2.2. Soil sampling and lab procedur

2.2. Soil sampling and lab procedures
Soil samplingwas conducted following harvest after the fifth year of
establishment to assess the effect of residue removal, tillage and N rate
effects on total carbon (TC) and nutrient stocks. Three samples per subsubplot
were taken with hand-held soil probes 20mmin diameter, to a
depth of 30 cm, and cores were divided into 0–15 and 15–30 cm depth
segments. Samples were air-dried and sieved to b2 mmbefore analysis
by standard methodology recommended for the North Central Region
(NCR, 1998). Soil pH and EC were determined 1:1 in water. Air dried
soil sieved to pass a 0.25-mm screen was used for determinations of
available P (Bray 1), and extractable K (atomic absorption). Air-dried,
finely-ground samples were used to determine TC and total nitrogen
(TN) by dry combustion on a Costech Elemental Analyzer (Model ECS
4010, Costech Analytical Technologies, Inc. CA). Two subsamples were
taken with a slide-hammer probe with a 5 cm core diameter in each
sub-subplot and at each depth to obtain bulk density values (Mg/m3)
via the core method (Blake and Hartge, 1986) to then convert TC and
nutrient concentrations (TN, P, and K) to a basis of weight per unit
area (Ellert and Bettany, 1995).
2.3. Statistical analysis
Data was analyzed using theMIXED procedure of SAS software version
9.4 (SAS Institute Inc., 2013). Residue removal (RR), tillage (T), and
N rates (Nr) were considered fixed effects, while replicates (blocks
within sites and sites) were considered random effects. Significance of
random effects was calculated with a Wald Z test statistic using the
COVTEST option in the MIXED procedure. Depth (D) was analyzed
using a repeated measures technique with variance-covariance structure
VC, variance components, selected on the basis of the lowest
Akaike's Information Criteria (Littell et al., 2006). Exact p-values from
the analysis of variance for each studied variable are reported in
Table 1. Pre-planned estimates were used for mean comparison purposes
setting the probability of Type I error or alpha level (α) at 0.05.

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