Earth Institute Center for Environmental Sustainability, Columbia University, New York, New York, 10027, USA.
Department of Geography, McGill University, Montreal, Quebec, H3A 0B9, Canada.
Ecol Appl. 2017 Mar;27(2):519-531. doi: 10.1002/eap.1462. Epub 2017 Feb 21.
In the study of shifting cultivation systems, fallow duration is seen as the key determinant of vegetation and soil dynamics: long fallows renew soil fertility, biomass, and biodiversity. However, long fallow systems are increasingly replaced around the world with short-medium fallow systems, and awareness is growing of the need to look across multiple (not just single) crop-fallow cycles to accurately understand observed soil and vegetation patterns. In a study from Peru that builds on 50+ years of field-level land-use histories, we found that, over multiple crop-fallow cycles, farmers' cropping practices mattered more than fallow duration for biodiversity and soil fertility. After initial clearing of primary forest, a precipitous decline occurred in tree species richness of fallows (>50%) with gradual but continued loss thereafter (~0.5 species/yr), which resulted in shifts in species composition over time. For soils, the decline in fertility was more gradual with each additional cycle of cropping resulting in lowered soil organic matter, available phosphorus, and exchangeable sodium levels, even in fields with long fallow durations. In the most intensively used sites, soils experienced a 16% decline of soil organic matter over 4+ cycles. In contrast to previous studies, biomass accumulation and carbon stocks were not related to cropping history or to the number and duration of cycles observed. This suggests that biodiversity-soils-biomass dynamics may not necessarily "move together" in these systems. These results point to the importance of the number of crop-fallow cycles over fallow duration in driving soil fertility and vegetation dynamics under shifting cultivation in the Peruvian Amazon. Overtime shifting cultivation may erode soil fertility and biodiversity levels even if long fallows persist. As the decline in soils appears slow, it may be possible to address this effect with the use of amendments, however biodiversity declines and species compositional changes may be much harder to reverse.
在轮作种植系统的研究中，休耕期被视为植被和土壤动态的关键决定因素：长期休耕可以更新土壤肥力、生物量和生物多样性。然而，随着短-中期休耕系统在全球范围内的日益取代，人们越来越意识到需要跨越多个（不仅仅是单个）作物-休耕周期来准确理解观察到的土壤和植被模式。在秘鲁的一项研究中，该研究基于 50 多年的实地土地利用历史，我们发现，在多个作物-休耕周期中，农民的种植方式比休耕期对生物多样性和土壤肥力更为重要。在最初清除原生林之后，休耕地的树种丰富度急剧下降（>50%），此后逐渐但持续下降（~0.5 种/年），导致物种组成随时间发生变化。对于土壤，肥力下降较为缓慢，每增加一个作物周期，土壤有机质、有效磷和可交换钠水平都会降低，即使在休耕期较长的田地也是如此。在使用最频繁的地点，土壤中的有机质在 4 个以上周期内减少了 16%。与之前的研究不同，生物量积累和碳储量与种植历史或观察到的周期数量和持续时间无关。这表明，在这些系统中，生物多样性-土壤-生物量动态不一定“齐头并进”。这些结果表明，在秘鲁亚马逊地区的轮作种植中，休耕期内的作物-休耕周期数量比休耕期长度对土壤肥力和植被动态的驱动更为重要。随着时间的推移，即使长期休耕持续存在，轮作种植也可能会侵蚀土壤肥力和生物多样性水平。由于土壤的下降似乎较为缓慢，因此通过使用改良剂可能可以解决这个问题，但是生物多样性的下降和物种组成的变化可能更难逆转。
