Carbon And Nitrogen Mineralization And Alkalinity Release Following Application Of Plant Materials To Acid Soils Differing In Initial PH

Soil acidification is a representative of soil degradation. Nowadays, croplands across China are reported undergoing soil acidification. When soil is getting acidified, it would have great impacts on soil microbes and crop productivity and then the sustainable development of agriculture. Mechanisms for soil acidification and its prevention and amelioration have long been focused in soil science, agronomy and environmental science, and a great progress has been made. Return of crop residues to field is an important measure to maintain soil fertility and improve crop productivity, and is highly-recommended world-wide. In order to improve the understanding of interaction between soil acidification and nutrient cycling (especially C and N), two paddy soils, Paleudalf and Plinthudult, were collected in this study and then pH gradients were produced through application of direct current (DC). Incubation experiments were conducted to investigate the carbon and nitrogen mineralization and alkalinity release following addition of plant materials to soils differing in initial pH. Main results are as follows:1. Influences of application of direct current (DC) on soil basic properties. Paleudalf and Plinthudult soil were tested in this study. Soil pH rapidly changed following application of DC, i.e. in the electrokenetic reactor, the pH of soil in the cathodic region significantly increased while substantially decreased in the anodic region. Correspondingly, soil exchangeable Ca2+and Mg2+accumulated in soils of the cathodic region, while large amount of soil exchangeable Al3+occurred in soils of the anodic region. Soil exchangeable Na+appeared to be less affected by DC treatment. In addition, great changes in soil mineral N were observed after DC treatment, while the application of DC has no significant impacts on soil total carbon, total nitrogen and particular size distribution.2. Differences in carbon mineralization after addition of plant materials to soils differing in initial pH. The results showed that addition of plant materials significantly (p<0.05) stimulated CO2emission irrespective of the initial pH of the soil. There were marked differences (p<0.05) in soil respiration rate between treatments receiving various plant materials. For all treatments, soil respiration rate peaked between1-3d after residue addition, then gradually slowed down with incubation time, and maintained stable at the later stage. At the end, cumulated CO2significantly differed between the residue amended soils (p<0.05), following the order of vetch residue> canola residue> rice straw. In general, cumulated CO2was negatively correlated with the C:N ratio of added plant materials. The initial pH of the soil has a significant influence on residue decomposition (p<0.05), the decomposition of plant materials usually accelerated with increasing soil pH.3. Nitrogen mineralization and pH change following application of plant materials to soils differing initial pH. Soil pH change after addition of plant materials was highly dependent on residue type and its decomposition. Vetch (leguminous residue) has a greater liming effect than canola residue and rice straw. The initial pH of the soil determined the subsequent pH change following addition of plant materials. This could largely be attributed to the contrasting effect of pH on nitrogen transformations. Nitrification was strongly depressed under low pH conditions, while ammonificaiton appeared not to be. Application of plant materials with high alkalinity and N content to strongly acidic soils could induce substantial liming effect and persisted for a long time. While application of plant materials of wide C:N ratio to weakly acidic soils might be more helpful in retarding further soil acidification.4. N mineralization and pH change following application of various rate of vetch to soils differing in initial pH. The liming effect of vetch residue on acid soils enhanced with increasing rate. Incorporation of vetch could significantly (p<0.05) improve soil mineral N status, but N immobilization possibly occurred at the early stage. Most of the N which mineralized from vetch residue existed in low pH soils in the form of NH4+-N, while accumulated in the form of NO3--N in soils of higher initial pH. Differences in effect of initial soil pH on N mineralization were observed between the Paleudalf soil and the Plinthudult soil. For instance, the N mineralization rates of vetch incorporating into the Plinthudult soil were greater at higher initial pH. In contrast, the N mineralization rates of vetch incorporating into the Paleudalf soil were greater at lower initial pH.5. Effects of application of plant materials on soil exchangeable cations. Application of plant materials increased soil exchangeable Ca, Mg, K and Na and effectively decreased exchangeable Al. Vetch was the most effective residue in decreasing Al, followed by canola, and then rice straw. However, this action was time-depending. This study demonstrated that soil exchangeable Al rapidly decreased during the initial two weeks after addition of plant materials, thereafter, exchangeable Al increased slowly but still lower than the control until the end of the incubation. Complexation of Al with organic matter and pH increase-induced Al precipitation might be the two major mechanisms involved, but respectively pronounced in different acid soils.The results obtained in this study also suggested that it could be feasible to regulate N transformation to minimize N loss in croplands through pH manipulation.