Pulse crops have been long associated with biological dinitrogen fixation and therefore improve the sustainability of cropping systems when included in rotation. However, studies indicate there may be additional benefits of including pulse crops in rotation. To quantify these potential benefits, soil processes and properties related to nitrogen (N) and carbon (C) cycling were examined in five crop rotations with and without field pea (<i>Pisum sativum</i> L.) in Scott, Saskatchewan. Gross mineralization and nitrification rates were determined using the 15N isotope dilution technique in intact soil cores. To estimate the proportion of nitrous oxide (N2O) emissions derived from nitrification related processes rather than denitrification processes tracer techniques using 15N were used. Field incubations were performed in 2008 at seeding (May 13), anthesis (July 8) and just after harvest (October 8). Mean mineralization and nitrification rates were not significantly different among rotations on any date and there was no significant difference in mean N2O emissions among rotations. From labeled 15NO3- cores, it was determined that nitrification-related processes were the major contributors to N2O emissions. There was no difference among the rotations in microbial biomass carbon (MB-C) or microbial biomass N (MB-N) with the exception of MB-C in the continuous field pea (FP) and the canola (<i>Brassica napus</i> L.)-wheat (<i>Triticum aestivum</i> L.)-field pea (CNL-W-FP) rotation at anthesis. There was no effect of rotation on dissolved organic carbon (DOC) and only seasonal differences were observed with DOC levels being lower before seeding than at anthesis and post-harvest. Based on the results obtained from a single growing season, our results show that N benefits of including field pea in rotation, beyond dinitrigen fixation, were not detectable and that the immediate N benefit of including field pea in rotation may be due simply to the direct effects of biological dinitrogen (N2) fixation. However, there have been reports of pulse crop benefits to succeeding crops in rotation. As a result, we investigated both the quantity and quality of crop residues, which can have an impact on soil properties and processes. Plants enriched with isotopic tracers can be used to trace crop residue decomposition to various C pools but only if the tracer is homogeneously distributed throughout the plant. In order to determine if repeat-pulse labeling could be used to trace crop residue decomposition, this method was followed using 13CO2 to enrich plant material of field pea and canola plants in a controlled environment. The distribution of 13C throughout the plant parts (roots, stem, leaves, and pod) and biochemical fractions [acid detergent fiber (ADF) and acid detergent lignin (ADL)] were determined. It was found that 13C was not homogeneously distributed throughout the plant parts or biochemical fractions. The pod fraction in particular was much less enriched in comparison to the other fractions. The ADL fraction was less enriched than the ADF fraction. Because of the heterogeneity of the label throughout the plant, modifications of the method are needed and 13C distribution through out the plant needs to be assessed before the repeat-pulse method can be used to trace C residue through various C pools. Nevertheless, root contributions to below-ground C were successfully determined from the enriched root material and the resulting enriched soil. It was found that canola contributed more above- and below-ground residues than field pea, however canola was also higher in ADF and ADL fractions indicating a more recalcitrant residue. Research should continue to better define the impact of pulse crop residues on C and N cycling and subsequent crops in rotation.

