Bringing function to structure: Root-soil interactions shaping phosphatase activity throughout a soil profile in Puerto Rico.

Large areas of highly productive tropical forests occur on weathered soils with low concentrations of available phosphorus (P). In such forests, root and microbial production of acid phosphatase enzymes capable of mineralizing organic phosphorus is considered vital to increasing available P for plant uptake.We measured both root and soil phosphatase throughout depth and alongside a variety of root and soil factors to better understand the potential of roots and soil biota to increase P availability and to constrain estimates of the biochemical mineralization within ecosystem models.We measured soil phosphatase down to 1 m, root phosphatase to 30 cm, and collected data on fine-root mass density, specific root length, soil P, bulk density, and soil texture using soil cores in four tropical forests within the Luquillo Experimental Forest in Puerto Rico.We found that soil phosphatase decreased with soil depth, but not root phosphatase. Furthermore, when both soil and root phosphatase were expressed per soil volume, soil phosphatase was 100-fold higher that root phosphatase.Both root and soil factors influenced soil and root phosphatase. Soil phosphatase increased with fine-root mass density and organic P, which together explained over 50% of the variation in soil phosphatase. Over 80% of the variation in root phosphatase per unit root mass was attributed to specific root length (positive correlation) and available (resin) P (negative correlation). Synthesis: Fine-root traits and soil P data are necessary to understand and represent soil and root phosphatase activity throughout the soil column and across sites with different soil conditions and tree species. These findings can be used to parameterize or benchmark estimates of biochemical mineralization in ecosystem models that contain fine-root biomass and soil P distributions throughout depth.

Co-occurring Fungal Functional Groups Respond Differently to Tree Neighborhoods and Soil Properties Across Three Tropical Rainforests in Panama.

Abiotic and biotic drivers of co-occurring fungal functional guilds across regional-scale environmental gradients remain poorly understood. We characterized fungal communities using Illumina sequencing from soil cores collected across three Neotropical rainforests in Panama that vary in soil properties and plant community composition. We classified each fungal OTU into different functional guilds, namely plant pathogens, saprotrophs, arbuscular mycorrhizal (AM), or ectomycorrhizal (ECM). We measured soil properties and nutrients within each core and determined the tree community composition and richness around each sampling core. Canonical correspondence analyses showed that soil pH and moisture were shared potential drivers of fungal communities for all guilds. However, partial the Mantel tests showed different strength of responses of fungal guilds to composition of trees and soils. Plant pathogens and saprotrophs were more strongly correlated with soil properties than with tree composition; ECM fungi showed a stronger correlation with tree composition than with soil properties; and AM fungi were correlated with soil properties, but not with trees. In conclusion, we show that co-occurring fungal guilds respond differently to abiotic and biotic environmental factors, depending on their ecological function. This highlights the joint role that abiotic and biotic factors play in determining composition of fungal communities, including those associated with plant hosts.

Dual-mycorrhizal plants: their ecology and relevance.

Dual-mycorrhizal plants are capable of associating with fungi that form characteristic arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) structures. Here, we address the following questions: (1) How many dual-mycorrhizal plant species are there? (2) What are the advantages for a plant to host two, rather than one, mycorrhizal types? (3) Which factors can provoke shifts in mycorrhizal dominance (i.e. mycorrhizal switching)? We identify a large number (89 genera within 32 families) of confirmed dual-mycorrhizal plants based on observing arbuscules or coils for AM status and Hartig net or similar structures for EM status within the same plant species. We then review the possible nutritional benefits and discuss the possible mechanisms leading to net costs and benefits. Cost and benefits of dual-mycorrhizal status appear to be context dependent, particularly with respect to the life stage of the host plant. Mycorrhizal switching occurs under a wide range of abiotic and biotic factors, including soil moisture and nutrient status. The relevance of dual-mycorrhizal plants in the ecological restoration of adverse sites where plants are not carbon limited is discussed. We conclude that dual-mycorrhizal plants are underutilized in ecophysiological-based experiments, yet are powerful model plant-fungal systems to better understand mycorrhizal symbioses without confounding host effects.

Phosphorus availability and dynamics in soil affected by long-term ruzigrass cover crop.

