Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Abstract:

Atmospheric nutrient deposition and climate change are expected to endanger the diversity of tropical forest ecosystems. Nitrogen (N) deposition might influence nutrient fluxes beyond the N cycle by a concomitant increased demand for other nutritional elements such as phosphorus (P). Organisms might respond to the increased P demand by enhanced activity of enzymes involved in releasing inorganic P from organic matter (OM). Our aims were to assess the effect of i) climate shifts (approximated by an altitudinal gradient), and ii) nutrient addition (N, P, N+P) on phosphatase activity (PA) in organic layer and mineral soil of a tropical montane rainforest in Southern Ecuador. A nutrient manipulation experiment (NUMEX) was set up along an altitudinal gradient (1000, 2000, and 3000 m a.s.l.). We determined PA and inorganic and total P concentrations. PA at 1000 m was significantly lower (mean ± standard error: 48 ± 20 µmol p-NP g-1 dm h-1) as compared to 2000 m and 3000 m (119 ± 11 and 137 ± 19, respectively). One explanation might be that very rapid decomposition of OM at 1000 m results in very thin organic layers reducing the stabilization of enzymes and thus, resulting in leaching loss of enzymes under the humid tropical climate. We found no effect of N addition on PA neither in the organic layer nor in mineral soil, probably because of the low nutrient addition rates that showed ambiguous results so far on productivity measures as a proxy for P demand. In the organic layers of P and N+P treatments, we found decreased PA and increased concentrations of inorganic P. This indicates that the surplus of inorganic P reduced the biosynthesis of phosphatase enzymes. PA in megadiverse montane rainforests is likely to be unaffected by increased atmospheric N deposition but reduced upon atmospheric P deposition.

Abstract:

Tropical ecosystems have an important role in global change scenarios, in part because they serve as a large terrestrial carbon pool. Carbon protection is mediated by soil aggregation processes, whereby biotic and abiotic factors influence the formation and stability of aggregates. Nutrient additions may affect soil structure indirectly by simultaneous shifts in biotic factors, mainly roots, and fungal hyphae, but also via impacts on abiotic soil properties. Here, we tested the hypothesis that soil aggregation will be affected by nutrient additions primarily via changes in arbuscular mycorrhizal fungal (AMF) hyphae and root length in a pristine tropical forest system. Therefore, the percentage of water-stable macroaggregates (> 250 ?m) (WSA) and the soil mean weight diameter (MWD) was analyzed, as well as nutrient contents, pH, root length, and AMF abundance. Phosphorus additions significantly increased the amount of WSA, which was consistent across two different sampling times. Despite a positive effect of phosphorus additions on extra-radical AMF biomass, no relationship between WSA and extra-radical AMF nor roots was revealed by regression analyses, contrary to the proposed hypothesis. These findings emphasize the importance of analyzing soil structure in understudied tropical systems, since it might be affected by increasing nutrient deposition expected in the future.

Abstract:

Studies in temperate systems provide evidence that the abundance of arbuscular mycorrhizal fungal (AMF) depends on soil nutrient availability, which is mainly explained in the context of resource stoichiometry and differential plant biomass allocation. We applied this concept to an understudied ecosystem – tropical montane forest – analyzing root and AMF abundance along an elevational gradient with decreasing nutrient availability, combined with responses to nitrogen (N) versus phosphorus (P) additions. At three sites from 1000 to 3000 m above sea-level we analyzed fine root length, AMF root colonization as well as extraradical AMF biomass (neutral lipid fatty acid 16:1?5, hyphal length and spore counts) in a nutrient manipulation experiment. We found a significant increase in root length as well as intra- and extraradical AMF abundance with elevation. Overall, P additions significantly increased, whereas N additions decreased AMF abundance, with differential though nonsystematic changes along the elevational gradient. Strongest effects were clearly observed at the intermediate site. These findings suggest a general dependency of roots and AMF on nutrient availability, though responses to N and P additions differed from previous studies in temperate systems. In the context of future nutrient depositions, results suggest diverging responses of AMF abundance depending on site characteristics.

