Carbohydrates and aggregation in lowland soils of Nigeria as influenced by organic inputs

We evaluated the influence of several organic matter management practices on the characteristics of carbohydrates in water-stable aggregates and soil aggregate stability at three Nigerian locations (Abakiliki, Nsukka and Umudike) where forests had been converted to arable farming. The effect of management practices to enhance aggregate stability was site-specific. The highest aggregate stability was obtained with Gliricidia sepium at Abakiliki, with Cajanus cajan followed by rice mill wastes (RW) at Nsukka and with the forested soil at Umudike. While none of the treatments at all sites was able to enhance the C and N contents of the soils to the levels obtained in the forested sites, a net improvement in carbohydrate and organic carbon (OC) content was found for some management practices. The carbohydrate status increased with G. sepium at Abakiliki, and with Dactylodenae bacterii alone or in combination with Pentaclethra species at Umudike, while at Nsukka all organic inputs increased carbohydrate content over the control and forested soils. However, neither total OC nor the carbohydrate content were significantly correlated to the variability in aggregate stability of these soils. The δ13C values found for acidic hydrolysates were constant within the soil aggregate sizes and generally distributed around −29 to −30‰, suggesting that the OC from these sites originated from C3 plants. Our results indicate that in these tropical Nigerian soils, aggregate stability and OC content are generally preserved by alley-cropping in well structured soil, whereas treatments with organic wastes are sustainable management practices in more fragile soils.