Fang et al, 2009).…”

“…Conversely, Tao and Song (2014) pointed out that nitrogen deposition notably inhibited carbon mineralization in macroaggregates, but enhanced it in microaggregates. Furthermore, Li et al (2020) reported that the rise in nitrogen mineralization due to nitrogen addition was mainly attributed to microaggregates. The addition of N increases the uncertainty faced by C and N mineralization in aggregates.…”