Effects of experimental nitrogen additions on plant diversity in an old-growth tropical forest

Response of plant biodiversity to increased availability of nitrogen (N) has been investigated in temperate and boreal forests, which are typically N-limited, but little is known in tropical forests. We examined the effects of artificial N additions on plant diversity (species richness, density and cover) of the understory layer in an N saturated old-growth tropical forest in southern China to test the following hypothesis: N additions decrease plant diversity in N saturated tropical forests primarily from N-mediated changes in soil properties. Experimental additions of N were administered at the following levels from July 2003 to July 2008: no addition (Control); 50 kg N ha−1 yr−1 (Low-N); 100 kg N ha−1 yr−1 (Medium-N), and 150 kg N ha−1 yr−1 (High-N). Results showed that no understory species exhibited positive growth response to any level of N addition during the study period. Although low-to-medium levels of N addition (≤100 kg N ha−1 yr−1) generally did not alter plant diversity through time, high levels of N addition significantly reduced species diversity. This decrease was most closely related to declines within tree seedling and fern functional groups, as well as to significant increases in soil acidity and Al mobility, and decreases in Ca availability and fine-root biomass. This mechanism for loss of biodiversity provides sharp contrast to competition-based mechanisms suggested in studies of understory communities in other forests. Our results suggest that high-N additions can decrease plant diversity in tropical forests, but that this response may vary with rate of N addition

Effects of nitrogen and phosphorus additions on soil microbial biomass and community structure in two reforested tropical forests

Elevated nitrogen (N) deposition may aggravate phosphorus (P) deficiency in forests in the warm humid regions of China. To our knowledge, the interactive effects of long-term N deposition and P availability on soil microorganisms in tropical replanted forests remain unclear. We conducted an N and P manipulation experiment with four treatments: control, N addition (15 g N m(-2).yr(-1)), P addition (15 g P m(-2).yr(-1)), and N and P addition (15 + 15 g N and P m(-2).yr(-1), respectively) in disturbed (planted pine forest with recent harvests of understory vegetation and litter) and rehabilitated (planted with pine, but mixed with broadleaf returning by natural succession) forests in southern China. Nitrogen addition did not significantly affect soil microbial biomass, but significantly decreased the abundance of gram-negative bacteria PLFAs in both forest types. Microbial biomass increased significantly after P addition in the disturbed forest but not in the rehabilitated forest. No interactions between N and P additions on soil microorganisms were observed in either forest type. Our results suggest that microbial growth in replanted forests of southern China may be limited by P rather than by N, and this P limitation may be greater in disturbed forests

Nitrogen input ¹⁵N signatures are reflected in plant ¹⁵N natural abundances in subtropical forests in China

Natural abundance of ¹⁵N (<i>δ</i>¹⁵N) in plants and soils can provide time-integrated information related to nitrogen (N) cycling within ecosystems, but it has not been well tested in warm and humid subtropical forests. In this study, we used ecosystem <i>δ</i>¹⁵N to assess effects of increased N deposition on N cycling in an old-growth broad-leaved forest and a secondary pine forest in a high-N-deposition area in southern China. We measured <i>δ</i>¹⁵N of inorganic N in input and output fluxes under ambient N deposition, and we measured N concentration (%N) and <i>δ</i>¹⁵N of major ecosystem compartments under ambient deposition and after decadal N addition at 50 kg N ha⁻¹yr⁻¹, which has a <i>δ</i>¹⁵N of −0.7 ‰. Our results showed that the total inorganic N in deposition was ¹⁵N-depleted (−10 ‰) mainly due to high input of strongly ¹⁵N-depleted NH₄⁺-N. Plant leaves in both forests were also ¹⁵N-depleted (−4 to −6 ‰). The broad-leaved forest had higher plant and soil %N and was more ¹⁵N-enriched in most ecosystem compartments relative to the pine forest. Nitrogen addition did not significantly affect %N in the broad-leaved forest, indicating that the ecosystem pools are already N-rich. However, %N was marginally increased in pine leaves and significantly increased in understory vegetation in the pine forest. Soil <i>δ</i>¹⁵N was not changed significantly by the N addition in either forest. However, the N addition significantly increased the <i>δ</i>¹⁵N of plants toward the ¹⁵N signature of the added N, indicating incorporation of added N into plants. Thus, plant <i>δ</i>¹⁵N was more sensitive to ecosystem N input manipulation than %N in these subtropical forests. We interpret the depleted <i>δ</i>¹⁵N of plants as an imprint from the high and ¹⁵N-depleted N deposition that may dominate the effects of fractionation that are observed in most warm and humid forests. Fractionation during the steps of N cycling could explain the difference between negative <i>δ</i>¹⁵N in plants and positive <i>δ</i>¹⁵N in soils, and the increase in soil <i>δ</i>¹⁵N with depths. Nevertheless, interpretation of ecosystem <i>δ</i>¹⁵N from high-N-deposition regions needs to include data on the deposition ¹⁵N signal

Effects of nitrogen and phosphorus additions on nitrous oxide emission in a nitrogen-rich and two nitrogen-limited tropical forests

