Nutrient Availability in Soil Amended with Wheat Straw and Legume Residue

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Study: Residue addition frequency influences respiration, microbial biomass and nutrient availability in soil amended with high and low C/N residue

In the image above I’ve basically highlighted the mature dried wheat straw in yellow with a C:N of 80:1 that was first applied to soil. After two weeks the same amount of the green young dry faba bean with a C:N of 20:1 was applied at differing amounts and frequency for two more weeks.

After application of the wheat straw you can see a decline in plant available nitrogen by 75% & phosphorus by 50% in the first two weeks.

After that two week period, adding the equivalent amount of faba bean residue then doubled the original available nitrogen and phosphorus availability, and it seems to me like it may have sustained much higher levels for longer had the study continued. Soil carbon priming in action.

The H1-L4 (High C:N wheat followed by 4 applications of Low C:N faba over two weeks) part of the study however is the most interesting for me. Instead of applying all the faba bean reside in one go, applying it in stages gradually increased (red line) the available N and P. This approach would probably be the most efficient nutrient wise as plant nutrient removal increases as the plant grows, so it makes sense to add the nutrients as it needs them. Plants typically remove nutrients in a sigmoid curve.

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Soil Carbon, Grazing & Cropping Grasslands of Alberta, Canada

114 site study.

Grazing:

  • Grazing increased plant diversity
  • Grazing increased introduced species in higher rainfall (>350mm May – Sept) areas
  • Grazing increased aboveground biomass productivity in high rainfall areas, decreased it in lower rainfall areas.
  • Grazing saw no change in SOC in the 6 study regions.
  • Grazing increased root growth in the top 30cm of soil in high rainfall areas.
  • Grazing increased decomposition of plant litter in high rainfall areas.
  • Grazing lowers CO2/N20 flux.
  • Rotational grazing (High intensity, low frequency) significantly lowered CH4 production.
  • Native grasslands store most carbon.

Conclusion: Rotational grazing native grasslands in high rainfall areas FTW.

Cropping:

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  • Perennial grasslands produce higher below ground biomass than above
  • Cultivation leads to a rapid loss 30-60% of soil C
  • Continuous wheat cropping led to 19% loss of C
  • Cropping saw 30-40% C loss after 5 years.
  • Silvopasture (perennial pasture system) produced least CO2
  • Naturally re-vegetated areas failed to recover even after 50 years.

Conclusion: Silvopasture intercropping FTW.

Basically what Colin Seis does with Pasture Cropping native grasses.

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Residue Amendment and Soil Carbon Priming for Richer or Poorer

Feed the microbes carbon in C-poor soil and they’ll have a party.
Feed the microbes carbon in C-rich soil and they’ll put it in the C-bank.
This quote is of particular interest, emphasis mine:

The shift of bacterial community composition in response to residue amendment contributes to the sequestration of residue-C in SOC fractions.

Predator-prey carbon sequestration? Sounds similar to the Arthropod predator results. May the shift be with you.

The study:

A 150-day incubation experiment was conducted with 13C-labelled soybean residue (4%) amended into two Mollisols differing in SOC (SOC-poor and SOC-rich soils). …

The amounts of residue-C incorporated into the coarse particulate organic C (POC), fine POC and mineral-associated C (MOC) fractions were 4.5-, 4.3– and 2.4-fold higher in the SOC-rich soil than in the SOC-poor soil, respectively.

Residue amendment led to negative SOC priming before Day 50 but positive priming thereafter.

The primed CO2 per unit of native SOC was greater in the SOC-poor soil than in the SOC-rich soil. This indicates that the contributions of residue-C to the POC and MOC fractions were greater in the SOC-rich soil while residue amendment had stronger priming effect in the SOC-poor soil, stimulating the C exchange rate between fresh and native SOC.

The shift of bacterial community composition in response to residue amendment contributes to the sequestration of residue-C in SOC fractions.

The fate of soybean residue-carbon links to changes of bacterial community composition in Mollisols differing in soil organic carbon