Changes in Soil Aggregate, Carbon, and Nitrogen Storages Following the Conversion of Cropland to Alfalfa Forage Land in the Marginal Oasis of Northwest China

Abstract

Maintenance of soil organic carbon (SOC) is important for sustainable use of soil resources due to the multiple effects of SOC on soil nutrient status and soil structural stability. The objective of this study was to identify the changes in soil aggregate distribution and stability, SOC, and nitrogen (N) concentrations after cropland was converted to perennial alfalfa (Medicago sativa L. Algonguin) grassland for 6 years in the marginal oasis of the middle of Hexi Corridor region, northwest China. Significant changes in the size distribution of dry-sieving aggregates and water-stable aggregates, SOC, and N concentrations occurred after the conversion from crop to alfalfa. SOC and N stocks increased by 20.2% and 18.5%, respectively, and the estimated C and N sequestration rates were 0.4 Mg C ha(-1) year(-1) and 0.04 Mg N ha(-1) year(-1) following the conversion. The large aggregate (>5 mm) was the most abundant dry aggregate size fraction in both crop and alfalfa soils, and significant difference in the distribution of dry aggregates between the two land use types occurred only in the >5 mm aggregate fraction. The percentage of water-stable macroaggregates (>2, 2-0.25 mm) and aggregate stability (mean weight diameter of water-stable aggregates, WMWD) were significantly higher in alfalfa soils than in crop soils. There was a significant linear relationship between total SOC concentration and aggregate parameters (mean weight diameter) for alfalfa soils, indicating that aggregate stability was closely associated with increased SOC concentration following the conversion of crops to alfalfa. The SOC and N concentrations and the C/N ratio were greatest in the >2 mm water-stable aggregates and the smallest in the 0.25-0.05 mm aggregates in crop and alfalfa soils. For the same aggregate, SOC and N concentrations in aggregate fractions increased with increasing total SOC and N concentrations. The result showed that the conversion of annual crops to alfalfa in the marginal land with coarse-texture soils can significantly increase SOC and N stocks, and improve soil structure.

Response of soil properties and C dynamics to land-use change in the west of Loess Plateau

“…The lower pH value at the surface soil in cropland relative to abandoned fields was probably related to applications of fertilizer and organic manure, particularly ammonium bicarbonate and calcium superphosphate (Su et al 2009). Nevertheless, EC decreased significantly at the 20-100 cm depth after cultivation and afforestation.…”

“…However, Wu et al (2003) also found a significant increase in SOC density for the irrigated silting soil in an arid climate zone in northwest China. On the other hand, significant accumulation of SOC was found after conversion of cropland to grassland in Europe (Poeplau and Don 2013) and the Loess Plateau (Fang et al 2012) and an oasis in the middle reach of the Heihe River (Su et al 2009) in China, and mainly occurred at the 0-10 cm depth. Cropland abandonment and subsequent cessation of agricultural activities occurs globally in various climatic zones.…”

Relaciones entre los hongos filamentosos y solubilizadores de fosfatos con algunas variables edáficas y el manejo de cafetales

“…El porcentaje de Agregados Estables en Agua (AEA) fue definido en cinco categorías, donde el subíndi-ce de cada AEA corresponde al diámetro promedio entre las dos aperturas de malla: AEA 3.4 (4.8-2mm), AEA 1.4 (2-0.84mm), AEA 0.7 (0.84-0.5mm), AEA 0.4 (0.5-0.25mm), AEA 0.1 (0.25-0mm). Los AEA se calcularon como la relación entre los pesos obtenidos previamente: AEA (x) = (PS 2 -PS 3 / PS 1 -PS 3 ) * 100 (Márquez et al 2004, Daraghmeh et al 2009, Su et al 2009). En el presente trabajo sólo se analizó el efecto de las fracciones superiores a 0.5mm (AEA 3.4 , AEA 1.4 y AEA 0.7 ) debido a que en ellas se desarrolla la mayor actividad fúngica y donde hay mayor disponibilidad de oxígeno (Ashman et al 2003, Six et al 2004, Six et al 2006.…”

Multi-scale variability of soil carbon and nitrogen in the middle reaches of the Heihe River basin, northwestern China

“…Populus simonii Carr and Elaeagnus angustifolia Linn are the main afforestation tree species. The zonal soils are gray brown desert soil, aeolian soil and irrigated desert soil according to Chinese Soil Taxonomy, which are equivalent to the Aridosols, Entisols and Inceptisols in terms of USDA Soil Taxonomy, respectively (Li et al, 2009;Su et al, 2009). …”

“…The SOC and TN contents in the topsoil of sand-fixing shrubs, irrigated cropland and shelter forest increased with time after the reclamation of desert land (Su et al, 2007(Su et al, , 2010a(Su et al, , 2010b. Su et al (2009) reported that SOC and TN accumulated at rates of 0.4 Mg C ha −1 year −1…”