Enhanced atmospheric phosphorus deposition in Asia and Europe in the past two decades

磷是植物生长必需的元素,磷缺乏将限制植被生产力及碳汇功能.为厘清全球大气磷沉降的时空格局,本文收集了1959-2020年发表的396条观测资料.结果发现,全球大气磷沉降的几何均值为0.32 kg ha~(-1)yr~(-1),全球大气磷库约4.4 Tg yr~(-1).与1959-2000年相比,近20年亚洲和欧洲大气磷沉降呈现上升趋势,主要来源是农业活动,沙尘传输和燃烧源排放.碳中和背景下,清洁空气行动是否会改变磷沉降的途径和形态,进而影响到生态系统的结构和功能,是需要回答的科学问题.

Winter soil respiration during soilfreezing process in a boreal forest in Northeast China

Aims Boreal forest is the largest and contains the most soil carbon among global terrestrial biomes.Soil respiration during the prolonged winter period may play an important role in the carbon cycles in boreal forests.This study aims to explore the characteristics of winter soil respiration in the boreal forest and to show how it is regulated by environmental factors,such as soil temperature,soil moisture and snowpack.Methods Soil respiration in an old-growth larch forest(Larix gmelinii Ruppr.)in Northeast China was intensively measured during the winter soilfreezing process in 2011 using an automated soil CO_(2) flux system.The effects of soil temperature,soil moisture and thin snowpack on soil respiration and its temperature sensitivity were investigated.Important Findings Total soil respiration and heterotrophic respiration both showed a declining trend during the observation period,and no significant difference was found between soil respiration and heterotrophic respiration until the snowpack exceeded 20cm.Soil respiration was exponentially correlated with soil temperature and its temperature sensitivity(Q10 value)for the entire measurement duration was 10.5.Snow depth and soil moisture both showed positive effects on the temperature sensitivity of soil respiration.Based on the change in the Q10 value,we proposed a‘freeze–thaw critical point’hypothesis,which states that the Q10 value above freeze–thaw critical point is much higher than that below it(16.0 vs.3.5),and this was probably regulated by the abrupt change in soil water availability during the soil-freezing process.Our findings suggest interactive effects of multiple environmental factors on winter soil respiration and recommend adopting the freeze–thaw critical point to model soil respiration in a changing winter climate.

Family-level leaf nitrogen and phosphorus stoichiometry of global terrestrial plants

Leaf nitrogen(N) and phosphorus(P) concentrations are critical for photosynthesis, growth, reproduction and other ecological processes of plants. Previous studies on large-scale biogeographic patterns of leaf N and P stoichiometric relationships were mostly conducted using data pooled across taxa, while family/genus-level analyses are rarely reported. Here, we examined global patterns of family-specific leaf N and P stoichiometry using a global data set of 12,716 paired leaf N and P records which includes 204 families, 1,305 genera, and 3,420 species. After determining the minimum size of samples(i.e., 35 records), we analyzed leaf N and P concentrations, N:P ratios and N^P scaling relationships of plants for 62 families with 11,440 records. The numeric values of leaf N and P stoichiometry varied significantly across families and showed diverse trends along gradients of mean annual temperature(MAT) and mean annual precipitation(MAP). The leaf N and P concentrations and N:P ratios of 62 families ranged from 6.11 to 30.30 mg g–1, 0.27 to 2.17 mg g–1, and 10.20 to 35.40, respectively. Approximately 1/3–1/2 of the families(22–35 of 62) showed a decrease in leaf N and P concentrations and N:P ratios with increasing MAT or MAP, while the remainder either did not show a significant trend or presented the opposite pattern. Family-specific leaf N^P scaling exponents did not converge to a certain empirical value, with a range of 0.307–0.991 for 54 out of 62 families which indicated a significant N^P scaling relationship. Our results for the first time revealed large variation in the family-level leaf N and P stoichiometry of global terrestrial plants and that the stoichiometric relationships for at least one-third of the families were not consistent with the global trends reported previously. The numeric values of the family-specific leaf N and P stoichiometry documented in the current study provide critical synthetic parameters for biogeographic modeling and for further studies on the physiological and ecological mechanisms underlying the nutrient use strategies of plants from different phylogenetic taxa.

