Pastures Of Different Ages Research Articles

Soil Water Depletion in Planted Alfalfa Pastures in an Alpine Pastoral Area

Alfalfa (Medicago sativa) has strong stress resistance, high nutritional value, good palatability for cattle, high yield and a drought tolerance mechanism, but long-term planting leads to soil desiccation. This research was carried out to examine the soil water conditions of alfalfa pastures with different planted ages, and determine the optimum time for alfalfa rotation in a plateau area of a sub-alpine monsoon climate. Soil water depletion, soil compaction and vegetation characteristics of alfalfa pastures of different ages (i.e., two, four and seven years) were assayed and compared with those of a cornfield which served as the control crop. Three 20 × 20 m plots and five random quadrats per plot were established at each field. Soil water contents at 0–400 cm depth and plant biomass were compared among different vegetation types, soil transects and planting years. The results showed that at the 250–400 cm depth, the soil water storage of the four- and seven-year-old alfalfa pastures was much lower than in the two-year-old alfalfa pasture and in the cornfield. Moreover, the degree of soil water storage deficit of the four- and seven-year-old alfalfa pastures was much higher than in the other fields. Soil compaction of alfalfa pastures increased with increasing planting age and reached a peak value in the seven-year-old alfalfa pasture. The highest above-ground biomass was observed in the four-year-old alpine alfalfa pasture. Thus, the best cultivation period for alfalfa pastures was four years from the perspective of higher yield and lower soil water consumption in pastoral sub-alpine areas. This study provided a basis for sustainable alfalfa pasture cultivation, timely harvest, rotation and water management measures to be implemented in alpine grazing lands.

Survey of long-term productivity and nutritional status of Leucaena leucocephala-grass pastures in subtropical Queensland

A survey of the productivity and nutritional status of leucaena (Leucaena leucocephala subsp. glabrata) pastures of different ages was conducted in subtropical Queensland from 2006 to 2008. Four leucaena stands (aged 8, 20, 31 and 38 years) growing on the same Vertosol soil type at Brian Pastures Research Station were surveyed. In the higher rainfall season of 2007–2008, leucaena yields and rainfall-use efficiency were highest in 8-year-old stands [2128 kg total dry matter (DM)/ha or 4.0 kg DM/ha.mm] and lowest in 38-year-old stands (978 kg total DM/ha or 1.9 kg DM/ha.mm). The reduced productivity in the 38-year-old leucaena pasture was associated with a decline in stem number/plant and leucaena plant density due to observed plant mortality. The reduced yield and vigour of aging leucaena was associated with nitrogen deficiency related to declining phosphorus and sulfur availability for adequate symbiotic N2 fixation and leucaena plant growth. Nutrient deficiencies of phosphorus and sulfur in leaf tissue were related to low to medium initial soil fertility (7–27 mg/kg of bicarbonate extractable phosphorus in the top 20 cm), coupled with inherent subsoil constraints (shallow soils, sodicity and high pH), and exacerbated by both long-term removal of nutrients by grazing animals and a reduction in soil phosphorus and sulfur availability over time. To maintain the productivity of leucaena pastures, plant nutritional status needs to be monitored in order to determine strategic fertiliser application.

Microbial community structure and resource availability drive the catalytic efficiency of soil enzymes under land-use change conditions

