Transect sampling for soil organic carbon monitoring in temperate alley cropping systems - A review and standardized guideline

Abstract

Agroforestry systems provide significant ecosystem services, including soil organic carbon sequestration. However, the structural complexity of these systems must be considered when taking samples. In particular, when sampling in the tree- and arable- or grassland strips of alley cropping systems (ACS), different levels of tree influence must be taken into account. The heterogeneity of soil sampling approaches in ACS has been creating challenges for comparability and the integration of findings in meta-analyses. Furthermore, some of the sampling approaches have led to biases of over- or underestimation of the tree influence for the whole system. We evaluated 48 studies of a published meta-analysis on soil organic carbon sequestration in temperate agroforestry systems, to identify, document and quantify potential biases associated with transect soil sampling in ACS. In all of the 23 transect designs evaluated in detail, at least one of the six identified biases was observed. Overall, 10 out of the 23 transect designs did not include soil sampling in the tree strip, which may lead to an underestimation of the tree influence. On the other hand, an overestimation of the tree influence may occur when the tree- and arable strips are not weighted for their respective area shares, which was considered only in three transect designs. To address the identified biases and enhance the accuracy and comparability of soil organic carbon analyses in ACS we propose a standardized guideline for transect sampling in temperate ACS.