Atmospheric Deposition Seasonal Patterns
From May 2018 until April 2019, the total rainfall collected by wet deposition (location depicted in Fig. 1) was 1524.6 mm, which represents 99% of the total rainfall reported by Lavras’ weather station for the same period (INMET 2019b). In comparison with climatological values, the sampling period showed a surplus of 63 mm (rainfall positive anomaly) in comparison with climatological values, such differences represent almost 5% above long-term annual average rainfall (1981–2010). Approximately 85% of the rainfall occurred in the wet season (October to March) (Fig. 2a), which is in accordance with the expected pattern of the region. Regarding the atmospheric bulk deposition, the total rainfall collected in the sampling period was 1050.4 mm, which in comparison with Lavras’ weather station represents 68%. In addition, such differences were expected due to wet and bulk deposition collection methods, moreover sample loss by evaporation is inherent to atmospheric deposition sampling. Conversely, we can assume that most of the atmospheric processes were represented by both atmospheric deposition samples collected.
Total Dissolved Nitrogen (TDN)
Deposition flux of TDN ranged from 0.328 to 2.869 kgha− 1month− 1 in June and December, respectively (Fig. 2b). According to the climatological normal for Lavras, December regularly presents the most rainfall and as expected, the highest rainfall amount collected. Alternatively, rainfall in June was 22 times lower than December in 2018. Moreover, TDN fluxes showed significant correlation (p-value < 0.05) with rainfall reported by Lavras weather station in the study period (r = 0.95). Thus, this pattern suggests that monthly variability of TDN deposition was influenced by the rainfall distribution pattern. Similar results were observed in the Northern Indo Gangetic Plain, Nepal (Bhattarai et al. 2021), in the Sichuan province, southwest China (Deng et al. 2018) and in Lagos Lagoon basin, Nigeria (Oladosu et al. 2017), where wet deposition flux of nitrogen increased with the increase in precipitation volume.
We calculated an annual TDN atmospheric deposition of 16.73 kgha− 1yr− 1, which was comparable with the global estimates higher than 8 kgha− 1yr− 1 at sites like eastern North America, Southern Brazil, Europe and Asia (Vet et al. 2014). In this perspective, Souza et al. (2015) (n = 40) reported a similar TDN bulk atmospheric deposition ranging from 12.1 to 17.2 kgha− 1yr− 1 in a forest area and a coastal urban area in the Southeast region of Brazil, where the increasing in the TDN deposition was not related to increasing in the annual rainfall, but due to the proximity of the Rio de Janeiro metropolitan region. In contrast, Araujo et al. (2015) (n = 20) monitored an urban area in the Northeast region of Brazil, with similar annual average precipitation (1500 mm) in comparison with our study region, however the bulk TDN deposition was four times lower (4.32 kgha− 1yr− 1). The results reported in the cited studies suggest that other factors control TDN fluxes besides rainfall, suggesting that agricultural sources are main drivers for nitrogen fluxes in the studied site.
Dissolved Inorganic and Organic Nitrogen (DIN and DON)
In the sampling campaign, NO3− fluxes ranged from 0.011 (August) to 0.831 kgha− 1month− 1 (January), while NH4+ deposition varied from 0.003 kgha− 1month− 1 in May to 0.343 kgha− 1month− 1 in March. The combined deposition flux of DIN (NO3− + NH4+) ranged between 0.016 (August) and 1.074 kgha− 1month− 1 (March), with NO3− contributing more than 56% of DIN throughout the study period. Regarding organic nitrogen, DON deposition fluxes ranged from 0.203 to 2.103 kgha− 1month− 1 in June and December, respectively. In this sense, DON was the predominant nitrogen species throughout the sampling campaign, with a monthly contribution relative to TDN ranging from 50–98%. In addition, same as that of TDN, the monthly deposition fluxes of inorganic and organic nitrogen species were controlled by the precipitation, since NO3−, NH4+ and DON presented strong and significant correlations (p-value < 0.05) with rainfall (r = 0.68, r = 0.79 and 0.87, respectively).