The turnover of nutrients bound to organic matter is largely mediated by extracellular hydrolytic enzymes (EHEs) produced by soil microorganisms. However, little is known about the environmental drivers (e.g., soil pH, C content, C:N ratio) of the catalytic properties of EHEs and their functional link to the structure of soil microbial communities. We linked catalytic properties, Km and Vmax, determined by Michaelis–Menten kinetics, to a set of environmental and microbial properties in the soils of a land-use sequence ranging from undisturbed natural forest to pastures of different ages and to secondary succession in the Andes of southern Ecuador. The sensitivity of the substrate affinity constant (Km) and the maximum rate (Vmax) of six EHEs (β-cellobiohydrolase (CBH), β-glucosidase (BG), N-acetylglucosaminidase (NAG), α-glucosidase (AG), xylanase (XYL), acid phosphomonoesterase (AP)) to changing environmental conditions was tested by fluorogenic substrates. We used the Vmax-to-Km ratio (Ka) as a proxy for the catalytic efficiency and the signature membrane phospholipid fatty acids as a proxy of microbial community structure.Microbial communities adapted to environmental changes, selected for enzymes with higher substrate affinity (Km) and catalytic efficiency (Ka) compared with pure cultures. Along the land-use sequence, catalytic efficiency increased from natural forest to young pasture, while it decreased during long-term pasture use and secondary succession. This is consistent with three to five times faster turnover of tested substrates (estimated based on Michaelis–Menten kinetic parameters) at the young pasture compared with the long-term pasture and secondary succession. Environmental drivers of the Km were enzyme-specific (e.g., the pH for XYL, the C:N ratio for AP, and the C availability for NAG) and differed from those for Vmax. A decoupled response of Vmax and Km to land-use changes observed for AG, BG, CBH, XYL, and AP, implies divers consequences for ecosystem processes mediated by these enzymes. A high abundance of Gram(−) bacteria triggered the catalytic properties (Km and/or Ka) of enzymes decomposing cellulose, hemicellulose, starch, and monophosphoesters. The importance of climatic factors for catalytic properties of EHEs was emphasized by the Ka values extracted from the literature and demonstrated good correspondence of Ka between soils from geographically distinct experimental plots.

Land-use and soil depth affect resource and microbial stoichiometry in a tropical mountain rainforest region of southern Ecuador

Global change phenomena, such as forest disturbance and land-use change, significantly affect elemental balances as well as the structure and function of terrestrial ecosystems. However, the importance of shifts in soil nutrient stoichiometry for the regulation of belowground biota and soil food webs have not been intensively studied for tropical ecosystems. In the present account, we examine the effects of land-use change and soil depth on soil and microbial stoichiometry along a land-use sequence (natural forest, pastures of different ages, secondary succession) in the tropical mountain rainforest region of southern Ecuador. Furthermore, we analyzed (PLFA-method) whether shifts in the microbial community structure were related to alterations in soil and microbial stoichiometry. Soil and microbial stoichiometry were affected by both land-use change and soil depth. After forest disturbance, significant decreases of soil C:N:P ratios at the pastures were followed by increases during secondary succession. Microbial C:N ratios varied slightly in response to land-use change, whereas no fixed microbial C:P and N:P ratios were observed. Shifts in microbial community composition were associated with soil and microbial stoichiometry. Strong positive relationships between PLFA-markers 18:2n6,9c (saprotrophic fungi) and 20:4 (animals) and negative associations between 20:4 and microbial N:P point to land-use change affecting the structure of soil food webs. Significant deviations from global soil and microbial C:N:P ratios indicated a major force of land-use change to alter stoichiometric relationships and to structure biological systems. Our results support the idea that soil biotic communities are stoichiometrically flexible in order to adapt to alterations in resource stoichiometry.

목초재배지 및 벼논 관리 변화에 따른 토양 탄소 및 미생물 활성도

This study investigated soil carbon storage and microbial activities influenced by different management practices in rice paddies and pastures. Soils under a single-crop farming of rice (CON) and rice-Italian ryegrass rotation farming (IRG) were compared in Jangheung, Jeollanam-do, Seocheon and Cheonan, Chungcheongnam-do. Soils from pastures were analyzed to investigate the effect of duration period (P1, P2, P3) in Namwon, Jeollabuk-do and Seosan, Chungcheongnam-do. In rice paddy, total and particulate carbon (PC) concentrations in the IRG soils were significantly higher than those in the CON soils both in Jangheung and Seocheon where the IRG has been established for three years, whereas carbon concentrations were not significantly different in Cheonan where IRG planting history is only one year. In rice paddy soils, PC was suggested as an early indicator to monitor changes in soil carbon storage followed by adopting different management practices. In pasture, total and PC concentrations increased with duration period especially in the 0-5 cm soils. Contrary to the rice paddy soils, the magnitude of change in PC concentration is not as great as that in total carbon concentration, implying that there is a need to develop a new early indicator other than PC using different fractionation scheme. The soil carbon storage in pasture also increased with years since establishment and the increasing rate was significantly greater in the early stage (0-5 yrs) than the later one (> 5 yrs). Microbial activities measured from fluorescein diacetate (FDA) hydrolysis analysis were significantly lower in the IRG soils than CON soils, whereas no difference was observed in the pastures of different ages. This shows that FDA activity is not a sensitive indicator to differentiate soil qualities influenced by management practices if it is used by itself.

