Pasture Age Research Articles

Effects of Kikuyu grass (Pennisetum clandestinum) age and different forage: concentrate ratios on methanogenesis

ABSTRACT Objective. To evaluate the effect of Kikuyu grass (Pennisetum clandestinum) harvested at two different ages and three forage: concentrate supplement ratios (F/C) on methane (CH4) production, dry matter digestibility (DMD), and fermentation profile using the in vitro gas production technique. Materials and methods. six treatments, resulting from the combination of pasture age (30 or 60 days) and F/C (100/0, 75/25, or 50/50) were evaluated using a 2x3 factorial design. The response variables were measured 6, 12, 24 and 48 hours after incubation. A repeated-measure over time design was used to analyze the data, and differences between means were determined with the LSMEANS procedure of SAS. Results. the youngest grass (30 days) was more digestible, produced less CH4 per gram of digestible dry matter (dDM) and more total volatile fatty acids (VFA) compared to the oldest grass (60 days; p <0.05). Reductions of the F/C ratio increased DMD and CH4 production per gram of dDM (p<0.05) but had no significant effect on VFA concentration (p>0.05). Conclusions. under in vitro conditions and pH close to neutrality, the older grass reduces DMD and increases CH4 production per gram of dDM, while a F/C reduction increases DMD and CH4 production per gram of dDM, which differs with reports conducted in vivo.

Carbon Stocks in Silvopastoral Systems: A Study from Four Communities in Southeastern Ecuador

AbstractAgriculture, particularly pasture, is the second largest source of greenhouse gas emissions from tropical regions. Silvopastoral systems may increase carbon pools in pastures while maintaining productivity. Adding trees to pasture provides carbon sinks in woody biomass, and may improve degraded soils and increase the stability of soil carbon pools. In this study we quantified the biomass carbon stored in spontaneous silvopastoral systems in southeastern Ecuador. We compared the stem density, basal area, aboveground biomass, and organic carbon in the top 20 cm of soil in 100 pastures, ranging from 3 to 250 hectares, in four different communities. Aboveground live woody biomass, calculated using allometric equations and two different wood densities, varied from 10.99 to 66.1 Mg per hectare. Soil organic carbon pools ranged from 85.0 to 97.6 Mg per hectare. Stem density, basal area, and aboveground live biomass all positively correlated with pasture age. We found no relationship between pasture age or stem density and soil organic carbon pools. We measured live woody biomass carbon pools of 34–1070 Mg of carbon per farm in these silvopastoral systems. We found no effects on productivity of the herbaceous layer, suggesting that having a low density of trees in pastures could substantially increase the number of trees and the associated carbon sequestration without affecting cattle production.

Soil carbon under perennial pastures; benchmarking the influence of pasture age and management

This paper reports baseline soil carbon stocks from a field survey of 19 sites; 8 pairs/triplet in the Monaro region of New South Wales. Site comparisons were selected by the Monaro Farming Systems group to demonstrate the influence of land management on soil carbon, and included: nutrient management, liming, pasture age and cropping history. Soil carbon stocks varied with parent material and with land management. The fertilised (phosphorus) native perennial pasture had a greater stock of soil carbon compared with the unfertilised site; 46.8 vs 40.4 Mg.C.ha to 0.50 m. However, the introduced perennial pasture which had been limed had a lower stock of soil carbon compared with the unlimed site; 62.8 vs 66.7 Mg.C.ha to 0.50 m. There was a greater stock of soil carbon under two of the three younger (<10 yr old) perennial pastures compared with older (>35 yr old) pastures. Cropped sites did not have lower soil carbon stocks at all sites; however, this survey was conducted after three years of above average annual rainfall and most sites had been cropped for less than three years. At all sites more than 20% of the total carbon stock to 0.50 m was in the 0.30 to 0.50 m soil layer highlighting the importance of considering this soil layer when investigating the implications of land management on soil carbon. Our baseline data indicates that nutrient management may increase soil carbon under perennial pastures and highlights the importance of perennial pastures for soil carbon sequestration regardless of age.

