Effect of different ratios of chicken manure N on organic matter accumulation and acidification of paddy soils

【Objectives】Soil acidification has been a major problem in the intensive agricultural systems. Irrational application of chemical and fertilizer was a major contributor to soil acidification in farmland. In this paper，the reasonable ratios of chicken manure N in the total N supply of rice production were studied through the accumulation of different labile fractions of soil organic matter and the variations of base cation contents. 【Methods】 A field experiment was successively conducted from 2003-2007, six treatments were designed: 1) No fertilizer (CK）; 2）100% organic N (M100）; 3）75% organic N（M75）; 4）50% organic N（M50）; 5）25% organic N（M25）; 6）100% chemical N（M0）. The soil samples in the fifth year were collected to determine the soil exchange cations, organic matter accumulation and soil buffering capacities. 【Results】 The rice yields are highest when 25%-50% of the total 270 kg/ha nitrogen are from organic fertilizer, with the average yield 5.1% higher than that from 100% chemical N, and 44.9% higher than that with CK, there are no significant differences between treatments with 25% and 50% organic N. The soil organic matter contents and soil carbon management index are increased with the increase of the ratio of organic fertilizer. Only the total organic matter content has positive correlation with the chick manure application ratio, and the mid-labile organic matter have positive correlation with the rice yield（P=0.0067**）. While high-labile organic matter, labile organic matter and total organic matter has no significant correlation with the rice yields (P=0.192，P=0.208，P=0.160, respectively). The increase of chicken manure application rate improves soil exchangeable base cation contents (Ca2+, Mg2+, K+ and Na+), resulting in cation exchange capacity and pH rise. The soil buffering capacity is from 2.07 to 2.36 cmol/kg and increased with the ratio of chicken manure, and there is a significant positive correlation between the cation exchange capacity and organic matter content. These results indicate that soil pH and soil buffering capacity are increased under the higher chicken manure application, and soil cations and organic matter accumulation are increased under the chicken manure application. 【Conclusions】 Continuous chicken manure application can increase soil organic matter content and labile organic matter content, significantly increase the cations exchange capacity, and a high soil buffering capacity as a concequence. The optimal substitute ratio of chicken manure N to chemical fertilizer is 25% to 50% for obtaining the satisfactory soil properties and high yield.