Lecture 7 - Crop Rotation

Crop Rotation and Its Benefits

Introduction

• Crop rotation involves planting different crops sequentially on the same land.
• The lecture will cover:
  • Assessing the benefits of crop rotation.
  • Effects on the abiotic (soil) environment (water, soil compaction, nutrients).
  • Effects on the biotic environment (weeds, rhizosphere organisms, pests, and diseases).

Historical Context

• Crop rotation is an ancient practice being rediscovered.
• Romans used a three-field system based on food, feed, and fallow.

Roman Three-Field System

• Wheat: Planted after a fallow period for human consumption.
• Oats: Planted after wheat to feed horses and livestock.
• Fallow: Bare soil period to increase nutrient levels via microbial decomposition of organic matter.
• Wheat, the most nutrient-demanding crop, was planted after the fallow period to maximize nutrient availability.
• Limitations:
  • Not suitable for areas with substantial winter rainfall (e.g., Northwest England) due to soil erosion and nutrient loss during the fallow period.

European Four-Field Rotation

• Introduced in the UK in the 1700s by Turnip Townsend.
• Crops:
  • Wheat: Planted after clover for human consumption.
  • Clover: Established for livestock grazing, providing nitrogen through biological nitrogen fixation.
  • Barley: Planted after wheat, used for brewing or livestock feed.
  • Turnips: Planted to provide livestock fodder during winter and alleviate soil compaction due to their voluminous root system.
• Benefits:
  • Eliminated the fallow period with clover, which has high protein levels and is nutritious for livestock.
  • Livestock returned manure to the soil.
  • Wheat exports increased ninefold within sixty years in the UK.
  • Improved livestock quantity and quality by ensuring sufficient winter feed.

Modern Challenges and Monoculture

• In the 1970s, the UK favored monoculture (continuous cropping of wheat) due to:
  • EU policies promoting high food production.
  • Market prices favoring wheat.
  • Incentives for high productivity.
  • Wheat's versatility (bread, livestock feed, biomass power stations).
  • Farmers felt wheat yields were sufficiently high to justify monoculture.
  • Farmers didn't want to diversify the machinery needed for different crop types.

Break Crop Benefits

• Planting something other than the primary crop to improve yields.
• Yield response to break crops varies.
  • Some studies show no benefit or even a disbenefit.
  • When successful, break crops can double wheat yields.
• Farmers may be reluctant to diversify due to lower market prices for break crops (oilseeds, legumes).
• The primary benefit comes in the subsequent year's yield increase.

Assessing Break Crop Benefits

• Compare monocultural continuous cropping (wheat, wheat, wheat) with crop rotations.
• Assess yield differences in parcels of land with the same crop but different preceding crops.
• Crop growth models can predict yields, but experimental approaches are preferred.

Mechanisms of Break Crops

• Sugarcane example in Australia:
  • Break crop benefit indicated by values greater than one compared to continuous sugarcane.
  • Treatments: fallow, methyl bromide application (destroys soilborne organisms), and various crops.
  • Results suggest multiple mechanisms by which break crops enhance yields.

Yield Decline in Monoculture

• Scientists have focused more on the benefits of break crops rather than the causes of yield decline in monoculture.
• Potential causes of yield decline:
  • Buildup of soilborne diseases.
  • Mycorrhizal fungi becoming parasitic.
  • Soil acidification due to nutrient management (high nitrogen fertilizers).
  • Depletion of soil organic matter and specific nutrients.
  • Changes in rhizosphere microbiology or microbiome.

Crop Rotation's Broader Benefits

• Crop rotation practiced since antiquity.
• Monoculture yields have increased over time due to agronomic practices, potentially overcoming yield decline.
• Crop rotation offers significant benefits, but variable magnitude makes farmers reluctant to adopt it.
• Crop rotations can:
  • Increase the soil matter, which is good for water retention.
  • Improve water usage.
  • Improve nitrogen fixation in the soil.
  • Help to suppress weed growth.
  • Control against pests/diseases in a field.

