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. 2022 Apr 13;11(8):1057.
doi: 10.3390/plants11081057.

Different Forms and Proportions of Exogenous Nitrogen Promote the Growth of Alfalfa by Increasing Soil Enzyme Activity

Yi Zhao  1 , Yuqiang Wang  1 , Shengnan Sun  1   2 , Wentao Liu  1 , Ling Zhu  1 , Xuebing Yan  1
Affiliations

Different Forms and Proportions of Exogenous Nitrogen Promote the Growth of Alfalfa by Increasing Soil Enzyme Activity

Yi Zhao et al. Plants (Basel). .

Abstract

Nitrogen fertilization is a simple and effective field management strategy for increasing plant productivity, but the regulatory mechanisms of nitrogen forms and proportions on soil nutrients and plant growth remain unclear. Therefore, we investigated soil enzyme activities and nutrient contents of alfalfa under different forms and proportions of exogenous nitrogen addition. Results showed that nitrogen input significantly increased the activity of three oxidoreductases (hydroxylamine reductase, nitrate reductase, and nitrite reductase) while having no significant effects on urease. A high proportion of ammonium nitrogen significantly increased neutral protease activity. The amylase activity markedly increased under mixed-nitrogen addition but decreased under single-nitrogen addition. Additionally, the contents of soil nutrients (soil organic matter, total nitrogen, nitrate nitrogen, ammonium nitrogen, available phosphorus, and available potassium) were significantly increased under different exogenous nitrogen inputs, which drove the changes in enzyme activities. Further, nitrogen addition also improved the biomass and nitrogen content of alfalfa. These findings indicated that applying different forms and proportions of exogenous nitrogen may stimulate soil enzyme activities, which will accelerate the transformation of nutrients and then promote alfalfa growth.

Keywords: ammonium nitrogen; amylase; nitrate nitrogen; nitrite reductase; nitrogen fertilization.

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Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Figure 1
Figure 1
Soil enzyme activities under different forms and proportions of exogenous nitrogen ((A) Urease, (B) Neutral protease, (C) Amylase, (D) Hydroxylamine reductase, (E) Nitrate reductase, (F) Nitrite reductase). Values are means ± standard error (n = 4). Different letters indicate significant differences between different treatment groups at the 0.05 level. N0 (control), N1 (NaNO3), N2 (NH4Cl), N3 (NaNO3:NH4Cl = 7:3), N4 (NaNO3:NH4Cl = 1:1), N5 (NaNO3:NH4Cl = 3:7).
Figure 2
Figure 2
Principal component analysis of soil enzyme activities under different forms and proportions of exogenous nitrogen. N0 (control), N1 (NaNO3), N2 (NH4Cl), N3 (NaNO3:NH4Cl = 7:3), N4 (NaNO3:NH4Cl = 1:1), N5 (NaNO3:NH4Cl = 3:7). UE (urease), NR (nitrate reductase), NiR (nitrite reductase), NPT (neutral protease), AL (amylase), and HR (hydroxylamine reductase).
Figure 3
Figure 3
Principal component analysis of soil physicochemical properties under different forms and proportions of exogenous nitrogen. N0 (control), N1 (NaNO3), N2 (NH4Cl), N3 (NaNO3:NH4Cl = 7:3), N4 (NaNO3:NH4Cl = 1:1), N5 (NaNO3:NH4Cl = 3:7). SOM (soil organic matter), TN (total nitrogen), NN (NO3−-N), AN (NH4+-N), AP (available phosphorus), AK (available potassium).
Figure 4
Figure 4
Results of multiple regression between soil enzyme activities and physicochemical properties ((A) Urease, (B) Neutral protease, (C) Amylase, (D) Nitrate reductase, (E) Nitrite reductase, (F) Hydroxylamine reductase). Each variable was standardized before comparing effect sizes (squares) to determine differences in the strength of predictor variables. Closed squares indicate mean of standardized regression coefficients and lines indicate standard errors. The abbreviations of variables are as follows: UE (urease), NR (nitrate reductase), NiR (nitrite reductase), NPT (neutral protease), AL (amylase) and HR (hydroxylamine reductase), SOM (soil organic matter), TN (total nitrogen), NN (NO3−-N), AN (NH4+-N), AP (available phosphorus), AK (available potassium).
Figure 5
Figure 5
Growth and biomass differences of alfalfa under different forms and proportions of exogenous nitrogen ((A) Plant height, (B) Root length, (C) Aboveground biomass, (D) Belowground biomass). Values are means ± standard error (n = 4). Different letters indicate significant differences between different treatment groups at the 0.05 level. Note: N0 (control), N1 (NaNO3), N2 (NH4Cl), N3 (NaNO3:NH4Cl = 7:3), N4 (NaNO3:NH4Cl = 1:1), N5 (NaNO3:NH4Cl = 3:7).
Figure 6
Figure 6
Nitrogen contents of alfalfa under different forms and proportions of exogenous nitrogen ((A) NH4+-N in aerial part, (B) NO3−-N in aerial part, (C) NH4+-N of root, (D) NO3−-N of root). Values are means ± standard error (n = 4). Different letters indicate significant differences between different treatment groups at the 0.05 level. Note: N0 (control), N1 (NaNO3), N2 (NH4Cl), N3 (NaNO3:NH4Cl = 7:3), N4 (NaNO3:NH4Cl = 1:1), N5 (NaNO3:NH4Cl = 3:7).
Figure 7
Figure 7
Structural equation model of direct and indirect effects of soil nitrate nitrogen and ammonium nitrogen enzyme activities on the biomass of alfalfa. The lines in black mean positive relationship, while those in red color mean negative relationship. The full and dashed lines represent direct and indirect effects, respectively. The width of lines is proportional to the strength of path coefficients. * and ** represent significant differences at the 0.05 and 0.01 level, respectively. This hypothetical model is consistent with our data, consistent with χ2 = 6.508, p = 0.164, GFI = 0.936, CFI = 0.979 values.
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