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Changes in land use may alter land cover, which results in carbon stock changes in biomass as well as in the soil. In China's loess plateau, vegetation restoration has been conducted since 1950s to control soil erosion and improve the ecosystem, with significant investment of money and manpower. Despite these efforts, soil erosion has still been severe. To reduce soil erosion and improve land quality, China initiated another state-funded project, Grain-for-Green, in 1999 in the loess plateau. However, it is not clear how effective this newly initiated project will be. In this study, we evaluated the effect of land-use conversion on soil organic carbon (SOC) and the potential effect of the current project on SOC sequestration in the Anjiapo catchment area of the loess hilly area of the loess plateau in China. This evaluation is based on SOC measurements in cropland versus in other converted land use types. We found that SOC sequestration mainly occurred in the surface soil after land use conversion took place. Land use conversion from cropland to shrubland or wild grassland (i.e. undisturbed land) was better for SOC sequestration than tree plantation in the semi-arid loess hilly area. By using the land use change in the study area as a scenario, the potential contribution of land use change on SOC sequestration due to the Grain-for-Green projectwas estimated. It was found that this project in the loess plateau of China would be helpful for SOC sequestration ifsuccessfully implemented.

The yield of spring wheat (Triticum aestivum L.), one of the major crops planted in the Loess Plateau, China, is mainly affected by available water. Straw mulch and irrigation are efficient ways of influencing wheat yield and water-use efficiency. To develop better semiarid crop and water management practices, a 13-year experiment in spring wheat monoculture was conducted at the Dingxi Soil and Water Conservation Institute of the Loess Plateau. The influence of rainfall during the growing season (March-July) on yields of rain-fed wheat was studied for 13 years (1982-1992 and 1997-1998). The influence of straw mulch and irrigation on wheat yield, and water-use efficiency, was studied for 2 years (1997)(1998). We found that growing season rainfall had a significant (P < 0.05) influence on biomass and grain yield of spring wheat in rain-fed conditions during the 13 years. Both biomass and grain yield were very low and varied significantly due to the low and significant variability of growing season rainfall. Straw mulch increased wheat yields significantly during both dry (1997) and wet (1998) years. It increased biomass and grain yield by 37 and 52%, respectively, in 1997, and by 20 and 26%, respectively, in 1998. Straw mulch also significantly decreased evapotranspiration (P < 0.05), soil water depletion (P < 0.01), and increased water-use efficiency (P < 0.001). Biomass and grain yield both increased (P < 0.01 orP < 0.001) with increasing irrigation in 1997 and 1998. The three irrigation levels increased the biomass yield from 34 to 66% in 1997, and from 34 to 77% in 1998. The irrigation levels also increased grain yield from 53 to 102% in 1997, and from 22 to 57% in 1998. Water-use efficiency for biomass and grain yield also increased with increasing irrigation. On the other hand, irrigation water-use efficiency for biomass and grain yield decreased with increasing irrigation. The results suggest that higher crop yields in the semiarid Loess Plateau may be achieved by using irrigation, or a proper combination of straw mulch and irrigation. #

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Abstract:Rainfall extremes (RE) become more variable and stochastic in the context of climate change, increasing uncertainties and risks of water erosion in the real world. Vegetation also plays a key role in soil erosion dynamics. Responses of water erosion to RE and vegetation, however, remain unclear. In this article, on the basis of the data measured on 15 plots (area: 10 m ð 10 m and 10 m ð 5 m) and the definition of World Meteorological Organization (WMO) on rainfall extremes, 158 natural rainfall events from 1986 to 2005 were analysed, and rain depth and maximal 30-min intensity (MI 30 ) were used to define RE. Then, water erosion process under RE and five vegetation types (spring wheat, alfalfa, sea buckthorn, Chinese pine, and wheatgrass) were studied in a key loess semiarid hilly area, NW China. The following findings were made: (1) The minimal thresholds of depth and MI 30 for defining RE were determined as 40Ð11 mm and 0Ð55 mm/min, respectively. Among the studied rainfall events, there were four events with both the variables exceeding the thresholds (REI), five events with depths exceeding 40Ð11 mm (REII), and four events with MI 30 exceeding 0Ð55 mm/min (REIII). Therefore, not only extreme rainstorm, but also events with lower intensities and long durations were considered as RE. Moreover, RE occurred mostly in July and August, with a probability of 46 and 31%, respectively. (2) Extreme events, especially REI, in general caused severer soil-water loss. Mean extreme runoff and erosion rates were 2Ð68 and 53Ð15 times of mean ordinary rates, respectively. The effect of each event on water erosion, however, becomes uncertain as a result of the variations of RE and vegetation. (3) The buffering capacities of vegetation on RE were generally in the order of sea buckthorn > wheatgrass > Chinese pine > alfalfa > spring wheat. In particular, sea buckthorn reduced runoff and erosion effectively after 3-4 years of plantation. Therefore, to fight against water erosion shrubs like sea buckthorn are strongly recommended as pioneer species in such areas. On the contrary, steep cultivation (spring wheat on slopes), however, should be avoided, because of its high sensitivities to RE.

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