Erick Fernandes

CARBON AND NUTRIENT ACCUMULATION IN SECONDARY FORESTS REGENERATING ON PASTURES IN CENTRAL AMAZONIA

Over the past three decades, large expanses of forest in the Amazon Basin were converted to pasture, many of which later degraded to woody fallows and were abandoned. While the majority of tropical secondary forest (SF) studies have examined post-deforestation or post-agricultural succession, we examined post-pasture forest recovery in 10 forests ranging in age from 0 to 14 years since abandonment. We measured above- ground biomass and soil nutrients to 45 cm depth and computed total site carbon (C) and nutrient stocks to gain an understanding of the dynamics of nutrient and C buildup in regenerating SF in central Amazonia. Aboveground biomass accrual was rapid, 11.0 Mg·ha 21 ·yr 21 , in the young SFs. Within 12-14 yr, they accumulated up to 128.1 Mg/ha of dry aboveground biomass, equivalent to 25-50% of primary forest biomass in the region. Wood nitrogen (N) and phosphorus (P) concentrations decreased with forest age. Aboveground P and calcium (Ca) stocks accu- mulated at a rate of 1.2 and 29.4 kg·ha 21 ·yr 21 ; extractable soil P stocks declined as forest age increased. Although soil stocks of exchangeable Ca (207.0 6 23.7 kg/ha) and extractable P (8.3 6 1.5 kg/ha) were low in the first 45 cm, both were rapidly translocated from soil to plant pools. Soil N stocks increased with forest age, probably due to N fixation, at- mospheric deposition, and/or subsoil mining. Total soil C storage to 45 cm depth ranged between 42 and 84 Mg/ha, with the first 15 cm storing 40-45% of the total. Total C accrual (7.04 Mg C·ha 21 ·yr 21 ) in both aboveground and soil pools was similar or higher than values reported in other studies. Tropical SFs regrowing on lightly to moderately used pasture rapidly sequester C and rebuild total nutrient capital following pasture abandonment. Translocation of some nutrients from deep soil (.45 cm depth) may be important to sustaining productivity and continuing biomass ac- cumulation in these forests. The soil pool represents the greatest potential for long-term C gains; however, soil nutrient deficits may limit future productivity.

ECOLOGICAL RESEARCH IN THE LARGE-SCALE BIOSPHERE– ATMOSPHERE EXPERIMENT IN AMAZONIA: EARLY RESULTS

The Large-scale Biosphere-Atmosphere Experiment in Amazonia (LBA) is a multinational, interdisciplinary research program led by Brazil. Ecological studies in LBA focus on how tropical forest conversion, regrowth, and selective logging influence carbon storage, nutrient dynamics, trace gas fluxes, and the prospect for sustainable land use in the Amazon region. Early results from ecological studies within LBA emphasize the var- iability within the vast Amazon region and the profound effects that land-use and land- cover changes are having on that landscape. The predominant land cover of the Amazon region is evergreen forest; nonetheless, LBA studies have observed strong seasonal patterns in gross primary production, ecosystem respiration, and net ecosystem exchange, as well as phenology and tree growth. The seasonal patterns vary spatially and interannually and evidence suggests that these patterns are driven not only by variations in weather but also by innate biological rhythms of the forest species. Rapid rates of deforestation have marked the forests of the Amazon region over the past three decades. Evidence from ground-based surveys and remote sensing show that substantial areas of forest are being degraded by logging activities and through the collapse of forest edges. Because forest edges and logged forests are susceptible to fire, positive feedback cycles of forest degradation may be initiated by land-use-change events. LBA studies indicate that cleared lands in the Amazon, once released from cultivation or pasture usage, regenerate biomass rapidly. However, the pace of biomass accumulation is dependent upon past land use and the depletion of nutrients by unsustainable land-management practices. The challenge for ongoing research within LBA is to integrate the recognition of diverse patterns and processes into general models for prediction of regional ecosystem function.

Management control of soil organic matter dynamics in tropical land-use systems

Abstract Given the rapid conversion of tropical forests to crop and pasture land, the economic constraints to widespread fertilizer use, and the potentially negative ecological impacts of fertilizer misuse, there is an urgent need to improve the management of organic inputs and soil organic matter (SOM) dynamics in tropical land-use systems. One desirable goal is the ability to be able to manipulate SOM dynamics via management practices so as to promote soil conservation, to ensure the sustainable productivity of agroecosystems, and to increase the capacity of tropical soils to act as a sink for, rather than a source of, atmospheric carbon. Soil organic matter dynamics are affected by management activities such as: (1) manipulation of the soil environment via tillage, mulching, and application of organic or inorganic fertilizers; (2) varying not only the quantity and quality of organic inputs, but also the placement and timing of application; and (3) manipulation of soil fauna. Although simulation models based on ecosystem concepts, such as Century, offer a dynamic conceptual framework to examine the effects of long-term management, the predictable management of SOM dynamics in tropical agroecosystems is constrained by the lack of appropriate methodologies to isolate SOM pools that are responsive to management.

