Soil Thresholds for Classification of Vegetation Types in Abandoned Cropland on the Loess Plateau, China

Soil erosion is a severe environmental problem on the Loess Plateau, China. Vegetation restoration is the most efficient method to control soil erosion and introducing late-successional plant species may accelerate natural succession. However, the progress is affected by soil conditions and the appropriate thresholds of soil condition for seed addition are needed. Our objectives were to identify the vegetation types, examine the key soil factors affecting the vegetation variation, and determine the thresholds of the soil factors for each vegetation type. Five vegetation types in secondary succession were identified: association (assoc.) Artemisia scoparia assoc. Bothriochloa ischaemum assoc. Artemisia gmelinii Artemisia giraldii assoc. Ostryopsis davidiana Rubus parvifolius L., Syringa oblate and assoc. Sophora viciifolia Years since abandonment (YEAR), alkali-hydrolysable nitrogen (AHN), soil water content (SW), and total phosphorus (TP) were the key factors used to discriminate among vegetation types. Assoc. A. scoparia developed in croplands that had been abandoned for less than 11 years. Assoc. B. ischaemum developed in croplands abandoned for more than 11 years with lower soil nutrient levels (0.30 <= TP <= 0.646 g kg(-1) and 16.72 <= AHN <= 32.62mg kg(-1)). Assoc. A. gmelinii + A. giraldii always required greater TP (0.646 <= TP <= 0.79 g kg(-1)) to develop. When soil water content was lower (6.6 <= SW <= 8.4%), the assoc. S. viciifolia developed; otherwise, other associations, such as assoc. O. davidiana, developed. These soil thresholds can be used as references for guiding restoration, such as specifying proper soil conditions for seed sowing.