Horst Fölster

Nutrient transfer to the atmosphere by burning of debris in eastern Amazonia

Abstract After replacement of tropical primary forests, fire becomes an important management tool. During establishment of plantations with fast-growing tree species and within slash-and-burn agriculture, the export of macronutrients and other elements to the atmosphere by burning was estimated. Two plots of clearcut 40-year-old and one plot of 7-year-old secondary forest with dry fuel weight 33.5 t ha−1 (plot A1), 95.2 t ha−1 (plot A2) and 31.2 t ha−1 (plot B1) were burnt. Total element losses including volatilisation, particle and leaching exports ranged from 94 to 98% C, 93–98% N, 30–47% P, 30–48% Na, 42–50% K, 13–35% Ca, 21–43% Mg and 66–76% S related to the initial nutrient. For the small-holder site (plot B1) where the drying period lasted 4 weeks with a total precipitation of 9 mm, this loss was to the atmosphere. Significant preburn losses due to leaching were observed on the moister plots (A1, A2) where the drying period lasted 80 days and 101 days with 250 mm and 296 mm precipitation, respectively. Here, element transfer to the atmosphere due to particle transport and volatilisation were 94–98% C, 95–98% N, 27–33% P, 17–23% Na, 16–31% K, 9–24% Ca, 17–43% Mg, 67–68% S of the initial element stock in the burnt debris.

Dynamic of soil chemical parameters in shifting agriculture in the Eastern Amazon

Abstract In Northeast Para, Brazil, shifting agriculture by land settlers has been practiced for around 100 years. After a common cropping period of 2 years the fields are left fallow for 4 to 8 years. Changes in extractable cations and C, N, and P of soils were studied on six fields which were under different phases of the rotational cycle. The chemical composition of soil solutions was also monitored for a period of 19 months. In topsoils, the differences in C and N contents and extractable Ca and Al could be related to the landuse history expressed as time elapsed since last burn. Repeated sampling on two ‘slash and burn’ plots, showed significant increases in pH, CEC, extractable K, Ca and Mg, but decreases in extractable Na and Al, C and N content in the plots from 7-year old fallow to the first-year cropping field. Soil solution chemistry at 105 cm depth did not change with increasing fallow age. On the ‘slash and burn’ field, big variations in solute composition at 10 cm depth were observed during the initial months of the cropping period. In the soil solutions after burning, Ca concentrations ranged from 4.8 to 30.1 mg l −1 , but decreased to values between 1.2 and 2.7 mg l −1 within 2 months. Slash burning and subsequent cropping lead to increases of Cl − and NO − 3 in the soil solution at 105 cm depth. A small dose of NPK fertilizer (11 kg N ha −1 , 13 kg P ha −1 and 17 kg K ha −1 ) did not increase NO − 3 concentration and chloride concentrations at this depth increased to a small extent. After the harvest of cassava, high nitrate concentrations of 11 mg N l −1 were observed. NO − 3 and Cl − concentrations on the cropping fields were significantly correlated ( n = 350) with the sum of the concentration of the M b cations (Na, K, Ca, Mg), (NO − 3 M b , r = 0.82; Cl − M b , r = 0.81). Nitrate also showed significant correlations with protons ( r = 0.59) and Al ( r = 0.47) concentrations. Ion concentrations in the soil solution can be reduced by agricultural management. During harvest, the aboveground vegetation should not be removed completely in one single step. If clear cutting cannot be avoided, the timing should be chosen as to keep some vegetation with high uptake potential at the beginning of the rainy season. A small dose of NPK-fertilizer stimulated the nutrient uptake by the vegetation and did not enhance nutrient concentrations in the seepage water.

