The effect of the presence of a forage legume on nitrogen and carbon levels in soils under Brachiaria pastures in the Atlantic forest region of the South of Bahia,Brazil

The impact of forest clearance, and its replacement by Brachiaria pastures, on soil carbon reserves has been studied at many sites in the Brazilian Amazonia, but to date there appear to be no reports of similar studies undertaken in the Atlantic forest region of Brazil. In this study performed in the extreme south of Bahia, the changes in C and N content of the soil were evaluated from the time of establishment of grass-only B. humidicola and mixed B. humidicola/Desmodium ovalifolium pastures through 9 years of grazing in comparison with the C and N contents of the adjacent secondary forest. The decline in the content of soil C derived from the forest (C3) vegetation and the accumulation of that derived from the Brachiaria (C4) were followed by determining the ¹³C natural abundance of the soil organic matter (SOM). The pastures were established in 1987, 10 years after deforestation, and it was estimated that until 1994 there was a loss in forest-derived C in the top 30 cm of soil of approximately 20% (9.1 Mg C ha^–1). After the establishment of the pastures, C derived from Brachiaria accumulated steadily such that at the final sampling (1997) it was estimated 13.9 Mg ha^–1 was derived from this source under the grass-only pasture (0–30 cm). Samples taken from all pastures and the forest in 1997 to a depth of 100 cm showed that below 40 cm depth there was no significant contribution of the Brachiaria-derived C and that total C reserves under the grass/legume and the grass-only pastures were slightly higher than under the forest (not significant at P=0.05). The more detailed sampling under the pastures showed that to a depth of 30 cm there was significantly (P<0.05) more C under the mixed pasture than the grass-only pasture. It was estimated that from the time of establishment the apparent rate of C accumulation (0–100 cm depth) under the grass/legume pastures (1.17 Mg ha^–1 yr^–1) was almost double that under the grass-only pastures (0.66 Mg ha^–1 yr^–1). The data indicated that newly incorporated SOM derived from the Brachiaria had a considerably higher C:N ratio than that present under the forest.     

Modeling changes in soil organic matter in Amazon forest to pasture conversion with the Century model

Land use and land cover changes in the Brazilian Amazon region have major implications for regional and even global carbon cycling. We analyzed the effects of the predominant land use change, conversion of tropical forest to pasture, on total soil C and N, using the Century ecosystem model and data collected from the Nova Vida ranch, Western Brazilian Amazon. We estimated equilibrium organic matter levels, plant productivity and residue carbon inputs under native forest conditions, then simulated deforestation following the slash and burn procedure. Soil organic matter dynamics were simulated for pastures established in 1989, 1987, 1983, 1979, 1972, 1951, and 1911. Using input data from the Nova Vida ranch, the Century model predicted that forest clearance and conversion to pasture would cause an initial decline in soil C and N stocks, followed by a slow rise to levels exceeding those under native forest. Simulated soil total C and N levels (2500 g C m^?2 and 245 g N m^?2 in the 0–20 cm layer) prior to conversion to pasture were close to those measured in the native forest. Simulated above‐ and below‐ground biomass for the forest and pasture were comparable with literature values from this region. The model predicted the long‐term changes in soil C and N under pasture inferred from the pasture chronosequence, but there was considerable variation in soil C stocks for pastures <20 years in age. Differences in soil texture between pastures were relatively small and could not account for much of the variability between different pastures of similar ages, in either the measured or simulated data. It is likely that much of the variability in C stocks between pastures of similar ages is related to initial C stocks immediately following deforestation and that this was the largest source of variability in the chronosequence. Internal C cycling processes in Century were evaluated using measurements of microbial biomass and soil δ¹³C. The relative magnitude and long‐term trend in microbial biomass simulated by the model were consistent with measurements. The close fit of simulated to measured values of δ¹³C over time suggests that the relative loss of forest‐derived C and its replacement by pasture‐derived C was accurately predicted by the model. After 80 years, almost 90% of the organic matter in the top 20 cm was pasture derived. While our analysis represents a single ‘case study’ of pasture conversion, our results suggest that modeling studies in these pasture systems can help to evaluate the magnitude of impacts on C and N cycling, and determine the effect of management strategies on pasture sustainability.     

