Agroforestry carbon stocks and greenhouse gas emission rates in central Alberta, Canada

Agroforestry systems (AFS) contribute to carbon (C) sequestration and reduction in greenhouse gas emissions from agricultural lands. However, previously understudied differences among AFS may underestimate their climate change mitigation potential. In this 3-year field study, we assessed various C stocks and greenhouse gas emissions across two common AFS (hedgerows and shelterbelts) and their component land uses: perennial vegetated areas with and without trees (woodland and grassland, respectively), newly planted saplings in grassland, and adjacent annual cropland in central Alberta, Canada. Between 2018 and 2020 (~April–October), nitrous oxide emissions were 89% lower under perennial vegetation relative to the cropland (0.02 and 0.18 g N m−2 year−1, respectively). In 2020, heterotrophic respiration in the woodland was 53% lower in shelterbelts relative to hedgerows (279 and 600 g C m−2 year−1, respectively). Within the woodland, deadwood C stock was particularly important in hedgerows (35 Mg C ha−1 or 7% of ecosystem C) relative to shelterbelts (2 Mg C ha−1 or < 1% of ecosystem C), and likely affected C cycling differences between the woodland types by enhancing soil labile C and microbial biomass in hedgerows. Deadwood C stock was positively correlated with annual heterotrophic respiration and total (to ~100 cm depth) soil organic C, water-soluble organic C, and microbial biomass C. Total ecosystem C was 1.90–2.55 times greater within the woodland than all other land uses, with 176, 234, 237, and 449 Mg C ha−1 found in the cropland, grassland, planted saplings treatment, and woodland, respectively. Shelterbelt and hedgerow woodlands contained 2.09 and 3.03 times more C, respectively, than adjacent cropland. Our findings emphasize the importance of AFS for fostering C sequestration and reducing greenhouse gas emissions and, in particular, retaining hedgerows (legacy woodland) and their associated deadwood across temperate agroecosystems to help mitigate climate change.

农林业系统（Agroforestry Systems, AFS）可助力农田碳（C）固存（carbon sequestration）并减少温室气体排放（greenhouse gas emissions）。然而，此前对农林业系统间差异的研究不足，可能会低估其气候变化减缓潜力（climate change mitigation potential）。本项为期3年的野外研究（field study）中，我们针对两种常见农林业系统（树篱林带（hedgerows）与防风林带（shelterbelts））及其组成土地利用类型的各类碳库（carbon stock）与温室气体排放展开了评估：分别为有林木与无林木的多年生植被区（perennial vegetated areas，即林地（woodland）与草地（grassland））、草地内新栽植的幼树（newly planted saplings）样地，以及加拿大阿尔伯塔省中部的邻近一年生农田（annual cropland）。2018年至2020年（约4月至10月）期间，多年生植被区的一氧化二氮（nitrous oxide）排放量较一年生农田低89%，二者分别为0.02与0.18 g N m⁻² year⁻¹。2020年，防风林带内林地的异养呼吸（heterotrophic respiration）速率较树篱林带低53%，二者分别为279与600 g C m⁻² year⁻¹。在两类林地中，树篱林带的枯木碳库（deadwood C stock）占比尤为突出，达35 Mg C ha⁻¹，约占其生态系统碳库的7%；而防风林带仅为2 Mg C ha⁻¹，不足生态系统碳库的1%。枯木碳库可通过提升树篱林带内的土壤活性碳（labile C）与微生物生物量（microbial biomass），进而影响两类林地的碳循环（carbon cycling）差异。枯木碳库与年际异养呼吸速率、总（至约100 cm深度）土壤有机碳（soil organic C）、水溶性有机碳（water-soluble organic C）及微生物生物量碳（microbial biomass C）均呈显著正相关。林地的生态系统总碳库（total ecosystem C）是其余所有土地利用类型的1.90至2.55倍，其中一年生农田、草地、新植幼树样地与林地的碳库分别为176、234、237与449 Mg C ha⁻¹。防风林带林地与树篱林带林地的碳库分别为邻近一年生农田的2.09倍与3.03倍。本研究结果凸显了农林业系统在促进碳固存与削减温室气体排放方面的关键价值，尤其指出在温带农业生态系统（temperate agroecosystems）中保留树篱林带（遗留林地）及其伴生枯木，将有助于推进气候变化减缓（climate change mitigation）工作。