The use of grasses as cover crops in the off-season of cash crops under no-till has been largely adopted. However, soil phosphorus (P) uptake was previously shown to be reduced when ruzigrass is introduced in the rotation, affecting the viability and sustainability of this cropping system. The objective of this study was to assess the effect of ruzigrass on soil P availability and desorption kinetics under different P fertilizer application rates. A long-term field experiment where soybean (Glycine max) has been grown in rotation with ruzigrass (Urochloa ruziziensis) or fallow for 10 years, with the application of 0, 13, and 26 kg ha-1 of P, was evaluated for two consecutive years. Soil P desorption kinetics was assessed using diffusive equilibrium (DET) and gradient in thin films (DGT) techniques, as well as the DGT-induced fluxes in soils model (DIFS). Microbial biomass P (MBP) was assessed to verify if soil solution P (PDET) was reduced due to immobilization by microorganisms. Ruzigrass reduced MBP and PDET especially when P fertilizer was applied. The concentration of labile P (PDGT) was also lower after ruzigrass than in fallow. The soil ability to resupply P to soil solution was lower after ruzigrass regardless of P rates due to a slower desorption in response to the perturbation imposed by DGT. Growing ruzigrass as cover crop in the soybean off-season decreases soil P availability regardless of P fertilizer application rates by fundamentally reducing P mobility and P resupply from soil solid phase into soil solution.

Leaf traits in Chilean matorral: sclerophylly within, among, and beyond matorral, and its environmental determinants.

Studies of leaf traits often focus on tradeoffs between growth and resource conservation, but little is known about variation in the mechanical traits that influence resource conservation. This study investigates how leaf mechanical traits vary across matorral vegetation in central Chile, how they correlate with environmental factors, and how these trends compare at a broader geographic scale. Leaf toughness, strength, stiffness, and associated traits were measured in five matorral types in central Chile, and relationships with soil N and P and climate variables were assessed. Trends with soil and climate were then analyzed across shrubland and woodland in Chile, Western Australia, and New Caledonia. Chilean species varied in leaf mechanics and associated traits, both within and among matorral types, with more species in sclerophyll matorral having strong, tough, and stiff leaves than in arid and littoral matorral. Overall, leaves with high leaf dry mass per area were stiffer, tougher, stronger, thicker, denser, with more fiber, lignin, phenolics and fiber per unit protein and less protein: tannin activity and N and P per mass, forming a broad sclerophylly syndrome. Mechanical traits of matorral species were not correlated with soil N or P, or predictably with climate variables, except flexural stiffness (EI W) which correlated positively with annual reference evapotranspiration (ET 0). However, soil P made strong independent contributions to variation in leaf mechanics across shrublands and woodlands of Chile, Western Australia, and New Caledonia, either separately (strength) or together with ET 0 (toughness) explaining 46-90% of variation. Hence ET 0 was predictive of EI W in Chilean matorral, whereas soil P was highly predictive of variation in leaf strength, and combined with ET 0 was highly predictive of toughness, at a broader geographic scale. The biological basis of these relationships, however, may be complex.

Long-term manure application effects on phosphorus speciation, kinetics and distribution in highly weathered agricultural soils.

Phosphorus (P) K-edge XANES and Fe K-edge EXAFS spectroscopies along with sequential P chemical fractionation and desorption kinetics experiments, were employed to provide micro- and macro-scale information on the long-term fate of manure application on the solid-state speciation, kinetics and distribution of P in highly weathered agricultural soils of southern Brazil. Soil test P values ranged from 7.3 up to 16.5 times as much higher than the reference soil. A sharp increase in amorphous Fe and Al amounts were observed as an effect of the consecutive application of manures. Whereas our results showed that the P sorption capacity of some manured soils was not significantly affected, P risk assessment indices indicated that P losses should be expected, likely due to the excessive manure rates applied to the soils. The much higher contents of amorphous Fe and Al (hydr)oxides (55% and 80% increase with respect to the reference soil, respectively) in manured soils seem to have counterbalanced the inhibiting effect of soil organic matter on P sorption by creating additional P sorption sites. Accordingly, the newly created P sorbing surfaces were important to prevent an even larger P loss potential. Phosphorus K-edge XANES lent complimentary hints on the loss of crystallinity and transformation of originally present Fe-P minerals into poorly crystalline ones as an effect of manuring, whereas Fe K-edge EXAFS provided insights into the structural changes underwent in the soils upon manure application and soil management.

Herbivory makes major contributions to ecosystem carbon and nutrient cycling in tropical forests.