Net changes of soil C stocks in two grassland soils 26 months after simulated pasture renovation including biochar addition

The use of deep‐rooting pasture species as a management practice can increase the allocation of plant carbon (C) below ground and enhance C storage. A 2‐year lysimeter trial was set up to compare changes in C stocks of soils under either deep‐ or shallow‐rooting pastures and investigate whether biochar addition below the top 10 cm could promote root growth at depth. For this i) soil ploughing at cultivation was simulated in a silt loam soil and in a sandy soil by inverting the 0 to 10 and 10‐ to 20‐cm‐depth soil layers, and a distinctive biochar (selected for each soil to overcome soil‐specific plant growth limitations) was mixed at 10 Mg ha^?1 in the buried layer, where appropriate and ii) three pasture types with contrasting root systems were grown. In the silt loam, soil inversion resulted in a general loss of C (2.0–8.1 Mg ha^?1), particularly in the buried horizon, under shallow‐rooting pastures only. The addition of a C‐rich biochar (equivalent to 7.6 Mg C ha^?1) to this soil resulted in a net C gain (21–40% over the non‐biochar treatment, P < 0.10) in the buried layer under all pastures; this overcame the loss of C in this horizon under shallow‐rooting pastures. In the sandy soil, all pastures were able to maintain soil C stocks at 10–20 cm depth over time, with minor gains of C (1.6–5.1 Mg ha^?1) for the profile. In this soil, the exposure of a skeletal‐ and nutrient‐depleted soil layer at the surface may have fostered root growth at depth. The addition of a nutrient‐rich biochar (equivalent to 3.6 Mg C ha^?1) to this soil had no apparent effect on C stocks. More research is needed to understand the mechanisms through which soil C stocks at depth are preserved.     

Key processes of the nitrogen cycle in an irrigated and a non-irrigated grazed pasture

The paper presents integrated measurements of N fixation, net mineralisation, pasture yield and change in soil mineral N over a 12 month period for dairy pastures on a sandy loam soil in the South East of South Australia. The two adjacent pastures studied were an irrigated perennial white clover-ryegrass and an annual non-irrigated subterranean clover with mixed annual grasses. This produced the most comprehensive mineral N balance reported for grazed pastures, to the authors' knowledge, allowing calculation of gaseous and leaching losses of N (210 kg ha^–1 in the irrigated and paddock and 81 kg ha^–1 in the non irrigated paddock) primarily from urine patches. In both paddocks these losses were about three times the N yield in milk (61 and 28 kg N ha^–1 respectively) and were replenished by biological N fixation (294 and 100 kg N ha^–1). However, mineralisation of soil organic N, excretal N and pasture residues (687 and 438 kg N ha^–1) was the major source of mineral N for cycling and losses. The results demonstrate the enormous impact of pasture management on N fluxes and reinforce the importance of livestock urine on the magnitude of N fluxes including gaseous and leaching losses.     

Carbon Sequestration in Reclaimed Minesoils

Minesoils are drastically influenced by anthropogenic activities. They are characterized by low soil organic matter (SOM) content, low fertility, and poor physicochemical and biological properties, limiting their quality, capability, and functions. Reclamation of these soils has potential for resequestering some of the C lost and mitigating CO₂ emissions. Soil organic carbon (SOC) sequestration rates in minesoils are high in the first 20 to 30 years after reclamation in the top 15 cm soil depth. In general, higher rates of SOC sequestration are observed for minesoils under pasture and grassland management than under forest land use. Observed rates of SOC sequestration are 0.3 to 1.85 Mg C ha^{? 1} yr^{? 1} for pastures and rangelands, and 0.2 to 1.64 Mg C ha^{? 1} yr^{? 1} for forest land use. Proper reclamation and postreclamation management may enhance SOC sequestration and add to the economic value of the mined sites. Management practices that may enhance SOC sequestration include increasing vegetative cover by deep-rooted perennial vegetation and afforestation, improving soil fertility, and alleviation of physical, chemical and biological limitations by fertilizers and soil amendments such as biosolids, manure, coal combustion by-products, and mulches. Soil and water conservation are important to SOC sequestration. The potential of SOC sequestration in minesoils of the US is estimated to be 1.28 Tg C yr^?1, compared to the emissions from coal combustion of 506 Tg C yr^{? 1}.     

