Submitted to: European Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 20, 2006
Publication Date: February 2, 2007
Citation: Olk, D.C., Samson, M.I., Gapas, P. 2007. Inhibition of nitrogen mineralization in young humic fractions by anaerobic decomposition of rice crop residues. European Journal of Soil Science. 58:270-281.
Interpretive Summary: Soil contains large amounts of bound nitrogen (N), which is released slowly into forms that are available for crop uptake. In tropical Asia, grain yield and crop uptake of the released N have decreased long-term in some fields that are planted two or three times each year with flooded rice. We tested the idea that the decrease in N availability might result from the conventional practice of maintaining flooded soil conditions when leftover straw and roots of harvested plants are plowed into the soil, causing undesirable changes in soil chemical properties. We added specially labeled N fertilizer to a rice field experiment that compared flooded versus aerated decomposition of straw and roots. As expected, we found that some labeled fertilizer was bound soon after addition to the soil. We found that the release of the bound labeled N into available forms during the rice-growing season was less when the straw and roots had decomposed in flooded soil than when they had decomposed in aerated soil. Rice crop uptake of unlabeled N, representing most bound N in the soil, was also substantially less when straw and roots had decomposed in flooded soil than in aerated soil. Our results demonstrate that decomposition of straw and roots in aerated soil provides for more timely release of bound N into available forms than does decomposition in flooded soil. These results will assist rice farmers in maintaining their grain yield levels and decreasing the need for N fertilizer. They will also assist rice scientists who are striving to understand the chemical processes that control availability of soil N.
Field observations indicate a decrease in crop uptake of N derived from soil organic matter under continuous production of irrigated lowland rice. Decreased availability has been associated with an accumulation in soil organic matter of phenolic lignin residues that can chemically bind N. To evaluate the hypothesis that the decreased availability results primarily from anaerobic decomposition of incorporated crop residues, 15N-labeled fertilizer was applied three times during one growing season in a field study that compared anaerobic decomposition with aerobic decomposition for annual rotations of rice-rice and rice-maize. Contents of 15N and total N during the growing season were measured for humic fractions and total soil organic matter. Results indicated an inhibition of N mineralization for the rice-rice rotation with anaerobic decomposition of crop residues, both for 15N that was immobilized after application and for total N. The inhibition was strongest for 15N that was applied at planting, and it became more evident as the season progressed, reaching significant levels during mid-season stages of plant growth when crop demand for N peaks. Trends were most significant for a young, phenolic-rich humic fraction, which was also active in 15N immobilization and remineralization. Less significant trends were evident for a more recalcitrant humic fraction and soil organic matter. Amounts of crop N uptake with the rice-rice rotation and anaerobic decomposition suggested inhibited uptake of soil organic N but uninhibited uptake of fertilizer N. Increased aeration of rice soils through aerobic decomposition of crop residues or crop rotation is a promising management technique for improving soil N supply in lowland rice cropping.