Brigitte Josefine Feigl

Nitrogen dynamics in soils of forests and active pastures in the western Brazilian Amazon Basin

To investigate the influence of forest conversion to pasture on soil N transformations, we compared soil inorganic-N pools and net mineralization and nitrification rates along two chronosequences of upland (terra firme) forest and pastures ranging in age from 4 to 82 years in the state of Rondonia in the western Brazilian Amazon Basin. Forest and pasture soils had similar total extractable inorganic-N pools at 0–5 and 5–10 cm depths. Ammonium-N and NO3−N pools were of similar magnitude in forest soils (2–10 μg N g−1 dry soil), while NH4+N dominated pasture soil inorganic-N pools. Annual average net N mineralization rates for the two chronosequences at 0–5 cm depth in the forests were 1.31–1.88 μg N g−1 d.s. d−1 and exceeded the annual average net N mineralization rates measured in pastures of −0.11-0.02 μg N g−1 d.s. d−1. Annual average net nitrification rates at 0–5 cm depth in forest (1.09–1.46 μg N g−1 d.s. d−1) were also higher than in pastures (0.24–0.25 μg N g−1 d.s. d−1). Pasture soils had lower net N mineralization and net nitrification rates than forest soils even though they had approximately equal or higher total C and total N content. Pasture age did not affect NH4+N pools or net nitrification rates, but decreased NO3−N pools and net N mineralization rates. Net N mineralization rate was unaffected by soil moisture, but net nitrification rate decreased at higher soil moisture. Higher net mineralization and nitrification rates in forest soils suggest a higher potential for NO3−N losses either through leaching or gaseous emissions from intact forests compared with established pastures.

Conversion of the Amazon rainforest to agriculture results in biotic homogenization of soil bacterial communities

The Amazon rainforest is the Earth’s largest reservoir of plant and animal diversity, and it has been subjected to especially high rates of land use change, primarily to cattle pasture. This conversion has had a strongly negative effect on biological diversity, reducing the number of plant and animal species and homogenizing communities. We report here that microbial biodiversity also responds strongly to conversion of the Amazon rainforest, but in a manner different from plants and animals. Local taxonomic and phylogenetic diversity of soil bacteria increases after conversion, but communities become more similar across space. This homogenization is driven by the loss of forest soil bacteria with restricted ranges (endemics) and results in a net loss of diversity. This study shows homogenization of microbial communities in response to human activities. Given that soil microbes represent the majority of biodiversity in terrestrial ecosystems and are intimately involved in ecosystem functions, we argue that microbial biodiversity loss should be taken into account when assessing the impact of land use change in tropical forests.

Changes in the origin and quality of soil organic matter after pasture introduction in Rondônia (Brazil)

We examined the effects of the conversion of tropical forest to pasture on soil organic matter (SOM) origin and quality along a chronosequence of sites, including a primary forest and six pastures. Bulk soil samples received a physical size-fractionation treatment to assess the contribution of each compartment to total SOM pool. Besides a general increase in total C and N stocks along the chronosequence, we observed a reduction of the relative contribution of the coarser fractions to total soil C content, and an increased concentration in the finer fractions. The origin of the C in each size fraction was established from measurements of13C abundance. After 80 years about 93% of the C in the least humified fraction of the top 10 cm of soil was of pasture origin, while in the most humified it was 82%. Chemical analyses indicated that the fine silt and coarse clay fractions contained the most refractory carbon.

Consequence of forest-to-pasture conversion on CH4 fluxes in the Brazilian Amazon Basin

Methane (CH4) fluxes between soils and the atmosphere were measured in two tropical forest-to-pasture chronosequences in the state of Rondonia, Brazil. Forest soils always consumed atmospheric CH4 with maximum uptake rates in the dry season. Pasture soils consumed atmospheric CH4 during the dry season, but at lower rates than those in the forests. When soil moisture increased in the pasture soils, they became a source of CH4 to the atmosphere. Integrated over the year, forest soils were a net sink of approximately 470 mg CH4-C/m2, while pastures were a net source of about 270 mg CH4-C/m2. Thus forest-to-pasture conversion resulted in a net source of CH4 from the soil of about 1 g CH4/m2/yr. The total pasture-related CH4 release for the entire Brazilian Amazon increased from 0.8 Tg CH4 in 1970 to about 2.5 Tg CH4 in 1990, with a maximum of 3.1 Tg CH4/yr in 1988. Soils accounted for a small part (about 5%) of the total CH4 release from the basin, while biomass burning and cattle emissions accounted for 95%. The average rate of increase in CH4 emission from pastures was about 0.2 Tg CH4/yr between 1975 and 1988. This represents between 12% and 14% of the global average rate of change in tropospheric CH4 content for this time period.