Changes in soil C, N and δ15N along three forest–pasture chronosequences in New Zealand

Changes in total soil carbon (C), nitrogen (N) and natural-abundance N isotopes (δ15N) were measured along three forest-to-pasture chronosequences on pumice soils in the Central North Island of New Zealand. On each of the three chronosequences, exotic pine forests had been converted to intensive dairy pastures 2–11 years before sampling and samples were also taken from remaining pine forests and long-term pastures (40–80 years old). The primary objective of the study was to test the hypothesis that surface-soil δ15N would increase over time following conversion of forest to pasture, due to greater N inputs and isotope-fractionating N losses (e.g. ammonia volatilisation) in pasture systems. Results supported our hypothesis, with linear regression revealing a significant (P &lt; 0.001) positive correlation between log-transformed pasture age (log10[pasture age + 1]) and surface-soil δ15N. There was also a positive correlation (P &lt; 0.001) between pasture age and total soil C and N, and a negative correlation of pasture age with C : N ratio. Surface-soil δ15N was also positively correlated (P &lt; 0.001) with total soil N, and negatively correlated with C : N ratio when C : N was &lt;13.6. These results suggested that as soils became more N-‘saturated’, isotope-fractionating N loss processes increased. Surface-soil δ15N in the pine forests was significantly less than subsoil δ15N, but there was no significant difference between the surface and subsoil in the long-term pastures, due to 15N enrichment of the surface soil. The difference in δ15N between the surface soil and subsoil may be a useful indicator of past land management, in addition to absolute δ15N values of surface soils.

Carbon sequestration under subtropical perennial pastures II: Carbon dynamics

Here we take advantage of the stable carbon isotope shift that occurs when a C4 plant is sown into a soil previously dominated by C3 vegetation, to explore the movement and fate of newly sequestered soil organic carbon (SOC) following establishment of subtropical perennial pastures in temperate regions of Australia. In kikuyu-based pastures up to 33 years of age, SOC accumulated exclusively in the coarse size fraction (&gt;50 μm) in the sandy soils of southern Western Australia. In South Australian loams, regardless of pasture age, new SOC was found to accumulate in both the coarse and fine (&lt;50 μm) size fractions. These differential results suggest that in soils with low clay content, new SOC remains in an unprotected form that is highly vulnerable to loss through decomposition and erosion. The Rothamsted Carbon Model, modified to track changes in stable isotopes, was able to represent the changes in total SOC stocks in both regions; however, the model over-predicted the incorporation of the new C4-SOC into the soil. This difference between data and model output could be reconciled if a greater proportion of new SOC is rapidly mineralised without being incorporated into any sort of stabilised pool.

Consequences of forest conversion to pasture and fallow on soil microbial biomass and activity in the eastern Amazon

AbstractThe main change in soil use in Amazonia is, after slash and burn deforestation followed by annual crops, the establishment of pastures. This conversion of forest to pasture induces changes in the carbon cycle, modifies soil organic matter content and quality and affects biological activity responsible for numerous biochemical and biological processes essential to ecosystem functioning. The aim of this study was to assess changes in microbial biomass and activity in fallow and pasture soils after forest clearing. The study was performed in smallholder settlements of eastern Brazilian Amazonia. Soil samples from depths of 0–2, 2–5 and 5–10 cm were gathered in native forest, fallow land 8–10 yr old and pastures with ages of 1–2, 5–7 and 10–12 yr. Once fallow began, soil microbial biomass and its activity showed little change. In contrast, conversion to pasture modified soil microbial functioning significantly. Microbial biomass and its basal respiration decreased markedly after pasture establishment and continued to decrease with pasture age. The increase in metabolic quotient in the first years of pasture indicated a disturbance in soil functioning. Our study confirms that microbial biomass is a sensitive indicator of soil disturbance caused by land‐use change.

