João Mielniczuk

Recomendação de adubação nitrogenada para o milho no RS e SC adaptada ao uso de culturas de cobertura do solo, sob sistema plantio direto

O nitrogenio, na maioria das situacoes, e o nutriente que mais influencia o rendimento do milho. O manejo da adubacao nitrogenada deve satisfazer o requerimento da cultura com o minimo de risco ambiental. Para tanto, e necessario que a recomendacao da dose de adubo nitrogenado seja a mais exata possivel. A generalizacao do uso do sistema de plantio direto e culturas de cobertura, no Sul do Brasil, criou a necessidade de ser a recomendacao da adubacao nitrogenada adaptada a este novo cenario agricola. O presente trabalho, alem de considerar o teor de MO e a expectativa do rendimento de graos de milho na recomendacao da adubacao nitrogenada conforme preconiza a CFS-RS/SC (1995), propoe a introducao de um terceiro parâmetro que e a contribuicao em N das culturas de cobertura antecedente. O efeito das culturas de cobertura foi considerado em tres situacoes: leguminosas em cultivo solteiro, gramineas em cultivo solteiro e consorciacoes. No caso de leguminosas e gramineas em cultivo solteiro, a influencia na disponibilidade de N foi considerada com base na producao de materia seca, enquanto, nas consorciacoes, a proporcao da leguminosa foi o principal fator considerado. A recomendacao de adubacao apresentada neste trabalho nao dispensa acompanhamento de campo, visando a ajustes que se fizerem necessarios, especialmente porque sistemas de producao, baseados em culturas de cobertura, dependem de processos biologicos influenciados por condicoes de clima, manejo e solo, que devem ser acompanhados localmente.

HUMIFICATION DEGREE OF SOIL HUMIC ACIDS DETERMINED BY FLUORESCENCE SPECTROSCOPY

The humification process increases semiquinone-type free radical concentrations (SFRC) in humus. Their quantification by electron spin resonance spectroscopy (ESR) has been a good indicator of the degree of humification of soil organic matter. As an alternative to relatively complex and expensive ESR spectroscopy, fluorescence spectroscopy was used in this study to evaluate the humification degree of 18 humic acids (HA) extracted from four Brazilian soils under different land use, tillage, or cropping systems. Two fluorescence humification indexes of HA were calculated based on work done by Zsolnay et al. and Kalbitz et al. in 1999, and a third new fluorescence index was proposed. Our proposal is to use the blue wavelength (465 nm) as the HA fluorescence excitation source. As this wavelength is more resonant with the humificated groups present in soil HA samples, our hypothesis is that the resultant fluorescence will provide information about these structures and, therefore, on the degree of humification of the HA. The SFRC varied from 1.90 × 1017 to 14.75 × 1017 spins/g HA, characterizing a wide range of the degree of humification of soil HA. The lowest SFRC occurred in native forests (1.90 × 1017 to 7.50 × 1017 spins/g HA) and pasture soils (2.30 × 1017 to 4.64 × 1017 spins/g HA). In cultivated sites, soil HAs from no-tillage soil were less humified (2.41 × 1017 to 13.30 × 1017 spins/g HA) than those in conventionally tilled soil (5.68 to 14.75 × 1017 spins/g HA). The three fluorescence indexes show the same tendency as SFRC. Our fluorescence method was closely related to procedures found in the works of Zsolnay et al. and Kalbitz et al. (R∼0.9, P < 0.0001) and with SFRC (R∼0.85, P < 0.02), showing their potential as a simple and sensitive technique for evaluating the effect of land use and soil management systems on the humification degree of soil humic acids.

Armazenamento de carbono em frações lábeis da matéria orgânica de um Latossolo Vermelho sob plantio direto

