Valorização agro-ambiental das leguminosas anuais como potencial fonte de azoto no solo
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2017-03-20
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As crescentes preocupações ambientais, os preços crescentes dos adubos minerais e a necessidade de dar respostas às limitações da fertilização azotada em agricultura biológica levam à necessidade de avaliar as culturas intercalares, nomeadamente as leguminosas, como uma potencial fonte de azoto (N). A reduzida investigação em Portugal sobre o tema e a reduzida investigação à escala mundial sobre as espécies de leguminosas forrageiras anuais conduziu-nos ao presente estudo sob duas perspetivas: uma perspetiva agronómica, em que se avaliaram as espécies de plantas que melhor se adaptam a determinadas condições edafo-climáticas e que melhor suplementam a cultura principal em N e, uma perspetiva científica/ambiental, em que se pretendeu conhecer melhor a dinâmica do N presente nas diferentes culturas intercalares. Para o efeito foram realizados dois ensaios: um ensaio de campo e um ensaio/incubação em laboratório.
No ensaio de campo que decorreu nos anos de 2007/08, 2008/09 e 2011/12, testaram-se as seguintes culturas intercalares: trevo balansa (Trifolium michelianum Savi) Cv. Paradana, trevo glandulífero (Trifolium glanduliferum Boiss.) Cv. Prima, trevo vesiculoso (Trifolium vesiculosum Savi) Cv. Cefalu, trevo encarnado (Trifolium incarnatum L.) Cv. Contea, serradela vulgar (Ornithopus sativus Brot.) Cv. Cadiz e tremocilha (Lupinus luteus L.) população regional, como leguminosas estremes, e ainda o azevém (Lolium multiflorum Lam.) Westerwoldicum diplóide Cv. Liforia, uma consociação de azevém com trevo balansa, e a vegetação espontânea. Procedeu-se à sementeira destas em duas datas diferentes (meados de setembro e de outubro). Na altura da plena floração procedeu-se ao corte e incorporação no solo. De seguida foi realizada a sementeira de milho, que completou o seu ciclo cultural, tendo sido efetuada a colheita em princípios de Setembro. Neste ensaio foram avaliados os teores de N mineral no solo, a quantidade de matéria seca (MS) produzida pelas culturas intercalares e pelo milho, o teor de N, P e K e o valor de N acumulado na biomassa aérea das mesmas.
No último ano de ensaio foi instalado um outro trabalho experimental no mesmo campo, onde foi avaliada a mineralização do N, através de uma incubação “in situ” durante o ciclo vegetativo do milho, nos tratamentos relativos ao azevém, tremocilha e trevo balansa.
O ensaio de laboratório foi realizado no Laboratório de Solos e Plantas da UTAD. Neste foi avaliado, ao longo de 196 dias, o N mineralizado pelos resíduos das plantas (azevém, tremocilha e trevo balansa) incorporados no solo, bem como a libertação de CO2 no processo de decomposição dos resíduos.
Das culturas intercalares estudadas, verificou-se ser a tremocilha que apresenta maior teor de N (41,3 g N kg-1) e maior quantidade de N acumulado (148 kg N ha-1). A MS produzida foi muito variável entre anos, realçando-se o efeito das condições climáticas sobre este parâmetro. O efeito da data de sementeira apenas se fez sentir no teor de N, obtendo valores mais elevados as culturas provenientes da segunda data de sementeira.
Quando analisámos o efeito destas culturas, na cultura principal (milho), verificou-se ser o tratamento relativo ao trevo balansa aquele que induziu um maior valor de N acumulado no milho (48,0 kg N ha-1) e também aquele que satisfez maior proporção da quantidade crítica de N para a cultura do milho (cerca de 70%). Em termos ambientais também este tratamento se mostrou bastante eficiente, pois no fim do ciclo cultural do milho restava um reduzido teor de N mineral no solo susceptível de ser lixiviado, não apresentando diferenças significativas com o tratamento onde se observou o menor teor de N mineral no solo (azevém).
