Infrared spectra of humic acid in 2012 (2012-HA) in different fertilization treatments of Black Soil.

<p>CK, no-fertilizer control treatment; NPK, inorganic fertilizers of nitrogen (N), phosphate (P), and potassium (K) fertilizer treatment; M, manure treatment; MNPK, treatment of manure combined with inorganic fertilizers of N, P, and K fertilizers.</p

Relative intensities of the main absorption peak of humic acid infrared spectra in different fertilization treatments of Black Soil (semi-quantity, cm⁻¹).

<p>Relative intensities of the main absorption peak of humic acid infrared spectra in different fertilization treatments of Black Soil (semi-quantity, cm⁻¹).</p

C distributions of different types of humic acid in solid-state polarization magic-spin ¹³C nuclear magnetic resonance spectroscopic analysis in Black Soil under different fertilization treatments.

<p>C distributions of different types of humic acid in solid-state polarization magic-spin ¹³C nuclear magnetic resonance spectroscopic analysis in Black Soil under different fertilization treatments.</p

Application amount of different fertilizers at wheat-soybean-maize rotation season.

<p>Application amount of different fertilizers at wheat-soybean-maize rotation season.</p

Three-dimensional excitation-emission matrices of humic acid under soil removal in four treatments: CK, no-fertilizer control treatment; NPK, inorganic nitrogen (N), phosphate (P), and potassium (K) treatment; M, manure fertilizer treatment; MNPK, treatment of manure combined with inorganic of N, P, and K fertilizers.

<p>Ex, excitation; Em, emissions. Peak A is the ultraviolet region type fulvic acid fluorescence peak; peak C is in the class of humic acid fluorescence peaks.</p

The results of solid-state polarization magic-spin ¹³C nuclear magnetic resonance spectroscopic analysis of humic acid in 2012 (2012-HA) in different fertilization treatments of Black Soil.

<p>CK, no-fertilizer control treatment; NPK, inorganic fertilizers of nitrogen (N), phosphate (P), and potassium (K) fertilizer treatment; M, manure treatment; MNPK, treatment of manure combined with inorganic fertilizers of N, P, and K fertilizers.</p

Differences in soil-derived organic carbon between the treatments with and without maize straw.

<p>Data expressed as the means ± SD (n = 3). Δ<i>C</i>_<i>S</i>, difference of soil-derived organic carbon between the treatment with and without maize straw. L-LV, low fertility Luvisol; H-LV, high fertility Luvisol; L-PH, low fertility Phaeozem; and H-PH, high fertility Phaeozem.</p

Locations of study sites.

<p>Locations of study sites.</p

Dynamic patterns of total nitrogen (TN) and C/N ratio in the treatments with maize straw.

<p>Data expressed as the means ± SD (n = 3). L-LV+m, low fertility Luvisol plus maize straw; H-LV+m, high fertility Luvisol plus maize straw; L-PH+m, low fertility Phaeozem plus maize straw; and H-PH+m, high fertility Phaeozem plus maize straw.</p

The ANOVA results of statistical items in this study.

<p><i>TOC</i> and <i>δ</i>^<i>13</i><i>C</i> used 96 samples in total; <i>f</i>_<i>M</i>, <i>f</i>_<i>S</i>, <i>r</i>_<i>SM</i>, <i>r</i>_<i>M</i>, <i>r</i>_<i>S</i>, </p><p></p><p></p><p></p><p><mi>r</mi></p><p></p><p><mi>S</mi><mo>′</mo></p><p></p><p></p><p></p><p></p><p></p> and Δ<i>C</i>_<i>S</i> used 48 samples in total. T, time; S, soil type; F, Fertility level; M, maize amendment. <i>TOC</i>, total organic carbon; <i>f</i>_<i>M</i>, the proportion of maize-derived carbon; <i>f</i>_<i>S</i>, the proportion of soil-derived organic carbon; <i>r</i>_<i>SM</i>, remaining rate of total organic carbon; <i>r</i>_<i>M</i>, remaining rate of maize-derived carbon in the treatment with maize straw; <i>r</i>_<i>S</i>, remaining rate of soil-derived organic carbon in the treatment with maize straw; <p></p><p></p><p></p><p><mi>r</mi></p><p></p><p><mi>S</mi><mo>′</mo></p><p></p><p></p><p></p><p></p><p></p>, remaining rate of soil-derived organic carbon in the treatment without maize straw; Δ<i>C</i>_<i>S</i>, difference in soil-derived organic carbon between the treatment with and without maize straw. *, **, ***, NS and-, indicate significant differences at <i>P<0</i>.<i>05</i>, <i>P<0</i>.<i>01</i>, <i>P<0</i>.<i>001</i>, no significance and no ANOVA analysis, respectively. <i>df</i> indicates degrees of freedom.<p></p><p>The ANOVA results of statistical items in this study.</p