Biomass,Carbon, and Nitrogen Pools in Mexican Tropical Dry Forest Landscapes

Tropical dry forest is the most widely distributed land-cover type in the tropics. As the rate of land-use/land-cover change from forest to pasture or agriculture accelerates worldwide, it is becoming increasingly important to quantify the ecosystem biomass and carbon (C) and nitrogen (N) pools of both intact forests and converted sites. In the central coastal region of México, we sampled total aboveground biomass (TAGB), and the N and C pools of two floodplain forests, three upland dry forests, and four pastures converted from dry forest. We also sampled belowground biomass and soil C and N pools in two sites of each land-cover type. The TAGB of floodplain forests was as high as 416 Mg ha^–1, whereas the TAGB of the dry forest ranged from 94 to 126 Mg ha^–1. The TAGB of pastures derived from dry forest ranged from 20 to 34 Mg ha^–1. Dead wood (standing and downed combined) comprised 27%–29% of the TABG of dry forest but only about 10% in floodplain forest. Root biomass averaged 32.0 Mg ha^–1 in floodplain forest, 17.1 Mg ha^–1 in dry forest, and 5.8 Mg ha^–1 in pasture. Although total root biomass was similar between sites within land-cover types, root distribution varied by depth and by size class. The highest proportion of root biomass occurred in the top 20 cm of soil in all sites. Total aboveground and root C pools, respectively, were 12 and 2.2 Mg ha^–1 in pasture and reached 180 and 12.9 Mg ha^–1 in floodplain forest. Total aboveground and root pools, respectively, were 149 and 47 kg ha^–1 in pasture and reached 2623 and 264 kg ha^–1 in floodplain forest. Soil organic C pools were greater in pastures than in dry forest, but soil N pools were similar when calculated for the same soil depths. Total ecosystem C pools were 306. The Mg ha^–1 in floodplain forest, 141 Mg ha^–1 in dry forest, and 124 Mg ha^–1 in pasture. Soil C comprised 37%–90% of the total ecosystem C, whereas soil N comprised 85%–98% of the total. The N pools lack of a consistent decrease in soil pools caused by land-use change suggests that C and N losses result from the burning of aboveground biomass. We estimate that in México, dry forest landscapes store approximately 2.3 Pg C, which is about equal to the C stored by the evergreen forests of that country (approximately 2.4 Pg C). Potential C emissions to the atmosphere from the burning of biomass in the dry tropical landscapes of México may amount to 708 Tg C, as compared with 569 Tg C from evergreen forests.     

Impacts of land‐use change on nitrogen status and mineralization in the Mulga Lands of Southern Queensland

Abstract: In the semiarid Mulga Lands of southern Queensland soil nitrogen (N) levels have declined after clearance of the native mulga (Acacia aneura F. Muell. ex Benth.) and conversion to grazed buffel grass (Cenchrus ciliaris) pasture. At three mulga sites, declines in soil total N ranged from 14% to 28% in the surface 10 cm of soil. In situ net N mineralization from December 2003 until November 2004 in the surface 10 cm was 49.5 kg N ha^?1 year^?1 in the mulga woodland, 48.2 kg N ha^?1 year^?1 in the young (<5 years old) buffel pasture (previously sown to wheat (Triticum aestivum L.) and 34.6 kg N ha^?1 year^?1 in the old buffel pasture (>20 years). Ammonium‐N was the dominant N pool under mulga in the top 30 cm, while nitrate‐N was dominant under the buffel pastures. Although ammonium‐N under mulga was significantly different to that for 21‐year‐old buffel pasture at all depths, nitrification and net N mineralization were not different between the three land uses at any depth or in the entire 90 cm profile. The Soil Nitrogen Availability Predictor model was used to predict field N mineralization rates for the mulga woodland and 21‐year‐old buffel pasture by using a medium‐term (6‐week) laboratory incubation to establish basal rates of N mineralization. The Soil Nitrogen Availability Predictor overestimated annual net N mineralization in the 0–30 cm depth of mulga by 9% and underestimated it by 28% for the old buffel pasture. The Soil Nitrogen Availability Predictor could be modified further to accurately predict net N mineralization for the mulga woodlands.     

