Dominant tree species and earthworms affect soil aggregation and carbon content along a soil degradation gradient in an agricultural landscape

Soil organic matter (SOM) is considered an important determinant of soil fertility in tropical agroecosystems. While numerous studies have shown the value of agroforestry in increasing soil nutrients and improving crop yield, few have addressed the systematic impacts of duration of cultivation on soil aggregation and C storage in such systems. A study was conducted in South Nandi (Kenya) to assess spatial influence of three dominant trees (Croton megalocarpus, Eucalyptus grandis and Zanthoxylum gilletii) on soil aggregation and C content in agroforestry systems. The study was conducted in a chronosequence experimental set-up where farms were continuously cultivated for 10, 16 and 62 years since conversion from primary forest. It was hypothesized that soil aggregates and whole soil and aggregate-associated C would decrease with duration of cultivation, with the magnitude of influence being reduced by the presence of trees and abundance of earthworms and termites. Greater abundance of small macroaggregates and microaggregates were recorded in soils under the canopy of Z. gilletii with an average weight of 62.8 g and 9.4 g 100 g−1 of soil compared to 53.9 g and 3.1 g 100 g−1 in soils under C. megalocarpus and 48.7 g and 3.9 g 100 g−1 in soils under E. grandis, respectively. These differences could be attributed to the high number of endogeic earthworm species, Nematogenia lacuum (Ocnerodrilidae) in soils under the canopy of Z. gilletii trees. Since N. lacuum is a small-sized species (40–55 mm long), it produces small faecal pellets and thus, we could infer that this species may have contributed to the fragmentation of large macroaggregates into small macroaggregates and microaggregates. The C content decreased by almost 40% in soils under longer duration of cultivation, with higher magnitude of differences associated with Z. gilletii trees. Increased microbial population in earthworms’ casts can increase mineralization rates, which may explain the low aggregate-associated C content under Z. gilletii trees where high number of N. lacuum were recorded. This study shows the significance of specific trees in shaping soil aggregation process and soil C content which could have far-reaching implications for the long-term C storage in the soil and hence net contributions to climate change mitigation.