Pulse crops have been long associated with biological dinitrogen fixation and therefore improve the sustainability of cropping systems when included in rotation. However, studies indicate there may be additional benefits of including pulse crops in rotation. To quantify these potential benefits, soil processes and properties related to nitrogen (N) and carbon (C) cycling were examined in five crop rotations with and without field pea (<i>Pisum sativum</i> L.) in Scott, Saskatchewan. Gross mineralization and nitrification rates were determined using the 15N isotope dilution technique in intact soil cores. To estimate the proportion of nitrous oxide (N2O) emissions derived from nitrification related processes rather than denitrification processes tracer techniques using 15N were used. Field incubations were performed in 2008 at seeding (May 13), anthesis (July 8) and just after harvest (October 8). Mean mineralization and nitrification rates were not significantly different among rotations on any date and there was no significant difference in mean N2O emissions among rotations. From labeled 15NO3- cores, it was determined that nitrification-related processes were the major contributors to N2O emissions. There was no difference among the rotations in microbial biomass carbon (MB-C) or microbial biomass N (MB-N) with the exception of MB-C in the continuous field pea (FP) and the canola (<i>Brassica napus</i> L.)-wheat (<i>Triticum aestivum</i> L.)-field pea (CNL-W-FP) rotation at anthesis. There was no effect of rotation on dissolved organic carbon (DOC) and only seasonal differences were observed with DOC levels being lower before seeding than at anthesis and post-harvest. Based on the results obtained from a single growing season, our results show that N benefits of including field pea in rotation, beyond dinitrigen fixation, were not detectable and that the immediate N benefit of including field pea in rotation may be due simply to the direct effects of biological dinitrogen (N2) fixation. However, there have been reports of pulse crop benefits to succeeding crops in rotation. As a result, we investigated both the quantity and quality of crop residues, which can have an impact on soil properties and processes. Plants enriched with isotopic tracers can be used to trace crop residue decomposition to various C pools but only if the tracer is homogeneously distributed throughout the plant. In order to determine if repeat-pulse labeling could be used to trace crop residue decomposition, this method was followed using 13CO2 to enrich plant material of field pea and canola plants in a controlled environment. The distribution of 13C throughout the plant parts (roots, stem, leaves, and pod) and biochemical fractions [acid detergent fiber (ADF) and acid detergent lignin (ADL)] were determined. It was found that 13C was not homogeneously distributed throughout the plant parts or biochemical fractions. The pod fraction in particular was much less enriched in comparison to the other fractions. The ADL fraction was less enriched than the ADF fraction. Because of the heterogeneity of the label throughout the plant, modifications of the method are needed and 13C distribution through out the plant needs to be assessed before the repeat-pulse method can be used to trace C residue through various C pools. Nevertheless, root contributions to below-ground C were successfully determined from the enriched root material and the resulting enriched soil. It was found that canola contributed more above- and below-ground residues than field pea, however canola was also higher in ADF and ADL fractions indicating a more recalcitrant residue. Research should continue to better define the impact of pulse crop residues on C and N cycling and subsequent crops in rotation.

A intensificação da agricultura convencional que ocorreu nas últimas décadas na região Sudeste da Província de Buenos Aires (Argentina), provocou degradação das propriedades do solo e redução da fertilidade nitrogenada, apesar dos níveis relativamente elevados de matéria orgânica nesse solo. A inclusão de pastagens (P) nas rotações agrícolas e a adoção da semeadura direta (SD), como manejos alternativos à lavoura convencional (LC), visaram deter a degradação desse solo. A adoção de diferentes sistemas de manejo influem na dinâmica do nitrogênio no solo, modificando a sua disponibilidade às plantas. Com base nesta hipótese, o objetivo geral deste trabalho foi o de quantificar os processos de mineralização, nitrificação e consumo brutos do nitrogênio em um Molisol do SE da Província de Buenos Aires, manejado durante 17 anos com LC e transformado para P ou SD. Os processos brutos do ciclo do nitrogênio foram quantificados pela técnica da diluição do isótopo 15N. Foram realizados os seguintes experimentos: (a) avaliação em laboratório de taxas de mineralização bruta (TMB), consumo bruto (TCB) e nitrificação bruta (TNB) em amostras deformadas e em condições de umidade e temperatura constantes; (b) avaliação dos efeitos do manejo na distribuição das frações granulométricas da matéria orgânica; (c) avaliação das relações entre os processos brutos do ciclo do nitrogênio e as frações granulométricas da matéria orgânica, (c) avaliação em laboratório dos efeitos da temperatura e da umidade do solo nas TMB e TCB e (d) determinações de TMB e TCB em campo durante o ciclo da cultura de milho nos manejos SD, LC e pastagem. A metodologia de diluição do isótopo 15N permitiu a determinação de taxas de mineralização e de consumo bruto em solo com diferentes sistemas de manejo, tanto em condições de laboratório, com amostras deformadas, quanto diretamente em campo, com amostras indeformadas. Na quantificação da taxa de nitrificação bruta, a