Foraging behavior and ruminal fermentation of dairy cows grazing ryegrass pasture alone or with white clover

The objective of this work was to evaluate the influence of pasture composition and regrowth age on the relationship between feeding behavior and ruminal fermentation in dairy cows grazing perennial ryegrass with or without white clover. The experiment was carried out in a 2x2 factorial arrangement, with two sward types and two ages of regrowth. Swards of perennial ryegrass sown alone (PRG) and of perennial ryegrass mixed with white clover (GC) were evaluated. Twelve late-lactation Holstein cows, fistulated at the rumen, were distributed in a 4x4 latin square experimental design with four 12-day periods. Daily distribution of grazing was similar in the PRG and the GC swards, but the concentration of rumen volatile fatty acids (VFA) was higher and the proportion of propionate was lower on mixed swards during the day. Daily distribution of grazing was similar in pastures of different ages. However, in the oldest swards, rumen fluid pH increased and VFA concentration decreased after evening milking. Time spent grazing does not influence ruminal fermentation, which depends on the changes that occur as different sward layers are grazed.

Soil biological attributes in pastures of different ages in a crop-livestock integrated system

The objective of this work was to evaluate the effect of the pasture (Urochloa brizantha) component age on soil biological properties, in a crop-livestock integrated system. The experiment was carried out in a Brazilian savannah (Cerrado) area with 92 ha, divided into six pens of approximately 15 ha. Each pen represented a different stage of the pasture component: formation, P0; one year, P1; two years, P2; three years, P3; and final with 3.5 years, Pf. Samples were taken in the 0-10 cm soil depth. The soil biological parameters - microbial biomass carbon (MBC), microbial biomass respiration (C-CO2), metabolic quotient (qCO2), microbial quotient (q mic), and total organic carbon (TOC) - were evaluated and compared among different stages of the pasture, and between an adjacent area under native Cerrado and another area under degraded pasture (PCD). The MBC, q mic and TOC increased and qCO2 reduced under the different pasture stages. Compared to PCD, the pasture stages had higher MBC, q mic and TOC, and lower qCO2. The crop-livestock integrated system improved soil microbiological parameters and immobilized carbon in the soil in comparison to the degraded pasture.

Soil water repellency in an old and young pasture in relation to N application

AbstractAgeing of pastures is likely to affect the degree of potential water repellency in the long term, whereas seasonal variation on a shorter term affects the actual repellency of soils. A 1‐year study on two pastures of different ages was conducted on a sandy soil to assess changes in the degree of potential and actual water repellency in relation to different levels of applied nitrogen (N). Sampling was carried out on four dates (April, June, August and October) to determine soil moisture content and both potential and actual water repellency at three depths (0–25, 25–50 and 50–75 cm). The relative number of potentially wettable samples for the young pasture (5 years) ranged from 64% (0–25 cm) to 96% (25–50 cm) to 100% (50–75 cm). For the old pasture (38 years) this ranged from 29 to 66 to 94%. The transition zone in which topsoil samples could be either wettable or water repellent ranged from 18 to 23% (v/v) for the young pasture compared with a range from 29 to 32% (v/v) for the old pasture. Thus, ageing pastures may result in both moving as well as decreasing transition zones over time. A positive relationship between increased inputs of fertilizer N, higher DM yields, lower soil water contents and higher degrees of water repellency was found for the old pasture for the August series. This suggests that seasonal variation in water repellency can be further re‐enforced through nutrient management.

