Background Yunnan is situated in a mountainous area, and there are only two seasons, rainy season and dry season.In rainy season, concentrated and heavy rainfall increases a great probability of soil erosion.As the main type of arable fields, sloping farmland is regarded as the key cultivated land type for soil and water loss.The contour reverse-slope terrace (CRT) increases soil water storage, root exudation, enzyme activities and reduces soil surface runoff and moisture loss, erosion and degradation, which improves the soil environment through changes of runoff and sediment yield, has been well documented, but the reasons remain unclear.In this study, we aim to elucidate the mechanisms of soil structure improvement and ecological remediation with the CRT in sloping farmland.

Methods A field location experiment was conducted in the watershed of Yizhe in Songhua dam in Yunnan with two measures: Above and below the terrace in CK sloping farmland (CK-A, CK-B), above and below the terrace in CRT sloping farmland (CRT-A, CRT-B).The composition and stability were analyzed by measuring 0-5, 5-10, 10-20 and 20-40 cm soil mechanical-stable aggregates, physical property and constructed minimum data set to evaluate soil structure.In order to evaluate the effect of CRT on soil structure improvement in sloping farmland, a soil structure evaluation index system based on aggregates properties was constructed.

Results 1) CRT measures improved the soil pore status, aggregates composition characteristics and stability significantly.2) Six indicators including soil bulk density, soil porosity, geometric mean diameter (GMD), stable aggregates (R_0.25), total aggregates (A_t) and >2-5 mm soil aggregates were identified as the main indicators for soil structure evaluation in the minimum data set.The verification of total data set suggested that the minimum data set reflected the effective information and accuracy evaluation for soil structure.3) The soil structure index in CRT, compared with CK, ranging from 0.26 to 0.58, increased by 23.1% and 36.2%, indicating that soil structure was on the middle level; meanwhile, below the terrace in CRT, soil structure index in the 0-5, 5-10 and 10-20 cm soil layer increased by 25.2%, 27.3% and 25.7% compared with that in CK, respectively.It is mainly due to the erosion protection from both the upper and lower steps.

Conclusions The CRT measures promotes soil structure via increasing the proportion of soil total aggregates and soil porosity, improving soil structure and therefore is the optimal ecological restoration measure of degraded soil ecosystem in sloping farmland.The soil structure evaluation method based on the minimum data set is applicable in this area of research and can represent the soil structure performance of sloping farmland with CRT measures accurately.