marcação do solo com amônio-15N não se mostrou adequado, levando a resultados muito variáveis, com evidencias de estimulação pelo substrato. Em todos os experimentos a TCB foi maior que a TMB e as quantidades totais consumidas superaram àquelas mineralizadas. Após seis anos da implantação dos manejos alternativos á LC (SD e P), os processos brutos de transformação do nitrogênio no solo apresentaram marcante estratificação, sendo maiores nas camadas superficiais do solo. As TMB da pastagem foram maiores às da LC, indicando melhora na condição biológica do solo, enquanto que SD somente manifestou uma tendência nesse sentido. Nos três manejos, grandes quantidades de C e N estiveram presentes nas frações <50 &#956;m, nas quais, a matéria orgânica tem alta proteção física e não foi modificada pelo manejo. A pastagem acumulou maior quantidade de C e de N nas frações leves >50 &#956;m que LC. Não se encontrou um padrão simples relacionando a TMB e a TCB com as variáveis das frações granulométricas. Equações de regressão múltiplas para LC incluíram o C e N das frações mais finas, enquanto que para P apareceram as frações leves maiores que 50 µm, sugerindo recuperação da MO do solo. A temperatura e a umidade do solo modificaram as TMB e TCB, sendo que as maiores taxas ocorreram a 25°C e a 70% MCR. LC não apresentou resposta nem a temperatura nem a umidade de solo. Em determinações feitas em campo, após de 9 anos de instalados os manejos, as variações das TMB e TCB foram associadas, principalmente, às variações na umidade do solo. As TMB e TCB na pastagem foram geralmente maiores que as de LC e SD. SD apresentou maiores TMB e TCB que LC no final do ciclo do milho. As quantidades mineralizadas e consumidas acumuladas por SD foram significativamente maiores às correspondentes de LC, embora a cultura de milho não foi beneficiada por essas diferenças devido a disponibilidade de N no solo não coincidir com os estádios de máxima demanda pelas plantas. / The intensification of conventional agriculture in Southeast of Buenos Aires Province (Argentina), caused (conducted to) soil degradation and reduction of nitrogen fertility, despite the relatively high levels of organic matter on this soil. No tillage systems (SD) and inclusion of pastures (P) in agricultural rotations are alternative soil managements to reduce the effects of conventional tillage (LC). Management systems affect the soil nitrogen dynamics modifying its availability to the plants. The objective of this work was to quantify the gross nitrogen mineralization, nitrification and consumption processes in a Molisol, which was previously under LC for 17 years. In 1994 it was transformed to P, SD or continued on LC. The gross nitrogen cycle processes were quantified using the isotope 15N dilution technique. The following experiments were carried out: (a) laboratory evaluation of gross mineralization (TMB), consumption (TCB) and nitrification rates (TNB) in disturbed soil samples with constant soil moisture and temperature conditions; (b) evaluation of the effect of management systems in the size distribution of soil organic matter; (c) evaluation of the relationships between the gross nitrogen processes and in the size fractions of organic matter, (c) laboratory evaluation of the effect of temperature and soil moisture on TMB and TCB, and (d) determination of TMB and TCB on field conditions during the growing season of maize under SD and LC and in a pasture during the same period. The isotope 15N dilution technique allowed the determination of gross mineralization rates for different management practices both in laboratory conditions with disturbed samples, and on field conditions with intact soil samples. For measurements of gross nitrification rates, soil label with 15N-ammonium was not adequate, producing highly variable results and evidences of substrate stimulation. In all the experiments, the TCB was greater than TMB and the consumed total amounts were higher than the mineralized ones. After six years since management’s installation, both the TMB and TCB gross nitrogen processes in soil were strongly depending of the soil depth, being the highest in the top 0-5 cm layer. The pasture TMBs were higher than LC ones, suggesting improvement in the soil biological condition, while SD only presented a trend in this way. For the three management systems, high levels of C and N contents were determined in <50 &#956;m fractions, in which, the organic matter has a high level of physical protection and it was not modified by the management. The largest relative increase in C and N contents occurred in light > 50 &#956;m fractions of pasture. A simple pattern relating the TMB and TCB with the organic matter size fractions was not found. Linear multiple regression analysis for LC included C and N content on the <50 &#956;m fractions, however for P models, the C and N contents on light >50 &#956;m are frequently included variables, suggesting recovery of soil OM. The TMB and TCB were affected by soil moisture and temperature, appearing as optimal conditions to gross mineralization temperature of 25°C and soil moisture of 70% of field capacity. For field determinations, after 9 years of management installation, the variations of TMB and TCB were mainly associates to the soil moisture variations. Pasture TMB and TCB values were higher than LC and SD.TMB and TCB values of SD management system were higher than LC only on the February and March soil samplings. For all the management systems TCB was higher than TMB, being the total consumed amounts, higher than the mineralized ones. Total mineralized N was higher on SD than LC; however, the corn crop was not benefited since N availability did not meet the period of highest N demand.

