Relationships between denitrification gene expression,dissimilatory nitrate reduction to ammonium and nitrous oxide and dinitrogen production in montane grassland soils

The montane grassland soils of Europe store significant amounts of nitrogen (N), and climate change might drive their volatilization due to the stimulation of gaseous nitrous oxide (N₂O) and dinitrogen (N₂) losses. Hence, a thorough, mechanistic understanding of the processes responsible for N loss and retention such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in these soils is urgently needed. Here we aimed to explore the relationships between denitrifier gene abundance and expression with N₂ and N₂O production and the importance of DNRA versus denitrification in nitrate consumption and N₂O production for typical montane grassland soils of Southern Germany. In a laboratory incubation experiment with glucose and nitrate addition, we combined direct measurements of N₂O and N₂ production with a molecular analysis of the denitrifier communities involved in nitrite, nitric oxide (NO) and N₂O reduction and with the quantification of DNRA. The soils originated from a space-for-time climate change experiment, where intact plant-soil mesocosms were exposed for three years either to ambient conditions at a high elevation site (“HE” control treatment) or to predicted climate change conditions (warming, reduced summer precipitation and reduced winter snow cover) by translocation to lower elevation (“LE” climate change treatment).The abundance (DNA) of cnorB genes was significantly reduced in LE soils, whereas the abundance of nosZ genes did not differ between the HE and LE soils. However, the decreased abundance of cnorB genes unexpectedly resulted in slightly increased rather than decreased potential N₂O emissions. This effect could be explained by the increased levels of cnorB mRNA and, therefore, the higher physiological activity of the NO reducers in the LE soils. In contrast with the DNA levels, the dynamics of the cnorB mRNA levels followed N₂O emission patterns, whereas the nosZ expression was strongly correlated with the N₂ emission (R² = 0.83). The potential rates of DNRA were approximately one-third of the rates of denitrification, and DNRA was not a source for N₂O.We conclude that DNRA significantly competes with denitrification in these soils, thus contributing to N conservation. This work demonstrates that the molecular analysis of nosZ gene expression has great potential to contribute to solving the enigmatic problem of understanding N₂ loss from soil.     

Spatial distribution of saplings in heavily worn urban forests: Implications for regeneration and management

We studied the spatial distribution of saplings in the vicinity of other saplings and mature trees in heavily worn urban forests. Our aim was to identify favorable microsites for saplings to regenerate under different levels of wear. We hypothesized that these safe microsites were situated close to tree trunks that might offer shelter from trampling caused by humans and their pet dogs. The distribution of saplings was explored at 0.1–0.6 m to the nearest sapling and 0.1–2 m to the nearest mature tree. Sorbus aucuparia was the most abundant sapling species, followed by Populus tremula, Betula pubescens and Picea abies. These species all tended to cluster with their conspecific saplings and were generally randomly distributed with respect to mature trees. Saplings of S. aucuparia and P. tremula favored growing close to mature P. abies (already at 0.4–0.8 up to 2 m from the trunk base, respectively) and S. aucuparia trees (at 0.2–0.4 m up to 2 m). Betula sp. and Acer platanoides grew close to Pinus sylvestris trees. Furthermore, with increased levels of wear, saplings clustered more likely together and close to tree trunks. The results are contrary to the gap regeneration hypothesis known from rural unworn forests where saplings often grow in canopy gaps. We suggest the idea of a ‘sheltering group’, i.e. tree groups and thickets of densely growing conspecific saplings, for the maintenance of regeneration of saplings and other vegetation in heavily worn recreational forests. Since urban forestry may strongly affect the existence and spatial location of a high variety of microhabitats, small-scale spatial exploration is needed to identify microsites that offer opportunities for natural regeneration under heavy recreational use. To maintain natural regeneration and the survival of saplings in worn urban forests, we recommend microhabitat-level species-specific forest management.     

