Nutrient limitations regulate soil greenhouse gas fluxes from tropical forests: evidence from an ecosystem-scale nutrient manipulation experiment in Uganda

Soil macronutrient availability is one of the abiotic controls that alters the exchange of greenhouse gases (GHGs) between the soil and the atmosphere in tropical forests. However, evidence on the macronutrient regulation of soil GHG fluxes from central African tropical forests is still lacking, limiting our understanding of how these biomes could respond to potential future increases in nitrogen (N) and phosphorus (P) deposition. The aim of this study was to disentangle the regulation effect of soil nutrients on soil GHG fluxes from a Ugandan tropical forest reserve in the context of increasing N and P deposition. Therefore, a large-scale nutrient manipulation experiment (NME), based on 40 m×40 m plots with different nutrient addition treatments (N, P, N + P, and control), was established in the Budongo Central Forest Reserve. Soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes were measured monthly, using permanently installed static chambers, for 14 months. Total soil CO2 fluxes were partitioned into autotrophic and heterotrophic components through a root trenching treatment. In addition, soil temperature, soil water content, and nitrates were measured in parallel to GHG fluxes. N addition (N and N + P) resulted in significantly higher N2O fluxes in the transitory phase (0–28 d after fertilization; p<0.01) because N fertilization likely increased soil N beyond the microbial immobilization and plant nutritional demands, leaving the excess to be nitrified or denitrified. Prolonged N fertilization, however, did not elicit a significant response in background (measured more than 28 d after fertilization) N2O fluxes. P fertilization marginally and significantly increased transitory (p=0.05) and background (p=0.01) CH4 consumption, probably because it enhanced methanotrophic activity. The addition of N and P (N + P) resulted in larger CO2 fluxes in the transitory phase (p=0.01), suggesting a possible co-limitation of both N and P on soil respiration. Heterotrophic (microbial) CO2 effluxes were significantly higher than the autotrophic (root) CO2 effluxes (p<0.01) across all treatment plots, with microbes contributing about two-thirds of the total soil CO2 effluxes. However, neither heterotrophic nor autotrophic respiration significantly differed between treatments. The results from this study suggest that the feedback of tropical forests to the global soil GHG budget could be disproportionately altered by increases in N and P availability over these biomes.

The Effects of Drought and Re-Watering on Non-Structural Carbohydrates of Pinus tabulaeformis Seedlings

Intense and frequent drought events strongly affect plant survival. Non-structural carbohydrates (NSCs) are important “buffers” to maintain plant functions under drought conditions. We conducted a drought manipulation experiment using three-year-old Pinus tabulaeformis Carr. seedlings. The seedlings were first treated under different drought intensities (i.e., no irrigation, severe, and moderate) for 50 days, and then they were re-watered for 25 days to explore the dynamics of NSCs in the leaves, twigs, stems, and roots. The results showed that the no irrigation and severe drought treatments significantly reduced photosynthetic rate by 93.9% and 32.6% for 30 days, respectively, leading to the depletion of the starch storage for hydraulic repair, osmotic adjustment, and plant metabolism. The seedlings under moderate drought condition also exhibited starch storage consumption in leaves and twigs. After re-watering, the reduced photosynthetic rate recovered to the control level within five days in the severe drought group but showed no sign of recovery in the no irrigation group. The seedlings under the severe and moderate drought conditions tended to invest newly fixed C to starch storage and hydraulic repair instead of growth due to the “drought legacy effect”. Our findings suggest the depletion and recovery of starch storage are important strategies for P. tabulaeformis seedlings, and they may play key roles in plant resistance and resilience under environmental stress.

Evaluating the role soil nutrients play in regulating soil greenhouse gas fluxes from the pristine tropical forests: evidence from a nutrient manipulation experiment in Uganda

&lt;p&gt;The exchange of the climate-relevant greenhouse gases (GHGs) at the soil-atmospheric interface is regulated by both abiotic and biotic controls. However, evidence on nutrient limitations of soil GHG fluxes from African tropical forest ecosystems is still rare. Therefore, an ecosystem-scale nutrient manipulation experiment (NME) consisting of nitrogen (N), phosphorus (P), N + P, and control treatments was set up in a tropical forest in northwestern Uganda. Soil carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;), methane (CH&lt;sub&gt;4&lt;/sub&gt;), and nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) fluxes were measured monthly using static vented chambers for 14 months. A root trenching treatment was also done in all the experimental plots in order to disentangle the contribution of root and microbial respiration to total soil CO&lt;sub&gt;2&lt;/sub&gt; effluxes. In parallel to soil GHG flux measurements, soil temperature, soil moisture, and mineral N were determined. Lifting the N limitation on the soil nitrifiers and denitrifiers through N fertilization significantly increased N&lt;sub&gt;2&lt;/sub&gt;O fluxes in N, and N + P addition plots in the transitory phase (0-28 days after N fertilization, p &lt; 0.01). However, sustained N fertilization did not significantly affect background (measured more than 28 days after fertilization) N&lt;sub&gt;2&lt;/sub&gt;O fluxes. Alleviation of the P limitation on soil methanotrophs through P fertilization marginally and significantly increased CH&lt;sub&gt;4&lt;/sub&gt; consumption in the transitory (p = 0.052) and background (p = 0.010) phases, respectively. Simultaneous addition of N and P (N + P) significantly affected transitory soil CO&lt;sub&gt;2&lt;/sub&gt; effluxes (p = 0.010), suggesting a possible co-limitation of N and P on soil respiration. Microbial CO&lt;sub&gt;2&lt;/sub&gt; effluxes were significantly larger than root CO&lt;sub&gt;2&lt;/sub&gt; effluxes (p &lt; 0.001) across all treatment plots so was the contribution of microbial respiration to the total soil CO&lt;sub&gt;2&lt;/sub&gt; effluxes (about 70 %, p &lt; 0.001). Despite the fact that soil respiration was affected through N + P fertilization, neither heterotrophic nor autotrophic respiration significantly differed in either the N + P or the other treatments. Overall, the study findings suggest that the contribution of tropical forests to the global soil GHG budget could be altered by changes in N and P availability in these biomes.&lt;/p&gt;&lt;p&gt;Key words: Soil greenhouse gas fluxes, nutrient manipulation experiment, soil nutrient limitation, and Ugandan tropical pristine forest.&lt;/p&gt;

