Soil microbial necromass as an essential phosphorus reservoir in forest nutrition

森林生态系统通过利用可及性最低的无机和有机磷形态并将磷保留在生态系统内，成功地应对了低磷供应的挑战。微生物有效地调动了植物无法获得的磷，但也与植物争夺磷。尽管微生物生物和坏死块能占土壤总磷的10%以上，但它们很少被认为是森林磷营养的主要磷贡献者。本研究旨在通过描述森林生态系统中磷分布的空间异质性，并对比获取和再循环森林生态系统中磷的循环，揭示微生物和坏死体作为山毛榉磷营养池的相关性。根据森林土壤中磷的有效性和物种形成，将分析单个微生物群在磷（im）动员、通过淋溶减少磷损失和微生物坏死体磷（Pnecro）再动员中的作用。通过用KH233PO4标记土壤和用灯芯法标记植物，在中生态系统中量化微生物生物量中根沉积物P和土壤溶液磷酸盐的固定化。在模拟降雨事件后，将对溶解有机磷和胶体排放量进行量化表征。通过氯仿熏蒸提取（33P- cfe）和33P磷脂脂肪酸（33P- plfa）分析，确定微生物P保留和微生物群对P固定的影响。为了确定Pnecro的来源，在随后的33P- cfe和33P- plfa孵育实验中，将以低效磷来源（33P磷灰石、fe -33P-磷酸和33P植物残留物）中的微生物P形成为目标。微生物稀释将揭示微生物群落的功能冗余和专业化。在确定Pnecro来源后，将通过应用33p标记的微生物坏死团来评估它们作为森林营养中间磷库的功能。利用异质分布的Pnecro进行的季节性根瘤菌试验，可以通过33P成像和磷酸酶酶谱图可视化山毛榉根际磷的再动员。随着与茎的距离增加，特别是考虑到外生菌根作为另一种Pnecro摄取途径，t形的中胚轴将允许定量测定Pnecro对山毛榉营养的贡献。在中央磷肥施用试验中，PLFA、CFE和磷酸酶活性在与茎的距离增加时进行分析，可以在田间规模上将Pnecro作用的结果转移到单株影响圈。因此，微生物坏死块作为中间磷库的功能及其在森林磷营养中的作用将在拟议的项目中评估，考虑到磷有效性和物种形成的对比。了解森林土壤微生物坏死体磷的空间和季节动态将从根本上提高我们对森林生态系统面对低磷供应能力的认识。

Forest ecosystems successfully meet the challenge of low phosphorous (P) supply by exploiting minimally accessible inorganic and organic P forms and retaining P within ecosystems. Microorganisms efficiently mobilize plant inaccessible P but also compete with plants for P. Although microbial bio- and necromass can account for greater than 10% of total soil P, they rarely were considered as primary P contributors to forest P nutrition. This proposal seeks to unravel the relevance of microbial bio- and necromass as P pools for beech P nutrition by characterizing the spatial heterogeneity of P distribution in forest ecosystems and contrasting P cycling in acquiring versus recycling forest ecosystems. The role of individual microbial groups in P (im)mobilization, reduction of P losses via leaching and re-mobilization of microbial necromass P (Pnecro) will be analyzed depending on P availability and speciation in forest soils.Immobilization of rhizodeposit P and soil solution phosphate in microbial biomass will be quantified in mesocosms by labeling of soils with KH233PO4 and plants via the wick method. After simulating rainfall events, dissolved organic P and colloidal discharge will be quantified and characterized. Microbial P retention and microbial groups accounting for P immobilization will be determined by chloroform fumigation extraction (33P-CFE) and 33P phospholipid fatty acid (33P-PLFA) analysis. To determine sources of Pnecro, microbial P formation from low available P sources (33P apatite, Fe-33P-phosphate and 33P plant residues) will be targeted in a subsequent incubation experiment with 33P-CFE and 33P-PLFA analysis. Microbial dilution will reveal functional redundancy and specialization within microbial communities. Following identification of Pnecro sources, their functions as an intermediate P reservoir for forest nutrition will be assessed by application of 33P-labelled microbial necromass. A seasonally harvested rhizotron experiment with heterogeneously distributed Pnecro will enable visualization of P re-mobilization in beech rhizosphere by 33P imaging and phosphatase zymography. T-shaped mesocosms will allow quantitative determination of Pnecro contribution to beech nutrition with increasing distance to the stem, especially considering ectomycorrhiza as an alternative Pnecro uptake pathway. Within the central P x N application experiment, PLFA, CFE and phosphatase activity analyzed at increasing distance to the stem will allow transferring these results on the role of Pnecro to single tree influence circles at the field scale.Consequently, functions of microbial necromass as an intermediate P reservoir and its role in forest P nutrition will be assessed within the proposed project considering sites of contrasting P availability and speciation. Understanding spatial and seasonal dynamics of microbial necromass P in forest soils will fundamentally enhance our understanding of forest ecosystems capability to face low P supply.