Data from: Transformation from NHx to NOy deposition aggravated China’s forest soil acidification
Data files
May 11, 2023 version files 16.05 KB
-
Datasets.xlsx
-
README.md
Abstract
Elevated nitrogen (N) deposition and changes in reduced or oxidized component contribution greatly affect soil acidification. China has experienced a significant transformation of N deposition components from NHx to NOy over the past 40 years, but the effects of component transformation on soil acidification are poorly understood. Therefore, long-term monitoring data and literature on N deposition, combined with the results of isotope experiments, were used to explore the contributions of different N forms on soil acidification in China’s forests. Here, all processes related to NHx and NOy, including the transformation to NH4+ and NO3-, and subsequent N cycling in the soil, were considered. We found that N-induced soil acidification in 80% area of China's forests was dominated by NHx deposition, and the other areas (South China) were dominated by NOy deposition in 2010s. From 1980 to 2019, the average contribution of NHx was higher than that of NOy but the latter contribution continued to increase. Meanwhile, the results showed that soil acidification increased with the decrease of the ratio of NHx to NOy (RNHx/NOy), this is mainly because NOy is more easily leached in the form of NO3- than that of NHx under the influence of different plant preferences and soil retention rates, resulting in a higher net proton production of NOy. Our research has powerful implications for policymaking, provides a theoretical basis for formulating different N reduction policies in different regions, and points out that the synergistic effect of RNHx/NOy changes should be considered to alleviate soil acidification.
Methods
Forests, which account for approximately 17% of the total soil area in China, are distributed between the latitudes of 18.46 °N and 52.97 °N and longitudes of 80.82 °E and 133.60 °E. The average annual temperature and precipitation in forests in China range from 4.3 to 25.1 °C and 158.9 to 2142.2 mm, respectively. There are five main types of forest: evergreen broad-leaf forests, deciduous broad-leaf forests, evergreen coniferous forests, deciduous coniferous forests, and coniferous and broad-leaf mixed forests. Forests in China received 19.80 ~ 41.2 kg N ha−1 yr−1 N deposition with the highest level in North and Central China during the 2010s. Forest soils in China have had a high acidification risk for a long time due to serve acid deposition, and N deposition appears to be the primary driver.
2.2 Quantifying the contribution of NHx and NOy deposition
The net H+ production of different N forms was used to quantify their contribution to soil acidification. Here, we considered all the H+ production and consumption processes induced by N deposition including the transformation process of NHx and NOy into NH4+ and NO3− and cycling process of NH4+ and NO3− in soil. First, protons are consumed and produced when NHx and NOy are transformed into NH4+ and NO3− via protonation and oxidation processes, respectively. The net H+ production through this process was calculated using Eqs. (1) and (2) below:
H+NHx = -NH4+in (1) |
H+NOy = NO3-in (2) |
Then NH4+ and NO3− undergo nitrogen cycling in the soil, accompanied by proton production and consumption. The net H+ production in the soil can also be calculated using Eqs. (3) and (4) [W De Vries and A Breeuwsma, 1987]:
H+NH4+ = NH4+in - NH4+L + NO3-LNH4+ (3) |
H+NO3- = NO3-LNO3- - NO3-in (4) |
Moreover, the total contribution of NHx and NOy is calculated using the sums as shown in Eqs. (5) and (6):
ConNHx = - NH4+L + NO3-LNH4+ (5) |
ConNOy= NO3-LNO3- (6) |
where H+NHx and H+NOy are the net H+ production from NHx protonated and NOy oxidized, respectively. NH4+in and H+NO3- are the inputs of NH4+ and NO3−. NH4+L is the amount of leached NH4+, which is usually so small that it is assumed to be zero. NO3-LNH4+ and NO3-LNO3- are the leached NO3− derived from inputs of NH4+ and NO3− (NH4+ is converted to NO3− by nitrification and then leached), respectively.
1.3 Isotope experimental methods and data sources
To quantify NO3− leaching from NHx and NOy depositions, integrated isotopic experimental methods were used. Nine paired 15N-tracer experiments in China’s forests through data integration were conducted . At all sites, only a few (0.1-0.2 kg15N ha−1 in Tieshanping and 0.25 kg15N ha−1 in the other sites) 15NH4+ or 15NO3− tracers were systematically added to the forest floors. The recovery of 15N was measured and calculated at each site to determine the fates of NH4+ and NO3− (including soil, plants, and loss). A regression model was used to scale up the spatial pattern of N loss. More detailed methods and data quality control are available in G A Gurmesa et al. [2022]. N loss consists of leaching and gaseous losses. We established a relationship between leaching losses and gaseous losses for Chinese forest. Finally, combined with the N deposition and isotope parameters, Eqs. (7) and (8) were used to calculate the contribution of NHx and NOy to soil acidification:
ConNHx = NH4+in * NLossNH4+*Rleach (7) |
ConNOy = NO3-in * NLossNO3−*Rleach (8) |
where NLossNH4+ and NLossNO3- are the loss fractions (15Nloss) for NH4+ and NO3−, respectively. Rleach is the ratio of the leaching loss to the total loss. All units are %. Finally, NH4+in and NO3-in are the input fluxes of NH4+ and NO3−, respectively, in units of kmolc ha−1yr−1.
2.4 Atmospheric nitrogen deposition data and analysis
The spatiotemporal pattern of N deposition data set (NHx and NOy depositions) in China from 1980 to 2019 was previously constructed by our group using network monitoring and data integration (Fig. S4). Network monitoring has been conducted in China since 2013 at 54 sites. Data integration collected 2,599 data points on N deposition from 1980 to 2019 (Fig. S5). And Kriging method was used to interpolate and rasterize the data set. More detailed data set on N deposition can be found in our previous studies [G Yu et al., 2019; J X Zhu et al., 2015]. Linear regression was used to analyze the relationship between the net H+ production per unit N deposition and RNHx/NOy. All analyses were performed using Origin 2018, SPSS 13.0, and ArcGIS 10.4. Differences were considered statistically significant at P<0.05.