In an Ecuadorian pasture soil the growth of Setaria sphacelata, but not of soil microorganisms, is co-limited by N and P

In the mountain rainforest region of southern Ecuador, soils of active pastures, established after slash-and-burn of the forest, are characterized by improved quantity and quality of soil organic matter favoring microbial conditions. However, these beneficial conditions decrease with increasing pasture age and burning frequency. As a consequence, rates of soil nutrient cycling decrease, supporting the infestation of bracken fern and, in turn, causing further decreases in pasture productivity. Finally, farmers are forced to abandon the degraded pastures and to establish new ones by continuous deforestation. To investigate whether an application of N and/or P nutrients to an extensively grazed pasture (active pasture) can improve grass productivity and maintain soil fertility, a pasture fertilization experiment was conducted. On an active pasture site, planted with Setaria sphacelata, moderate rates of urea (50kgNha−1a−1), rock phosphate (10kgPha−1a−1), and a combination of both were applied. It was examined whether soil mineralization (gross and net N mineralization, SOC mineralization) and microbial community structure (PLFA-analysis), as well as quantity and quality of the grass biomass, were affected by fertilization. Furthermore, the impact of fertilization on in situ soil respiration rates was studied. The combined application of urea and rock phosphate increased the pasture yield by 2Mgha−1a−1 most efficiently, indicating a co-limitation of growth. Additionally, the fodder quality was improved by a 67% higher content of P and by a 7% higher content of Ca in the grass biomass compared to the control. While carbon, nitrogen, and phosphorus in the microbial biomass remained unaffected and the microbial activity increased only temporarily, the relative abundance of fungi (18:2n6,9) increased significantly due to fertilizer addition. Urea addition induced a short-lived increase in the in situ soil CO2-C effluxes, assuming a positive priming effect due to an activation of soil microbes. In total, plots amended with urea emitted 0.8MgCO2-Cha−1a−1 more than the control. Results reveal that already moderate fertilization significantly improved pasture productivity and maintained soil quality. However, it is expected that higher loads of NP fertilizer will increase pasture productivity at the expense of soil organic carbon sequestration due to enhanced soil CO2-C losses. Hence, to establish a sustainable pasture management in the study region, the soil C management must also be carefully considered.

Nutrient stocks and phosphorus fractions in mountain soils of Southern Ecuador after conversion of forest to pasture

Understanding pasture degradation processes is the key for sustainable land management in the tropical mountain rainforest region of the South Ecuadorian Andes. We estimated the stocks of total carbon and nutrients, microbial biomass and different P fractions along a gradient of land-uses that is typical of the eastern escarpment of the Cordillera Real i.e., old-growth evergreen lower montane forest, active pastures (17 and 50 years-old), abandoned pastures 10 and 20 years old with bracken fern or successional vegetation. Conversion of forest to pasture by slash-and-burn increased the stocks of SOC, TN, P and S in mineral topsoil of active pasture sites. Microbial growth in pasture soils was enhanced by improved availability of nutrients, C:N ratio, and increased soil pH. Up to 39 % of the total P in mineral soil was stored in the microbial biomass indicating its importance as a dynamic, easily available P reservoir at all sites. At a 17 years-old pasture the stock of NH4F extractable organic P, which is considered to be mineralisable in the short-term, was twice as high as in all other soils. The importance of the NaOH extractable organic P pool increased with pasture age. Pasture degradation was accelerated by a decline of this P stock, which is essential for the long-term P supply. Stocks of microbial biomass, total N and S had returned to forest levels 10 years after pasture abandonment; soil pH and total P 20 years after growth of successional bush vegetation. Only the C:N ratio increased above forest level indicating an ongoing loss of N after 20 years. Soil nutrient depletion and microbial biomass decline enforced the degradation of pastures on the investigated Cambisol sites.