A intensificação da agricultura convencional que ocorreu nas últimas décadas na região Sudeste da Província de Buenos Aires (Argentina), provocou degradação das propriedades do solo e redução da fertilidade nitrogenada, apesar dos níveis relativamente elevados de matéria orgânica nesse solo. A inclusão de pastagens (P) nas rotações agrícolas e a adoção da semeadura direta (SD), como manejos alternativos à lavoura convencional (LC), visaram deter a degradação desse solo. A adoção de diferentes sistemas de manejo influem na dinâmica do nitrogênio no solo, modificando a sua disponibilidade às plantas. Com base nesta hipótese, o objetivo geral deste trabalho foi o de quantificar os processos de mineralização, nitrificação e consumo brutos do nitrogênio em um Molisol do SE da Província de Buenos Aires, manejado durante 17 anos com LC e transformado para P ou SD. Os processos brutos do ciclo do nitrogênio foram quantificados pela técnica da diluição do isótopo 15N. Foram realizados os seguintes experimentos: (a) avaliação em laboratório de taxas de mineralização bruta (TMB), consumo bruto (TCB) e nitrificação bruta (TNB) em amostras deformadas e em condições de umidade e temperatura constantes; (b) avaliação dos efeitos do manejo na distribuição das frações granulométricas da matéria orgânica; (c) avaliação das relações entre os processos brutos do ciclo do nitrogênio e as frações granulométricas da matéria orgânica, (c) avaliação em laboratório dos efeitos da temperatura e da umidade do solo nas TMB e TCB e (d) determinações de TMB e TCB em campo durante o ciclo da cultura de milho nos manejos SD, LC e pastagem. A metodologia de diluição do isótopo 15N permitiu a determinação de taxas de mineralização e de consumo bruto em solo com diferentes sistemas de manejo, tanto em condições de laboratório, com amostras deformadas, quanto diretamente em campo, com amostras indeformadas. Na quantificação da taxa de nitrificação bruta, a marcação do solo com amônio-15N não se mostrou adequado, levando a resultados muito variáveis, com evidencias de estimulação pelo substrato. Em todos os experimentos a TCB foi maior que a TMB e as quantidades totais consumidas superaram àquelas mineralizadas. Após seis anos da implantação dos manejos alternativos á LC (SD e P), os processos brutos de transformação do nitrogênio no solo apresentaram marcante estratificação, sendo maiores nas camadas superficiais do solo. As TMB da pastagem foram maiores às da LC, indicando melhora na condição biológica do solo, enquanto que SD somente manifestou uma tendência nesse sentido. Nos três manejos, grandes quantidades de C e N estiveram presentes nas frações <50 &#956;m, nas quais, a matéria orgânica tem alta proteção física e não foi modificada pelo manejo. A pastagem acumulou maior quantidade de C e de N nas frações leves >50 &#956;m que LC. Não se encontrou um padrão simples relacionando a TMB e a TCB com as variáveis das frações granulométricas. Equações de regressão múltiplas para LC incluíram o C e N das frações mais finas, enquanto que para P apareceram as frações leves maiores que 50 µm, sugerindo recuperação da MO do solo. A temperatura e a umidade do solo modificaram as TMB e TCB, sendo que as maiores taxas ocorreram a 25°C e a 70% MCR. LC não apresentou resposta nem a temperatura nem a umidade de solo. Em determinações feitas em campo, após de 9 anos de instalados os manejos, as variações das TMB e TCB foram associadas, principalmente, às variações na umidade do solo. As TMB e TCB na pastagem foram geralmente maiores que as de LC e SD. SD apresentou maiores TMB e TCB que LC no final do ciclo do milho. As quantidades mineralizadas e consumidas acumuladas por SD foram significativamente maiores às correspondentes de LC, embora a cultura de milho não foi beneficiada por essas diferenças devido a disponibilidade de N no solo