The functional role of herbivores in tropical rainforests remains poorly understood. We quantified the magnitude of, and underlying controls on, carbon, nitrogen and phosphorus cycled by invertebrate herbivory along a 2800 m elevational gradient in the tropical Andes spanning 12°C mean annual temperature. We find, firstly, that leaf area loss is greater at warmer sites with lower foliar phosphorus, and secondly, that the estimated herbivore-mediated flux of foliar nitrogen and phosphorus from plants to soil via leaf area loss is similar to, or greater than, other major sources of these nutrients in tropical forests. Finally, we estimate that herbivores consume a significant portion of plant carbon, potentially causing major shifts in the pattern of plant and soil carbon cycling. We conclude that future shifts in herbivore abundance and activity as a result of environmental change could have major impacts on soil fertility and ecosystem carbon sequestration in tropical forests.

Dessorção de fósforo avaliada por extrações sucessivas em amostras de solo provenientes dos sistemas plantio direto e convencional

The phosphorus is retained in the soil solid phase with several degrees of energy, which depends on the characteristics and amount of the soil colloids. The no tillage system (NT) may promote phosphorus accumulation in different forms from those observed in the conventional tillage (CT) and thus, may modify the dessorption capacity of the soil. The objective of this work was to estimate the phosphorus dessorption capacity of different soils submitted to NT and CT. Samples from three soils with variable clay contents (Rhodic Hapludox - 680g kg-1 clay, Rhodic Hapludox - 530g kg-1 clay and Rhodic Paleudult - 220g kg-1 clay) cultivated under NT and CT were submitted to successive extractions with anion exchange resin (AER) and adjusted first order kinetics equations to estimate the maximum phosphorus dessorption of the soil. The dessorption by the first extraction and maximum dessorption were higher in the superficial layer of the soil under NT, comparatively to CT. Only one extraction with AER represented, on average, 38, 46 and 49% of the maximum dessorption on soils with high, medium and low clay content, respectively, while that estimated by Mehlich 1 extractor represented, for the same soils, 32, 46 and 89% of the maximum dessorption.

O fósforo é retido à fase sólida do solo com diversas energias de ligação, dependentes das características e quantidade dos colóides do solo. A adoção do sistema plantio direto (SPD) pode promover o acúmulo de fósforo em formas diferentes daquelas observadas no sistema de cultivo convencional (SCC) e, assim, modificar as características de dessorção do fósforo do solo. O objetivo deste trabalho foi avaliar a capacidade de dessorção de fósforo em solos submetidos a sistemas de cultivo. Três solos com teores variáveis de argila (Latossolo Vermelho Distroférrico - 680g kg-1 de argila, Latossolo Vermelho Distrófico - 530g kg-1 de argila e Argissolo Vermelho Distrófico - 220g kg-1 de argila) cultivados sob SPD e SCC foram submetidos a extrações sucessivas com resina trocadora de ânions (RTA) e ajustadas equações de cinética de primeira ordem para estimar a capacidade de dessorção máxima do solo. A dessorção da primeira extração e dessorção máxima de fósforo foram maiores na camada superficial do solo sob SPD, comparativamente ao SCC. Uma única extração com RTA representou, em média, 38, 46 e 49% do fósforo dessorvível para o solo muito argiloso, argiloso e arenoso, respectivamente, enquanto a estimativa pelo extrator Mehlich 1 representou, para os mesmos solos, 32, 46 e 89% da dessorção máxima de fósforo.

Dessorção de fósforo avaliada por extrações sucessivas em amostras de solo provenientes dos sistemas plantio direto e convencional

The phosphorus is retained in the soil solid phase with several degrees of energy, which depends on the characteristics and amount of the soil colloids. The no tillage system (NT) may promote phosphorus accumulation in different forms from those observed in the conventional tillage (CT) and thus, may modify the dessorption capacity of the soil. The objective of this work was to estimate the phosphorus dessorption capacity of different soils submitted to NT and CT. Samples from three soils with variable clay contents (Rhodic Hapludox - 680g kg-1 clay, Rhodic Hapludox - 530g kg-1 clay and Rhodic Paleudult - 220g kg-1 clay) cultivated under NT and CT were submitted to successive extractions with anion exchange resin (AER) and adjusted first order kinetics equations to estimate the maximum phosphorus dessorption of the soil. The dessorption by the first extraction and maximum dessorption were higher in the superficial layer of the soil under NT, comparatively to CT. Only one extraction with AER represented, on average, 38, 46 and 49% of the maximum dessorption on soils with high, medium and low clay content, respectively, while that estimated by Mehlich 1 extractor represented, for the same soils, 32, 46 and 89% of the maximum dessorption.

