Effect of Land Use Types and Soil Macrofauna on Soil Aggregate Stability and Carbon Storage in the Central Highlands of Kenya

Abstract

Changes from natural forests and bush lands to various land use types has been shown to have great influence on soil structure and carbon storage. However, little is still known about how these changes affect soil aggregate stability and C storage over the long-term, especially in Africa. A study was conducted in central highlands of Kenya to evaluate the influence of four land use types on soil aggregation and aggregates C content, as affected by the abundance of earthworms and termites. The four land use types included: (1) an undisturbed natural bush land dotted with Lantana camara L. shrubs and Croton macrostachyus Hochst. ex Delile. trees, (2) over 50 years old grazed pasture dominated by a mixture of Bermuda grass (Cynodon dactylon (L.) Pers.) and Kikuyu grass (Pennisetum clandestinum Hochst. ex Chiov.), (3) over 80 years old coffee plantation with Coffea arabica L. cv. SL 28, and (4) a maize field that had been cultivated continuously for the last 3 years. Sampling for soil aggregates and soil macrofauna was done using monolith method to a depth of 0.3 m. Separation of stable soil aggregates was done through wet sieving process, where seven aggregate fractions were obtained; large macroaggregates (LM), small macroaggregates (SM), microaggregates (m), silt and clay (s+c), coarse particulate organic matter (cPOM), microaggregates-within-macroaggregates (mM) and silt and clay within macroaggregates (s+cM). The aggregate C content was analyzed using wet oxidation method. Soil macrofauna were handpicked from the monolith soil and preserved in 75% ethanol and identified to genera or species where possible, using morpho-anatomical keys and comparison with reference collections. It was hypothesized that soil aggregates and aggregate C would decrease with increasing level of soil disturbance, with the magnitude of these effects being reduced by the abundance of earthworms and termites. Generalized linear models (GLM) were used to test the effects of land use types on aggregate fractions and aggregate-associated C using R statistical software However, to test the effects of the two factors on soil macrofauna data, negative binomial regression was chosen as an extension of the Poisson distribution. When analysis of variance (ANOVA) showed significant effects, Tukey’s post-hoc tests were performed at α = 0.05. Results showed that land use types had significant influence on soil aggregation. Notable differences were observed in large macroaggregates (LM) fraction, with higher weight in bush land (14.4 g 100 g-1 soil) and grazed pasture (12.4 g100 g-1 soil) compared to coffee plantation (3.9 g 100 g-1 soil) and maize field (0.6 g100 g-1 soil). On the contrary, microaggregate fraction weight was higher in maize field (41.2 g 100 g-1 soil) and lowest in bush land (18.8 g 100 g-1 soil). Land use type also had significant effects on soil aggregate-associated C, with the differences similar to those of the aggregate fractions. Among the soil macrofauna recovered, myriapods was the only group that showed significant differences, where Spirostreptidae sp. showed the highest abundance in bush land (149.3 individuals m2) compared to grazed pasture (42.7 individuals m2) coffee plantation (53.3 individuals m2) and maize field (5.3 individuals m2). Soil macrofauna considered ‘ecosystem engineers’ (earthworms and termites), and have been suggested to have significant effects on soil aggregation also showed no significant differences. Their abundance also showed weak or no correlation with soil aggregates and aggregate-associated C content across the land use types. This study shows the significance of land use change in shaping soil aggregation process and soil C content which could have far-reaching implications on the long-term C storage in the soil. This could be important especially in the tropics where mitigation and adaptation to climate change are ideally closely linked due to the critical importance of soil C for crop production.