In order to characterize the seasonal variability, monthly deposition fluxes of NO3−, NH4+ and DON were combined according to seasons: the wet season (October – March) and the dry season (April – September). Nitrogen deposition fluxes in the wet season reached values of 3.38, 1.23 and 8.11 kgha− 1 for NO3−, NH4+ and DON respectively, which were 5.15, 6.74 and 2.56 times higher than deposition fluxes of these species in the dry season, revealing a significant distinct seasonal pattern (ANOVA test; p-value < 0.05). It is important to note that the highest deposition fluxes for both inorganic and organic nitrogen species occurred in the months that concentrate the fertilizers application period in coffee crop plantations (October - March) (Freitas 2017), suggesting that agricultural emissions induce strong seasonal variations in the deposition fluxes, driven largely by the seasonality of agricultural operations.
Based on the monthly deposition flux data, we found a strong positive correlation between NH4+ and NO3− (ρ = 0.78; p < 0.05), indicating likely sources, mainly agricultural production, suggesting NH4NO3 formation. Among the inorganic species, NH4+ had a higher rate of increase when comparing the dry and wet seasons (574%), also a higher correlation with rainfall (r = 0.79) than NO3− (r = 0.68). This behavior emphasizes the influence of fertilizer application and rainfall on NH4+ deposition and reveals a slight presence of non-agricultural factors in NO3− deposition, likely fossil fuel combustion sources.
It should be noted that DON is constitutive of a variety of organic compounds, of which many originate from diverse natural and anthropogenic sources (Cape et al. 2011). In this sense, previous studies carried out in agricultural regions indicated that DON in the atmosphere may be related with the application of urea and organic fertilizers as livestock manure (Ham and Tamiya 2006; Zhang et al. 2012). Figure 2b shows DON inputs with the lowest rate of increase across both seasons (156%), and the highest correlation with rainfall (r = 0.87), suggesting that DON is more influenced by rainfall and other mixed sources, such as litterfall and biomass burning. Moreover, the moderate correlations between DON and NH4+ (ρ = 0.67; p < 0.05) and between DON and NO3− (ρ = 0.55; p < 0.10) suggest that DON also may be partially derived from other sources, such as soil dust, biological emissions, biomass burning and industrial emissions. Although the Southern Minas Gerais region could not be described as an industrialized area, Pereira et al. (2021) showed that cement manufacturing and forest fires influenced atmospheric deposition chemistry in the region.
NO3− / NH4+ Ratio & pH
Several studies have been using the NO3−/NH4+ ratio as a reliable proxy for assessing the relative contributions of oxidized and reduced nitrogen species in the atmospheric deposition (Hunová et al. 2017; Xing et al. 2017; Zheng et al. 2020). In our study, the monthly mean values of NO3−/NH4+ ratios varied between 1.36 in April and 33.7 in May, with an annual average of 8.25 (Fig. 3). This pattern reveals monthly NO3−/NH4+ ratios > 1 for the entire study period, thereby indicating that oxidized nitrogen played a more important role for the nitrogen deposition flux. It is noteworthy that pH values were lower in May (pH = 5.75) than April (pH = 6.03), which was expected, since NO3− is well recognized as an acidity proxy, whereas NH4+ is associated with NH3 gas (Seinfeld and Pandis 1998). In addition, despite NO3−/NH4+ ratios > 1, none of the samples presented pH values below the limit of 5.60, a critical value for determining acid rain samples. Thus, all samples were alkaline with an average pH of 5.99 (5.73–6.19).