Does a warmer climate with frequent mild water shortages protect grassland communities against a prolonged drought?

While irrigation of farm dairy effluent (FDE) to land is becoming popular in New Zealand, it can lead to increased emissions of the greenhouse gas nitrous oxide (N₂O). This paper reports the results from trials on N₂O emissions from irrigation of FDE to two dairy-grazed pastures on two poorly drained silt-loam soils located at Waikato and Manawatu, New Zealand. These pasture soils were periodically irrigated with FDE under contrasting soil moisture conditions with water-filled pore-space (WFPS) ranging between 26% and 94%. Nitrous oxide emissions were measured from the FDE irrigated and unirrigated sites using large numbers of static chambers (12–20). Irrigation of FDE generally increased N₂O emissions compared to the control. N₂O emissions varied with changes in climatic conditions and soil WFPS. Overall N₂O emissions from effluent-derived N ranged between 0.01% and 4.93% depending on irrigation time and soil WFPS. Lower N₂O emissions from FDE were attributable to very low soil WFPS conditions during the dry seasons. Higher N₂O emissions were measured from application of FDE to a recently grazed pasture on wet soil. Our results suggest strategic application of FDE during dry summer and autumn seasons can reduce N₂O emissions from application of FDE. Delaying effluent-irrigation after grazing events could further reduce N₂O emissions by reducing the levels of surplus mineral-N.     

Nitrogen Oxide Fluxes and Nitrogen Cycling during Postagricultural Succession and Forest Fertilization in the Humid Tropics

The effects of changes in tropical land use on soil emissions of nitrous oxide (N₂O) and nitric oxide (NO) are not well understood. We examined emissions of N₂O and NO and their relationships to land use and forest composition, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and patterns of carbon (C) cycling in a lower montane, subtropical wet region of Puerto Rico. Fluxes of N₂O and NO were measured monthly for over 1 year in old (more than 60 years old) pastures, early- and mid-successional forests previously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertilized tabonuco forest. N₂O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg N · ha⁻¹· y⁻¹). Among the unfertilized sites, the expected pattern of increasing emissions with stand age did not occur in all cases. The mid-successional forest most dominated by leguminous trees had the highest emissions (9.0 kg N · ha⁻¹· y⁻¹), whereas the mid-successional forest lacking legumes had the lowest emissions (0.09 kg N · ha⁻¹· y⁻¹). N oxide fluxes from late-successional forests were higher than fluxes from pastures. Annual N oxide fluxes correlated positively to leaf litter N, net nitrification, potential nitrification, soil nitrate, and net N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissions. We conclude that (a) N oxide fluxes were substantial where the availability of inorganic N exceeded the requirements of competing biota; (b) species composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) the established ecosystem models that predict N oxide loss from positive relationships with soil ammonium may need to be modified. Received 22 February 2000; accepted 6 September 2000.     

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.     

