Fire in the Brazilian Amazon 2. Biomass, nutrient pools and losses in cattle pastures
Fire in the Brazilian Amazon 2. Biomass, nutrient pools and losses in cattle pastures
复制标题
巴西亚马逊火灾 2. 牛牧场的生物量、养分库和损失
作者：
J. Kauffman;D. Cummings;D. Ward
摘要 在巴西亚马孙地区已被砍伐的约42.6万平方千米的热带森林中，最常见的用途是转变为养牛牧场。然而，人们对这些牧场的生物量、碳、养分库及其对频繁发生的火灾的反应知之甚少。我们对三个具有典型但不同土地利用历史的亚马孙养牛牧场的生物量、养分库以及它们在火灾中的损失或转化进行了采样。地上总生物量（TAGB）介于53到119兆克/公顷之间。曾经占据这些地点的森林残留的木材碎片构成了地上总生物量的大部分（47% - 87%）。细小燃料（主要是牧场草类）的生物量约为16 - 29兆克/公顷。草类含有高达地上钾库52%的钾，草类和凋落物成分加起来构成高达地上磷库88%的磷。火灾消耗了21% - 84%的地上总生物量。释放到大气中的碳损失量介于11到21兆克/公顷之间，氮损失量介于205到261千克/公顷之间。硫、磷、钙和钾的损失量小于33千克/公顷。与相邻的原始森林相比，牧场表层土壤（0 - 10厘米）的养分浓度没有变化。养牛牧场火灾频繁发生（即大约每两年一次），而且牧场现在可能是亚马孙地区最常见的着火土地类型。一个牧场存在的前6年可能包括原始森林砍伐后的火灾以及三次牧场火灾。根据我们的研究结果，这些火灾导致的氮累计损失量将达到1935千克/公顷（相当于原始森林地上氮库的94%）。老牧场火灾后的地上碳库低至相邻原始森林的3%。原始森林砍伐后的火灾以及牧场中反复发生的火灾导致大部分地上碳和养分库通过燃烧过程而非分解过程释放。
Abstract Conversion to cattle pasture is the most common fate of the ≈426,000 km2 of tropical forest that has been deforested in the Brazilian Amazon. Yet little is known about the biomass, C, nutrient pools, or their responses to the frequent fires occurring in these pastures. We sampled biomass, nutrient pools and their losses or transformation during fire in three Amazonian cattle pastures with typical, but different, land-use histories. Total aboveground biomass (TAGB) ranged from to 53 to 119 Mg ha−1. Residual wood debris from the forests that formally occupied the sites composed the majority of TAGB (47–87%). Biomass of fine fuels, principally pasture grasses, was ≈16–29 Mg ha−1. Grasses contained as much as 52% of the aboveground K pool and the grass and litter components combined composed as much as 88% of the aboveground P pool. Fires consumed 21–84% of the TAGB. Losses of C to the atmosphere ranged from 11 to 21 Mg ha−1 and N losses ranged from 205 to 261 kg ha−1. Losses of S, P, Ca, and K were <33 kg ha−1. There were no changes in surface soil (0–10 cm) nutrient concentration in pastures compared to adjacent primary forests. Fires occur frequently in cattle pastures (i.e., about every 2 years) and pastures are now likely the most common type of land burned in Amazonia. The first 6 years of a pastures existence would likely include the primary forest slash fire and three pasture fires. Based upon our results, the cumulative losses of N from these fires would be 1935 kg ha−1 (equivalent to 94% of the aboveground pool of primary forest). Postfire aboveground C pools in old pastures are as low as 3% of those in adjacent primary forest. The initial primary forest slash fire and the repeated fires occurring in the pastures result in the majority of aboveground C and nutrient pools being released via combustion processes rather than decomposition processes.