Aiming to identify seasonal patterns, we calculated an average NO3−/NH4+ ratio of 12.8 and 4.48 in the dry and wet period, respectively. In general, NH4+ can be directly attributed to ammonia (NH3) emissions, mainly due to agricultural activities, such as fertilizer application and livestock production (Zheng et al. 2020), and to a lesser extent due to the combustion of fossil fuels (Liu et al. 2016). Regarding the driving factors of NO3−, although it is linked to high NOx emissions mainly from vehicle exhaust systems in large urban centers (Cui et al. 2014; Wu et al. 2018), recent findings identified soil emissions as a significant origin of high emissions of NOx in agricultural regions (Molina-Herrera et al. 2017; Almaraz et al. 2018). In this sense, the decrease of the NO3−/NH4+ ratio in the wet season implies an increase of NH3 emission in the wet season. In another perspective, the NO3−/NH4+ ratio closer to the unit also shows the importance of NOx emissions from the fertilized soil. This behavior suggests a strong seasonality of agricultural activities in the studied region, due to nitrogen fertilizers application.
Moreover, fertilization coupled with heavy rain and high temperature promote strong volatility of NH3 and NOx, which could lead to volatilization rates of 8% and 11%, respectively (Kurvits and Matta 1998; Calvo-Fernández et al. 2017; Freitas 2017). By contrast, the higher NO3−/NH4+ ratio in the dry season reflects the NOx emissions from anthropogenic sources, as well as the lower temperatures and higher relative humidity contributes to higher formation of nitrate due to gas-particles reactions (Dong et al. 2020). It is valuable to note that NO3−/NH4+ ratios displayed statistically non-significant differences between the dry and wet seasons, which may be associated with high ratio values in January (10.1) and low values in August (2.10). This non-seasonal behavior of nitrogen species could be associated with the negative rainfall anomaly in January (-129 mm) and positive in August (+ 53 mm).
Worldwide Deposition Comparisons
On an annual basis, NO3− and NH4+ deposition fluxes were 4.04 and 1.41 kgha− 1year− 1, accounting for 24% and 8% of the TDN, respectively. The NO3− deposition flux was similar to reported by Souza et al. (2015) in a coastal urban area in Brazil and 2.6 fold lower than reported in urban and agricultural areas in China (Cui et al. 2014; Deng et al. 2018), Table 1. Same as NO3−, the NH4+ deposition flux was lower than reported in agricultural and urban areas in China (Cui et al. 2014; Deng et al. 2018). In addition, it agrees with Ounissi et al. (2021), (Cao et al. 2019) and by Izquieta-Rojano et al. (2016) in Algeria coastal, Japanese forest and Iberian Peninsula urban areas, respectively.
Table 1
Worldwide comparison of nitrogen species (TDN, DON, NO3− and NH4+) in atmospheric deposition in kgha− 1yr− 1
Reference | TDN | DON | NO3− | NH4+ | Sampling Site | Background | Period |
This study | 16.7 | 11.3 | 4.04 | 1.41 | Lavras, Minas Gerais - Brazil | Agricultural | May 2018 to Apr 2019 |
Schroth et al. (2001) | 5.50 | 2.30 | 1.40 | 1.80 | Central Amazonia - Brazil | Forest/Agricultural | Apr 1996 to Mar 1997 |
Souza et al. (2015) | 12.1 | 4.00 | 5.00 | 3.10 | Rio de Janeiro - Brazil | Forest | Aug 2008 to Aug 2009 |
17.2 | 9.90 | 3.90 | 3.40 | Coastal urban area |
Tu et al. (2013) | 114 | 26.9 | 24.9 | 61.9 | Liujiang, Sichuan - China | Forest | Jan to Dec 2009 |
Izquieta-Rojano et al. (2016) | 7.96 | 3.17 | 2.90 | 1.89 | Iberian Peninsula | Urban | Jun to May (2012–2013) |
9.23 | 3.11 | 3.54 | 2.58 | Background |
22.0 | 12.3 | 3.18 | 6.55 | Agricultural |
2.84 | 1.08 | 1.08 | 0.68 | Urban |
