Soil water stable aggregates and carbon and nitrogen storage enhanced by conversion of farmland to shrub and grass in China Loess Plateau: the influence of conversion cultivation time

【Objectives】Conversion of farmland to forests and grassland is the most important factor to aid in the prevention and control of soil erosion as well as to restore soil fertility in western China. Sites subjected to farmland conversion in Shenmu and Wuqi counties (in northern Shaanxi province on the Loess Plateau) were selected to study the effect that conversion to grasses (Alfalfa) and shrubs (Seabuckthorn) had on soil over differing time scales. In this study, we focused specifically on the effects that forest conversion had on the storage of surface soil organic carbon, both total and available nitrogen as well as soil water stable aggregates. This research was part of the Grain for Green Program, with the objective to reveal the mechanisms in forest conversion that affect soil quality of sloped land. The obtained results are expected to provide a scientific basis for improving soil fertility of sloped lands in China.【Methods】Using a spatial method, we selected three different types of conversion with same soil types and slope gradient. Taking measurements at various points across the sloped landscape, we analyzed organic carbon, etc after collecting soil samples.【Results】Our results show that soil organic carbon, available nitrogen and total nitrogen storage as well as soil water stable aggregates increased significantly along with the restoration time. When these measurements were compared to those from cultivated hillslopes, stocks of soil organic carbon (SOC), total nitrogen (TN) and available nitrogen (AN)all increased after Seabuckthorn planting for 5-years by 1.4, 0.5 and 0.5 times, respectively. Following Seabuckthorn planting for 10 years, SOC, TN and AN levels were found to increase by 6.6, 2.4, 1.5 times, respectively. When assessing the 5-year restorative effects of alfalfa planting, SOC, TN and AN increased by 0.4, 0.1, 5.0 times when compared to cultivated hillslopes, whilst these measurements increased by 0.7, 0.3, 5.2 times when extended to 10 years of alfalfa planting on hillslopes, respectively. Seabuckthorn planting on hillslopes significantly improved SOC and TN levels when compared to slopes planted with alfalfa. However, the available nitrogen in soil was significantly greater in alfalfa-planted hillslopes compared to Seabuckthorn-planted hillslopes. The differences in observed SOC and TN levels between Seabuckthorn-and alfalfa-planted hillslopes were due to the initial levels of these soil components prior to the conversion to forests and grassland. Whereas differences observed in AN between seabuckthorn-planted and alfalfa-planted hillslopes were attributed to the differing nitrogen fixation mechanisms between the two plants. As alfalfa root nitrogen fixations were greater than seabuckthorn, the amount of soil available nitrogen increased significantly in alfalfa-planted soil. Soil water stable aggregate content in 0.25 mm class increased by 41% and 56%, respectively, in hillslopes planted with Seabuckthorn for 5 and 10 years, respectively. This enhanced water stable aggregate in converted forest vegetation is attributed predominantly to increased particle size of 0.25-2 mm water stable aggregate content.【Conclusions】Our study suggests that grass and shrub vegetation play an important role in enhancing soil organic carbon and nitrogen storages as well as contributing to the stability of the soil structure. These effects should be considered when choosing measures to enhance soil fertility in sloped farmlands in China and it must be acknowledged how differing management of sloped lands can affect soil organic carbon reserves when using space substitute for time method in small regional scale.