4290.86 g C kg−^1 ) treatments without any N addition in a winter wheat – summer fallow
rotation over seven decades (Wuest et al. 2005 ). Furthermore, burning of residues
can reduce arbuscular mycorrhizal fungal (basidiomycetes and total glomalin) and
earthworm activities, that are responsible for stabilizing soil aggregates and enhance
soil water infiltration (Wuest et al. 2005 ).
Carbon (C) sequestration in soil occurs when C in plant residues, root exudates,
and microbial biomass is incorporated into soil organic matter (SOM) and not lost
to the atmosphere as CO 2. A dynamic balance exists in the soil between the addi-
tions of plant residue, the conversion of plant residues into SOM, and loss of SOC
to the atmosphere as CO 2. Increasing SOM and hence C sequestration occurs when
the production of SOM is greater than its decomposition by soil microbes (and loss
by erosion). Based on this premise, SOM accumulation would be unattainable in
conventional tillage WF systems predominant in the IPNW and Great Plains because
of insufficient residues (one crop in 2 years) and rapid SOM oxidation of buried
residues. Crop residue burning exacerbates SOM depletion by further reducing the
amount of residues returned to the soil under conventional tillage. Annual cropping
systems, even under conventional tillage, tend to maintain SOM although most of it
is located in the depth of incorporation (Machado et al. 2006 ). Under no-till systems
residues remain at the surface and are protected from decomposition and conse-
quently do not significantly add to SOM buildup as has been shown in other studies
(Soon 1999 ; Hooker et al. 2005 ). There were no differences in SOM content between
treatments with residues returned and removed (Soon 1999 ; Hooker et al. ( 2005 ) or
burned (Rumpel 2008 ). Roots play an important role in SOM accretion under no-till
systems (Gale and Cambardella 2000 ). Under no-till, increasing cropping intensity
increased SOM accretion due to root biomass production (Franzluebbers et al.
1994 ). The little or no increase in SOM when conventional tillage WF systems were
converted to no-till WF systems in the INPNW and Great Plains was probably
attributed to less root biomass produced as a result of fallow. Cropping intensity has
been shown to increase SOM accretion in no-till cropping systems (West and Post
2002 ; Luo et al. 2010 ; Hansen et al. 2012 ).
There is an increasing body of knowledge, however, that indicates SOM buildup
is not solely dependent on C input but also on crop residue quality. Kirkby et al.
( 2013 , 2014 ) observed that SOM buildup was influenced by humification effi-
ciency, the conversion of C inputs to humus or the stable or fine fraction of
SOM. The conversion of C inputs into SOM was influenced by the carbon-nutrient
stoichiometry and more stable SOM was formed when elements (N, P, S) were
added to crop residues to match the stoichiometry of these nutrients in stable
SOM. Soils that were supplemented with nutrients to match this ratio sequestered
more carbon than soils without additional nutrients (Kirkby et al. 2013 , 2014 ).
Crop residue quality, therefore, play an important role in SOC dynamics. In the
CR-LTE the MN and PV treatments had higher SOC than treatments receiving
artificial N only (Fig. 4 ). The MN and PV added 3.68 and 2.54 MG C ha−^1 year−^1 ,
and supplied about 110 and 45 kg N ha−^1 year−^1 , respectively. Crop quality effects
were clearly demonstrated by comparing SOC under PV and other N treatments.
Dryland Agriculture in North America