Soil carbon dynamics in regrowing forest of eastern Amazonia

The future flora of Amazonia will include significant areas of secondary forest as degraded pastures are abandoned and secondary succession proceeds. The rate at which secondary forests regain carbon (C) stocks and re-establish biogeochemical cycles that resemble those of primary forests will influence the biogeochemistry of the region. Most studies have focused on the effects of deforestation on biogeochemical cycles. In this study, we present data on the recuperation of carbon stocks and carbon fluxes within a secondary forest of the eastern Amazon, and we compare these measurements to those for primary forest, degraded pasture, and productive pasture. Along a transect from a 23-y-old degraded pasture, through a 7-y-old secondary forest, through a 16-year-old secondary forest, and to a primary forest, the δ¹³C values of soil organic matter (SOM) in the top 10 cm of soil were – 21.0, – 26.5, – 27.4, and – 27.9‰, respectively, indicating that the isotopic signature of SOM from C3 forest plants was rapidly re-established. The degraded pasture also had significant inputs of C from C3 plants. Radiocarbon data indicated that most of the C in the top 10 cm of soil had been fixed by plants during the last 30 years. Differences in soil C inventory among land use types were small compared to uncertainties in their measurement. Root inputs were nearly identical in primary and secondary forests, and litterfall in the secondary forest was 88% of the litterfall rate of the primary forest. In contrast, the secondary forest had only 17% of the above ground biomass. Because of rapid cycling rates of soil C and rapid recovery of C fluxes to and from the soil, the below ground C cycle in this secondary forest was nearly identical with those of the unaltered primary forest.     

Afforestation with Norway spruce on a subalpine pasture alters carbon dynamics but only moderately affects soil carbon storage

There is a strong trend toward reforestation of abandoned grasslands in alpine regions which may impact the carbon balance of alpine ecosystems. Here, we studied the effects of afforestation with Norway spruce (Picea abies L.) on an extensively grazed subalpine pasture in Switzerland on soil organic carbon (SOC) cycling and storage. Along a 120-year long chronosequence with spruce stands of 25, 30, 40, 45, and >120 years and adjacent pastures, we measured tree biomass, SOC stocks down to the bedrock, natural ¹³C abundances, and litter quality. To unravel controls on SOC cycling, we have monitored microclimatic conditions and quantified SOC decomposability under standardized conditions as well as soil respiration in situ. Stocks of SOC were only moderately affected by the afforestation: in the mineral soil, SOC stocks transiently decreased after tree establishment, reaching a minimum 40–45 years after afforestation (?25 %) and increased thereafter. Soils of the mature spruce forest stored the largest amount of SOC, 13 % more than the pasture soils, mainly due to the accumulation of an organic layer (23 t C ha^?1). By comparison, C accumulated in the tree biomass exceeded the SOC pool by a factor of three in the old forest. In contrast to the small impact on C storage, afforestation strongly influenced the composition and quality of the soil organic matter (SOM). With increasing stand age, δ¹³C values of the SOM became consistently more positive, which can be interpreted as a gradual replacement of grass- by spruce-derived C. Fine roots of spruce were enriched in ¹³C, in lignin and had a higher C/N ratio in comparison to grass roots. As a consequence, SOM quality as indicated by the lower fraction of readily decomposable (labile) SOM and higher C:N ratios declined after the land-use change. Furthermore, spruce plantation induced a less favorable microclimate for microbial activity with the average soil temperature during the growing season being 5 °C lower in the spruce stands than in the pasture. In situ soil respiration was approximately 50 % lower after the land use conversion, which we primarily attribute to the colder conditions and the lower SOM quality, but also to drier soils (?25 %) and to a decreased fine root biomass (?40 %). In summary, afforestation on subalpine pastures only moderately affected SOC storage as compared to the large C sink in tree biomass. In contrast, SOC cycling rates strongly decreased as a result of a less favorable microclimate for decomposition of SOM, a lower C input by roots, and a lower litter quality.     

Soil organic carbon stocks in estuarine and marine mangrove ecosystems are driven by nutrient colimitation of P and N

