ESPM 131 Lecture Notes - Lecture 19: Humus, Aeration, Lignin
14 May 2018
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Lecture 19: SOM/C-Persistence
●Decomp of organic residues:
○In oxic soils: complex polymers → monomers/dimers → CO2/H20/biomass/E
○In anoxic soils : complex polymers → monomers/dimers → CO2/CH4/alcs/org
acids/NH3/less E and biomass
○Factors controlling rates of decomp and mineralization:
■Location and size of residues: at surface/in, particle size
■C/N ratio of organic residues (higher decomps slower)
■Soil fauna: soil mixing → incorp residues into soil, fragmentation reduces size,
important role in N-mineralization
■Env conditions: pH, moisture, aeration, temp
○Kinetics of residue decay:
■Zero-order : m = -k, used for 20-30 yrs
■First-order: exponential, log scale has slope of -k
■Glucose > cellulose > lignin decomp rate
●Soil C transformations:
○Plant debris (mostly roots) undergo microbial transformation into humus
○OM protected from decomp thru:
■Stabilization with minerals like clay/Fe-oxides
■Phys isolation w/in aggregates
■New soil continuum model
●Why does OM persist for 100s-1000s of yrs?
○Phys, chem, bio context → relative probability of decomposition
○Phys protection: extensive interactions
■Surfaces
■3D spatial protection in aggregates
○Chem resistance:
■Large mol size, nondifusible, complex, globular, sometimes hydrophobic, only
attacked by extracellular enz
■varied/random structures → enzyme attack dificult
○Environment/climate: extensive interactions
○Presence of degraders:
■Phys separation, dificulty getting around
■OM biologically reactive → binds, inactivates enzymes
○Presence of roots can “ prime ”
■Accel: increased number of microbes, primary C source
■Decel: simple substrates (exudates) used pref to litter, drying out surrounding
soil
●SOM in soil - why care?
○Huge C reservoir, slow-release
○Bank acct for N and nutrients, P, S
○Growth-promoting compounds
○Water holding capacity
Document Summary
In o(cid:481)i(cid:283) soils: (cid:283)omple(cid:481) pol(cid:482)mers monomers/dimers co(cid:1681)/h(cid:1681)(cid:1679)/(cid:282)iomass/e. In ano(cid:481)i(cid:283) soils : (cid:283)omple(cid:481) pol(cid:482)mers monomers/dimers co(cid:1681)/ch(cid:1683)/al(cid:283)s/or(cid:322) a(cid:283)ids/nh(cid:1682)/less e and (cid:282)iomass. Fa(cid:283)tors (cid:283)ontrollin(cid:322) rates o(cid:321) de(cid:283)omp and mineralization: Lo(cid:283)ation and size o(cid:321) residues: at sur(cid:321)a(cid:283)e/in, parti(cid:283)le size. C/n ratio o(cid:321) or(cid:322)ani(cid:283) residues (cid:1827)hi(cid:322)her de(cid:283)omps slo(cid:476)er(cid:1828) Soil (cid:321)auna: soil mi(cid:481)in(cid:322) in(cid:283)orp residues into soil, (cid:321)ra(cid:322)mentation redu(cid:283)es size, important role in n-mineralization. Zero-order : m = -k, used (cid:321)or (cid:1681)(cid:1679)-(cid:1682)(cid:1679) (cid:482)rs. First-order: e(cid:481)ponential, lo(cid:322) s(cid:283)ale has slope o(cid:321) -k. Glu(cid:283)ose > (cid:283)ellulose > li(cid:322)nin de(cid:283)omp rate. Plant de(cid:282)ris (cid:1827)mostl(cid:482) roots(cid:1828) under(cid:322)o mi(cid:283)ro(cid:282)ial trans(cid:321)ormation into humus. Ph(cid:482)s, (cid:283)hem, (cid:282)io (cid:283)onte(cid:481)t relati(cid:475)e pro(cid:282)a(cid:282)ilit(cid:482) o(cid:321) de(cid:283)omposition. Lar(cid:322)e mol size, nondi(cid:498)(cid:321)usi(cid:282)le, (cid:283)omple(cid:481), (cid:322)lo(cid:282)ular, sometimes h(cid:482)dropho(cid:282)i(cid:283), onl(cid:482) atta(cid:283)ked (cid:282)(cid:482) e(cid:481)tra(cid:283)ellular enz. Om (cid:282)iolo(cid:322)i(cid:283)all(cid:482) rea(cid:283)ti(cid:475)e (cid:282)inds, ina(cid:283)ti(cid:475)ates enz(cid:482)mes. Presen(cid:283)e o(cid:321) roots (cid:283)an (cid:1849) prime (cid:1850) A(cid:283)(cid:283)el: in(cid:283)reased num(cid:282)er o(cid:321) mi(cid:283)ro(cid:282)es, primar(cid:482) c sour(cid:283)e. De(cid:283)el: simple su(cid:282)strates (cid:1827)e(cid:481)udates(cid:1828) used pre(cid:321) to litter, dr(cid:482)in(cid:322) out surroundin(cid:322) Som in soil - (cid:476)h(cid:482) (cid:283)are? soil. Bank a(cid:283)(cid:283)t (cid:321)or n and nutrients, p, s.
