| Long-term field experiments play an important role in studying agro-ecological process of croplands. Samples from the experiments with historical information are helpful to researchers to assess the environmental change. In this dissertation observations from long-term fertilization and tillage experiments were used to evaluate soil organic carbon (SOC) and crop productivity dynamics of China’s croplands.Data were collected from publications reporting SOC measurements under long-term agro-ecosystem experiments with different fertilization and tillage treatments that were available in the Chinese Journals Database (CNKI) and the Chinese Journals of Science and Technology Database (VIP Info) from 1979 to 2008.For assessing the SOC dynamics under different fertilization schemes in the long run, the publications were screened using the following criteria:(1) The experiments was designed with well sustained fertilization schemes with a control treatment (no fertilizer application) and a minimum plot area of a single treatment no smaller than 10m2; (2) The experiments were conducted for no shorter than 6 years, with clear initial and final monitoring years;(3) SOC data available for topsoil with clear initial and final SOC values, the sampling and SOC measurements with wet digestion was consistent throughout the monitoring duration. And relative annual change (RAC) was defined to assess SOC dynamics. This analysis was done for 6 different categories of long-term fertilizations:N (inorganic nitrogen fertilizer only), NP (compound inorganic nitrogen and phosphorus fertilizers), NPK (compound inorganic Nitrogen, Phosphorus and Potassium fertilizers), O (organic fertilizers only), OF (the combined of inorganic fertilizers with organic amendments) and Others (other unbalanced fertilizations: N, P, K, PK, NK). The soils represented here covered 16 dominant types of cropland soils from China. The data set of fertilization treatments comprised of 481 observations (135 and 346 from rice paddies and dry croplands, respectively) of SOC under different fertilization at 70 experimental sites (28 for rice paddies and 42 for dry croplands) across 23 provinces of mainland China. The relative change in SOC content was analyzed with respect to the initial SOC content, and rice paddies and dry croplands soils were compared.An overall increase in SOC was projected for both types of croplands, with a relative annual change (RAC, g/kg/yr) of-0.14～0.60 (0.13 on average) for dry cropland soils, and-0.12～0.70 (0.19 on average) for rice paddy soils. RAC values decreased as experiment duration increased, with experiment durations shorter than 10 years leading to larger estimates of RAC values than those with longer durations. While enhanced SOC accumulation was much higher under fertilizer treatments with organic amendments (those of O and OF), compound inorganic fertilization also resulted in a significantly higher RAC (double that under N fertilization). There was significant correlation of RAC values with annual nutrient application rates for rice paddies under 5 defined categories of fertilizations though this correlation was not applicable for dry croplands. The results presented here indicate an overall trend of SOC accumulation in China’s croplands under long-term fertilization experiments, which can be enhanced greatly under compound fertilization of inorganic fertilizers, and combined fertilization of inorganic fertilizers with organic amendments. China’s croplands may have significant potential for C accumulation when cereal production is supported with well managed fertilization schemes.The 156 observations with both SOC content and crop yield were selected from the whole dataset of 481 observations mentioned above to analysis the relationship of mean RAC values and relative yield increment. An overall increase in SOC was projected for both types of croplands. Results showed a positive relationship of crop yield and SOC increment both dry cropland crops (wheat and corn) and rice paddy crop (rice). While relative yield increment of 173.5%,204.4% and 45.6% contained for wheat, corn and rice, SOC increment values of 1.00,1.25 and 2.17 g/kg in croplands matched them respectively.In this dissertation, data of topsoil SOC in long-term tillage experiments of China from publications available from 1979 to 2008 was collected to summarize and analyze the dynamics of SOC. The data set comprised of 88 observations (51 and 37 from dry croplands and rice paddies, respectively) of SOC under different conservation tillage measures at 59 experiment sites (31 for dry croplands and 28 for rice paddies) across 18 provinces of mainland China. The soils represented by the experiments covered 12 dominant types of cropland soils from China. The relative increase in SOC content was analyzed with respect to initial SOC stock, and rice paddies and dry croplands soils were compared. This analysis was done for 2 different categories of long-term conservation tillage: no-tillage (reduced tillage) and straw mulching.An overall increase in SOC was projected for both types of croplands, with a relative annual change (RAC, g/kg/yr) of-0.30～0.75 (0.21 on average) for dry cropland soils, and-0.20～2.71 (0.51 on average) for rice paddy soils. Initial and final SOC values in rice paddies were higher significantly than that in dry croplands. While enhanced SOC accumulation was much higher under straw mulching, there was no significant difference between no-tillage and reduced tillage.Conservation tillage is a key technique of sustainable agriculture, and carrying out research of application and extension of conservation tillage in China is of profound significance. It is necessary to strengthen the study for improving crop productivity, enhancing SOC accumulation and change the traditional idea of farmers so as to accelerate the application of conservation tillage in China. |
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