The role of rhizosphere pH in regulating the rhizosphere priming effect and implications for the availability of soil-derived nitrogen to plants

Abstract

Background and aims: A comprehensive understanding of the rhizosphere priming effect (RPE) on the decomposition of soil organic carbon (SOC) requires an integration of many factors. It is unclear how N form-induced change in soil pH affects the RPE and SOC sequestration.

Methods: This study compared the change in the RPE under supply of NO3-N and NH4-N. The effect of the RPE on the mineralization of soil N and hence its availability to plant and microbes was also examined using a 15N-labelled N source.

Key results: The supply of NH4-N decreased rhizosphere pH by 0.16-0.38 units, and resulted in a decreased or negative RPE. In contrast, NO3-N nutrition increased rhizosphere pH by 0.19-0.78 units, and led to a persistently positive RPE. The amounts of rhizosphere-primed C were positively correlated with rhizosphere pH. Rhizosphere pH affected the RPE mainly through influencing microbial biomass, activity and utilization of root exudates, and the availability of SOC to microbes. Furthermore, the amount of rhizosphere primed C correlated negatively with microbial biomass atom% 15N (R2 0.77-0.98, n = 12), suggesting that microbes in the rhizosphere acted as the immediate sink for N released from enhanced SOC decomposition via the RPE.

Conclusion: N form was an important factor affecting the magnitude and direction of the RPE via its effect on rhizosphere pH. Rhizosphere pH needs to be considered in SOC and RPE modelling.

Keywords: 13C natural abundance; 15N; N form; microbial N immobilization; rhizosphere acidification.

Fig. 1.

Soil pH of the no-plant control and the rhizosphere of wheat and white lupin supplied with NO₃-N or NH₄-N at days 42 and 80. Error bars represent ±s.e.m. of three replicates. Different italic letters above the bars indicate significant differences among treatments at each harvest time (Tukey’s test, P < 0.05).

Fig. 2.

Total below-ground CO₂ efflux (A), δ¹³C values of CO₂ (B) and primed soil C (C) under wheat and white lupin fed with NO₃-N and NH₄-N or no-plant control at days 42 and 80. Error bars represent s.e.m. of four replicates. Different italic letters above the bars indicate significant differences among treatments at each harvest time (Tukey’s test, P < 0.05).

Fig. 3.

Relationships between rhizosphere-primed soil C and rhizosphere pH (A) and microbial biomass atom% ¹⁵N for wheat (B) and white lupin (C) fed with NO₃-N and NH₄-N at days 42 and 80. Open and closed circles denote data for wheat and white lupin, respectively. Open and closed triangles denote the data for urea-fed wheat and N₂-fixing white lupin, respectively, from Wang et al. (2016).

Fig. 4.

Atom% ¹⁵N in the rhizosphere inorganic N (A), plant shoots (B) and rhizosphere microbial biomass (C) of wheat and white lupin supplied with NO₃-N or NH₄-N at days 42 and 80. Error bars represent s.e.m. of three replicates. Different italic letters above the bars indicate significant differences among treatments at each harvest time (Tukey’s test, P < 0.05).

Fig. 5.

Conceptual diagram of the effects of N form (NO₃-N vs. NH₄-N)-induced pH changes on the rhizosphere priming effect (the RPE) and involved N immobilization mechanisms.