Bacteria & Nitrogen

I've read that adding plant residues high in Nitrogen to a soil can actually cause a Nitrogen deficiency in plants because the bacteria consume the Nitrogen before plants get to uptake any of it.

The solutions I've read for this are: 1. put plant residues into the soil at a different time of year when the plant isn't in as much need of Nitrogen and 2. to compost plant resiues first, lowering the C:N ratio.

Is the reasoning behind these two options that the bacteria will have their chance to consume the Nitrogen quickly, but then will die and slowly realease N back into the soil for plants at a later time? How does lowering the C:N ratio in compost help plants get Nitrogen more efficiently than having a compost with a high C:N ratio?

Hi Scott - be clear to distinguish between high in N and high C:N ratio

A plant residue high in N has a low C:N ratio. A C:N ratio <25:1 (low) will lead to net mineralization as the food source contains more N than required by the microbes - thus excess N will be available to plants. A plant residue (incorporated into the soil) with a high C:N ratio e.g. >50:1 will lead to net immobilization as soil microbes require more N than in their food source, thus available soil N will be incorporated into microbial biomass (and not be available to plants).

Without seeing the specific reference you have read.... you may be referring to the priming effect - Addition of a "low" C:N ratio litter to the soil leading the an exponential growth in the microbial population, thus at least temporarily tying up available N.

In this part of the world, typically plant residues are incorporated into the soil either at the end of the growing season (i.e. crop residues) and/or in early spring if a cover crop (often a legume mix) is being grown, so that as the soil warms up, and microbial activity increases decomposition of the newly incorporated SOM releases nutrients to the growing crop.

Organic matter with a wide C:N ratio may be composted prior to incorporation into the soil in order to narrow the C:N ratio. Fundamentally micro-organisms start to decompose the OM, immobilizing the N in the food source, and subsequent micro-organisms obtain C & N from dead micro-organisms in addition to the original organic material, thus reducing the C:N ratio over time (i.e. closer to 25:1). Recall that micro-organisms have a C:N ratio of 8:1.

Thank you - that was very helpful.

I was reading high and low ratios the wrong way.

So basically, although micro-organisms tie up N (immobilize it) at first, they they die and because their bodies have very low C:N ratios, N gets put back into the system. I think I was understanding it as N being unavailable once immobilized for a much longer time frame.