Historical land management alters new soil carbon inputs by annual and perennial bioenergy crops

Bioenergy and bioproduct markets are expanding to meet demand for climate friendly goods and services. Perennial biomass crops are particularly well suited for this goal because of their high yields, low input requirements, and potential to increase soil carbon (C). However, it is unclear how much C is allocated into belowground pools by perennial bioenergy crops and whether the belowground benefits vary with nitrogen (N) fertilizer inputs. Using in situ 13C pulse-chase labeling, we tested whether the sterile perennial grass Miscanthus × giganteus (miscanthus) or annual maize transfers more photosynthetic C to belowground pools. The experiment took place at two sites in Central and Northwest (NW) Iowa with different management histories and two nitrogen (N) fertilizer rates (0 and 224 kg N ha-1 yr-1) to determine if the fate of plant-derived soil C depends on soil fertility and crop type (perennial or annual). Maize allocated a greater percentage of total new 13C to roots than miscanthus, but miscanthus had greater new 13C in total and belowground plant biomass. We found strong interactions between site and most soil measurements – including new 13C in mineral and particulate soil organic matter (SOM) pools –which appear to be driven by differences in historical fertilizer management. The NW Iowa site, with a history of manure inputs, had greater plant-available nutrients (phosphorus, potassium, and ammonium) in soils, and resulted in less 13C from miscanthus in SOM pools compared to maize (approximately 64% less in POM and 70% less in MAOM). In more nutrient-limited soils (Central site), miscanthus transferred 4.5 times more 13C than maize to the more stable mineral-associated SOM pool. Our results suggest that past management, including historical manure inputs that affect a site’s soil fertility, can influence the net C benefits of bioenergy crops.

为满足气候友好型商品与服务的市场需求，生物能源与生物制品产业正持续扩张。多年生生物质作物因兼具高产、低投入以及提升土壤碳（C）储量的潜力，尤其契合这一发展目标。然而，目前学界尚不明确多年生生物能源作物向地下碳库分配的光合碳量究竟有多少，且地下碳库增益是否会随氮肥（N）施用量发生变化。本研究采用原位13C脉冲追踪标记法（in situ 13C pulse-chase labeling），对比了无菌多年生草本芒草（Miscanthus × giganteus）与一年生玉米向地下碳库分配光合碳的差异。试验设置于美国爱荷华州中部与西北部（NW）的两个试验点，两地土壤管理历史存在显著差异，并设置两种氮肥施用量（0与224 kg N ha⁻¹ yr⁻¹），旨在明确植物源土壤碳的归趋是否取决于土壤肥力与作物类型（多年生或一年生）。研究结果显示，玉米分配至根系的新形成13C占总新13C的比例高于芒草，但芒草的总植株生物量与地下植株生物量中的新13C总量更高。我们发现试验点与多数土壤指标（包括矿物结合态与颗粒态土壤有机质（SOM）库中的新13C含量）之间存在显著交互作用，这一现象似乎由历史氮肥管理的差异所驱动。爱荷华州西北部试验点曾有粪肥施用历史，其土壤中的植物有效养分（磷、钾与铵态氮）含量更高，且与玉米相比，芒草在土壤有机质库中的13C占比更低（颗粒态有机质（POM）库中低约64%，矿物结合态有机质（MAOM）库中低约70%）。在养分限制更为严格的中部试验点土壤中，芒草向更稳定的矿物结合态土壤有机质库分配的13C量是玉米的4.5倍。本研究结果表明，包括影响土壤肥力的历史粪肥施用在内的过往农田管理措施，会对生物能源作物的净碳增益产生显著影响。