Chronic nitrogen addition differentially affects gross nitrogen transformations in alpine and temperate grassland soils

Abstract

Nitrogen (N) deposition can profoundly alter soil N cycling of grassland ecosystems.Substrates and soil acidification are expected to modify soil N transformations in response to elevated N deposition. Here, we carried out 15 N tracing studies to test the effects of N addition rates (low: 30 kg N ha -1 and high: 90/120 kg N ha -1 ) and soil acidification on gross N transformation rates using two typical Chinese grassland soils, an alpine calcareous soil and a temperate neutral soil. We found that N addition significantly increased the ratio of gross nitrification rate to gross ammonia immobilization rate (N/I) in both soils, but gross N transformation rates changed differently as a function of N addition rates and soil types. In the calcareous soil, N addition increased soil gross N transformations, largely due to mineral N substrates, SOC, TN and fungal dominance. In contrast, low N addition did not affect gross N transformation rates in the neutral soil, but high N addition significantly decreased gross N transformation rates. Although both SOC and TN were increased with N addition in the neutral soil, N-induced soil pH decline decreased gross N transformation rates. Our results indicate that the effects of N addition on grassland soil gross N transformations are highly dependent on mineral N substrates, SOC and TN. Soil acidification played a more important role than SOC and TN in gross N transformation rate changes in response to elevated N deposition. These findings suggest that the different changes of gross N transformation rates in response to N deposition and soil properties (e.g. SOC, TN and soil pH) should be integrated into biogeochemical models to better predict grassland ecosystem N cycling in the future scenarios of N deposition.

Source Apportionment of Atmospheric Ammonia at 16 Sites in China Using a Bayesian Isotope Mixing Model Based on δ15N–NHx Signatures

“…Elevated N deposition affects terrestrial and aquatic ecosystems, with impacts on the soil N cycle, ecosystem functioning, and structures. , Knowledge of the source contributions of NH x deposition would benefit in evaluating the ecological risks and developing abatement strategies for agricultural and industrial activities. The quantified NH x dry deposition fluxes at the monitoring sites ranged from 6.0 to 65.6 kg N ha –1 yr –1 , with a mean of 21.0 kg N ha –1 yr –1 , higher than the national estimate of 12.2 kg N ha –1 yr –1 reported by Wen et al The dry deposition fluxes were estimated using the measured concentrations and modeled V d .…”

“…However, two background sites (DL and BK) were surrounded by temperate and alpine grasslands, respectively, but had similar seasonal variations to the rural sites, probably due to NH 3 emissions from several N addition experiments being conducted within 1 km of the two sampling sites. 76 These spatial and seasonal differences across sites and seasons clearly indicate human activity-induced variations in NH 3 emissions and concentrations.…”

Decrease in soil inorganic nitrogen supply capacity under long‐term areca nut plantations in the tropics

“…I NH4 was significantly and positively correlated to M (Figure 2h). This indicated that decreasing NH 4 + substrates due to inhibitory effects on M was the most probable reason for altered NH 4 + immobilization rate in the areca nut plantations (Hao et al, 2020; Zhu et al, 2011). C/N ratio also remained unchanged (Table 1), indicating that the C/N ratio was not the main factor influencing N mineralization and immobilization processes of areca nut plantations in tropical regions.…”

“…The key factor was the decline in inorganic N production, for instance, the decline in M and O Nrec in long‐time areca nut plantations, which is related to agricultural practices, predominantly due to litter management. This decreased the levels of SOC and TN in areca nut plantations, with negative effects on M and O Nrec (Hao et al, 2018; Hao et al, 2020). Meanwhile, long‐term areca nut plantation induced soil acidification that could have also inhibited the gross N mineralization rates, limiting the INS.…”

Higher N Addition and Mowing Interactively Improved Net Primary Productivity by Stimulating Gross Nitrification in a Temperate Steppe of Northern China

“…The various experimental results from different individual studies were reported in grassland ecosystems that had been conducted to explore how N deposition or fertilization affects N turnover processes in grassland ecosystems, especially in soil N turnover (i.e., N ammonification, nitrification, denitrification, and immobilization) and total N pool [ 18 , 19 ]. Previous studies showed that N addition increased, decreased, or had no effects on soil N turnover [ 14 , 20 , 21 , 22 ].…”