Chemical characterization of two organic matter fractions in irrigated lowland rice soils
1994
Olk, D.C. | Belleza, E.B. | Cassman, K.G. (International Rice Research Inst., Los Banos, Laguna (Philippines). Agronomy, Plant Physiology and Agroecology Div.)
Much of the N fertilizers applied to irrigated rice paddies cycles through soil organic matter (SOM). Chemical characterization of SOM may help predict soil N supply and N balance. Characterization is often performed on SOM extracted with NaOH. Soil Ca can bind to SOM, protecting it from complete microbial degradation -- thus facilitating its maturation into more recalcitrant forms. NaOH extraction of SOM with or without initial soil decalcification allows fractionation of younger, more labile SOM called mobile humic acids (MHA) from more recalcitant SOM bound to Ca (Ca-humates, Ca-HA). Both SOM fractions were extracted from a field experiment in which rice was double-cropped for 10 yrs without N inputs and N-source treatments with equivalent N inputs as prilled urea, azolla, Sesbania rostrata, or rice straw (116 kg N/ha and 58 kg N/ha in the dry and wet seasons, respectively). The MHA had 22 percent higher total N and 50 percent more acid-hydrolyzable amino acids than the Ca-HA. The MHA may have a smaller molecular weight and be less aromatic and condensed than the Ca-HA. Although the quantity of total organic carbon and HA was greater in soil with organic N-sources. Chemical characteristics varied only slightly by N2 source. These results indicate that using organic N sources can increase SOM content without changing the chemical composition of young and older HA fractions in soils cropped under submerged conditions
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