Abstract:

The increasing anthropogenic influence on the environment on a global scale has led to a shift in nutrient cycles which are not fully understood yet. These shifts could alter the properties of ecosystems and therefore change habitats and species composition. Therefore it is crucial to understand nutrient cycles and how ecosystems react to these nutrient availability shifts. Especially the phosphorus and nitrogen cycles are highly important because of their direct influence on plant growth as primary macronutrients. Even in some so far undisturbed regions, a human caused rise in atmospheric input of phosphorus-containing particles is expected. One of the reasons therefor is land-use change in rural areas with associated combustions, from which the particles are transported downwind. This study simulates the prognosticated increased nutrient input and focuses on the following fate of the phosphorus in soil. For this, in 2008 a nutrient manipulation experiment was established to fertilize an old-growth tropical montane forest with moderate phosphorus (10 kg P ha-1 yr-1) and/or nitrogen additions (50 kg N ha-1 yr-1). This experimental setup was conducted for the first time at three altitudes (1000m a.s.l., 2000m a.s.l., 3000m a.s.l.) to compare the effects along an altitudinal gradient. After seven years the total phosphorus and bioavailable phosphorus concentrations of these locations were investigated in litterfall, the organic layer and in mineral soil to see if the added phosphorus is retained and enriched. The results showed that the application of the phosphorus fertilizer resulted in increased total and bioavailable phosphorus concentrations mainly in the organic layer. The phosphorus increase in mineral soil was not significant and indicated that the added phosphorus is not leached, but retained in the ecosystems. There were no effects on phosphorus concentrations after nitrogen addition. The altitudinal gradient referred to a pronounced difference between 1000m and the other altitudes (2000m and 3000m), due to the accumulated organic matter at the higher altitudes. Overall this study shows that the sustainable anthropogenic phosphorus increase may cause a change in the ecosystem’s characteristics and nutrient cycles.

Abstract:

Ecosystems worldwide face increasing nutrient depositions mainly caused by anthropogenic processes. In particular, tropical ecosystems react sensitively to altering nutrient supply. The deposition of nutrients might influence the nutrient cycles, primarily of N and P in tropical montane rainforests. Increased nutrient supply leads to an enhanced biomass production and therefore other nutrients become limited for plants and microorganisms. For this reason, the aim of this thesis is to study the response of phosphatase activity (PA) on moderate fertilization along an altitudinal gradient in a tropical montane rainforest in South Ecuador. The experiment was conducted on the NUMEX study sites including three different elevation levels 1000, 2000 and 3000m a.s.l. The different plots were treated with N, P, N+P to simulate increased nutrient depositions and one control plot. Further, organic layer and mineral soil was sampled and phosphomono- and phosphodiesterase activity (PMEA and PDEA) were determined. The N fertilized plots showed only small effects compared to the control; presumably due to low amounts of added fertilizer. PA in the P addition plots showed reduced activity compared to the control with significant results of PMEA in the organic layer of the study sites on 2000 and 3000m a.s.l. The reason might be sufficient quantities of inorganic P which suppresses the production of phosphatases. Further, PA in N+P plots showed lower PA compared to the control than in the P addition plots. This effect could be caused through the dominating inhibitory effect of P in contrast to the stimulating effect of N on PA. Altitudinal differences were observed comparing the control plots at different elevation levels (1000, 2000 and 3000m a.s.l.). The results for the organic layer showed increasing PA along the altitudinal gradient with the lowest PA at 1000m a.s.l. The findings are contrary to the present literature which states that lowland tropical forests are characterized by high decomposition rates coming up with high PA. The findings cannot support this hypothesis; therefore more research is needed in the studied area in South Ecuador.

