Effect of long-term application of organic residues on the distribution of organic matter and nitrogen in some rice soil profiles

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Abstract
Long-term field experiments were established in 1963 on the slowly permeable Gray Lowland soil at Kyushu National Agricultural Experiment Station Farm to monitor the effect of organic matter application on the soil fertility status for rice cultivation. In the spring of 1979, soil horizon samples were taken in the profile to a maximupi depth of 74 cm. Application of organic matter in the form of compost, or residues of rice straw and Italian ryegrass had a pronounced effect in an increase of organic C, and total N content in the soil profiles. However, most of the increases occurred in the two upper horizons (Apg and A12g). Organic C content was significantly correlated with total N content (r=0.993***), and with organic N content (r=0.993***). Based on the organic C content throughout the soil profiles, it is estimated that 13 to 25% of the organic matter returned to the soil was incorporated into soil organic matter during the study period. Strikingly, little changes of organic C and total N content in the surface horizon were observed in the control plot relative to that present at the start of the experiment. In general, a small portion of the total N remained in the extractable inorganic N form; however, fixed NH«+-N was the dominant form of inorganic N accounting for 4 to 41% of the total N. Narrowing of organic C/total N ratio with depth was due to the increase with depth in the relative amounts of soil N as fixed NH4+-N, regardless of whether organic matter was applied. Organic matter treatments gave higher rice yields, though not to a great extent, due to the higher availability of the N pool in the surface horizons, as reflected by the increase in mineralizable N content. The content of mineralizable N was significantly correlated with the total N content (r=0.975***), and organic N content (r=0.973***). The positive correlation of organic N content with the content of mineralizable N clearly indicated the presence of undecomposable organic N by microbial activities which amounted to approximately 410 ppm. Some practical aspects of the findings are discussed.