Abstract
While the effects of carbon on soil nitrogen (N) cycle have been extensively studied, it is not clearly understood how co-existing macronutrients, such as phosphorus (P), affect the N cycle in agroecosystems. In this study, P amendment effects on nitrification in a fertile agricultural soil were investigated under a typical N-P amendment rate. In a laboratory incubation study, soils were amended with urea, monopotassium phosphate and a mixture of urea and monopotassium phosphate at the same rate. In soils that received no amendments (control), P only, urea only, and urea plus P amendment, nitrification occurred within the first five days, with an average net nitrification rate of 5.30, 5.77, 16.66 and 9.00 mg N kg−1d−1, respectively. Interestingly, nitrification in urea-treated soils was retarded by P addition where a N:P ratio seemed to be a key factor impeding nitrification. This was also supported by the response of ammonia-oxidizing bacteria (AOB), which was more sensitive to P addition than ammonia-oxidizing archaea (AOA). The outcome of this study showed that application of P fertilizer suppressed the nitrification process in urea amended soil, suggesting that a synergistic aspect of N and P nutrient management should be further explored to retard N losses from agricultural systems.
Highlights
Accepted: 31 May 2021Nitrification is an important biological oxidation process in the soil N cycle
The significant differences in nitrification or abundance of ammonia oxidizers between these amendments were not likely due to the changes of soil pH, which can be supported by the correlation analysis between pH and net nitrification rate, and the abundance of bacterial and archaeal amoA gene (Figures 2 and 3, Tables 1 and 4)
This study showed that phosphorus addition stimulated the nitrification of native N via mediating soil NH4 + -N availability directly, whereas it suppressed nitrification indirectly in soil treated with urea and P simultaneously through regulating the soil NH4 + -N/AP ratio associated with the abundance in ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA)
Summary
Accepted: 31 May 2021Nitrification is an important biological oxidation process in the soil N cycle. Ammonia monooxygenase encoded by amoA gene serves as a crucial enzyme to catalyze the reaction of ammonia oxidation [1,2,3], which is the first rate-limiting step in nitrification. Both ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) bearing with ammonia monooxygenase are identified to drive the conversion of ammonia to nitrite [4,5]. The contrasting responses occurring between AOA and AOB may be attributed to their different physiology and metabolic pathways as AOA ecotypes contain urease-encoding genes and the strong affinity of AOA on ammonia facilitates its adaptation
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