Microbial biomass, microbial diversity, soil carbon storage, and stability after incubation of soil from grass–clover pastures of different age

A laboratory incubation study with clover grass pasture soils of seven different ages (0, 1, 2, 3, 4, 5, and 16 production years) was carried out to determine initial soil carbon (C) and nitrogen (N) stocks and potentials for greenhouse gas emissions (N2O and CO2). Compared with the soil from the recently established pasture, an increase of total soil C and N was observed along with pasture age. Greenhouse gas emissions were low and not significantly different among the soils from younger pastures (0–5 years), but especially N2O emissions increased markedly in the soil from 16-year-old grass–clover. Low emissions might mainly be due to an early C limitation occurring in the soils from younger pastures, which was also corroborated by decreasing levels of cold water-extractable C and early shifts within the microbial community. However, higher emissions from the old pasture soil were offset by its increase in total soil C. A longer ley phase without soil disturbance may therefore be beneficial in terms of overall C sequestration in systems with temporary grass–clover swards.

Consequences of Buffelgrass Pasture Development for Primary Productivity, Perennial Plant Richness, and Vegetation Structure in the Drylands of Sonora, Mexico

In large parts of northern Mexico native plant communities are being converted to non-native buffelgrass (Pennisetum ciliare) pastures, and this conversion could fundamentally alter primary productivity and species richness. In Sonora, Mexico land conversion is occurring at a regional scale along a rainfall-driven gradient of primary productivity, across which native plant communities transition from desert scrub to thorn scrub. We used a paired sampling design to compare a satellite-derived index of primary productivity, richness of perennial plant species, and canopy-height profiles of native plant communities with buffelgrass pastures. We sampled species richness across a gradient of primary productivity in desert scrub and thorn scrub vegetation to examine the influence of site productivity on the outcomes of land conversion. We also examined the influence of pasture age on species richness of perennial plants. Index values of primary productivity were lower in buffelgrass pastures than in native vegetation, which suggests a reduction in primary productivity. Land conversion reduced species richness by approximately 50% at local and regional scales, reduced tree and shrub cover by 78%, and reduced canopy height. Land conversion disproportionately reduced shrub species richness, which reflects the common practice among Sonoran ranchers of conserving certain tree and cactus species. Site productivity did not affect the outcomes of land conversion. The age of a buffelgrass pasture was unrelated to species richness within the pasture, which suggests that passive recovery of species richness to preconversion levels is unlikely. Our findings demonstrate that land conversion can result in large losses of plant species richness at local and regional scales and in substantial changes to primary productivity and vegetation structure, which casts doubt on the feasibility of restoring native plant communities without active intervention on the part of land managers.

Carbon content in Amazonian Oxisols after forest conversion to pasture

Soil plays an important role in the C cycle, and substitution of tropical forest by cultivated land affects C dynamic and stock. This study was developed in an area of expansion of human settlement in the Eastern Amazon, in Itupiranga, State of Pará, to evaluate the effects of native forest conversion to Brachiaria brizantha pasture on C contents of a dystrophic Oxisol. Soil samples were collected in areas of native forest (NF), of 8 to 10 year old secondary forest (SF), 1 to 2 year old SF (P1-2), 5 to 7 year old SF (P5-7), and of 10 to 12 year old SF (P10-12), and from under pastures, in the layers 0-2, 2-5 and 5-10 cm, to evaluate C levels and stocks and carry out separation of OM based on particle size. After deforestation, soil density increased to a depth of 5 cm, with greater increase in older pastures. Variation in C levels was greatest in the top soil layer; C contents increased with increasing pasture age. In the layers 2-5 and 5-10 cm, C content proved to be stable for the types of plant cover evaluated. Highest C concentrations were found in the silt fraction; however, C contents were highest in the clay fraction, independent of the plant cover. An increase in C associated with the sand fraction in the form of little decomposed organic residues was observed in pastures, confirming greater sensitivity of this fraction to change in soil use.

