Abstract
Soil nitrification via ammonia oxidation is a key ecosystem process in terrestrial environments, but little is known of how increasing irrigation of farmland soils with saline waters effects these processes. We investigated the effects of long-term irrigation with saline water on the abundances and community structures of ammonia-oxidizing bacteria (AOB) and archaea (AOA). Irrigation with brackish or saline water increased soil salinity (EC1:5) and NH4-N compared to irrigation with freshwater, while NO3-N, potential nitrification rates (PNR) and amoA gene copy numbers of AOA and AOB decreased markedly under irrigation regimes with saline waters. Moreover, irrigation with brackish water lowered AOA/AOB ratios. PNR was positively correlated with AOA and AOB amoA gene copy numbers across treatments. Saline and brackish water irrigation significantly increased the diversity of AOA, as noted by Shannon index values, while saline water irrigation markedly reduced AOB diversity. In addition, irrigation with brackish or fresh waters resulted in higher proportions of unclassified taxa in the AOB communities. However, irrigation with saline water led to higher proportions of unclassified taxa in the AOA communities along with the Candidatus Nitrosocaldus genus, as compared to soils irrigated with freshwater. AOA community structures were closely associated with soil salinity, NO3−N, and pH, while AOB communities were only significantly associated with NO3−N and pH. These results suggest that salinity was the dominant factor affecting the growth of ammonia-oxidizing microorganisms and community structure. These results can provide a scientific basis for further exploring the response mechanism of ammonia-oxidizing microorganisms and their roles in nitrogen transformation in alluvial grey desert soils of arid areas.
Highlights
Nitrification in soils is a central component of N biogeochemical cycling and involves the oxidation of ammonia (NH+4 ) to nitrite (NO−2 ), along with further oxidation and agricultural impacts on N nutrient availability tfoorniptrlaantets(,NNOO−3 )3.−TNheleparcohciensgs,haansdsiggrneiefinchanotuesnevgiarosensmeemnitsa-l sions[1,2,3]
Increased irrigation water salinity was associated with significant increases (P < 0.001) in the EC1:5, Soil water content (SWC), and NH4-N contents of soils, but significant decreases (P < 0.001) in soil pH, Soil organic carbon (SOC), total nitrogen (TN), and NO3-N contents (Table 1)
These results are consistent with those of Ying et al.[56] wherein ammonia-oxidizing bacteria (AOB) community structure variation was associated with the combined effects of numerous environmental factors including soil pH and N availability, even when gradients were minimal. These findings indicate that pH was likely an important factor associated with changes in the ammonia oxidizing Archaea (AOA) and AOB structure in these alluvial gray desert soils. These results indicate that irrigation with saline or brackish waters significantly reduced soil NO3-N contents and potential nitrification rates (PNR), while soil PNR was significantly and positively correlated with amoA gene copy numbers of AOB and AOA
Summary
Nitrification in soils is a central component of N biogeochemical cycling and involves the oxidation of ammonia (NH+4 ) to nitrite (NO−2 ), along with further oxidation and agricultural impacts on N nutrient availability tfoorniptrlaantets(,NNOO−3 )3.−TNheleparcohciensgs,haansdsiggrneiefinchanotuesnevgiarosensmeemnitsa-l sions[1,2,3]. The extensive development of molecular biology in recent decades has led to an increasing number of studies that have investigated the ecology of AOA and AOB via amoA gene surveys Such studies have evaluated the effects of different environmental factors on AOA and AOB abundances[6,7], in addition to influences on their community structures[8,9], as well as the relative contributions of AOA and AOB to nitrification[10,11]. Previous studies have shown that the inhibition of nitrification increased with increasing soil salt levels[14] and that the abundances of AOA and AOB are negatively correlated with soil salinity[15]. We leveraged field experiments to investigate the effects of saline water irrigation on (i) the abundances and community structures of AOB and AOA, (ii) the relationships between soil properties and the abundances and community structures of AOB and AOA
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