Abstract. Land use is known to exert a dominant impact on a range of essential soil functions like water retention, carbon sequestration, matter cycling and plant growth. At the same time, land use management is known to have a strong influence on soil structure, e.g. through bioturbation, tillage and compaction. However, it is often unclear whether differences in soil structure are the actual cause for differences in soil functions or just co-occurring. This impact of land use (conventional and organic farming, intensive and extensive meadow, extensive pasture) on the relationship between soil structure and short-term carbon mineralization was investigated at the Global Change Exploratory Facility, in Bad Lauchstädt, Germany. Intact topsoil cores (n = 75) were sampled from each land use type at the early growing season. Soil structure and microbial activity were measured using X-ray computed tomography and respirometry, respectively. Grasslands had a greater microbial activity than croplands, both in terms of basal respiration and rate of carbon mineralization during growth. This was caused by a larger amount of particulate organic matter (POM) in the topsoil of grasslands. The frequently postulated dependency of basal respiration on soil moisture was absent even though some cores were apparently water limited. This finding was related to microenvironments shaping microbial hotspots where the decomposition of plant residues was obviously decoupled from water limitation in bulk soil. Differences in microstructural properties between land uses were surprisingly small, mainly due huge variability induced by patterns of compacted clods and loose areas caused by tillage in cropland soils. The most striking difference was larger macropore diameters in grasslands soil due to the presence of large biopores that are periodically destroyed in croplands. Variability of basal respiration among all soil cores amounted to more than one order of magnitude (0.08–1.42 µg CO₂-C h⁻¹ g⁻¹ soil) and was best described by POM mass (R² = 0.53, p < 0.001). Predictive power was hardly improved by considering all bulk, microstructure and microbial properties jointly. The predictive power of image-derived microstructural properties was low, because aeration was not limiting carbon mineralization and was sustained by pores smaller than the image resolution limit (< 30 µm). The rate of glucose mineralization during growth was explained well by substrate-induced respiration (R² = 0.84) prior to growth, which was in turn correlated with total microbial biomass, basal respiration and POM mass and again not affected by pore metrics. These findings stress that soil structure had little relevance in predicting carbon mineralization in well-aerated soil, as this predominantly took place in microbial hotspots around degrading POM that was detached from the pore structure and moisture of the bulk soil. Land use therefore affects carbon mineralization in well-aerated soil mainly by the amount and quality of labile carbon.

中文翻译：

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土地利用对碳矿化的影响主要是由颗粒有机质含量的变化而不是土壤结构引起的

摘要。众所周知，土地利用对一系列基本土壤功能产生主要影响，如保水、固碳、物质循环和植物生长。同时，众所周知，土地利用管理对土壤结构有很大影响，例如通过生物扰动、耕作和压实。然而，通常不清楚土壤结构的差异是土壤功能差异的真正原因还是共同发生。位于德国巴特劳赫施塔特的全球变化探索设施调查了土地利用（传统和有机农业、集约化和粗放草地、粗放牧场）对土壤结构和短期碳矿化之间关系的影响。在早期生长季节从每种土地利用类型中采样完整的表土核心（n = 75）。分别使用 X 射线计算机断层扫描和呼吸测量法测量土壤结构和微生物活动。就基础呼吸和生长过程中的碳矿化速率而言，草地比农田具有更大的微生物活性。这是由草地表层土壤中大量的颗粒有机物（POM）引起的。尽管一些核心显然受水限制，但不存在经常假设的基础呼吸对土壤水分的依赖性。这一发现与塑造微生物热点的微环境有关，其中植物残留物的分解与大块土壤中的水分限制明显脱钩。土地利用之间微观结构特性的差异出奇地小，主要是由于耕地土壤中耕作造成的压实土块和松散区域的模式引起的巨大变异。最显着的差异是草原土壤中较大的大孔直径，这是由于农田中存在周期性破坏的大生物孔。所有土壤核心之间基础呼吸的变化达一个数量级以上（0.08–1.42 µg CO₂ -C h ^-1  g ^-1 土壤），最好用 POM 质量描述（R ²  = 0.53，p < 0.001）。综合考虑所有的体积、微观结构和微生物特性，预测能力几乎没有提高。图像衍生的微观结构特性的预测能力很低，因为曝气不限制碳矿化，而是由小于图像分辨率限制 (< 30 µm) 的孔隙维持。生长过程中葡萄糖矿化的速率可以用底物诱导的呼吸作用很好地解释（R ² = 0.84) 在生长之前，这又与总微生物生物量、基础呼吸和 POM 质量相关，并且同样不受孔隙指标的影响。这些发现强调土壤结构与预测通气良好的土壤中的碳矿化几乎没有相关性，因为这主要发生在降解 POM 周围的微生物热点中，该热点与大块土壤的孔隙结构和水分分离。因此，土地利用主要通过不稳定碳的数量和质量来影响通气良好的土壤中的碳矿化。