Agriculture Reference
In-Depth Information
The soil C increases of 50 g C m
−2
yr
−1
in the Biologically Based system
(Syswerda et al. 2011) are particularly intriguing. Syswerda et al. examined SOC
in the full soil profile (to 1 m), whereas most studies including the aforementioned
one by Senthilkumar et al. (2009a) sampled surface soils (to 15 or 20 cm depth:
Table 5.3). The soils in the Biologically Based system received no manure or com-
post and soil disturbance in this system was more intensive than in the Conventional
system because weed populations were controlled with cultivation. Although this
system has a legume cover crop, total biomass inputs are not higher than those
in the Conventional system, which produces more grain crop biomass. Thus, C
accumulation in this system cannot be attributed to reduced soil disturbance or the
quantity of biomass or other C inputs, but is instead due to some other factor that is
changing the processing of SOM. One possible explanation is that the incorporation
of legume cover crops in the rotation cycle (Table 5.1) is altering the decomposition
and stabilization of SOM (Willson et al. 2001). Differences in soil communities, the
biochemistry of plant inputs, or their interaction may have influenced soil aggrega-
tion or other factors that regulate SOM dynamics (Grandy and Robertson 2007).
Although the mechanism is not yet clear, the most likely explanation for enhanced
aggregation and soil C in the Biologically Based system is related to the red clover
cover crop.
Additionally, SOM change in the Biologically Based system appears to vary with
topography. On undulating glacial terrain, topography is one of the most influential
factors governing spatial patterns of SOM at field scales, while strongly interacting
with land use and management. Senthilkumar et al. (2009b) observed a tendency
for greater gain in SOC in topographic depressions (“valleys”) of the Biologically
Based system than in higher areas of plots (Fig. 5.1). Muñoz et al. (2008) con-
cluded that greater crop biomass inputs together with differences in residue quality
contributed to greater SOC accumulation in depressions in spite of higher levels of
soil moisture for decomposition. Considering that these results were obtained from
small-scale (1-ha) plots of the MCSE, where terrain variations are substantially
less than those typical of whole watersheds (e.g., average MCSE terrain slopes are
around 1°, ranging from 0-5°), one can suspect that the interactions between man-
agement strategies and topographical effects are even more important elsewhere.
Erosion has not yet been explicitly measured in KBS soils, which is an important
omission. Voroney et al. (1981), by using the van Veen and Paul (1981) model to
predict long-term levels of SOC, found that the inclusion of the universal soil loss
equation (Wischmeier and Smith 1961) greatly altered the predicted levels of SOC.
Irrespective of topography and historical soil erosion, there are other notice-
able differences in the spatial patterns of SOC under different land management
practices at KBS LTER (Kravchenko et al. 2006). Semivariance analysis shows
that SOC in the MCSE No-till and Biologically Based systems is more spatially
autocorrelated than in the Conventional system at scales of tens of meters (Fig. 5.2).
Another study observed stronger spatial structure in the Poplar system than in the
Conventional system even at 1-2 m scales (Stoyan et al. 2000).
That no differences in spatial patterns among MCSE plot locations were evi-
dent in the 1988 spatial analyses (prior to establishment of the experimental plots
in 1989; Robertson et al. 1997) indicates that these system-specific patterns have