Carbon storage in the soil organic matter (SOM) is an important strategy to mitigate carbon dioxide concentration in the atmosphere. The objective of this study was to evaluate the effect of the no-tillage (NT) use for six years, under four autumn (dry-season) cover crops based maize and soybean crop rotations (pigeon pea- pearl millet, bengan hemp-sorghum, sunflower-black oat, and wild radish-maize), on C storage in the particulate (>53 µm) and mineral-associated (<53 µm) SOM fractions of a clayey Oxisol from Brazilian Cerrado region. In comparison to conventional tillage (CT) under summer cash-crops, NT increased the total organic carbon (TOC) stocks in the surface soil layer (0-5 cm). In the top 20 cm layer, NT soil under wild radish-maize had 9% (4.66 Mg ha -1 ) more TOC than the conventionally tilled soil. On the other hand, C storage in NT soil under other cropping systems was statiscally similar to the CT soil. The C stocks in the particulate SOM fraction increased by 37 to 52% in NT soil (0-20 cm) in comparison to CT. The higher sensitivity to soil management changes made the particulate organic matter a more adequate soil quality index compared to the TOC stocks. The C stock in the mineral-associated SOM fraction was not affected by soil management systems, which can be related to the short-term under NT and or to the highly stable soil microaggregates in this clayey Oxisol. The preferential C storage in the labile SOM fraction is an environmental benefit, which is expected to occur only under continuous no-tillage and crop residues addition.

Adição de carbono e nitrogênio e sua relação com os estoques no solo e com o rendimento do milho em sistemas de manejo

Long-term experiments are essential for research about organic matter dynamics of soils. This paper reports results of an experiment installed in September 1985 on a Paleudult soil degraded under inadequate management for 16 years. The experiment was conducted at the Experimental Station of the Federal University of Rio Grande do Sul (Brazil) in Eldorado do Sul county. Three soil tillage methods (conventional tillage-CT, reduced tillage-RT and no-tillage-NT), three crop systems (oat/corn-O/C, vetch/corn-V/C and oat + vetch/corn + cowpea-O + V/C + Cp), and two N rates applied to corn as urea (0 and 139 kg ha-1) were tested. A split-plot design arranged in randomized blocks with three replications was used with soil tillage as main plot, the cropping systems as sub-plots and nitrogen rates as sub-blocks. Carbon and nitrogen addition by crops was estimated for the experimental period of 13 years. In September 1998, the soil was sampled in six layers down to 0.30 m depth, and total organic carbon (TOC) and total nitrogen (TN) contents were determined in each layer and then calculated for the 0-0.175 m and 0.175-0.30 m layers. Legumes were responsible for the greatest annual soil C and N increase by crops (4.17 to 8.39 Mg ha-1 and -21 to 178 kg ha-1, respectively). The highest C and N additions were positively related to these elements stocks in the soil under NT (0-0.175 m layer), and promoted gradual corn yield increase in all soil tillage treatments. The annual rate of C addition (A) necessary to maintain de initial condition (dC/dt = zero) was estimated at 4.2 Mg ha-1 for NT, 7.3 Mg ha-1 for RT and 8.9 Mg ha-1 for CT. Similarly, the annual rate of N addition required to maintain the initial condition (dN/dt = zero) was 5 kg ha-1 for NT, 31 kg ha-1 for RT, and 94 kg ha-1 for CT. An estimation by the angular coefficient of the linear regression, which relates the annual C addition rates and annual TOC stock variation in the surface layer, calculated that 12.9 % for NT, 8.1 % for RT, and 11.5 % for CT of the C added to the soil was retained in the soil organic matter, representing approximately the humification coefficient (k1). Likewise, it was estimated that 49.7 % of the net annual N addition under NT, 21.0 % under RT, and 33.1 % under CT was retained as TN in the soil. The TOC loss coefficient from the soil (k2), calculated for the condition dC/dt = zero, was 0.0166 yr-1 under NT, 0.0182 yr-1 under RT, and 0.0314 yr-1 under CT. No-tillage systems with the reduction of TOC loss (k2), and legume based crop systems with the high C and N additions are good alternatives to recover the soil TOC and TN stocks and increase corn yield in the subtropical region of southern Brazil.

Effect of cropping systems on soil chemical characteristics, with emphasis on soil acidification*

The soil under intensive cultivation and low addition of crop residues is exposed to erosion and reduction of organic matter. Increases in soil organic matter, cation exchange capacity (CEC) and nutrient availability may occur in no-till systems with legumes and with large additions of organic residues. Nevertheless, some legumes may increase soil acidification through the carbon and nitrogen cycles. An experiment was carried out over 10 years, with 10 cropping systems on a Dark Red Podzolic soil (Paleudult) to evaluate the effect of no-till cropping systems on soil chemical characteristics. Legume cropping systems resulted in the greatest soil organic C gain and the highest ECEC to a depth of 17.5 cm. The increase was greatest at 0 - 2.5 cm layer. Clover systems resulted in the highest soil acidification at 2.5 - 7.5 and 7.5 - 17.5 cm depths. The rate of soil pH decrease at 2.5 - 7.5 cm depth under clover+ t Spergula/maize system was 0.1 unit year-1. Differences in soil acidification affected soil ECEC. Soil exchangeable cation data indicate that nitrate leaching increased soil acidification. Maize yields were greatest in legume systems due to increased N supply.