Parron et al. (2011) | 12.6 | 3.80 | 1.40 | 7.40 | Brasília in the Federal District - Brazil | Forest | Apr 2011 to May 2002 |
Decina et al. (2018) | 8.88 | 3.65 | 1.22 | 4.01 | Boston, Massachusetts -USA. | Urban | May to Oct 2015 |
Cao et al. (2019) | 11.1 | 7.90 | 1.53 | 1.66 | Mt. Norikura, central Japan | Forest | May 2015 to Apr 2018 |
Deng et al. (2018) | 37.1 | 10.8 | 10.4 | 16.0 | Sichuan Agriculture University Far - China | Urban | Jan 2015 to Dec 2016 |
36.6 | 9.14 | 8.71 | 18.8 | Intensive agricultural area |
27.9 | 7.52 | 7.04 | 13.3 | Agricultural |
Ounissi et al. (2021) | 2.30–3.36 | 0.48–0.89 | 0.94–1.32 | 0.65–1.36 | Annaba - Algeria | Coastal | 2012 to 2017 |
Cui et al. (2014) | 30.5–37.4 | 5.71–10.6 | 7.42–10.4 | 16.3–17.3 | Southeastern China | Agricultural | Jan 2011 to Dec 2012 |
Regarding DON, the annual deposition flux was of 11.3 kgha− 1year− 1, which is similar to areas with agricultural background in the Iberian Peninsula and China (Cui et al. 2014; Izquieta-Rojano et al. 2016; Deng et al. 2018), as well as urban areas in Brazil and China (Souza et al. 2015; Deng et al. 2018). Moreover, DON deposition flux was about 5 fold higher than reported by Schroth et al. (2001) in forest areas. DON represented the major fraction of the TDN (68%), similar to Cao et al. (2019) in a forest area in Japan. Such high relative contributions also were reported in Chinese inland regions (Liu et al. 2016; Wu et al. 2018) and are reasonable due to fertilizer application in croplands.
In general, NH4+ showed the lowest values when comparing with all locations, whereas NO3− levels in the studied region were similar to urban zones and DON has values comparable to urban and agricultural areas. In addition, when comparing all agricultural background locations (Table 1), one can assume that nitrogen deposition in this study is within the levels estimated by other studies, thereby indicating that agricultural regions, especially Lavras, are hotspots of nitrogen deposition.
Ecological Effects of Wet Nitrogen Deposition
Due to TDN values observed in our study, we suggest that nitrogen supply via wet deposition should be considered when calculating optimum nitrogen fertilizer application rates, since the study region has a rural background, mostly agricultural. Moreover, nitrogen species deposition by wet deposition provided 16.73 kgha− 1yr− 1, which is about 15% of the annual nitrogen fertilizer application in the region, suggesting a major input of nitrogen species, specifically in the wet season.
Regarding N-DIN species, the annual value observed (2.011 kgNha− 1yr− 1) via wet deposition was lower than the empirical critical nitrogen load proposed by Pardo et al. (2011) for tropical and subtropical humid forests (5–10 kgNha− 1yr− 1). However, such value does not account for N-DON nitrogen inputs that would surpass such critical range. The excesses of nitrogen deposition over critical loads could lead to environmental impacts in the region, such as soil acidification, which has implications for the chemical availability of metals and other nutrients, influencing plants nutritional status (Stevens et al. 2018).
Excess nitrogen may also cause damage in freshwater aquatic ecosystems (Chen et al. 2018; Clark et al. 2018). Baron et al. (2011) estimated an acidification critical load of 8.0 kgNha− 1yr− 1 and an even lower nutrient enrichment critical load, ranging from 3.5 to 6.0 kgNha− 1yr− 1, for lakes of the northeastern United States. Thus, our results showed that nitrogen atmospheric deposition of N-DIN (2.011 kgNha− 1yr− 1) has the potential to increase acidification and eutrophication in water bodies in the studied region by atmospheric input by 25% and 33%, respectively.