Abstract:

Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO3?N, NH4?N and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15?0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural 15N abundance in NO3 during the passage of rain water through the ecosystem and bulk d15N values in soil to detect N transformations. Depletion of 15N in NO3 and increased NO3 fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO3 concentrations progressively decreased and were enriched in 15N but did not reach the d15N values of solid phase organic matter (d15N = 5.6?6.7%). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the d15N value of NO3 in litter leachate was smaller (d15N = -1.58%) than in the other quarters (d15N = -9.38 ± SE 0.46%) probably because of reduced mineralization and associated fractionation against 15N. Nitrogen isotope fractionation of NO3 between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO3-N from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO3 gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.

Abstract:

Management of natural forestsmight be one option to reduce the high deforestation rate in Ecuador. We therefore evaluated the response of water and nutrient cycles in a natural tropical montane forest to improvement fellings with the aim of favoring economically valuable target trees which will later be harvested with additional ecosystem impacts not considered here. The study was conducted at ca. 1900?2200 m above sea level in the south Ecuadorian Andes on the east-exposed slope of the east cordillera. In June 2004, one of two paired ca. 10-ha large catchments was thinned by felling 10.2% of the initial basal area (dbh longer than 10 cm) on 30% of the catchment. The stems remained in situ. We measured ecosystem fluxes from rainfall via throughfall and stemflow to soil solution (litter leachate, soil solution at 15 and 30 cm depth) and stream flow between May 2004 and May 2005. After the fellings, soil solutions were extracted from the gaps created by the felled trees and the forest next to the gaps. We determined aboveground water fluxes by direct measurement and soil water fluxes with a budget approach. In the solutions, we measured concentrations of NH4-N, NO3-N, total dissolved N, PO4-P, total dissolved P, Ca, Mg, K, Na, and Cl-. Fluxes were calculated as volumeweighted mean (vwm) concentrations times water fluxes. Dry deposition was estimated using chloride as inert tracer. The fellings increased concentrations of N, K, Ca, and Mg in the organic layer of the resulting gaps compared with the forest next to the gaps (vwm concentrations of N: 6.4 mg/l in the forest next to the gap/8.7 mg/l in the gaps, K: 9.8/11, Mg: 1.8/3.0, Ca: 3.4/5.8). Lower nutrient concentrations and fluxes in the mineral soil of the gaps than in forest next to the gaps suggested that these nutrientswere taken up by ground vegetation and target trees. The paired modified and undisturbed catchments had similar water and nutrient budgets. The fellings did not have a significant impact on the water and nutrient budget at the catchment scale.

Abstract:

In tropical montane forests, soil properties change with altitude and tree growth decreases. In a tropical montane forest in Ecuador, we determined soil and tree properties along an altitudinal transect between 1960 and 2450 m above seal level. In different vegetation units height, basal area, and diameter growth of trees were recorded, and all horizons of three replicate profiles at each of eight sites were sampled. We determined pH and total concentrations of Al, C, Ca, K, Mg, Mn, N, Na, P, S, Zn, polyphenols, and lignin in all soil horizons and in the mineral soil additionally the effective cation-exchange capacity (ECEC). The soils were Cambisols, Planosols, and Histosols. The concentrations of Mg, Mn, N, P, and S in the O horizons and of Al, C, and all nutrients except Ca in the A horizons correlated significantly negatively with altitude. The C/N, C/P, and C/S ratios increased and the lignin concentrations decreased in O and A horizons with increasing altitude. Forest stature, tree basal area, and tree growth decreased with altitude. An ANOVA analysis indicated that macronutrients (e.g., N, P, Ca) and micronutrients (e.g., Mn) in the organic layer and in the soil mineral A horizon were correlated with tree growth. Furthermore, lignin concentrations in the organic layer and the C/N ratio in soil affected tree growth. These effects were consistent, even if the effect of altitude was accounted for in a statistical hierarchical model. This suggests a contribution of nutrient deficiencies to reduced tree growth possibly caused by reduced organic matter turnover at higher altitudes.