Agriculture Reference
In-Depth Information
Table 5.8. Effect of corn and wheat roots on nitrogen (N) uptake during crop
growth and nitrogen and carbon (C) mineralization subsequent to cropping.
a
N Supplied by Soil
(kg ha
-1
)
N Mineralized after Crop
(mg N kg
-1
soil)
C Mineralized after Crop
(mg C kg
-1
soil)
Bare soil
108
70
525
Corn
168
22
780
Wheat
116
45
680
a
The field study was conducted at the KBS Living Field Lab, adjacent to the MCSE.
Note:
Bare soil = microplots with roots excluded.
Source:
Sánchez et al. (2002).
including clover, play in SOM accumulation was described earlier in this chapter.
Their ability to provide N for subsequent crops indicates that such rotations should
continue to be a dominant component of sustainable cropping systems.
Summary
Twenty years of KBS LTER research have produced a remarkable consensus
about controls on SOM dynamics and has furthered our general understanding of
long-term changes in SOM in agricultural ecosystems. This understanding includes
the concept that although SOM takes many years to develop and includes a con-
tinuum of young and old materials, it is sensitive to management changes that can
either improve or hinder its role as one of the major controllers of soil fertility and
ecosystem functioning. Differences in SOM levels within the soil profile and across
landscapes can foster greater biological diversity both above- and belowground,
but the high spatial variability of SOM makes it especially difficult to interpret data
collected at low frequencies, particularly from only one or two sampling points in
time. Thus, long-term studies that allow resampling of the same locations across
time offer one of the best opportunities to detect gradual changes in SOM levels,
characteristics, and dynamics relative to vegetation influences, cultivation, fertiliz-
ers, and cover crops (Robertson and Paul 2000). These will all be affected by global
environmental change and the need for increased production of food and biofuel
crops in the future.
Results from KBS show that variation in SOM content and dynamics occurs
across the landscape even on relatively flat terrain with only shallow depressions.
Lower, wetter areas accumulate SOM most rapidly. Soils in these areas receive
higher inputs of crop residue and also receive eroded materials from upper slopes,
thus increasing their SOM content and water-holding capacities. Soils in so-called
steady state after many years of cultivation can continue to slowly lose SOM. Both
erosion and the change in SOM, with consequent alterations in aggregation, can
significantly affect soil bulk density, an important soil physical property. Changes
in bulk density can introduce uncertainty into estimates of soil C change when
measurements made from soil cores are compared across time.