Mangroves play an important role in carbon sequestration, but soil organic carbon (SOC) stocks differ between marine and estuarine mangroves, suggesting differing processes and drivers of SOC accumulation. Here, we compared undegraded and degraded marine and estuarine mangroves in a regional approach across the Indonesian archipelago for their SOC stocks and evaluated possible drivers imposed by nutrient limitations along the land‐to‐sea gradients. SOC stocks in natural marine mangroves (271–572 Mg ha^?1 m^?1) were much higher than under estuarine mangroves (100–315 Mg ha^?1 m^?1) with a further decrease caused by degradation to 80–132 Mg ha^?1 m^?1. Soils differed in C/N ratio (marine: 29–64; estuarine: 9–28), δ¹⁵N (marine: ?0.6 to 0.7‰; estuarine: 2.5 to 7.2‰), and plant‐available P (marine: 2.3–6.3 mg kg^?1; estuarine: 0.16–1.8 mg kg^?1). We found N and P supply of sea‐oriented mangroves primarily met by dominating symbiotic N₂ fixation from air and P import from sea, while mangroves on the landward gradient increasingly covered their demand in N and P from allochthonous sources and SOM recycling. Pioneer plants favored by degradation further increased nutrient recycling from soil resulting in smaller SOC stocks in the topsoil. These processes explained the differences in SOC stocks along the land‐to‐sea gradient in each mangrove type as well as the SOC stock differences observed between estuarine and marine mangrove ecosystems. This first large‐scale evaluation of drivers of SOC stocks under mangroves thus suggests a continuum in mangrove functioning across scales and ecotypes and additionally provides viable proxies for carbon stock estimations in PES or REDD schemes.     

The change of soil carbon stocks and fine root dynamics after land use change from a native pasture to a pine plantation

A published meta-analysis of worldwide data showed soil carbon decreasing following land use change from pasture to conifer plantation. A paired site (a native pasture with Themeda triandra dominant, and an adjacent Pinus radiata plantation planted onto the pasture 16 years ago) was set up as a case study to assess the soil carbon reduction and the possible reason for the reduction under pine, including the change in fine root (diameter <2 mm) dynamics (production and mortality). Soil analysis confirmed that soil carbon and nitrogen stocks to 100 cm under the plantation were significantly less than under the pasture by 20 and 15%, respectively. A 36% greater mass of fine root was found in the soil under the pasture than under the plantation and the length of fine root was about nine times greater in the pasture. Much less fine root length was produced and roots died more slowly under the plantation than under the pasture based on observations of fine root dynamics in minirhizotrons. The annual inputs of fine root litter to the top 100 cm soil, estimated from soil coring and minirhizotron observations, were 6.3 Mg dry matter ha⁻¹ year⁻¹ (containing 2.7 Mg C and 38.9 kg N) under the plantation, and 9.7 Mg ha⁻¹ year⁻¹ (containing 3.6 Mg C and 81.4 kg N) under the pasture. The reduced amount of carbon, following afforestation of the pasture, in each depth-layer of the soil profile correlated with the lower length of dead fine roots in the layer under the plantation compared with the pasture. This correlation was consistent with the hypothesis that the soil carbon reduction after land use change from pasture to conifer plantation might be related to change of fine root dynamics, at least in part.     

Rhizosphere respiration varies with plant species and phenology: A greenhouse pot experiment

Assessment of particulate (>53-m) and mineral-associated (<53-m) soil organic matter (SOM) fractions is a useful approach to understand the dynamic of organic matter in soils. This study aimed to compare the long-term (9-yr) effects of no-tillage (NT) and conventional tillage (CT) on C and N stocks in the two above mentioned organic fractions in a Brazilian Acrisol. The degree of SOM humification, which has been associated with the concentration of semiquinone-type free radicals (`spin') determined by electron spin resonance (ESR), was also evaluated. Soil under no-tillage had 7.55 Mg ha<sup>–1</sup> (25%) more C and 741 kg ha<sup>–1</sup> (29%) more N than conventionally tilled soil in the 0–175-mm depth. Both particulate and mineral-associated SOM increased in the no-tilled soil. The increase of C and N stocks in the mineral-associated SOM accounted for 75% and 91% of the difference in total soil C and N stocks between NT and CT, respectively. Averaged across tillage systems, C and N stocks were respectively 4.6 and 16.8 times higher in the mineral-associated SOM than in particulate SOM. The higher C and N stocks were associated with greater recalcitrance of mineral-associated SOM to biological decomposition, resulting, probably, from its interaction with variable charge minerals. This is corroborated by a positive relationship between concentrations of C and iron oxides and kaolinite in the 53–20, 20–2 and <2-m particle size classes, of the 0–25-mm soil layer. The degree of SOM humification, assessed by ESR, decreased in both the 53–20 and 20–2-m fractions under NT. However, it was unaffected by tillage in the <2-m fraction, which normally presented the lowest `spin' concentration. Since quality as well as quantity of SOM improved in the no-tillage soil, adoption of this system is highly recommended for amelioration of degraded tropical and subtropical soils.     

