Agroecosystem diversification with legumes or non-legumes improves differently soil fertility according to soil type.

Plant diversification through crop rotation or agroforestry is a promising way to improve sustainability of agroecosystems. Nonetheless, criteria to select the most suitable plant communities for agroecosystems diversification facing contrasting environmental constraints need to be refined. Here, we compared the impacts of 24 different plant communities on soil fertility across six tropical agroecosystems: either on highly weathered Ferralsols, with strong P limitation, or on partially weathered soils derived from volcanic material, with major N limitation. In each agroecosystem, we tested several plant communities for diversification, as compared to a matching low diversity management for their cropping system. Plant residue restitution, N, P and lignin contents were measured for each plant community. In parallel, the soil under each community was analyzed for organic C and N, inorganic N, Olsen P, soil pH and nematode community composition. Soil potential fertility was assessed with plant bioassays under greenhouse controlled climatic conditions. Overall, plant diversification had a positive effect on soil fertility across all sites, with contrasting effects depending on soil type and legumes presence in the community. Communities with legumes improved soil fertility indicators of volcanic soils, which was demonstrated through significantly higher plant biomass production in the bioassays (+18%) and soil inorganic N (+26%) compared to the low diversity management. Contrastingly, communities without legumes were the most beneficial in Ferralsols, with increases in plant biomass production in the bioassays (+39%), soil Olsen P (+46%), soil C (+26%), and pH (+5%). Piecewise structural equation models with Shipley's test revealed that plant diversification impacts on volcanic soil fertility were related to soil N availability, driven by litter N. Meanwhile, Ferralsols fertility was related to soil P availability, driven by litter P. These findings underline the importance of multifactorial and multi-sites experiments to inform trait-based frameworks used in designing optimal plant diversification in agroecological systems.

Shade trees have higher impact on soil nutrient availability and food web in organic than conventional coffee agroforestry.

Conventional, intensively managed coffee plantations are currently facing environmental challenges. The use of shade trees and the organic management of coffee crops are welcome alternatives, aiming to reduce synthetic inputs and restore soil biological balance. However, little is known about the impacts of the different types of shade tree species on soil functioning and fauna. In this paper, we assess soil nutrient availability and food web structure on a 17-year old experimental coffee plantation in Turrialba in Costa Rica. Three shade types (unshaded coffee, shaded with Terminalia amazonia, and shaded with Erythrina poepiggiana) combined with two management practices (organic and conventional) were evaluated. Total C and N, inorganic N and Olsen P content, soil pH, global soil fertility, and nematode and microarthropod communities were measured in the top 10 cm soil layer, with the objective of determining how shade tree species impact the soil food web and soil C, N and P cycling under different types of management. We noted a decrease in soil inorganic N content and nematode density under conventional management (respectively -47% and -91% compared to organic management), which suggested an important biological imbalance, possibly caused by the lack of organic amendment. Under conventional management, soil nutrient availability and fauna densities were higher under shade, regardless of the shade tree species. Under organic management, only soils under E. poeppigiana, a heavily pruned, N2-fixing species, had increased nutrient availability and fauna density, while T. amazonia shade had a null or negative impact. The effects of coffee management and shade type on soil nutrient availability were mirrored by changes in soil food web structure. Higher fertility was recorded in soil with balanced food webs. These results emphasize the importance of the choice of shade tree species for soil functions in low input systems, more so than in fertilized systems.

Soil microbial diversity drives the priming effect along climate gradients: a case study in Madagascar.