No estudo da mineralização do N “in situ”, verificou-se ser o tratamento relativo à tremocilha aquele que registou maior taxa de mineralização diária (0,71 mg N kg-1 dia-1), e um maior valor de N mineralizado (82 mg N kg-1).
Da análise laboratorial feita à mineralização dos resíduos incorporados ao solo, verificou-se uma maior mineralização nos resíduos da tremocilha, na qual, ao fim de 196 dias, mais de 40% do N adicionado foi mineralizado. O resíduo que libertou uma maior quantidade de CO2 durante o seu processo de decomposição foi a mistura da parte aérea e parte radicular do azevém (2142 mg C kg-1 solo).
Genericamente comprovou-se a eficiência das leguminosas anuais como fonte de N para a cultura principal, para as nossas condições edafo-climáticas. Apesar destas não fornecerem todo o N necessário à cultura principal, verificou-se conseguirem fornecer uma grande parte do N e no seu processo de decomposição verificou-se serem eficientes em reter C no solo, apresentando taxas reduzidas de emissão de CO2.
The growing environmental concerns, rising prices of mineral fertilizers and the need to answer the limitations of nitrogen fertilization in organic farming led us to consider the cover crops, especially annual legumes, as a potential source of nitrogen (N). The limited research in Portugal on the subject and the limited research worldwide on the species of annual forage legumes led us to this study from two perspectives: an agronomic one where the plant species better appropriate to environmental conditions were evaluated to find the better to supplement N to the main crop and also a scientific / environmental perspective, which sought to better understand the N dynamics present in the crop residues. For this purpose two experiments were conducted: a field and a laboratory experiment. The field experiment was conducted in Agrarian School of Viseu, Portugal. In this trial, which took place in the years 2007/08, 2008/09 and 2011/12, we tested the following intercropping: balansa clover (Trifolium michelianum Savi) Cv. Paradana, gland clover (Trifolium glanduliferum Boiss.) Cv. Prima, Arrowleaf clover (Trifolium vesiculosum Savi) Cv. Cefalu, crimson clover (Trifolium incarnatum L.) Cv. Contea, French serradella (Ornithopus sativus Brot.) Cv. Cadiz and yellow lupine (Lupinus luteus L.) regional population, as annual legumes, Italian ryegrass (Lolium multiflorum Lam.) Westerwoldicum diploid Cv. Liforia, as a grass test, a mixture of Italian ryegrass with balansa clover and weed vegetation (natural vegetation). These plant treatments were sown in two different dates (mid-September and mid-October). At the time of full bloom (late April) we proceeded to cut and incorporate into the soil. Then a main crop of corn was seeded. Completed maize maturation was harvested in early September. The content of N in the soil, dry matter (DM) yield, N content and N uptake of cover crops and maize were evaluated. In the last year of experiment another experimental work was installed in the same field: an "in situ" incubation, during the maize cycle, for N mineralization estimate in ryegrass, yellow lupine and balansa clover treatments. A laboratory experiment was carried out to evaluate the N mineralized and CO2 emission of the residues decomposition process. Plant residues (ryegrass, yellow lupine and balansa clover) were incorporated into the soil by 196 days. These residues are separated in roots, stems and mixture of them. Yellow lupine was the cover crop with higher N content (41.3 g N kg-1) and greater N uptake (148 kg N ha-1). DM production was very variable between years, highlighting the effects of different environmental conditions of the experimental years. Sowing date effect was only significant in N content, with higher values from the latter sowing date. The analysis of the effect of cover crops in the main crop (maize) showed that balansa clover induced a higher value of N uptake in maize (48.0 kg N ha-1) and also satisfied most the amount of critical concentration of N for maize (about 70%). In environmental requisites, balansa clover was very efficient, because at the end of cultural cycle of maize low N content remained in the soil available to leaching. In the experiment of “in situ” incubation the yellow lupine treatment has a higher N mineralization rate (0.71 mg N kg-1 day-1) and a greater N mineralized (82 mg N kg-1). In laboratory analysis of residues mineralization, yellow lupine was the best treatment because more than 40% N added was mineralized in 196 days. Italian ryegrass (mixture roots and stems) was the residue that, during the process of decomposition release higher CO2 content (2142 mg C kg-1soil). Generally it was shown the efficiency of annual legumes as nitrogen source for the main crop, to our soil and climate conditions. Although these do not provide all the main crop N needed, it deliver a large portion of N and its process of decomposition were found to be effective in storage C in soil because they have low rates of CO2 emission.
