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Characterization of Humic Acid Fractions Improves Estimates of Nitrogen Mineralization Kinetics for Lowland Rice Soils

^a B.V. Nguyen, Dep. of Crop Sciences, Can Tho Univ., Can Tho, Vietnam
^b USDA-ARS, National Soil Tilth Laboratory, 2150 Pammel Dr., Ames, IA 50011
^c Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE 68583-0915

View larger version (19K): [in a new window]   Fig. 1. The relationships between properties of humic acids (HA) extracted from predried soil versus fresh soil for the 0- to 15-cm depth of two fertilizer treatments at three sites of the Long-Term Fertility Experiment: (a) amount of N extracted as HA (HA-N), and (b) C/N ratio of HA (HA-C/N). The mobile humic acid fraction is the MHA, and the calcium humate fraction is the CaHA. Each data point represents one field replicate of a fertilizer treatment at each site.

View larger version (18K): [in a new window]   Fig. 2. The relationship between the amount of N extracted as the mobile humic acid fraction (MHA) or calcium humates (CaHA) versus (a) soil organic C and (b) total soil N for the 0- to 15-cm depth of 14 tropical lowland rice soils. The mean coefficient of variation across the 14 soils in the amount of extracted N contained in the MHA and CaHA fractions was 4% for the three field replicates of each soil. For soil organic C and for total soil N the mean coefficient of variation was 1% for three laboratory replicates of a composite soil.

View larger version (24K): [in a new window]   Fig. 3. Net cumulative NH4–N accumulation during anaerobic incubation of (a) soils having N1 < 75 mg NH4–N kg–1 soil, (b) soil having 75 < N1 < 150 mg NH4–N kg–1 soil, and (c) soil having N1 > 150 mg NH4–N kg–1 soil, where N1 is the total size of the rapidly mineralizable N pool. Curves shown are regression lines based on the two-pool first- and zero-order model using parameters given in Table 5. Soil was sampled from the 0- to 15-cm depth.

View larger version (17K): [in a new window]   Fig. 4. The relationship between N1 (total size of the rapidly mineralizable N pool) and soil organic C for the 0- to 15-cm depth of 12 tropical lowland rice soils. Standard errors for the regression equation are 75 (intercept), 7.4 (x term), and 0.169 (x2 term). Each data point represents the mean of three replicates.