não coincidir com os estádios de máxima demanda pelas plantas. / The intensification of conventional agriculture in Southeast of Buenos Aires Province (Argentina), caused (conducted to) soil degradation and reduction of nitrogen fertility, despite the relatively high levels of organic matter on this soil. No tillage systems (SD) and inclusion of pastures (P) in agricultural rotations are alternative soil managements to reduce the effects of conventional tillage (LC). Management systems affect the soil nitrogen dynamics modifying its availability to the plants. The objective of this work was to quantify the gross nitrogen mineralization, nitrification and consumption processes in a Molisol, which was previously under LC for 17 years. In 1994 it was transformed to P, SD or continued on LC. The gross nitrogen cycle processes were quantified using the isotope 15N dilution technique. The following experiments were carried out: (a) laboratory evaluation of gross mineralization (TMB), consumption (TCB) and nitrification rates (TNB) in disturbed soil samples with constant soil moisture and temperature conditions; (b) evaluation of the effect of management systems in the size distribution of soil organic matter; (c) evaluation of the relationships between the gross nitrogen processes and in the size fractions of organic matter, (c) laboratory evaluation of the effect of temperature and soil moisture on TMB and TCB, and (d) determination of TMB and TCB on field conditions during the growing season of maize under SD and LC and in a pasture during the same period. The isotope 15N dilution technique allowed the determination of gross mineralization rates for different management practices both in laboratory conditions with disturbed samples, and on field conditions with intact soil samples. For measurements of gross nitrification rates, soil label with 15N-ammonium was not adequate, producing highly variable results and evidences of substrate stimulation. In all the experiments, the TCB was greater than TMB and the consumed total amounts were higher than the mineralized ones. After six years since management’s installation, both the TMB and TCB gross nitrogen processes in soil were strongly depending of the soil depth, being the highest in the top 0-5 cm layer. The pasture TMBs were higher than LC ones, suggesting improvement in the soil biological condition, while SD only presented a trend in this way. For the three management systems, high levels of C and N contents were determined in <50 &#956;m fractions, in which, the organic matter has a high level of physical protection and it was not modified by the management. The largest relative increase in C and N contents occurred in light > 50 &#956;m fractions of pasture. A simple pattern relating the TMB and TCB with the organic matter size fractions was not found. Linear multiple regression analysis for LC included C and N content on the <50 &#956;m fractions, however for P models, the C and N contents on light >50 &#956;m are frequently included variables, suggesting recovery of soil OM. The TMB and TCB were affected by soil moisture and temperature, appearing as optimal conditions to gross mineralization temperature of 25°C and soil moisture of 70% of field capacity. For field determinations, after 9 years of management installation, the variations of TMB and TCB were mainly associates to the soil moisture variations. Pasture TMB and TCB values were higher than LC and SD.TMB and TCB values of SD management system were higher than LC only on the February and March soil samplings. For all the management systems TCB was higher than TMB, being the total consumed amounts, higher than the mineralized ones. Total mineralized N was higher on SD than LC; however, the corn crop was not benefited since N availability did not meet the period of highest N demand.