O fósforo é retido à fase sólida do solo com diversas energias de ligação, dependentes das características e quantidade dos colóides do solo. A adoção do sistema plantio direto (SPD) pode promover o acúmulo de fósforo em formas diferentes daquelas observadas no sistema de cultivo convencional (SCC) e, assim, modificar as características de dessorção do fósforo do solo. O objetivo deste trabalho foi avaliar a capacidade de dessorção de fósforo em solos submetidos a sistemas de cultivo. Três solos com teores variáveis de argila (Latossolo Vermelho Distroférrico - 680g kg-1 de argila, Latossolo Vermelho Distrófico - 530g kg-1 de argila e Argissolo Vermelho Distrófico - 220g kg-1 de argila) cultivados sob SPD e SCC foram submetidos a extrações sucessivas com resina trocadora de ânions (RTA) e ajustadas equações de cinética de primeira ordem para estimar a capacidade de dessorção máxima do solo. A dessorção da primeira extração e dessorção máxima de fósforo foram maiores na camada superficial do solo sob SPD, comparativamente ao SCC. Uma única extração com RTA representou, em média, 38, 46 e 49% do fósforo dessorvível para o solo muito argiloso, argiloso e arenoso, respectivamente, enquanto a estimativa pelo extrator Mehlich 1 representou, para os mesmos solos, 32, 46 e 89% da dessorção máxima de fósforo.

Níveis críticos de fósforo para o estabelecimento de gramíneas forrageiras em Latossolo Vermelho Amarelo, álico: II. Experimento de campo

The objectives of the present study, were to determine the critical levels of soil P by the Mehlich I and Resin extractors, and in the plant, for the establishment of Brachiaria decumbens Stapf., Brachiaria brizantha (Ilochst.) Stapf cv. Marandu and Panicum maximum Jack., in a Red Yellow Latosol. The experimental design was a randomized complete block with four replications and 21 treatments (7 levels of P x 3 species). The levels of P corresponded to 0, 20, 40, 80, 160, 320 e 640 kg of P(2)0(5)/ha. Two cuts of plant shoots were performed: the first, eighty days after sowing and the second, thirty days later. Differences in P requeriments for the establishment among the species, for 80% of maximum growth, were evident. The critical levels of soil P, in the first cut, were 4.7; 10.8 and 8.0 ppm (Mehlich I) and 10.7; 21.8 and 16.0 ppm (Resin), for the Brachiaria decumbens, Brachiaria brizantha and Panicum maximum, respectively. For the second cut, the levels were 3.0 and 3,5 ppm (Mehlich I) and 5.8 and 6.5 ppm (Resin), for Brachiaria decumbens and Panicum maximum, respectively. The critical levels of P in the plant were, in the same order, 0.08; 0.13 and 0.06% for the first cut and 0.09 and 0.08%, for the second cut.

O estudo teve por objetivos determinar os níveis críticos de P no solo, pelos extratores de Mehlich I e Resina, e na planta, para o estabelecimento das gramíneas Brachiaria decumbens Stapf., Brachiaria brizantha (Hochst.) Stapf. cv. Marandu e Panicum maximum Jacq., em Latossolo Vermelho Amarelo, álico. O delineamento experimental foi o de blocos casualizados com quatro repetições, no esquema fatorial 7 (doses de P) x 3 (gramíneas). As doses de P corresponderam a 0, 20, 40, 80, 160, 320 e 640 kg de P(2)0(5)/ha. Foram realizados dois cortes, o primeiro, 80 dias após a semeadura, e o segundo com 60 dias de rebrota. Diferenças de exigência em P para o estabelecimento das forrageiras, foram bem evidentes entre as três espécies. Os níveis críticos de P no solo, para 80% dos crescimentos máximos, no primeiro corte, foram de 4,7, 10,8 e 8,0 ppm (Mehlich I) e 10,7, 21,8 e 16,0 ppm (Resina), respectivamente, para a Brachiaria decumbens, Brachiaria brizantha e o Panicum maximum. No segundo corte foram de 3,0 e 3,5 ppm (Mehlich I) e de 5,8 e 6,5 ppm (Resina), respectivamente, para a Brachiaria decumbens e o Panicum maximum. Os níveis críticos de P na planta foram de 0,08; 0,13 e 0,06%, no primeiro corte e de 0,09 e 0,08%, no segundo corte, na mesma ordem acima, respectivamente.

