Carlos Valarezo

Change in water quality during the passage through a tropical montane rain forest in Ecuador

We studied five 20-m transects onthe lower slope under tropical lower montanerain forest at 1900–2200 m above sea level. We collectedsamples of soil and of weekly rainfall,throughfall, litter leachate, and stream waterbetween 14 March 1998 and 30 April 1999 anddetermined the concentrations of Al, totalorganic C (TOC), Ca, Cl−, Cu, K, Mg, Mn,NH4+-N, NO3−-N, total N (TN), Na, P, S, and Zn. The soils were shallowInceptisols; pH ranged 4.4–6.3 in the Ohorizons and 3.9–5.3 in the A horizons, totalCa (6.3–19.3 mg kg−1) and Mgconcentrations (1.4–5.4) in the O horizon weresignificantly different between the transects.Annual rainfall was 2193 mm; throughfall variedbetween 43 and 91% of rainfall, cloud waterinputs were ≤ 3.3 mm a−1 except forone transect (203). The volume-weighted mean pHwas 5.3 in rainfall and 6.1–6.7 in throughfall.The median of the pH of litter leachate andstream water was 4.8–6.8 and 6.8, respectively.The concentrations of Ca and Mg in litterleachate and throughfall correlatedsignificantly with those in the soil (r =0.76–0.95). Element concentrations inthroughfall were larger than in rainfallbecause of leaching from the leaves (Al, TOC,Ca, K, Mg), particulate dry deposition (TOC,Cu, Cl−, NH4+-N), and gaseousdry deposition (NO3−-N, total N, S).Net throughfall (= throughfall-rainfalldeposition) was positive for most elementsexcept for Mn, Na, and Zn. High-flow eventswere associated with elevated Al, TOC, Cu, Mn,and Zn concentrations.

Soil properties on a chronosequence of landslides in montane rain forest, Ecuador

To assess the impact of landslides on soil fertility in south Ecuador, we compared the properties of shallow translational debris slides with those of adjacent undisturbed soils. A chronosequence of four small landslides, ca. 0.5, ca. 2–3, ca. 8–10, and ca. 20 years old, was selected on a 30–50°, forest-covered, east-facing slope of the eastern cordillera at 1900–2300 m above sea level (a.s.l.). All soils were Dystric Cambisols. The mean mass of the soil organic layer (sum of all O horizons) increased in the order landslide head (46 t ha−1)<central part (“track”, 101 t ha−1)<foot (102 t ha−1)<reference soils (291 t ha−1), implying a variation in nutrient storage of the same order. In the head area, only the oldest landslide had an organic layer. All mineral soils were acidic (pH in H2O of the A horizon: 3.4–5.2), the effective CEC of the A horizon ranged from 22 to 141 mmolc kg−1, and the base saturation from 13% to 54%; none of these properties was consistently different between landslide and reference soils. The A horizons of the soil in the landslide foot area had greater concentrations of most nutrients than those in the landslide head area. However, differences were not significant. The nutrient concentrations in the landslide foot area resembled those in the A horizons of the reference soils. Thus, the most obvious change in soil properties caused by the landsliding was partial or complete removal of the organic layer, which was not restored during the ca. 20 years covered by the chronosequence. This decreased the topsoil fertility of the landslide area.

The nitrogen cycle of tropical montane forest in Ecuador turns inorganic under environmental change

Water-bound nitrogen (N) cycling in temperate terrestrial ecosystems of the Northern Hemisphere is today mainly inorganic because of anthropogenic release of reactive N to the environment. In little-industrialized and remote areas, in contrast, a larger part of N cycling occurs as dissolved organic N (DON). In a north Andean tropical montane forest in Ecuador, the N cycle changed markedly during 1998–2010 along with increasing N deposition and reduced soil moisture. The DON concentrations and the fractional contribution of DON to total N significantly decreased in rainfall, throughfall, and soil solutions. This inorganic turn of the N cycle was most pronounced in rainfall and became weaker along the flow path of water through the system until it disappeared in stream water. Decreasing organic contributions to N cycling were caused not only by increasing inorganic N input but also by reduced DON production and/or enhanced DON decomposition. Accelerated DON decomposition might be attributable to less waterlogging and higher nutrient availability. Significantly increasing NO3-N concentrations and NO3-N/NH4-N concentration ratios in throughfall and litter leachate below the thick organic layers indicated increasing nitrification. In mineral soil solutions, in contrast, NH4-N concentrations increased and NO3-N/NH4-N concentration ratios decreased significantly, suggesting increasing net ammonification. Our results demonstrate that the remote tropical montane forests on the rim of the Amazon basin experienced a pronounced change of the N cycle in only one decade. This change likely parallels a similar change which followed industrialization in the temperate zone of the Northern Hemisphere more than a century ago.

