Influences on nutrient release from cover crops a) Decomposition of cover crops in the soil begins with primary consumers that consume the cover crop residue. These include large invertebrates such as earthworms, millipedes, sowbugs, and slugs.
As the large organisms consume the plant materials, they shred it, creating greater surface area for microscopic invertebrates, such as nematodes, and microbes such as bacteria and fungi to continue the decomposition process. b) Temperature and moisture conditions affect the level of microbial activity (lower bioactivity at cooler temperatures, and under dry or waterlogged conditions) c) Location of the residue: Residue may be left on the soil surface (as in conservation tillage) or incorporated into the soil i. Incorporation into the top 6–8 inches of the soil:
With adequate moisture, will decompose most rapidly due to high oxygen levels and large populations of decomposing organisms ii. Leaving cover crop residue on soil surface: Will decompose more slowly due to drying. However, in some systems and under certain conditions (especially warm temperatures and adequate moisture), soil organisms may move surface residue below ground, facilitating decomposition. iii. Below 6–8 inches:
May decompose more slowly due to lower oxygen levels and fewer decomposing organisms d) Composition of the cover crop residue i. The carbon to nitrogen (C:N) ratio of the cover crop residue has a close link with N mineralization. Higher cereal biomass produces higher C:N, while more legume-heavy biomass produces lower C:N.
Microbes need both C and N, the C in carbohydrates to combust for energy and the N for building amino acids necessary for maintenance and reproduction. The relative amount of C and N available for microbes determines how large the pool of mineralized nitrate in the soil will be. • C:N ratios of 22:1 or less result in net mineralization of N.
Nitrate is liberated into soil solution and is thus available for plant uptake or leaching. • C:N ratios above 22:1 result in net immobilization of N. Nitrogen is bound in organic forms and is unavailable for plant uptake. Nutrient deficiencies may result if this state is prolonged.
Because the C:N ratio of cover crops increases as they age, it is generally recommended that cover crops be harvested or incorporated into the soil when close to full bloom (but prior to seed set) to assure a C:N ratio of 22:1 or less so that net mineralization occurs ii. The presence of lignins and tannins in cover crop residue slows the rate of decomposition. Lignins and tannins are organic compounds that are produced by plants and have high C:N.