Níveis críticos de fósforo para o estabelecimento de gramíneas forrageiras em Latossolo Vermelho-Amarelo, álico. I: ensaio em casa de vegetação

The objectives of the present study, carried out in a greenhouse, were to determine the critical levels of soil P, by the Mehlich I and Resin extractors, and in the plant, for the stablishment of pastures of Brachiaría decumbens Stapf., Brachiaría brizaníha (Ilochst.) Stapf cv. Marandu and Panicum maximum JACK., on a Red Yellow Latosol. The experimental design was a randomized complete block with four replications and 21 treatments (7 levels of P x 3 species). The levels of P were: 0; 17.5; 35.0; 70.0; 140.0; 280.0 and 560.0 ppm. Two shoot cuts of plants were done: the first, fifty days after sowing and the second, thirty days later. No differences in P requeriment for crop establishment were observed among the species, for 80% of maximum growth. The critical level of soil P, in the first cut, varied from 8.0 to 12.0 ppm (Mehlich I) and 13.0 to 18.0 ppm (Resin); for the second, cut, the levels were 11.5; 12.0 and 14.0 ppm (Mehlich I) and 18.0; 22.0 and 24.0 ppm (Resin), for the Brachiaria decumbens, Brachiaria brizaníha and Panicum maximum, respectively. The critical levels of P in the plant were, in the same order, 0.11; 0.13 and 0.12% for the first cut and 0.09; 0.12 and 0.10%, for the second cut.

O estudo, desenvolvido em casa de vegetação, teve por objetivos determinar os níveis críticos de P no solo, avaliados pelos extratores de Mehlich I e Resina e na planta, para o estabelecimento das gramíneas Brachiaria decumbens Stapf., Brachiaria brizantha (Ilochst.) Stapf. cv. Marandu e Panicum maximum Jacq., em Latossolo Vermelho Amarelo, álico. O delineamento experimental foi o de blocos casualizados com quatro repetições, no esquema fatorial: 7 (doses de P) x 3 (gramíneas). As doses de P corresponderam a 0; 17,5; 35,0; 70,0; 140,0; 280,0 e 560,0 ppm. Foram realizados dois cortes, o primeiro 50 dias após a semeadura e o segundo com 30 dias de rebrota. Não foram constatadas diferenças entre as espécies quanto à exigência em P para o seu estabelecimento. Os níveis críticos de P no solo, para 80% do crescimento máximo, no primeiro corte, variaram de 8,0 a 12,0 ppm (Mehlich I) e de 13,0 a 18,0 ppm (Resina) para as três espécies forrageiras. No segundo corte foram de 11,5; 12,0 e 14,0 ppm (Mehlich I) e de 18,0; 22,0 e 24,0 ppm (Resina), para a Brachiaria decumbens, Brachiaria brizaníha e o Panicum maximum, respectivamente. Os níveis críticos de P na planta foram, na mesma ordem, de 0,11; 0,13 e 0,12%, no primeiro corte e de 0,09; 0,12 e 0,10%, no segundo corte.

Níveis críticos de fósforo para o estabelecimento de gramíneas forrageiras em Latossolo Vermelho-Amarelo, álico. I: ensaio em casa de vegetação

The objectives of the present study, carried out in a greenhouse, were to determine the critical levels of soil P, by the Mehlich I and Resin extractors, and in the plant, for the stablishment of pastures of Brachiaría decumbens Stapf., Brachiaría brizaníha (Ilochst.) Stapf cv. Marandu and Panicum maximum JACK., on a Red Yellow Latosol. The experimental design was a randomized complete block with four replications and 21 treatments (7 levels of P x 3 species). The levels of P were: 0; 17.5; 35.0; 70.0; 140.0; 280.0 and 560.0 ppm. Two shoot cuts of plants were done: the first, fifty days after sowing and the second, thirty days later. No differences in P requeriment for crop establishment were observed among the species, for 80% of maximum growth. The critical level of soil P, in the first cut, varied from 8.0 to 12.0 ppm (Mehlich I) and 13.0 to 18.0 ppm (Resin); for the second, cut, the levels were 11.5; 12.0 and 14.0 ppm (Mehlich I) and 18.0; 22.0 and 24.0 ppm (Resin), for the Brachiaria decumbens, Brachiaria brizaníha and Panicum maximum, respectively. The critical levels of P in the plant were, in the same order, 0.11; 0.13 and 0.12% for the first cut and 0.09; 0.12 and 0.10%, for the second cut.