Nutrient input-output budget of shifting agriculture in Eastern Amazonia

A case study on the nutrient input-output budget of slash and burn agriculture was carried out in Northeast-Pará, Brazil, where such a land-use system has been practised for about 100 years. A common cropping period lasts for two years and the fields lie fallow for 4 to 8 years. We quantified rates of deposition, fertilization, and losses due to the burn, harvest and leaching. Six fields of different phases in the rotational cycle were under study during a 19 month period. During the fallow period, the input of Na, K, Mg, N, P and S via deposition exceeded the estimated losses with the seepage water. The Ca budget was almost balanced. The balance of fields in the transition from the fallow to the cropping phase was negative for Na, K, Ca, Mg, N, and S. The P balance was positive when NPK fertilizer was applied, and negative without fertilizer application. The nutrient balance for K, Mg, Ca, N, and P was also negative on the field in transition from the cropping to the fallow period. The nutrient budget for an entire land-use cycle of 9 years was estimated by the “false time series approach”. In the case of an NPK fertilization during the cropping period there were net losses of 75 kg K ha−1, 125 kg Ca ha−1, 16 kg Mg ha−1, 285 kg N ha−1 and 13 kg S ha−1. Na (86 kg ha−1) and P (11 kg ha−1) were accumulated. The harvest was the most important flux for the K (61%) and P (62%) output. The element transfer into the atmosphere during the burn caused the main losses of N (60%), S (65%), Ca (58%) and Mg (41%). The most important path of Na loss was leaching (92%). The net K losses were severe as they represented 45% of the K store found extractable in the soil down to 1m depth and in the above ground biomass. The presented results may be useful in planning a sustainable and environmentally protective method of land-use within a shifting cultivation system. It is strongly recommended that slash burning be abandoned in order to keep the nutrients in the ecosystem.

Nutrient balance of shifting cultivation by burning or mulching in the Eastern Amazon – evidence for subsoil nutrient accumulation

For over a hundred years shifting cultivation with slash-and-burn land preparation has been the predominant type of land use by smallholders in the Bragantina region of the Brazilian Eastern Amazon. This study contrasts the nutrient balance of slash-and-burn agriculture with a fire-free cultivation. Therefore, one half of a 3.5-year-old (28.7 t DM ha–1) and a 7-year-old woody fallow vegetation (46.5 t DM ha–1) was burnt and the other half mulched, leaving the biomass as a surface residue. Subsequently, a sequence of maize, beans and cassava was cropped for 1.5 year. Burning the 3.5- and 7-year-old fallow removed 97 and 94% of the C, 98 and 96% of the N, 90 and 63% of the P-stocks, and between 45 and 70% of the cations K, Mg and Ca of the aboveground biomass by volatilization or ash-particle transfer. These losses were avoided with the slash-and-mulch land preparation. Mulching did not increase the losses of nutrients by leaching, despite the high amount of rapidly decomposing surface mulch. Also the length of preceding fallow had no significant influence on leaching losses. At a depth of 3 m, leached nutrients were quantitatively negligible in both treatments. Comparing the nutrient fluxes at soil depths of 0.9 m, 1.8 m and 3 m, the amounts of all mobile nutrients, and also of chloride and sodium were markedly reduced during percolation and must have been retained. It is likely that nutrient retention in the subsoil layer is only temporary, emphasizing the need for a rapid re-establishment of the naturally deep-rooting secondary vegetation after abandonment of sites to enable uptake of these nutrients. The overall nutrient balance was highly negative for slash-and-burn. 291 and 403 kg N ha–1, 21 and 18 kg P ha–1, and 70 and 132 kg K ha–1 were removed from the burnt plots with a preceding fallow of 3.5 and 7 years, respectively. A reduced fallow period (3.5 years), which is a common trend in the region, resulted in a higher mean annual rate of nutrient loss averaged over the duration of the cycle than a fallow period of 7 years. Eliminating the burning losses by mulching brought the agricultural system back to an equilibrated or even slightly positive nutrient balance, even after a reduced fallow period. Thus, slash-and-mulch is a viable alternative to maintain agricultural productivity and ecosystem functioning.

Soil-vegetation relationship in base-deficient premontane moist forest-savanna mosaics of the Venezuelan Guayana

