Agroforestry systems and environmental quality: introduction.

Investments in agroforestry research during the past three decades-albeit modest-have yielded significant gains in understanding the role of trees on farmlands, and the ecological and economic advantages of integrated farming systems. While early research focused mostly on farm or local levels, broader-level ecosystem services of agroforestry systems (AFS) have raised high expectations in recent years. The nine papers included in this special collection deal with three of such environmental benefits of AFS: water-quality enhancement, carbon sequestration, and soil improvement. These benefits are based on the perceived ability of (i) vegetative buffer strips (VBS) to reduce surface transport of agrochemical pollutants, (ii) large volumes of aboveground and belowground biomass of trees to store high amounts of C deeper in the soil profile, and (iii) trees to enhance soil productivity through biological nitrogen fixation, efficient nutrient cycling, and deep capture of nutrients. The papers included have, in general, substantiated these premises and provided new insights. For example, the riparian VBS are reported to increase the reservoir life, in addition to reducing transport of agrochemicals; the variations in C storage in different soil-fraction sizes suggest that microaggregate (250-53 μm) dynamics in the soil could be a good indicator of its C-storage potential; and the use of vector analysis technique is recommended in AFS to avoid consequences of inaccurate and overuse of fertilizers. The papers also identified significant knowledge gaps in these areas. A common theme across all three environmental quality issues covered is that more and varied research datasets across a broad spectrum of conditions need to be generated and integrated with powerful statistical tools to ensure wide applicability of the results. Furthermore, appropriate management practices that are acceptable to the targeted land users and agroforestry practitioners need to be designed to exploit these environmental benefits. The relative newness of research in environmental quality of AFS will pose some additional challenges as well. These include the lack of allometric equations for tree-biomass determination, absence of standardized norms on soil sampling depth, and limitations of fixed-effect models arising from issues such as pseudo-replication and repeated measures that are common in studies on preexisting field plots. Overall, this special collection is a timely effort in highlighting the promise of AFS in addressing some of the environmental quality issues, and the challenges in realizing that potential.