Neuza Asakawa

Carbon and nitrogen dynamics in ageing earthworm casts in grasslands of the eastern plains of Colombia

Abstract The effects of a large species of anecic earthworm, Martiodrilus carimaguensis Jiménez and Moreno, on soil C and N dynamics were investigated in a native savanna and a man-made pasture of the eastern plains of Colombia. We compared, across time (11 months), the total C, total N, NH+4 and NO–3 contents in the earthworm casts, the underlying soil and the adjacent soil. Additional sampling of root biomass and macrofauna was performed. In the two management systems, the total C and N contents were higher in casts (4.33–7.50%) than in the bulk soil (2.81–4.08%), showing that the earthworms selected food substrates with high organic contents. In general, C contents significantly increased during cast ageing (+100%), possibly because of CO2 fixation processes, dead root accumulation and/or macrofaunal activities in casts. In fresh casts, NH+4 levels were very high (294.20–233.98 μg g–1 dry cast) when compared to the soil (26.96–73.95 μg g–1 dry soil), due to the intense mineralisation processes that occurred during the transit of soil and organic matter through the earthworm gut. During the first week of cast ageing, NH+4 levels sharply decreased, while NH–3 levels showed successive peaks in the casts, the underlying soil and the adjacent soil. These results suggested the rapid production of NO–3 by nitrification processes in the fresh casts, followed by diffusion to the nearby soil, first vertically, then horizontally. After 2 weeks of cast ageing, NH+4 and NO–3 levels only showed slight variations, likely because of organic matter protection in stable dry casts. The root biomass was higher (1.6–4.7 times) below the old earthworm casts. The ecological significance of these results is discussed.

Nitrogen fixation by three tropical forage legumes in an acid-soil savanna of Colombia

Abstract The proportion of nitrogen derived from fixation was measured by 15N isotope dilution in three tropical grass-legume pastures on two Oxisols differing in texture over a 3 year period using three non-fixing controls. The use of the companion pasture grass, Brachiaria dictyoneura, was satisfactory as a non-fixing control when compared with a non-fixing Panicum maximum ecotype KK16 and native savanna grasses. Amounts of nitrogen fixed ranged from 0.3 to 40 kg N ha−1 12 weeks−1 during the wet season and were greatest with Stylosanthes capitata followed by Centrosema acutifolium and Arachis pintoi mainly as a result of greater legume biomass in the former compared with the latter two species. There were little or no differences amongst the three legumes in kg N fixed t legume DM−1. Amounts of nitrogen fixed per unit area decreased over the 3 year period mainly as a result of a loss of legume biomass. The % N derived from fixation (%Ndfa) was generally greater than 80% in all legumes on both soil types and there were little or no differences in %Ndfa with two levels of fertilization given at establishment. The %Ndfa did not change over time. In a separate experiment with A. pintoi, %Ndfa did decrease with increasing legume proportion in the sward. The results indicate that in tropical pastures sown on low fertility acid soils the amounts of nitrogen fixed by forage legumes are dependent on legume growth and persistence. Soil type, level of fertilization of age had little effect on the N2-fixation process as measured by %Ndfa. Based on these results approximate amounts of nitrogen fixed may be estimated by simple measurements of legume biomass-N × 0.8.