Effects of Shelterbelt Transformation on Soil Aggregates Characterization and Erodibility in China Black Soil Farmland

Abstract

Farmland shelterbelts are widely used to reduce wind erosion, maintaining the ecological and food security of the black soil plain region of northeast China. In recent years, the protective effect of shelterbelts has been reduced due to tree degeneration. Efforts have been made to transform the construction of shelterbelts to conserve the stability of soil aggregates and enhance protection against erosion, however, the results are not well understood. To evaluate the impact of shelterbelt transformation on the stability of farmland soil aggregates and soil erodibility, three transformation modes of shelterbelts were selected, including pure Pinus sylvestris var. mongolica (ZC), pure Picea asperata (YS), and mixed Populus × xiaohei–Pinus sylvestris var.mongolica (ZY), with a degraded Populus × xiaohei shelterbelt (TYC) used as a control. We set up soil sampling points at 0.5H, 1H, 1.5H, 3H, 5H, 7H, and 9H from the shelterbelts and analyzed aggregate composition, mean weight diameter (MWD), geometric mean diameter (GMD), fractal dimension (D), soil erodibility (K-value), and their relationships to soil properties of the 0–10 cm, 10–20 cm and 20–40 cm soil layers and the shelterbelt structure by using dry and wet sieving and equation estimation methods. The results show that dry (d) sieved soil samples from the transformed shelterbelt-protected farmlands are mainly composed of 2–5 mm and >5 mm grain size aggregates; the sum of the two particle sizes ranged from 48.67% to 51.27%, significantly larger than in the degraded shelterbelts (15.37%), decreasing with increasing distance from the shelterbelts. The effect is most obvious in the 0–10 cm soil layer. Wet (w) sieved soil samples are all dominated by <0.25 mm and 0.25–0.5 mm grain size aggregates; the sum of the two particle sizes ranged from 78.25% to 80.82%, which do not vary significantly with the mode of shelterbelts. The dMWD and dGMD show significantly higher mean values in samples from transformed shelterbelt-protected farmland than in soil from degraded shelterbelt-protected farmland; their magnitudes differ depending on the transformation mode, showing a pattern of ZC > ZY > YS and decreasing with increasing distance from shelterbelts, while the opposite is true for D and K. The difference between wMWD and wGMD for different shelterbelts protected farmland is not significant and is significantly lower than that between dMWD and dGMD. Clay and silt content was highly significantly positively correlated with aggregates dMWD and dGMD, weakly positively correlated with wMWD, wGMD and wD, and highly significantly negatively correlated with dD and K values. This shows that particle composition parameters can be used to reflect the sensitivity of agricultural soils to wind erosion. Farmland shelterbelt porosity is the main factor driving changes in soil aggregates stability, soil erodibility, and other soil properties. The transformation of degraded farmland shelterbelts can decrease the porosity and reduce wind speed, resulting in improved stability and erosion resistance of the farmland soil aggregates by increasing the clay content of the farmland soils. These results are useful in renovating degraded shelterbelts, providing novel insights into how to regulate the stability of soil aggregates and soil erodibility characteristics at the shelterbelt network scale.