COMPONENTES DA PARTE AÉREA E RAÍZES DE PASTAGENS DE Brachiaria spp. EM DIFERENTES IDADES APÓS A REFORMA, COMO INDICADORES DE PRODUTIVIDADE EM AMBIENTE DE CERRADO

This study aimed to evaluate the production components of aboveground and total root stock biomass of Brachiaria pastures of different ages after renovation. Three B.brizantha pastures, with one, seven, and nine years after renovation through the Barreirao system, and one B. decumbens pasture twenty years after traditional renovation were evaluated. The areas were located at Goiânia, Brazil (16o35'12;S, 49o21'14;W, 730 m). The one year after renovation pasture showed the highest productivity, 197% higher than the twenty year after renovation pasture. With increasing pasture renovation age, there was a production and regrowth rate decrease due to the degradation process. This aspect may be observed in the root system development, since the total root productivity to 100 cm depth was reduced as pasture age increased, with 9.14 Mg ha-1, 4.87 Mg ha-1, and 2.90 Mg ha-1 for pastures of seven, nine, and twenty years after renovation, respectively. The one year pasture did not show appreciable root contribution. The Brachiaria regrowth pasture was found to be dependent on system N availability; hence, the litter production showed to be a good pasture degradation indicator. The N stocks down to 100 cm depth were of 7.54 Mg ha-1, 8.70 Mg ha-1, and 9.47 Mg ha-1, in the pastures with one, seven, and twenty years, respectively. These values were much lower than those in the nine year after renovation pasture (14.2 Mg ha-1). KEY-WORDS: Brachiaria; pastures; roots; litter.

Rates and controls of nitrous oxide and nitric oxide emissions following conversion of forest to pasture in Rondônia

Tropical soils are important sources of nitrous oxide (N2O) and nitric oxide (NO) emissions from the Earth’s terrestrial ecosystems. Clearing of tropical rainforest for pasture has the potential to alter N2O and NO emissions from soils by altering moisture, nitrogen supply or other factors that control N oxide production. In this review we report annual rates of N2O and NO emissions from forest and pastures of different ages in the western Brazilian Amazon state of Rondonia and examine how forest clearing alters the major controls of N oxide production. Forests had annual N2O emissions of 1.7 to 4.3 kg N ha −1 y −1 and annual NO emissions of 1.4 kg N ha −1 y −1 . Young pastures of 1–3 years old had higher N2O emissions than the original forest (3.1–5.1 kg N ha −1 y −1 ) but older pastures of 6 years or more had lower emissions (0.1 to 0.4 kg N ha −1 y −1 ). Both soil moisture and indices of soil N cycling were relatively poor predictors of N2O, NO and combined N2O + NO emissions. In forest, high N2O emissions occurred at soil moistures above 30% water-filled pore space, while NO emissions occurred at all measured soil moistures (18–43%). In pastures, low N availability led to low N2O and NO emissions across the entire range of soil moistures. Based on these patterns and results of field fertilization experiments, we concluded that: (1) nitrification was the source of NO from forest soils, (2) denitrification was not a major source of N2O production from forest soils or was not limited by NO − supply, (3) denitrification was a major source of N2O production from pasture soils but only when NO − was available, and (4) nitrification was not a major source of NO production in pasture soils. Pulse wettings after prolonged dry periods increased N2O and NO emissions for only short periods and not enough to appreciably affect annual emission rates. We project that Basin-wide, the effect of clearing for pasture in the future will be a small reduction in total N2O emissions if the extensive pastures of the Amazon continue to be managed in a way similar to current practices. In the future, both N2 Oa nd NO fluxes could increase if uses of pastures change to include greater use of N fertilizers or N-fixing crops. Predicting the consequences of these changes for N oxide production will require an understanding of how the processes of nitrification and denitrification interact with soil type and regional moisture regimes to control N2 Oa nd NO production from these new anthropogenic N sources.

The relationship between pasture degradation and soil properties in the Brazilian amazon: a case study