Nutrient limitations regulate soil greenhouse gas fluxes from tropical forests: evidence from an ecosystem-scale nutrient manipulation experiment in Uganda

Tropical forests contribute significantly to the emission and uptake of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). However, studies on the soil environmental controls of greenhouse gases (GHGs) from African tropical forest ecosystems are still rare. The aim of this study was to disentangle the regulation effect of soil nutrients on soil GHG fluxes in a tropical forest in northwestern Uganda. Therefore, a large-scale nutrient manipulation experiment (NME) based on 40 m × 40 m plots with different nutrient addition treatments (nitrogen (N), phosphorus (P), N + P, and control) was established. Soil CO2, CH4, and N2O fluxes were measured monthly using permanently installed static chambers for 14 months. Total soil CO2 fluxes were partitioned into autotrophic and heterotrophic components through a root trenching treatment. In addition, soil temperature, soil water content, and mineral N were measured in parallel to GHG fluxes. N addition (N, N + P) resulted in significantly higher N2O fluxes in the transitory phase (0–28 days after fertilization, p

Conflict-induced perceptual filtering: A mechanism supporting location-specific control?

Cognitive control can adapt to the level of conflict present in the environment in a proactive (pre-stimulus onset) or reactive (post-stimulus onset) manner. This is evidenced by list-wide and location-specific proportion congruence effects, reduced interference in higher conflict lists or locations, respectively. Proactive control in the flanker task is believed to be supported by a conflict-induced-filtering (CIF) mechanism. The goal of the present set of experiments was to test if CIF also supports reactive location-specific control in the flanker task. To measure CIF, we interspersed a visual search task with a flanker task. After reproducing evidence for CIF using a two-location, list-wide proportion congruence manipulation (Experiment 1), we examined if a similar pattern emerges using a location-specific proportion congruence manipulation in Experiments 2 - 5. We found minimal evidence that reactive location-specific control employs a CIF mechanism. What was clear, however, is that the location-specific proportion congruence effect is susceptible to disruption from an intermixed task that dilutes the location-conflict signal. This highlights the need for alternative approaches to elucidate whether CIF or another mechanism supports reactive, location-specific control.

Disproportionate Changes in the CH4 Emissions of Six Water Table Levels in an Alpine Peatland

The Zoige alpine peatlands are one of the highest and largest alpine peatlands in the world and play an important role in the global carbon cycle. Drainage is the main disturbance at Zoige, and the drawdown of the water table level changes CH4 emissions. There is still much uncertainty relating to how CH4 emissions respond to multiple water table levels. Here, we simulated six gradients (−30 cm, −20 cm, −10 cm, 0 cm, 10 cm, and 20 cm) of the water table level through a mesocosm manipulation experiment in the Zoige peatlands. The water table level had a significant effect on CH4 emissions. CH4 emissions did not change with water table levels from −30 cm to −10 cm, but significantly increased as the water table level increased above −10 cm. A significant log-linear relationship (R2 = 0.44, p < 0.001) was found between CH4 emissions and a water table level range from −10 to 20 cm. This study characterized the responses of CH4 emissions to multiple water table levels and provide additional data for accurately evaluating CH4 emissions. The results of this study also have several conservation implications for alpine peatlands.

Timing outweighs amount of rainfall in shaping population dynamics

Climate variability has been widely documented to have bottom-up effects on the population dynamics of animals1,2, but the mechanisms underlying these effects have been rarely investigated through field manipulative experiments that control for confounding factors3. Here, we examined the effects of different rainfall patterns (i.e. timing and amount) on the population size of Brandt’s voles Lasiopodomys brandtii in semi-arid steppe grassland in Inner-Mongolia by conducting a 10-year (2010-2019) rainfall manipulation experiment in twelve 0.48 ha field enclosures. We found that moderate rainfall increase during the early rather than late growing season drove marked increases in population size through increasing the biomass of preferred plant species, whereas heavily increased rainfall produced no further increase in vole population growth. The increase in vole population size was more coupled with increased reproduction of overwintered voles and increased body mass of young-of-year than with better survival. Our results provide the first experimental evidence for the bottom-up effects of changing rainfall on the population growth of small mammals, and highlight the importance of rainfall timing on the population dynamics of wildlife in the steppe grassland environment.