Soil biogeochemistry across Central and South American tropical dry forests

<b>Abstract</b><br/><p>The availability of nitrogen (N) and phosphorus (P) controls the flow of carbon (C) among plants, soils, and the atmosphere, thereby shaping terrestrial ecosystem responses to global change. Soil C, N, and P cycles are linked by drivers operating at multiple spatial and temporal scales: landscape-level variation in macroclimate, seasonality, and soil geochemistry; stand-scale heterogeneity in forest composition and structure; and microbial community dynamics at the soil pore scale. Yet in many biomes, we do not know at which scales most of the biogeochemical variation emerges, nor which processes drive cross-scale feedbacks. Here, we examined the drivers and spatial/temporal scales of variation in soil biogeochemistry across four tropical dry forests spanning steep gradients of climate, soil parent material, and plant community structure. To do so, we quantified soil C, N, and P pools, extracellular enzyme activities, and microbial community structure across wet and dry seasons in sixteen plots located in Colombia, Costa Rica, Mexico, and Puerto Rico.</p> <p>Soil biogeochemistry exhibited marked heterogeneity across the sixteen plots, with total organic C, N, and P pools varying four-fold, and inorganic nutrient pools by an order of magnitude. Most soil characteristics changed more across space (i.e., among sites and plots) than over time (between dry and wet season samplings). We observed stoichiometric decoupling among C, N, and P cycles, which may reflect their divergent biogeochemical drivers. Organic C and N pool sizes were positively correlated with the relative abundance of ectomycorrhizal trees and legumes. By contrast, the distribution of soil P pools was driven by soil geochemistry, with larger inorganic P pools in soils with P-rich parent material.</p> <p>Most earth system models assume that soils within a texture class operate similarly, and ignore sub-grid cell variation in soil properties. Here we reveal that soil nutrient pools and fluxes exhibit as much variation among four Neotropical dry forests as is observed across terrestrial ecosystems at the global scale. Thus, the biogeochemical patterns we observed across the Neotropical dry forest biome challenge representation of soil processes in ecosystem models.</p>

<b>摘要</b><br/>
<p>氮（N）与磷（P）的有效性调控着植物、土壤与大气之间的碳（C）流动，进而塑造陆地生态系统对全球变化的响应。土壤碳、氮、磷循环由多时空尺度的驱动因子所联结：宏观气候、季节动态与土壤地球化学的景观尺度变异；森林组成与结构的林分尺度异质性；以及土壤孔隙尺度下的微生物群落动态。然而在诸多生物群区中，我们既未明确大部分生物地球化学变异产生的具体尺度，也未能厘清驱动跨尺度反馈的核心过程。本研究聚焦于横跨气候、土壤母质与植物群落结构显著梯度的四个热带干旱森林，探究了土壤生物地球化学变异的驱动因子与时空尺度。为此，研究团队在哥伦比亚、哥斯达黎加、墨西哥与波多黎各的16个样地中，于干湿两季定量测定了土壤碳、氮、磷库、胞外酶活性（extracellular enzyme activities）以及微生物群落结构。</p>
<p>16个样地的土壤生物地球化学特征呈现出显著异质性：总有机碳、氮、磷库的变异幅度达4倍，无机养分库的变异幅度更是达到一个数量级。多数土壤特性在空间尺度（即不同样点与样地间）的变化幅度大于时间尺度（干湿季采样间的差异）。研究团队观测到碳、氮、磷循环之间存在化学计量解耦现象，这或反映了三者各自迥异的生物地球化学驱动机制。有机碳与氮库的规模与外生菌根（ectomycorrhizal）树木和豆科植物的相对丰度呈显著正相关。相较之下，土壤磷库的分布主要受土壤地球化学调控，在富磷母质发育的土壤中，无机磷库规模更大。</p>
<p>多数地球系统模型（earth system models）假设同一质地类别的土壤具有相似的功能，并忽略了土壤属性的亚网格尺度变异。本研究揭示，新热带区（Neotropical）四个干旱森林的土壤养分库与通量的变异程度，堪比全球尺度陆地生态系统间的变异水平。因此，我们在新热带干旱森林生物群区中观测到的生物地球化学模式，对生态系统模型中土壤过程的表征方式提出了挑战。</p>