Rhizosphere priming effects differ between two tree species, Norway spruce (Picea abies) and Scots pine (Pinus sylvestris), under two levels of light intensity

Abstract

Rhizosphere priming effect (RPE) is a vital progress to regulate soil organic matter decomposition which would influence the carbon cycle in the environment. In my study, Norway spruce and Scots pine were considered to have different RPE because their photosynthetic intensity are different. Spruce had better adaptability of light intensity that it has lower light saturation point (LSP) and lower light compensation point (LCP) than pine. To test whether RPE is different in spruce and pine and the effect of light intensity, I used a 13CO2 pulse-chase experiment which could test for SOM decomposition and the 15N pool-dilution method to test N assimilation. I conclude that the priming effect was significant different between spruce and pine. Spruce had positive priming effect while pine had negative priming effect. Three possible theories, preferential substrate utilization, the plant microbial competition and soil moisture were considered to explain why negative priming effect occurred in the soil with pine seedlings instead of in the soil with spruce seedlings. At last, plant microbial competition was proved to be the main reason for the negative priming effect by the results. It also has been confirmed that the RPE would be affected by different levels of light intensity in this study. In contrast to the second hypothesis, spruce had higher priming effect when kept under low light intensity, which could be explained by the higher root growth and the higher root/shoot ratio in the pots in this treatment. In addition, the abundance of ectomycorrhizal fungi was suspected to be the reason why stronger negative priming effect of soil organic matter (SOM) decomposition was found in the low light treatment, but the priming effect of N mineralization did not response to the assumptions.

Popular Abstract

Climate change is a serious problem in the past decades. Rhizosphere priming effect (RPE) is a vital progress to regulate soil organic matter (SOM) decomposition which would influence the carbon cycle in the environment. Many factors would be considered to affect the RPE, but few researchers studied that the RPE varies in plant species. In my study, Norway spruce and Scots pine were considered to have different RPE because they have different photosynthetic intensity. Also, under better light intensity, both roots and shoots could grow better and thus I also assumed that under high light intensity plants would have higher RPE.

A 13CO2 pulse-chase experiment was used to analyze root exudates and the 15N-pool dilution method was used to estimate SOM decomposition, gross N transformation rates and priming. I used the 13CO2 and 15NH4Cl to label the plants. The following lab work would estimate the content of different C and N as I can track the change of 13C and 15N to understand plant growth and microbial activities in the soil. The RPE of soil planted with pine seedlings was negative which were -9.2% (under high light intensity) and -19.5% (under low light intensity). However, soil planted with spruce had positive priming effect which were 11.3% under high light intensity and 44.5% under low light intensity.

I assumed three theories to explain why negative RPE was observed in the soil with pine rather than in the soil with spruce. The first one was called preferential substrate utilization which means when the nutrient availability was high in the soil, microorganisms could use labile root exudates instead of decomposing SOM. The second one was called the plant-microbial competition theory which means when the nutrient availability was low, plants might compete. In my study, based on the results of gross N mineralization and concentration of inorganic N, the nutrient availability was found to be lower in the soil with pine. So the preferential substrate utilization theory was excluded. The last one is about soil moisture. Although when the soil had more water content, the decomposition of SOM increased, the difference in soil water content between cut and intact seedlings was smaller for pine than for spruce. This result was not consistent with the result of SOM decomposition that the difference in SOM decomposition in soil with intact and cut plants was greater for pine than for spruce. Thus, the plant-microbial competition theory might be the most plausible explanation to the observed differences in RPE of the two species.

In contrast to the hypothesis, seedlings under low light intensity had stronger priming effect which might also be caused growth of roots and ectomycorrhizal fungi. Then I found in the low light condition, both of the root biomass of spruce and the root/shoot ratio were higher. I guess that in the short time period of this experiment and at this stage of seedling development, the nutrient uptake by plants might preferably be used for the growth of shoots rather than the growth of roots. Thus, the higher RPE was observed in the soil with spruce under low light intensity might because there were more roots in the soil under low light than under high light intensity. I also suspected that there were more ectomycorrhizal fungi in the soil with pine under low light intensity. But the results of the RPE of SOM decomposition and the RPE of N mineralization were opposite. Thus, the reason why the RPE of SOM decomposition was stronger in the soil with pine under low light intensity is still unknown.


Master’s Degree Project in Biology, 45 credits, 2018
Department of Biology, Lund University

Advisor: Per Bengtson
Department of Biology

• author: Zhou, Moyan
• supervisor: Per Bengtson
• organization: Degree Projects in Biology
• course: BION02 20172
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8960259
• date added to LUP: 2018-10-15 13:51:24
• date last changed: 2018-10-15 13:51:24