Abstract
We used a seven-year urea gradient applied field experiment to investigate the effects of nitrogen (N) applications on soil N hydrolytic enzyme activity and ammonia-oxidizing microbial abundance in a typical steppe ecosystem in Inner Mongolia. The results showed that N additions inhibited the soil N-related hydrolytic enzyme activities, especially in 392 kg N ha−1 yr−1 treatment. As N additions increased, the amoA gene copy ratios of ammonia-oxidizing archaea (AOA) to ammonia-oxidizing bacteria (AOB) decreased from 1.13 to 0.65. Pearson correlation analysis showed that the AOA gene copies were negatively related with NH4+-N content. However, the AOB gene copies were positively correlated with NO3−-N content. Moderate N application rates (56–224 kg N ha−1 yr−1) accompanied by P additions are beneficial to maintaining the abundance of AOB, as opposed to the inhibition of highest N application rate (392 kg N ha−1 yr−1) on the abundance of AOB. This study suggests that the abundance of AOB and AOA would not decrease unless N applications exceed 224 kg N ha−1 yr−1 in temperate grasslands in Inner Mongolia.
Highlights
Results showed that urea-N additions of up to 150 kg N ha−1 yr−1 led to significant increases in ammonia-oxidizing bacteria (AOB), but not in Ammonia-oxidizing archaea (AOA), which suggested that AOB was more sensitive to N addition than AOA19
Nitrogen additions resulted in significant decreases in soil pH, and increased soil organic carbon (SOC) and inorganic N contents (Fig. 1)
Given the fact that their N addition amount was 100 kg N ha−1 yr−1, which was much less than the N applied in this study, we assumed that the amount of N addition in their study was not enough to stimulate C sequestration in grassland ecosystem
Summary
Ammonia oxidation is thought to be the primary step of nitrification, in which ammonia oxidizers are responsible to limit the subsequent nitrification rate, regulating the balance between ammonium, nitrate, and nitrite This process can affect soil nitrogen availability; and is vital to the nitrogen cycle in terrestrial ecosystems[12]. The main objectives were to (1) explore the effects of N applications on soil N hydrolytic enzyme activities; (2) quantify the abundance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) at different N application levels without limiting P; and (3) find a threshold for N applications that the enzyme activities, AOA and AOB will be changed if the N application is above the threshold
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