The priming effect in soil is proposed to be generated by two distinct mechanisms: 'stoichiometric decomposition' and/or 'nutrient mining' theories. Each mechanism has its own dynamics, involves its own microbial actors, and targets different soil organic matter (SOM) pools. The present study aims to evaluate how climatic parameters drive the intensity of each priming effect generation mechanism via the modification of soil microbial and physicochemical properties. Soils were sampled in the center of Madagascar, along climatic gradients designed to distinguish temperature from rainfall effects. Abiotic and biotic soil descriptors were characterized including bacterial and fungal phylogenetic composition. Potential organic matter mineralization and PE were assessed 7 and 42 days after the beginning of incubation with 13C-enriched wheat straw. Both priming mechanisms were mainly driven by the mean annual temperature but in opposite directions. The priming effect generated by stoichiometric decomposition was fostered under colder climates, because of soil enrichment in less developed organic matter, as well as in fast-growing populations. Conversely, the priming effect generated by nutrient mining was enhanced under warmer climates, probably because of the lack of competition between slow-growing populations mining SOM and fast-growing populations for the energy-rich residue entering the soil. Our study leads to hypotheses about the consequences of climate change on both PE generation mechanisms and associated consequences on soil carbon sequestration.

Fertility and acidity status of latossolos (oxisols) under pasture in the Brazilian Cerrado.

The Cerrado region, with over 50 million hectares of cultivated pasture, provides 55% of Brazilian beef production. Previous investigations have shown that about 70-80% of this pasture is affected by some kind of degradation, leading to low productivity. However, until now, few surveys have been carried out on a regional scale. The aim of the present work is both to assess the fertility and acidity levels of Cerrado soils under pasture and compare the variability of the soils characteristics on a regional scale. Two soil depths were sampled in different places within the studied area: (1) a surface horizon (0.0-0.2 m) in order to evaluate its fertility and acidity status for pasture, and (2) a subsurface horizon (0.6-0.8 m), used for classification. Most of soils had levels of nutrients below the reference values for adequate pasture development. Whatever the texture, about 90% of soils had low or very low availability of phosphorus. Only 7 to 14% of soils had low pH, high exchangeable aluminum, and aluminum saturation above the critical acidity level. Except for nitrogen, no significant difference was found between Latossolos Vermelhos and Latossolos Vermelho-Amarelos.

Fertility and acidity status of latossolos (oxisols) under pasture in the Brazilian Cerrado

The Cerrado region, with over 50 million hectares of cultivated pasture, provides 55 percent of Brazilian beef production. Previous investigations have shown that about 70-80 percent of this pasture is affected by some kind of degradation, leading to low productivity. However, until now, few surveys have been carried out on a regional scale. The aim of the present work is both to assess the fertility and acidity levels of Cerrado soils under pasture and compare the variability of the soils characteristics on a regional scale. Two soil depths were sampled in different places within the studied area: (1) a surface horizon (0.0-0.2 m) in order to evaluate its fertility and acidity status for pasture, and (2) a subsurface horizon (0.6-0.8 m), used for classification. Most of soils had levels of nutrients below the reference values for adequate pasture development. Whatever the texture, about 90 percent of soils had low or very low availability of phosphorus. Only 7 to 14 percent of soils had low pH, high exchangeable aluminum, and aluminum saturation above the critical acidity level. Except for nitrogen, no significant difference was found between Latossolos Vermelhos and Latossolos Vermelho-Amarelos.

A região do Cerrado, com 50 milhões de hectares de pastagens cultivadas, responde por 55 por cento da carne bovina produzida no Brasil. Há estimativas indicando que entre 70 e 80 por cento destas pastagens apresentam algum grau de degradação, o que leva a baixa produtividade. No entanto, até o presente, poucos trabalhos foram desenvolvidos em escala regional. O objetivo deste trabalho foi avaliar a fertilidade e a acidez de Latossolos sob pastagens na região do Cerrado e comparar a variabilidade das características dos solos em escala regional. A amostragem foi realizada em duas profundidades, superficial (0,0-0,2 m) para avaliação da fertilidade e acidez e subsuperficial (0,60,8 m), utilizada para classificar os solos. A maioria dos solos apresentou teores de nutrientes abaixo do recomendado para o adequado desenvolviento das pastagens. Independentemente da textura, aproximadamente 90 por cento dos solos apresentaram baixa ou muito baixa disponibilidade de P. Apenas 7-14 por cento dos solos apresentaram baixo pH, alta saturação por Al e acidez acima do nível crítico. Exceto para o N, não houve diferença significativa nos níveis de fertilidade e acidez entre os Latossolos Vermelhos e os Latossolos Vermelho-Amarelos.

