Agriculture Reference
In-Depth Information
Effect of Management on Soil Organic Matter Levels, Temporal
Trends, and Spatial Variability Patterns
Management strongly affects SOM accumulation, especially in the uppermost soil
horizons (Robertson et al. 1993, 1997) and is primarily restricted to the top 7 cm
of the soil profile (De Gryze et al. 2004, Syswerda et al. 2011). While using con-
centration (e.g., g C kg
−1
) to describe SOC content is suitable in many situations,
estimates of ecosystem C storage must be made on a surface-area basis (e.g., g C
m
−2
, kg C ha
−1
), which incorporates soil bulk density and depth of soil sampling.
Changes in both bulk density and distribution of SOC in the soil profile are known
to occur in response to changes in land use, including agricultural management
(Morris et al. 2007).
Twelve years after establishment of the seven MCSE systems on previously
cropped soils at the LTER main site (Table 5.1), the Early Successional system had
gained 380 g C m
−2
in the upper 5 cm of surface soil, representing an increase of
61% compared to the Conventional system (Grandy and Robertson 2007). Other
systems also gained SOC, in the order Alfalfa (341 g C m
−2
, 55% greater) > No-till
(264 g C m
−2
, 43% greater) > Poplar (229 g m
−2
C, 37% greater) > Biologically
Based (148 g m
−2
C, 24% greater) > Reduced Input (107 g m
−2
C, 17% greater).
In contrast, the soils of the Deciduous Forest and Mown Grassland (never tilled)
systems had about 2.5 times more SOC than the Conventional system in the upper-
most 5 cm.
Annual SOC accumulation rates of the MCSE main site systems relative to
the Conventional system ranged from 32 g C m
−2
yr
−1
in the Early Successional
community down to 8.9 g C m
−2
yr
−1
in the Reduced Input system. Whether these
accumulation rates represent absolute gains of SOC or slower losses of SOC rela-
tive to the Conventional system depends on whether Conventional system soils
were stable or were losing C over the 12-year study period. In 2006 and 2007,
Senthilkumar et al. (2009a, b) remeasured SOC at geo-referenced locations in
the MCSE, where SOC had previously been measured in 1988, and in the nearby
Interactions Experiment, where SOC had previously been measured in 1986. The
studied sites had a common history of conventional agricultural management for at
least the prior 70-100 years. In both experiments, SOC content appeared to decline
under conventional tillage management (Table 5.3). Carbon losses were less in the
No-till and Biologically Based systems, consistent with other studies (Grandy and
Robertson 2007; Syswerda et al. 2011) that show SOC gains in these systems rela-
tive to the Conventional system.
That C should be lost from these long-cultivated soils is surprising, and may be
related to wintertime warming; Senthilkumar et al. (2009a) documented a signifi-
cant local increase in the number of days per year with above-freezing air tempera-
tures over the 20-year study period (see also Robertson and Hamilton 2015, Chapter
1 in this volume). These findings are consistent with other studies around the world;
increasing temperatures are associated with observed soil C losses (Bellamy et al.
2005, Stevens and van Wesemael 2008). However, the contribution of accelerated
erosion over this period cannot be ruled out (Wischmeir and Smith 1961), although
the adoption of chisel in place of moldboard plowing in 1996 should have slowed
