How plant diversity benefits soil structure

Soil health and, particularly, the concern about increasing soil degradation and its effect on ecosystems, is a significant area of research. We’ve previously written about crop diversity and how it can have positive effects on soil organic matter and how crop residue return increases microbial biomass and soil health. However, a new article in Ecology Letters noted that many studies have focused on the biological effects of plant diversity on soil. Therefore, these researchers designed a study to look at the direct physical effects of species diversity on soil properties, leaving aside the biological effects.

Method

The experiment was conducted in two parts: a mesocosm trial and a field trial.

The mesocosm trial used 64 mesocosms with 24 plants in each. The type of plant added depended on the treatment. Six plants were used; two grasses (Lolium perenne and Anthoxanthum odoratum), two forbs (herbs) (Plantago lanceolata and Achillea millefolium) and two legumes (Trifolium repens and Lotus corniculatus). Therefore, some mesocosms were bare soil (control), 6 were monocultures of one of the six plants, and the remainder contained each available combination of two or more of the six plants. The 64 mesocosms were replicated again as some of the subsequent soil testing procedures were destructive. The 128 mesocosms were randomly arranged across 4 glasshouses and grown for 18 months after which the aboveground biomass was measured and soil cores were taken for root trait analysis. The soil cores were used to test for stability through three methods:

  1. slaking (rapid immersion in water);
  2. microcracking (slowly wetting); and
  3. mechanical breakdown (agitated in conical flask).

The replicated samples were tested for water conductivity (water flow through the soil) and soil strength, being its ability to withstand being displaced when a mechanical ram applied force to the soil column.

The field experiment used the grassland experiment set up at Jena in Germany in 2002. 82 plots with different treatments consisting of the same species used in the mesocosm experiment save for the L. perenne were combined with a number of other species to provide differing levels of diversity. Plots contained either 1, 2, 4, 8, 16 or 60 different species.

Topsoil was extracted and analysed for aggregate stability, root trait analysis, organic matter content and glomalin-related protein content in similar methods to those used in the mesocosm analysis.

Results and discussion

The data collected from the two experiments indicated that although species diversity increased aggregate stability, certain species of plants, particularly the grasses with their particular root traits, played the most significant role in this improved stability. With improved stability came improved productivity indicated by an increase in above-ground plant biomass significantly correlated with the improved aggregate stability.

Plant diversity Figure 1

Figure 1 from article. Figures (a) – (c) contain aggregate stability data from the mesocosm experiments while (d) – (f) contain the same data from the field experiment. Across the experiment, increasing plant diversity resulted in increased soil aggregate stability when assessed as a function of the remaining mean weight soil diameter.

Rooting strategies of the grasses resulted in a significant increase in resistance to slaking even when planted as a monoculture. The researchers hypothesise that this may be due to the grasses having finer root systems for greater resource uptake, their root length being the greatest and narrowest out of the 3 types of plants in the experiment. Across the experiment it was shown that root length density and root density were positively correlated with increased aggregate stability. Further, finer roots are decomposed easier than thicker roots which is likely to lead to higher organic matter which has independently been shown to increase aggregate stability.

L. perenne, one of the grasses, had the greatest root length and impacted on aggregate stability the most. When it was planted in a mesocosm with a diversity of other plants, the root length density of the mesocosm was not correlated with the soil aggregate stability but, in mesocosms with a diversity of plants but without L. perenne, the correlation between stability and root length density was again apparent (see Figure 2). The data points in the figure (at least for slaking aggregate stability) appear to generally show greater stability overall when L. perenne was included, indicating the significant effect the root system of this grass has on aggregates.

Plant Diversity Figure 2

Figure 2 from article. Without L. perenne (circles), increasing root length density, correlated with species diversity, resulted in increased resistance to being broken down. But the presence of L. perenne skewed this correlation (dots), seeming to have single-handed effect on increasing aggregate stability.

Adding to this finding that the effects were more complicated than just the effect of species diversity alone, the two legume plants resulted in reduced aggregate stability with associated reduced root length but thicker roots and greater root mass. Even in field plots with legumes included in the diversity treatments, a reduction in the aggregate stability of the treatment was observed compared to when the legumes were not included.

These abundant short, thick roots had other advantages though – water permeability and resistance to soil displacement from the force of the mechanical ram increased in the legume treatments.

Conclusion

What does this mean? As almost always, its complicated. However, we can be confident that increasing crop diversity is likely to result in increase soil aggregate stability. Aggregate stability increases the nutrient holding capacity of the soil, assisting plant growth. But some plants in the combination will have a greater affect than others on traits such as stability or resistance to displacement. If you have  sandy soil struggling to form or maintain aggregates and therefore losing nutrients, planting grasses are likely to help to a bigger extent than many other plant types. But if you have a clay soil that struggles to drain water and in which plants struggle to form a solid root system, a legume crop is more likely to increase water permeability and assist in soil aeration.

A point that is not obvious from the paper but could be of interest is whether, or how, legume crops adversely affect root length in a diverse planting. Grasses are considered to have their particular root strategy to scavenge as many nutrients as possible. But if planted next a legume, does the easy accessibility to nitrogen fixed by the root nodules of the legume plant result in the grass roots not growing to the depth they would have otherwise grown to? Perhaps this has been answered before but, if not, it could explain this particular piece of data.

A great experiment with some helpful data for farmers, agronomists and gardeners.

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