Forest-to-pasture conversion influences on soil organic carbon dynamics in a tropical deciduous forest

On a global basis, nearly 42% of tropical land area is classified as tropical deciduous forest (TDF) (Murphy and Lugo 1986). Currently, this ecosystem has very high deforestation rates; and its conversion to cattle pasture may result in losses of soil organic matter, decreases in soil fertility, and increases in CO₂ flux to the atmosphere. The soil organic matter turnover rate in a TDF after pasture conversion was estimated in Mexico by determining natural abundances of¹³C. Changes in these values would be induced by vegetation changes from the C₃ (forest) to the C₄ (pasture) photosynthetic pathway. The rate of loss of remnant forest-soil organic matter (fSOM) was 2.9 t ha^–1 year^–1 in 7-year-old pasture and decreased to 0.66 t ha^–1 year^–1 by year 11. For up to 3 years, net fSOM level increased in pastures; this increment can be attributed to decomposition of remnant forest roots. The sand-associated SOM fraction was the most and the silt-associated fraction the least depleted. TDF conversion to pasture results in extremely high rates of loss of remnant fSOM that are higher than any reported for any tropical forest.     

CO2 flux from soil in pastures and forests in southwestern Amazonia

Stocks of carbon in Amazonian forest biomass and soils have received considerable research attention because of their potential as sources and sinks of atmospheric CO₂. Fluxes of CO₂ from soil to the atmosphere, on the other hand, have not been addressed comprehensively in regard to temporal and spatial variations and to land cover change, and have been measured directly only in a few locations in Amazonia. Considerable variation exists across the Amazon Basin in soil properties, climate, and management practices in forests and cattle pastures that might affect soil CO₂ fluxes. Here we report soil CO₂ fluxes from an area of rapid deforestation in the southwestern Amazonian state of Acre. Specifically we addressed (1) the seasonal variation of soil CO₂ fluxes, soil moisture, and soil temperature; (2) the effects of land cover (pastures, mature, and secondary forests) on these fluxes; (3) annual estimates of soil respiration; and (4) the relative contributions of grass‐derived and forest‐derived C as indicated by δ¹³CO₂. Fluxes were greatest during the wet season and declined during the dry season in all land covers. Soil respiration was significantly correlated with soil water‐filled pore space but not correlated with temperature. Annual fluxes were higher in pastures compared with mature and secondary forests, and some of the pastures also had higher soil C stocks. The δ¹³C of CO₂ respired in pasture soils showed that high respiration rates in pastures were derived almost entirely from grass root respiration and decomposition of grass residues. These results indicate that the pastures are very productive and that the larger flux of C cycling through pasture soils compared with forest soils is probably due to greater allocation of C belowground. Secondary forests had soil respiration rates similar to mature forests, and there was no correlation between soil respiration and either forest age or forest biomass. Hence, belowground allocation of C does not appear to be directly related to the stature of vegetation in this region. Variation in seasonal and annual rates of soil respiration of these forests and pastures is more indicative of flux of C through the soil rather than major net changes in ecosystem C stocks.     

