A question about lecture 2, slide 22: Why do perennial crops encourage larger aggregates? Is it because there is less soil disturbance (for instance limited tillage) ?
Hello. For starters, perennial vegetation such as forages and grasses are not disturbed by tillage. So larger aggregates can form without being broken by the impact of cultivation equipment in breaking them apart. However, we have cropped soils that are not tilled but don’t build as good soil structure as under perennials. Perennials invest more energy and resources into building root systems than annual crops. Perennials rely on roots to store energy and nutrients for growth in spring. Many perennials can grow sooner than annual in spring and continue to grow in the fall. Thus perennials for the most part, grow for a longer portion of the year than annual crops in our climate. This all results in more carbon input into soil with perennial via the root system. Further perennial roots are growing and dieing constantly, this adds carbon to soil and feeds microbes. The roots also encourage micorrhizal association (roots with a fungus, we will learn more on this later). The mycorrhizae have thin threads or filaments of cells. These excrete sticky glycoproteins to bind soil together. They also physically make a mesh around aggregates. Roots also can bind microaggregates together to make larger aggregates, partly because of excretion of sticky carbon substances and also because they push and pack soil as they grow and expand in pores. So you see its about carbon input for a longer period of time, roots, feeding soil microbes and lack of disturbance. Sorry for the long answer.
Thank you for the answer!
It makes sense that more carbon input and a stronger root system helps strengthen soil structure. I never knew that mycorrhizae excreted glycoproteins; I can certainly see how that would favour aggregate formation!
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