Response of Dissolved Carbon and Nitrogen Concentrations to Moderate Nutrient Additions in a Tropical Montane Forest of South Ecuador

In the past two decades, the tropical montane rain forests in south Ecuador experienced increasing deposition of reactive nitrogen mainly originating from Amazonian forest fires, while Saharan dust inputs episodically increased deposition of base metals. Increasing air temperature and unevenly distributed rainfall have allowed for longer dry spells in a perhumid ecosystem. This might have favored mineralization of dissolved organic matter (DOM) by microorganisms and increased nutrient release from the organic layer. Environmental change is expected to impact the functioning of this ecosystem belonging to the biodiversity hotspots of the Earth. In 2007, we established a nutrient manipulation experiment (NUMEX) to understand the response of the ecosystem to moderately increased nutrient inputs. Since 2008, we have continuously applied 50 kg ha-1 a-1 of nitrogen (N), 10 kg ha-1 a-1 of phosphorus (P), 50 kg + 10 kg ha-1 a-1 of N and P and 10 kg ha-1 a-1 of calcium (Ca) in a randomized block design at 2000 m a.s.l. in a natural forest on the Amazonia-exposed slopes of the south Ecuadorian Andes. Nitrogen concentrations in throughfall increased following N+P additions, while separate N amendments only increased nitrate concentrations. Total organic carbon (TOC) and dissolved organic nitrogen (DON) concentrations showed high seasonal variations in litter leachate and decreased significantly in the P and N+P treatments, but not in the N treatment. Thus, P availability plays a key role in the mineralization of DOM. TOC/DON ratios were narrower in throughfall than in litter leachate but their temporal course did not respond to nutrient amendments. Our results revealed an initially fast, positive response of the C and N cycling to nutrient additions which declined with time. TOC and DON cycling only change if N and P supply are improved concurrently, while NO3-N leaching increases only if N is separately added. This indicates co-limitation of the microorganisms by N and P. The current increasing reactive N deposition will increase N export from the root zone, while it will only accelerate TOC and DON turnover if P availability is simultaneously increased. The Saharan dust-related Ca deposition has no impact on TOC and DON turnover.

Current Regulating and Supporting Services: Nutrient Cycles

The study forest regulates nutrient cycles as a supporting ecosystem service mainly via retention in the biosphere and the soil organic layer. How tight the nutrient cycles are depends on environmental conditions. In this chapter, we focus on the roles of (1) deposition from the atmosphere, (2) soil moisture regime, and (3) conversion to pasture in the nutrient cycle. Between 1998 and 2010, there were a seasonal deposition of salpetric acid, an episodic deposition of Ca and Mg from Sahara dusts, and a continuous increase in reactive N inputs related to Amazonian forest fires, the El Nino Southern Oscillation cycle, and the economic development, respectively. Simultaneously, soils became increasingly drier enhancing nutrient release by mineralization. An increasing number of rain storms could considerably increase the export of N and base metals (K, Ca, Mg) via fast surface-near lateral transport in soil. Land-use change from forest to pasture introduces alkaline ashes and grass-derived organic matter. The resulting increases in soil pH and nutrient and substrate supply increase nutrient cycling rates because of enhanced microbial activity.

Aluminum toxicity to tropical montane forest tree seedlings in southern Ecuador: Response of nutrient status to elevated Al concentrations

AimsWe determined the reasons why in nutrient solution increasing Al concentrations >300 μM inhibited shoot biomass production of Cedrela odorata L., Heliocarpus americanus L., and Tabebuia chrysantha (Jacq.) G. Nicholson while 300 μM Al stimulated root biomass production of Tabebuia chrysantha.MethodsNutrient concentrations in plant tissue after a hydroponic growth experiment were determined.ResultsIncreasing Al concentrations significantly decreased Mg concentrations in leaves. Phosphorus concentrations in roots of C. odorata and T. chrysantha were significantly highest in the treatment with 300 μM Al and correlated significantly with root biomass.ConclusionsShoot biomass production was likely inhibited by reduced Mg uptake, impairing photosynthesis. The stimulation of root growth at low Al concentrations can be possibly attributed to improved P uptake.

Aluminum toxicity to tropical montane forest tree seedlings in southern Ecuador

AimsWe determined the reasons why in nutrient solution increasing Al concentrations >300 μM inhibited shoot biomass production of Cedrela odorata L., Heliocarpus americanus L., and Tabebuia chrysantha (Jacq.) G. Nicholson while 300 μM Al stimulated root biomass production of Tabebuia chrysantha.MethodsNutrient concentrations in plant tissue after a hydroponic growth experiment were determined.ResultsIncreasing Al concentrations significantly decreased Mg concentrations in leaves. Phosphorus concentrations in roots of C. odorata and T. chrysantha were significantly highest in the treatment with 300 μM Al and correlated significantly with root biomass.ConclusionsShoot biomass production was likely inhibited by reduced Mg uptake, impairing photosynthesis. The stimulation of root growth at low Al concentrations can be possibly attributed to improved P uptake.

Nutrient Additions Affecting Matter Turnover in Forest and Pasture Ecosystems

Nutrient inputs into ecosystems of the tropical mountain rainforest region are projected to further increase in the next decades. To investigate whether important ecosystem services such as nutrient cycling and matter turnover in native forests and pasture ecosystems show different patterns of response, two nutrient addition experiments have been established: NUMEX in the forest and FERPAST at the pasture. Both ecosystems already responded 1.5 years after the start of nutrient application (N, P, NP, Ca). Interestingly, most nutrients remained in the respective systems. While the pasture grass was co-limited by N and P, most tree species responded to P addition. Soil microbial biomass in the forest litter layer increased after NP fertilization pointing to nutrient co-limitation. In pasture soils, microorganisms were neither limited by N nor P. The results support the hypothesis that multiple and temporally variable nutrient limitations can coexist in tropical ecosystems.