Background Sloping farmland is the main type of farmland in Yunnan province, but at present, there are issues of serious soil erosion and low productivity in sloping farmland. The contour reverse-slope terrace (CRT) is an effective soil and water conservation measure for the restoration of degraded sloping farmland, but its intrinsic restoration mechanism has not yet been clarified. In this study, we investigated the changes in the content of glomalin-related soil proteins (GRSP) under this measure from soil aggregates, so as to elucidate the mechanism of CRT on the improvement of soil structure of sloping farmland.

Methods Field-based experiments were conducted to compare the composition and stability parameters of soil aggregates, along with the variations in glomalin-related soil protein (GRSP) and soil organic carbon (SOC) within different aggregate fractions (5.000– > 2.000, 2.000– > 0.250, 0.250– > 0.053, and ≤ 0.053 mm), between unmodified (CK) and contour reverse-slope terrace (CRT) measures. The study examined different seasons (dry and rainy), soil layers (0–20 cm and 20–40 cm), and terrace positions (above and below the terrace) in sloping farmland. The objective was to investigate the accumulation of GRSP under CRT and its mechanistic role in enhancing aggregate stability.

Results 1) The CRT measures significantly improved the soil aggregates structure of sloping farmland, the large soil aggregates (5.000– > 2.000 mm) below the terrace increased by 5.35%–6.45% (dry season) and 8.54%–8.61% (rainy season). 2) The CRT measures significantly enhanced the stability of soil aggregates below the terrace of sloping farmland (P < 0.05), CRT compared with CK, the MWD, GMD and R_0.25 of the aggregates under CRT increased by 0.18%–15.80%, 1.42%–41.10% and 0.94%–4.96%, respectively. 3) CRT compared with CK, the contents of EE-GRSP, T-GRSP and SOC in aggregates of each particle size fraction under CRT increased significantly (P < 0.05) in the dry and rainy seasons, with increases of 4.08%–73.40% and 13.30%–163.00% for EE-GRSP, 1.24%–16.90% and 1.14%–35.00% for T-GRSP, 0.03%–57.70% and 0.59%–59.80% for SOC, respectively; the CRT measures increased the production and accumulation rates of GRSP and promoted the SOC sequestration in aggregates with a particle size of > 0.250 mm. 4) Compared with CK, the content of GRSP in each particle size of soil aggregates in sloping farmland was positively correlated with aggregate stability parameters (MWD, GMD and R_0.25) by CRT. In addition, the results of redundancy analysis show that CRT measures improve the influence of T-GRSP on the stability of aggregates.

Conclusions CRT measures may improve the secretion of GRSP, increase the content of GRSP in aggregates, enhance the proportion of soil macroaggregates, and increase the stability of soil aggregates, thus improving the soil structure of sloping farmland, and the deployment of CRT measures may promote the restoration of degraded soils in sloping farmland.