Evaluation of a Resource Allocation Model for Regulation of Litter Decomposition

分解是一个重要的生态系统过程，导致植物生长所需的养分循环利用。大多数现有的分解模型预测的速率间接形成了对区域气候和凋落物化学成分的测量。根据微生物活动的实际水平建立的凋落物腐烂的机制模型仍然缺乏。这降低了我们预测生态系统干扰的影响的能力，例如全球气候变化预期的影响。我们将通过评估一个直接基于参与垃圾分解的酶的微生物产生的分解模型来解决这个问题。我们的模型基于可检验假设，即微生物种群通过优化胞外酶的相对产量来增长，胞外酶从环境中获得必要的碳(C)和氮(N)。我们认为，微生物将根据环境中N化合物的可获得性来改变(C-获得酶的产生)的数量。如果无机N化合物很容易获得，微生物应该将更多的资源分配给生产获得C的酶。这应该会导致更大的微生物生长和更快的垃圾分解。如果无机氮供应不足，微生物应该产生更多的有机氮获取酶，并分配更少的资源来获取C化合物。这应该会减少垃圾的微生物生长和分解速度。为了评估这一模型，我们将在森林土壤地块上放置包含三种类型的树叶凋落物的网袋。其中一些地块将在两个水平上接受氮肥。其他人将不会得到氮肥。这将提供一系列无机氮有效性的变化，用于测试该模型。在分解过程中，将收集垃圾样本并分析微生物生物量和几种酶的活性，这些酶涉及从有机化合物中获得C和N。这些酶的活性将作为微生物资源分配的指标，以获取养分。我们的模型的意义在于，它机械地将N的有效性与分解速率联系起来。如果得到验证，它可能有助于在田间研究中评估营养限制，并为营养循环建模提供新的信息。这些信息还应提高我们预测森林生态系统对大气氮污染输入的反应的能力，目前在农业和城市地区附近的森林中发现了这种水平。

Decomposition is a vital ecosystem process that results in the recycling of nutrients needed for plant growth. Most existing decomposition models predict rates indirectly form measures of regional climate and litter chemical composition. Mechanistic models of litter decay, based on the actual level of microbial activity, are still lacking. This reduces our ability to predict the effects of ecosystem disturbances such as those anticipated form global climate change. We will address this problem by evaluating a decomposition model based directly on microbial production of enzymes involved in litter breakdown. Our model is based o the testable hypothesis that microbial populations grow by optimizing the relative production of extracellular enzymes that obtain essential carbon (C) and nitrogen (N) from their environment. We propose that microbes will change the amount of (C-acquiring enzymes produced depending on the availability of N compounds in the environment. If inorganic N compounds are easily obtained, microbes should allocate more of their resources to producing C-acquiring enzymes. This should result in grater microbial growth and faster decomposition of litter. If inorganic N is in low supply, microbes should produce more organic N- acquiring enzymes and allocate less resources towards acquiring C compounds. This should result in both reduced microbial growth and decomposition rats for litter. To evaluate this model, we will place mesh bags containing three of types of leaf litter on forest soil plots. Some of these plots will receive N fertilizer at two levels of application. Others will not receive N fertilizer. This will provide a range of variation in inorganic N availability over which to test the model. During decomposition, litter samples will be collected and analyzed for microbial biomass and the activities of several enzymes involved obtaining C and N form organic compounds. These enzyme activities will serve as measures of microbial resource allocation to nutrient acquisition. The significance of our model is that it mechanistically links N availability to decomposition rates. If validated, it may facilitate assessments of nutrient limitation in field studies as well as contribute new information for nutrient cycle modeling. Such information should also improve our ability to predict forest ecosystem responses to inputs of atmospheric N pollution at levels now found in forests near agricultural and urban areas.