Nitrous oxide emissions from forests and pastures of various ages in the Brazilian Amazon

Nitrous oxide emissions from tropical forest soils are thought to account for 2.2–3.7 Tg N yr−1 of the total annual global production of 10–17 Tg N yr−1. Recent research suggests that clearing of tropical forest for pasture can increase N2O emissions but that the period of elevated emissions may be limited and fluxes from older pastures may be lower than from the original forest. Here we report N2O emissions from two land-use sequences in the Brazilian Amazons state of Rondonia. Each sequence includes a forest and a set of pastures of different ages. One sequence contains a newly created pasture that we studied intensively through its first 2 years, including forest cutting, burning, and the planting of forage grasses. Emissions from the newly created pasture were about two and one half times the forest emissions during the first 2 years (5.0 kg N2O-N ha−1 yr−1 versus 1.9 kg N2O-N ha−1 yr−1). Nitrous oxide fluxes from pastures older than 3 years were on average about one third lower than fluxes from uncut forest (1.4 kg N2O-N ha−1 yr−1 versus 1.9 kg N2O-N ha−1 yr−1). The best predictor of N2O flux across the chronosequences was the magnitude of the NO3 pool in the upper 10 cm of soil measured at the time of gas sampling. Using a simple cohort model combined with deforestation rates estimated from satellite images by Brazils Instituto de Pesquisas Espaciais (INPE) for the period 1978 through 1997, we estimate that for the Brazilian Amazon the basin-wide flux of N2O-N from pasture soils was 0.06 Tg in 1997. This is ∼8% of the combined forest plus pasture flux of 0.78 Tg N2O-N we estimate for the Brazilian part of the basin in 1997. In the absence of any forest-to-pasture conversion in the Brazilian part of the basin, we estimate that the basin-wide flux of N2O-N would have been only slightly larger: 0.80 Tg in 1997. Through a second modeling analysis we estimate that for the whole of the Amazon Basin, including parts of the basin outside of Brazil, the N2O-N emissions from forests averaged 1.3 Tg yr−1 over the period 1978–1995.

Seasonal variation of soil chemical properties and CO2 and CH4 fluxes in unfertilized and P-fertilized pastures in an Ultisol of the Brazilian Amazon

Abstract Conversion of the tropical forest to pasture results in changes in the quality and quantity of the soil organic matter (SOM) and other physical and chemical soil properties. Most studies concerning the carbon (C) cycle have focused on soil C stocks. However, little information is available on tropical soil respiration rates. The objectives of this paper are, firstly, to determine how conversion of a natural forest to a pasture and P fertilization affect (1) some soil properties and carbon and nutrient content, and (2) CO2 and CH4 exchanges at the soil–atmosphere interface. For such purposes, a forest-to-pasture chronosequence (pasture established in 1983, 1987, and 1994) was selected at the Fazenda Nova Vida in Rondonia. Soil sampling was done during the dry season (July 1996) and 6 months later during the rainy season (January 1997). Results show increased pH levels, exchangeable cations, and carbon stocks with the pasture installation. On the contrary, the level of available phosphorus decreased with the increasing age of the pastures, justifying the phosphorus fertilization practice. Regarding the CO2 and CH4 fluxes, we found that the forest and pasture soils had greater respiration (CO2 release) rates during the wet season than during the dry season but no specific and significant relationships between the emission and the pasture age was found. Forest soil CH4 consumption rates were three times lower during the wet season, whereas pasture soils showed a net emission of CH4 even during the dry season.

SOIL MICROBIAL BIOMASS IN AMAZONIAN SOILS: EVALUATION OF METHODS AND ESTIMATES OF POOL SIZES

The suitability of the fumigation-incubation (FI), fumigation-extraction (FE) (with specific k-factor determined by 14C-labelling) and substrate induced respiration (SIR) methods to obtain measures of the microbial biomass in forest soils from the Amazonian basin was examined. Several representitive topsoil (0–10 cm) samples were studied. A reasonable estimate of microbial biomass C in two acid Oxisols (pH <4.5) with a high clay content was obtained by the FI method, provided no correction was applied for basal CO2 respiration. However, on two Ultisols and one Alfisol more consistent estimates of microbial C were obtained by the FI method, where correction for basal respiration was obtained using the respiration of non-fumigated (NF) soils between 0–10 d, or fumigated (F) soils between 10–20 d. Estimates of microbial C by the FE method did not differ significantly from those obtained by the FI method when calculated using a specifically-determined k-factor of 0.26 to convert the extractable-C flush to microbial biomass C. In the Ultisols and the Alfisol, the SIR method gave similar estimates of microbial C to those obtained by the FI and FE methods, but in both Oxisols microbial C was underestimated when a standard conversion factor was applied. Possibly the SIR assay was adversely affected by a high proportion of dormant organisms and a slow gaseous release of respired CO2 from these heavy clay soils. When estimated using the FE method, microbial C was in the range 890–1100 μg C g−1 soil in the Oxisols, 340–470 μg C g−1 soil in the Ultisols and around 580 μg C g−1 soil in the Alfisol. Microbial C comprised 3–4% of the total soil organic C and microbial N 4–7% of total N. Microbial C-to-N ratios averaged 6.9 ± 0.8. Although the FI method can be used on these Amazonian soils, the FE method is more convenient and generally appears suitable for estimating both microbial C and N in the same extract. The SIR method is not recommened for use with Oxisols.