The growing environmental concerns, rising prices of mineral fertilizers and the need to answer the limitations of nitrogen fertilization in organic farming led us to consider the cover crops, especially annual legumes, as a potential source of nitrogen (N). The limited research in Portugal on the subject and the limited research worldwide on the species of annual forage legumes led us to this study from two perspectives: an agronomic one where the plant species better appropriate to environmental conditions were evaluated to find the better to supplement N to the main crop and also a scientific / environmental perspective, which sought to better understand the N dynamics present in the crop residues. For this purpose two experiments were conducted: a field and a laboratory experiment. The field experiment was conducted in Agrarian School of Viseu, Portugal. In this trial, which took place in the years 2007/08, 2008/09 and 2011/12, we tested the following intercropping: balansa clover (Trifolium michelianum Savi) Cv. Paradana, gland clover (Trifolium glanduliferum Boiss.) Cv. Prima, Arrowleaf clover (Trifolium vesiculosum Savi) Cv. Cefalu, crimson clover (Trifolium incarnatum L.) Cv. Contea, French serradella (Ornithopus sativus Brot.) Cv. Cadiz and yellow lupine (Lupinus luteus L.) regional population, as annual legumes, Italian ryegrass (Lolium multiflorum Lam.) Westerwoldicum diploid Cv. Liforia, as a grass test, a mixture of Italian ryegrass with balansa clover and weed vegetation (natural vegetation). These plant treatments were sown in two different dates (mid-September and mid-October). At the time of full bloom (late April) we proceeded to cut and incorporate into the soil. Then a main crop of corn was seeded. Completed maize maturation was harvested in early September. The content of N in the soil, dry matter (DM) yield, N content and N uptake of cover crops and maize were evaluated. In the last year of experiment another experimental work was installed in the same field: an "in situ" incubation, during the maize cycle, for N mineralization estimate in ryegrass, yellow lupine and balansa clover treatments. A laboratory experiment was carried out to evaluate the N mineralized and CO2 emission of the residues decomposition process. Plant residues (ryegrass, yellow lupine and balansa clover) were incorporated into the soil by 196 days. These residues are separated in roots, stems and mixture of them. Yellow lupine was the cover crop with higher N content (41.3 g N kg-1) and greater N uptake (148 kg N ha-1). DM production was very variable between years, highlighting the effects of different environmental conditions of the experimental years. Sowing date effect was only significant in N content, with higher values from the latter sowing date. The analysis of the effect of cover crops in the main crop (maize) showed that balansa clover induced a higher value of N uptake in maize (48.0 kg N ha-1) and also satisfied most the amount of critical concentration of N for maize (about 70%). In environmental requisites, balansa clover was very efficient, because at the end of cultural cycle of maize low N content remained in the soil available to leaching. In the experiment of “in situ” incubation the yellow lupine treatment has a higher N mineralization rate (0.71 mg N kg-1 day-1) and a greater N mineralized (82 mg N kg-1). In laboratory analysis of residues mineralization, yellow lupine was the best treatment because more than 40% N added was mineralized in 196 days. Italian ryegrass (mixture roots and stems) was the residue that, during the process of decomposition release higher CO2 content (2142 mg C kg-1soil). Generally it was shown the efficiency of annual legumes as nitrogen source for the main crop, to our soil and climate conditions. Although these do not provide all the main crop N needed, it deliver a large portion of N and its process of decomposition were found to be effective in storage C in soil because they have low rates of CO2 emission.
Descrição
Tese de Doutoramento em Ciências Agronómicas e Florestais
Palavras-chave
Azoto , Incubação , Leguminosas , Milho , Mineralização , Sideração