Agriculture Reference
In-Depth Information
y = 244.50x + 2.50
30
MCSE System
R 2 = 0.40
Conventional
No-till
Reduced Input
Biologically Based
Poplar
Alfalfa
Early Successional
Mid-successional
Coniferous Forest
Deciduous Forest
20
10
0
0
0.02
0.04
0.06
0.08
0.1
Total Dissolved P (mg L -1 )
Figure 11.7 . Total dissolved phosphorus (TDP) and nitrate (NO 3 - ) in soil waters collected
with tension samplers at 1.2 m depth from MCSE systems. Row-crop systems receiving fer-
tilizer are shown with solid symbols. Each symbol represents the mean of 100-150 samples
collected at a particular site from 2000-2003; n = 3 replicate sites for each system. The regres-
sion line is fit to all points and is highly significant (P fit 0.001). Data from Kurzman (2006).
Although soil water NO 3 and TDP concentrations are correlated across MCSE
systems, these two ions are differentially retained by surface soils, as illustrated by
following changes in N:P ratios. The molar N:P ratio of NO 3 + NH 4 + to TDP in
precipitation is ~70-86, based on the NADP/NTN or USGS N data, respectively,
assuming TDP is ~0.03 mg L −1 (Table 11.1). Soils tend to retain P but lose N by
NO 3 leaching. The molar N:P ratio in soil water beneath the root zone, averaged
across all MCSE cropping systems, was 749 based on the ratio of NO 3 + NH 4 + to
TDP concentrations (Kurzman 2006). Thus, the water percolating out of the root
zone is enriched in N, largely in the form of NO 3 , relative to P. This increase in dis-
solved N:P ratios as precipitation infiltrates the soil reflects not only gains in N but
also the tendency for P to be retained in the soil. As a result, soils in the root zone
of the MCSE contain much more P relative to N; Robertson et al. (1997) reported
a molar N:P in the upper 15 cm of MCSE soils of 0.3, based on extractable inor-
ganic N (NO 3 + NH 4 + ) and total P. Given the considerable depth of the unsaturated
zone below the root zone (and the depth of soil water sampling), and the predomi-
nance of inorganic phosphate in the soil water (>90% of TDP, on average: Kurzman
2006), it is possible that much of the P that is transported downward in percolating
water becomes sequestered with minerals below the depth of sampling, but possi-
bly within the reach of deeply rooted plants. This is consistent with the lower TDP
concentrations typical of groundwater pumped from water-supply wells (means,
0.011 mg L −1 in well waters vs. 0.027 mg L −1 in soil waters). This selective reten-
tion of P in soils helps to explain why N is the most frequently limiting nutrient
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