O estudo, desenvolvido em casa de vegetação, teve por objetivos determinar os níveis críticos de P no solo, avaliados pelos extratores de Mehlich I e Resina e na planta, para o estabelecimento das gramíneas Brachiaria decumbens Stapf., Brachiaria brizantha (Ilochst.) Stapf. cv. Marandu e Panicum maximum Jacq., em Latossolo Vermelho Amarelo, álico. O delineamento experimental foi o de blocos casualizados com quatro repetições, no esquema fatorial: 7 (doses de P) x 3 (gramíneas). As doses de P corresponderam a 0; 17,5; 35,0; 70,0; 140,0; 280,0 e 560,0 ppm. Foram realizados dois cortes, o primeiro 50 dias após a semeadura e o segundo com 30 dias de rebrota. Não foram constatadas diferenças entre as espécies quanto à exigência em P para o seu estabelecimento. Os níveis críticos de P no solo, para 80% do crescimento máximo, no primeiro corte, variaram de 8,0 a 12,0 ppm (Mehlich I) e de 13,0 a 18,0 ppm (Resina) para as três espécies forrageiras. No segundo corte foram de 11,5; 12,0 e 14,0 ppm (Mehlich I) e de 18,0; 22,0 e 24,0 ppm (Resina), para a Brachiaria decumbens, Brachiaria brizaníha e o Panicum maximum, respectivamente. Os níveis críticos de P na planta foram, na mesma ordem, de 0,11; 0,13 e 0,12%, no primeiro corte e de 0,09; 0,12 e 0,10%, no segundo corte.

Níveis críticos de fósforo para o estabelecimento de gramíneas forrageiras em Latossolo Vermelho Amarelo, álico: II. Experimento de campo

The objectives of the present study, were to determine the critical levels of soil P by the Mehlich I and Resin extractors, and in the plant, for the establishment of Brachiaria decumbens Stapf., Brachiaria brizantha (Ilochst.) Stapf cv. Marandu and Panicum maximum Jack., in a Red Yellow Latosol. The experimental design was a randomized complete block with four replications and 21 treatments (7 levels of P x 3 species). The levels of P corresponded to 0, 20, 40, 80, 160, 320 e 640 kg of P(2)0(5)/ha. Two cuts of plant shoots were performed: the first, eighty days after sowing and the second, thirty days later. Differences in P requeriments for the establishment among the species, for 80% of maximum growth, were evident. The critical levels of soil P, in the first cut, were 4.7; 10.8 and 8.0 ppm (Mehlich I) and 10.7; 21.8 and 16.0 ppm (Resin), for the Brachiaria decumbens, Brachiaria brizantha and Panicum maximum, respectively. For the second cut, the levels were 3.0 and 3,5 ppm (Mehlich I) and 5.8 and 6.5 ppm (Resin), for Brachiaria decumbens and Panicum maximum, respectively. The critical levels of P in the plant were, in the same order, 0.08; 0.13 and 0.06% for the first cut and 0.09 and 0.08%, for the second cut.

O estudo teve por objetivos determinar os níveis críticos de P no solo, pelos extratores de Mehlich I e Resina, e na planta, para o estabelecimento das gramíneas Brachiaria decumbens Stapf., Brachiaria brizantha (Hochst.) Stapf. cv. Marandu e Panicum maximum Jacq., em Latossolo Vermelho Amarelo, álico. O delineamento experimental foi o de blocos casualizados com quatro repetições, no esquema fatorial 7 (doses de P) x 3 (gramíneas). As doses de P corresponderam a 0, 20, 40, 80, 160, 320 e 640 kg de P(2)0(5)/ha. Foram realizados dois cortes, o primeiro, 80 dias após a semeadura, e o segundo com 60 dias de rebrota. Diferenças de exigência em P para o estabelecimento das forrageiras, foram bem evidentes entre as três espécies. Os níveis críticos de P no solo, para 80% dos crescimentos máximos, no primeiro corte, foram de 4,7, 10,8 e 8,0 ppm (Mehlich I) e 10,7, 21,8 e 16,0 ppm (Resina), respectivamente, para a Brachiaria decumbens, Brachiaria brizantha e o Panicum maximum. No segundo corte foram de 3,0 e 3,5 ppm (Mehlich I) e de 5,8 e 6,5 ppm (Resina), respectivamente, para a Brachiaria decumbens e o Panicum maximum. Os níveis críticos de P na planta foram de 0,08; 0,13 e 0,06%, no primeiro corte e de 0,09 e 0,08%, no segundo corte, na mesma ordem acima, respectivamente.