Continuous soil carbon storage of old permanent pastures in Amazonia

Amazonian forests continuously accumulate carbon (C) in biomass and in soil, representing a carbon sink of 0.42–0.65 GtC yr^?1. In recent decades, more than 15% of Amazonian forests have been converted into pastures, resulting in net C emissions (~200 tC ha^?1) due to biomass burning and litter mineralization in the first years after deforestation. However, little is known about the capacity of tropical pastures to restore a C sink. Our study shows in French Amazonia that the C storage observed in native forest can be partly restored in old (≥24 year) tropical pastures managed with a low stocking rate (±1 LSU ha^?1) and without the use of fire since their establishment. A unique combination of a large chronosequence study and eddy covariance measurements showed that pastures stored between ?1.27 ± 0.37 and ?5.31 ± 2.08 tC ha^?1 yr^?1 while the nearby native forest stored ?3.31 ± 0.44 tC ha^?1 yr^?1. This carbon is mainly sequestered in the humus of deep soil layers (20–100 cm), whereas no C storage was observed in the 0‐ to 20‐cm layer. C storage in C4 tropical pasture is associated with the installation and development of C3 species, which increase either the input of N to the ecosystem or the C:N ratio of soil organic matter. Efforts to curb deforestation remain an obvious priority to preserve forest C stocks and biodiversity. However, our results show that if sustainable management is applied in tropical pastures coming from deforestation (avoiding fires and overgrazing, using a grazing rotation plan and a mixture of C3 and C4 species), they can ensure a continuous C storage, thereby adding to the current C sink of Amazonian forests.     

Soil carbon stocks and land use change: a meta analysis

The effects of land use change on soil carbon stocks are of concern in the context of international policy agendas on greenhouse gas emissions mitigation. This paper reviews the literature for the influence of land use changes on soil C stocks and reports the results of a meta analysis of these data from 74 publications. The meta analysis indicates that soil C stocks decline after land use changes from pasture to plantation (?10%), native forest to plantation (?13%), native forest to crop (?42%), and pasture to crop (?59%). Soil C stocks increase after land use changes from native forest to pasture (+ 8%), crop to pasture (+ 19%), crop to plantation (+ 18%), and crop to secondary forest (+ 53%). Wherever one of the land use changes decreased soil C, the reverse process usually increased soil carbon and vice versa. As the quantity of available data is not large and the methodologies used are diverse, the conclusions drawn must be regarded as working hypotheses from which to design future targeted investigations that broaden the database. Within some land use changes there were, however, sufficient examples to explore the role of other factors contributing to the above conclusions. One outcome of the meta analysis, especially worthy of further investigation in the context of carbon sink strategies for greenhouse gas mitigation, is that broadleaf tree plantations placed onto prior native forest or pastures did not affect soil C stocks whereas pine plantations reduced soil C stocks by 12–15%.     

Forest- and pasture-derived carbon contributions to carbon stocks and microbial respiration of tropical pasture soils

The clearing of tropical forest for pasture leads to important changes in soil organic carbon (C) stocks and cycling patterns. We used the naturally occurring distribution of¹³C in soil organic matter (SOM) to examine the roles of forest- and pasture-derived organic matter in the carbon balance in the soils of 3- to 81-year-old pastures created following deforestation in the western Brazilian Amazon Basin state of Rondônia. Different ¹³C values of C3 forest-derived C (-28) and C4 pasture-derived C (-13) allowed determination of the origin of total soil C and soil respiration. The ¹³C of total soil increased steadily across ecosystems from -27.8 in the forest to -15.8 in the 81-year-old pasture and indicated a replacement of forest-derived C with pasture-derived C. The ¹³C of respired CO₂ increased more rapidly from -26.5 in the forest to -17 in the 3- to 13-year-old pastures and indicated a faster shift in the origin of more labile SOM. In 3-year-old pasture, soil C derived from pasture grasses made up 69% of respired C but only 17% of total soil C in the top 10 cm. Soils of pastures 5 years old and older had higher total C stocks to 30 cm than the original forest. This occurred because pasture-derived C in soil organic matter increased more rapidly than forest-derived C was lost. The increase of pasture-derived C in soils of young pastures suggests that C inputs derived from pasture grasses play a critical role in development of soil C stocks in addition to fueling microbial respiration. Management practices that promote high grass production will likely result in greater inputs of grass-derived C to pasture soils and will be important for maintaining tropical pasture soil C stocks.