Managing carbon sequestration in soils: concepts and terminology

The rapidly growing scientific literature on various aspects of carbon storage in soils has given rise to the introduction of several terms when discussing the amounts of carbon that are, or could be, stored in soils. The term “carbon sequestration potential”, in particular, is used with different meanings, sometimes referring to what might be possible given a certain set of management conditions with little regard to soil factors which fundamentally determine carbon storage. An attempt is made to clarify some of the main issues by adopting terminology developed in plant physiology and crop modelling research. This, together with examples from the tropics, is used to clarify some of the issues as relating to mineral soils. The term “Attainablemax” is defined and is suggested as the preferred term for carbon sequestration in mineral soils, being more relevant to management than “potential” and thereby of greater practical value.

The effects of repeated soil wetting and drying on lowland rice yield with system of Rice Intensification (SRI) methods

In lowland rice farming, water management is the most important practice that determines the productivity of other inputs, e.g. nutrients, herbicides, pesticides, farm machinery, microbial activity, mineralization rates. Deliberate flooding or poor drainage that keeps soil saturated is detrimental to crops and degrades soil quality. This study evaluated whether rice grain yield could be increased relative to continuous flooding by using the management practices of the System of Rice Intensification (SRI). The effects of SRIs repeated wetting and drying cycles plus different plant populations were investigated at Sapu Research Station in The Gambia, on an alluvial soil between 2000 and 2002. The water management practices proposed for SRI were found to be beneficial to rice growth. At 20 cm spacing, average grain yield with SRI practice was 7.3 t ha−1 compared with 2.5 t ha−1 under continuous flooding. At 30 cm spacing SRI practice yielded 6.6 t ha−1, while under continuous flooding, grain yield was only 1.7 t ha−1. Even wider spacing did not produce higher yield. At 40 cm spacing, SRI management gave 4.7 t ha−1, while continuous flooding yielded 1.3 t ha−1. Thus overall, SRI practices gave better results than continuous flooding. This was probably as a result of increased nutrient availability and superior growing conditions which enhanced physiological development and grain yield. Rewetting dry soil reportedly facilitates nitrogen mineralization. The phenomenon of having a flush of nitrogen mineralization occurring after rewetting dry soil was first reported by Birch in 1958. This intensive pathway of nitrogen mineralization and nitrogen availability has potential to increase lowland rice yields in ways consistent with sustainable agricultural production.

Above- and belowground biomass, nutrient and carbon stocks contrasting an open-grown and a shaded coffee plantation

Coffee (Coffea canephora var robusta) is grown in Southwestern Togo under shade of native Albizia adianthifolia as a low input cropping system. However, there is no information on carbon and nutrient cycling in these shaded coffee systems. Hence, a study was conducted in a mature coffee plantation in Southwestern Togo to determine carbon and nutrient stocks in shaded versus open-grown coffee systems. Biomass of Albizia trees was predicted by allometry, whereas biomass of coffee bushes was estimated through destructive sampling. Above- and belowground biomass estimates were respectively, 140 Mg ha−1 and 32 Mg ha−1 in the coffee–Albizia association, and 29.7 Mg ha−1 and 18.7 Mg ha−1 in the open-grown system. Albizia trees contributed 87% of total aboveground biomass and 55% of total root biomass in the shaded coffee system. Individual coffee bushes consistently had higher biomass in the open-grown than in the shaded coffee system. Total C stock was 81 Mg ha−1 in the shaded coffee system and only 22.9 Mg ha−1 for coffee grown in the open. Apart from P and Mg, considerable amounts of major nutrients were stored in the shade tree biomass in non-easily recyclable fractions. Plant tissues in the shaded coffee system had higher N concentration, suggesting possible N fixation. Given the potential for competition between the shade trees and coffee for nutrients, particularly in low soil fertility conditions, it is suggested that the shade trees be periodically pruned in order to increase organic matter addition and nutrient return to the soil.