Urea fertilisation affected soil organic matter dynamics and microbial community structure in pasture soils of Southern Ecuador

In the mountain rainforest region of the South Ecuadorian Andes natural forests have often been converted to pastures by slash-and-burn practice. With advanced pasture age the pasture grasses are increasingly replaced by the tropical bracken leading to the abandonment of the sites. To improve pasture productivity a fertilisation experiment with urea was established. The effects of urea on soil organic matter (SOM) mineralisation and microbial community structure in top soil (0–5 cm depth) of an active and abandoned pasture site have been investigated in laboratory incubation experiments. Either 14C- or 15N-labelled urea (74 mg urea-N kg −1 dw soil) was added to track the fate of 14C into CO 2 or microbial biomass and that of 15N into the KCl-extractable NH 4-N or NO 3-N or microbial biomass pool. The soil microbial community structure was assessed using phospholipid fatty acid analysis (PLFA). In a second experiment two levels of 14C-labelled urea (74 and 110 mg urea-N kg −1 dw soil) were added to soil from 5 to 10 cm depth of the respective sites. Urea fertilisation accelerated the mineralisation of SOC directly after addition up to 17% compared to the non-fertilised control after 14 days of incubation. The larger the amount of N potentially available per unit of microbial biomass N the larger was the positive priming effect. Since in average 80% of the urea-C had been mineralised already 1 day after amendment, the priming effect was strong enough to cause a net loss of soil C. Although the structure of the microbial community was significantly different between sites, urea fertilisation induced the same alteration in microbial community composition: towards a relative lower abundance of PLFA marker characteristic of Gram-positive bacteria and a higher one of those typical of Gram-negative bacteria and fungi. This change was positively correlated with the increase in NH 4, NO 3 and DON availability. In addition to the activation of different microbial groups the abolishment of energy limitation of the microbes seemed to be an important mechanism for the enhanced mineralisation of SOM.

Objective indicators of pasture degradation from spectral mixture analysis of Landsat imagery

Degradation of cattle pastures is a management concern that influences future land use in Amazonia. However, “degradation” is poorly defined and has different meanings for ranchers, ecologists, and policy makers. Here we analyze pasture degradation using objective scalars of photosynthetic vegetation (PV), nonphotosynthetic vegetation (NPV), and exposed soil (S) derived from Landsat imagery. A general, probabilistic spectral mixture model decomposed satellite spectral reflectance measurements into subpixel estimates of PV, NPV, and S covers at ranches in western and eastern Amazonia. Most pasture management units at all ranches fell along a single line of decreasing PV with increasing NPV and S, which could be considered a degradation continuum. The ranch with the highest stocking densities and most intensive management had greater NPV and S than a less intensively managed ranch. The number of liming, herbiciding, and disking treatments applied to each pasture management unit was positively correlated with NPV and negatively correlated with PV. Although these objective scalars revealed signs of degradation, intensive management kept exposed soil to &lt;40% cover and maintained economically viable cattle production over several decades. In ranches with few management inputs, the high PV cover in young pastures declined with increasing pasture age, while NPV and S increased, even where grazing intensity was low. Both highly productive pastures and vigorous regrowth of native vegetation cause high PV values. Analysis of spectral properties holds promise for identifying areas where grazing intensity has exceeded management inputs, thus increasing coverage of senescent foliage and exposed soil.

