Soil Science: Organic Matter Increases with Crop Diversity

A recent study published in Ecology Letters tested whether rotating an increasing number of crops in a plot without adjusting any other soil management practices would effect soil structure, microbial community structure and activity, and soil organic matter.

Unfortunately access to the article requires payment. A previous meta-analysis by the same researchers on this topic and which formed the platform from which this study launched is freely available here.

In the meta-analysis the researchers found 122 studies that compared the effects of diversified agriculture with that of a monoculture for at least a three year period. Those studies reported on the affect of the rotation of one or a combination of the following soil characteristics:

  • Total soil Carbon or Nitrogen; and
  • Microbial biomass Carbon or Nitrogen.

Using the data from the studies they found that there was a general increase in carbon and nitrogen pools and a substantially large increase in microbial carbon and nitrogen when one or more crops were added in rotation to a monoculture. The effect was reduced after adding more than 3 crops in rotation and the most significant effect on soil organic matter was where a cover crop (being a crop not grown for cultivation and sale but manage soil) was used as one of the rotations.

Although the findings supported the positive effects of crop diversity on soil organic matter the researchers pointed out that it was impossible to separate the effects of cover crops from the effects of the diversification of crops grown. They also conceded that the studies did not control for changes in other management practices at the same time as the diversification of crops used, introducing a variable that could have significant effect on soil properties aside from the rotation of crops.

This question of the effect of rotation on soil properties devoid of the adjustment of other variables was the basis for the study published in Ecology Letters.

The Experiment

The researchers performed a 12 year experiment with the hypothesis that an increase in crop diversity through rotation = alteration in microbial community = positive effects on microbial activity = enhanced soil aggregation and stability of aggregates = increased soil organic matter accumulation = increases in soil fertility. The Biodiversity Gradient experiment has a website.

In soil science aggregates of clay particles and organic matter are crucial to soil fertility. Smaller (micro) aggregates usually contain more processed and harder to release organic carbon and nitrogen which remain in the soil longer, while larger (macro and mega) aggregates contain more readily available matter but can degrade quickly, therefore allowing the organic matter to be, for example, washed away. Greater stability of aggregates usually means greater fertility of the soil and increased microbial activity has been linked to increase stability of aggregates. A history of the research into the link between aggregates, microbes and organic matter is well set out in this paper.

The 12 year experiment compared a corn monoculture, to a

  • Corn and soy rotation;
  • Corn and cover crop (red clover);
  • Corn and soy and wheat;
  • Corn and soy and wheat and red clover; and
  • Corn and soy and wheat and red clover and rye cover.

The figure below is reproduced from the article and shows how the rotations were organised through the study.

rotational crops

My first question of the methods used in the study is around the timing of the sample. In the paper they suggested that all the samples were taken while the plots were under the same treatment (corn), however the figure above suggests this isn’t the case. If the figure is correct there may be some treatment bias. My second question is whether there is any usefulness sampling the soil at the start of the trial. The researchers mention that there was some greater input in some plots prior to the trial starting, and therefore my question is whether this affected the result. It may be that the plot preparation and the length of time of the study means that any difference at the beginning is likely to be of nil effect at the end of the study.

Results

Some main results taken from the study were:

  1. More water stable mega-aggregates were found in the high diversity rotations, the greatest in the corn and soy and wheat and red clover plot;
  2. Mega-aggregate stability correlated significantly with diversity and the soil organic carbon, total nitrogen and fungal abundance found in those aggregates;
  3. The concentration of soil organic carbon and total nitrogen in all aggregates (when corrected for sand content in those aggregates) increased with increasing rotations; and
  4. Statistically significant positive correlations existed between increasing diversity and labile (readily available/liable to change) carbon, nitrogen and microbial enzymes.

Discussion

In discussing their findings the researchers stated their concern that the use of legume cover crops may have exaggerated the effects of diversity given the nitrogen inputs created by legume crops. However they were able to show that there were diversity effects beyond just the effects of the legume crops, particularly the linear positive relationship between the diversity of crops and microbial communities the found. This may have played an important role in the finding that as crop diversity increased so did the stability of mega-aggregates, soil organic carbon concentration and total nitrogen concentration. The figure below from the paper represents these findings:

aggreages

Trajectory of aggregate and SOM formation and stabilization
under a high diversity rotation (e.g. SWC2) versus monoculture crop (e.g.
Cm). (a) Greater quantity and quality of residues entering soils in high
diversity rotations enhances microbial activity with positive impacts on
rates and extent of (b) mega-aggregate formation and stabilization. This
also leads to (c) enhanced microbial activity in high diversity microaggregates and concomitant increases in microbial by-products that
accelerates micro-aggregate formation, resulting in (d) increasing stocks of
stable SOC and TN.

Conclusion

It appears likely that diversifying crops leads to greater organic matter content in soils. If you were to take one point home to your veggie garden, market garden, community garden or farm it would be that rotating your crop with at least one legume cover crop will have a definite positive effect on your soil health; rotating in two or three food crops with a legume cover crop will be probably be even better.

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2 thoughts on “Soil Science: Organic Matter Increases with Crop Diversity

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