Ion activity and distribution of heavy metals in acid mine drainage polluted subtropical soils.

The oxidative dissolution of mine wastes gives rise to acidic, metal-enriched mine drainage (AMD) and has typically posed an additional risk to the environment. The poly-metallic mine Dabaoshan in South China is an excellent test site to understand the processes affecting the surrounding polluted agricultural fields. Our objectives were firstly to investigate metal ion activity in soil solution, distribution in solid constituents, and spatial distribution in samples, secondly to determine dominant environment factors controlling metal activity in the long-term AMD-polluted subtropical soils. Soil Column Donnan Membrane Technology (SC-DMT) combined with sequential extraction shows that unusually large proportion of the metal ions are present as free ion in the soil solutions. The narrow range of low pH values prevents any pH effects during the binding onto oxides or organic matter. The differences in speciation of the soil solutions may explain the different soil degradation observed between paddy and non-paddy soils.

Caracterização da solução de solos do Bioma Cerrado sob pastagens / Characterization of the soil solution of brazilian Cerrado soils under pasture

O objetivo deste trabalho foi caracterizar a solução de solos e avaliar a especiação do Al em solos sob pastagens no Bioma Cerrado. Para a coleta das soluções dos solos utilizou-se tubo de PVC com a extremidade inferior acoplada a uma ponta porosa de cerâmica especial. Para quantificar as formas iônicas do alumínio, foram comparadas duas metodologias: método químico do “aluminon” e o método WHAM (modelo geoquímico de especiação). Os solos utilizados no presente estudo foram coletados às margens das rodovias GO – 060 e BR – 154, no trecho entre os municípios de Goiânia, GO e Barra do Garças, MT. Os resultados permitiram concluir que as espécies iônicas de alumínio predominantes variaram em função do tipo de solo, pH e concentração de NO3 - na solução do solo. Verificou-se também que as espécies iônicas mais abundantes foram: Al-M.O., Al3+ e Al(OH)4.

The aim of the present work was to characterize the soils solution and to examine Al's speciation in soilsunder pasture in Brazilian Cerrado region. The soils used in this study were collected in the spacebetween the towns of Goiânia, GO and Barra do Garças, MT, to the margins of GO ­ 060 and BR ­ 154highways. For the collection of soil solutions it was used a special equipment built with a PVC tubewhose inferior extremity was coupled to a porous tip of special ceramic. To quantify the aluminum ionicspecies, two methods were used: chemical method of " aluminon " and the method WHAM (geochemicalspeciation model). The results allowed to conclude that the predominant aluminum ionic species variedin function of soil type, pH and concentration of NO3- in the soil solution. More abundant Al's speciesin the soils were Al-M.O., Al3+ and Al(OH)4- , in this order.

Assessment and control of the bioavailability of nickel in soils.

Nickel, a potentially toxic metal, is present in all soils with an average concentration of 20 to 30 mg/kg, sometimes exceeding 10,000 mg/kg (e.g., ultramafic soils). The ecotoxicological risk of Ni in soils to organisms is controlled by its availability. It is therefore essential to identify an efficient and reliable method for the evaluation of this risk. This paper presents a complete study of the effect of Ni origin, localization, and soil properties on its availability as assessed with the isotopic exchange kinetics (IEK) method and compares plant response to isotopically exchangeable properties of Ni in soils. We performed IEK on 100 soil samples representing a worldwide range of Ni fate, and concentrations showed that pH was the main influencing parameter and that labile Ni (i.e., isotopically exchangeable Ni, Et) could be reasonably well assessed by a single diethylene triamine pentaacetic acid extraction. The identification of the soil mineral phases that bear Ni (bearing phases) in 16 Ni-rich samples selected among the 100 soils showed a strong effect of the mineralogy of the bearing phases on Ni availability (IEK). Plants with different Ni accumulation strategies all took up Ni from the same labile pool of Ni in four contrasting soils, and the amount taken up by hyperaccumulator plants could be anticipated with the IEK parameters, thus confirming the usefulness of isotopic dilution methods for risk assessment.