Soil organic carbon storage in grazing pasture converted from forest on Andosol soil

AbstractIn order to clarify the effect of land‐use change from forest to grazing pasture on the organic carbon storage in Andosol soil, the Rothamsted carbon turnover model for volcanic soil was applied to a pasture situated at the National Livestock Breeding Center (37°09′N, 140°03′E). The top 25‐cm soil layer was considered to be an active soil carbon pool. The carbon storage in the soils of native forest surrounding the pastures ranged 111–163 t C ha−1 with an average of 133 t C ha−1, which was adjusted according to an equivalent soil weight of pasture. The pasture soil carbon stocks ranged 88–135 t C ha−1, with variations according to site and/or pasture age. The carbon inputs to the soil through the above‐ and below‐ground dead material from pasture plants and cattle feces were estimated to be 1.1, 1.8 and 0.9 t C ha−1 year−1, respectively. As the model outputs of 14C content of the soil, which is an index of carbon dating corresponding to nuclear weapons testing, showed a relatively close agreement with the observations, the modeling was acceptable for the purpose of predicting the turnover of organic carbon in Andosol soil. The model simulation demonstrated that, in order to maintain the average forest carbon level, 3–4 t ha−1 year−1 of the organic carbon input would be needed. These inputs would be provided in a grazing pasture producing 8–9 t ha−1 year−1 of above‐ground dry matter.

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.

Regeneration of Seasonal Deciduous Forest Tree Species in Long‐Used Pastures in Central Brazil

ABSTRACTWe tested the relationship between the length of pasture use and the density, richness, and composition of naturally regenerating tropical seasonal deciduous forest in pastures. We sampled regenerating trees in 25 pastures distributed in four age classes ranging from &lt; 6 to 40 yr of use. Density and composition of regenerating trees did not change with pasture age, but richness was lower in 25‐ and 40‐yr‐old pastures. Nonetheless, a number of species seem to be able to resprout even after 40 yr of ranching.

Temporal nutrient variation in soil and vegetation of post-forest pastures as a function of soil order, pasture age, and management, Rondônia, Brazil

Abstract Understanding pasture degradation processes is key for sustainable development in the Amazon region. Pasture degradation can be indexed by pasture nutritional status, which is impacted by environmental and human related factors. Most studies of the biogeochemistry of Amazonian pastures have been restricted to highly acidic soils with little focus on how management affects pasture biogeochemistry. We investigated soil physical and chemical properties and grass nutrients under different edaphic conditions and management practices in Rondonia state, in the southwestern Amazon region. Our objectives were to analyze the impacts of soil order, pasture age, and pasture management on soil fertility, bulk density and plant nutrients, in order to understand sustainability needs for the future of this region. We sampled 17 study sites on Oxisols, Ultisols, and Alfisols. The pasture age classes we used consisted of 6–10, 11–15, and >16 years including forest as the control. We also evaluated the impact of beef and dairy management strategies on pastures installed on Ultisols. Pastures on Alfisols showed the highest soil P, Ca, and base saturation, properties that decreased gradually after 5 years of pasture installation. In contrast, pastures on Oxisols showed low and stable levels of nutrients regardless of age. Foliar nutrient concentrations appear to be influenced by edaphic conditions. Soil P and foliar P concentration were highly correlated. In general, pasture age was a poor indicator of soil fertility and foliar chemistry because of high variability among pastures belonging to the same soil order. In terms of the impacts of pasture management, we did not observe significant differences in soil or foliar properties in pasture used for beef and dairy on Ultisols.

Nitrous oxide fluxes and nitrogen cycling along a pasture chronosequence in Central Amazonia, Brazil