农田土壤碳氮及其与气象因子的关系

为揭示土壤碳氮受外界环境因子的影响,基于辽宁省51个农业气象站的代表性,分析了土壤有机碳和全氮的剖面分布及其与气象因子、地理因子的关系。结果表明,土壤表层（0~20 cm）有机碳和全氮含量总体高于土壤下层（20~40 cm）;不同土层土壤有机碳和全氮受年降水量、年平均气温的影响程度不同。土壤表层有机碳和全氮受年降水量、年平均气温的显著或极显著影响（P〈0.05,P〈0.01）;土壤下层有机碳和全氮均与年平均气温呈显著负相关（P〈0.01）,而与年降水量不成显著相关。经度与土壤表层有机碳和土壤下层全氮呈显著正相关（P〈0.05）,而纬度和海拔不影响土壤有机碳和全氮。不同土层有机碳、全氮与气象因子的回归方程为农田生态系统的碳收支评估及养分循环提供基础资料。     

辽宁地区近50年无霜期时空演变特征

结合GIS技术、气候倾向率以及突变分析方法,利用近50a辽宁省52个气象观测站的逐日气温资料对无霜期初日、终日以及持续日数的时空演变特征进行分析。结果表明:(1)区域平均无霜期初日以2.2d/10a的速率提前(P0.01),在1989年前后发生突变;终日和持续日数分别以1.9d/10a和3.9d/10a的速率延后和增多(P0.01),终日在1975年前后发生突变。辽宁大部地区无霜期日数以3～4d/10a速率增加(P0.01)。(2)不同基准年的气候平均值(A时段:1961-1990年,B时段:1971-2000年)有明显差异,B时段较A时段初日提前、终日延后、无霜期日数延长。(3)从年代际变化来看,无霜期日数在20世纪70年代的高值和低值区域较60年代有所扩大,80年代高值区再次扩大,低值区缩小,90年代较前者空间格局变化不大,2000年以后高值区已覆盖辽宁大部地区,低值区明显缩小。     

东北玉米根系生物量模型的构建

开展根系生物量的观测和研究,建立通用性的根系生物量模型对于开展生态系统生物量的监测和评估具有重要意义。为得到根系生物量的实时信息,2016年9月末利用挖土法和根系扫描系统,获取玉米根系的生物量及生态指标,分析了玉米根系生物量的垂直分布特征并建立了根系生物量与根系生态指标之间的模拟方程。结果表明：玉米根系生物量主要集中于0~30 cm,占玉米根系垂直分布量的94.44%。利用普通最小二乘法建立根系生物量模型均存在异方差问题,增加根长作为自变量建立的根系生物量模型显著提高了模拟精度,决定系数（R²）达0.91以上。采用对数转换消除方程的异方差及比较不同的模拟方程后发现,玉米根系生物量与根径和根长的组合变量（D²H）建立的指数函数是模拟玉米根系生物量的最优方程,决定系数（R²）最高,为0.90,平均绝对误差（MAE）、估计值的标准误差（SEE）、平均预估误差（MPE）均最小,满足了模拟方程的精度要求。对该方程进一步验证发现,模拟值和实测值之间的相关系数为0.92,说明此模型能较好地模拟根系生物量。利用根系生物量模型结合微根管法,可解决根系生物量实时观测难的问题。     

3月北半球海冰趋势变化的区域特征及影响因子分析

利用海冰密集度资料和海冰范围指数,分析了3月北半球海冰在1979-1995年和1996-2014年2个时间段内的变化趋势以及趋势差异的区域特征.结果表明:北半球3月总海冰范围以稳定速率减少,速率无明显加速.而各个分海域的海冰变化表现出一定的区域差异.其中,巴伦支海海冰在2个时期内以稳定的速率减少;白令海海冰不减反增;鄂霍次克海海冰缩减速度在后期减小;圣劳伦斯湾的海冰范围在90年代中期以前显著增大,后期缓慢缩减. 3月海冰趋势变化的空间特征与同期表面气温和表面风场的趋势变化相一致.     