Mechanisms by Which Break Crops Enhance Yields

Soil Water

• Example: Grain yield of wheat in Australia related to water use.
• Farmers rely on residual soil moisture at depth when planting wheat after rain.
• Different preceding crops (fallow, lentils, barley, canola) leave different amounts of residual soil moisture.
• Waterlogging can be a risk in poorly drained soils. Lucerne (legume) can be used to extract excess water through transpiration.
• Drier years can produce more wheat yield due to reduced water stress.

Biological Drilling

• Deep-rooted plants like lucerne can break plough pans and change subsoil pore structure.
• Following crops utilize biopores created by vigorous root systems (e.g., canola).
• Soil moisture and water accessibility impact crop success.

Soil Structure

• Crops impact soil structure (aggregation of soil particles) through:
  • Root growth creating biopores.
  • Root exudates (carbohydrates, nitrogen compounds, organic acids).
  • Root exudates (organic acids disperse soil particles, polysaccharides aggregate soil).
  • Microbial assemblages around roots.
  • Mycorrhizal colonization and glomalin production.
  • Turnover of roots introducing organic matter.

Nitrogen Fixation by Legumes

• Legumes acquire nitrogen through:
  • Direct nitrogen fixation via nodulation.
  • Direct soil uptake.
• High soil nitrate levels can inhibit nodulation.
• Residues break down, releasing nitrogen, though denitrification and nitrate losses can occur.
• Nitrogen sparing: successful biological nitrogen fixation means the legume takes up less soil nitrogen, making it available to subsequent crops.
• The success of nodulation and biological nitrogen fixation is highly variable.
• Legume benefits in crop rotations depend on effective nodulation.
• Nitrogen loss occurs through crop offtake (yield x nitrogen content in grain).
• EU studies show limited legume benefits (0-20 kg N/ha left behind), contrasting with Australian/North American examples.

Biological Mechanisms

Weed Control

• Crop rotation disrupts weed life cycles.
• Greater diversity in crop and herbicide rotations reduces blackgrass populations.
• Crops suppress weeds through:
  • Smothering via vigorous canopy expansion.
  • Crop residues providing a physical barrier.
  • Allelopathy: exuding chemicals that suppress weed seed germination or seedling establishment (e.g., sorghum producing sorgholeon).

Plant Growth Promoting Rhizobacteria

• Rice-Burseem clover rotation in Egypt.
• Burseem clover (legume) fixes nitrogen and leaves residues in the soil.
• Clover changes the prevalent rhizobacteria in the soil.
• Inoculating rice crops with rhizobacteria isolated from clover enhances yield.
• Associative nitrogen fixation benefits rice even without synthetic nitrogen fertilizer.
• Even with adequate nitrogen, rhizobacteria provide yield uplift, likely due to hormonal effects enhancing root growth.

Pest and Disease Control

• Potato Cyst Nematode:
  • Avoided by not planting potatoes sequentially to prevent egg buildup.
  • Natural attrition of eggs occurs over time due to physical factors (temperature, moisture).
  • Longer crop rotations offer greater pest control.
• Take-all Disease of Wheat:
  • Yields decline with continuous wheat planting, then recover over time.
  • Disease-suppressive rhizosphere develops with bacteria antagonistic to the take-all fungus.
  • Pseudomonad bacteria produce a toxin that inhibits the take-all fungus.
  • Continuous wheat plantings can become disease-suppressive, though yields may not reach maximum.
  • Disease-suppressive soils can build up, maximizing profits requires diverse crop rotation.

Conclusion

• Crop rotations offer multiple benefits by planting different crops sequentially.
• The preceding crop impacts root distribution, soil, water, nutrient availability.
• The effects can be positive or negative for the current crop, depending on resource requirements.
• Multiple mechanisms of weed, pest, and disease suppression:
  • Allelopathy
  • Competition
  • Rhizosphere changes
  • Time (pest and disease populations die off)