Abstract
Nitrogen (N) deposition is a key factor that affects terrestrial biogeochemical cycles with a growing trend, especially in the southeast region of China, where shortage of available phosphorus (P) is particularly acute and P has become a major factor limiting plant growth and productivity. Arbuscular mycorrhizal fungi (AMF) establish a mutualistic symbiosis with plants, and play an important role in enhancing plant stress resistance. However, the response of AMF to the combined effects of N deposition and P additions is poorly understood. Thus, in this study, a field experiment was conducted in 10-year Chinese fir forests to estimate the effects of simulated nitrogen (N) deposition (low-N, 30 kg ha−1 year−1 and high-N, 60 kg ha−1 year−1) and phosphorus (P) addition treatments (low-P, 20 mg kg−1 and high-P, 40 mg kg−1) on AMF since April 2017, which was reflected in AMF root colonization rates and spore density of rhizosphere soil. Our results showed that N deposition significantly decreased AMF root colonization rates and spore density. In N-free plots, P addition significantly decreased AMF root colonization rates, but did not significantly alter spore density. In low-N plots, colonization rates significantly decreased under low P addition, but significantly increased under high P addition, and spore density exhibited a significant decline under high P additions. In high-N plots, colonization rates and spore density significantly increased under P additions. Interactive effects of simulated N deposition and P addition on both colonization rates and spore density were significant. Moderate N deposition or P addition can weaken the symbiotic relationship between plants and AMF, significantly reducing AMF colonization rates and inhibiting spore production. However, a moderate addition of P greatly enhances spore yield. In the case of interactive effects, the AMF colonization rates and spore density are affected by the relative content of N and P in the soil.
Highlights
Nitrogen (N) deposition is a key factor that affects terrestrial biogeochemical cycles with a growing trend, especially in the southeast region of China, where shortage of available phosphorus (P) is acute and P has become a major factor limiting plant growth and productivity
In the N (60 kg·ha−1·year−1) treatment (N60) plots, colonization rates increased under N60 + P (20 mg·kg−1) treatment (P20) treatment and decreased under N60 + P (40 mg·kg−1) treatment (P40) treatment
Arbuscular mycorrhizal fungi (AMF) spore density increased significantly under the N60 + P40 treatment, indicating that under adequate soil nutrients conditions, spores are used to store nutrients and promote their continuous growth and reproduction in order to cope with an unpredictable future environment[53,54]. Under both simulated N deposition and P addition treatments, AMF colonization rates were significantly reduced, the symbiotic relationship was weakened due to a lack of C resources needed by AMF from the roots of the fir trees, and spore propagation was inhibited
Summary
Nitrogen (N) deposition is a key factor that affects terrestrial biogeochemical cycles with a growing trend, especially in the southeast region of China, where shortage of available phosphorus (P) is acute and P has become a major factor limiting plant growth and productivity. 5–10% of photosynthetically fixed C is allocated to the fungal p artner[8] Owing to their filamentous organization, fungi exploit diverse substrates on the basis of their nutritional s trategy[9] and significantly contribute to the uptake of soil nutrients, increase plant biomass, and improve the plant resistance to nutrient stress. The availability of P in soil affects the colonization of AMF on plants, and is not conducive to the formation of mycorrhiza if out of range[25] It has been reported by Zhang et al.[26] that AMF mycorrhizal colonization rates, mycelium density, and spore numbers of corn (Zea mays) were significantly higher at the P50 level (50 mg·kg−1) than those without P addition. As the P treatment increased to 200 mg·kg−1 and 500 mg·kg−1, mycorrhizal colonization rates declined
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