Abstract Pasture degradation is one of the greatest problems related to land use in the Amazon region, forcing farmers to open new forest areas. Many studies have identified the causes and the factors involved in this degradation process, in an attempt to reverse the situation. The purpose of this study was to examine the relationship between pasture degradation and some soil properties, to try to identify the most significant soil features in the degradation process. A cattle raising farm in the eastern Amazon region, with pastures of different ages and degrees of degradation, was used as the site for this study: a primary forest area, PN; three Guinea grass (Panicum maximum Jacq.) pastures in an increasingly degraded sequence—P1, P2 and P3; one Gamba grass (Andropogon gayanus Kunth) pasture following an extremely degraded Guinea grass pasture, P4. Aboveground phytomass data showed differences between the pastures, reflecting initially observed degradation levels. Grass biomass decreased sharply from P1 to P2 and disappeared at P3. Pasture recovery with Gamba grass at P4 was very successful, with grass biomass higher than P1 and weed biomass smaller than P2 and P3. Root biomass also decreased with pasture degradation. Soil bulk density increased with pasture decrease at the topsoil layer. Results from the soil chemical analysis showed that there were no signs of decrease in organic carbon and total nitrogen after the forest was transformed into pasture. In all pastures, degraded or not, the soil pH, the sum of bases and the saturation degree were higher than in the forest soil. The extractable phosphorus content, lower in forest soil, remained quite stable in pasture soils, but it could become a limiting factor for the maintenance of Guinea grass. Results indicated that pasture degradation does not seem to be directly related to the modification of the chemical features of soils.

Effects of forest conversion to pasture on soil carbon content and dynamics in Brazilian Amazonia

Abstract Soils play an important role in the carbon cycle, and deforestation in the tropics affects both soil carbon storage and CO2 release into the atmosphere. The consequences of deforestation and conversion to pasture for soil carbon content and dynamics were examined in two soil types differing mainly by their texture. Two chronosequences were selected, each consisting of an intact forest and three pastures of different ages (4, 8, 15 years and 3, 9, 15 years, respectively). One chronosequence is located in the central part of the Brazilian Amazon basin, where the soils are clayey ferralsols, and the second in the Eastern Brazilian Amazon Basin, where the soils are sandy clayey acrisols. In the upper layer the C content of clayey soils was three times higher than in the sandy soils, but despite the differences in soil texture, the C distribution in the particle-size fractions was quite similar. In the two chronosequences, the conversion to pasture induced a slight increase in C content. Bulk density increases were greater on soils with lower clay contents. The 13 C measurements, which allowed to calculate the distribution of C derived from forest and from pasture, showed that all the particle-size fractions incorporated C derived from pasture and that a significant proportion of the young organic matter is rapidly trapped in the finest fractions. Although the proportions of pasture-derived C were higher in the sandy soils than in the clayey soils, the amounts of pasture-derived C in the particle-size fractions were 2–3 times larger in the clayey soils than in the sandy soils.

Root biomass and carbon in a tropical evergreen forest of Mexico: changes with secondary succession and forest conversion to pasture

Conversion of tropical evergreen forests to crops or pastures results in significant depletions of terrestrial carbon (C) pools. Root biomass and root C pools were quantified in tropical evergreen primary forest, and in secondary forests and pastures of different ages, in the Los Tuxtlas Region, Veracruz, Mexico. Total root biomass to 1-m depth ranged from 19 to 27 Mg ha-1 in primary forest, from 5.5 to 22.5 Mg ha-1 in secondary forests (8-, 20- and 30-y-old), and from 3.1 to 5.4 Mg ha-1 in pastures (12-, 20- and 28-y-old). Large roots (> 20 mm in diameter) were largely absent below 40 cm depth in secondary forests and pastures. Roots in the 0–40 cm soil depth represented 60–76% of the total root biomass in primary forest, 77–93% in secondary forests, and 89–96% in pastures. Root biomass comprised 4.7–6.2% of the total biomass in primary forests and between 6.8–8.5% in secondary forests. These low values, the relatively high concentration of roots in the top 40 cm of soil, and the shallow depth at which large roots occurred in secondary forests suggest forest susceptibility to natural disturbances. Root C pools ranged from 7.9 to 11.6 Mg ha-1 in primary forests, from 2.1 to 9.6 Mg ha-1 in secondary forests and from 1.0 to 1.9 Mg ha-1 in pastures. The estimated total ecosystem C pool in primary forest was 415 Mg ha-1, it ranged from 187-246 Mg ha-1 in secondary forests, and was 179 Mg ha-1 in pastures. Tropical forest conversion to pasture decreased the root C pool by nearly 80% and represented a 94% loss of C in ecosystem biomass. Absolute losses of root C were nevertheless small when compared with the above-ground C loss. Carbon distribution among ecosystem biomass components is key to adequately understanding the consequences of land-use/cover change on C dynamics in tropical regions.

