Agriculture Reference
In-Depth Information
developed since 1988 in response to different management practices. In particular,
the reduction in soil disturbance in the No-till and Poplar systems apparently led
to more pronounced site-specific variations in factors affecting soil C storage and
C and N mineralization (Kravchenko and Hoa 2008). Likewise, replacement of
uniform fertilizer applications under Conventional management with cover crops
under Biologically Based management may also have led to differences in spatial
variability, in this case because of a greater spatial variability of biomass inputs
(Kravchenko et al. 2006, Muñoz et al. 2008).
Roots also play a major role in controlling SOM dynamics through their effects
on aggregation and the movement of C to depth (Kavdir and Smucker 2005). Crop
type and growth habits can potentially affect root-derived SOM, though probably
over long time scales. Genetically engineered Bt corn, which is thought to produce
litter that decomposes more slowly than traditional varieties, did not alter soil C
and N pools relative to that of nonengineered corn over 7 years at the KBS LTER
(Kravchenko et al. 2009). In another example, an herbivorous insect infestation of
the Poplar system was found to affect the N use and mineralization of the defoliated
poplar plants, but did not detectably alter soil N pools (Russell et al. 2004).
Research at KBS LTER has also documented the destabilizing effect of land-
use intensification on soil aggregation and SOC mineralization. The conversion of
long-term grasslands and other set-aside lands in the USDA Conservation Reserve
Program (CRP) to row-crop agriculture (Feng and Babcock 2010) poses a potential
threat to stored SOC (CAST 2011). Recent experiments at KBS demonstrated the
rapid and destabilizing effect of tilling grasslands on aggregation and the poten-
tial implications for SOM dynamics. Grandy and Robertson (2006a, b) plowed a
portion of the Mown Grassland (never tilled) community and followed changes
in soil aggregation, CO
2
emissions, and microbial activity. They found that after
a single spring tillage event, aggregates in the 2000-8000 µm size class declined
substantially from 34% to 19% of total aggregates. Associated with these changes,
the newly cultivated sites lost an average of 1.4 g C m
−2
d
−1
, derived from both
plant litter inputs and native SOM, between May and October over 3 years. Such
results raise concerns about the long-term environmental implications of expand-
ing crop production into CRP and other grasslands to support the biofuel industry
(Robertson et al. 2011, Gelfand and Robertson 2015, Chapter 12 in this volume).
More work is needed to better understand and perhaps mitigate changes in SOM
under different scenarios of land-use change.
Afforestation is another land-use change that impacts the terrestrial C cycle
through changes in SOM (Morris and Paul 2003). Soil carbon accrual in affor-
ested former agricultural lands at KBS, at the nearby Kellogg Forest, and at the
Russ Forest in Cass County, Michigan—all on the same soil series and with stands
of 50-60 years age—are shown in Fig. 5.3 (Morris et al. 2007). Changes were
ascertained by comparison with adjacent agricultural fields. In contrast to C, the N
content in afforested soils appeared to decrease in the MCSE and Kellogg Forest
soils, but increase in the Russ Forest soils. The N accumulation at Russ Forest was
in excess of that expected based on localized atmospheric N deposition (Morris
et al. 2007). One explanation for the different responses is that afforested soils at
KBS may have higher N concentrations because of manure applications when the
