Application of fractal theory in assessing soil aggregates in Indian tropical ecosystems

Soil aggregation varies according to climatic and management factors, and is difficult to measure because of irregular shapes and sizes of soil aggregates. We applied fractal theory to assess soil aggregation as affected by (1) land use change from forest to savanna, (2) nutrient additions in forest, ecotone, and savanna ecosystems, and (3) tillage practice and residue treatments in an agro-ecosystem. We used fractal dimensions nonlinear (Dnon-lin) and linear (Dlin) based on number of aggregates (N) and mass of aggregates (M) (the range of values were 2.6–2.89 and 2.69–3.41, respectively) to capture the variations in the sizes of soil aggregates due to land use and treatments/management in these ecosystems. The variation in the values of non-linear fractal dimension based on mass (DMnon-lin) was smaller in forest and savanna ecosystems with and without nutrient additions, while the variation was wider in agro-ecosystems with different management practices. Linear fractal dimensions based on number (DNlin) and mass (DMlin) of aggregates varied marginally in these ecosystems and did not capture the variations in soil aggregates well. The variations in non-linear fractal dimension indicate that continued nitrogen loading in forest accelerates the formation of macro-aggregates, whereas in savanna the situation was reversed. The values of non-linear fractal dimensions did not show significant change after 6 years of nutrient additions in the ecotone; reflecting a buffering mechanism of this system in soil aggregate formation. On the basis of non-linear fractal dimension values, we conclude that residue retention and minimum tillage are appropriate for proper maintenance of soil aggregate stability for sustained crop production in the Indian dry land agro-ecosystems.