Historical bank-breachings of the lower Yellow River as influenced by drainage basin factors

The Yellow River is famous for sediment-related disasters in history. The bank-breaching frequency of the lower Yellow River has been adopted as a proxy index to study the historical sediment-related disasters, and then it has been related to various indices describing the natural and human factors of the Yellow River basin. Consequently, some correlations have been established.Climate has profound influences on the bank-breaching frequency of the lower Yellow River. Before the Sui and Tang Dynasties (681–907 AD), natural vegetation in the middle Yellow basin was not yet significantly affected by man. With the increase in the humid index, growth of vegetation became greater and the bank-breaching frequency of lower Yellow River declined. After the 8th Century, however, the relation between bank-breaching frequency and the humid index became inverse, i.e., with the increase in the humid index, bank-breaching frequency also increased. This is because, to a great degree, the natural vegetation had been destroyed since the 10th Century.The bank-breaching frequency can be positively correlated to the frequencies of both large flood and drought disasters. The positive correlation between bank-breaching frequency and frequency of large droughts is because the Yellow River channel decreased in size during dry periods; when large floods came, bank-breaching was more likely to occur.Human activities are the major factor influencing sediment-related disasters in the lower Yellow River. Historically, the increase in population led to a higher bank-breaching frequency, and the periods of low population corresponded to the periods with low frequency of bank-breaching. The increase in the bank-breaching frequency of the lower Yellow River occurred in association with the northward shift of the agriculture–animal husbandry transitional zone, and when the agriculture–animal husbandry transitional zone shifted southward, the bank-breaching frequency declined.     

Influence of balsam poplar tannin fractions on carbon and nitrogen dynamics in Alaskan taiga floodplain soils

The feedbacks between plant and soil processes play an important role in driving forest succession. One poorly understood feedback mechanism is the interaction between plant secondary chemicals and soil microbes. In the Alaskan taiga, changes in nutrient cycling caused by balsam poplar (Populus balsamifera) secondary chemicals may affect the transition from alder (Alnus tenuifolia) to balsam poplar on river floodplains. We examined the effects of four poplar condensed tannin fractions on N cycling in alder and poplar soils. Tannins were added to forest floor samples from both poplar and alder sites. Samples were incubated for 1 month in the laboratory with soil respiration rates measured over the course of the incubation. At the end of the incubation we measured both net and gross nitrogen mineralization and nitrification, microbial biomass C and N, and the activity of various exoenzymes. In all soils, tannin additions reduced N availability, however, the mechanisms differed depending on the molecular weight of the tannin and the native soil microbial community. Low molecular weight tannin fractions served as a labile C source in poplar Oi, poplar Oe, and alder Oe horizons but were toxic to microbes in alder Oi. High molecular weight tannin fractions appeared to act primarily by binding extracellular substrates and thus limiting C and N mineralization, with the strongest effects observed in the alder soils.     

Non-stationary thermal time accumulation reduces the predictability of climate change effects on agriculture

Current modeling studies on the impacts of climate change on agriculture widely assume that thermal time accumulation of crops during the growing season remains constant under various climate conditions. However, in this study, a 20-year single rice variety, experimental dataset indicates that the thermal time accumulation for the entire growing season is not constant. As a result, a crop model based on constant thermal time accumulation significantly underestimates the observed phenological trend exhibited over the two decades of research—despite comparably accurate simulations of short periods. This deviation can result in misleading yield simulations, whereas the model simulations, using observed phenology data, show a similar yield trend as the observation. This study casts serious doubt on the assumptions of constant thermal time accumulation made in previous modeling studies, and, moreover, it highlights the critical requirements needed to improve phenology simulations on a larger scale so that predictions of the eventual yield trends due to climate change can more accurately reflect the results of yield trends in reality.