RESUMO: A pastagem nativa é uma importante fonte de forragem para produção de bovinos e ovinos no bioma Pampa. A introdução do azevém e a fertilização nitrogenada são alternativas que visam intensificar a exploração pecuária de forma mais sustentável, podendo causar alterações em vários atributos do solo. Este trabalho teve por objetivo avaliar os efeitos de longo prazo de fertilização nitrogenada sobre atributos indicadores da saúde do solo em pastagem nativa com introdução de azevém anual. O experimento consistiu de uma sucessão secundária da pastagem natural submetida ao pastejo contínuo por bovinos e ovinos, com oferta de forragem constante. Em 1996, anteriormente ao inicio do experimento, a área experimental foi calcareada para correção da acidez do solo. O experimento foi então iniciado, sendo constituído da aplicação de três doses de N em cobertura, correspondendo a 0, 100 e 200kg ha-1 ano-1 de N. Em 2010, amostras de solo foram coletadas nas camadas de 0-20 e 20-40cm para análise de C e N total, particulado e associado aos minerais, além de uma área de referência (campo nativo sem pastejo). Análises microbiológicas foram conduzidas nas camadas de 0-5 e 5-10cm. A adubação nitrogenada de longo prazo em solos de campo nativo com introdução de azevém não altera os estoques totais de C e N, mas aumenta a fração particulada de N na dose de 100kg ha-1 ano-1. Em relação à atividade microbiana, o aumento na dose do fertilizante aumenta a respiração dos microrganismos, assim como o N da sua biomassa.

ABSTRACT: Native pastures are of great importance for cattle and sheep nutrition in the Pampa biome. However, due to its low productivity, the Italian ryegrass introduction and the nitrogen (N) fertilization are alternatives proposed to intensify livestock production in a sustainable manner. The objective of this study was to evaluate the effects of long-term N application on soil health indicators in a native pasture with Italian ryegrass introduction in southern Brazil. The experiment consists of a secondary native pasture under continuous grazing and constant herbage allowance. In 1996 experimental area was broadcast limed and the experiment was initiated, testing three N topdressing rates (0, 100 and 200kg N ha-1 year-1). In 2010 soil of experimental and reference area of non grazed native grassland was sampled in the soil layers of 0-20 and 20-40cm. Total, particulate and mineral associated carbon (C) and N stocks were evaluated. Soil microbiological attributes were evaluated in 0-5 and 5-10cm soil layers. The long-term N fertilization in soils with native pasture and Italian ryegrass introduction did not affect total C and N stocks. However, increases in N particulate fraction were seen with 100kg ha-1 year-1 of N rate of fertilization. Furthermore, the increase in N rates increased N microbial biomass and respiration.

The overseeding of ryegrass (Lolium multiflorum Lam) on sub-tropical native pastures increases the availability and quality of forage in the cold season. However, soil chemical amendment and nitrogen use can affect the floristic composition, with consequences to the primary and secondary production. In a randomized complete block design with three replications, the effect of nitrogen fertilization (40, 90 and 140 kg/ha of nitrogen) on native pasture overseeded with ryegrass in two consecutive years was assessed regarding production and composition of pasture and performance of beef calves. In both years, the forage accumulation rate and forage production responded linearly to the increase in nitrogen rates as a result of increased participation of ryegrass. The lower efficiency of responses in 2008 was due to the increased participation of dead material and the reduction of ryegrass in the forage sward. In 2007 there was an increasing substitution of the participation of native grasses by ryegrass with increasing nitrogen rates. Native legumes and other species were almost not affected by nitrogen fertilization. The average daily weight gain of the animals responded linearly to nitrogen rates in 2007, and quadratically in 2008. The stocking rate and gain per hectare increased linearly in both years, with greater responses in 2007. The production and composition of forage and livestock production are improved by the use of nitrogen, but the efficiency of responses can be affected by sowing date and nitrogen fertilization.