Nitrogen immobilization in decomposing litter contributes to productivity decline in ageing pastures of green panic (Panicum maximum var. trichoglume)

Abstract

The extent and rate of N release from nylon bags containing green panic {Panicum maximum var. trichoglume) litter was measured for up to 319 days (long-term studies) in 1978/79 and 1979/80 in Gayndah, Australia. Dry matter (DM) decomposition rates were measured in 41 periods of 39 days and related to environmental variables and initial litter N concentrations (short-term study).About half of litter DM decomposed during the long-term studies, while N concentration in the remaining litter increased from an initial average of 0-57 % N, to 0-95 % N. Net release of N from bags began when its concentration in the residue increased to c. 0-65% N (or when the C:N ratio decreased to 75:1). Only a net 20-30 % of the initial N was released for potential plant uptake by the end of the study. The short-term study showed that DM decomposition was rapid and independent of pasture age. Decomposition rate increased with soil moisture and average daily temperature but was unaffected by initial litter N concentration. Release of N from decomposing litter was slow, despite rapid DM decomposition. It was concluded that a major cause of declining productivity in sown grass pastures is the immobilization of N in decomposing grass litter.

Yield and botanical composition of a mixed grass-legume pasture in response to maintenance fertilization

“…Andrade & Braga, 2005), central Brazil (Drudi et al, 1995;Oliveira et al, 2001) and Australia (Robbins et al, 1989;Robertson et al, 1993) soil nitrogen availability is one of the main causes of this phenomenon. The causes of the decreased soil N availability have been attributed to an accumulation of low-quality plant litter and root materials in C4 grasslands and, consequently, to an increase in net immobilization by soil microorganisms (Robbins et al, 1989;Robertson et al, 1993;Urquiaga et al, 1998). The annual N deficit in tropical grass pastures have been estimated in 60 to 100 kg/ha (Myers & Robbins, 1991;Cadisch et al, 1994).…”

Nutrient cycling in tropical pasture ecosystems

“…Litter decomposition of tropical grass pastures is limited by high C:N ratio, lignin:N ratio, and acid detergent insoluble N (ADIN) concentration. Under these circumstances, nutrient immobilization occurs to a great extent and is considered one of the major causes of pasture decline (Robbins et al, 1989) in tropical ecosystems. Cantarutti (1996) observed that during the first week of incubation of soil samples with litter of creeping signalgrass [Brachiaria humidicola (Rendle) Schweick], 600 to 800 g kg -1 of all soil mineral N was immobilized and 300 to 500 g kg -1 remained immobilized after 150 days.…”

Dinâmica de decomposição e mineralização de nitrogênio em função da qualidade de resíduos de gramíneas e leguminosas forrageiras

“…A deficiência de nitrogênio acentua-se com o declínio nos teores de matéria orgânica, com a grande quantidade de N retido nos resíduos vegetais (liteira) de elevada relação C:N depositados e a imobilização pelos microrganismos durante a decomposição destes resíduos. Assim, a taxa com que a liteira é reciclada afeta a produtividade das pastagens (Robbins et al, 1989), sendo que o declínio na taxa de crescimento e na qualidade da forrageira relaciona-se com a diminuição na disponibilidade do nitrogênio no solo.…”

“…O manejo do pastejo pode contribuir para acentuar a deficiência de N, uma vez que pastagens com excesso de resíduos orgânicos aumentam a quantidade de N imobilizado em forma não-disponível (Cantarutti & Boddey, 1997;Robbins et al, 1989). Por outro lado, a desfolha intensiva aumenta as exportações do nitrogênio e as perdas devidas à maior exposição do solo.…”