Technical note: A bootstrapped LOESS regression approach for comparing soil depth profiles

Aidan M Keith,Peter A Henrys,Rebecca L Rowe,Niall P Mcnamara

doi:10.5194/bg-13-3863-2016

Abstract

Abstract. Understanding the consequences of different land uses for the soil system is important to make better informed decisions based on sustainability. The ability to assess change in soil properties, throughout the soil profile, is a critical step in this process. We present an approach to examine differences in soil depth profiles between land uses using bootstrapped LOESS regressions (BLRs). This non-parametric approach is data-driven, unconstrained by distributional model parameters and provides the ability to determine significant effects of land use at specific locations down a soil profile. We demonstrate an example of the BLR approach using data from a study examining the impacts of bioenergy land use change on soil organic carbon (SOC). While this straightforward non-parametric approach may be most useful in comparing SOC profiles between land uses, it can be applied to any soil property which has been measured at satisfactory resolution down the soil profile. It is hoped that further studies of land use and land management, based on new or existing data, can make use of this approach to examine differences in soil profiles.

Highlights

Understanding the consequences of different land uses for the soil system is important to better inform decisions based on sustainability (Foley et al, 2005; Haygarth and Ritz, 2009)
It is becoming more evident that, in addition to there being a large proportion of total soil organic carbon (SOC) stocks resident in the subsoil, important C dynamics may occur deeper in the soil (Jobbágy and Jackson, 2000; Lorenz and Lal, 2005)
Differences in SOC across transitions and soil depth profiles can be tested with both land use and depth included as fixed factors in an interaction model, and appropriate random terms to account for non-independence of depth increments within the same core and/or plots

Summary

Introduction

Understanding the consequences of different land uses for the soil system is important to better inform decisions based on sustainability (Foley et al, 2005; Haygarth and Ritz, 2009). SOC (sensu organic matter) is generally concentrated in the top 30 cm of the soil and so LUC is generally expected to have the greatest impact on SOC in these upper layers (Lorenz and Lal, 2005; Laganière et al, 2010) Even within this surface soil, the magnitude and sometimes direction of the effects of LUC on SOC can depend on the depth that is being considered (Guo and Gifford, 2002; Popelau et al, 2011). Altered land use or management may impact the translocation of particulate and dissolved organic C likely to occur down the soil profile via effects on leaching. Such mechanisms may produce more complex relationships between soil depth and soil characteristics, and even discontinuous horizonation, rather than linear gradients

Existing approaches to model soil depth profiles

Applying a BLR approach – an example of bioenergy land use change

Findings

Conclusions