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The trend in HFOC data was very similar to that of LFOC. The rotation treatments significantly impacted the HFOC almost at every depth at each site, except 30-60 cm at Brookings (P <0.18) and 15-30cm at Beresford (P <0.43).
In general, SOC was positively correlated with most of the studied properties that include: TN, POM, SOM, AS, HFOC and LFOC (P< 0.001) and negatively correlated with bulk density (P < 0.01).
SOC is a heterogeneous material and can be separated into LFOC and HFOC using densitometry techniques (Janzen et al.
Results on LFON content in 3-year experiment showed that in 0-5 cm soil layer [T.sub.1], and [T.sub.5] increased LFOC content from 5.1 mg x [kg.sup.-1] in CT ([T.sub.9]) to 7.9 and 9.6 mg x [kg.sup.-1] without CR, and to 10.3, 11.5 and 13.1 mg x [kg.sup.-1] with crop residue @ 2, 4 and 6 [tha.sup.-1], respectively (Table 4).
A tillage x irrigation interaction had significant effects for the PON, LFOC and LFON in the surface layer only (Table 4&5).
There was a significant improvement in water stable aggregation and proportion of macro-aggregates; water soluble C, particulate and light fraction organic matter organic C (POC), particulate organic N (PON), light fraction organic C (LFOC), and light fraction organic N with the application of 4-6 [tha.sup.-1] rice residue along with recommended rate of NPK to wheat.
The differences of soil, litterfall, overstory and understory vegetation carbon storage, LFOC, and soil respiration between the thinned, underplanted, and unthinned stands were examined by one-way ANOVA and Duncan multiple comparisons ([alpha] = 0.05).
The LFOC of the topsoil (0-15 cm) in the thinned and underplanted stands was significantly higher than that of the unthinned stand, with no significant differences detected between the thinned and underplanted stands.
Moreover, the higher LFOC of the topsoil in the thinned and underplanted stands provided readily decomposable substrate for soil microorganisms and thus increased [CO.sub.2] release.
There were no differences in LFOC, POC, and WSOC among the CK, S0, and S1 treatments, while S2 and S3 increased WSOC, LFOC, and POC by 17.7-28.6%, 33.9-81.3%, and 35.3-82.4% compared with S0 respectively.
The LFOC:TOC ratio decreased by 16.8-18.8% for S0-S1, but not change for S2.
Analysis of MBC, WSOC, LFOC, POC, KMn[O.sub.4]-C, and TOC levels revealed that they were significantly correlated with each other (Table 2).