Leguminosas e adubação mineral como fontes de nitrogênio para o milho em sistemas de preparo do solo

O nitrogenio e um nutriente requerido em grande quantidade pelo milho. O solo tem limitada capacidade de atender a esta demanda, sendo por isto utilizadas outras fontes de N como leguminosas e adubacao mineral. A combinacao destas duas fontes de N pode contribuir para a sustentabilidade do agroecossistema. Com o objetivo de avaliar a disponibilidade de N em sistemas de preparo do solo e cultura, foi realizada esta pesquisa, utilizando um experimento de longa duracao na UFRGS (RS) em um Argissolo Vermelho-Escuro distrofico. Os tratamentos utilizados foram tres sistemas de preparo: convencional, reduzido e direto; tres sistemas de cultura: aveia/milho, aveia + ervilhaca/milho + caupi e ervilhaca/milho, e tres doses de N: 0, 90 e 180 kg ha-1 aplicadas em cobertura no milho. A associacao do sistema plantio direto com o uso de leguminosas foi eficiente em promover o aumento do estoque de N total no solo. Embora tenha sido constatada maior velocidade de decomposicao de residuos no preparo convencional do que no plantio direto, ambos os sistemas apresentaram potencial de sincronismo da liberacao de N dos residuos com a demanda do milho. Esta cultura no sistema plantio direto apresentou menor producao de materia seca e absorcao de N do que no preparo convencional, porem nao houve diferenca no rendimento entre estes sistemas de preparo. A equivalencia em fertilizante nitrogenado foi estimada em 55 e 38 kg ha-1, respectivamente, para a ervilhaca e aveia + ervilhaca.O nitrogenio e um nutriente requerido em grande quantidade pelo milho. O solo tem limitada capacidade de atender a esta demanda, sendo por isto utilizadas outras fontes de N como leguminosas e adubacao mineral. A combinacao destas duas fontes de N pode contribuir para a sustentabilidade do agroecossistema. Com o objetivo de avaliar a disponibilidade de N em sistemas de preparo do solo e cultura, foi realizada esta pesquisa, utilizando um experimento de longa duracao na UFRGS (RS) em um Argissolo Vermelho-Escuro distrofico. Os tratamentos utilizados foram tres sistemas de preparo: convencional, reduzido e direto; tres sistemas de cultura: aveia/milho, aveia + ervilhaca/milho + caupi e ervilhaca/ milho, e tres doses de N: 0, 90 e 180 kg ha-1 aplicadas em cobertura no milho. A associacao do sistema plantio direto com o uso de leguminosas foi eficiente em promover o aumento do estoque de N total no solo. Embora tenha sido constatada maior velocidade de decomposicao de residuos no preparo convencional do que no plantio direto, ambos os sistemas apresentaram potencial de sincronismo da liberacao de N dos residuos com a demanda do milho. Esta cultura no sistema plantio direto apresentou menor producao de materia seca e absorcao de N do que no preparo convencional, porem nao houve diferenca no rendimento entre estes sistemas de preparo. A equivalencia em fertilizante nitrogenado foi estimada em 55 e 38 kg ha-1, respectivamente, para a ervilhaca e aveia + ervilhaca.

UMA VISÃO SOBRE QUALIDADE DO SOLO

Soil quality (SQ) has become a major research issue among soil scientists since the early 1990s, following the recognition of the role soils play in terms of agricultural sustainability and environmental quality. This review aims to discuss the state of the art concerning soil quality and look into the proposed indicators for its evaluation. Most part of the research work in the literature is focused on the identification of a soil attribute that would serve as a SQ indicator (SQI) underlying the evaluation of soil degradation and the estimation of research needs and funding, as well as a critical evaluation of soil management practices. Some authors have proposed criteria for the definition of a SQI. In this literature review three major approaches were clearly identified: search for soil attributes to be used as SQI; organic matter as SQI; and SQ as a result of processes in the soil-plant system. It was concluded that the understanding of the soil as a system represents the most significant advance and that therefore the identification of soil management practices to achieve SQ are more important than the effort to obtain a SQ indicator. From this point of view, soil should be treated as an open system where the non-equilibrium thermodynamics theory is applicable and the SQ emerges from the interactions of the soil-plant-biota system. The agricultural systems that enhance SQ are those where the soil is intensively cultivated with plants preferably of different species, and without tilling.

