Chemical analysis of rainfall and throughfall in the Tapajós National Forest, Belterra, Pará, Brazil

The Tapajós National Forest (FLONA Tapajós) has 600,000 hectares of protected forest, and is situated 50 km south of the city of Santarém, Pará, Brazil, a port city of 250,000 inhabitants that is located at the confluence of the Tapajós and Amazon Rivers. There is a lot of farmland in the region, which offers many opportunities to study changes in land use. Selective wood harvesting is one type of land use that is particularly important to the economy of Santarém. Wet and dry deposition of organic material can be an important source of nutrients for plants, and this is especially true when the soil is poor, which is the case in Santarém-Belterra plateau region, the study area of this research. In this region, the natural atmospheric deposition of nutrients is often enhanced by the burning of biomass, which releases a large part of the above-ground biomass nutrients into the atmosphere. The objectives of this study were: 1 - estimate the total wet deposition via direct precipitation and through the canopy, including dry deposition; 2 - verify potential sources of nutrients found in the total wet deposition and dry deposition; and 3 - investigate the effects of coverage vegetation on nutrient content in precipitation and throughfall. The study was conducted in FLONA Tapajós at km 67 of Santarém- Cuiabá Highway, south of the city of Santarém. The study area consisted of a portion of 100 x 100 m transects divided into 10 x 10 m plots. The area was located next to a meteorological tower 65 m tall that measures various climate parameters such as rainfall, wind speed and direction, solar radiation, temperature and humidity, among others. Direct precipitation (PD) and internal precipitation (IP) collectors consisted of 2 L polyethylene bottles with a 115 mm diameter funnel. Samples were collected weekly from April 2003 to March 2006. The volume of the sample was measured individually for each collector (25 traps for internal precipitation and 4 for direct precipitation). The conclusions that can be drawn from this study are: 1 - the dry season has the highest variation in ion flux; 2 - seasonality has a strong influence on the concentration of basic cations; 3 - dry deposition is one of the most important ways that nutrients are acquired in FLONA Tapajos; 4 - there is a significant inflow of nutrients Cl and Na due to intensive grain farming nearby; 5 - dry deposition is the most important process for the enrichment of water that reaches the forest floor; 6 - principal component analysis facilitates the interpretation and characterization of rainwater and in this study shows the influence of anthropogenic sources such as agriculture, biomass burning, and dust.

塔帕若斯国家森林公园（FLONA Tapajós）占地60万公顷保护林，坐落于巴西帕拉州圣塔伦市以南50公里处。圣塔伦是拥有25万人口的港口城市，地处塔帕若斯河与亚马逊河的汇流地段。该区域分布有大量农田，为土地利用变化的相关研究提供了诸多契机。选择性木材采伐是对圣塔伦经济尤为关键的土地利用类型之一。

有机物质的湿沉降与干沉降是植物重要的养分来源，在土壤贫瘠的条件下这一作用尤为突出——而本研究的研究区圣塔伦-贝尔特拉高原区域，正是土壤贫瘠的典型区域。在该区域内，自然大气养分沉降过程常因生物质燃烧活动得到强化，生物质燃烧会将大量地上生物量所储存的养分释放至大气中。

本研究的目标如下：
1. 通过直接降水（direct precipitation, PD）与林内穿透降水（internal precipitation, IP），估算包含干沉降在内的总湿沉降量；
2. 探明总湿沉降与干沉降中养分的潜在来源；
3. 探究植被覆盖对降水及穿透降水中养分含量的影响。

本研究于圣塔伦-库亚巴公路67公里处的FLONA Tapajós开展，地点位于圣塔伦市南部。研究区域为一片100×100米的样带，被划分为10×10米的样方。该区域毗邻一座高65米的气象塔，可监测降雨、风速与风向、太阳辐射、温度与湿度等多项气候参数。

直接降水与林内穿透降水的收集器采用容积2升、漏斗直径115毫米的聚乙烯瓶。采样周期为2003年4月至2006年3月，每周采集一次样本。每个收集器单独测定样本体积：林内穿透降水采样陷阱共25个，直接降水采样陷阱共4个。

本研究可得出如下结论：
1. 旱季的离子通量变异程度最高；
2. 季节动态对碱性阳离子浓度具有显著影响；
3. 干沉降是塔帕若斯国家森林公园获取养分的最重要途径之一；
4. 附近大规模谷物种植导致Cl与Na养分大量输入；
5. 干沉降是到达林地表层的水体养分富集的最关键过程；
6. 主成分分析有助于雨水水样的解译与特征表征，本研究中该方法揭示了农业活动、生物质燃烧与扬尘等人为源的影响。