Using soil aggregate stability and erodibility to evaluate the sustainability of large-scale afforestation of Robinia pseudoacacia and Caragana korshinskii in the Loess Plateau

Abstract

Revegetation in fragile ecosystems is an efficient means to increase aggregate stability and thus reduce soil erosion. However, the influence of large-scale afforestation on soil aggregate stability and erodibility in the Loess Plateau is not well understood. To assess the sustainability and suitability of widespread, long-term planting of plantations in terms of soil aggregate stability (mean weighted diameter (MWD) and geometric mean diameter (GMD)) and erodibility (K), we performed a large-scale investigation of soil aggregate stability and soil erodibility of Robinia pseudoacacia (RP) and Caragana korshinskii (CK) plantations on the Loess Plateau. The results showed that the soil macroaggregate fraction (>0.25 mm) content under RP and CK plantations had a decreasing trend with increasing latitude. Moreover, soil aggregate stability and soil organic carbon (SOC) and total nitrogen (TN) contents in RP and CK plantations decreased with increasing latitude. RP and CK plantations did not always result in improvement of soil aggregate stability and the accumulation of SOC and TN, which depended on the latitude and precipitation conditions. Specifically, RP planting in the south warm temperate forest subzone (STFZ) and north warm temperate forest subzone (NTFZ) could enhance the macroaggregate content (>5 mm), soil aggregate stability and soil nutrients, while CK planting for the improvement of the soil macroaggregate content, soil aggregate stability and soil nutrients in the temperate forest steppe subzone (TFSZ) was more effective than was RP planting. Correlation analysis showed that the latitude, longitude and mean annual precipitation (MAP) were significantly correlated with soil MWD, GMD, K values, SOC and TN across the RP and CK plantations, and soil MWD, GMD, and K values were significantly correlated with SOC and TN in RP and CK plantations. The changing trends of SOC and TN with latitude and longitude were consistent with that of soil aggregate stability with latitude and longitude under RP and CK plantations, indicating that changes in the SOC and TN concentrations will cause changes in soil aggregate stability and erodibility. Additionally, the effects of RP and CK plantations on the soil aggregate stability, erodibility, SOC and TN content were different and distinguished by latitude and MAP. RP plantation resulted in more SOC and TN accumulation, enhanced the soil aggregate stability and decreased erodibility compared with CK plantation in areas with MAP >500 mm or latitude <36° N; in contrast, in sites with MAP <500 mm or latitude >36° N, CK plantation had slight advantages over RP plantation. This study indicated that the changes in soil aggregate stability, erodibility, SOC and TN accumulation following RP and CK plantations had different patterns along latitude and precipitation gradients. Therefore, the 36° N or 500 mm precipitation threshold may be the dividing line for planting RP and CK plantations. Overall, our study produces unique insights into the relative significance of environmental factors that influence aggregate stability at large scales and is useful for selecting suitable afforestation species and locations to optimize forest management with sustainable production.