Abstract
While over-use of N fertilizers can suppress microbial biomass, application of urease inhibitors is known to be a potential way to rebuilt microbial diversity and improve soil functions. However, the hypothesis of this study is that the application of N fertilizers regardless of the source would increase soil microbial biomass and reduce soil respiration. A two-year field experiment was conducted to assess the effects of enhanced-efficiency N sources on soil microbial biomass, and soil respiration. The experiment was set up in a randomized block design in a 3 × 4 + 1 factorial scheme, with four replicates. Treatments comprised three sources (conventional uncoated urea, NBPT (N-(n-butyl) thiophosphoric triamide)-treated urea, and polymer-coated urea) and four rates (30, 60, 90 and 120 kg ha-1) of N, in addition to a control treatment (no fertilizer application). Microbial biomass C (MBC) and microbial biomass N (MBN), and soil respiration (C-CO2 and qCO2) were determined in upland rice rhizosphere in each crop season. Responses of soil microbial properties to N fertilization were dependent on the N rates, but no significant effect of the N sources was observed. All measured parameters, except MBC in the first season and C-CO2 in the second season, were increased with increasing N rates. However, the application of N higher than 60 kg ha-1 suppressed soil microbial biomass, as well as soil respiration.  Therefore, the lack of response by added urease inhibitors to the N sources indicate that optimizing N rates for upland rice production is a far more effective option for improving soil microbial community than using enhanced-efficiency N sources.
Highlights
Nitrogen (N) is one of the most important elements in natural and agricultural ecosystems (Krivtsov et al, 2011)
The enhanced-efficiency N fertilizers had no influence on the microbial soil properties at any growing season of the upland rice crop
No significant interaction effect of N sources and N rates occurred for the microbial parameters
Summary
Nitrogen (N) is one of the most important elements in natural and agricultural ecosystems (Krivtsov et al, 2011). The N addition to agricultural fields can negatively affect microbial communities and restrict crop productivity depending on many factors, such as fertilizer type, N rate, soil pH, soil texture, and the crop (Lupwayi et al, 2012). Previous studies reported that N fertilizers can have no effect or act either positively or negatively on soil microorganisms (Lupwayi et al, 2012; Yu et al, 2018). Li et al (2013) found that microbial activity decreased in soils with increasing NH4+-N concentration under application of ammonium sulfate and urea, suggesting that microbial functional diversity can occur with a N gradient. Zhalnina et al (2015) reported that the application of ammonium-based fertilizers decreased soil pH, reduced microbial abundance because some species cannot tolerate acidic environment. Liu et al (2020) found that the content of microbial biomass C (MBC) and N (MBN) in the rhizosphere soil were increased following fertilizer applications
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