Development of the soil macrofauna community under silvopastoral and agrosilvicultural systems in Amazonia

Summary In the Brazilian Amazon region, millions of hectares of forest land have been converted into cattle pastures and then been abandoned. Agroforestry is a potential option for the transformation of in parts degraded lands into productive agricultural systems. The re-establishment of a diversified soil macrofauna can help in the process of recuperation of the often compacted soil structure of the pastures. The soil macrofauna community was studied during the rainy season in four different agroforestry systems near Manaus in Central Amazonia: 1. a high-input silvopastoral system (ASPh), 2. a low-input silvopastoral system (ASPl), 3. a palm based system with four tree crop species (AS1) and 4. a high-diversity tree crop system with ten tree crop species (AS2), plus a spontaneous fallow for comparison. The sampling method recommended by the Tropical Soil Biology and Fertility Programme was used. The highest diversity of fauna groups was observed in the ASPh and ASPl where trees were associated with the leguminous cover crop, Desmodium ovalifolium. The cover crop exerted a favorable effect on the soil fauna presumably by maintaining the soil moist and shaded and providing litter as a substrate. Of the 15 soil fauna groups that were found in all systems, four were absent from AS1. Within the AS2 system a significantly greater density of the soil fauna was observed under peach palm (Bactris gasipaes) and cupuacu (Theobroma grandiflorum) (3107 and 524 ind.m−2, respectively) than under the other three tree species. The soil under peach palm and cupuacu also tended to have a higher number of soil fauna groups. In AS1, the soil under peach palm had a higher fauna density than the soil under cupuacu, probably caused by the abundant residues of the heart of palm harvest on the soil. The earthworm biomass was particularly high in AS1. Under cupuacu approximately 7 times more earthworms were found in AS1 (17.9) than in AS2 (2.4). The study of the macrofauna community, including both the litter layer and the superficial soil layers, allows to identify the plant species/management combinations which favour the increase of the diversity of the invertebrates.

Indigenous fruit trees of Madagascar: potential components of agroforestry systems to improve human nutrition and restore biological diversity

Biodiversity in Eastern Madagascar is threatened by slash and burn agriculture, which is resulting in species extinction, land and soil degradation and rural impoverishment. An ethnobotanical study was undertaken to determine the domestication potential of indigenous fruit tree species as components of agroforestry systems. Four major selection criteria were used: nutritional and income needs of the population, diversification of the agroecosystem, and protection of plant and animal diversity. At three sites, Andasibe, Masoala and Ranomafana, in the humid primary forest region of Eastern Madagascar, a total of 150 wild fruit species from 82 genera and 42 families, of which 85% were indigenous and 92% of woody habit, were identified. In contrast to most of the deforested areas in Madagascar, the rural population in these areas possess an intimate knowledge of indigenous plant resources. Most of the indigenous fruits are collected from the forest but for a few species, domestication is initiated by managing naturally established species or by planting individual trees in agricultural fields. Wild fruits supplement the daily diet, substitute for exotic fruits, gain importance during periods of food shortage and are most appreciated by children. Commercialization of wild fruits is mainly undertaken by the poorer section of the population. Gender related differences in knowledge and preferences on species were identified and related to the respective household responsibilities. A list of the 26 priority species was established based on the preferences of children, women and men at the three sites. Local, fruit-eating lemur species are also highly dependent on indigenous fruit trees and are crucial for successful regeneration of forest vegetation.

Weed management through short-term improved fallows in tropical agroecosystems

Weeds in tropical agricultural systems can cause serious economic damage, and their control often requires the commitment of substantial resources in the way of labor, capital, or pesticides. Tropical shifting cultivation systems employ a fallow period to help overcome weed infestations in addition to improving soil productivity and reducing other pest populations. Short-term, intensive fallow systems that employ herbaceous and woody species to facilitate rapid restoration of soil productivity have evolved as an alternative to long-term fallows. Short-term fallows can impact weeds at all growth stages and play a role in the integrated management of weeds and crops. Fallow management that promotes vegetative soil cover may reduce weed recruitment due to attenuation of soil temperature and/or shift in light quality at the soil surface. Residues or litterfall from fallow species may alter the chemical and microbial ecology of the soil to favor losses from the weed seed bank due to germination, loss of seed vigor, or seed decay. In addition, fallow vegetation can influence weed seed predation. Enhancement of soil productivity should increase the vigor of crop growth and enable crops to better compete with weeds. The burning of fallow species residues may result in weed seed death due to extreme temperatures or may induce seed germination by the release of mechanical dormancy or chemical germination cues. Certain weeds may serve as improved fallow species due to their high nutrient scavenging efficiency in low-fertility environments and their ease of establishment. Short-term improved fallows can be an important component of integrated weed management, particularly by promoting the prevention and tolerance of weeds in crops.

Development of Forest Structure and Leaf Area in Secondary Forests Regenerating on Abandoned Pastures in Central Amazônia

The area of secondary forest (SF) regenerating from pastures is increasing in the Amazon basin; however, the return of forest and canopy structure following abandonment is not well understood. This study examined the development of leaf area index (LAI), canopy cover, aboveground biomass,