Natural vegetation mosaics of heterogeneous forests, shrubs and savanna are characteristic for the thinly populated Venezuelan Guayana. Vegetation studies and 14C data suggested that the mosaic represents a stage in a long-term process of savannisation, which is originally triggered by forest fires. In the paper, we are looking for the possible role the soil plays in this process. Vegetation structure and soil chemical conditions were analyzed in 74 plots. We further studied the composition of soil solution or water extract, fine root distribution, aboveground biomass and total nutrient stores in selected sites. The soils are humic ferralsols and under woody vegetation, always covered by an organic layer (8–114 t ha−1). The mineral soil is characterized by high organic matter content (0–50 cm: 113–165 t C ha−1), a low cation exchange capacity of the mineral soil (0.94 cmolc kg−1 of clay), and a dominant Al saturation of the exchange complex. Under forest, Ca concentrations on the exchange complex, and in the soil solution/water extract, rank below that of NH4, K, Mg and Fe. Ca-deficiency is considered the main element of the chemical stress responsible for the extremely shallow rooting (<30 cm), which renders the forest systems sensitive to drought stress. NO3 in the soil solution showed high variability and often occurred below the rooting zone, which may indicate disturbance of fine roots. Status of vegetation development and soil properties are not consistently related. Selected case studies show that decreasing aboveground biomass result in changes of soil properties (C, base cation stores), but with a time lag. Forest instability is supposedly conditioned by a combination of constant soil chemical and episodic drought stress.

Reaction of root systems of grand fir (Abies grandis Lindl.) and Norway spruce (Picea abies Karst.) to seasonal waterlogging

Abstract Effects of seasonal waterlogging on rooting depth and rooting intensity (root counts per unit profile wall area) were analysed on profile walls in 14 stands of pure grand fir ( Abies grandis Lindl.) and pure Norway spruce ( Picea abies Karst.), varying from 20 to 31 years old, in a mountainous region in western Germany. The sites vary from non-waterlogged to strongly seasonally waterlogged. We found that seasonal waterlogging in compacted soils is a main stress factor that strongly influences the root distribution of both species. However, roots of grand fir were more tolerant, expanded to greater depths (in general more than 20 cm), and attained higher densities (more than 30%) than roots of Norway spruce. The roots of grand fir even penetrated into the compact Sd-horizon of pseudogleys, whereas the Sw-horizon (seasonally waterlogged horizon) prevented deeper rooting of spruce and contributed to its plate-shaped root system. In addition, a study on the dynamics of fine root biomass and chemistry from waterlogged and non-waterlogged periods was carried out by core sampling in a 25-year-old grand fir stand. During the waterlogged period, the fine roots had considerably greater dead (5558 kg ha −1 ) than live biomass (3594 kg ha −1 ). At the beginning of the non-waterlogged period, the total biomass of live fine roots increased by 50%, and the dead fine roots decreased by 27%. The influence of waterlogging on the vitality of the fine roots is recognizable in their chemical composition: under waterlogged condition, live fine roots contained distinctly less K, Mg and Zn, but more Mn, than during the non-waterlogged period.

Rainfall partitioning and related hydrochemical fluxes in a diverse and in a mono specific (Phenakospermum guyannense) secondary vegetation stand in eastern Amazonia

Abstract Rainfall partitioning into throughfall and stemflow was studied in a diverse and in a mono specific stand of secondary vegetation in Eastern Amazonia. The nutrient concentrations in the water were analysed in order to quantify the related hydrochemical fluxes. Secondary vegetation forms the fallow in the local shifting cultivation system and is usually dominated by shrubs and trees. Phenakospermum guyannense (Strelitziaceae), a banana-like herb, is one of the predominant non-woody species. The study was conducted during an 18-month period in a 2.5-year-old relatively species-rich stand and a 10-year-old stand dominated by P. guyannense. In a year with 1956 mm of rainfall 65% (1281 mm) of this quantity reached the soil as throughfall in the diverse stand and 38% (743 mm) in the mono specific stand. Stemflow was estimated to be 23% and 41% respectively. P. guyannense and Banara guianensis (Flacourtiaceae), a tree species, were causing these high funnelling effects. In the young diverse stand B. guianensis had a stemflow of more than 200 l year−1 and P. guyannense had a median flux of 77 l year−1 per pseudostem. In the older stand the taller plants of P.␣guyannense collected 644 l year−1 per pseudostem on the median. The reason for these high values could be the banana-like growth form of P. guyannense and the crown morphology of B. guianensis, which has inclined branches. The low proportion of throughfall and the high stemflow values differ from all previous studies in Amazonian primary forests. The proximity to the Atlantic Ocean strongly influenced the nutrient fluxes via rainfall at our study site. This becomes obvious from the high Na and Cl fluxes with rainfall (19.7 kg Na ha−1 year−1, 37.2 kg Cl ha−1 year−1) which were approximately equal to the Na and Cl fluxes with the sum of throughfall and stemflow for both stands. K fluxes in throughfall and stemflow in both stands were higher than in rainfall by a factor of 8. The high K enrichment during the crown passage is assumed to be caused by a␣high K concentration in the leaf tissue resulting in enhanced leaching from the leaves. In months with low␣rainfall the concentrations of Ca, Mg, S and Cl in throughfall of the diverse stand were significantly higher than in months with high rainfall. This was mainly due to vegetation burns in the dry period, which resulted in ash deposition on the canopy and subsequent wash-off and solution of ash particles.

