Impact of soil tillage and land use on soil organic carbon decline under mediterranean conditions

Gottlieb Basch,José Barros,Mário Carvalho,José Calado

doi:10.31285/agro.16.666

Abstract

Soils under Mediterranean climate conditions frequently have low to very low levels of soil organic matter (SOM), as a result of low biomass production under the predominantly rainfed conditions and the intensive tillage operations commonly practiced. In order to assess both short and long-term impacts of soil tillage and land use on soil organic carbon, two sets of experiments were performed. One consisted in the identification and soil analysis of 3 pairs of sites under different soil types and land use over 5 to 30 years; in the second experiment a long-term fallow area was repeatedly submitted to different types of soil tillage management (mouldboard plough + disc harrow; non-inversion tine cultivation; no-till) over 3 years. Soil texture, bulk density and SOM were analysed along the whole soil profile in the first experiment, whereas bulk density and SOM to a depth of 30 cm was measured before the first tillage operations and at the end of the observation period in the second experiment. The results clearly indicate that tillage based land use, irrespective of the type of land use, caused a considerable decline in SOM content in the tilled soil layer. Very small and inconsistent differences in SOM between paired soil profiles were observed in the lower part of the profiles. In the second experiment with three types of tillage systems, SOM content decreased with tillage intensity. Avoidance of soil disturbance is an important step towards halting SOM decline under Mediterranean climate conditions.

Highlights

Soil organic carbon (SOC) changes in agricultural soils mainly occur because of soil management practices that alter the decomposition rates of soil organic matter and the amount of organic topsoil C that is lost through erosion [1, 2, 3]
Land use trial For the purpose of this trial we identified several “promising” locations in the Alentejo district in South Portugal, where we performed a previous soil sampling and an inquiry of the land owner regarding the land use history
Both approaches analysed in this study tried to collect data on how differentiated land-use or soil management could affect SOC gains or losses under Mediterranean climate conditions

Summary

Introduction

Soil organic carbon (SOC) changes in agricultural soils mainly occur because of soil management practices that alter the decomposition rates of soil organic matter and the amount of organic topsoil C that is lost through erosion [1, 2, 3]. Mainly warm and wet autumns and springs, lead to high mineralization rates of the organic matter, which, associated to low biomass production under rainfed conditions, intensive soil tillage used for crop establishment, straw removal and grazing of the stubble and soil erosion, are the main causes for the soil organic carbon depletion of Mediterranean cropland. Average soil organic matter (SOM) contents in the top layer frequently are around 1% These low SOC contents affect crop and overall soil productivity in different ways; through a) reduced water infiltration and retention capacity, b) reduced cation exchange capacity and nutrient cycling efficiency, c) deficient soil structure and root growth. In this study, integrated in a research project on the potential of no-till for carbon sequestration in agricultural soils, we wanted to understand the medium and long-term effects of different land use on the potential accumulation/loss of organic carbon along the soil profile, and how tillage would affect SOM sequestered over a period of several years of grazed fallow

Methods

Results

Conclusion