Nitrogen additions inhibit nitrification in acidic soils in a subtropical pine plantation: effects of soil pH and compositional shifts in microbial groups

Plantation forests play a pivotal role in carbon sequestration in terrestrial ecosystems, but enhanced nitrogen(N) deposition in these forests may affect plantation productivity by altering soil N cycling. Hence,understanding how simulated N deposition affects the rate and direction of soil N transformation is critically important in predicting responses of plantation productivity in the context of N loading. This study reports the effects of N addition rate(0, 40, and 120 kg N ha^(-1) a^(-1)) and form(NH_4Cl vs. NaNO_3) on net N mineralization and nitrification estimated by in situ soil core incubation and on-soil microbial biomass determined by the phospholipid fatty acid(PLFA) method in a subtropical pine plantation. N additions had no influences on net N mineralization throughout the year. Net nitrification rate was significantly reduced by additions of both NH_4Cl(71.5) and NaNO_3(47.1%) during the active growing season, with the stronger inhibitory effect at high N rates. Soil pH was markedly decreased by 0.16 units by NH_4Cl additions. N inputs significantly decreased the ratio of fungal-to-bacterial PLFAs on average by 0.28(49.1%) in November. Under NH_4Cl additions, nitrification was positively related with fungal biomass and soil pH. Under NaNO_3 additions,nitrification was positively related with all microbial groups except for bacterial biomass. We conclude that simulated N deposition inhibited net nitrification in the acidic soils of a subtropical plantation forest in China,primarily due to accelerated soil acidification and compositional shifts in microbial functional groups. These findings may facilitate a better mechanistic understanding of soil N cycling in the context of N loading.

Litter mixing significantly affects decomposition in the Hulun Buir meadow steppe of Inner Mongolia, China

Aims We explored the decomposition rates of single-and mixed-species litter,the litter-mixing effect and the effect of component litters in a mixture on decomposition.Methods In a litter bag experiment,shoot litters from two dominant grasses(Leymus chinensis and Stipa baicalensis)and one legume(Melissitus ruthenica)were decomposed separately and as a mixture from May 2010 to September 2011 in the Hulun Buir meadow steppe of Inner Mongolia,China.We separated the litter mixture into its individual component litters(i.e.the different single-species litters)and analyzed the changes in litter mass remaining and litter nitrogen(N)remaining during single-and mixed-species litter decomposition.Important Findings(i)Litter mixing had significant positive effects on litter decomposition.The litter-mixing effect was strongest for the mixture of S.baicalensis and L.chinensis litters,followed by the mixture of S.baicalensis and M.ruthenica litters.(ii)Single-species component litters decomposed faster in the mixtures than separately(positive effect),but these effects were not significant for legume species M.ruthenica litter.Relative to single-species litter decomposition,the decomposition rates of the two grass(S.baicalensis and L.chinensis)litters significantly increased when they were mixed with each other or with M.ruthenica litter.(iii)For each species litter type,the percentage of litter N remaining during decomposition(NR)differed between the single-species litter and mixed litter treatments.The NR of S.baicalensis litter was higher when it was decomposed in the mixture than in isolation.However,the NR of L.chinensis litter was lowest in its mixture with M.ruthenica among the treatments.Regardless of its decomposition in the mixture or in isolation,the NR of M.ruthenica litter varied little among treatments.There was a significant positive relationship between the NR and percentage of initial litter mass remaining in both the single litter and mixed litter treatments.These results suggest that N transfer may happen among component litters in mixture and further affect the decomposition.

Energy partitioning and environmental influence factors in different vegetation types in the GEWEX Asian Monsoon Experiment

Environmental influences upon energy balance in areas of different vegetation types （i.e., forest at Kog-Ma in Thailand and at Yakutsk in Russia, grassland at Amdo in Chinese Tibet and at Arvaikheer in Mongolia, and mixed farmland at Tak in Thailand） in the GEWEX Asian Monsoon Experiment were investigated. The sites we investigated are geographically and climatologically different; and consequently had quite large variations in temperature （T）, water vapor pressure deficit （VPD）, soil moisture （SM）, and precipitation （PPT）. During May- October, the net radiation flux （Rn） （in W·m^-2） was 406.21 at Tak, 365.57 at Kog-Ma, 390.97 at Amdo, 316.65 at Arvaikheer, and 287.10 at Yakutsk. During the growing period, the Rn partitioned into latent heat flux （2E/Rn） was greater than that partitioned into sensible heat flux （H/Rn） at Tak and at Kog-Ma. In contrast, 2E/Rn was lower than H/Rn at Arvaikheer, H/Rn was less than 2E/Rn between DOY 149 and DOY 270 at Amdo, and between DOY 165 and DOY 235 at Yakutsk. The R, partitioned into ground heat flux was generally less than 0.15. The short-wave albedo was 0.12, 0.18, and 0.20 at the forest, mixed land, and grass sites, respectively. At an hourly scale, energy partitions had no correlation with environmental factors, based on average summer half- hourly values. At a seasonal scale energy partitions were linearly correlated （usually p 〈 0.05） with T, VPD, and SM. The 2E/Rn increased with increases in SM, T, and VPD at forest areas. At mixed farmlands, ）λE/Rn generally had positive correlations with SM, T, and VPD, but was restrained at extremely high values of VPD and T. At grasslands, λE/Rn was enhanced with increases of SM and T, but was decreased with VPD.

基于碳-水-氮耦合过程改进模型的温带草地生态系统生产力模拟研究

预测气候变化背景下生态系统总初级生产力的响应是全球变化生态学研究领域的一项核心任务。然而,对模型研究领域来说,准确模拟干旱生态系统总初级生产力的年际变异仍然是一个巨大的挑战。土壤含水量和总初级生产力对土壤水敏感性的精确模拟,是预测干旱生态系统中总初级生产力年际变异的两个关键方面。为此,本研究以一个广泛应用的生态系统模型(Biome-BGC模型)为例,旨在改进温带草地生态系统的模型模拟效果。一方面,通过对蒸散模块、土壤水沿剖面的垂直分布和田间持水量计算的改进和调整,模型实现了对土壤水模拟的更新。另一方面,我们改进了影响水-氮关系的函数,从而调节了总初级生产力对土壤水的敏感性。研究结果表明,原有模型高估了土壤含水量,低估了总初级生产力敏感性的年际变异,从而导致模拟总初级生产力的年际变异低于观测值。例如,原模型严重低估了总初级生产力在干旱年份的减少。相比之下,改进后的模型准确地模拟了观测土壤水的季节和年际变化,特别是表层土壤水。通过优化影响氮矿化的参数,改进后的模型改善了总初级生产力对土壤水敏感性的模拟,使其更接近观测值。因此,改进后模型对总初级生产力年际变异的模拟得到了很大程度的提高。我们的结果表明,在对干旱生态系统总初级生产力年际变异进行模拟时,应优先考虑表层土壤水及其对氮有效性的影响。