Earthworms and litter management contributions to ecosystem services in a tropical agroforestry system

The development of sustainable agricultural systems depends in part upon improved management of non-crop species to enhance the overall functioning and provision of services by agroecosystems. To address this need, our research examined the role of earthworms and litter management on nutrient dynamics, soil organic matter (SOM) stabilization, and crop growth in the Quesungual agroforestry system of western Honduras. Field mesocosms were established with two earthworm treatments (0 vs. 8 Pontoscolex corethrurus individuals per mesocosm) and four litter quality treatments: (1) low-quality Zea mays, (2) high-quality Diphysa robinioides, (3) a mixture of low-and high-quality litters, and (4) a control with no organic residues applied. Mesocosms included a single Z. mays plant and additions of N-15-labeled inorganic nitrogen. At maize harvest, surface soils (0-15 cm) in the mesocosms were sampled to determine total and available P as well as the distribution of C, N, and N-15 among different aggregate-associated SOM pools. Maize plants were divided into grain and non-grain components and analyzed for total P, N, and N-15. Earthworm additions improved soil structure as demonstrated by a 10% increase in mean weight diameter and higher C and N storage within large macro-aggregates (>2000 mu m). A corresponding 17% increase in C contained in micro-aggregates within the macro-aggregates indicates that earthworms enhance the stabilization of SOM in these soils; however, this effect only occurred when organic residues were applied. Earthworms also decreased available P and total soil P, indicating that earthworms may facilitate the loss of labile P added to this system. Earthworms decreased the recovery of fertilizer-derived N in the soil but increased the uptake of N-15 by maize by 7%. Litter treatments yielded minimal effects on soil properties and plant growth. Our results indicate that the application of litter inputs and proper management of earthworm populations can have important implications for the provision of ecosystem services (e.g., C sequestration, soil fertility, and plant production) by tropical agroforestry systems.