Changes in humus carbon fractions in paddy soil given different organic amendments and mineral fertilizers

The effects of 4 years of combined application of mineral fertilizer with a variety of organic materials on soil humus carbon fractions were evaluated in a low-fertility paddy soil. Five treatments were selected: (1) no fertilizer, (2) mineral fertilizer alone, (3) mineral fertilizer plus green manure at 3.6 Mg ha(-1), (4) mineral fertilizer plus cattle manure at 4.7 Mg ha(-1), and (5) mineral fertilizer plus rice straw at 3.0 Mg ha(-1). The following properties were determined: water soluble substance carbon, water floating substance carbon, humic acid carbon, fulvic acid carbon, total alkali-extractable humic carbon, and humin carbon. The ratio of humic acid carbon /fulvic acid carbon and precipitation ratio (the percentage of humic acid in the sum of humic acid and fulvic acid) were calculated. The results indicated that application of mineral fertilizer with organic amendments respectively increased the concentrations of humic acid carbon and total alkali-extractable humic carbon by 18.3-31% and 11.7-21.3% in 0-5 cm, humin carbon by 14.9-22.5% in 5-10 cm, compared to mineral fertilizer alone. However, no differences in fulvic acid carbon concentration were recorded among the fertilization treatments in 0-5 cm. Furthermore, organic inputs exhibited minor influences on humus carbon fractions (-4.2-22% and 2.8-8% for humic acid carbon;-0.6-13% and L6-4.4% for total alkali-extractable humic carbon; 0.3-9.5% and-0.4-4.4% for humin carbon;-0.7-5.3% and 0.4-1.3% for fulvic acid carbon) in 10-20 cm and 20-30 cm, respectively. Among the three organic amendments, surface placement of cattle manure resulted in the highest concentrations of water soluble substance carbon (0.26 g kg(-1), 0.22 g kg(-1), and 0.21 g kg(-1)), humic acid carbon (4.6 g kg(-1), 3.2 g kg(-1), and 2.1 g kg-(1)), total alkali-extractable humic carbon (7.1 g kg-1, 5.6 g kg-1, and 4.2 g kg-1), and humin carbon (11.4 g kg(-1), 11 g kg(-1), and 7.8 g kg(-1)) in upper soil layer (0-5 cm, 5-10 cm, and 10-20 cm). In addition, the precipitation ratio value tended to be significantly higher (23.2% in 0-5 cm, 24.6% in 5-10 cm, and 16.5% in 10-20 cm, P < 0.05) in mineral fertilizer with cattle manure additions than mineral fertilizer alone. There were significant correlations between rice grain yield and total soil organic carbon (R-2 = 0.72, n = 15, P < 0.01), humic acid carbon (R-2 = 0.79, n = 15, P < 0.001), and humin carbon (R-2 = 0.59, n = 15, P < 0.05) in the 0-5 cm soil depth. In summary, our results showed that combined application of mineral fertilizer with cattle manure was the most effective management practice to increase humus carbon pool, which could be regarded as a useful indicator of the carbon sequestration process in paddy soil.