EFEITO DE SISTEMAS DE PREPARO E DE CULTURA NA DINÂMICA DA MATÉRIA ORGÂNICA E NA MITIGAÇÃO DAS EMISSÕES DE CO2

In this study, a first order exponential model was adjusted to the experimental data of soil organic carbon (OC) and total nitrogen (TN) of a 9-year-old experiment in southern Brazil. Parameters of soil organic matter dynamics were determined and the soil OC and TN stocks simulated in a Paleudult soil under three tillage systems (conventional tillage: CT, reduced tillage:RT, and no-tillage:NT) and three cropping systems (oat/maize-O/M, oat + vetch/maize-O + V/M and oat + vetch/maize + cowpea-O + V/M + C). Yearly, less soil turn over decreased the coefficient of soil organic matter losses (k2) from 0.054 year-1 under CT to 0.039 year-1 and 0.029 year-1 under RT and NT, respectively. The model showed that the soil OC and TN stocks from O/M under CT tended to decrease from 30.78 Mg ha-1 and 2.200 kg ha-1 in 1990 to constant values of 16.11 Mg ha-1 and 1.396 kg ha-1. In 1990, the soil OC and TN stocks from O + V/M + C under NT would increase from 32.52 Mg ha-1 and 2.690 kg ha-1 to constant amounts of 54.83 Mg ha-1 and 7.966 kg ha-1, respectively. The conservative soil management systems with no tillage and high addition of crop residues to soil also showed a positive effect on the mitigation of CO2 emissions. In the year of 1994, CT-O/M showed a net CO2 efflux to atmosphere (soil emission > crop fixation by photosynthesis) of 2.08 Mg ha-1, while the NT-O + V/M + C showed a net influx (soil emission < crop fixation by photosynthesis) of 1.79 Mg CO2 ha-1. By the time the system reached the constant soil OC level, as determined by the exponential model, the soil OC storage in NT-O + V/M + C soil will have represented a C sink of 142 Mg CO2 ha-1, in comparison with conventionally tilled soil and O/M system.

Sistemas de cultivo e características do solo afetando a estabilidade de agregados

This research was carried out in Rio Grande do Sul State, Southern Brazil, during the 1990/1991 summer harvest. The main objective was to evaluate the aggregation and aggregate stability of two clayey soils classified as Dusky Latosol (Haplorthox) and Dark-Red Podzolic (Paleoudult). Two treatments were used in the Haplorthox: grass (Setaria anceps L.); and crop rotation under conventional tillage (wheat (Triticum aestivum) soybean (Glycine max L.)). On the other hand, four treatments were used in the Paleoudult: grass (Digitaria decumbens L.), leguminous (Macroptilium atropurpureum L.), crop rotation under no-tillage (oat (Avena bizantina) corn (Zea mays)) and bare soil. Soil aggregation and aggregate stability were greatly enhanced through the action of the grass roots. Soil aggregation was also related to the contents of organic carbon, oxalate-extractable iron and aluminum, clay and water-dispersed clay. However, these factors were not sufficient to explain differences observed in the weighed mean diameter of soil aggregates among the treatments.

Tillage and cropping system effects on soil humic acid characteristics as determined by electron spin resonance and fluorescence spectroscopies

Abstract Long-term (5- and 9-year) effects were evaluated of two tillage regimes (conventional tillage: CT; and no-tillage: NT) and two cropping systems (oat/maize: O/M; and oat+vetch/maize+cowpea: O+V/M+C) on characteristics of humic acids (HAs) from surface layer (0 to 25 mm) of a subtropical Brazilian Paleudult soil. Generally, soil HA samples from conservation management systems with no soil disturbance (NT) and high crop residue addition (O+V/M+C) showed lowest humification degree, as demonstrated by lowest concentration of semiquinone-type free radicals, determined by electron spin resonance (ESR), and lowest total fluorescence (TF), which is proportional to area under fluorescence spectrum; however, cropping systems had less effect than tillage regimes on two spectroscopic parameters. Since all HA samples presented a maximum fluorescence emission peak around 518 nm, when excited with blue light, we believe that differences in the fluorescence intensity could be attributed to concentration of similar condensed aromatic moiety. This assumption was consistent with strong correlation with stable semiquinone-type free radicals (r=0.84, P