Carbon sequestration in tropical and temperate agroforestry systems: a review with examples from Costa Rica and southern Canada
Deforestation in the tropics, and fossil fuel burning in temperate regions contribute to the largest flux of CO2 to the atmosphere. Therefore, land-use systems that increase the soil organic matter (SOM) pool and stabilize soil organic carbon (SOC) need to be implemented. Agroforestry systems have the potential to sequester atmospheric carbon (C) in trees and soil while maintaining sustainable productivity. The potential to sequester C in agroforestry systems in tropical and temperate regions is promising, but little information is available to date. The objective of this paper is to give an overview of the history of agroforestry and to outline differences in management practices between tropical and temperate systems. This review focuses on C inputs, SOC pools and SOC stabilization with highlights from Costa Rican and Canadian systems, and their role in C sequestration and trading. The potential to sequester C in aboveground components in agroforestry systems is estimated to be 2.1×109 Mg C year−1 in tropical and 1.9×109 Mg C year−1 in temperate biomes. However, the type of agroforestry systems and their capacity to sequester C vary globally. For example, alley cropping is an agroforestry practice where trees are integrated with crops, therefore storing C in the woody components of the trees and in the soil, with a continual addition of organic material from tree prunings and crop residues. Studies from Costa Rica have shown that a 10-year-old system with E. poeppigiana sequestered C at a rate of 0.4 Mg C ha−1 year−1 in coarse roots and 0.3 Mg C ha−1 year−1 in tree trunks. Tree branches and leaves are added to the soil as mulch, contributing 1.4 Mg C ha−1 year−1 in addition to 3.0 Mg ha−1 year−1 from crop residues. This resulted in an annual increase of the SOC pool by 0.6 Mg ha−1 year−1. Despite the two crop rotations in tropical agroforests, C input from crop residues is similar between the two biomes. The total organic matter input, however, is still greater in tropical systems due to the larger addition from tree prunings. This greater input does not necessarily increase the SOC pool significantly when compared to a temperate system of similar age as a result of faster turnover rates of the SOM pool.