Soil Respiration And Organic Carbon Of Agro-ecosystems In Loess Plateau

Loess Plateau locates in the ecotone between the pastoral areas in Mongolian Plateau and Qinhai- Tibetan Plateau and cropping areas in Hexi corridor and Northern China Plain, hereby is one of areas with the most abundant agriculture systems in the world and has the longest cropping history in China. In this thesis, the soil respiration and soil organic carbon of cropping systems (annual crops- cash crops and grain crops), sown grassland system (perennial crops- alfalfa), rangeland systems, silvo-pastoral systems (elm-rangeland system, poplar-rangeland system) and agroforestry systems (poplar-alfalfa system) were researched in northern part of Loess Plateau in 2006. Vegetation biomass, soil nitrogen and phosphorus were also involved in this study to analyze the soil carbon balance.This research gained the main results as follows:1. Soil respiration of agriculture systemsSoil respiration rate of every agro-ecosystem was the highest in August and the lowest in late May. In late May and August, elm silvo-pastoral system had the highest soil respiration rate among the systems, which were about 1.54μmolCO₂·m^-2·s^-1 and 2.05μmolCO₂·m^-2·s^-1 respectively, and cropland system had the lowest one. In December, sown grassland system's soil respiration is about 0.48μmolCO₂·m^-2·s^-1 and which was the highest among the systems. And rangeland system is lowest in this period. Environmental factors has different influences on different systems, but the environmental temperature has a positive correlation effect on systems, and air moisture and soil water content of 10cm below ground has a negative correlation with most of systems.In Aug, soil respiration diurnal changes of sown grassland system, cropland system and agroforestry system showed a single peak curve, but elm and poplar silvo-pastoral system showed a double peak curve. The highest soil respiration rat of sown grassland system and agroforestry system was observed at 16:00. And the minimum value was observed between 1:00 and 6:00. The minimum value of cropland and rangeland system was observed at 1:00, but the peak value of cropland system is 2.07μmolCO₂·m^-2·s^-1 at 14:00 and rangeland system is 1.84μmolCO₂·m^-2·s^-1 at 12:00. The two peak value of diurnal changes soil respiration of elm silvo-pastoral system were observed at 14:00 and 1:00, and the peak values were 2.54μmolCO₂·m^-2·s^-1 and 2.51μmolCO₂·m^-2·s^-1. The two peak value of diurnal changes soil respiration of poplar silvo-pastoral system were observed at 14:00 and 1:00, and the peak values were 2.58μmolCO₂·m^-2·s^-1 and 2.66μmolCO₂·m^-2·s^-1. The series of gross soil emission CO₂ of systems were ranged: elm silvo-pastoral system > poplar silvo-pastoral system > sown grassland system > agroforestry system > cropland system > rangeland system, and the value were as follow: 23.55 gC·m^-2·d^-1,23.05gC·m^-2·d^-1,18.23gC·m^-2·d^-1,15.24gC·m^-2·d^-1,13.31 gC·m^-2·d^-1 and 10.33 gC·m^-2·d^-1. 2. Organic carbon of agriculture systemsThere was a digressive trend of the soil organic carbon vertical distribution in the soil. The soil organic density of the sown grassland system, cropland system, agroforestry system, poplar silvo-pastoral system, elm silvo-pastoral system and rangeland system was 3.91 kg·m^-2,4.20 kg·m^-2,3.77 kg·m^-2,3.40 kg·m^-2, 5.19 kg·m^-2 and 3.93 kg·m^-2, respectively. The series of vegetation carbon density of aboveground were ranged: poplar silvo-pastoral system > elm silvo-pastoral system > agroforestry system > sown grassland system > cropland system > rangeland system. The series of vegetation carbon density of belowground were ranged: sown grassland system > agroforestry system > poplar silvo-pastoral system > rangeland system > elm silvo-pastoral system > cropland system.3. Soil nitrogen of of agriculture systemsAmong the system, elm silvo-pastoral system has a highest soil total nitrogen and available nitrogen of 0.57 kg·m^-2 and 0.031 kg·m^-2. The series of soil total nitrogen density were ranged: elm silvo-pastoral system > sown grassland system > rangeland system > agroforestry system > poplar silvo-pastoral system> cropland system. The series of soil available nitrogen density were ranged: elm silvo-pastoral system > sown grassland system> rangeland system > agroforestry system > poplar silvo-pastoral system and cropland system.4. Soil phosphorus of agriculture systemsAmong the system, agroforestry system has a highest soil total phosphorus of 0.52 kg·m^-2, and poplar silvo-pastoral system was lowest. The series of soil total phosphorus density were ranged: agroforestry system > cropland system > elm silvo-pastoral system > rangeland system > sown grassland system > poplar silvo-pastoral system. The series of soil available phosphorus density were ranged: cropland system > agroforestry system and rangeland system > poplar silvo-pastoral system > elm silvo-pastoral system5. Water content of agriculture systemsCropland system has higher soil water content in May and August , and rangeland system was lowest. In December, agroforestry system's soil water content was highest and poplar silvo-pastoral system was lowest. From May to December, the series of system water consumption were ranged: poplar silvo-pastoral system > elm silvo-pastoral system> sown grassland system > rangeland system > cropland system > agroforestry system.