NEECF: a project of nutrient enrichment experiments in China’s forests

Anthropogenic nitrogen(N)emissions to atmosphere have increased dramatically in China since 1980s,and this increase has aroused great concerns on its ecological impacts on terrestrial ecosystems.Previous studies have showed that terrestrial ecosystems in China are acting as a large carbon(C)sink,but its potential in the future remains largely uncertain.So far little work on the impacts of the N deposition on C sequestration in China’s terrestrial ecosystems has been assessed at a national scale.Aiming to assess and predict how ecological processes especially the C cycling respond to the increasing N deposition in China’s forests,recently researchers from Peking University and their partners have established a manipulation experimental network on the ecological effects of the N deposition:Nutrient Enrichment Experiments in China’s Forests Project(NEECF).The NEECF comprises 10 experiments at 7 sites located from north to south China,covering major zonal forest vegetation in eastern China from boreal forest in Greater Khingan Mountains to tropical forests in Hainan Island.This paper introduces the framework of the NEECF project and its potential policy implications.

Temperature and substrate availability regulate soil respiration in the tropical mountain rainforests,Hainan Island,China

Aims Tropical forest plays a key role in global C cycle;however,there are few studies on the C budget in the tropical rainforests in Asia.This study aims to(i)reveal the seasonal patterns of total soil respiration(R_(T)),litter respiration(R_(L))and soil respiration without surface organic litter(R_(NL))in the primary and secondary Asian tropical mountain rainforests and(ii)quantify the effects of soil temperature,soil moisture and substrate availability on soil respiration.Methods The seasonal dynamics of soil CO_(2) efflux was measured by an automatic chamber system(Li-8100),within the primary and secondary tropical mountain rainforests located at the Jianfengling National Reserve in Hainan Island,China.The litter removal treatment was used to assess the contribution of litter to belowground CO_(2) production.Important Findings The annual R_(T) was higher in the primary forest(16.73±0.87 Mg C ha−1)than in the secondary forest(15.10±0.26 Mg C ha−1).The rates of R_(T),R_(NL) and R_(L) were all significantly higher in the hot and wet season(May–October)than those in the cool and dry season(November–April).Soil temperature at 5cm depth could explain 55–61%of the seasonal variation in R_(T),and the temperature sensitivity index(Q_(10))ranked by R_(L)(Q_(10)=3.39)>R_(T)(2.17)>R_(NL)(1.76)in the primary forest and by R_(L)(4.31)>R_(T)(1.86)>R_(NL)(1.58)in the secondary forest.The contribution of R_(L) to R_(T) was 22–23%,while litter input and R_(T) had 1 month time lag.In addition,the seasonal variation of R_(T) was mainly determined by soil temperature and substrate availability.Our findings suggested that global warming and increased substrate availability are likely to cause considerable losses of soil C in the tropical forests.

ECOLOGICAL EFFECTS OF NITROGEN DEPOSITION ON URBAN FORESTS: AN OVERVIEW

The global urban area is expanding continuously,resulting in unprecedented emissions and deposition of reactive nitrogen(N)in urban environments.However,large knowledge gaps remain in the ecological effects of N deposition on urban forests that provide key ecosystem services for an increasing majority of city dwellers.The current understanding of the spatial patterns and ecological effects of N deposition in urban forests was synthesized based on a literature review of observational and experimental studies.Nitrogen deposition generally increases closer to cities,resulting in an urban hotspot phenomenon.Chemical components of N deposition also shift across urban-suburban-rural gradients,showing higher ratios of ammonium to nitrate in and around urban areas.The ecological effects of N deposition on urban forest ecosystems are overviewed with a special focus on ecosystem N cycling,soil acidification,nutrient imbalances,soil greenhouse gas emissions,tree growth and forest productivity,and plant and soil microbial diversity.The distinct effects of unprecedented N deposition on urban forests are discussed in comparison with the common effects in natural forests.Despite the existing research efforts,several key research needs are highlighted to fill the knowledge gaps in the ecological effects of N deposition on urban forests.