The ebb and flow of soil

Gully erosion at a cattle crossing
Gully erosion at a cattle crossing. J. Kelley (CC BY 2.0),

Loss of soil, although not recognised by many, is one of the major problems that society has to face. It provides habitat for a variety of species, influences the hydrological cycle by its water holding capacity and, therefore, affects vegetation. Soil has a large impact on agriculture because of its nutrient content. Looking at soil from this angle, it really is the basis for human nourishment . Soil is one of the most important factors considering the health of almost every ecosystem of our world! But still, Fire, Wind, Water, Earth; guess which one gets the least attention?

Different non-governmental organisations (NGO‘s) indicate there are major problems induced by erosion of soil. The top 10 – 15 cm of the ground, referred to as topsoil, is a nutrient rich layer in which almost all biological processes take place. It is estimated that half of the earth’s topsoil  has been lost in the past 150 years. It’s high time to not only bring more media and political attention to this problem but also to learn more about the mechanisms behind the phenomenon to better understand its origins and work how the best solutions.

Different processes affect the sediment transport and movement of topsoil. Water, wind and gravity are the most important erosive factors on a short time scale and they influence the shape of earth’s surface. As long as these forces exist (hint: forever!), landscapes will change, soil will change and that ultimately influences people’s lifes in many ways.

To properly manage the effects and reduce negatives of soil erosion we have to understand the processes behind it. On steep slopes, gravity is the major factor that moves soil, sometimes even in a large scale, leading to great time-lapses movies. Gravity is not the only factor, vegetation has a huge impact on vulnerability of soil to mass movements. The roots of trees reinforce the soil, connecting different layers and stabilising soils by managing water content. Vegetation also reduces surface flow and wind speed which, in turn, reduces water and wind erosion.

Due to the immediate danger associated with landslides in mountainous areas, a lot of research has been done on short term effects of soil movement and the processes are relatively well known. But what is happening in low-gradient landscapes? What are long-term influences of different types of vegetation? How much do we really know about the processes and how can these findings be used in the future for a more sustainable development? The prediction of soil loss and landscape development has become one major factor in planning and management practices for agriculture. Contour Farming, for example, can reduce soil loss by 50% as can soil stabilising crop rotations.

Chartwell Basin. James Harper (CC BY-NC 2.0),

Matthew Hughes from the University of Canterbury, Peter Almond from Lincoln University and Joshua Roering, from the University of Oregon, question the stabilising effect of forests on low slope (<30%) hill sites. Over long timescales, the sediment transport of smooth landscapes increased bioturbation in a forest environment which increases sediment flux, as suggested in their article “Increased sediment transport via bioturbation at the last glacial-interglacial transition“. The geomorphic response to climate change is evident, but the understanding of how vegetation (and especially vegetation change) contributes to soil movement is poorly understood. Although numerical models start to incorporate these effects, there is not a lot quantitative field data for calibration and validation.

The authors used the Charwell Basin on the South Island, NZ as a study site to investigate sediment flux rates. This low-gradient, unchanneled valley transect was vegetated by forest during the Holocene, roughly 14,000 years ago,  and grass/shrubland during the Pleistocene, from 2.6 million years to 14,000 years ago. The fluctuations in soil accumulation suggests that climate and vegetation have a long-term impact on soil transport. The results of the study show that the Holocene transport rate is nearly twice the one on the Pleistocene. This highlights the significance of biogenic processes in estimating erosion and soil movement processes. The article suggests that, over short time scales, root networks effectively stabilise soils, but in the long term, tree turnover and bioturbation increases downslope movement of soil.

Lewis Pass slip
A slip in Lewis Pass. Trees aren’t always able to stabilize soil. Image from Adrian Paterson.

This research agrees with other studies that examine the major impact of vegetational change on local forest landscapes. The rate of change during the Holocene is very similar to conclusions of Denny and Goodlett (1956). Hughes, Almond and Roering state that steep slopes soil movement via landslides will always exceed the influence of bioturbation. Although in low-gradient slopes, such as the study site, the opposite seems to be the case. A few recent models have included climatic and vegetational influence on soil transport and soil properties.

This type of study allows us to understand the relation of ecological processes and landscape evolution as well as estimating the effect of climate change on different landscapes around the globe. But more importantly, it is one step further in understanding the complex processes that influence the roots of our existence.

To attack future problems of food supply in a world with a growing population but limited resources, effective soil management is crucial for sustainability. Additionally, we must be prepared for changes in our landscapes, especially our agricultural land. As modelling becomes more precise, the more we efficiently and sustainably manage our resources, the longer they will last.

The study in Hughes, Almonds and Roerings article is clearly just the beginning of research on the impact of climate induced ecosystem shifts and their impacts on landscapes but it has already influenced further reserch on the topic. For more information on the topic I can recommend looking into other publications of the authors.

The author Adrian Balasch is a postgraduate student in the Master of Natural Resources Management and Ecological Engineering taught jointly at Lincoln University and BOKU, Vienna. He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.

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