Nitrogen (N) deposition is generally considered to increase soil nitrous oxide (N2O) emission in N-rich forests. In many tropical forests, however, elevated N deposition has caused soil N enrichment and further phosphorus (P) deficiency, and the interaction of N and P to control soil N2O emission remains poorly understood, particularly in forests with different soil N status. In this study, we examined the effects of N and P additions on soil N2O emission in an N-rich old-growth forest and two N-limited younger forests (a mixed and a pine forest) in southern China to test the following hypotheses: (1) soil N2O emission is the highest in old-growth forest due to the N-rich soil; (2) N addition increases N2O emission more in the old-growth forest than in the two younger forests; (3) P addition decreases N2O emission more in the old-growth forest than in the two younger forests; and (4) P addition alleviates the stimulation of N2O emission by N addition. The following four treatments were established in each forest: Control, N addition (150 kg N ha(-1) yr(-1)), P addition (150 kg P ha(-1) yr(-1)), and NP addition (150 kg N ha(-1) yr(-1) plus 150 kg P ha(-1) yr(-1)). From February 2007 to October 2009, monthly quantification of soil N2O emission was performed using static chamber and gas chromatography techniques. Mean N2O emission was shown to be significantly higher in the old-growth forest (13.9 +/- 0.7 mu g N2O-N m(-2) h(-1)) than in the mixed (9.9 +/- 0.4 mu g N2O-N m(-2) h(-1)) or pine (10.8 +/- 0.5 mu g N2O-N m(-2) h(-1)) forests, with no significant difference between the latter two. N addition significantly increased N2O emission in the old-growth forest but not in the two younger forests. However, both P and NP addition had no significant effect on N2O emission in all three forests, suggesting that P addition alleviated the stimulation of N2O emission by N addition in the old-growth forest. Although P fertilization may alleviate the stimulated effects of atmospheric N deposition on N2O emission in N-rich forests, this effect may only occur under high N deposition and/or long-term P addition, and we suggest future investigations to definitively assess this management strategy and the importance of P in regulating N cycles from regional to global scales

Averting biodiversity collapse in tropical forest protected areas

The rapid disruption of tropical forests probably imperils global biodiversity more than any other contemporary phenomenon¹⁻³. With deforestation advancing quickly, protected areas are increasingly becoming final refuges for threatened species and natural ecosystem processes. However, many protected areas in the tropics are themselves vulnerable to human encroachment and other environmental stresses⁴⁻⁹. As pressures mount, it is vital to know whether existing reserves can sustain their biodiversity. A critical constraint in addressing this question has been that data describing a broad array of biodiversity groups have been unavailable for a sufficiently large and representative sample of reserves. Here we present a uniquely comprehensive data set on changes over the past 20 to 30 years in 31 functional groups of species and 21 potential drivers of environmental change, for 60 protected areas stratified across the world’s major tropical regions. Our analysis reveals great variation in reserve ‘health’: about half of all reserves have been effective or performed passably, but the rest are experiencing an erosion of biodiversity that is often alarmingly widespread taxonomically and functionally. Habitat disruption, hunting and forest-product exploitation were the strongest predictors of declining reserve health. Crucially, environmental changes immediately outside reserves seemed nearly as important as those inside in determining their ecological fate, with changes inside reserves strongly mirroring those occurring around them. These findings suggest that tropical protected areas are often intimately linked ecologically to their surrounding habitats, and that a failure to stem broad-scale loss and degradation of such habitats could sharply increase the likelihood of serious biodiversity declines.Keywords: Ecology, Environmental scienc

Carbon Footprint Analysis of Tourism Life Cycle: The Case of Guilin from 2011 to 2022

Low-carbon tourism is an important way for the tourism industry to achieve the United Nations Sustainable Development Goals and the goals of carbon peaking and carbon neutrality. In order to promote the development of Guilin as a world-class tourism city and ensure the sustainable development of the tourism industry in Guilin, this paper combines the concept of carbon footprint and the theory of life cycle to build a tourists’ carbon footprint life cycle analysis model of Guilin. Taking tourists in Guilin as an example, the composition and changes of tourists’ carbon footprint are dynamically analyzed. The research shows that: (1) The overall tourism carbon footprint of Guilin showed an upward trend during 2011–2019. From 2020 to 2022, due to the impact of COVID-19, Guilin’s tourism carbon footprint has decreased significantly. The per capita carbon footprint of tourism in Guilin showed a downward trend from 2011 to 2022; (2) The order of the size of Guilin’s tourism carbon footprint is tourism transportation > tourism catering > tourism accommodation > tourism activities; (3) From 2011 to 2022, the carbon footprint of tourism transportation in Guilin showed an obvious narrowing state, while the carbon footprint of tourism accommodation, tourism activities, and tourism catering showed an obvious expanding trend. Based on the characteristics of the carbon footprint of Guilin’s tourism and the current situation of the development of Guilin’s tourism, this paper puts forward suggestions on reducing carbon emissions, forms a new tool for evaluating and constructing low-carbon tourism, and provides a scientific basis and practical reference significance for the sustainable development of low-carbon tourism in Guilin