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 Amazon's state of Rondônia. 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 Brazil's 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.

COMPARISON OF BIOLOGICAL NITROGEN FIXATION IN FOUR PAIRS OF CONVENTIONAL AND ALTERNATIVE MIXED CROPPING FARMS WITH THREE ROTATIONAL STAGES OF PASTURE ESTABLISHMENT IN CANTERBURY, NEW ZEALAND

It is generally recognized that in order for arable farming systems to remain sustainable in the long term, they need to be productive and their soil fertility must be maintained to continue to provide viable yields with minimum detrimental effects on soil and environmental quality. The use of biological nitrogen fixation (BNF) as the main source of nitrogen (N) for crops is an important sustainable management strategy. Amounts of BNF in four pairs of conventional and organic mixed cropping farms, each with three stages of pasture establishment in Canterbury, New Zealand were compared under farmers' field conditions over a period of one year using the 15N isotope dilution technique. Composite topsoil (0–75 mm) samples were taken from all sites at the beginning and end of the trials. These were analyzed for soil pH and available nutrients according to New Zealand “MAF Quick Test” methods. In addition, at the end of the trials, soil samples at four depths (0–50, 50–100, 100–150, 150–200 mm) were taken from inside and outside the experimental plots in pastures of three different ages in one of the pairs of conventional and organic farms and analyzed for total soil N. Results obtained showed that the amounts of biologically-fixed N varied from 19 to 68 kg N ha−1 year−1. There were no significant differences between paired conventional and organic farms or between pastures of different ages. This was attributed to the large variability present and the possibility that the farms studied were at a steady state condition in terms of BNF as supported by total soil N and available soil nutrient data. The BNF was significantly correlated with clover biomass and was also affected significantly by seasons.

Comparison of different methodologies for field measurement of net nitrogen mineralization in pasture soils under different soil conditions

Net mineralization was measured in free-draining and poorly drained pasture soils using three different field incubation methodologies. Two involved the use of enclosed incubation vessels (jar or box) containing C2H2 as a nitrification inhibitor. The third method confined soil cores in situ in an open tube in the ground, with an anion-exchange resin at the base to retain leached NO3– (resin-core technique, RCT). Measurements were made on three occasions on three free-draining pastures of different ages and contrasting organic matter contents. In general, rates of net mineralization increased with pasture age and organic matter content (range: 0.5–1.5 kg N ha–1 day–1) and similar rates were obtained between the three techniques for a particular pasture. Coefficients of variation (CVs) were generally high (range: 10.4–98.5%), but the enclosed incubation methods were rather less variable than the RCT and were considered overall to be the more reliable. The RCT did not include C2H2 and, therefore, newly formed NO3– may have been lost through denitrification. In a poorly drained pasture soil, there were discrepancies between the two enclosed methods, especially when the soil water content approached field capacity. The interpretation of the incubation measurements in relation to the flux of N through the soil inorganic N pool is discussed and the drawbacks of the various methodologies are evaluated.

The use of stable carbon isotopes for estimating soil organic matter turnover rates

In natural ecosystems, soil organic carbon (C) is derived almost exclusively from the residues of plants growing in situ. In agroecosystems, it has at least two origins: one is the remains from the previous native vegetation, and the other is the remains of the crop and the decomposition of its residues. Where vegetation has changed from plants with the C3 photosynthetic pathway to C4 pathway or vice versa, changes in the natural abundance of 13C in soil organic matter (SOM) over time can be used to identify sources of organic C in soil and to determine the turnover rate of SOM. For example, large areas of C3 tropical forest have been replaced with C4 pasture or cropland. Changes in the δ 13C values of soil organic C in these areas reflect soil organic matter turnover rate, and provide insight regarding the functional role of tropical ecosystems in the global C cycle. This paper illustrates how the stable isotope 13C can be used to estimate SOM turnover rates and the sensitivity of different models and different model parameters, using a chronosequence of forest and pastures of different ages from the Brazilian Amazon. A single-compartment exponential decay model and a two-compartment model in which SOM was divided into stable and labile components yielded similar estimates of soil C turnover time at the surface but divergent estimates at depth. The one-compartment model gave the least variable estimates of model parameters and turnover times and was also relatively insensitive to individual C stocks in single pastures of a particular age. Estimates of soil stable and labile C pools obtained using changes in forest soil δ 13C with depth differed from estimates obtained using the chronosequence. This suggests that upon burning and pasture creation, a portion of the previously stable soil C pool is rendered less stable. Model r 2 was a poor criterion for selecting an appropriate soil C turnover model to apply to chronosequence data. In the absence of substantial justification for segregating SOM into different compartments based on lability, modeling should be done with the simplest models possible.

