Contents of soil organic carbon and nitrogen in water-stable aggregates in abandoned agricultural lands in an arid ecosystem of Northwest China

Soil organic matter content in water-stable aggregates（WSA） in the arid ecosystems（abandoned agricultural lands especially） of China is poorly understood. In this study, we examined the WSA sizes and stability, and soil organic carbon（OC） and nitrogen（N） contents in agricultural lands with abandonment ages of 0, 3, 12, 20, 30 and 40 years, respectively, in the Minqin Oasis of Northwest China. The total soil OC and N contents at depths of 0–20, 20–40 and 40–60 cm in abandoned agricultural lands were compared to those in cultivated land（the control）. Agricultural land abandonment significantly（P0.25 mm） as the age of agricultural land abandonment increased. The effect of abandonment ages of agricultural lands on MWD was determined by the changes of OC and N accumulation in WSA sizes ＆gt;2 mm. The total OC and N contents presented a stratification phenomenon across soil depths in this arid ecosystem. That is, both of them decreased significantly at depths of 0–20 and 40–60 cm while increased at the depth of 20–40 cm. The WSA sizes ＆lt;0.053 mm had the highest soil OC and N contents（accounting for 51.41%–55.59% and 42.61%–48.94% of their total, respectively）. Soil OC and N contents in microaggregates（sizes 0.053–0.25 mm） were the dominant factors that influenced the variations of total OC and N contents in abandoned agricultural lands. The results of this study suggested that agricultural land abandonment may result in the recovery of WSA stability and the shifting of soil organic matter from the silt＋clay（＆lt;0.053 mm） and microaggregate fractions to the macroaggregate fractions. However, agricultural land abandonment did not increase total soil OC and N contents in the short-term.

Effects of Long-Term Winter Planted Green Manure on Distribution and Storage of Organic Carbon and Nitrogen in Water-Stable Aggregates of Reddish Paddy Soil Under a Double-Rice Cropping System

In agricultural systems, maintenance of soil organic matter has long been recognized as a strategy to reduce soil degradation. Manure amendments and green manures are management practices that can increase some nutrient contents and improve soil aggregation. We investigated the effects of 28 yr of winter planted green manure on soil aggregate-size distribution and aggregateassociated carbon（C） and nitrogen（N）. The study was a randomized completed block design with three replicates. The treatments included rice-rice-fallow, rice-rice-rape, rice-rice-Chinese milk vetch and rice-rice-ryegrass. The experiment was established in 1982 on a silty light clayey paddy soil derived from Quaternary red clay（classified as Fe-Accumuli-Stagnic Anthrosols） with continuous early and late rice. In 2009, soil samples were collected（0-15 cm depth） from the field treatment plots and separated into water-stable aggregates of different sizes（i.e., 〉5, 2-5, 1-2, 0.5-1, 0.25-0.5 and 〈0.25 mm） by wet sieving. The long-term winter planted green manure significantly increased total C and N, and the formation of the 2-5-mm water-stable aggregate fraction. Compared with rice-rice-rape, rice-rice-Chinese milk vetch and rice-rice-ryegrass, the rice-rice-fallow significantly reduced 2-5-mm water-stable aggregates, with a significant redistribution of aggregates into micro-aggregates. Long-term winter planted green manure obviously improved C/N ratio and macro-aggregate-associated C and N. The highest contribution to soil fertility was from macro-aggregates of 2-5 mm in most cases.

Relations between the Underground Biomass and Soil Organic Carbon and Nitrogen of the Alpine Meadow at the Eastern Margin of the Qinghai-Tibet Plateau

This article, by combining field investigation with laboratorial analysis, studies diverse alpine meadow at the Eastern Margin of the Qinghai-Tibet Plateau for the underground biomass dynamics, vertical distribution of the content of soil carbon and nitrogen, the connection between the biomass and the content of carbon and nitrogen. The studies show that underground biomass in the herb layer of upland meadow is more than that in the terrace meadow, while underground biomass in the upland shrubland is the most. The vertical distribution of underground biomass of each type is obvious as in shape of＂T＂. As to the distribution of the content of soil organic carbon in the three sample grounds, it showed that the deeper the soil the less the content of soil organic carbon. In May, unlike at terrace meadow, the underground biomass and the content of soil organic carbon in positive proportion, such revelation at upland meadow and upland shrubland is not apparent. In July, at upland meadow and terrace meadow the underground biomass and the content of soil total nitrogen in positive proportion, such revelation at upland shrubland is not apparent either.

Effects of land use, climate, topography and soil properties on regional soil organic carbon and total nitrogen in the Upstream Watershed of Miyun Reservoir, North China

Soil organic carbon （SOC） and total nitrogen （TN） contents as well as their relationships with site characteristics are of profound importance in assessing current regional, continental and global soil C and N stocks and potentials for C sequestration and N conservation to offset anthropogenic emissions of greenhouse gases. This study investigated contents and distribution of SOC and TN under different land uses, and the quantitative relationships between SOC or TN and site characteristics in the Upstream Watershed of Miyun Reservoir, North China. Overall, both SOC and TN contents in natural secondary forests and grasslands were much higher than in plantations and croplands. Land use alone explained 37.2% and 38.4% of variations in SOC and TN contents, respectively. The optimal models for SOC and TN, achieved by multiple regression analysis combined with principal component analysis （PCA） to remove the multicollinearity among site variables, showed that elevation, slope, soil clay and water contents were the most significant factors controlling SOC and TN contents, jointly explaining 70.3% of SOC and 67.1% of TN contents variability. Only does additional 1.9% and 3% increase in the interpretations of SOC and TN contents variability respectively when land use was added to regressions, probably due to environment factors determine land use. Therefore, environmental variables were more important for SOC and TN variability than land use in the study area, and should be taken into consideration in properly evaluating effects of future land use changes on SOC and TN on a regional scale.