Holocene Autochthonous Forest Degradation in Southeast Venezuela

On marginally impoverished soils of southeastern Venezuela, forests have been degrading towards savannas at least since 3500 years BP. In an environment with a sparse human population, natural or man-made forest fires often represent the primary disturbing event. The degradation itself proceeds via successional stages with transitional patchy or individual die-back. Examples are described from two localities. The cause of this degradation is a low vitality and resilience of the forest vegetation conditioned by chemical and drought stress. The paper concentrates on the morphological and structural aspects of the degradation process.

Assessment of cation and anion exchange and pH buffering in an Amazonian Ultisol

Abstract Knowledge about cation and anion exchange and pH buffering in soils is essential for developing an efficient nutrient management system, especially in tropical soils where nutrient retention capacity may be low. Sorption characteristics and pH buffering of an Amazonian Ultisol were studied in sequential batch experiments. The objective was to gain additional information on the cation and anion exchange and pH buffer properties of this soil by using a coupled equilibrium model. The CEC values in the surface soil and subsoil were below 30 mmolc kg−1, indicating that the soil is highly limited in retaining nutrient cations against leaching. The anion exchange capacity (AEC) of the soils for both depths was small (1–4 mmolc kg−1), indicating insignificant retention of NO3 and its enhanced losses in seepage water following fertilization. The exchange coefficients (Gaines–Thomas formulation) of the surface soil followed the order Al>Ca>K>Mg>Na, whereas the order for the subsoil was Al>K>Na>Mg=Ca, indicating that K transferred by leaching from the surface soil to deeper depths may be retained preferentially in the subsoil. More protons could be buffered in the surface soil than in the subsoil. Addition of protons to the surface soil released equivalent amounts of Mb (Ca, Mg, K, Na) cations because of CEC reduction, and an insignificant amount of Al. However, input of protons to the subsoil released large amounts of Al. The model satisfactorily predicted the sorption values for various elements in both depths, indicating that the main processes are understood, but some deviations between modeled and experimental values were noted for the pH in the surface soil. The results indicated a very poor capacity of the soil for nutrient retention and pH buffering and a high risk of Al toxicity. Management options should substitute ‘slash and burn’ practices (which result in further nutrient losses) with mulch producing technologies and strive for only small additions of fertilizer (to reduce nutrient losses and avoid further soil acidification).

Comparison between colorimetric and titration methods for organic carbon determination in acidic soils

Dry combustion CO2 determination is an accurate method to measure organic carbon in soil, but is not always used because it requires expensive equipment. Wet oxidation methods are then widely used. The oxidation conditions in these chemical procedures are still searching and in some cases can be difficult to decide the appropriate method for organic carbon (OC) determination. The objective of this paper was to compare the efficiency of organic carbon oxidation of the Walkley and Black titration (OCWB) and the Anderson and Ingram colorimetric (OCC) methods in acid soils from two humid premontane regions of Venezuela. Both methods were applied to 184 samples of sandstone- and schist-derived soils covering different soil depth and different vegetation (forest to savanna). The standard procedure to evaluate the efficiency of the wet oxidation methods was the dry combustion (OCDC) using a C analyzer. The OCWB method recovered 100% of OC in sandstone-derived soils and 94% in schist-derived soils. The OCC method, however, underestimated the OC, recovering only 53% of OC in the sandstone-derived, and 41% of OC in the schist-derived soils. Statistical analysis indicates that the relationship between OCWB and OCC methods is independent of parent material which allows us to use a conversion factor (0.49) between these methods.