need help on a problem for my problem set, thanks in advance! You are studying t
ID: 69227 • Letter: N
Question
need help on a problem for my problem set, thanks in advance!
You are studying the decomposition and mineralization from leaf litter in a forest. The litter has the following characteristics:
Material
% of litter mass
C/N ratio
Physiological material
20
5
Cellulose
60
No N
Lignin
20
No N
Three groups of organisms decompose this material
Organism
C/N Ratio
Substrate Use Efficiency
They eat:
Bacteria
5
0.50
Only physiological material
Cellulytic fungi
20
0.40
Everything except lignin
Lignolytic fungi
20
0.25
Everything
Assume:
Growth rates:
A- Bacteria grow faster than fungi and have first crack at any material that they can metabolize (physiological material and dead microbes).
B- Cellulytic fungi grow next fastest and will use resources that bacteria don’t.
C- Lignolytic fungi grow the slowest of all three groups (i.e. they have last crack at material).
D- Microbes will only process a material if they have the nitrogen they need to do so.
1. Given those assumptions, run calculations for the first cycle of decomposition. Calculate:
A-How much of each plant carbon pool is decomposed?
B-How much biomass of each group of microorganisms is produced?
C-.Is any nitrogen mineralized? If so, how much?
Material
% of litter mass
C/N ratio
Physiological material
20
5
Cellulose
60
No N
Lignin
20
No N
Explanation / Answer
lant residues – these are shoot and root residues found on the soil surface and in the soil. They are broken down relatively quickly (weeks to years), and provide an important source of energy for soil microorganisms.
Particulate organic carbon – is defined as pieces of plant debris 0.053–2 mm in size. Particulate organic carbon also decomposes relatively quickly (years to decades) and provides an important source of energy for soil microorganisms. It also plays an important role in maintaining soil structure and providing soil nutrients. Plant residues and particulate organic carbon are often referred to as ‘labile carbon’ because they cycle in the soil relatively quickly.
Humus – is composed of decomposed material less than 0.053 mm in size, and is usually found attached to soil minerals. This type of carbon is more resistant to decomposition by soil microorganisms, and so tends to turn over more slowly (over decades to centuries). It plays a role in all key soil functions, and is particularly important in the provision of nutrients.
Recalcitrant organic carbon – is organic material resistant to decomposition and, in Australian soils, is dominated by charcoal. Recalcitrant organic carbon can take centuries to thousands of years to decompose, and is largely unavailable to microorganisms. Highly weathered soils and soils with a history of burning have a high proportion of recalcitrant organic carbon.
Carbon pools and soil health
The proportion of each of the different types of carbon that are present in a soil can provide important information about soil health. The proportion of total carbon made up of plant residues and particulate organic carbon is primarily influenced by the amount of new organic material added to the soil each year. Systems that produce high amounts of organic material, such as well managed pastures or native vegetation, will have higher amounts of residue and particulate organic carbon. Agricultural systems characterised by continuous cropping, long fallow periods and management practices that accelerate the breakdown of organic material (such as tillage and/or stubble burning/grazing) typically have low amounts of these carbon fractions.
Because labile carbon fractions provide an important energy source for soil microorganisms, the proportion of the organic matter in these fractions can indicate how biologically fertile a soil is. Two soils may have the same total organic matter content, but a soil with 50 % of its total soil organic matter present in the labile pool suggests a more biologically active soil, with greater potential for nutrient turnover, and better soil structure, than a soil with just 5 % of its total organic matter in labile fractions (figure 2). Similarly, a soil that has a high proportion of its total carbon present in the recalcitrant organic pool is likely to be less fertile than a soil with the same total carbon content but a lower proportion of recalcitrant organic carbon.
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