Land cover mapping and carbon pools estimates in Rondonia, Brazil

In order to estimate changes in carbon pools and fluxes to the atmosphere, we used Landsat TM data to calculate the extent of areas converted to pasture to different lengths of time in a 100km by 92km area of the southwestern Brazilian Amazon. Image processing and the supervised classification allowed the production of an accurate land cover map including forest and pastures of different ages. Estimates of carbon pools and fluxes to the atmosphere were carried out using the results of Landsat image analysis and published data about carbon stocks in vegetation and soil. The results showed that a large amount of carbon (12 103 g cm2) is released after three years under pasture. From the initial burning and after fifth year under pasture, the system functioned as a source of CO2 to atmosphere. After this period the balance is slightly negative and the amount of CO2 released is lower than the amount released in the initial period.

SOIL CARBON AND NITROGEN STOCKS FOLLOWING FOREST CLEARING FOR PASTURE IN THE SOUTHWESTERN BRAZILIAN AMAZON

Tropical soils contain large stocks of carbon and nitrogen that can be altered by clearing for agriculture. In the Brazilian Amazon, cattle pasture is the predominant use for cleared forest lands. We examined changes to soil bulk density and C and N stocks in seven chronosequences, each consisting of an intact forest and pastures of different ages created directly from cleared forest (7 forests, 18 pastures), along a 700-km transect in Rondônia in the southwestern Amazon Basin. The transect included sites with a similar climate but a range of soil types. We used soil δ13C distributions to determine the origin of soil C and to infer changes to soil C cycling patterns after forest clearing. Soil bulk density increased under pasture; these increases were significant in 6 of 18 pastures examined. Changes in C stocks to a depth of 30 cm under pasture ranged from a loss of 0.72 kg/m2 to an increase of 1.77 kg/m2. Soil C stocks increased in 14 of 18 pastures, but these increases were significant in only 4 pastures. Changes in soil N stocks to a depth of 30 cm ranged from a loss of 0.25 kg/m2 to a gain of 0.23 kg/m2 and showed a similar pattern to C, except in one site where we measured significant N loss. Five of 18 pastures accumulated significant amounts of N, and one pasture lost a significant amount of N. Soil δ13C values were greater in pastures than in the original forests, and δ13C values increased with a longer time under C4 pasture vegetation. Bulk density increases were greater on soils with higher clay contents. Carbon accumulation increased with pasture age but was independent of soil texture. Soil C increases to a depth of 30 cm of up to 1.77 kg/m2 amounted to an increase of >50% of the original soil C stock and represented up to 12% of the C in the biomass of forest vegetation. In contrast, changes to soil N stocks in the range of 0.25 kg/m2 approximately equaled the N stock in the original forest vegetation. Our results indicated that when site history was controlled by considering only pastures formed directly from cleared forest, C and N accumulation was the dominant trend in pasture soils. Absence of a correlation between C and N accumulation and soil texture suggested that site history and management may be more important than soil type as determinants of the direction and magnitude of changes in soil C and N stocks.

The colonization of dung pats and molehills in permanent pastures

The colonization of areas affected by molehills and dung pats was monitored in three adjacent pastures of different ages. Seedling establishment was infrequent, and most colonization took place by rhizomes, e.g., Agropyron repens and Poa compressa, or by stolons, e.g., Trifolium repens. Tillering species, e.g., Dactylis glomerata, Holcus lanatus, and Lolium perenne, were poor colonizers of disturbed areas. Invasion of gaps by a species was not proportional to its abundance in the pasture. Individual plants are eliminated at random by the disturbances. Small-scale disturbances tend to decrease community heterogeneity. Key words: disturbance, colonization, dung, molehill, pasture.