Abstract. We studied nitrous oxide (N2O) fluxes and soil nitrogen (N) cycling following forest conversion to pasture in the central Amazon near Santarém, Pará, Brazil. Two undisturbed forest sites and 27 pasture sites of 0.5 to 60 years were sampled once each during wet and dry seasons. In addition to soil-atmosphere fluxes of N2O we measured 27 soil chemical, soil microbiological and soil physical variables. Soil N2O fluxes were higher in the wet season than in the dry season. Fluxes of N2O from forest soils always exceeded fluxes from pasture soils and showed no consistent trend with pasture age. At our forest sites, nitrate was the dominant form of inorganic N both during wet and dry season. At our pasture sites nitrate generally dominated the inorganic N pools during the wet season and ammonium dominated during the dry season. Net mineralization and nitrification rates displayed large variations. During the dry season net immobilization of N was observed in some pastures. Compared to forest sites, young pasture sites (≤2 years) had low microbial biomass N and protease activities. Protease activity and microbial biomass N peaked in pastures of intermediate age (4 to 8 years) followed by consistently lower values in older pasture (10 to 60 years). The C/N ratio of litter was low at the forest sites (~25) and rapidly increased with pasture age reaching values of 60-70 at pastures of 15 years and older. Nitrous oxide emissions at our sites were controlled by C and N availability and soil aeration. Fluxes of N2O were negatively correlated to leaf litter C/N ratio, NH4+-N and the ratio of NO3--N to the sum of NO3--N + NH4+-N (indicators of N availability), and methane fluxes and bulk density (indicators of soil aeration status) during the wet season. During the dry season fluxes of N2O were positively correlated to microbial biomass N, β-glucosidase activity, total inorganic N stocks and NH4+-N. In our study region, pastures of all age emitted less N2O than old-growth forests, because of a progressive decline in N availability with pasture age combined with strongly anaerobic conditions in some pastures during the wet season.

Relationship Between Gross Nitrogen Cycling and Nitrous Oxide Emissionin Grass-clover Pasture

Replacement of high-input N fertilized pastures with low-input grass-legume pastures may provide a mitigation option to reduce agricultural N2O emissions. This study examined the relationship between N-cycling rates and N2O production and evolution from the root zone of grass-clover pastures of various ages (production year 1, 2 and 8). The experimental approach included cross-labelling pasture monoliths with 15N-enriched substrates to identify sources of N2O, in combination with assessment of gross N mineralization and nitrification. Nitrous oxide emissions were generally low, fluctuating between 82 and 136μg N2O–N m−2 d−1, independent of pasture age. The 15N labelling indicated that at least 50% of the N2O was derived from the soil NH 4 + pool, approaching 100% in June. In the two year old pasture the NH 4 + pool contributions to N2O emissions varied significantly with sampling time. Emission rates of N2O correlated positively with soil NH 4 + concentrations and the NH 4 + supply as expressed by gross mineralization. The N2O emissions showed a significant inverse relationship with soil NO 3 − , but was not correlated with the supply of NO 3 − as expressed by gross nitrification. The ratio N2O vs. nitrification averaged 0.05% (range 0.004 to 0.29%) and varied with sampling time showing the lowest value in wet soil conditions.

Regional variation in soil carbon and δ13C in forests and pastures of northeastern Costa Rica

Recent studies suggest that the direction and magnitude of changes in soil organic carbon (soil C) pools following forest-to-pasture conversion in the tropics are dependent upon initial soil conditions and local factors (e.g. pre-conversion soil C content, soil texture, vegetation productivity, and management practices). The goal of this study was to understand how landscape-scale variation in soil-forming factors influenced the response of soil C pools to forest clearing and pasture establishment in northeastern Costa Rica. We measured soil C and its stable isotopic composition in 24 paired pasture and reference forest sites distributed over large gradients of edaphic characteristics and slope throughout a 1400 km2 region. We used the large difference in stable C isotopic signatures of C3 vegetation (rain forest) versus C4 vegetation (pasture grasses) as a tracer of soil C dynamics. Soil C pools to 30 cm depth ranged from 26% lower to 23% higher in pastures compared to paired forests. The presence of non-crystalline clays and percent slope explained between 27 and 37% of the variation in the direction and magnitude of the changes in soil C storage following pasture establishment. Stable carbon isotopes (δ13C) in the top soil (0–10 cm) showed a rapid incorporation of pasture-derived C following pasture establishment, but the vegetation in these pastures never became pure C4 communities. The amount of forest-derived soil C in pasture topsoils (0–10 cm) was negatively correlated to both pasture age and the concentrations of non-crystalline iron oxides. Together these results imply that site factors such as soil mineralogy are an important control over soil C storage and turnover in this region.