Net greenhouse gas fluxes in Brazilian ethanol production systems

Biofuels are both a promising solution to global warming mitigation and a potential contributor to the problem. Several life cycle assessments of bioethanol have been conducted to address these questions. We performed a synthesis of the available data on Brazilian ethanol production focusing on greenhouse gas (GHG) emissions and carbon (C) sinks in the agricultural and industrial phases. Emissions of carbon dioxide (CO2) from fossil fuels, methane (CH4) and nitrous oxide (N2O) from sources commonly included in C footprints, such as fossil fuel usage, biomass burning, nitrogen fertilizer application, liming and litter decomposition were accounted for. In addition, black carbon (BC) emissions from burning biomass and soil C sequestration were included in the balance. Most of the annual emissions per hectare are in the agricultural phase, both in the burned system (2209 out of a total of 2398 kg Ceq), and in the unburned system (559 out of 748 kg Ceq). Although nitrogen fertilizer emissions are large, 111 kg Ceq ha−1 yr−1, the largest single source of emissions is biomass burning in the manual harvest system, with a large amount of both GHG (196 kg Ceq ha−1 yr−1). and BC (1536 kg Ceq ha−1 yr−1). Besides avoiding emissions from biomass burning, harvesting sugarcane mechanically without burning tends to increase soil C stocks, providing a C sink of 1500 kg C ha−1 yr−1 in the 30 cm layer. The data show a C output: input ratio of 1.4 for ethanol produced under the conventionally burned and manual harvest compared with 6.5 for the mechanized harvest without burning, signifying the importance of conservation agricultural systems in bioethanol feedstock production.

Propriedades químicas de um Neossolo Quartzarênico sob diferentes sistemas de manejo no Cerrado mato-grossense

O objetivo deste trabalho foi avaliar o efeito de diferentes usos da terra e sistemas de manejo do solo, nas caracteristicas quimicas de um Neossolo Quartzarenico, no Cerrado de Mato Grosso. Os tratamentos estudados foram: cerrado nativo; uma pastagem de baixa produtividade com 22 anos de implantacao; plantio convencional com 1 e 4 anos de uso e diferentes sucessoes de culturas; e plantio direto com 5 anos de implantacao. Foram determinados os teores de carbono e nitrogenio, pH, acidez potencial, capacidade de troca de cations (CTC), fosforo disponivel, bases trocaveis e saturacao por bases do solo. Os valores mais baixos de pH, fosforo disponivel, potassio, calcio e magnesio foram observados nas areas sob cerrado e pastagem, e a menor concentracao de carbono no solo sob pastagem refletiu menor CTC do solo. A aplicacao do calcario nos tratamentos com plantio convencional e direto favorece o aumento das concentracoes de calcio e magnesio no solo. Nesses sistemas, a saturacao por bases (V%) do solo e mais elevada nas areas com maior tempo de implantacao (4 e 5 anos). Embora o Neossolo Quartzarenico possua limitacoes para o cultivo, em razao de sua baixa fertilidade natural, se adequadamente manejado, este solo apresenta potencial para o cultivo agricola.

Effects of pasture introduction on soil CO2 emissions during the dry season in the state of Rondonia, Brazil

Soil CO2 evolution rates, soil temperatures and moisture were measured during the dry season in two forest-to-pasture chronosequences in Rondônia, Brazil. The study included pastures ranging from 3 to 80 years-old. Mean dry-season CO2 evolution from the forest in chronosequence 1, 88.8 mg CO2-C m−2h−1 was lower than from the pastures which ranged from 111 to 158 mg CO2-C m−2h−1. We found that temperature was not a good predictor of CO2 emissions from pasture but that there was a significant relationship (r = 0.72,p < 0.05) between soil moisture and pasture emissions. The δ13C of the soil CO2 emissions also was measured on chronosequence I; δ13C of the CO2 emitted from the C3 forest was −29.43%. Pasture13CO2 δ values increased from −17.91%. in the 3 year-old pasture to −12.86% in the 80 year-old, reflecting the increasing C4 inputs with pasture age. Even in the youngest (3 year-old) pasture, 70 percent of the CO2 evolved originated from C4 pasture-derived carbon.