Maokui Lyu defended at the School of Geographical Sciences, Fujian Normal University, Fuzhou, Fujian, China. His dissertation, entitled “Belowground carbon processes feedback to climate change in tropical and subtropical forests: plant mediation and microbial acclimation” was conducted at Laupāhoehoe in the mean annual temperature (MAT) gradient plots.
Tropical and subtropical forests exert a large influence on the terrestrial carbon (C) balance, but the effects of future increases in temperature on tropical forest C cycling are uncertain. Currently, there is no consensus on how a changing climate will impact C inputs to, C cycling within and C loss from tropical ecosystems, with effects on belowground C processes being particularly understudied. Therefore, it’s crucial to understand how belowground C processes in response to climate change in tropical and subtropical forests. We quantified soil-surface CO2 efflux, aboveground litterfall, and total belowground C flux (TBCF) across a highly constrained 5.2°C mean annual temperature (MAT) gradient in tropical forests in Hawaii U.S.A and a 4.0°C MAT gradient in subtropical forests in Wuyi mountain China to examine how climatic differences affect belowground C fluxes in tropical and subtropical forests, and the mechanisms of soil C cycling in response to rising temperatures by using model MAT gradient and doing soil cores transplant, mycorrhizal exclusion and litterfall manipulation experiments in subtropical forests. There are several important findings as follows:
(1) Rising MAT will increase belowground C fluxes regardless of interannual rainfall patterns; total annual precipitation will strongly modify the response of belowground C fluxes to rising temperatures, with wet years seeing a reduced effect of temperature on belowground C fluxes, whereas dry years see an accentuation of MAT effects on belowground C fluxes. Increasing annual precipitation will depress productivity in tropical montane wet forest ecosystems, making soil respiration in these systems less sensitive to climate warming. In contrast, declines in annual precipitation will increase the productivity of these forests, making soil respiration more sensitive to rising temperatures.
(2) In subtropical forests, soil microbial respiration in high-altitude site is sensitive to simulated climate change, while there is no significant change at low altitudes. Divergent response of soil microbial respiration to climate change across elevation gradient can be partly explained by the difference in soil moisture content. Simulated climate warming increased the sensitivity of soil microbial respiration at high altitudes but decreased the sensitivity of soil microbial respiration at medium and low altitudes. The adaptation of soil fungal communities at different altitudes to climate change and soil nutrients availability are the main factors that driving soil microbial respiration in subtropical montane forests.
(3) Soil fungal communities were significantly changed by mycorrhizal exclusion and rising temperatures in subtropical montane forests. The risk of soil C and nitrogen (N) loss was significantly increased after mycorrhizal isolation at different elevations, mainly because the relative abundance of saprophytes was significantly increased after mycorrhizal isolation, and its open N utilization strategy aggravated the decomposition of soil organic matter. After transplanting high-altitude soil to low-altitude, soil respiration in the treatment without isolated mycorrhiza increased significantly, even greater than that in the new host soil, which mainly caused by the nutrients competition between the original saprophytes in the high-altitude soil and the newly invaded ectomycorrhizal fungi. The effects of climate change and mycorrhizal fungi on soil N availability play a decisive role in soil C cycle. In the future, due to climate change and upward migration of vegetation at different altitudes and elevations, new mycorrhiza invasion may, at least in the short-term, greatly increase soil C and N loss.
(4) Priming effects (PE) induced by high-quality litter might be more sensitive to climate change compared with low-quality litter, as indicated by the PE caused by high-quality litter at warm site were 140% greater than low-quality litter while no significant difference were observed at cool site. The divergent PEs induced by the high- and low-quality litters are mainly regulated by the microbial metabolic efficiency and the investment of litter-derived energy for microbial P-mining rather than N-mining in subtropical forests. Although both high- and low-quality litters could induce positive PEs in high and low altitudes, the new added C could compensate the soil C loss caused by positive PE, resulting in no significantly impact on soil C storage.
(5) Soil respiration in low altitude are more sensitive to changes in quantity of litter than at high altitudes, because litter removal significantly reduces soil respiration at low altitudes but has no significant effect on high altitudes. Increasing litters results in positive PE at high altitude and negative PE at low altitude. We predicate that a future increase in litterfall of 30% with an increase in atmospheric CO2 concentrations of 150 ppm could release about 0.04 t C ha-1 yr-1 from high-altitude forest soil, but could accumulate about 0.28 t C ha-1 yr-1 in low-altitude soil.
Overall, our results strongly suggest that, within the MAT and MAP range studied here, increasing annual precipitation will depress productivity in tropical montane wet forest ecosystems, making soil respiration in these systems less sensitive to climate warming. In contrast, declines in annual precipitation will increase the productivity of these forests, making soil respiration more sensitive to rising temperatures. With both scenarios, our results highlight that effects on soil respiration may be driven largely by productivity effects on the amount of C sent belowground by trees, rather than impacts on the decomposition rates of SOC. In subtropical forests, the response of soil respiration to climate change is mainly driven by microbial acclimation across the elevation gradient. Therefore, climate warming might not increase the CO2 released by microbial respiration as we expected. The rapid adaptation of microorganisms to climate change can mitigate the impact of climate change on the carbon cycle process of forest ecosystems. We speculate that in the future climate warming, the input of litters increased by the increase of atmospheric CO2 concentration may partially offset the effect of soil C decomposition caused by warming, so climate warming would have little effect on soil carbon storage. Consequently, we suggest that, belowground C cycling processes from tropical forest ecosystem could be used to predict the responses of subtropical forest ecosystems to future climate change.
Keywords: Tropical and subtropical forests; Elevation gradient; Soil respiration; Forest productivity; Microbial acclimation; Priming effects; Litterfall; TBCF; Climate change